Paul Slangen Student identification number 1143565
Prefeasibility Study on Askjelldalsvatn Power Plant
MSc. Thesis report
Bergen, 25 February 2008
Delft University of Technology Faculty of Civil Engineering and Geosciences Section of Hydraulic Engineering
In association with Bergenshalvøens Kommunale Kraftselskap
Examination committee Prof. drs. ir. J.K. Vrijling (TU Delft) ir.J. van Duivendijk (TU Delft) ir. R.J. Labeur (TU Delft)
Supervisor J. Matre (BKK Rådgiving)
Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
PREFACE
This report is the result of a study performed by the author for his graduation thesis for the Master of Science title from Delft University of Technology, Faculty of Civil Engineering, for the Section of Hydraulic Engineering.
The study is performed in association with the power company Bergenshalvøens Kommunale Kraftselskap (BKK) in Bergen, Norway. The author has investigated the feasibility of constructing a new power plant within an existing power scheme. This report presents the process and findings of this study.
Decision makers are encouraged to read not only the summary and conclusions but also sections 1.1 and 9.4 for a better understanding of the selected solution.
For readers who are already familiar with hydropower in Norway and the project area the chapters 5, 9, 10 and 11 will be of main interest.
Chapter 8 and sections 10.2 and 10.3 contain the necessary information for an in-depth economic analysis of the project.
Readers who are interested in the background for the design, the results and the process are welcome to read the complete report.
My thanks go out to Jostein Matre for the opportunity he has given me to write my thesis study with BKK and for his work as supervisor of the project. I would like to express my gratitude to Han Vrijling, professor of the Section of Hydraulic Engineering, for his willingness to support this thesis work abroad and for his valuable comments. I would also like to thank Hans van Duivendijk and Robert-Jan Labeur who have supervised and valuably criticised the thesis work from their positions at TU Delft. Many thanks also go out to everybody of BKK Rådgiving for their help in any way, early lunch, wise advice and just good company. Finally I would like to thank my parents, my sister and Ina for their great support during the last six months.
Bergen, February 25 2008
Paul Slangen
i Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
EXECUTIVE SUMMARY
Bergenshalvøens Kommunale Kraftselskap (BKK) is the owner of the Evanger hydropower scheme and has requested to investigate the feasibility of a new power plant in this scheme. The objective of this study is to identify the technical and economical feasibility of a hydro power plant that utilizes the water flow between Holskardvatn and Askjelldalsvatn. A combined pump turbine plant with an installed capacity of 8 MW is found to be the optimal feasible solution. The net present value and construction costs are estimated as 46.2 MNOK and 98.7 MNOK respectively. The pumping up of 50 Mm3 yearly from Askjelldalsvatn to Holskardvatn will give a better balance in the system and lead to a large shift from summer to winter production in the Evanger plant.
The free power market in Norway results in high energy prices in winter. The precipitation is then small and the energy demand is large as a result of the cold. In summer the runoff is high and with a low energy demand, the price is low. This difference in price and the topography in Norway make it beneficial to store water in reservoirs in summer and utilize it for energy production in winter.
The main power plant in the Evanger scheme is the Evanger power plant, with a head of 770m. The intake reservoir for this plant is the relatively small Askjelldalsvatn and apart from the winter months the water level is at the highest regulated water of 805 m.a.s.l.. Holskardvatn is a large reservoir above Askjelldalsvatn, where water is stored in summer. The water is transferred through a tunnel system to Askjelldalsvatn in winter to ensure maximum production in the Evanger plant. The available head between these reservoirs varies from 25m to 70m.
In the identified range of flows both Francis and Kaplan turbines can be selected. The turbine characteristics are essential for the design, because of the widely varying available head. Several experience values for design have been found in literature and are presented.
The main requirement for the new power plant is that it may not become a bottleneck for the Evanger plant. To fulfil this requirement it is decided to keep the existing gate system, beside the new plant. To find the most economic design of the power plant, two starting points of design are used: minimum costs and maximum value. The main design variables are the outlet level, the installed capacity and a pumping mode.
For all alternatives one unit is selected and a Francis turbine preferred over a Kaplan turbine, because it is cheaper. For all alternatives drawings, a Bill of Quantities and an energy estimate have been provided. The latter two have been used as input for the economic analysis.
Three basically different preliminary designs are proposed, based on the two starting points for the design: - Alternative I aims to minimize the costs. The outlet level at the highest regulated water level in Askjelldalsvatn makes a short and cheap alignment possible. However, only 40% of the available water can be utilized for energy production for this alternative, the rest needs to be bypassed because the available head is outside the range of the turbine. - Alternative II is designed as a compromise between maximum energy and minimum construction costs. The water is bypassed around the existing gates, through the power plant and into the existing lower diversion tunnel with an outlet at 783 m.a.s.l.. The large costs for the underground works however result in a small profit for this alternative. Installing a combined pump turbine in this design shows very good results. 50 Mm3 water is pumped up in summer and almost 65
ii Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
GWh of energy production shifts from summer to winter, mainly in the Evanger plant. This shift results in additional yearly benefits of approximately 2 MNOK. - Alternative III is designed to maximize the potential energy and the outlet is placed at 770 m.a.s.l. This makes it possible to utilize almost all available water for energy production. This design showed similar economic results as the previous alternative (also for the combined pump turbine), but with both higher costs and value.
The combined pump turbine solution is studied in more detail. The optimization process focuses on the outlet level and installed capacity. Both combined pump turbines from the preliminary design phase, plus a design with a lower outlet at 760 m.a.s.l. have been optimized for the installed capacity. An outlet level of 768 m.a.s.l. with an installed capacity of 8 MW for the pump turbine is an optimum solution. With the new plant, the water level in Askjelldalsvatn does not go below this water level as a part of the snow melt can be pumped up to Holskardvatn and thus does not need to be stored in Askjelldalsvatn. The selected installed capacity is the highest installed capacity that is fully utilized; one additional MW would only be used incidentally and is therefore not found to be beneficial.
It is recommended to apply for a license for the plant as soon as a study has been performed to estimate the costs for the transmission system in more detail. For a more detail design of the plant it is also necessary to acquire more detailed topographic maps and more specific information on turbines from turbine manufacturers
iii Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
TABLE OF CONTENTS
Preface ...... i Executive Summary...... ii Table of Contents ...... iv List of Figures and Tables ...... vi Glossary of abbreviations, definitions and units ...... vii Introduction ...... ix 1. Design Process ...... 1 1.1. General Approach...... 1 1.2. Methodology...... 2 2. Hydropower in Norway ...... 3 2.1. Hydropower ...... 3 2.2. Topography...... 4 2.3. Climate...... 5 2.4. Hydrology ...... 5 2.5. Power Market in Norway...... 6 2.6. Hydropower Plants in Norway ...... 8 3. Project area ...... 10 3.1. Evanger Power Scheme...... 10 3.2. Project Site ...... 13 3.3. Reservoir Operation...... 16 3.4. Potential Energy...... 21 4. Turbines...... 22 4.1. Turbine Type ...... 22 4.2. Turbine Selection ...... 22 4.3. General Design Considerations ...... 23 4.4. Performance Characteristics...... 27 4.5. Case Studies ...... 29 4.6. Preliminary Turbine Design ...... 29 5. Schedule of requirements ...... 32 5.1. Concept ...... 32 5.2. Boundary Conditions...... 32 5.3. Requirements and Assumptions...... 33 5.4. Design of Alternatives...... 34 6. Dimensioning Project Components ...... 38 6.1. Access Roads and Tunnels ...... 38 6.2. Weirs and Intake Works ...... 38 6.3. Waterways ...... 39 6.4. Powerhouse-Civil Works ...... 41 6.5. Electrical and Mechanical Equipment ...... 41 6.6. Contingencies ...... 44 6.7. Planning and Administration ...... 45 6.8. Financing Cost ...... 45 6.9. Transmission ...... 45 6.10. Increase of Cost Price 2005 – 2007 ...... 47 7. Energy Estimate ...... 48 7.1. Head Losses ...... 48 7.2. Energy Estimate from Reservoir Operation ’97-‘07 ...... 48 7.3. Energy Estimate Using Vansimtap ...... 49 8. Economic Analysis ...... 52 9. Preliminary design ...... 54 9.1. Alterative I - Minimum Construction Costs ...... 55 9.2. Alternative II – Compromise ...... 61 9.3. Alternative III – Maximum Value ...... 65 9.4. Selection of the Best Alternative...... 68 10. Detailed design ...... 72
iv Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
10.1. Optimization ...... 72 10.2. Final Design ...... 79 10.3. Construction Schedule ...... 80 10.4. Reservoir Operation ...... 81 11. Conclusions and Recommendations...... 82 11.1. Conclusions...... 82 11.2. Recommendations ...... 83 References ...... 84
APPENDIX I PROJECT AREA APPENDIX II PRELIMINARY DESIGN DRAWINGS APPENDIX III DETERMINE HEAD LOSS APPENDIX IV BILLS OF QUANTITIES APPENDIX V TURBINE CHARACTERISTICS APPENDIX VI SURGE SHAFT REQUIREMENT APPENDIX VII ENERGY ESTIMATE IN VANSIMTAP APPENDIX VIII DETAILED DESIGN DRAWINGS APPENDIX IX DETAILED DESIGN TABLES
v Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
LIST OF FIGURES AND TABLES
Figure 1 Major components of a hydropower plant Figure 2 Diagram showing precipitation – runoff in 2000 for Fosse/Bergevatn Figure 3 Monthly energy spot prices Nord Pool Figure 4 Aura power plant is a typical high head power plant in Norway Figure 5: Location project area in Norway Figure 6 Layout Evanger Power Scheme Figure 7 L Section Evanger Power Scheme Figure 8 L-Section Transfer Tunnel Holskardvatn - Askjelldalsvatn and Vassøyane Figure 9 Water levels in Holskardvatn 1997-2007 on weekly basis Figure 10 Outflow of Holskardvatn in m3/s Figure 11 Holskardvatn water levels vs. outflows 1997-2007 Figure 12 Water levels in Askjelldalsvatn 1997-2007 on weekly basis Figure 13 Outflow Askjelldalsvatn 1997-2007 derived from weekly data Figure 14 Gross head Holskardvatn - Askjelldalsvatn vs. outflows Holskardvatn Figure 15 Determine net positive suction head at runner outlet Figure 16 Examples of performance diagrams of a Francis turbine and a Kaplan turbine Figure 17 Influence of outlet level on energy production Figure 19 Decision schedule for the estimate of produced energy in Askjelldalsvatn plant Figure 20 Sketch showing the principle alignment of the suggested alternatives Figure 21 Optimization outlet 782.9 m.a.s.l. Figure 22 Optimization outlet at 770 m.a.s.l. Figure 23 Optimization outlet at 760 m.a.s.l. Figure 24 Optimization outlet level of Askjelldalsvatn power plant Figure 25 Reservoir operation characteristics for outlet 770 m.a.s.l and 8 MW Figure 26 Holskardvatn and Askjelldalsvatn water levels for outlet 760 m.a.s.l. and 8 MW Figure 27 Characteristics of Holskardvatn for outlet 770 m.a.s.l. and 9 MW
Table 1 Salient Features Evanger Power Plant Table 2 Salient Features Holskardvatn and Askjelldalsvatn Table 3 Hydrology of Project Area Table 4 Energy estimates based on ten years of production data (1997-2007) Table 5 Result of energy estimate with Vansimtap Table 6 Parameters for economic analysis Table 7 Rehabilitation of hydro power plants Table 8 Construction costs Structural Design Alternative I a Table 9 Construction costs Structural Design Alternative I b Table 10 Construction costs Structural Design Alternative I c Table 11 Comparison structural designs for minimum costs Table 12 Comparison installed capacities for alternative I Table 13 Salient features Alternative I Table 14 Comparison installed capacities alternative II Table 15 Salient features Alternative II: turbine and combined pump turbine solution Table 16 Comparison installed capacities alternative II Table 17 Salient features Alternative III Table 18 Comparison of alternatives Table 19 Optimization outlet 782.9 m.a.s.l. Table 20 Optimization outlet at 770 m.a.s.l. Table 21 Optimization outlet at 760 m.a.s.l. Table 22 Optimization outlet level of Askjelldalsvatn power plant Table 23 Salient features Askjelldalsvatn power plant
vi Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
GLOSSARY OF ABBREVIATIONS, DEFINITIONS AND UNITS
Abbreviations B/C Benefit cost ratio HRWL Highest regulated water level LRWL Lowest regulated water level m.a.s.l. Meter above sea level MNOK Million Norwegian kroner NOK Norwegian kroner NPV Net present value NVE Norwegian Water and Energy resources Directorate
Definitions *H Design head (m) * Hn Net design head (m) * Hg Gross design head (m) Hnmin Minimum net head for the turbine (m) Hnmax Maximum net head for the turbine (m) *Q Best efficiency turbine discharge (m3/s) ºQ Maximum turbine discharge (m3/s)
Units Volume: m3 and Mm3 (=106 m3) Power or capacity: kilowatt (kW) or megawatt (MW) Energy: kilowatt hour (kWh) or gigawatt hour (GWh) Force: Newton (N) or kilo Newton (kN) Rotational speed: rounds per minute (RPM)
vii Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
viii Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
INTRODUCTION
Electrical energy in Norway is (and has been) almost solely produced by hydro power plants. At the beginning of the twentieth century the construction of hydro power plants started close to villages and towns to supply the local communities with electricity. Over time larger schemes have been developed and local transmission networks have been connected to each other to form a national grid. Nowadays almost all economically and technically feasible large power schemes have been developed, with some promising undeveloped hydro power potential being protected for environmental reasons. Norwegian power companies mainly focus on small scale hydro and a further optimisation of existing power schemes to increase their energy production.
Bergenshalvøens Kommunale Kraftselskap (BKK) is the largest electricity supplier on the west coast of Norway, with the head office located in Bergen. The largest power scheme BKK owns is the Evanger power scheme, which consists of a long head race tunnel to which the water from several natural lakes is diverted. BKK wants to optimize this scheme and has asked their consultant to investigate the technical and economic feasibility of a new power plant between two existing reservoirs in the Evanger scheme; Holskardvatn and Askjelldalsvatn.
The objective of this study is to investigate the feasibility of a hydro power plant that utilizes the water between Holskardvatn and Askjelldalsvatn. In this report it will be shown that a combined pump turbine plant is the best solution.
The work has been divided in three parts. First all information has been acquired necessary to arrive at the schedule of requirements. In a second phase, several alternatives have been generated, based on the schedule of requirements. For all alternatives costs and energy estimates have been made on which the economic analysis has been based. After comparing the technical and economic feasibility, the combined pump turbine has been selected for the final phase: a detailed optimization.
The first Chapter describes the design process which has been followed for this work. For a good understanding of the project, Chapter 2 presents some background information of hydropower in Norway. The Evanger scheme is described in Chapter 3. The turbine is a key component for the design of this power plant, because it is a large cost component and its performance and hydraulic range are important for the energy that can be produced. Therefore Chapter 4 discusses the possibilities of turbines. Chapter 5 is a key chapter in that it links the preceding chapters to the preliminary design of the alternatives. The schedule of requirements is deduced from the preceding chapters and gives a direction to the preliminary design study in the form of starting points for the design. The project components that can be found in the preliminary designs are discussed in general in Chapter 6 and the methodology to estimate the energy production of the alternatives is presented and discussed in Chapter 7. The short Chapter 8 describes the method that is used for the economic evaluation of the projects. The preliminary alternatives are presented in Chapter 9. An optimization for the power plant is performed in Chapter 10. The final design of the plant is also presented in this Chapter. The last Chapter 11 contains the conclusions and recommendations.
ix Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
1. DESIGN PROCESS
This Chapter describes the design process for this prefeasibility study. A general introduction to the problem is described and the methodology that has been followed in this study is presented.
1.1. General Approach
1.1.1. Objective The Evanger scheme is located close to Voss in western Norway. BKK Rådgiving has been asked by BKK Produksjon to investigate the possibility of constructing a new power plant in the existing Evanger scheme, between the reservoirs Holskardvatn and Askjelldalsvatn. The Evanger scheme is described in Chapter 3.
This request is also the subject for this thesis and the main objective for this thesis is formulated as:
Investigate the technical and economic feasibility of a hydro power plant which utilizes the water between the reservoirs Holskardvatn and Askjelldalsvatn
1.1.2. Concept The function of the project is to produce energy utilizing the water flow between the reservoirs Holskardvatn and Askjelldalsvatn. The concept is a hydropower plant which can include dams, intakes, conveyance systems, a powerhouse, spillways, an outlet or other structures related to the power plant.
1.1.3. Value The benefits of the project come from the sales of energy. A distinction can be made between the benefits gained from the sale of new energy and the benefits that are gained by shifting the production of energy to a more valuable time, in this case from summer to winter.
1.1.4. Costs The costs of the project are determined for different phases in the life of the power plant. - Costs for the planning process - Construction costs are associated with the construction of a new hydro power plant. Besides the direct contract costs, this includes financing costs, costs for administration, environmental costs, costs for land etc. - During operation yearly costs for operation and maintenance as well as irregular costs for refurbishment occur.
1 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
1.2. Methodology To achieve the main objective of this thesis study, the work is divided in three phases; the problem is described in a literature study, several alternatives are generated in the preliminary design phase and one alternative is prepared further for a detailed design.
1.2.1. 1st Phase: literature study This first part of the study intends to provide the necessary information to be able to design different options. This phase forms the basis of the feasibility study and the main objective is to give insight into the design problem and the important aspects that should be given attention in the design process. For a complete understanding of the problem the following aspects need to be discussed in this phase: - hydropower in Norway in general - the system in which the new plant will operate: the Evanger power scheme - the project area and the operation regime of the relevant part of the Evanger power scheme: Askjelldalsvatn and upstream - the possibilities and limitations for turbines need to be examined as this is a major influence on both the cost and benefit side of the design A schedule of requirements is prepared from this information.
1.2.2. 2nd Phase: preliminary design The objective of the preliminary design study is to identify the best solution for a hydropower plant that utilizes the water flow between Holskardvatn and Askjelldalsvatn. The best solution is defined as a technically feasible solution that has the most promising economic results. Several alternatives will be generated, trying to incorporate each of the design variables in different ways. For each alternative the technical feasibility will be discussed, a cost estimate will be prepared, the energy that can be produced will be estimated and an economic analysis will be performed. It is anticipated that the different alternatives are roughly optimized for the sake of the comparison.
The actions in this phase can be summarized as follows: - generate different alternatives - estimate construction costs - estimate the energy that can be produced - perform an economic analysis - compare the alternatives and select the best solution
1.2.3. 3rd Phase: detailed design The detailed design phase is necessary to give a more accurate estimate of the economic feasibility of the project. The best solution of the preliminary design phase is optimized in more detail so that the construction costs and possible energy can be determined with satisfactory accuracy.
This phase will entail the following actions: - optimize the selected alternative - detail design of main project components - provide a construction schedule - analyse the economic feasibility of the project in more detail
2 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
2. HYDROPOWER IN NORWAY
Climate and topography in Norway are favourable for hydropower. High heads and short tunnel alignments are possible, because of the flat mountain plateaus and short distances from the mountain tops to the sea. The high precipitation of up to 3m per year results in large flows. The Nordic power market is a free market and is characterized by large demands in winter and moderate demands in summer. Because most runoff occurs in the summer months, the energy price is lowest in the summer and at its highest in winter. Storage plants are therefore designed to be able to store the water in the summer so that it can be utilized for energy production in winter.
2.1. Hydropower The energy potential is an indication of the value that a hydropower plant can create. Each of the main components of a power plant has a different function. All components together are required to produce electricity in an operational power plant.
2.1.1. Energy Potential Hydropower plants utilize the energy of water to generate electricity. The energy potential is determined by the static head (difference between upstream and downstream water level) and the available water:
E = m ⋅ g ⋅ H = ρ ⋅V ⋅ g ⋅ H [J]
For hydropower plants the energy per unit time (power) is usually considered:
P = E = ρ ⋅V ⋅ g ⋅ H available t t
= ρ ⋅ Q ⋅ g ⋅ H [W]
E = energy of water [J] m = mass of water [kg] g = acceleration of gravity = 9.81 [m/s2] ρ = density of water = 1000 [kg/m3] H = gross head [m] t = time [s] V = water volume [m3] Q = discharge [m3/s] Pavailable = available power [W]
However, not all available energy can be converted as losses occur in the hydraulic system, the convergence from hydraulic to mechanical energy in the turbine and in the convergence from mechanical to electrical energy in the generator. These losses can be summed and an overall efficiency for the power plant can be introduced: ηoverall. The electric energy that can be produced by a hydropower plant is then defined as:
Pplant = ρ ⋅ Q ⋅ g ⋅ H ⋅η overall [W]
2.1.2. Hydropower Plant The main function of a hydropower plant is to produce electricity. The turbine converts the hydraulic energy into mechanical energy and the generator converts the rotational energy into electrical energy. A hydropower plant consists of several components which are all responsible for a part of the process to create electricity from water, see Figure 1:
3 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
- The dam raises the water level to create sufficient head. It can have an additional function to create a reservoir. A reservoir can store water and can be used to control the water flow to offer more flexibility in the production of energy. The conveyance system (in the figure a penstock) transports the water from the intake to the turbine. It is a closed system which can withstand the pressure of the water. - The turbine converts the hydraulic energy (discharge and head) into rotational energy. - The generator is connected to the turbine via a shaft and converts the rotational energy of the turbine into electrical energy. - The powerhouse provides place for the electro- mechanical equipment. - The transmission lines transport the electricity from the power plant to the users.
Figure 1 Major components of a hydropower plant
2.2. Topography1 Topographically, Norway is characterized by a moderate height mountain range along the full length of the country. The mountains have been cut by rivers and glaciers, creating narrow gorges and wide valleys. The bottoms of these cuts are currently above and below sea level (fjords).
Around 30% of Norway’s land is at elevations between 0 and 300 meters above sea level and another 30% between 300m and 600m. The remaining land is mostly at elevations between 600m and 1200m in the form of wide mountain valleys and plateaus. The highest peak is Galdhøpiggen at 2469m above sea level.
Some 60 million years ago the flat slab of land that has now become the Scandinavian Peninsula, was lifted out of the ocean. The western part was more elevated, so that a steep slope towards the sea was formed, while the flat plateau gently sloped eastwards. Already during this period the rivers started cutting deep valleys into the mountains, flowing west towards the sea.
1 This Section is based on information from [HVEDING, 1992]
4 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
In the following glacial periods the ice slowly moved towards the edges of the plateau, creating large U-shaped valleys. The ice also cut depressions in the plateau which have later filled up to become countless lakes of all different sizes. Today 5% of the land area in Norway is covered with fresh water.
2.3. Climate The climate in Norway is determined by the northerly location of the country and the warm current flowing from the Gulfstream towards the coast of Norway. This results in a relatively mild climate in Norway compared to other countries at these northerly latitudes.
The weather however fluctuates significantly in both time and place. The average annual temperature is some 8 °C along the western coast and in the mountains, large areas have an average annual temperature of -4 °C or less. The average temperature in winter is above freezing point all along the coast up to the Lofoten; while the temperature in inland areas has winter averages around -15°C. Extreme minima in the inland can be as low as -45°C. January and February are the coldest months, while the warmest period is mid July in the inland and a bit later in coastal and mountain regions.
Some areas east of the mountain plateau experience hardly any rain, in the order of a couple of 100mm, whereas mountainous areas a bit inland of the western coast record annual rainfalls up to 3000 mm. The western coast receives most of its precipitation in autumn and winter. Typical annual average rainfall for the coast is 2000 mm in the south to 1000 mm in the North. [MET.NO]
2.4. Hydrology The Western coast of Norway is known for its high rainfall, with annual averages up to 3000 mm. In winter, which lasts for 4 to 6 months in the mountains, precipitation is retained as snow. The snow melts from May to July and swells rivers to spring floods, with the severest floods occurring late July, when melting has also reached the mountains. Evaporation and transpiration are modest throughout the country. It is less then 10% of the total precipitation in the mountains, where the runoff can go up to 100 l/s/ha. [HVEDING, 1992]
As an example Figure 2 shows the difference between the runoff into Bergevatn reservoir at 500 m.a.s.l., and the precipitation measured at the nearby located power plant Fosse at 402 m.a.s.l. It is meant to illustrate that most precipitation falls in the beginning and end of the year: in autumn and winter. It is clear that the runoff reaches its maximum between weeks 17 and 28; from late April to mid July. The precipitation before this time is mostly in the form of snow which does not runoff into the reservoir; although some part either falls on the reservoir area or melts and reaches the reservoir after all. The snow starts melting in April and at this time the runoff is much larger than the precipitation. From week 33 (mid August) the precipitation and runoff are more or less equal: the precipitation falls in the form of rain, and because the rock only has a small ground water flow and a small sandy overburden the water runs of almost instantaneously.
5 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Precipitation Runoff Diagram Fosse - Bergevatn
250
200
150 Prec ipitation
100 Runoff
50
Precipitation/Runoff (mm) Precipitation/Runoff 0 0 5 10 15 20 25 30 35 40 45 50 Week No.
Figure 2 Diagram showing precipitation – runoff in 2000 for Fosse/Bergevatn2
The above is typical for the mountains in Western Norway and the differences between runoff and precipitation are even larger at higher altitudes, where a larger percentage falls as snow which melts and creates high runoff peaks in the summer.
2.5. Power Market in Norway
2.5.1. Free Market Norway deregulated its electricity market in 1991 with the Energy Act and the state owned company Statnett was assigned the task of transmission system operator as a monopolist. Statnett established the power exchange for trading electricity in 1993, back then only covering the Norwegian market. The purpose of the power exchange was to make the electric power sector more efficient. Before the Energy Act every power company was obliged to provide power to all customers in their assigned region. To ensure energy in all situations, also dry years, each power company needed spare installed capacity: hence a larger installed capacity than the average demand was necessary. The Energy Act took away the obligation to only provide power for customers in the region and it thus also took away the need for spare capacity for the driest years. The result was a spread for spare capacity over the whole country and more spare capacity in the form of import from abroad: hence an efficiency upgrade on regional scale was achieved. Because the power sector pressures the environment tremendously, the investment in the sector should be both economically and environmentally sound. This is thought to be achieved by letting the consumer decide from which producer he will buy his energy. Approximately 200 utilities are competing to supply electricity to Norwegian customers in an open market, where customers can pick an energy provider at will and at no fixed cost. The results for the consumers are believed to be both lower energy prices and better service. The transmission system operation and extension are taken care of by Statnett.
In 1996 the power exchange name was changed to Nord Pool, with the collaboration with the Swedish power market. [NORDPOOL.NO]
2 Production data from BKK Produksjon. The Fosse power plant and the Bergevatn reservoir are located close to Bergen in the municipality of Vaksdal in the county of Hordaland.
6 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
With the Energy Act the immediate need for new power plants became less urgent, as the rearranging of the spare capacity from a regional to a national level lead to a smaller required total spare capacity in the whole country. The power companies were now able to use parts of their spare capacity for regular production instead of building new power plants.
The public opinion and politics also turned against construction of new (large) power plants which has resulted in the Norwegian energy balance being negative since the middle of the nineties: the ever increasing demand was not met by an increase in production. Norway has thus changed from an exporting energy country before the nineties to an importing country of energy (in an average year), see the data below.
Salient Features Power Market Norway [nve.no]
Area: 324,220 square kilometres Population: 4.6 million Electric consumption: 125.9 TWh (2005) Electric generation in an average year: 119.7 TWh (from 2005) of which: Hydropower: 99 per cent Other (thermal) power: 1 per cent
2.5.2. Prices The price of energy in Norway will fluctuate, depending on demand and supply. Figure 3 shows that the prices do vary over the year with minimum average in May, just below 200 NOK/MWh and the maximum average in December at more than 270 NOK/MWh. Low rainfall and low reservoir levels led to a record price level in the winter of 2002/2003 with the price for on MWh more than 530 NOK.
Prices at Nord Pool Spot
600
500
AVG Monthly Prices 400 2000 2001 2002 300 2003 2004 200 2005
Price (NOK/MWh) 2006
100
0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month
Figure 3 Monthly energy spot prices Nord Pool The figure also shows the trend of an increasing energy price per year, which is not only the result of inflation. It is also explained by an increasing energy demand as a result of economic growth whereas the supply of energy has not kept up at the same pace because of the aversion of constructing new large power plants. 2006 was an exceptional dry year.
7 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
The variation of the price of energy over the year is the result of an increased demand in the cold autumn and winter months, with modern heating in Norwegian households mainly depending on electricity (and some waste burning). But also the supply side has its share in the higher prices in autumn and winter; although precipitation is high, the water inflow in the hydropower plants is rather low due to snow. Hence the large demand for energy, together with a limited amount of available energy in the form of water stored in reservoirs, results in higher energy prices in autumn and winter. [SSB.NO]
2.6. Hydropower Plants in Norway Norway provides some good possibilities for hydropower development. The flat mountain plateau has many natural lakes with large catchments which can be tapped and dammed to turn them into large reservoirs. The distance between the watershed and the sea is generally short, which has created many smaller water courses, most of them rather steep and short. Precipitation, especially along the coast and in the mountains is high so that naturally a lot of water is available at high altitudes: an ideal situation for hydropower development. There are thus many convenient sites for hydropower development on different scales all over the country.
Power plants in Norway are typically of a high head type: water is taken into a conveyance system (often a tunnel) at the top of the mountain and transported to the power house at the foot of the mountain. The geology makes it possible to construct (cheap) tunnels in rock with only few stability measures. A small dam is often built at the outlet of a lake, which can then be utilized as an intake reservoir which will also be able to store water. The lake is usually regulated a couple of meters above and many meters below natural water level. In this way a large reservoir can be created at relatively low costs.
The Aura power plant is a typical Norwegian high head power plant with reservoirs to store water and several brook intakes. Holbu reservoir functions as the intake reservoir and Aursjø reservoir is used to increase the storage capacity. A tunnel and shaft connect the intake reservoir to the powerhouse, some 850m lower. To increase the available water, several brook intakes are diverted into the reservoirs or directly into the head race tunnel.
Figure 4 Aura power plant is a typical high head power plant in Norway
8 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
The reservoirs are able to store water until the prices on the exchange market are high. The owner of the plant will try to store the water that runs into their reservoirs in the summer (when the energy price is low) and use most of it in winter to produce energy. The reservoir will thus be filled in the summer months with the direct runoff from rainfall and the melt water from snow that has accumulated in the catchment. The water is utilized for energy production in winter and the reservoir will be drawn down almost completely during this period. The inflow in winter is small as the precipitation usually falls in the form of snow. The reservoir needs to be partially empty just before the summer to create storage space for the melt water of the snow. The energy production thus follows the energy price quite closely, except for the period just before summer when it is necessary to draw down the reservoir to provide storage space for the snow melt water. The energy price is often low in these summer months, but the power plant needs to produce energy with to empty the reservoir.
Besides this annual production cycle, these power plants often have a daily cycle as well: the plant is operating close to its capacity during day time to supply consumers with peak power, while it may be shut down at night when the energy demand is low.
Hydropower plants with sufficient storage are thus able to follow the energy demand quite closely in both a daily and an annual production cycle.
9 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
3. PROJECT AREA
The project area is located around 60˚70’ N and 6˚10’E, in western Norway, somewhat north of Voss, see Figure 5. The Evanger power plant is the main plant in the Evanger power scheme with Askjelldalsvatn as the intake reservoir. Holskardvatn is a reservoir located above Askjelldalsvatn. The water flow between the two reservoirs is nowadays not utilized for energy production.
Figure 5: Location project area in Norway [1], close to Voss and Bergen [2]. This study will look at the feasibility of a hydropower plant that utilizes the water flow from Holskardvatn to Askjelldalsvatn (the project site) [3].
3.1. Evanger Power Scheme
3.1.1. Lay Out Evanger Power Scheme Two power plants have been constructed in the Evanger power scheme; the Evanger power plant and the Oksebotn power plant. The Evanger power plant is the main plant in this scheme and described in Table 1 and Figure 7 and Figure 6. From the underground power house a short steep steel lined shaft goes up to the fore bay, at 721 m.a.s.l. A surge shaft is connected to the fore bay. The slightly inclined head race tunnel is some 35 km long and connects the fore bay to the main intake reservoir Askjelldalsvatn, picking up several water streams under way. The water from the eight brook intakes and the lakes Kvandalsvatn and Eide-Fannadal flows into the head race tunnel by gravity; hence their intake levels are all above the highest regulated water level (HRWL) of Askjelldalsvatn. The regulated water level of
10 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Grøndalsvatn is below the HRWL of the intake reservoir and the water is pumped into the headrace tunnel. More water is diverted into Askjelldalsvatn by a tunnel system upstream of the reservoir, with Holskardvatn and Vassøyane as the most important diversions.
Evanger Power Plant Municipality Voss Catchment Area 254,3 km2 Average Annual Inflow 700,8 Mm3 Gross Head 770 m
Specific Energy (gross head) 2.1 kWh/m3 Turbine 3 Pelton wheels Capacity 330 MW Avg. Annual Production 1267 GWh Headrace Tunnel Length 34,4 km Cross Section 30 m2
Table 1 Salient Features Evanger Power Plant
Figure 6 Layout Evanger Power Scheme
11 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Figure 7 Evanger Power Scheme L-Section Through Tunnel in Figure 6 The Oksebotn plant utilizes the energy in an upstream part of the catchment (Volavatn is the intake reservoir). The tail race water ends in the Eide-Fannadal Lake, from which the water is diverted into the head race tunnel of the Evanger power plant. Oksebotn power plant has an average annual production of 44 GWh.
3.1.2. Design Considerations of the Evanger Power Scheme At the time of construction of the Evanger power scheme in 1965, the head race tunnel of 35 km was the longest hydropower tunnel in the world. The head race tunnel picks up water from a lot of smaller catchments and creates a situation a large total runoff is available at high altitude for the Evanger plant. It was found to be beneficial to build one large power plant instead of several smaller plants.
The large scale of the Evanger scheme itself is not enough to make it beneficial, but it helps. Regulation is another important aspect of the economy (and thus design) of the scheme. Regulation is acquired by having large volumes available in which water can be stored. The importance of regulation is the ability to provide energy at the right time, which was even more important when the plant was build. Before the Energy Act BKK had an obligation to deliver electricity to all customers in their assigned region only (see Section 2.5). Today the regulation is still beneficial as the energy can be delivered at the right time from an economic point of view. The many lakes at the mountain plateau provide an easy and cheap opportunity to create these storage volumes: only small dams need to be constructed to regulate the lakes downwards from their natural water level. These regulation measures can be constructed at relatively low costs compared to the total investments. Hence, besides several brook intakes, all major lakes of the scheme are regulated, see also Figure 6.
The operation of the Evanger power scheme is almost primarily determined by the Evanger plant, simply because its potential energy is much larger than that of the Oksebotn plant. The latter is thus mainly steered by the functioning of the Evanger plant. The water that flows into the head race tunnel from the unregulated brook intakes is utilized for energy production almost immediately, as the only possible place to store it, Askjelldalsvatn, is relatively small and often almost completely filled. The water in the regulated reservoirs is stored at least from night to day, but it is also possible to store some part of the water from summer to winter. It is tried to have full reservoirs
12 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
at the start of the winter period and to draw them down almost completely during one winter. Holskardvatn is the only reservoir, large enough to store all water from summer to winter. Holskardvatn is seldom completely drawn down, because the storage volume is larger than the average annual inflow. The water from Vassøyane is diverted into Holskardvatn, because it can be stored here (and not in Askjelldalsvatn where the water would naturally go to). Holskardvatn has been included in the Evanger scheme for two reasons: - to increase the water inflow for the Evanger plant - to increase the seasonal storage capacity in the Evanger scheme (which was important to be able to deliver electricity to all customers in the region before the Energy Act and is important today to be able to deliver energy at the desired time from an economic point of view)
The energy production in the Evanger plant thus consists of a base part, which closely coincides with the runoff of water throughout the year and a ‘winter’ part which comes from the stored water in the reservoirs.
3.2. Project Site The feasibility of a power plant that utilizes the water flow between Holskardvatn and Askjelldalsvatn is the subject of this study. The project site is then also defined as the area between these reservoirs, see picture 3 in Figure 5.
3.2.1. Location and topography The lowest point is Askjelldalsvatn, with a regulated water level between 750 m.a.s.l. and 805 m.a.s.l. The highest point is Sørdalsfjellet with 1215 m.a.s.l. The terrain is steep and the area has plenty of smaller and larger lakes. No significant vegetation, flat plains or glaciers are found in the area. Holskardvatn and Kvanngrødvatn are regulated reservoirs, while the other lakes of the power scheme are kept at a constant water level. Digital topographic maps are available with 20m contour lines and original scale 1:50.000. See also the topographic map in Appendix I V1.
3.2.2. Existing Tunnel System Holskardvatn is diverted via a tunnel into Askjelldalsvatn. Several other tunnels divert the lakes Kvanngrødvatn, Sørdalsvatn and Vassøyane plus two brook intakes into Holskardvatn. See also Figure 7 and Figure 6, plus the topographic map in Appendix I V1. Catchment areas and average annual inflow for each diversion are given in Table 3 in the next Section.
Holskardvatn, Askjelldalsvatn and Vassøyane The connection between the three reservoirs is sketched in a schematic L section in Figure 8, and a more detailed schematic sketch in Appendix I 18968. Pictures of the outlet of the tunnel into Askjelldalsvatn, the gate house and access tunnel for the gates and access tunnel for the Vassøyane tunnel can be found in Appendix I.
Holskardvatn is a large reservoir with sufficient storage capacity (127% storage volume relative to the average annual inflow), while Askjelldalsvatn is a relatively small reservoir (29%). Vassøyane is diverted into Holskardvatn.
From the bottom outlet of Holskardvatn a tunnel has been constructed in a straight line to Askjelldalsvatn. The outlet of this tunnel is at 782.9 m.a.s.l. and closed by a low pressure gate A2. Upstream of this gate a shaft is blasted to a level 806 m.a.s.l.. From
13 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
here a tunnel ends at Askjelldalsvatn at 805 m.a.s.l.. A high pressure gate, A1, is constructed in this tunnel. Vassøyane is diverted in the main diversion tunnel from Holskardvatn to Askjelldalsvatn. A1 is a high pressure, fixed roller gate and measures 1m x 1.5m. It is used when the water level in Holskardvatn is above 820 m.a.s.l. Gate A1 can regulate the flow. A2 is a low pressure gate, fixed roller 2m x 2m, and is used when the water level in Holskardvatn is below 820 m.a.s.l. to completely empty the reservoir. This gate is either in an open or closed position. The gate capacities for different water levels are graphically presented in Appendix I 12558.
At the outlet of Vassøyane a dam has been built and the water level is permanently raised to 865.5 m.a.s.l. Dams were also constructed at the outlets of Askjelldalsvatn and Holskardvatn to be able to raise the water level. Salient features of these two reservoirs are presented in Table 2.
Holskardvatn Askjelldalsvatn HRWL (m.a.s.l) 865.5 HRWL (m.a.s.l) 805 LRWL (m.a.s.l) 796 LRWL (m.a.s.l) 750 Total Annual Inflow (Mm3) 187 Total Annual Inflow (Mm3) 300 Storage Volume (Mm3) 241.5 Storage Volume (Mm3) 86,7 Relative storage (= storage volume 130 Relative storage (%) 28.9 / average annual inflow (%)) Table 2 Salient Features Holskardvatn and Askjelldalsvatn
The brook intakes Askjelldal East and West are connected to the tunnel system that connects Holskardvatn, Askjelldalsvatn and Vassøyane. They connect upstream of the gates and are therefore accounted as inflow into Holskardvatn.
Kvanngrøvatn and Sødalsvatn Kvanngrøvatn has been dammed and the water level is kept at 870.5 m.a.s.l. The water flows over an outlet weir, into a shaft and further through a tunnel into Holskardvatn. A bottom outlet has also been constructed, but this is only used in winter. It is then allowed to draw down the water to 853.5 m.a.s.l.
Sørdalsvatn has a similar tapping construction as Kvanngrødvatn, but without the bottom outlet. The water level has been permanently lowered 3m to 926 m.a.s.l.; hence it was not necessary to construct a dam.
14 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Figure 8 L-Section Transfer Tunnel Holskardvatn - Askjelldalsvatn and Vassøyane Diversion. High pressure gate A1, low pressure gate A2.
3.2.3. Hydrology Hydrological data is necessary to estimate the energy a power plant can produce. The catchment boundaries of the different diversions and catchment areas are shown in Appendix I V1. The average annual runoff is determined with runoff data from 1960- 1990 from NVE’s database3. For all sub catchments the average annual runoff has been established, see Table 3.
Catchment Area (km2) Runoff (Mm3/y) Specific Runoff (l/km2/yr) Holskardvatn 29.6 88.8 95.1 Vassøyane 22.4 63.9 90.5 Kvanngrødvatn 3.9 12.4 100.8 Sørdalsvatn 5.0 16.5 104.6 Askjelldal brook intake west 1.2 3.8 100.4 Askjelldal brook intake east 0.5 1.6 101.5 TOTAL 62.6 187 94.7
Table 3 Hydrology of Project Area Reconstructed flow data are available for the Holskardvatn catchment, in the form of measurement series -B, from 1961-1990. These data have been correlated to the measurement series in Brakestad in the Eksingedal river basin. The total runoff data can be correlated to the daily flow data from a gauging station around the project site. To correlate daily flow data from another gauging station to the runoff of the project site is justified if the catchment characteristics are quite similar; hence elevation and slope gradient, vegetation, amount of lakes are more or less the same for the two catchments4.
3 NVE: Norwegian Water Resources and Energy Directorate. Their database, called Regine, contains among others runoff data which can be linked to a topographical map to estimate the total runoff for a defined catchment. 4 This data is part of the computer program Vansimtap that will be used to model the preliminary designs to estimate their energy production, see Chapter 7.
15 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
3.2.4. Geology The rock in the area is of good quality. The area is rich of quartz5, which could be an indication of a rather strong rock with expected medium blastibility (assumed a low to moderate anisotropy of the rock mass and a low to moderate degree of jointing, and a general high mechanical strength of quartz containing rocks). However, because of the hardness of quartz poor drillabality and high bit ware values are anticipated. [NILSEN AND THIDEMANN, 1993]
All larger fault zones in the area strike some 40 degrees north-west and dip some 75 to 90 degrees in a south-west direction.
3.2.5. Transmission System An existing 22kV, 13 km long transmission line provides the gate house with power. This line connects to the regional grid at the village of Trefall, south of Askjelldalsvatn and the alignment is on the west bank of Askjelldalsvatn. The electricity is put into the national grid at the transformer station of Evanger. Because of low local demand, almost all electricity needs to go into the national grid via the transformer station at Evanger. This station is currently operating close to its capacity.
For a new power plant at Askjelldalsvatn both the local transmission line and the transformer station need to be upgraded. The costs for the upgrade are dependent on the maximum output of the new power plant. 6
3.2.6. Road access There is road access to the gate house for the outlet gates of Holskardvatn during summer. The alignment is through Norddalen and connects to the county road at Trefall. The dam site of Holskardvatn can also be reached by road.
3.3. Reservoir Operation In this Section the reservoir operation of Askjelldalsvatn and Holskardvatn will be discussed. 10 years of production data from 1997-2007 for the Evanger power scheme are presented in several diagrams in this Section. These data are important for a good understanding of the design problem.
3.3.1. Holskardvatn Holskardvatn is regulated between 865.5 m.a.s.l and 796 m.a.s.l and has a live storage volume of 241.5 Mm3. The outflow into Askjelldalsvatn can be regulated with two gates close to Askjelldalsvatn.
The water levels from the last ten years are shown in Figure 9 which shows that the reservoir quite regular is almost full in winter, but that it has never been fully drawn down. Only in 2001 and 2003 the water level has been below 820 m.a.s.l, which means that only those two years the low pressure gate was used to release water. With the water level at 820 m.a.s.l, the reservoir still contains 51.5 Mm3 or 22% of the total storage. A stage-volume curve is provided in Appendix I 11162.
The water level in the years succeeding a dry year are influenced by the dry year; the water levels in 2002 and 2004 (and also 2005) do not reach to the HRWL, but one can detect a steadily increase of the maximum annual water levels in the period 2003 -
5 According to J. Matre, BKK Rådgiving. The geological report of the area is not available. 6 According to B. Otterås of BKK Nett.
16 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
2005. This is explained by the fact that Holskardvatn is a multi-annual storage reservoir (relative storage 127%).
Holskardvatn water level
870
860
850
840
830 masl
820
810
800
790 jun.97 jun.98 jun.99 jun.00 jun.01 jun.02 jun.03 jun.04 jun.05 jun.06 jun.07 Date Holskardvatn water level HRWL LRWL
Figure 9 Water levels in Holskardvatn 1997-2007 on weekly basis
Holskardvatn outflow in m3/s (derived from outflow in Mm3 per week)
Holskardvatn outflow m3/s 35,00
30,00
25,00
20,00 m3/s 15,00
10,00
5,00
0,00 jun.97 jun.98 jun.99 jun.00 jun.01 jun.02 jun.03 jun.04 jun.05 jun.06 jun.07 Date
Figure 10 Outflow of Holskardvatn in m3/s. Calculated as average flow from the weekly release in Mm3.
17 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Figure 10 shows the outflow of Holskardvatn in m3/s from the last ten years. It reveals the capability of Holskardvatn to store the water that flows into the reservoir in the summer and to use it for energy production in the winter months: the outflows are concentrated in the period from November to April. The outflow of Holskardvatn has reached values of 25 m3/s for one week in six out of ten years. 15 m3/s has been exceeded per week in all years. In the last ten years 30.5 m3/s has been the maximum outflow.
From these data also stage-duration and flow-duration curves have been made, which can be found in Appendix I D1 & D2. The water level in Holskardvatn is above 835 m.a.s.l. for 80% of the time and evenly divided over all water levels above 835 m.a.s.l. The stage duration curves drops sharply below this water level; the lower water levels are less frequently reached than the water levels above 835 m.a.s.l. The outflow from Holskardvatn is 0 more than 30% of the time and most flows occur between 23 m3/s and 2 m3/s on average per week.
When the water level in Holskardvatn is presented in a diagram versus the relative (to the total) flows (Figure 11), it shows that the outflows from Holskardvatn occur are spread evenly over all water levels above 825 m.a.s.l.. Only five percent of the outflows from Holskardvatn occurred through the low pressure gate;. at water levels in Holskardvatn below 820 m.a.s.l.
HOLSKARDVATN: OUTFLOWS vs WATER LEVEL for period 1997-2007
870,0
860,0
850,0
840,0
830,0
820,0
810,0
800,0
790,0 0 % 10 % 20 % 30 % 40 % 50 % 60 % 70 % 80 % 90 % 100 %
percentage of flow above water level
HOLSKARDVATN WL vs FLOW HRWL HOLSKA RDV A TN LRWL HOLSKARDVATN
Figure 11 Holskardvatn water levels vs. outflows 1997-2007
3.3.2. Askjelldalsvatn Askjelldalsvatn has a live storage of 86.7 Mm3 and the water level is regulated between 805 m.a.s.l and 750 m.a.s.l. Figure 12 and Figure 13 show the water levels and outflow of Askjelldalsvatn between 1997 and 2007.
Figure 12 shows that the water level in Askjelldalsvatn has reached its maximum at 805 m.a.s.l every year. In most occasions the water level was drawn down to 770 m.a.s.l, only in 2001 and 2003 the water level reached a minimum of 760 m.a.s.l. These are the same years that the minimum levels in Holskardvatn were reached.
18 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Askjelldalsvatn water level
810
800
790
780 masl 770
760
750
740 jun.97 jun.98 jun.99 jun.00 jun.01 jun.02 jun.03 jun.04 jun.05 jun.06 jun.07 Date Askjelldalsvatn water level HRWL LRWL
Figure 12 Water levels in Askjelldalsvatn 1997-2007 on weekly basis
Askjelldalsvatn outflow in m3/s (derived from outflow in Mm3 per week) Askjelldalsvatn outflow m3/s 45,00
40,00
35,00
30,00
25,00 m3/s 20,00
15,00
10,00
5,00
0,00 jun.97 jun.98 jun.99 jun.00 jun.01 jun.02 jun.03 jun.04 jun.05 jun.06 jun.07 Date
Figure 13 Outflow Askjelldalsvatn 1997-2007 derived from weekly data
The outflow of Askjelldalsvatn is presented in Figure 13 and reveals that the reservoir is not able to store the water from the wet into the dry season: the outflow occurs almost continuously, although the annual maxima usually do occur in the winter period. The high water level causes a significant spill of water; in the period June 1997- June 2007 some 90 Mm3 was spilled over the spillway.
19 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
For Askjelldalsvatn stage-duration curves and flow-duration curves have been made, which can be found in Appendix I D3 & D4. The water level in Askjelldalsvatn is usually above 790 m.a.s.l. (80% of the time). Lower water levels are less frequently reached. The average weekly outflows from Askjelldalsvatn vary quite evenly between 0 m3/s and 36 m3/s.
3.3.3. Operation strategy In winter it is tried to keep Askjelldalsvatn at its HRWL, while Evanger power plant is producing energy. This can be achieved quite easily, because the reservoir is rather small and has a large upstream water body, Holskardvatn, which can feed it. Almost all inflow in this season comes from Holskardvatn as the precipitation in winter usually falls in the form of snow. In these situations the inflow of Askjelldalsvatn equals the outflow. Askjelldalsvatn is drawn down every year, just before June, see Figure 12. This is necessary to supply storage volume for the expected runoff of melt water from the snow that has accumulated in the catchment area. The snow represents approximately 90 Mm3 water, hence the complete storage volume in Askjelldalsvatn is needed to store the melt water of the snow. The required storage volume is created by utilizing water and thus producing electricity in the Evanger plant, although the energy price is low in this period. The reservoir is full again in the beginning of July as a result of the snow melt. From this month until December all runoff comes directly from rainfall. The Evanger plant produces this runoff almost instantaneously and the water level in Askjelldalsvatn is kept around its highest regulated water level. There is hardly any inflow from Holskardvatn during this period. From approximately December the majority of the precipitation falls as snow and Askjelldalsvatn needs to be fed with the water from Holskardvatn to have sufficient water available for the Evanger plant.
The water from Holskardvatn flows into Askjelldalsvatn and the former reservoir is operated in close connection with the latter. Because of its size it is possible to store the water until the energy price is at its highest. The risk of spilling water is also an important factor for the reservoir operation, so that Holskardvatn is kept below its highest regulated water level when snow melt is expected in June. This has resulted in a negligible amount of spilled water over the last 10 years (only 0.1 Mm3). When Askjelldalsvatn is drawn down in June, the outflow from Holskardvatn is stopped and water is stored continuously until approximately December. During this period no water flows to Askjelldalsvatn. From December Holskardvatn is used to feed Askjelldalsvatn with water and as a result the water level in Holskardvatn falls gradually. The water in Holskardvatn is thus mainly used for energy production in winter, although it is seldom found worthy to utilize all stored water in one winter (hence, to completely draw down the water).
When the outflow from Holskardvatn and the gross head between Holskardvatn and Askjelldalsvatn are compared (Figure 14), it is noticed that 94% of the flows occur above a head of 30m. 50 % of the flows occur at heads of 50m or higher and the other 50% occur at lower heads. The maximum gross head is just above 70m. The absolute minimum for the period 1997 – 2007 is 15,7m. The graph of the gross heads versus flows is steep in the beginning and the end; from 0-5 % and 90 – 100%. The flows in between are more evenly spread over the different heads between 65 and 35 m.
20 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
HOLSKARDVATN: OUTFLOWS vs GROSS HEAD HOLSKARDV. - ASKJELLDALSV. for period 1997-2007
70,0
60,0
50,0
40,0
30,0
20,0
10 , 0
0,0 0 % 10 % 20 % 30 % 40 % 50 % 60 % 70 % 80 % 90 % 100 % percentage of flow above gross head
Figure 14 Gross head Holskardvatn - Askjelldalsvatn vs. outflows Holskardvatn 1997-2007 Several conclusions can be drawn from the description of the reservoirs, although it should be kept in mind that the period of analysis is only 10 years.
- The water level in Holskardvatn is usually not lower than 820 m.a.s.l. (only 5% of the time) ; hence the low pressure gate is seldom used. - The water level in Askjelldalsvatn does not drop below 770 m.a.s.l., apart from very dry years. - The maximum weekly average outflow from Holskardvatn usually varies between 15 m3/s and 25 m3/s, with a maximum of 30.5 m3/s. - The maximum available head for energy production between Holskardvatn and Askjelldalsvatn is slightly higher than 70m. This is smaller than the theoretical maximum head which is the difference between the HRWL of Holskardvatn and the LRWL of Askjelldalsvatn ; 865.5 – 750 = 115.5m. - The minimum available head depends on the outlet level of the new plant. If the outlet is at HRWL of Askjelldalsvatn, the minimum head is around 15m. This occurred in dry years, otherwise the minimum head is around 25m. If the outlet is below the lowest water level in Askjelldalsvatn the minimum available head is around 30m. - If inflow occurs in Askjelldalsvatn when the water level in this reservoir is at HRWL, the inflow equals the outflow, under reservation that no spill of water occurs. - The average available head can be assumed to be 50m.
3.4. Potential Energy A quick calculation gives an estimate of the possible energy potential. With an average head of 50m (see Figure 14) and an average runoff in Holskardvatn of 187 Mm3 (see Table 3) and an overall efficiency estimated at 90% (hydraulic, turbine, generator losses included) the potential energy that a new plant can produce estimated as:
E = ρ ⋅ g ⋅ H ⋅V ⋅η = 1000 ⋅9.81⋅50 ⋅187 ⋅106 ⋅ 0.9 = 8.26 ⋅1013 J tot 8.26 ⋅1013 /(109 ⋅3600) = 22.9GWh
21 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
4. TURBINES
The turbine is a key component of the power plant as it has major influences on both the cost and benefit side. It will thus have a large influence on the design. The installed capacity and number of turbines determine a large part of the construction costs. The performance characteristics of the turbines determine how much water can be utilized for energy production and how much water needs to be bypassed around the turbine. The latter can be necessary if for example the available head is outside the head range of the turbine.
4.1. Turbine Type7 The hydraulic characteristics, i.e. head range, of turbines are determined by the geometry of the runner. Therefore the first selection of turbine types is usually performed on the head. In the head range H = 15 to 70 m, as has been identified in the previous Chapter, only reaction type turbines seem applicable and the following types of turbines can be selected: - Francis turbine: H = 20 to 700 m - Kaplan turbine H = 3 to 80 m
All these turbines have a runner that turns in an immersed space. The pressure drop takes place over the runner, which absorbs this energy plus the energy that is transferred by the change of velocity direction from the runner inlet to the runner outlet (impulse part).
4.1.1. Francis Turbine The Francis turbine is a radial turbine, i.e. the flowing water hits the runner blades in a radial direction. It is a single regulated turbine: the flow can only be changed by steering the regulating ring which adjusts the guide vane openings. By adjusting the guide vane opening, the flow behaviour of the turbine can be adjusted to changed heads, so that the flow follows the runner as good as possible to achieve an optimal efficiency. The peak efficiency of a Francis turbine is in the range of 93-96%.
4.1.2. Kaplan Turbines The Kaplan turbine is of the axial type and the runner can be equipped with up to 8 blades (for the highest heads). It is a double regulated turbine type: the flow behaviour can be controlled by adjusting the guide vane openings and by changing the pitch of the blades of the runner. The double regulation is the main reason that the Kaplan turbine has a higher efficiency over a wider range of heads and flows than the Francis turbine, although the width of the range is only slightly larger than the width of the Francis turbine. The peak efficiency of a Kaplan turbine is in the range of 93-95%.
4.2. Turbine Selection When selecting a turbine configuration for a plant, one has to find an optimal (economic) solution where the characteristics of the turbine configuration fit best to the head and flow range at which the plant has to operate. The latter can be influenced by the design and an optimal turbine solution will be found by an iterative process.
7 This section is based on information from [ANDRITZ.COM], [VAN DUIVENDIJK, 2007] and [VIOGG AND ELSTAD, 2003]
22 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Besides the considerations that have been described previous in this chapter, several other aspects that should be considered are described below.
4.2.1. Number of Turbines The installation of fewer machines tends to reduce the costs of civil and hydraulic engineering structures, as well as the costs of mechanical equipment. Installing a single unit, one has to be aware of the situation where the turbine is out of order, due to breakdown or repair. If the water cannot bypass the machine, only one turbine can become a bottleneck in a larger power scheme. Several units will make it possible to run the turbines close to the best efficiency or to enlarge the operational range of the plant.
4.2.2. Turbine Type Based on the available head range Francis and Kaplan turbines are all possible turbine solutions. Each turbine has certain characteristics regarding efficiency, head and flow range, costs and setting. These are described in later sections.
4.2.3. Initial investments Kaplan turbines are some 30% - 50% more expensive than Francis turbines, although their higher speeds and corresponding smaller units may reduce excavation and generator costs. Kaplan turbines require deeper settings which again can increase excavation costs.
4.2.4. Maintenance requirements None of the turbines in this operation range is particularly vulnerable to sand erosion. Operation, maintenance and replacement costs are higher for Kaplan turbines than for Francis turbines.
4.2.5. Installed capacity The installed capacity will be the result of the anticipated head and flow. The generator and the transmission network have to comply with maximum output of the turbine. Very often the installed capacity is less than the capacity of the turbine so that under maximum head the plant cannot utilize maximum flow. This is often decided by economic parameters.
4.3. General Design Considerations When designing a hydro power plant, several aspects have to be considered with respect to the mechanical equipment. The most important are briefly described below.
4.3.1. Design head The design head is a decision parameter, which can serve as a tool for the design of a hydro power plant. It can serve as an indication for energy output and construction sizes and may be used to place the optimum efficiency of a turbine (together with a design flow). It is usually the gross head between the intake water level and the outlet water level minus head losses. In a reservoir these water levels can be approximated as the centre of gravity of the live storage of a reservoir.
When selecting the design head one should consider the following:
23 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
- Turbines have a limited range of heads in which they can operate. For reaction turbines the head range is between 65% and 125% of the design head, based on experience values.[WALTERS AND BATES, 1976] - Efficiencies tend to decrease more rapidly toward the lower range of the head than toward the range of higher heads, which limits output in the lower range. Higher heads result in reduced turbine diameters, which reduce the cost of the machinery and powerhouse. With increased speeds however, a deeper setting of the turbine below tail water level is required which again can increase the costs for the power house. - Lower design heads conform to increased wheel diameter and greater costs of machinery and powerhouse. Reduced speeds will however result in reduced excavation costs for the power house as a consequence of the setting of the turbine with respect to the tail water level. [MOSONY, 1987]
4.3.2. Water Quality Physical components such as floating debris, sediments and ice can damage the mechanical equipment. The area around Holskardvatn and Askjelldalsvatn is rich of quartz, which in the past has caused damage to the turbine runner in the Evanger power plant. Quartz particles used as a runway at the inverts of the tunnels were flushed down the tunnels with the moving water, and finally flowing through the runner. It has to be assessed to what extend quartz is likely to cause problems in the new power plant as well. Ice blocks can cause problems when the water level is almost equal to the intake level. A trash rack at the intake will prevent large pieces of rock and ice to enter the tunnel. A settling basin can prevent smaller particles from entering the conveyance system, although the particles in suspension will not be caught in the basin. The necessity of such a basin has to be examined.
The intake from Holskardvatn is deeply submerged, hence air entrainment is unlikely.
4.3.3. Transportation The access road may limit the transport size and weight of the turbine components. It is assumed that the electro mechanical equipment will be of such size that transportation will not restrict the design.
4.3.4. Mechanical Power The mechanical power P is generated by the turbine by converting hydraulic energy: the flowing water creates a torque T on the turbine runner with an angular velocity ω.
P = T *ω [Nm ] Where P = power [Nm/s] s ω = angular velocity [rad/s] 2*π *n 1 n = angular speed [rpm] With ω = = const [ ] r = inner lever arm [m] 60 s 3 Q = flow [m /s] T = F *r [Nm] ∆V = change of water velocity [m/s] F = ρ *Q*ΔV [N] T = torque exerted on runner [Nm] F = force exerted on runner [Nm]
The angular velocity is constant to match synchronous speed, required for the generator, see also the next Section. The mechanical (and electrical) energy output thus fluctuates with the torque created by the flow and the change of velocity vector in the runner. In practice the output is regulated by regulating the flow. The flow is dependent on the head and the size of the opening of the turbine, the latter can be controlled by steering the guide vanes.
24 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
The turbine thus has to rotate at a constant speed, while the energy output will vary as a function of the net head and flow within the limits of cavitation, vibrations, efficiency drop and mechanical properties of the turbine and generator. [VINOGG AND ELSTAD, 2003]
4.3.5. Generator The generator will convert the mechanical energy of the turbine in electrical energy. As hydraulic turbines are designed to operate under a specific range of conditions, the generator is also unique in that its mechanical and electrical design has to conform to both the characteristics of the turbine and to the characteristics of the electrical system it is operating in. The electrical system operates at a fixed frequency, in Norway 50 Hz, from which only slight deviations are allowed (+/- 0.1 Hz). Deviations are caused by a disturbed balance between input and output of energy. The frequency is kept constant by larger power plants which have an additional function in the national grid as frequency regulators (besides energy production).
All devices, producing or using energy, connected to the grid thus operate at 50 Hz. One therefore speaks of the generator being synchronous, as its rotational speed has to match with the frequency of the grid. A frequency regulator is usually required on all machines with capacities above 1 MW. A rotational speed n is a synchronous speed if the following condition is satisfied:
60* f n = [rpm] Z With f is frequency of the grid = 50Hz Z is number of generator pole pairs
The rotational speed of the turbine is determined by the grid and the installed number of generator pole pairs. 8
4.3.6. Runaway Speed When the electric load on a generator is disconnected, the turbine speed will accelerate until the torque on the runner balances with the friction in the machine. This will happen when the torque on the runner is reduced to almost zero, because the flow does not fit the blade angles. This runaway speed is approximately 190% of the design speed for low head Francis units. The runaway speed of a Kaplan turbine is largely dependent on the guide vane angle and the blade angle and can vary between 150%-300% of the design speed. The turbine and the generator have to be designed for these high speeds, although the duration of the uncontrolled speed is limited, because a shut off valve or the intake gate closes when this situation occurs.
4.3.7. Cavitation and Suction Head Where high velocities occur, the pressure can decrease to below the vapour pressure and cavities are created in the water. This takes also place in hydro turbines. Especially the runner outlet and the inlet of the draft tube in reaction turbines are vulnerable to cavitation. The air bubbles will collapse when they come into regions with a higher pressure. This is accompanied by a specific sound. If cavities collapse on surfaces of for example the runner blade or the draft tube wall, they can cause serious erosion.
8 More information can be found in for instance [MOSONY, 1987] and [VAN DUIVENDIJK, 2007]
25 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Low pressure regions occur if the flow lines no longer follow the surfaces of the guide vanes and runner. The flow lines through the turbine are influenced by the available net head and the flow. Hence, these two parameters limit the operational range of a turbine, to prevent cavitation.
Low pressure regions can be avoided to some extend by ensuring sufficient back pressure on the turbine. In this relation the net positive suction head (NPSH) is often used and defined as the absolute hydraulic head at the runner outlet. See Figure 15. Using level Y of the draft tube exit as datum, Bernouilli’s theorem is applied to the water leaving the runner (v3) and the water leaving the draft tube (v4) respectively:
H 3 = H 4 p v 2 p v 2 + 3 + h + y = 0 + 4 + y + Δh [m] γ 2g s γ 2g
,where ∆h is the loss in energy head inside the draft tube. The absolute pressure head at the runner outlet can be written as: 2 2 p p0 v3 − v4 NPSH = = − h − + Δh [m] γ γ s 2g
Figure 15 Determine net positive suction head at runner outlet
The NPSH needs to be at least larger than the vapour pressure, to avoid cavitation. A substantial safety margin is however required since the pressure has a non-uniform distribution and thus falls far below average at some parts of the vanes. For the required NPSH formulae have been derived from extended research. [MOSONY, 1987] These formulae can be used as a first approximation for the design of a power plant and are presented in Section 4.6.
Higher rotational speeds are more prone to cavitation than lower speeds, under otherwise similar conditions. This is also found by analysing the formula above: a higher rotational speed results in a higher water velocity at the runner outlet (v3), which reduces the absolute pressure at the runner outlet. Higher speeds thus require deeper settings (smaller or even negative values of hs) of the turbine with respect to the tail water level. Although deeper settings will increase excavation works, they will
26 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
allow for higher speeds, hence smaller and cheaper turbines. It often turns out to be more economic to have negative suction head. Kaplan turbines can operate at higher rotational speeds and are therefore more vulnerable to cavitation. They require a deeper setting of the turbine compared to a Francis runner.
4.4. Performance Characteristics For large power plants turbines are Taylor-made and they are designed for a specific range of conditions under which they will operate. For small hydropower a standard turbine design is often found to be the best solution. The turbine design and production is usually done by the turbine manufacturer. Turbines are optimized at a best efficiency point for a specific head and discharge. It is also possible to run the turbine outside its best efficiency point with reduced efficiency. The latter depends on the turbine discharge and the head and is usually displayed in a performance diagram of the turbine (or efficiency hill diagram). Performance diagrams are only valid for geometrically similar turbines.
A performance diagram is acquired by performing model tests (and up scaled to be valid for the prototype) or prototype tests when the turbine is installed. The limits of running the turbine are given by the manufacturer and are usually a result of cavitation; at certain discharges and heads the flow doesn’t follow the runner blades anymore and air bubbles are sucked in which when they collapse cause damage to the runner.
Figure 16 shows performance diagrams of a Francis and a Kaplan turbine. The hill diagram of the Kaplan turbine is much flatter than the one of the Francis turbine; hence the overall efficiency (for both variable flows as well as for variable heads) of the Kaplan is higher than that of the Francis. The higher efficiency of the Kaplan turbine is explained by the double regulation of this type of turbine, whereas a Francis is only single regulated. Both diagrams do show several similarities. The diagrams are egg shaped which means that the efficiency varies more for varying flows than for varying heads;. Looking only at the variation over the head another similarity is noticed. The efficiency falls much faster for low heads than for high heads. Efficiencies around 90% can be reached at maximum heads, close to the design flows, while the efficiency is between 80% and 85% for minimum heads (for Francis and Kaplan turbines respectively).
To determine the operation range of turbines, performance diagrams are thus a very useful tool. Turbine manufacturers indicate the range of operation on these diagrams, but even then in practice other limits can be found, as is clear from the Blåfally case study, see the next Section. However, turbine manufacturers are very reluctant to supply performance diagrams of their turbines to others than the buyer of the specific turbine. In a preliminary design one therefore usually has to be satisfied with some basic design rules, or some performance diagrams available at instances such as the United States Department of the Interior Bureau of Reclamation (USBR). 9
9 These performance diagrams can be found in [WALTERS AND BATES, 1976]
27 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Figure 16 Examples of performance diagrams of a Francis turbine (lower graph) and a Kaplan turbine (upper graph)
28 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
4.5. Case Studies This section will deal with two Francis turbines which are already installed and for which performance data are available. The objective of this Section is to identify some practical limits for turbines which operate in a wide head range. No sufficient data has been found for Kaplan turbines performing in the same operation range.
Norconsult10 supplied us with data of a Francis turbine, with an optimum efficiency for H = 45m and Q is 24.3 m3/s. The upper limit of operation for head is 60 m and the lower limit is approximately 38 m. There is quite some uncertainty about the lower limit; performance below this limit, down to some 31.5 m, could not be guaranteed by the turbine manufacturer. Vibration and cavitation were the main reason for this uncertainty.
For another project, Blåfally V, performance diagrams have been acquired. This confidential performance data for a Francis turbine with He = 56 m and Pmax = 8.5 MW reveals that the turbine, which was to have an operational range of heads between 34 m and 62.5 m creates problems when running the turbine at low heads, below some 42m of head. Although the manufacturer designed the turbine for a limited range of flows for the low heads, in practice the turbine is very noisy and unsteady due to vibration in this lower head range. The owner has finally decided not to use the turbine in the low head and discharge range. The turbine functions well in the head range between 62.5 and 42 m, under flows close to the design flow of 16.5 m3/s.
It is concluded that variations in head up to of about 20 m can be expected to be achieved with guarantees from the manufacturer, for design heads between 40 and 60m.
4.6. Preliminary Turbine Design In literature design rules for the design of a hydro power plant with respect to turbines can be found. These rules of thumb are discussed and sometimes slightly altered based on the experience of the case studies.
4.6.1. Specific Speed The specific speed is a parameter that is used for the classification of hydraulic turbines. A similar specific speed represents similar relative flows in geometrically similar turbine. Similarity considerations on hydraulic turbines are an attempt to describe the performance of a given machine by comparison with the experimentally known performance of another machine under modified operating conditions, such as a change of speed. The latter is important since the performance of turbines cannot be determined accurately enough by analytical or numerical methods; it can only be established by model or prototype tests (with prototype here meaning the installed ‘real’ turbine). This is also the reason why preliminary design rules use the specific speed.
The specific speed nq of a turbine to be designed for a specific flow, head and rotational speed, is the rotational speed of an imaginary turbine that is geometrically similar to the turbine to be designed. The dimensions of the imaginary turbine are chosen such that at best efficiency a 1m3/s discharge flows through the turbine with a head of 1m.
10 Norconsult is the consultant for BKK for technical advice on mechanical equipment
29 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
A specific speed for unit turbine power (P, in most literature still in metric HP) under 1 m head is noted as ns.
1/ 2 Where P 3 ns = n ⋅ 5 / 4 Q = flow [m /s] H n P = power [HP] γ ⋅Q ⋅ H n = rotational speed of turbine [rpm] P = η ⋅ = 13.33⋅η ⋅Q ⋅ H Hn = net head [m] 75 η = efficiency of turbine and generator [-]
The formulae can also be used to compare geometrically similar turbines and their performances.
4.6.2. Head Range A technical publication of USBR suggests a head range for reaction turbines of 65% to 125% of the design head. [WALTERS AND BATES, 1976]
As a rule of thumb, [VAN DUIVENDIJK, 2007] suggests a minimum to maximum head ratio of 2/3 for Kaplan turbines and 3/4 for Francis turbines.
[MOSONY, 1987] states a head range to design head ratio of 0.7 for Francis turbines and somewhat higher for Kaplan turbines.
It is decided to take the head range of reaction turbines from 70% to 125% of the design head as case studies showed that cavitation and vibration occurred at lower heads.
4.6.3. Suction Head
D. Thoma has given the following formula for determining the maximum draft head hs (vertical distance between the tail water level and the runner outlet, negative if the runner outlet is below the tail water level) [MOSONY, 1987]:
hs ≤ B −σH [mWC] Where B = 0.95(10.3-0.0011*E) [m] E = elevation above sea level [m] σ = cavitation coefficient [-]
For this formula [MOSONY, 1987] suggests the following values for σ (without safety margin). Modern Kaplan turbines at full load: n3 / 2 σ = s 25200
This can be increased with 10% - 15% to achieve the optimum design concerning cavitation control.
For Francis units ns 200 – 500 RPM (without safety margin): n3 / 2 σ = s 31500
The maximum allowable draft head has to be determined for the whole range of operation, since other than the design parameters might be prevailing.
30 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
4.6.4. Maximum Specific Speed For practical purposes, for every head there exists a maximum value of the highest specific speed nsmax [MOSONY, 1987]: - conventional Kaplan turbines: 3000 ns max = H n
- Low head Francis turbines (ns =200-500 rpm): 3970 ns max = 0.6 H n
Where Hn = net head nsmax = maximum specific speed (metric)
These limitations are the result of research of data from turbine manufacturers. It also shows, as stated before, that Kaplan turbines can be designed for higher speeds.
4.6.5. Runaway Speed
[WALTERS AND BATES, 1976] found that from field test the runaway speeds are not expected to exceed the following: n r = 0.63(n )1/ 5 n s
H max 1/ 2 nmax = nr ( ) H d
Where Hd = design head Hmax = maximum head n = rotational speed nmax = runaway speed at maximum head nr = runaway speed at design head and full gate ns = specific speed at design head and full gate
4.6.6. Turbine Size When the maximum speed is determined with help of the formulae above, and adjusted for a synchronous speed, the preliminary size of the turbine and expected performance can be determined from the efficiency hill diagram. By placing the unit speed (n11) at best efficiency point, the inlet diameter for the runner (D) can be determined with which the unit discharge (Q11) and the performance diagram are fixed.
Q Where Q = 3 11 2 Q = flow [m /s] D H H = head [m] n* D n = rotational speed [rpm] n = 11 H
31 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
5. SCHEDULE OF REQUIREMENTS
The schedule of requirements forms the basis for the design process. First the concept of a hydropower plant is discussed. After this the boundary conditions are presented point wise; they follow from the problem description in the preceding chapters. After this, the requirements and assumptions which are valid for any power plant are listed. Finally the starting points for the generation of different alternatives are discussed.
5.1. Concept The function of the power plant is to produce electricity by converting the energy of the water between Holskardvatn and Askjelldalsvatn into electrical energy.
The value of the project comes from the sale of energy. A distinction can be made between the benefits gained from the sale of new energy and the benefits that are gained by shifting the production of energy to a more valuable time, in this case from summer to winter.
The costs of the project are determined for different phases in the lifetime of the power plant. - Costs for the planning process - Construction costs are all the costs that are associated with the construction of a new hydro power plant. This includes besides the direct contract costs, financing costs, costs for administration, environmental costs, costs for land etc. - During operation yearly costs for operation and maintenance as well as irregular costs for refurbishment occur. Costs associated with the closure of the power plant at the end of its lifetime are not taken into account.
The potential energy available has been estimated as 23 GWh per year, see Section 3.4. This potential is available under favourable conditions for a power plant: the water is regulated, a tunnel system is already in place and the available head around 50m and the maximum flows around 30 m3/s indicate that economic dimensions of the plant can be. Both the potential value of the project as well as the expected possibilities for relatively low construction costs justify the design of a power plant on a preliminary level.
5.2. Boundary Conditions
5.2.1. Holskardvatn - Holskardvatn storage volume is 241.5 Mm3 (for stage volume curve, see Appendix I 11162) - Holskardvatn highest regulated water level at 865.5 m.a.s.l. - Holskardvatn lowest regulated water level at 796 m.a.s.l.
5.2.2. Askjelldalsvatn - Askjelldalsvatn storage is 86.7 Mm3 - Askjelldalsvatn probable maximum flood level of is 806.4 m.a.s.l. - Askjelldalsvatn highest regulated water level at 805 m.a.s.l. - Askjelldalsvatn lowest regulated water level at 750 m.a.s.l.
5.2.3. Transmission System - The frequency of the grid is 50 Hz.
32 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
- The transmission system needs to be upgraded to comply with the selected output of the plant. Both the local transmission line and the transformer at Evanger need to be upgraded.
5.2.4. Other - The construction period is limited to 6 months per year when the area is snow free. - The existing tunnel system is part of the project area and described in Chapter 3.
5.3. Requirements and Assumptions
5.3.1. Functional Requirements - The new power plant may not become a bottleneck in the existing power scheme. To fulfil this requirement the bypass capacity of the plant has to be equal to the capacity of the tunnel system as it currently exists: 38 m3/s. The bypass capacity is required for three reasons. Water transfer to Askjelldalsvatn should be possible to prevent loss of benefits if: the power plant has a breakdown the water levels are outside the head range of the turbine the desired water inflow into Askjelldalsvatn for the Evanger plant is larger than the turbine capacity. - The plant has to be remote controlled, because of the difficult access in winter. - The lifetime of the civil works is 100 years. - A winter entrance for personal is required to be able to access the power house in winter for unexpected repairs - The entrance of the access tunnel needs to be above the probable maximum flood level of Askjelldalsvatn - The leakage water from the pressure system needs to be kept at a minimum - All parts of the conveyance system need to be accessible for dry inspection. It is thus necessary to be able to drain all parts of the conveyance system, such as head- and tailrace tunnel, draft tube, turbine etc.
5.3.2. Requirements regarding the surrounding - Any damage to the surrounding for construction purposes need to be restored to their original form - Tunnel rock deposits need to be fit into the natural surrounding as good as possible
5.3.3. Technical Requirements - The lifetime of the civil works is 100 years - Any underground works need sufficient permanent stability measures - To guarantee stable underground caverns, the distance between (parallel) walls of different caverns needs to be at least 15m - A rock overburden of 60% of the local pressure is needed for unlined tunnels
5.3.4. Construction Requirements - Any design should try to avoid the need for special construction equipment - Maximum inclination for tunnels is 1:7
33 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
5.3.5. Maintenance Requirements - Tunnels need to be designed such, that no maintenance is necessary during the lifetime of the plant. Maintenance should only be allowed in case of unexpected events. - The powerhouse needs to be designed trying to minimize the maintenance works. This is a result of the remote location and difficult winter access - Any electro-mechanical parts of the power plant need to be accessible for repair and refurbishment. It should be possible to replace any electro-mechanical part
5.3.6. Assumptions - The rock in the area is of good quality, it is a strong rock with medium blastibility and poor drillability - Upgrading the transmission system up to 9 MW output is technically feasible11 - Kaplan and Francis turbines can cover a head range between 125% and 70% of the net design head. - Environmental impacts of the plant are only associated with the construction works. The power plant will not change the existing water course as nowadays the water is diverted through a tunnel system.
5.4. Design of Alternatives The objective of the preliminary design is to identify the best solution for a hydropower plant that utilizes the water flow between Holskardvatn and Askjelldalsvatn. The best solution is a technical feasible solution that has the most promising economic results.
A net present value larger than zero is required in order to make the power plant economically feasible, otherwise money would be invested in another project. The net present value will be used as the criteria for the economic feasibility of the project.
Alternatives that will be proposed in this report are technically feasible (otherwise they would have been rejected at some stage in the design process). The preliminary alternatives will have to cover the whole spectrum of possible solutions that make any (economic) sense. The economic best solution will have the largest difference between value and costs. To cover the whole spectrum of economic solutions, two main starting points for the design are formulated: minimum costs and maximum value.
Now that the main starting points of the design have been identified, the generation of alternatives can start. The spectrum of possible solutions is however, besides the boundary conditions and requirements, not specified. Here too, the starting points for the design can be used.
Maximum value The energy production of the new power plant (estimated maximum of 23 GWh/y) will be small compared to the Evanger power plant (1270 GWh/y), as is the head of the new plant (approximately 50m) to the head of the Evanger plant (770m). These numbers illustrate the significance of the new plant in the Evanger scheme: it will be small. It seems fair to say that the Evanger scheme with the new plant will operate quite similar to the scheme as it exists today considering that the operation of the Evanger scheme has been optimized. This statement does allow for a change of operation strategy of the scheme in the future, but assumes that the change will be rather small. The alternative that will produce maximum energy is thus believed to fall within the operation range of the reservoirs as it is today. In Section 3.3 some extremes for the
11 According to B. Otterås of BKK Nett, see also Section 6.9.
34 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
reservoir operation have been identified and they will also be used to give some more information about boundaries of the spectrum of possible economic solutions: - The water level in Holskardvatn is usually not lower than 820 m.a.s.l. - The maximum weekly average outflow from Holskardvatn to Askjelldalsvatn varies between 15 m3/s and 25 m3/s. - The outflow from Holskardvatn occurs mainly in winter over a period of 5 months (the energy price in winter is higher than in summer) - The maximum head between the reservoirs is around 70m in practice, opposite to the theoretical head of 115.5 m which never occurs. - The minimum head between the two reservoirs varies between 15m and 25m. - The water level in Askjelldalsvatn usually does not drop below 770 m.a.s.l.
Maximum value can be seen from different perspectives. A hydro power plant can utilize the full energy potential and thus maximize its value seen from this point of view. The full energy potential will be utilized if the maximum available head is used and no water is lost.
Maximum value is however not only maximum energy, it is also a matter of being able to produce electricity at the right time, when the price is high. A power plant that is to utilize all available water when the price is high will create a high value. At least more value than a similar power plant that utilizes the available runoff water immediately. To be able to produce electricity at the right time, a storage volume and a sufficient capacity to produce the water during a limited period of time are necessary. Holskardvatn is available for this project as a storage reservoir and the plant can thus be designed such, that the installed capacity is sufficient to produce electricity during the winter.
Finally, we can look to maximum value from a broader view: the whole power scheme. More value can be created by increasing the energy that is produced and by producing more energy at the right time; in winter. The Evanger plant can produce more energy if the water level in Askjelldalsvatn is higher. This is however rather unlikely as it has been shown that the water level in Askjelldalsvatn is already at its highest regulated level most of time. But energy production can be shifted from summer to winter by a pumped storage plant between Holskardvatn and Askjelldalsvatn. Water can be pumped from Askjelldalsvatn to Holskardvatn in summer and it can be utilized for energy production in winter in both the new plant and, more important, the Evanger plant. Askjelldalsvatn is a small reservoir and needs to be drawn down almost completely before the summer to provide storage volume for the expected melt water. At this time the energy price is rather low, but production in the Evanger plant is necessary to create the storage volume in Askjelldalsvatn. A pump turbine can turn the disadvantageous of the low energy price and low head (as a result of emptying the reservoir) into an advantage: the low energy price makes it rather cheap to pump up the water from Askjelldalsvatn to Holskardvatn. In this way the energy does not need to be produced at a low price and empty space is created for the expected melt water. The water can be stored in the larger Holskardvatn until winter when the energy price is high again. The water can then be utilized for energy production in both the new plant and the Evanger plant. Approximately 20% of energy is lost in the pumping – production cycle in the new plant. The difference between summer and winter energy prices and the fact that the water can also be utilized in the high head Evanger plant are believed to make a pumping mode beneficial.
Minimum costs The direct construction costs make up for a large part the total costs of a power plant. The direct construction costs are usually divided in civil works costs and electro mechanical costs. The latter are mainly determined by the installed capacity and the civil works costs mainly depend on the selected alignment.
35 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
To minimize the construction costs the plant is designed such that no special (hence expensive) equipment needs to be used. A short alignment that can be constructed with standard equipment is often the cheapest solution. Horizontal alignments such as tunnels are easier to construct than vertical shafts and are therefore significantly cheaper per running meter. To further minimize the construction costs, the following aspects are taken into account when generating alternatives: - A large part of the existing tunnel system will be used to reduce the construction costs. The existing bottom outlet of Holskardvatn will be used as an intake for the new power plant and a major part of the existing tunnel system will be used as a conveyance system for the plant. - To meet the requirement for the bypass capacity, it is anticipated to be able to use the existing gate system parallel to the new power plant. This is more cost effective than constructing a new bypass system. The project area is then defined as the area around the existing tunnel system, close to (and including) the outlet in Askjelldalsvatn, see drawings PFS 000a & 000b in Appendix I.
5.4.1. Design variables The alignment of the power plant can be seen a design variable, but it is rather difficult to quantify this variable in a single number. The outlet level of the new plant is a design variable that is easier to quantify. It tells something about the alignment and influences both the cost and benefit side of a project. A lower outlet level of the plant will give a larger possible head, but also a longer and more expensive alignment. The sketch in Figure 17 shows the influence of the outlet level of the new power plant on the potential energy. A power plant with an outlet below the lowest water level in Askjelldalsvatn can utilize the total available head. If the tunnel outlet is placed higher, the water level in Askjelldalsvatn will sometimes be below the outlet level and the total available energy potential is not utilized.
Figure 17 Influence of outlet level on energy production. The upper lay out shows an outlet level of the new plant below the lowest water level in Askjelldalsvatn. The upper lay out shows an outlet level above this lowest water level.
36 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
A second design variable that influences both value and costs is the installed capacity. A larger installed capacity is beneficial in that flood spill or bypass can be minimized or even avoided and more peak electricity can be produced, but the electro mechanical equipment will also be more expensive. As a third design variable the pumping mode is considered. A combined pump turbine can be considered for an alternative or not.
The different alternatives intend to use different values for the identified design variables. This is done by proposing alternatives at different outlet levels for each alternative. The installed capacity will be roughly optimized and a pumping mode will be considered if it looks promising within the suggested design.
37 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
6. DIMENSIONING PROJECT COMPONENTS
In this Chapter the project components that are used in the preliminary designs are discussed. This Chapter explains the consequences and selection of the important components.
The preliminary alternatives are presented in Chapter 9 and this Chapter contains some references to that Chapter if the components are specific for one alternative only. The titles of the different Sections refer to the titles used in the Bill of Quantities for the generated alternatives, see Appendix IV. The prices are taken from figures from [NVE, 2005] and corrected to a 2007 price level. The prices mentioned in this chapter refer to the 2005 price level, they have been adjusted in the Bill of Quantities, see the last section in this chapter.
6.1. Access Roads and Tunnels Access Roads An existing gravel road leads to the project site. It crosses a bridge over the outlet of the upper diversion tunnel. The bridge is assumed to be strong enough to support all construction vehicles with loads that will have to pass; it has been used for the construction works for the gates before. The strength of the bridge should be confirmed in a later stage. No extra costs for access roads are taken into account
Access Tunnels The maximum inclination for the tunnel is 1:7 for wheel based machinery. The maximum inclination and the elevation of the power house determine the length of the tunnel. The entrance of the access tunnel at 809 m.a.s.l. is above maximum flood level in Askjelldalsvatn. The tunnel costs per running meter include stability measures (rock bolts and shotcrete) ventilation, drainage equipment and a cable shaft. To allow standard equipment a minimum tunnel cross section of 20m2 is taken.
Example: access tunnel alternative II
Alternative II has a powerhouse floor at 783 m.a.s.l.. The vertical distance from the entrance to the powerhouse is 26m. The tunnel length is thus determined by the maximum inclination of 1:7. Hence a tunnel length of 26 x 7 = 273m is assumed.
The enlargement of tunnel cross sections is estimated per m3 of additional excavation. The costs are estimated as NOK 220/m3 for blasting and transport plus 15% for stability measures plus 30% start-up costs. Hence 330 NOK per m3 is used.
6.2. Weirs and Intake Works Weirs A concrete overflow weir at the outlet is needed for Alternatives I to keep the tail water level at 805 m.a.s.l.. The tail race level is needed to maintain a negative suction head over the turbine, see Section 6.5.1. The weir is estimated to be 4m high, including foundation and 8m long so that the water level in the outlet pool only rises slightly above 805 m.a.s.l. at maximum turbine discharge. The top of the weir needs to be at 804 m.a.s.l.
38 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Example: weir height alternative I-a
The outlet level of 805 m.a.s.l. needs to be maintained to provide sufficient suction head for the turbine. When the water level in Askjelldalsvatn is at its highest regulated water level, this requirement is fulfilled no matter what the weir height is.
To guarantee the outlet level when the water level in Askjelldalsvatn is below 805 m.a.s.l., the weir needs to have a certain height. This situation is estimated as a board crested overflow weir (the approach velocity is estimated as 1 m/s), see Figure:
He = 805 – x - Hv [m] x = top of the crest [m.a.s.l] 2 Hv = velocity head = 1 /(2 x 9,81) = 0.05 [m]
The effective length of the weir L is estimated as 7,8m and the weir coefficient as 1,7 [Novak, 2001]. The maximum discharge for this alternative is 13 m3/s.
3/2 With Q = C x L x He He is quickly found to be 1.0m and from the first equation the top of the crest is found to be approximately 804 m.a.s.l.
The weir length in Alternative I-b is 4 m, as it will be constructed in the outlet of the tunnel. In this case the top of the outlet weir needs to be at approximately 803.5 m.
Intake Works The bottom outlet from Holskardvatn is used as an intake for all alternatives. A trash rack is not necessary here, because of the depth of the intake below water level. No additional costs for intake works are taken into account in any of the alternatives.
6.3. Waterways Trench Two different types of trenches are used for pipes: trenches in rock (in the tunnels in Alternatives I-a and I-b) and trenches in gravel in the rock deposit (Alternative I-c). The pipes in the trenches will be covered with gravel. The depth of the trench is taken
39 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
as the pipe diameter plus a cover of 1m. The width is the diameter of the pipe plus 20 cm space on both sides.
Head and Tail Race Tunnel The tunnel cross section is determined for a flow velocity around 1m/s. From experience [GUNNES] it is known that an optimized unlined tunnel crosses section (comparing marginal tunnel costs and head losses) will usually result in velocities around this value.
However, rock tunnels have a practical minimum cross section of some 20m2 because of the size of the equipment used by most contractors: large drilling rigs and trucks. For all alternatives the tunnel cross-section is therefore selected as 20m2, which results in velocities well below 1m/s. The tunnels will be unlined unless stated otherwise.
Concrete Casing A concrete casing forms the transition from the tunnel to the pipe, just upstream of the powerhouse. It is needed to prevent water leakage into the powerhouse. The length of the concrete casing is estimated as 1/20 of the local water pressure with a minimum of 5m.
Shaft Lined pressure shafts are constructed the rock and a glass reinforced plastic (GRP) pipe is constructed in the shaft. The space between the shaft wall and the pipe is filled with concrete. These shafts are used in Alternative I-a and I-b.
Pipes GRP pipes are selected because of their low price and the relatively low water pressure (<85 m). The diameter is selected so that the design velocity is around 3m/s, which is in the range of the expected economically optimized velocity in pipes. [GUNNES].
Surge Shaft Calculations show that a surge shaft is necessary for every alternative to achieve a stable governor situation. The surge shaft will reduce the inertia of the water masses. It is constructed as a raised bored shaft from the head race tunnel, as close as possible to the powerhouse. The location of the shaft is limited to areas where the surface is several meters above the highest regulated water level in Holskardvatn. The surging water should stay within the shaft.
A surge shaft is needed for every alternative and constructed as a raised bored shaft with a diameter D = 2m. The top of the shaft is enlarged according to the required cross section and the surge.
The necessity of a surge shaft is determined by the quality of the system regarding governor stability and the need to reduce the transient pressure wave.
For good governor stability, the inertia of the water masses needs to be small enough relative to the inertia of the inertia of the rotating masses of the turbine and generator. This will be illustrated by an example. Suppose the load of the grid decreases, the speed of the turbine will then increase (less electrical resistance). The governor will try to regulate the speed by steering the wicket gate to close a bit. This will reduce the flow and thus the torque on the turbine; hence as a result the rotational speed will decrease. However, if the inertia of the water masses is too large, a small closure of the wicket gate will result in a pressure rise in front of the turbine. The pressure rise will increase the torque on the turbine and the rotational speed of the turbine will increase again, where as the initial intention was to slow down the speed.
40 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
For good quality governor stability this last effect needs to be controlled; the inertia of the water masses needs to be sufficiently small compared to the inertia of the turbine and generator. The inertia of the generator is determined by its own mass and the electrical resistance of the transmission net. A surge shaft creates an open water surface closer to the turbine. The water volume involved thus reduces and hence, so does the inertia of the water masses.
A pressure wave occurs when a sudden disturbance in the flow is created, by for example closure of the turbine. The pressure system needs to be designed for this pressure wave. A surge shaft can be needed to decrease the design pressure for the mechanical machinery. The pressure wave is sufficiently small for the proposed alternatives.
Appendix VI provides the calculations for all alternatives that show the need for the surge shaft to achieve sufficient governor stability. It also determines the pressure wave and dimensions of the top of the surge shaft.
6.4. Powerhouse-Civil Works Powerhouse The location of the underground powerhouses is checked to have a minimum rock overburden of 60% of the local water pressure. The costs for an underground powerhouse are calculated by multiplying the excavation volume with a unit price. The following formula is used to estimate the necessary excavation volume V:
V = 78 * H0.5 * Q0.7 * n0.1 [m3] [NVE, 2005] H = design head Q = design discharge n = number of turbines
The values found in this way are rounded of to 50m3. The unit price is taken from the relevant figure as 1500 NOK/m3. This is increased by 10% for alternatives II and V to provide additional measures for water tightness.
For the powerhouse in open air the total civil construction costs can be read directly from the relevant figure. The large amounts of snow will require a solid structure. This could increase the costs of the outside powerhouse, but this has not been taken into account in the Bill of Quantities.
Additional blasting For Alternatives I-a and I-c additional blasting is necessary for the outlet pool (see also 6.2, Weirs). A 10*15*5 (length*width*depth) basin is anticipated.
6.5. Electrical and Mechanical Equipment
6.5.1. Turbine Turbine Type In this Section the selection of the turbine is motivated. The Francis turbine is selected instead of the Kaplan turbine for two reasons.
The first reason is the cost aspect. The turbine is a large cost item and a Francis unit is some 30% to 50% cheaper than a Kaplan unit. A Kaplan unit also requires a deeper setting of the turbine, which will give extra costs for civil works.
41 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Secondly, the double regulation of the Kaplan unit is not necessary in these designs. Holskardvatn is a large reservoir and the turbine can thus always be operated close to its best flow-efficiency. The double regulation of the Kaplan unit is thus not used in this respect. What remains is the variation in head. Here the Kaplan unit is beneficial compared to the Francis unit, however the differences in efficiency between the units is not so large for varying heads (the difference is much larger for varying flows).
Hence because the double regulation of the Kaplan unit is not optimally used, the additional investment for this turbine type is believed not to be beneficial and a Francis type is selected.
Installed Capacity The head is selected differently for each alternative (see Chapter 9), but the head range is always determined as follows: - maximum turbine head as 125% of the design head - minimum turbine head as 70% of the design head
The discharge has been roughly optimized for all alternatives. The initial design discharge was selected based on two assumptions: - The period the turbine is in operation is 5 months (winter period) - The volume of water that goes through the turbines is determined by the water flows between the reservoirs that occurred within the selected head range the last 10 years. The head-flow diagrams for Holskardvatn and the gross head from Holskardvatn to Askjelldalsvatn were used to determine the latter, see Figure 11 and Figure 14.
The maximum discharge through the turbine is selected 15% larger than the design discharge: Qmax = k x Qdes = 1.15 x Qdes
With the discharge and head selected, the installed capacity has been determined with the following formula (overall turbine efficiency ηturb is selected at 90%):
6 Pturb = ηturb x ρ x g x Hn,des x Qmax /10 [MW]
The value found in this way is rounded up to a multiple of 1 MW to allow sufficiently large flows at high heads.
The installed capacities for the alternatives have been roughly optimized; see the description of the alternatives in Chapter 9. The design discharge has been determined more accurately using the head loss, see hereafter.
Note: the selected discharge can be delivered for all heads in the turbine range. The existing gate capacities for different heads are presented in Appendix I 12558. It can be assumed that the head loss of the power plant will be somewhat different from the gate capacities, but not so much. The figure in the Appendix shows that a discharge of 15m3/s can be achieved in the existing system with a head difference of 8m between Holskardvatn and Askjelldalsvatn. Hence the discharge is only dependent on the guide vane opening of the turbine and not on the available head.
42 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Example: initial installed capacity Alternative I
The outlet level of Alternative I is at 805 m.a.s.l.
The maximum turbine head is selected equal to the maximum available head: 865.5 – 805 = 60.5 m The turbine design head is selected as: 60.5 / 1.25 = 48 m, or 853 m.a.s.l. The minimum turbine head is selected as: 48.4 x 0.7 = 34 m, or 839 m.a.s.l.
Hence the turbine range is 839 m.a.s.l. < Hturb < 865.5 m.a.s.l.
Figure 11 shows that approximately 65% of all flows from Holskardvatn occur when the water level is above 839 m.a.s.l. 65% of the flows corresponds to 0.65 x 187 = 122 Mm3 water per year available for the power plant (the rest needs to be bypassed).
The initial design discharge of the plant is estimated with full production during the winter period of 5 months: 6 3 Qdes = V / t = 122 x 10 / ( 5 x 30 x 24 x 3600)= 9.4 m /s The maximum turbine discharge is selected 15% larger: 3 Qmax = k x Qdes = 1.15 x 9.4 = 10.8 m /s
With a design head of 48m, the installed capacity is found to be: 6 Pturb = ηturb x ρ x g x Hn,des x Qmax /10 = 0.9 x 1000 x 9.81 x 48 x 10.8 / 106 = 4.6 MW
This is rounded of to 5 MW to allow large flows at high heads. The design discharge is then selected as: 6 3 Qdes = 5.0 x 10 / (1.15 x 0.9 x 9.81 x 48 x 1000) = 10.3 m /s
Preliminary turbine design A preliminary design of the turbine is prepared to estimate the required suction of the turbine, which is important for the turbine setting and thus the civil work costs component. The formulae presented in Section 4.6 are used for this purpose.
The suction head and rotational speed of the turbine are closely correlated. A larger rotational speed requires a larger suction head to avoid cavitation problems. The rotational speed has to match a synchronous speed, which is determined by the frequency of the grid.
Synchronous speeds have been selected that give a suction head of approximately - 4m. Larger suction heads would require a shaft between the turbine and generator (with the generator above flood water level) that would become so long that additional bearings are required. Hence the results are suction heads around -4m with synchronous rotational speeds that are a bit lower than the maximum rotational speeds that turbine manufacturers suggest for the selected head range (the latter is presented in Section 4.6).
This exercise to determine rotational speed and setting has been performed for each alternative and can be found in Appendix V.
43 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
6.5.2. Combined Pump Turbine For alternatives II and III a combined pump turbine has also been considered. The same turbine characteristics are assumed as for a regular turbine. The pump characteristics are based on the formulae suggested by [STELZER, R.S. AND WALTERS, R.N. (1977)].
The calculation of the combined pump turbine characteristics can be found in Appendix VI. The maximum and minimum pumping heads fall within the head range of the turbine. The maximum pumping head is selected at 60m, the minimum at 40m. The maximum discharges are found from the generator capacity at both pumping heads, assuming efficiencies of 95% and 94% for the generator and turbine respectively.
The pumping mode is more prone to cavitation problems than the generating mode and the setting of the combined pump turbine is determined by the former. Required settings of approximately -15m below tail water level are found in this way.
A combined pump turbine is approximately 25% more expensive than a regular turbine. Other additional investments for a pump turbine are needed for the longer tunnels (as a result of the deeper setting), more complicated control equipment and more expensive transmission grid upgrade.
6.5.3. Miscellaneous Equipment This includes cooling and drainage equipment and the powerhouse crane. For Alternatives II and II 40% costs are added for the drainage pumps. The pumps for these alternatives need to overcome large heads of 30m and 50m, whereas drainage pumps for only a couple of meters are taken into account in the NVE estimate.
6.5.4. Electro-technical Equipment The cost for the generator (plus 10% for vertical setting), the transformer, control equipment (for generator, remote control, battery etc.) and costs for the connection to grid (excluding any new transmission lines) are mentioned in the Bill of Quantities under this heading. The transformer will be placed in the powerhouse, separated from the machine hall by walls.
The control equipment is some 25% more expensive in case of a combine pump turbine.
6.5.5. Outlet gate An outlet gate is necessary in Alternative II and III to be able to drain the draft tube. A 2m x 2m gate is selected. For the other Alternative the water level in Askjelldalsvatn will have to be lowered in case the draft tube needs to be inspected.
6.6. Contingencies To take the uncertainty into account, contingency charges are taken into account. The prices provided by NVE are average unit prices which resulted from studying several projects. The uncertainty is between +/- 25% and 30% for most civil works, with the underground powerhouse as the major exception: the uncertainty here is from -50% to +100%. Uncertainty for electro technical works is estimated to be +/- 10% to 20%. For mechanical works, such as turbine and pipes, the uncertainty is estimated at +/- 20%, see also [NVE, 2005].
44 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Contingencies are taken as 10% of the estimated construction costs (sum of costs mentioned in Sections 6.1 - 6.5) for the low risk profile alternatives I. The variants for this alternative either have construction works mainly in open air (I-a and I-c) and/or have underground works in well known rock (I-b). Unforeseen costs are believed to be higher for alternatives II and III as large underground works are required. These works are associated with larger risks for unexpected events such as high pressure water entrance in the tunnel during construction. Contingencies are therefore estimates at 15% of the construction costs.
6.7. Planning and Administration This is taken as 10% of the estimated construction costs (sum of costs mentioned in Sections 6.1 - 6.5).
6.8. Financing Cost This is taken as 5% of the estimated construction costs (sum of costs mentioned in Sections 6.1 - 6.5) for a estimated construction period of 1 year for Alternative I and 7% of the estimated construction costs for Alternatives II and III (estimated 2 years construction period).
Construction can only take place in the snow free summer period and the number of months that work can be done is therefore limited to some 6-8 months per year. It is assumed that the draw down of Askjelldalsvatn around June can be combined with the required lowering of the reservoir for tunnel works. Hence no extra costs for lowering Askjelldalsvatn during the construction period are taken into account.
6.9. Transmission12 For outputs of the new power plant up to 9 MW no major technical and economical problems are expected. Outputs higher than 9 MW can cause unacceptable large variations of voltage in the local transmission line and other measures than upgrading the local transmission line need to be taken.
The 22 kV transmission line will have to be upgraded over its whole length, some 13 km. Because of the length large voltage variations are expected over the line. This variation is limited to 2% of the output for a 22kV line. This can be achieved by producing sufficient reactive energy. It is advised to have a generator with power factor (defined as ratio of the real power to the apparent power) cos Φ = 0.86 for this purpose.
A majority of the produced electricity has to be transported to the national grid, because of the low local use. This is done at transformer stations at Evanger and Mynster. These transformers are operating at the limit of their capacities, while several power projects are planned from which will also need transformer capacity these stations. Depending on the demand, a new higher capacity transformer can be installed at costs to be paid by these new projects. Several other projects have requested an increase of capacity of the transformers at Evanger.
The exact upgrade and costs of the transmission system have to be determined by a separate study by an electrical engineer. This kind of study will be performed if the results of the prefeasibility study on the Askjelldalsvatn power plant look promising. The costs for the upgrade of the transmission system in this report will be based on the prefeasibility study on the Skjerjovatn13 power plant.
12 This Section is based on information from an interview with B. Otterås from BKK Nett 13 The report ‘Skjerjovatn Kraftverk, Versie 1999’ .
45 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
The transmission costs can be divided in two: one sum for the upgrade of the local transmission line and another sum for the installation of a new transformer plus miscellaneous equipment to transport the produced electricity in the national grid.
For this study the costs are based on transmission costs for the Skjerjovatn project, see also Section 3.2.5. In this 1999 evaluation the transmission cost for upgrading the line over a length of 10 km down to Trefall was estimated as 4.18 MNOK (for a 7MW project). The costs for a new transformer and an upgrade of a 132 kV line at Evanger for a 16 MW project were estimated at 4.40 + 4.0 = 8.4 MNOK.
These figures are corrected for inflation to 2005 (assumed 3%) and linearly correlated to the installed capacity to estimate the transmission costs for the alternatives
Example: transmission costs Alternative I 5 MW
For the Askjelldalsvatn power plant an extra 3km of transmission line should be upgraded, hence the local transmission costs are estimated for a 7 MW project (price level 1999): 13 / 10 x 4.18 = 5.43 MNOK
Corrected for inflation (assumed 3%, total 19% over 6 years) the 2005 price would become (7 MW) 5.43 x 1.19 = 6.5 MNOK
For different outputs the price is linearly correlated to the output. For alternative I (5 MW): 6.5 x 5 / 7 = 4.64 MNOK
The same has been done to estimate the costs for a new transformer and upgrade of a high voltage line at Evanger. In 1999 these costs were 8.4 MNOK for a 16 MW project. The costs versus the output are assumed to be linear. These costs are also corrected for inflation (3% per year). The transformer and high voltage line costs for a 5 MW project are estimated as: 5 / 16 x 8.4 x 1.19 = 3.12 MNOK
Hence the total transmission costs in 2005 for Alternative I are estimated as: 4.64 + 3.12 = 7.76 MNOK
Combined pump turbine With a combined pump turbine the direction of the current will vary. As a result the energy loss over the transmission line and the available voltage for the local users will also vary from the pumping mode to the generating mode. Additional measures need to be taken to achieve acceptable voltage levels for local users. 10% additional costs for transmission costs are accounted for in case of a combined pump turbine design.
46 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
6.10. Increase of Cost Price 2005 – 2007 The prices in the Bill of Quantities are corrected to 2007 using the price indices given by [NVE, 2007]. For different components the following price increases have been used for the period 2005-2007: - Turbines +14% - Gates +9% - GRP pipe +7% - Valves +10% - Miscellaneous equipment +10% - Generator +20% - Transformer +20% - Control equipment +20% - Grid connection -10% - Transmission +15%
The Bills of Quantities show the 2007 prices, which have been corrected with the above price increases over this period.
47 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
7. ENERGY ESTIMATE
The energy an alternative is able to produce is estimated using the computer model ‘Vansimtap’. For this purpose the head losses are determined first. The energy estimates from Vansimtap for the preliminary alternatives (see Chapter 9) are validated by comparing them to energy estimates based on the last ten years of production data of the reservoirs.
7.1. Head Losses For each alternative the head losses are determined in a similar manner. An explanation and the calculation of the head losses for all alternatives can be found in Appendix III.
Friction and local losses occur both in the existing tunnel system and the new conveyance system of an alternative.
For existing drill and blast tunnel a Strickler’s coefficient M = 29 m1/3/s is suggested for drill and blast tunnels by [GARNAYAK, 1999].
For the pipe f = 0.018 is used:
v 2 L Friction losses in pipes: Δh = f [m] f 2g D V 2 L Friction losses in tunnels: Δh = [m] f M 2 R 4 / 3
For horseshoe shaped tunnels, the following approximation for the hydraulic radius R is used: R = 0.265 A [m]
Local losses are calculated with the following formula, k-values are taken from [RÖSSERT, 1992]: v 2 Δh = k [m] l l 2g
The head losses are used to determine the net heads of the turbines, which are used to determine the energy that an alternative can produce.
7.2. Energy Estimate from Reservoir Operation ’97-‘07 A rough energy estimate is used to validate the results for the energy production from Vansimtap.
An estimate of the energy production of an alternative is made using the production data from the period 1997 – 2007 for the Evanger plant.
The new Askjelldalsvatn power plant will not influence the operation of the Evanger power scheme a lot as the Evanger plant has a much larger capacity and production than the new plant. Hence the operation will almost be similar to the situation today. This assumption is used to make a preliminary estimate of the energy that the new plant can produce, based on the reservoir operation data of the Evanger scheme.
With a selected head, discharge and capacity for each alternative, the exercise as shown in Figure 18, is performed on the weekly production data from 1997-2007. The
48 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
data is used to estimate the production of energy per week for the whole period. From these 10 years annual energy estimates are deduced.
For alternatives I, III and IV the net head is determined as the difference between the water level in Holskardvatn and the outlet level at 805 m.a.s.l. minus the head loss. For alternatives II and V the net head is determined as the difference between the water level in Holskardvatn and Askjelldalsvatn or the outlet level (depending on the water level compared to the outlet level), minus the head loss.
Figure 18 Decision schedule for the estimate of produced energy in Askjelldalsvatn power plant, based on reservoir operation data from 1197-2007.
7.2.1. Results The energy that is produced over the ten year period is summed and divided by 10 to arrive at an estimate for the average annual energy that can be produced.
The results are given in Table 4 for the optimized alternatives.
Maximum Average annual Installed Alternative turbine flow H (m) H (m) energy (GWh) capacity (MW) n,min n,max (m3/s) I 11,2 5,0 12,9 30,1 53,7 II 13,2 7,0 14,2 37,9 67,7 III 14,0 7,0 14,4 38,0 67,8
Table 4 Energy estimates based on ten years of production data (1997-2007)
7.3. Energy Estimate Using Vansimtap A more accurate energy estimate is made for each alternative using the computer model Vansimtap. In this section the program and its (dis)advantages will be briefly discussed and the results of the energy estimate will be presented.
7.3.1. Vansimtap The computer program Vansimtap basically consists of two different models.
49 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
One part of the program models the energy market and decides how much energy will be produced in thy power plants of the owner (in this case BKK has modelled al of its power plants in Vansimtap). This model analyses the energy market and decides on the energy that is bought, sold and produced for a certain week, based on data from market research and the value of water in the owner’s hydro power plants. For this simulation the average value of the water in all hydropower plants of the client is calculated and serves as an input parameter for the energy market model.
When the amount of energy that will be produced in the power plants is decided upon, the second model spreads the production over the individual power plants (represented by a module), producing the energy with the lowest value first. For example unregulated water, or water that is about to spill over a reservoir has a low value.
An iterative process between these two models determines the optimum amount of water that should be produced. In this way Vansimtap is able to determine the energy production for multiple developed river basins. It is able to calculate the energy that will be produced for (hydro) power plants. It optimizes the production for a certain system and it is noted that it is not able to optimize the system itself.
The program Vansimtap has two disadvantages for the simulation of this power plant. The first problem is that the results from Vansimtap do split between flood spill and bypassed water. Although the model steers this water to the correct module, the results from the Holskardvatn module only make a difference between water that has been utilized for energy production and water that has not been utilized for energy production (which equals flood spill plus bypass). This problem is qualitatively checked by graphically presenting the water levels, flows and flood spills plus bypass of three normative years (a dry, regular and wet year) in a diagram; examples of these diagrams can be found in Chapter 10. It can then easily be seen if water is bypassed or whether it has flooded over the reservoir. The latter can only occur if the water level in the reservoir is at the highest regulated water level. The visual analysis of these results showed no inconsistencies of the results produced by Vansimtap.
A second point of attention is that the efficiency is only given for the design head and no minimum or maximum turbine head can be given: the power – discharge (PQ) curve can only be given for the design head. This problem is solved by assuming a 1% lower efficiency all over the flow range in the PQ curve. Even if the head falls below the minimum head, energy is produced. This problem can not be solved by restricting the flows for lower heads: the program will then see this lower head as the lowest regulated water level in the reservoir. In practice using the head discharge restriction will result in large flood spills from the reservoir and a reduced energy production in the system. It is decided to ‘clean’ the results for these lower heads, by simply deleting the energy produced when the minimum gross head is not exceeded from the results. The efficiency can neither be adjusted for different heads, but because the efficiency changes less for varying heads than for varying flows,
7.3.2. Input For each module (which in Vansimtap consists of a reservoir and a hydro power plant) three sets of input parameters can be given: characteristics for the module, restrictions and steering functions. A module consists of an intake reservoir plus the connected power plant. The Holskardvatn module is the only one that changes as a result of the new plant, the other modules in the system and the market data remain unchanged.
The turbine characteristics are given as a power – discharge curve (PQ curve). These curves are provided in Appendix V, using the estimated efficiency
50 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
As an example the input parameters for the Holskardvatn module Alternative I are described and explained in Appendix VII. In this Appendix the energy estimates for the other alternatives can also be found.
7.3.3. Results The energy that an alternative will produce is compared to the situation as it is today, the basic situation. The most interesting results of the basic model are the energy production; the flow utilized for energy production and the combined flood spill and bypassed water (per week, per season and per year). The power plants and reservoirs that are physically influenced by the new power plant are Evanger, Oksebotn, Myster, Nygard and Steinsland (power plants) and Skjerjevatn, Grøndalsvatn, Piksvatn, Volavatn, Holskardvatn, Askjelldalsvatn, Nesevatn and Stølsvatn (reservoirs). These power plants and reservoirs together form the basic model. The energy that an alternative can be accounted for is compared to this basic model; this also explains how a negative energy production in summer can occur.
The energy estimates made with Vansimtap of the different alternatives plus the estimates that have been determined from the production data in the previous Section are presented in Table 5. These are the optimized energy estimates from the alternatives described in Chapter 9. The results of alternative I do not differ too much from the indicative energy estimate from Section 7.2. Alternatives II and III show a larger difference; this can be explained by the fact that Vansimtap tries to maintain the head at the best efficiency point. This is possible in Alternative III because of the low outlet.
A full detail of the results from Vansimtap for alternatives I to III can be found in Appendix VII.
Average annual energy (GWh). Vansimtap: Vansimtap: Vansimtap: Determined from Alternative Average annual Summer energy Winter Energy production data 97- energy (GWh) (GWh) (GWh) 07, see previous Section. 0 2659 1033 1626 I 11,2 11,9 -3,9 15,8 II 13,2 17,4 -3,9 21,3 III 14,0 21,9 -1,5 23,4
Table 5 Result of energy estimate with Vansimtap compared to estimate based on reservoir operation data.
51 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
8. ECONOMIC ANALYSIS
An economic analysis has been performed for each alternative to quantify the economic profit. The alternatives can be compared with these results to identify the best solution. In this Chapter the analysis is explained. The results can be found in Chapter 9.
The analysis is done for the basic year 2007. The Net Present Value and Benefit / Cost ratio have been determined for each of the alternatives.
The input consists of the total construction costs, as identified in the Bill of Quantities and the energy produced, which can both be found in Appendix IV. Other parameters for the economic analysis are given in Table 6. Future market prices have been determined by BKK and are also used in the economic analysis. A difference is made between the energy price in summer and winter. These figures are confidential and are therefore not included in this report.
No taxes have been included in the economic analysis.
Refurbishment is taken into account as a percentage of capital costs according to Table 7.
Period of analysis (yr) 60 Operation & Maintenance % of total 1 construction costs Construction period (years) 1 or 2 Discount rate (%) 7 Transmission loss (%) 3
Table 6 Parameters for economic analysis
Rehabilitation 1 Rehabilitation 2 Rehabilitation 3 Year Percentage Year Percentage Year Percentage Civil Works 20 3% 40 6% 60 6% Generator 25 10% 45 40% 70 75% Turbine 25 10% 45 40% 70 75% Miscellaneous 25 10% 50 50% 75 75% Equipment Control 20 20% 40 50% 60 25% Equipment Transmission 20 5% 40 10% 60 50% Transformer 0 0% 40 5% 50 100%
Table 7 Rehabilitation of hydro power plants14
14 This information was provided by BKK and is used by the company for evaluating hydro power projects.
52 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
The following definitions have been used:
NPV = PVB – PVC [MNOK]
B/C Ratio = PVB / PVC [-]
n PV = An * 1 / (1 + r) [MNOK]
NPV = Net Present Value [MNOK] PVB = Present Value of Benefits [MNOK] PVC = Present Value of Total Cost [MNOK]
PV = Present Value [MNOK] An = annual investment in year n [MNOK] r = discount rate [-] n = year from basic year [y]
53 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
9. PRELIMINARY DESIGN
This Chapter describes the main features of three alternatives for a power plant that utilizes the water from Holskardvatn to Askjelldalsvatn. Several aspects that have been considered during the design process are discussed for each alternative. The project components are described in Chapter 6. The methodology to estimate the energy that an alternative can produce and the economic analysis are presented in chapter 7 and 8 respectively.
The schedule of requirements has been discussed in the Chapter 5. All suggested alternatives comply with the given requirements and boundary conditions. The alternatives use different values for the design variables.
The starting points of design are used to cover the whole spectrum of practical solutions. This has resulted in three alternatives (with different outlet levels): - Alternative I: minimum construction costs (805 m.a.s.l) - Alternative II: compromise between minimum costs and maximum value (783 m.a.s.l.) - Alternative III: maximize energy value (770 m.a.s.l.)
Both alternative II and III are also tested for the effect of pumping water to Holskardvatn.
The alternatives have been roughly optimized with regard to the installed capacity. This is the most influential factor that can be optimized for a certain selected alignment. Cost items as the tunnel cross section and pipe diameter are much smaller. The optimization is done by comparing the ratio construction cost over annual energy for different installed capacities. This is used as a first indication of the optimum design; a marginal cost benefit approach will be used to optimize the selected alternative in the detailed design phase.
A more detailed discussion of the design variables can be found in the next sections. A principle sketch of the alternatives is presented in Figure 19.
Figure 19 Sketch showing the principle alignment of the suggested alternatives
54 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
9.1. Alterative I - Minimum Construction Costs This alternative is designed to minimize the construction costs. Therefore a short alignment is selected, using as much as possible of the existing tunnel system. By selecting an outlet level equal to the highest regulated water level in Askjelldalsvatn, the power house floor can be positioned just above the maximum flood water level of Askjelldalsvatn. This has cost saving effects: - only a short new conveyance system needs to be constructed - no pumps are required to drain the powerhouse as gravitational flow is possible - no risk of damaging the electrical equipment in case of floods
To ensure that the proposed design has minimum costs, different structural designs are proposed for the same hydraulic design. This is necessary as different alignments can be selected with different types of construction works; underground or open air. The influence of these type of works are not know and will be identified by proposing different structural designs. The energy that can be produced by the proposed variants is almost equal (differences occur because of different head losses), but the construction costs might differ. For each structural design the head loss will be determined for a certain discharge (the arbitrary value of 11 m3/s is selected) with the purpose to enable a quantitative comparison of these production costs, see Appendix III. All structural designs are priced for the initially selected installed capacity (see Section 6.5.1) of 5 MW. The estimated construction costs and the head loss of the different structural designs are compared and one structural variant is selected in Section 9.1.5.
The energy that can be produced is only determined for the selected structural design.
9.1.1. Hydraulic Design The intake structure is the bottom outlet of Holskardvatn. A large part of the existing tunnel system will be used and the water will be lead around the high pressure gate. The existing intake gate can be used to empty the head race tunnel for inspection.
The outlet water level is at the highest regulated water level in Askjelldalsvatn at 805 m.a.s.l.. The powerhouse floor will be at 807 m.a.s.l., which is above the probable maximum flood level in Askjelldalsvatn (806.4 m.a.s.l.) The water will normally flow through the powerhouse and out into Askjelldalsvatn. If water needs to be bypassed around the powerhouse, the existing gate system can be used. To avoid cavitation problems, the turbine needs to be placed several meters below tail water level as is shown in Appendix V Table 1. To guarantee this suction head an outlet pond and overflow weir are constructed. The available head is thus independent on the water level in Askjelldalsvatn and is therefore a sort of minimum head.
Because the turbine is a large cost item, one Francis turbine is. The selected installed capacity and energy that can be produced will be discussed after the structural design has been selected.
55 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
9.1.2. Structural Design Alternative I a – Short Alignment in Open Air This structural design opts for a short alignment with minimum underground tunnel works. The major civil works are related to the powerhouse and installation of a pipe.
Drawings of this structural design of this alternative can be found in Appendix II PFS 001 & 006.
The power house is placed outside, close to the existing gate house. An outlet pond is blasted and a concrete overflow weir is constructed between the pond and Askjelldalsvatn.
In the existing access tunnel to the gates, a 3 m deep trench will be blasted. From the power house a glass reinforced plastic (GRP) pipe goes up through the trench and via a concrete casing into an 18m long shaft. This shaft ends halfway the existing shaft that connects the existing head race tunnel to the high pressure gate. The space between the GRP pipe and the new shaft will be filled with concrete to fix the position of the pipe. The GRP pipe in the trench is covered with rock so that the access tunnel can be used to access the gates.
A surge shaft is necessary to reduce the inertia of the water masses for good governor stability. The necessity will be shown for the selected structural design, see Section 6.3.
The head loss and construction costs for Alternative I-a are presented in Table 8, the corresponding Bill of Quantities can be found in Appendix IV Table 1. The head loss is determined in Appendix III.
Station MNOK 32,3 Construction Costs Transmission MNOK 8,9 Total MNOK 41,2 Head loss Q = 11 m3/s m 2,63
Table 8 Construction costs Structural Design Alternative I a
9.1.3. Structural Design Alternative I b – Short Alignment Underground This structural design opts for a short alignment with almost all construction works underground. The major civil works are related to the underground powerhouse and two short tunnels.
Drawings of this structural design of this alternative can be found in Appendix II PFS 004 & 009.
The power house will be constructed underground partially in the existing access tunnel to the gates. The access tunnel will go through the machine hall at 809 m.a.s.l and on to the hoist mechanism for the gates. For the transport of the turbine and generator the cross sectional area has to be enlarged. The cross section of the access tunnel is 14m2 and it is estimated that it will be necessary to enlarge it to 20m2.
The tail race tunnel is constructed from the draft tube outlet to the existing upper diversion tunnel. The upper diversion tunnel is deepened 3m over its whole length and a small concrete overflow weir is constructed at the end to ensure that the tail water level stays at 805 m.a.s.l., even if the water level in Askjelldalsvatn is lower.
56 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
A 10m inclined lined shaft will be constructed from the powerhouse to the existing shaft between the two gates. Upstream of the lined shaft the existing tunnel system will be used to the intake.
A surge shaft is also required for this structural design.
The build up of the construction costs for Alternative I-b are presented in Table 9, the corresponding Bill of Quantities can be found in Appendix IV Table 2.
Station MNOK 31,1 Construction Costs Transmission MNOK 8,9 Total MNOK 40,0 Head loss Q = 11 m3/s m 1,76
Table 9 Construction costs Structural Design Alternative I b
9.1.4. Structural Design Alternative I c – Simple Construction Works This structural design will require no underground and only simple construction works. The alignment is a straight GRP pipe from the access tunnel to the Vassøyane tunnel to the powerhouse. The advantage of this design is the possibility for good cost control: the uncertainty in the costs items for the construction of a GRP pipe and outside powerhouse are smaller than for underground works (see Section 6.6); they can be estimated rather precisely after a detailed planning.
Drawings of this structural design of this alternative can be found in Appendix II PFS 003 & 008.
The access tunnel to the tunnel that leads to Vassøyane is used to tap the water out of the existing tunnel system. The water flows from the access tunnel through a GRP pipe into the powerhouse.
The powerhouse will be built close to Askjelldalsvatn, at the foot of the rock deposit, see pictures 7 & 8 in Appendix I. The tail water level will be kept at 805 m.a.s.l. by constructing an 8 m long and 4 m high concrete overflow weir between the outlet pond and Askjelldalsvatn.
From the powerhouse a pipe will go up 164m through a trench in the rock deposit and into the access tunnel to the Vassøyane tunnel. A GRP pipe is selected to cope with the expected settlements of the underground, which can be minimized by compacting. The last 65 m of the pipe will be constructed in the access tunnel to the Vassøyane tunnel. The transition from the pipe to the tunnel is a concrete casing, located 65 m from the entrance of the tunnel. The location is determined by the restriction that an unlined tunnel needs a roc overburden at leas equal to 60% of the local pressure. Upstream of the concrete casing the existing tunnel system is used.
The fact that the tunnel to Vassøyane is used results in a weir in the system at 836 m.a.s.l.. If the water level in Holskardvatn is below approximately 838 m.a.s.l. the water cannot go via the power house (to avoid air entrainment) and needs to be bypassed through the high and low pressure gate.
A surge shaft is also required for this structural design, see Appendix VI Table 1.
The build up of the construction costs for Alternative I-c are presented in Table 10, the corresponding Bill of Quantities can be found in Appendix IV Table 3.
57 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Station MNOK 32,6 Construction Costs Transmission MNOK 8,9 Total MNOK 41,5 Head loss Q = 11 m3/s m 2,72
Table 10 Construction costs Structural Design Alternative I c
9.1.5. Selection of Structural Design The best structural design will have the lowest construction costs and the smallest head loss. In this case both parameters are in favour of Alternative I-b, which has only underground works and the shortest alignment, see Table 11.
Structural Design I a I b I c Total Construction Costs MNOK 41,2 40,0 41,5 Head loss m 2,63 1,76 2,72
Table 11 Comparison structural designs for minimum costs
Structural design I b has minimum costs; both the construction costs and head loss are lower than for the other proposed designs. Although underground works are associated with a larger uncertainty in costs than open air works, it is believed that the uncertainty is already acceptably small. The fact that the existing tunnel works around the high and low pressure gate have been performed without significant problems, gives an indication that the risk for future underground construction problems is small.
The short alignment and small head loss of variant I-b give relatively low costs and therefore this variant is selected as the minimum costs alternative. Table 11 shows that the differences in costs are rather small: there is not much difference in costs for underground works (I-b) or works in open air (I a). In this case the shortest alignment with underground works came out best.
9.1.6. Turbine Selection The initial design head was selected such that the maximum turbine head (125% o the design head) corresponded with the maximum available head of the plant. With the outlet level at 805 m.a.s.l. and the HRWL in Holskardvatn at 865.5 m.a.s.l., the initial maximum turbine gross head was selected at 60.5 m. The initial design capacity was selected to give rounded of MW outputs (The initial discharge was selected to allow all water to run through the turbine in winter, see Section 6.5.1) In this way a turbine capacity of 5 MW has been found.
It has been tested to change the properties of the turbine, without changing the capacity (and thereby assuming not changing the costs). The whole head range in which the turbine can operate is lowered with 5m, as it is thought that much more water is stored in the lower water levels (around 837.5 m.a.s.l.) than in the upper 5m. The upper 5m is often not used as this volume is kept empty to anticipate floods. The lower 5m is used every year as Holskardvatn is drawn down.
It has furthermore been tested if it is beneficial to select a 6 MW turbine. For this 6 MW turbine, the construction costs will increase to 45.7 MNOK (Appendix IV Table 4), but the energy production will also increase. A lower design head and larger discharge have also been tried for this installed capacity.
The average construction costs per unit energy are lowest for the 5 MW turbine (see Table 12), with the increased maximum discharge and reduced turbine head. This is
58 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
used as an indication for the optimum installed capacity at this preliminary design level.
Installed Capacity MW 5,0 5,0 (large Q) 6,0 6,0 (large Q) Construction Costs MNOK 40,0 40,0 45,7 45,7 Total Additional Energy Evanger Scheme GWH/y 9,8 11,9 12,8 13,1 Construction Costs per Unit Energy NOK/kWh 4,10 3,38 3,57 3,50
Table 12 Comparison installed capacities for alternative I15
Hence a 5 MW installed capacity is selected, with a maximum turbine head at 55.5m.
The difference between the desired levels for the machine hall floor and the generator (807 m.a.s.l.) and the turbine outlet (801 m.a.s.l.) is so large, that a vertical setting of the turbine and generator is found to be the only practical solution.
To avoid cavitation at full flow at low heads, calculations show that the turbine outlet needs to be 3.5m below tail water level, see Appendix V Table 1. To avoid a too deep setting of the turbine and so to avoid increasing the construction costs for a larger turbine shaft and other construction works, a lower rotational speed (428 RPM) of the turbine is selected than that is practically possible (500 RPM according to turbine manufacturers).
9.1.7. Results A detailed overview of the estimated energy production for this alternative can be found in Appendix VII.
As Table 13 shows, the new plant produces slightly less than 8 GWh per year. However, almost 60% (115 Mm3) of the available water needs to be bypassed. Of this 25 Mm3 is bypassed because an inflow of water, larger than the plant capacity is required for the Evanger plant. The rest is bypassed because the available head falls outside the head range of the turbine. A larger installed capacity has been tested, but was not found to be beneficial.
When analysing the production data in Appendix VII, the following is noticed. The additional 3.9 GWh per year is produced by the Evanger plant. Flood spill from Askjelldalsvatn is practically the same as the situation without the new plant. The additional energy the Evanger plant produces is explained by the fact that the water level in Askjelldalsvatn is kept at a higher level for a longer period when the reservoir is drawn down in winter. A possible explanation is the increased capacity of the diversion tunnel. The increased capacity (bypass plus power plant) between Holskardvatn and Askjelldalsvatn makes larger flows from Holskardvatn possible, which allows Askjelldalsvatn to maintain a higher water level for a longer period of time with full production in the Evanger plant.
4 GWh per year is changed from summer to winter production. This shift is accomplished entirely in the Evanger plant. The water stored in Holskardvatn has a higher value with the new power plant than without. Whereas in the basic situation some water flowed from Holskardvatn to Askjelldalsvatn in summer and was produced in Evanger to avoid flood spill, in the new situation a higher probability of flood spill from Holskardvatn is accepted against the benefit that the water can be utilized for energy production in winter in both the new power plant and the Evanger power plant.
15 Detailed production data of the different variants can be found in Appendix VII
59 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Turbine Capacity MW 5,0 Maximum discharge m3/s 12,9 energy equivalent at *H kWh/m3 0,105 Summer GWh 0,0 Energy Plant Winter GWh 7,9 Total GWh 8,0 Summer GWh -3,9 Energy BKK System Winter GWh 15,8 Total GWh 11,9 Flood spill & bypass Mm3/y 115,7 Production Mm3/y 77,8 Outlet level m.a.s.l. 805,0 Maximum m.a.s.l. 860,5 Holskardvatn Water Level Minimum m.a.s.l. 836,8 Maximum m.a.s.l. 805,0 Askjelldalsvatn Water Level Minimum m.a.s.l. 805,0 Maximum net turbine head m 53,7 Design net head m 43,0 Minimum net turbine head m 30,0 Station MNOK 31,1 Construction Costs Transmission MNOK 8,9 Total MNOK 40,0
Net Present Value 2007 MNOK 9,8
Construction costs / annual average energy NOK/kWh 3,37
Table 13 Salient features Alternative I
60 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
9.2. Alternative II – Compromise The idea for this alternative is a compromise between minimum construction costs and maximum energy value. Construction costs are minimized by bypassing the water around the low pressure gate and through the power house. The water will flow from the powerhouse through a new tunnel and into the existing lower diversion tunnel again. Water tunnels need to be constructed for this ‘bypass’ purpose only and can therefore be short. Hence a large part of the existing tunnel system will be used.
More energy is available compared to Alternative I, because the lower outlet level results in a larger head.
A combined pump turbine can be fitted into this design with only small structural changes. The idea is to pump up water in summer to Holskardvatn where it can be stored. The water can then be utilized for energy production in winter in both the new plant and the Evanger plant. It is thought that by pumping up a certain volume of water, a large part of the energy production can be shifted from summer to winter.
9.2.1. Hydraulic and Structural Design The Bill of Quantities for this alternative can be found in Appendix IV. The intake structure is the bottom outlet of Holskardvatn. The existing intake gate can be used to empty the head race tunnel for inspection. A large part of the existing tunnel system will be used and the water will be lead around the low pressure gate. A sort of bypass tunnel will be constructed which starts in the lower diversion tunnel upstream of the gate and ends downstream of the gate in the same tunnel.
Drawings for this alternative can be found in Appendix II PFS 002 & 007.
The tailrace outlet is the outlet of the existing lower diversion tunnel at 782.9 m.a.s.l.. The power house is underground and is accessed by a 182m long access tunnel. The location of the powerhouse is east of the existing tunnel. Extra measures are anticipated for water tightness of the power house and drainage pumps to remove leakage water. An outlet gate will be installed to make drainage of the draft tube possible. The tail race tunnel is 51 m long and connects to the low pressure tunnel downstream of the low pressure gate.
From the power house a new 34 m long head race tunnel is blasted to the existing head race tunnel to Holskardvatn. To minimize water leakage from the conduit system into the power house, a GRP pipe will be constructed in this new tunnel. A concrete casing will form the transition from the tunnel flow to the pipe flow.
A surge shaft is foreseen upstream of the new head race tunnel, connecting the tunnel system to the ground level above at 865.5 m.a.s.l. A surge shaft is needed for governor stability. See Appendix VII Table 2 for calculations.
9.2.2. Turbine Selection An initial installed capacity of 6 MW was suggested, see Section 6.5. The maximum turbine head is selected corresponding to a gross head of 69m. This head is selected to give a head range for the turbine that corresponds to the highest occurring heads between the reservoirs over the past ten years, excluding the maximum heads, see Figure 14. It is expected that a larger volume of water can be lead through the turbines with turbine heads below these extreme heads. It is also expected that these
61 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
high heads can be avoided by operating the reservoirs in a proper way with respect to the new power plant. Larger installed capacities have been tested for similar design heads to arrive at a satisfactory optimum solution for the preliminary design (see Appendix IV Tables 5 – 7 for the Bills of Quantities and Appendix VII for the energy estimate). An installed capacity of 7 MW is selected as a result of this optimization, see Table 14.
Installed Capacity MW 6,0 7,0 8,0 Construction Costs MNOK 58,1 64,5 70,4 Total Additional Energy Evanger Scheme GWH/y 14,3 17,4 18,2 Construction Costs per Unit Energy NOK/kWh 4,06 3,70 3,87
Table 14 Comparison installed capacities alternative II
A larger installed capacity requires more expensive and larger electro mechanical equipment and, to be able to place the equipment, a larger and more expensive powerhouse.
To avoid cavitation at full flow at low heads, the turbine outlet must be 4.0 m below tail water level for the selected rotational speed of 428.57 RPM. A higher rotational speed of 500 RPM could have been selected for this head range according to literature, see Appendix V Table 2. This would however require a larger suction head and result in a deeper power house with accompanied longer access and water tunnels. A horizontal turbine setting is selected because it is possible at these depths below tail water level and it is cheaper than a vertical setting.
9.2.3. Alternative II – Combined Pump Turbine A further increase of value can possibly be achieved by installing a combined pump turbine instead of regular turbine.
Drawings for this combined pump turbine design can be found in Appendix II PFS 002a & 007a.
Additional costs compared to a regular turbine plant originate from the deeper setting that is required for the pumping mode. The pumping mode is governing for the setting of the turbine. In this case the turbine outlet needs to be at 766 m.a.s.l.: 17 m below the tail race tunnel outlet to provide sufficient suction head in the pumping mode, see also Section 6.5.2. Longer access, head and tail race tunnels are needed for this deeper setting, otherwise the lay out is similar to the regular turbine solution. Other cost items that become more expensive are the pump turbine, control equipment and transmission equipment.
A reversible pump turbine is assumed to be installed, with the same turbine characteristics as the selected turbine solution of 7 MW. The combined turbine solution has only been tested for the optimized installed capacity that has been determined for the regular turbine solution. At a maximum pumping head of 60m 10.2 m3/s can be pumped up. At a minimum head of 40 m 14.2 m3/s can be pumped. See also Appendix V Table 4.
9.2.4. Results Detailed energy production estimates and for both the turbine and combined pump turbine can be found in Appendix VII. The salient features, including a summary of the energy production estimate, are presented in Table 15.
62 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Alternative II Turbine The new power plant produces an average of 16.9 GWh energy per year, with 133 Mm3 of water. A large amount of water can be lead through the turbine, instead of bypassing it as is necessary in alternative I. Only 60 Mm3 of water needs to be bypassed or spilled, of which 15 Mm3 of water needs to be bypassed because an inflow of water, larger than the plant capacity is required for the Evanger plant. The rest is bypassed because the available head falls outside the head range of the turbine. A larger installed capacity has been tested, but was not found to be beneficial.
The additional increase of energy produced in the system (0.5 GWh) can be explained by analyzing the energy estimate in Appendix VII. The water level in Askjelldalsvatn can be kept at a high water level for a longer period of time during the draw down (winter season) because of increase capacity between Holskardvatn and Askjelldalsvatn. In this case the additional benefit is 1 GWh per year. In the Nygard pumping plant 0.5 GWh is lost as a result of increased pumping of water. The flood spill water from Holskardvatn flows into Stølsvatn which serves as the intake reservoir for the Steinsland power plant and as the pumping pond for the Nygard power plant. A change of flood spill volume from Holskardvatn could explain the difference in operation of both Steinsland and Nygard power plant, as they are also responsible for the shift from summer to winter production of 3.9 GWh. This can however not be verified, as the computer model does not make any difference between flood spill and bypassed water from Holskardvatn.
Turbine Pump Turbine Turbine Capacity MW 7,0 7,0 Maximum discharge m3/s 14,2 14,2 energy equivalent at *H kWh/m3 0,132 0,132 Summer GWh 0,9 -6,8 Energy Plant Winter GWh 16,1 22,3 Total GWh 16,9 15,5 Summer GWh -3,9 -70,8 Energy BKK System Winter GWh 21,3 87,6 Total GWh 17,4 16,8 Flood spill & bypass Mm3/y 60,1 61,2 Production Mm3/y 133,2 132,0 Outlet level m.a.s.l. 782,9 782,9 Maximum m.a.s.l. 865,5 865,5 Holskardvatn Water Level Minimum m.a.s.l. 822,0 822,0 Maximum m.a.s.l. 805,0 805,0 Askjelldalsvatn Water Level Minimum m.a.s.l. 782,9 782,9 Maximum net turbine head m 67,7 67,7 Design net head m 54,2 54,2 Minimum net turbine head m 37,9 37,9 Station MNOK 52,0 64,1 Construction Costs Transmission MNOK 12,5 13,7 Total MNOK 64,5 77,8
Net Present Value 2007 MNOK 5,2 21,3 Construction costs / annual average NOK/kWh 3,7 4,6 energy Table 15 Salient features Alternative II: turbine and combined pump turbine solution
63 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Alternative II Combined Pump Turbine Approximately 51 Mm3 water (6.8 GWh) is pumped up from Askjelldalsvatn to Holskardvatn in summer. This delays the production of energy in the Askjelldal power plant and in the Evanger power plant from summer to winter; a total of circa 65 GWh is shifted from summer to winter production, which gives an additional annual benefit of a bit more than 2 MNOK. The net present value of the combined pump turbine solution is estimated at 21 MNOK. The total amount of produced energy is somewhat less than in the pure turbine solution, because of the energy loss in the pumping – generating cycle. Hence the installation of a combined pump turbine is beneficial compared to a regular turbine for this alternative.
64 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
9.3. Alternative III – Maximum Value This alternative is designed to be able to give a maximum energy value. This is achieved by selecting the outlet level at 770 m.a.s.l.; the lowest water level in Askjelldalsvatn that is reached in practice. This outlet level does not only give larger heads than the previous alternatives, but it also thought that the whole range of heads occurring between the two reservoirs can be utilized. The latter can be concluded by looking at Figure 14 in Section 3.3: some 80% of the flows occur at heads between 35m and 70m, which a single turbine should be able to handle. Hence the selected outlet level is expected to reduce the volume of water that needs to be bypassed around the turbine to a minimum.
A combined pump turbine is tested for this alternative as well, for the same argumentation as alternative II: pumping up water from Askjelldalsvatn to Holskardvatn in summer, is believed to give a relative large shift in energy production from summer to winter in both the new plant and especially the high head Evanger plant.
9.3.1. Hydraulic and Structural Design The bottom outlet of Holskardvatn is used as the intake structure. The existing intake gate can be used to empty the head race tunnel for inspection. The existing diversion tunnel is used for the power plant until some 20m upstream of the low pressure gate. At this point the water flows into a new tunnel and later into the power house. The outlet level of 770 m.a.s.l. is only reached in the deeper central part of Askjelldalsvatn.
Drawings for this alternative can be found in Appendix II PFS 005 & 010. The Bill of Quantities of this alternative can be found in Appendix IV.
The power house is underground and can be accessed by a 294m long access tunnel. Extra costs for measures for water tightness of the power house and drainage pumps to remove leakage water are anticipated in the bill of quantities.
The tail race tunnel is 438 m long and goes from the draft tube outlet directly into Askjelldalsvatn. The tunnel outlet is at 770 m.a.s.l. and this water depth is only available a couple of hundred meters west of the existing tunnel system.
From the power house a 184 m long head race tunnel is blasted to connect to the existing head race tunnel to Holskardvatn. To prevent water leakage into the power house, a GRP pipe will be constructed in the new tunnel, transmission to the tunnel will be formed by a concrete casing.
A surge shaft is foreseen upstream of the new head race tunnel, connecting the tunnel system to the ground level above at 865.5 m.a.s.l. See Appendix VI Table 3 for calculations. The surge shaft is required for governor stability.
9.3.2. Turbine Selection Initially, similar preliminary turbine characteristics have been suggested for this alternative as for alternative II with a maximum turbine head of 69m and an installed capacity of 6 MW, see also Section 6.5.1. A rough optimization of the installed capacity resulted in a selected installed capacity of 7 MW, see Table 16. Bills of Quantities and energy estimates of the different installed capacities can be found in Appendix IV Tables 8-10 and Appendix VII respectively.
65 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Installed Capacity MW 6,0 7,0 8,0 Construction Costs MNOK 75,8 82,3 88,8 Total Additional Energy Evanger Scheme GWH/y 19,9 21,9 22,0 Construction Costs per Unit Energy NOK/kWh 3,81 3,76 4,03
Table 16 Comparison installed capacities alternative II A larger installed capacity requires more expensive and larger electro mechanical equipment and, to be able to place the equipment, a larger and more expensive powerhouse.
To avoid cavitation at full flow at low heads, the turbine outlet must be 4.0 m below tail water level at a rotational speed of 428.57 RPM. A higher rotational speed of 500 RPM could have been selected for this head range according to literature, see Appendix V Table 3. This would however require a larger suction head and result in a deeper power house with accompanied longer access and water tunnels. A horizontal turbine setting is selected because it is possible at these depths below tail water level and it is cheaper than a vertical setting.
9.3.3. Alternative III – Combined Pump Turbine Drawings for this alternative can be found in Appendix II PFS 005a & 010a.
A combined pump turbine is also tested for this alternative. A similar alignment as for the regular turbine variant is used, but the powerhouse needs to placed at 753 m.a.s.l. to achieve sufficient suction head in the pumping mode (in this way the required setting of 17m below the outlet level is achieved, see also Section 6.5.2) The deeper setting of the powerhouse and pumping mode result in additional costs as is described for alternative II in Section 9.2.3.
The combined pump turbine has not been optimized; hence a 7MW capacity is selected. The turbine characteristics of the reversible pump turbine are similar to the regular turbine variant. At a maximum pumping head of 60m 10.2 m3/s can be pumped up. At a minimum head of 40 m 14.2 m3/s can be pumped. See also Appendix V Table 5.
9.3.4. Results Detailed energy production estimates and for both the turbine and combined pump turbine can be found in Appendix VII. The salient features, including a summary of the energy production estimate, are presented in Table 17.
Alternative III Turbine The new power plant produces an average of 21.8 GWh energy per year, with 169 Mm3 of water. Hence only a small amount of water is bypassed, mainly because of an inflow of water, larger than the plant capacity is required for the Evanger plant. Hardly any water needs to be bypassed around the turbine because almost all flows occur within the head range of the turbine. This alternative is very energy effective.
The additional increase of energy produced in the system (1.6 GWh) is produced by the Nygard pumping plant and the Steinsland power plant. The shift from summer to winter energy also comes from these two plants. Similar to alternative II, the reason for this production change can only be guessed as the computer model output does not make any difference between flood spill and bypassed water from Holskardvatn. The flood spill water from Holskardvatn flows into Stølsvatn which serves as the intake reservoir for the Steinsland power plant and as the pumping pond for the Nygard
66 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
power plant. A change of flood spill volume from Holskardvatn could explain the difference in operation of both Steinsland and Nygard power plant.
Alternative III Combined Pump Turbine Approximately 50 Mm3 water (6.6 GWh) is pumped up from Askjelldalsvatn to Holskardvatn in summer. This delays the production of energy in the Askjelldal power plant and in the Evanger power plant from summer to winter; a total of circa 65 GWh is shifted from summer to winter production, which gives an additional annual benefit of a bit more than 2 MNOK. The net present value of the combined pump turbine solution is estimated at 23 MNOK. The total amount of produced energy is somewhat less than in the pure turbine solution, because of the energy loss in the pumping – generating cycle. Hence the installation of a combined pump turbine is beneficial compared to a regular turbine for this alternative.
Turbine Pump Turbine Turbine Capacity MW 7,0 7,0 Maximum discharge m3/s 14,4 14,4 energy equivalent at *H kWh/m3 0,132 0,132 Summer GWh 1,7 -6,6 Energy Plant Winter GWh 20,1 26,4 Total GWh 21,8 19,9 Summer GWh -1,5 -72,3 Energy BKK System Winter GWh 23,4 93,0 Total GWh 21,9 20,8 Flood spill & bypass Mm3/y 26,2 28,3 Production Mm3/y 168,9 165,0 Outlet level m.a.s.l. 770,0 770,0 Maximum m.a.s.l. 865,5 865,5 Holskardvatn Water Level Minimum m.a.s.l. 809,2 809,2 Maximum m.a.s.l. 805,0 805,0 Askjelldalsvatn Water Level Minimum m.a.s.l. 770,0 770,0 Maximum net turbine head m 67,8 67,8 Design net head m 54,3 54,3 Minimum net turbine head m 38,0 38,0 Station MNOK 70,2 78,6 Construction Costs Transmission MNOK 12,5 13,7 Total MNOK 82,3 92,3 Net Present Value 2007 MNOK 3,8 23,2 Construction costs / annual average NOK/kWh 3,8 4,4 energy Table 17 Salient features Alternative III
67 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
9.4. Selection of the Best Alternative The best alternative is defined as a technical feasible solution, with the most promising economic results. All proposed alternatives are found to be technically and economically feasible. A detailed design study is necessary to reduce the uncertainties in the design. Only the best alternative is selected for a more detailed study. This Section aims to motivate the selection of the best alternative and will therefore first briefly compare the proposed alternatives before concluding on the selected design.
9.4.1. Comparison All alternatives have one Francis turbine with negative suction head to cope with the wide range of available heads. The pumping mode requires a deeper setting of the turbine below water level than the generating mode to avoid cavitation. Sufficient bypass capacity is provided by the already existing gate system. The bottom outlet of Holskardvatn and a large part of the existing tunnel system are also used in the new plant.
Alternatives I aims to minimize the construction costs and has an outlet level corresponding to the highest regulated water level in Askjelldalsvatn, 805 m.a.s.l.. The water level in Holskardvatn is thus the only variable parameter for the gross head. A short, underground alignment has been found the cheapest solution. This alternative is able to utilize only a limited (ca. 40%) part of the available water flows between the reservoirs because the available head is often outside the head range of the turbine. The majority of the water flows thus needs to be bypassed around the power house. A net present value of 9.8 MNOK is estimated for this alternative.
Alternatives II (a compromise between maximum value and minimum construction costs) and III (maximum value) have an outlet below the highest regulated water level in Askjelldalsvatn, at 782.9 m.a.s.l. and 770 m.a.s.l. respectively. The available head is now mainly dependent on the difference in water levels between Holskardvatn and Askjelldalsvatn. The head can be kept within a certain band width, which makes it possible for the turbine to utilize a large part of the available flows: 70% and more than 85% for alternative II and III respectively. These two alternatives are also able to produce significantly more energy than the alternatives with a fixed outlet level. Net present values are 5.2 and 3.8 MNOK for turbine solutions of alternative II and III respectively. A combined pump turbine solution has been considered for alternatives II and III and found to be beneficial in both cases. By pumping approximately 51 Mm3 up from Askjelldalsvatn to Holskardvatn, a shift of some 65 GWh in energy production from summer to winter can be achieved for both alternatives. Additional benefits of some 2 MNOK per year result outweigh the additional construction costs for a deeper setting and more expensive turbine, transmission and control equipment. Net present values are 21.3 and 23.2 MNOK for combined pump turbine solutions of alternative II and III respectively.
A comparison of all alternatives is presented in Table 18 on the next page.
68 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
7,0 7,0 4,4 -6,6 14,4 14,4 26,4 19,9 93,0 20,8 28,3 67,8 54,3 38,0 78,6 13,7 92,3 23,2 -72,3 -72,3 0,132 0,132 165,0 770,0 865,5 809,2 805,0 770,0 Alternative 3 Pump 7,0 7,0 4,6 -6,8 12,1 12,1 22,3 15,5 87,6 16,8 61,2 54,0 43,2 30,2 64,1 13,7 77,8 21,3 -70,8 -70,8 0,105 0,105 132,0 805,0 860,5 836,7 805,0 805,0 Alternative 2 Pump 7,0 7,0 1,7 3,8 3,8 -1,5 14,4 14,4 20,1 21,8 23,4 21,9 26,2 67,8 54,3 38,0 70,2 12,5 82,3 0,132 0,132 168,9 770,0 865,5 809,2 805,0 770,0 Alternative 3 7,0 7,0 0,9 5,2 3,7 -3,9 14,2 14,2 16,1 16,9 21,3 17,4 60,1 67,7 54,2 37,9 52,0 12,5 64,5 0,132 0,132 133,2 782,9 865,5 822,0 805,0 782,9 Alternative 2 5,0 5,0 0,0 7,9 8,0 9,8 9,8 -3,9 12,9 12,9 15,8 15,8 11,9 77,8 53,7 43,0 30,0 26,3 13,7 40,0 3,37 0,105 0,105 115,7 115,7 805,0 860,5 836,8 805,0 805,0 Alternative 1
3 /y /y /s /s 3 3
3 m m m MW m GWh GWh GWh GWh GWh GWh MNOK MNOK MNOK MNOK Mm Mm m.a.s.l. m.a.s.l. m.a.s.l. m.a.s.l. m.a.s.l. kWh/m NOK/kWh NOK/kWh
Total Total Total 2007 Winter Winter Winter Winter Station Summer Summer Minimum Minimum Maximum Maximum Transmission
n
max turb turb *H P Q n,turb,min n,turb,max H H Production Production Outlet level level Outlet Energy Plant Holskardvatn WL Holskardvatn Net Present Value Value Present Net Askjelldalsvatn WL Construction Costs Flood spill & bypass & bypass spill Flood Energy BKK System Constr. Cost per energy energy equivalent at *H
Table 18 Comparison of alternatives
69 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
9.4.2. Conclusions With a good technical feasibility and promising economy it is recommended to study the Askjelldalsvatn project in more detail. A combined pump turbine solution creates a better balance in the Evanger power scheme than use of the reservoirs in the scheme today. A combined pump turbine design is able to shift a considerable amount of energy production from summer to winter period and shows therefore by far best economic results.
It is recommended to study this combined pump turbine solution in more detail, where an optimization needs to be found mainly with regard to the outlet level in Askjelldalsvatn and installed capacity. Both the pumping options for alternatives II and III should thus be considered in the optimization process.
Three different alternatives have been proposed for a power plant that utilizes the water flows from Holskardvatn to Askjelldalsvatn. Two of these alternatives have also been investigated for pumping possibilities. As no major technical obstacles have been identified, all alternatives are found to be technically feasible. All alternatives also show promising economic results, but the pumping option for Alternatives II and III are superior to the other alternatives in this respect. The net present benefit of these pumping alternatives is more than twice as large as the best regular turbine solution.
A distinction needs to be made between the proposals with an outlet level at the highest regulated water level in Askjelldalsvatn (alternatives I) and the alternatives with an outlet level below this maximum water level (alternatives II and III). Alternatives II and III have two major advantages over the first alternative.
Firstly, much more energy is produced, because a larger part of the available water flows can be utilized in Alternatives II and III than for the other alternatives. The fluctuations of the water levels in Holskardvatn and Askjelldalsvatn over the year are more or less equal, especially in the winter period when both reservoirs are drawn down. As a result the available head between the two reservoirs can be kept within a certain band width over a large period of time. A power plant with an outlet below the maximum water level in Askjelldalsvatn is able to utilize almost all flows, because the turbine is able to follow the available head almost continuously. The head of Alternatives I is only dependent on the water level in Holskardvatn and this head fluctuates much more. A large part of the water flows cannot be utilized in these alternatives and needs to be bypassed around the power plant as the turbine is not able to operate in a large part of the available heads. Thus, Alternatives II and III are able to produce more energy than the other alternatives, with Alternative III being superior at this point because of its lower outlet level.
Secondly, a combined pump turbine has been considered in alternatives II and III, whereas this is not possible in the other alternatives. Each summer approximately 51 Mm3 of water can be pumped up from Askjelldalsvatn to Holskardvatn. This water is stored in Holskardvatn and utilized for energy production in winter in both the new power plant and the Evanger plant. A better balance is created in the relative storage between Askjelldalsvatn and Holskardvatn; the relative storage in Holskardvatn decreases from 1.29 to 1.01 and the relative storage in Askjelldalsvatn increases from 0.29 to 0.35. This production shift from summer to winter gives an additional benefit of approximately 2 MNOK per year, compared to a standard turbine solution.
The pumping options for alternatives II and III have a small difference in net present benefits. An optimum solution needs to be found for the outlet level of the power plant. It is recommended to consider the pumping option for both alternatives II and III in a
70 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
more detailed study to arrive at this optimum solution and to be able to specify the costs and benefits with smaller uncertainties.
In this case it is found that the maximization of the energy value is of more importance for the economy of the project than the construction costs. Because of the existing imbalance in storage between the reservoirs Holskardvatn and Askjelldalsvatn and the high head power Evanger power plant, pumping water in the melting season is found to be very beneficial.
The optimization of the project should be undertaken more accurately. The focus should be on the same design variables that have been considered in the preliminary design phase: the outlet level of the power plant and the installed capacity of the reversible pump turbine. Other components that can to be optimized are shaft, tunnel and pipe dimensions.
71 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
10. DETAILED DESIGN
This Chapter will deal with the detailed design of the selected alternative. The technical feasibility of a combined pump turbine plant has already been confirmed and the objective of the detailed design phase is to give a more accurate estimate of the economic feasibility of the project. The result of this phase will be an optimized, detailed design of the Askjelldalsvatn hydropower plant, including reliable cost and benefit estimates that can be used in an economic analysis (to be performed by a engineering economist). For the latter a construction schedule will also be provided.
10.1. Optimization In the preliminary design phase (see Chapter 9) it is shown that a combined pump turbine power plant has superior economic results. The small difference in profit for two alternatives (II and III) with different outlet levels, given that the alternatives only have been roughly optimized, necessitates a new optimization step. In this Section the optimum installed capacity and outlet level are determined. To do so, the installed capacity will be optimized for different outlet levels by comparing discounted marginal costs and benefits. The optimized solutions for different outlet levels will then be compared to end up with the most economic design for the Askjelldalsvatn power plant. The outlet levels of alternatives II and III will be examined, 783 m.a.s.l. and 770 m.a.s.l. respectively, plus a new lower outlet level of 760 m.a.s.l. The latter is necessary to test if the outlet level of 770 m.a.s.l. is really the minimum water level that is reached (and thus gives maximum heads). The project components are described in Chapter 6 and the energy estimate is described in Chapter 7.
10.1.1. Approach For each outlet level, different installed capacities are tested. The marginal annual benefits are compared with the marginal annual costs and presented in a diagram. The optimum installed capacity is found where the marginal benefits equal the marginal costs. The optimized installed capacities for different outlet levels are compared with a similar marginal cost benefit approach.
Three different installed capacities are tested, which will result in two points in the diagram. A straight line is drawn between these points. Any other than a linear relationship is thought to give a false indication of the accuracy of the optimization. There lies an uncertainty in the actual turbine performance (which can only be determined after a turbine manufacturer has put in a bid). Vansimtap also has its limitations; the turbine performance can be given as input for one head only and weekly data is used instead of daily data. Trying to optimize for half MW would therefore be too accurate. Therefore optimum installed capacities are selected as multiplications of 1 MW.
For each installed capacity the costs are determined and presented in the Bill of Quantities, see Appendix IV. A larger installed capacity gives extra costs for the electro mechanical equipment, the powerhouse (larger capacity results in a larger turbine, which requires a larger powerhouse) and the transmission system. The total marginal costs can then be found for different installed capacities. The total marginal costs are converted into the annual marginal costs by multiplying it with a factor based on the so called capital recovery factor [GUNNES]:
72 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
A i(1+ i) N = P (1+ i) N −1 With A = equal annual amounts during the period of analysis N to result in a present value P at t = 0 P = net present value at t = 0 N = years of analysis i = discount rate
With a period of analysis of 60 years, and a discount rate of 7% the capital recovery factor A/P = 0.07123. Adding 1% for operation and maintenance gives a multiplication factor 0.08123 to convert total costs at t = 0 to annual marginal costs.
The average annual benefits are calculated by multiplying the estimated future summer and winter energy prices16 with the estimated produced energy from Vansimtap for the whole Evanger scheme. The annual marginal benefits can then be determined.
10.1.2. Optimum installed capacity for an outlet at 782.9 m.a.s.l. The Bill of Quantity for this alternative can be found in Appendix IV. This outlet level is the same as in preliminary alternative II, hence the corresponding design is used, see Section 9.2. A 7 MW installed capacity is found to be the optimum, see Figure 20 and Table 19. A larger installed capacity is not efficiently used as not much extra water is lead through the turbine, see the energy estimate in Appendix VII. The water volume that needs to be bypassed is quite similar for both 7 MW and 8 MW and limited by the fact that available head is too low for the turbine.
Optimum installed capacity masl 782.9
1,20
1,00
0,80
0,60
0,40
0,20
0,00 6,50 7,50 Turbine Capacity (M W) Marginal Costs masl 782.9 Marginal benefits masl 782.9
Figure 20 Optimization outlet 782.9 m.a.s.l.
16 This information is provided by BKK and confidential. Therefore only the results will be presented in this report.
73 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Turbine Capacity (MW) 6,00 7,00 8,00 Construction Costs (MNOK) 69,73 77,41 83,16 Ann. Avg. Construction Costs (MNOK) 5,65 6,32 6,76 Winter Energy (GWh) 84,0 87,6 87,5 Summer Energy (GWh) -70,4 -70,8 -70,1 Annual Avg. Benefits (MNOK) 7,46 8,56 8,77 Marg. Benefits (MNOK) 1,10 0,21 Marg. Construction Costs (MNOK) 0,68 0,43
Table 19 Optimization outlet 782.9 m.a.s.l.
10.1.3. Optimum installed capacity for an outlet at 770 m.a.s.l. The Bill of Quantity for this alternative can be found in Appendix IV, the detailed energy estimate in Appendix VII.
The outlet level at 770 m.a.s.l. is the same as in alternative III, see Section 9.3. Figure 21 shows that the optimum installed capacity is expected to be just below 8.5 MW. Going up from 8 MW to 9 MW only leads to a change from summer to winter energy; no additional energy is produced. This will be discussed hereafter. These extra benefits do not weigh up against the larger construction costs. Therefore an installed capacity of 8 MW is selected.
Optimum installed capacity masl 770
2,50
2,00
1,50
1,00
0,50
0,00
7,50Turbine Capacity (MW) 8,50 Marginal Costs masl 770 masl Marginal benefits masl 770
Figure 21 Optimization outlet at 770 m.a.s.l.
Turbine Capacity (MW) 7,00 8,00 9,00 Construction Costs (MNOK) 88,20 97,46 103,72 Ann. Avg. Construction Costs (MNOK) 7,16 7,92 8,43 Winter Energy (GWh) 93,0 85,0 88,5 Summer Energy (GWh) -72,3 -55,4 -58,2 Annual Avg. Benefits (MNOK) 9,99 12,34 12,69 Marg. Benefits (MNOK) 2,36 0,35 Marg. Construction Costs (MNOK) 0,75 0,51
Table 20 Optimization outlet at 770 m.a.s.l.
74 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Results The energy production for the different installed capacities requires some explanation: 1 MW extra installed capacity from 7 MW to 8 MW seems to give a change in the operation strategy. The change from 8 MW to 9 MW is much smaller.
The following changes in the energy production of the system are identified for the increase from 7 MW to 8 MW: - Additional production of 9 GWh electricity, solely achieved at the Evanger plant - Less energy production in winter: - 8 GWh - Smaller energy reduction in summer: 55.4 GWh instead of 72.3 GWh - 20 Mm3 water from Grøndalsvatn is pumped into Askjelldalsvatn instead of free flow into Askjelldalsvatn (1.5 GWh) - The water level in Askjelldalsvatn doesn’t drop below 767 m.a.s.l.
The sudden change in strategy as sketched above requires some explanation before the results can be accepted.
All the changes can be coupled to the larger turbine capacity. The pump capacity is found to be of less importance as almost similar amounts of water are pumped up for both capacities (in fact the full installed pump capacity is seldom used). In the first three months of the year the energy price is high and the Askjelldalsvatn power plant is producing at its full capacity for both the 7 MW and 8 MW option. At the same time the Evanger plant is also producing a lot of energy and because the outflow of Askjelldalsvatn is smaller than the inflow, the water level goes down. However, for the 8 MW option, the water level goes down more gradually than for the 7 MW option. It goes down so gradually that it is found beneficial not to draw down Askjelldalsvatn completely (as is the case for 7 MW) but to keep the water level around 770 m.a.s.l.. The storage below this water level is not needed for expected snow melt, since a part of this snow melt can be pumped up to Holskardvatn again. With this higher water level in Askjelldalsvatn it is necessary to pump water from Grøndalsvatn into the head race tunnel from Askjelldalsvatn. The explanation for less winter and more summer production can be found in the fact that the summer and winter energy are determined as output for absolute week numbers (summer between weeks 16 and 40), whereas the computer simulation uses an energy price that varies per week and not per season. The higher water level in Askjelldalsvatn is thus responsible for the extra 9 GWh of energy production in the Evanger plant.
For the 7 MW option, the inflow from Holskardvatn into Askjelldalsvatn is not enough in the first couple of months of the year and therefore the water in the bottom of Askjelldalsvatn is also needed for production in the Evanger plant. Hence the water level in Askjelldalsvatn goes down to its lowest regulated water level and the water from Grøndalsvatn can flow into the head race tunnel without pumping. The lower water level in Askjelldalsvatn results in a lower energy production in the Evanger plant.
10.1.4. Optimum installed capacity for an outlet at 760 m.a.s.l. The Bill of Quantity for this alternative can be found in Appendix IV, the detailed energy estimate in Appendix VII.
Extra costs for a power plant with an outlet at 760 m.a.s.l., compared to an outlet that is 10m higher, come from a longer access and tail race tunnel. The optimum installed capacity is found to be 8 MW, see Figure 22 and Table 21.
75 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Optimum installed capacity masl 760
1,00 0,90 0,80 0,70 0,60 0,50 0,40 0,30 0,20 0,10 0,00 7,50 8,50 Turbine Capacity (M W) Marginal Costs masl 760 masl Marginal benefits masl 760
Figure 22 Optimization outlet at 760 m.a.s.l.
Turbine Capacity (MW) 7,00 8,00 9,00 Construction Costs (MNOK) 94,30 100,92 107,21 Ann. Avg. Construction Costs (MNOK) 7,66 8,20 8,71 Winter Energy (GWh) 81,1 83,9 88,7 Summer Energy (GWh) -53,3 -53,6 -58,1 Annual Avg. Benefits (MNOK) 11,59 12,53 12,80 Marg. Benefits (MNOK) 0,94 0,27 Marg. Construction Costs (MNOK) 0,54 0,51
Table 21 Optimization outlet at 760 m.a.s.l.
10.1.5. Optimized design In this Section the optimized design will be decided upon. In the previous three sections the optimum installed capacity has been determined for three different outlet levels. These optimum designs for different outlet levels are compared with a marginal cost benefit approach here. From this optimization an outlet level slightly above 765 m.a.s.l. is found to be an optimum, see Table 22 and Figure 23 below. The selected optimum design is at 768 m.a.s.l. and will be discussed hereafter.
Outlet level (m.a.s.l.) 782,90 770,00 760,00 Turbine capacity (MW) 7,00 8,00 8,00 Construction Costs (MNOK) 77,84 97,46 100,92 Ann. Avg. Construction Costs (MNOK) 6,32 7,92 8,20 Winter Energy (GWh) 87,6 85,0 83,9 Summer Energy (GWh) -70,8 -55,4 -53,6 Annual Avg. Benefits (MNOK) 8,56 12,34 12,53 Marg. Benefits (MNOK) 1,46 0,09 Marg. Construction Costs (MNOK) 0,62 0,14 NPV 2007 (MNOK) 21,31 45,30 43,89
Table 22 Optimization outlet level of Askjelldalsvatn power plant
76 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Optimum outlet level
778
776
774
772
770
768
766
764 0,00 0,20 0,40 0,60 0,80 1,00 1,20 1,40 1,60 M arginal Costs/ Benefits (M NOK)
Marginal Costs WL Marginal benefits WL
Figure 23 Optimization outlet level of Askjelldalsvatn power plant
Discussion For the Askjelldalsvatn power plant an outlet level of 768 m.a.s.l. and an installed capacity of 8.0 MW are suggested.
The optimization resulted in an installed capacity of 8.0 MW for the options with outlet levels at 770 and 760 m.a.s.l. This installed capacity is also selected for the outlet level at 768 m.a.s.l.
Outlet level The outlet level of 768 m.a.s.l. is selected after analyzing the reservoir operation for both outlet levels. In both cases the water level in Askjelldalsvatn does not drop below 768 m.a.s.l. and it would therefore not yield any extra benefit to construct a power plant with an outlet below this water level, see the upper diagrams in Figure 24 and Figure 25.
The reason the water level does not drop below this level is already discussed in Section 10.1.3 and will be briefly repeated here. The discharge through the Askjelldalsvatn power plant is large enough to provide sufficient inflow into Askjelldalsvatn to maintain the water level above 768 m.a.s.l. Askjelldalsvatn does not need to be drawn down completely to achieve sufficient storage volume for the snow melt, as a part of this snow melt can be pumped up to Holskardvatn in the melting season, see the diagrams showing the outflow from Holskardvatn in Figure 24 and Figure 26 The energy price is low in this period, hence pumping up water is cost effective.
With the outlet of the plant at 768 m.a.s.l. the Askjelldalsvatn power plant is able to utilize (almost) all available water for energy production; a small volume of water needs to be bypassed because of insufficient capacity. Both reservoirs can be operated in such a way that the available head always falls within the working range of the turbine.
77 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Askjelldalsvatn water level 8 MW 770 m.a.s.l. Askjelldalsvatnet 8 MW outlet 760 m.a.s.l.
810,000 810,000
800,000 800,000
790,000 790,000
780,000 780,000
770,000 770,000
760,000 760,000
750,000 750,000 jan f eb mar apr mai jun jul aug sep okt nov des jan f eb mar apr mai jun jul aug sep okt nov des
Wet year Dr y year Aver age year Ar i thmetic aver age Wet year Dr y year Aver age year Ar i thmetic aver age
Holskardvatnet water level 8 MW 770 m.a.s.l. Holskardvatnet water level 8 MW outlet 760 m.a.s.l.
870,0 870,0
860,0 860,0
850,0 850,0
840,0 840,0
830,0 830,0
820,0 820,0
810,0 810,0
800,0 800,0 jan f eb mar apr mai jun jul aug sep okt nov des jan f eb mar apr mai jun jul aug sep okt nov des
Wet year Dr y year Aver age year Ar i thmetic aver age Wet year Dr y year Aver age year Ar i thmetic aver age
Figure 25 Holskardvatn and Askjelldalsvatn water levels for outlet 760 m.a.s.l. and 8 MW
Holskardvatnet outflow 8 MW 770 m.a.s.l. Holskar dvatnet outf low 9 MW 770 m.a.s.l.
13 , 0 13,0
8,0 8,0
3,0 3 3,0
-2,0 -2,0 -7,0 -7,0 -12,0 -12,0 jan f eb mar apr mai jun jul aug sep okt nov des jan f eb mar apr mai jun jul aug sep okt nov des
Vå t t Tørt M iddels Gjennomsnitt Wet year Dr y year Aver age year Ar i thmeti c aver age
Holskardvatnet flood & bypass 8 MW 770 m.a.s.l. Holskardvatnet f lood spill & bypass 9 MW 770 m.a.s.l.
12 , 0 14 ,0 12 ,0 10 , 0 10 ,0 8,0 8,0 6,0 6,0
4,0 4,0
2,0 2,0
0,0 0,0 jan f eb mar apr mai jun jul aug sep okt nov des jan feb mar apr mai jun jul aug sep okt nov des
Wet year Dr y year Aver age year Ar i thmeti c aver age Vått Tørt Mi ddel s Gj ennomsni tt
Figure 24 Reservoir operation characteristics for Figure 26 Characteristics of Holskardvatn for outlet outlet 770 m.a.s.l and 8 MW 770 m.a.s.l. and 9 MW
Capacity The outflow and flood spill plus bypass from Holskardvatn are shown in Figure 24 and Figure 26 for an outlet at 770 m.a.s.l. and an installed capacity of 8 MW and 9 MW respectively. These diagrams show that some bypass is always required to provide water for the Evanger plant in a very dry year. The benefit of a 9 MW turbine is that it is able to cope with more extreme situations: less water needs to be bypassed, compare the
78 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
diagrams for the different capacities. However, the turbine is used less efficient. The almost straight lines representing the outflow from Holskardvatn at 8 MW from January to March indicate efficient use of the turbine (Figure 24 left). With a 9 MW installed capacity such an efficient use of the turbine is only found in a dry year, otherwise the outflow graphs are much rougher. It is also noted that the full capacity of the pump is seldom used for both installed capacities.
The reservoir operation characteristics verify that an installed capacity of 8 MW is an optimum solution. The turbine is used very efficient, with the pumping capacity being of less importance. An additional 1 MW only takes care of the extremes and is not used optimal and therefore not found to be beneficial.
10.2. Final Design Detailed drawings of the design are presented in Appendix VIII. This includes a lay out and section of the plant, including tunnel systems and a more detailed plan and section of the powerhouse.
The Bill of Quantities and a detailed energy estimate can be found in Appendix IX. The turbine and pump characteristics, as well as the surge shaft design can also be found in this Appendix.
Turbine Capacity MW 8,0 Maximum discharge m3/s 16,7 energy equivalent at *H kWh/m3 0,130 Summer GWh -7,0 Energy Plant Winter GWh 27,1 Total GWh 20,2 Summer GWh -53,6 Energy BKK System Winter GWh 83,9 Total GWh 30,4 Flood spill & bypass Mm3/y 14,9 Production Mm3/y 178,4 Outlet level m.a.s.l. 768,0 Maximum m.a.s.l. 865,5 Holskardvatn Water Level Minimum m.a.s.l. 809,2
Askjelldalsvatn Water Maximum m.a.s.l. 805,0 Level Minimum m.a.s.l. 768,0 Maximum net turbine m 66,6 head Design net head m 53,3 Minimum net turbine head m 37,3 Maximum pumping head m 60,0 Minimum pumping head m 40,0 Station MNOK 83,0 Construction Costs Transmission MNOK 15,7 Total MNOK 98,7 Net Present Value 2007 MNOK 46,2 Construction costs / NOK/kWh 3,25 annual average energy Table 23 Salient features Askjelldalsvatn power plant
79 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
The Askjelldalsvatn power plant will utilize the water flows between Holskardvatn and Askjelldalsvatn. A combined pump turbine of 8 MW will be installed, which allows the pumping up of water from Askjelldalsvatn to Holskardvatn in the summer. The plant is remote controlled and personal is only needed in the powerhouse in case of maintenance or inspection. The pumping mode dictates the setting of the turbine. To be able to use the outlet level in Askjelldalsvatn also in the pumping mode, the turbine outlet is at 751 m.a.s.l.
A 392 m long access tunnel leads to the underground powerhouse. The cable box is placed in the access tunnel. The powerhouse is designed as a widened tunnel cavern. This is beneficial because the horizontal turbine setting does not require a large depth below the machine hall floor. The turbine and generator are installed at the end of the cavern, where the tail and head race tunnels connect to the powerhouse. The cavern allows for truck passage to the machine hall and the outlet gate. From the powerhouse a 448 m long tail race tunnel is constructed in a South Western direction to arrive at the required outlet level in Askjelldalsvatn. The 183m long head race tunnel will be constructed from the powerhouse in a North Eastern direction and connects to the existing diversion tunnel between Holskardvatn and Askjelldalsvatn, upstream of the existing gates. These gates can be used to bypass water around the power plant in the future. A concrete casing will be constructed in the head race tunnel 20m upstream of the powerhouse to avoid water leakage into the power house. A GRP pipe will form the transition from the casing to the turbine. The existing tunnel system will be used upstream of the connection to the new head race tunnel. Hence the bottom outlet and gates at Holskardvatn will function as the intake for the new plant.
In the existing diversion tunnel a surge shaft will be constructed with a surge pool of 800 m3. The top of the pool needs to be at 875 m.a.s.l. because of the up surge that occurs at turbine stop. Salient features of the Askjelldalsvatn power plant are presented in Table 23.
10.3. Construction Schedule The Askjelldalsvatn power plant can be constructed within a 2 year period, see the construction schedule in Appendix IX. Construction works can start in May. Because of the difficult winter conditions, a winter break is anticipated from November to April.
It is possible to construct the power plant without influencing the operation of Askjelldalsvatn too much. The first year from May to October is used to construct the new tunnel system and to excavate the powerhouse cavern. Concrete works in the powerhouse will start and the outlet gate will be installed. The existing tunnel system can still be used in the first year to transfer water from Holskardvatn to Askjelldalsvatn. The second year starts with preparing the tunnel system and powerhouse for the filling of the tailrace tunnel. The break through of the tail race tunnel can be done in open air in May with Askjelldalsvatn drawn down. Before the large snowmelt the outlet gate of the powerhouse and the breakthrough should be finished. Hence during this period no water can be diverted into Askjelldalsvatn, which usually is not necessary. The existing tunnel system upstream of the powerhouse will be drained and a breakthrough will be made to connect the existing tunnel system to the new head race tunnel. At this moment the surge shaft can be constructed from the existing diversion tunnel. By installing the concrete casing the powerhouse becomes ‘watertight’ and the head race tunnel can also be filled. From this moment on, the existing tunnel system can be used again.
80 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
Transmission works can continue throughout the whole construction period, except for the winter months.
The plant can be ready in September in the second year.
10.4. Reservoir Operation The Askjelldalsvatn power plant, with a combined pump turbine will affect the reservoir operation of both Askjelldalsvatn and Holskardvatn.
The Evanger plant will also in combination with the new plant be the most influential factor for the reservoir operation. This leads to the bypass of water around the new power plant in case of extreme drought. In other situations, the bypass gates will seldom be used.
Two important aspects for the reservoir operation are the limitations of the pump turbine: - The head between Holskardvatn and Askjelldalsvatn should be kept between 70m and 40m in the production season (November to April) to allow use of the turbine in the new plant and avoid the need of bypassing water. - The maximum pumping head is 60m. The head between the reservoirs should be kept below this maximum in the pumping period from June to August.
The production season starts roughly in November. Both the Askjelldalsvatn plant and the Evanger plant start producing energy and the water levels in both Askjelldalsvatn and Holskardvatn will slowly go down. From January to April full production in both plants is expected as a result of which the water levels in both reservoirs fall fast. To store the snow melt water, Askjelldalsvatn needs to be drawn down to around 770 m.a.s.l.. Complete draw down is not necessary as a part of the melt water can be pumped up to Holskardvatn from June to August. The storage capacity for melt water and pumped water in Holskardvatn is sufficient if the water level is taken at approximately 820 m.a.s.l. at the end of the production season. In wet years this level will not be reached, in dry years the water level can go even lower. From June to October the Evanger plant will (occasionally, depending on dry or wet conditions) produce energy to avoid flood spill from Askjelldalsvatn. The Askjelldalsvatn plant will hardly be used between the pumping and production season in the months September and October. In that period Holskardvatn is filled up with water for winter production.
81 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
11. CONCLUSIONS AND RECOMMENDATIONS
11.1. Conclusions It is technically and economically feasible to build the Askjelldalsvatn power plant that utilizes the water flows between Holskardvatn and Askjelldalsvatn, in the Evanger power scheme. This study shows that a combined pump turbine plant is superior over a regular turbine plant for two reasons. Firstly, the pump turbine plant creates a better balance between the reservoirs by pumping up water from Askjelldalsvatn, where it otherwise could not have been stored, to Holskardvatn, where the water can be stored from summer to winter. An annual benefit of approximately 2 MNOK is the result of this storage as the water can now be utilized for energy production in winter instead of summer in both the Askjelldalsvatn plant and the Evanger plant. The second advantage is that Askjelldalsvatn does not need to be drawn down completely to store the snow melt water in the summer. Instead, a part of the melt water is pumped up to Holskardvatn. The higher water level in summer results in a higher head for the Evanger plant and thus a larger energy production. Hence the main requirement that the new plant may not become a bottleneck in the system is more than fulfilled with this pump turbine design.
An optimization of the design shows that an outlet at the lowest practical water level at 768 m.a.s.l. in Askjelldalsvatn is the best solution. In combination with an installed capacity of 8 MW the plant is able to utilize almost all water flows from Holskardvatn for energy production with a single turbine. Only in extremely dry years water needs to be bypassed through the existing gates into Askjelldalsvatn to guarantee sufficient water for energy production in the Evanger plant. A larger installed capacity would only be used in these dry years and is therefore not beneficial.
The Askjelldalsvatn power plant can be constructed in two years and the construction costs are estimated at 99 MNOK. This includes a rather rough cost estimate of almost 16 MNOK for the upgrade of the transmission system. The new plant will produce 20 GWh annually and the Evanger plant will produce 10 GWh extra per year because of the increased water level in Askjelldalsvatn. Approximately 50 Mm3 will be pumped up from Askjelldalsvatn to Holskardvatn yearly. The net present value of the project is estimated to be 46 MNOK.
The preliminary design shows that the alternative that tries to minimize the construction costs is also technically and economically feasible. This design with an outlet at the highest regulated water level in Askjelldalsvatn is rather energy inefficient. A large part of the water would needs to be bypassed because the available head falls outside the head range of the turbine. A lower outlet level is able to utilize almost all flows with only a single turbine. This in combination with a pumping option, makes a combined pump turbine plant much more beneficial than a standard turbine power plant.
82 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
11.2. Recommendations This study shows the feasibility of the Askjelldalsvatn plant and based on this study it is recommended to apply for a license for the plant. It should be prioritized to get a better design and cost estimate for the upgrade of the transmission system. The plant can then be designed in more detail, for which it is necessary to undertake the following actions: - Acquire more detailed topographic maps with the exact locations of the tunnel and contour lines. - Contact turbine manufacturers for more specified pump turbine characteristics. These parameters will be important for the final optimization. - Make a site visit with an engineering geologist to investigate the geology. This is necessary for the final alignment and to reduce the uncertainty for the expected costs of the underground works. - Optimize the plant in detail. If Vansimtap will be used, care should be taken to the input of the turbine characteristics. Maybe other methods are available to take the varying turbine performance over both head and flow into account. Daily data should be used in combination with the weekly data that has been used in this study. - Let the financial analysis be performed by an engineering economist.
83 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
REFERENCES
Books
GUNNES, O. (unknown) Lecture notes on technical and economic analysis in hydropower planning. Norwegian University of Science and Technology. Dept. of Hydraulic and Environmental Engineering, Trondheim. 51
HVEDING, V. (1992) Hydropower development. Volume No. 1 Hydropower development in Norway.Norwegian University of Science and Technology. Dept. of Hydraulic and Environmental Engineering, Trondheim. 9-16
KJØLLE, A. (2001) Hydropower in Norway. Mechanical Equipment. Norwegian University of Science and Technology. Dept. of Hydraulic and Environmental Engineering, Trondheim. 3.1-3.16, 7.1-8.10
MOSONY, E. (1987) Water power development. Volume one: low-head power plants. Akademiai Kiado, Budapest. 610-746
NIELSEN, T. (unknown) Lecture notes: dynamic dimensioning of hydro power plants. Norwegian University of Science and Technology. Vannkraftlaboratoriet, Trondheim.
NILSEN, B. AND THIDEMANN, A. (1993) Hydropower development. Volume No. 9. Rock Engineering. University of Science and Technology. Dept. of Hydraulic and Environmental Engineering, Trondheim. 95-108
NOVAK, P. (2001) Hydraulic Structures. Spon Press, London. 191-201
RÖSSERT, R. (1992) Hydraulik im Wasserbau. Oldenbourg, Műnchen. 119-150
VAN DUIVENDIJK, J. (2007) Water power engineering. Principles and characteristics. Delft University of Technology – Section Hydraulic Engineering (VSSD), Delft
VIOGG, L. AND ELSTAD, I. (2003) Hydropower development. Volume No. 12: mechanical equipment. Norwegian University of Science and Technology. Dept. of Hydraulic and Environmental Engineering, Trondheim. 9-43
Publications
GARNAYAK, M.K. (2001) Hydraulic head losses in an unlined pressure tunnel of a high head power plant. Post graduate diploma work at École Polytechnique Fédérale de Lausanne. 13-14
NVE (2005) Cost estimates for hydro power projects. NVE’s hustrykkeri, Oslo.
NVE (2007) Indices for cost estimates for hydro power projects January 2005 – January 2007. NVE’s hustrykkeri, Oslo.
US ARMY CORPS OF ENGINEERS (1988) Engineering and design. Selecting reaction type hydraulic turbines and pump turbines and hydroelectric generators and generator motors. Technical letter No. 1110-2-317 (CECW EE)
STELZER, R.S. AND WALTERS, R.N. (1977) Estimating Reversible Pump-Turbine Characteristics. United States Department of the Interior Bureau of Reclamation, Denver.
84 Paul Slangen MSc. Thesis Prefeasibility Study on Askjelldalsvatn Power Plant
WALTERS, R.N. AND BATES, C.G. (1976) Selecting hydraulic reaction turbines. A water resources technical publication. Office of Design and Construction Engineering and Research Centre, Denver, Colorado. United States of the Interior Bureau of Reclamation.
Websites www.andritz.com: homepage of turbine manufacturer Andritz www.met.no: homepage of the Norwegian Meteorological Institute www.nordpool.no: homepage of Nordpool www.nve.no: homepage of the Norwegian Water Resources and Energy Directorate www.ssb.no: homepage of Statistics Norway
85