DEVELOPMENT OF SMALL SCALE HYDROPOWER (<10 MW) FOR RESTORATION OF IMPOUNDMENT AT DAM

ML Griffioen 1, S. Natha 2 and B. Barta3

1&2Department of Civil Engineering Science, University of , 3Retired Professional Engineer

Hydropower is a useful conversion of renewable energy derived typically from the constant head and variable water flows. Currently, of the 45 500 MW installed generation capacity in , hydropower contributes a mere 5%. This is primarily due to relatively cheap and plentiful energy available in SA for many decades mainly from burning fossil fuels (i.e. coal and oils). Along with the world-wide tendencies in cutting down on the greenhouse gases emissions the SA Government created an enabling environment in introducing the Integrated Resource Plan 2010 (IRP 2010) and the National Energy Efficiency Strategy (NEES) to encourage energy use efficiency and implementation of renewable energy technologies including hydropower. The University of Johannesburg recognizes a need for capacity building in renewable energy technology development and offers the guidance in development and implementation of such projects. The development of small scale hydropower (<10 MW) installation for restoration of impoundment at the Hartbeespoort Dam is the first such project compiled and guided at the university. The Hartbeespoort Dam impoundment has been seriously affected with for many years and its storage capacity has dramatically been reduced by sedimentation. The Department of Water Affairs (DWA) as a custodian of this water source will have access to the sustainable and substantial source of energy to be made available for mitigation of the negative environmental impacts and gradual restoration of critical water storage. Given the current state of electricity supply from the national grid, the estimated contribution of 19,5 GWh obtainable annually from the hydropower installation at the Hartbeespoort Dam could ultimately help ESKOM in making this amount of energy available to other consumers elsewhere.

INTRODUCTION

At present two important areas of the national economy which are attracting attention are energy efficiency and the development of renewable resources available in South Africa. The University of Johannesburg recognizes a need for capacity building in renewable energy technology development and offers the guidance in development and implementation of such projects. A project is currently underway where the aim is to develop a small scale hydropower (<10 MW) installation for restoration of impoundment at the Hartbeespoort Dam.

Small scale hydroelectricity technology (typically below 10 MW of capacity) is recognized as well tested and efficient renewable energy technology in the electricity generation sector. This is mainly due to the most efficient conversion process of energy from moving water into electricity. Hydropower is a useful conversion form of renewable energy derived typically from the constant head and variable water flows. Currently, of the 45 500 MW installed generation capacity in South Africa, hydropower contributes a mere 5%. Along with the world-wide tendencies in cutting down on the greenhouse gases emissions, the SA Government created an enabling environment in introducing the Integrated Resource Plan 2010 (IRP 2010) and the National Energy Efficiency Strategy (NEES) to encourage energy use efficiency and implementation of renewable energy technologies which includes hydropower. The Hartbeespoort Dam impoundment has been seriously affected with eutrophication for many years and its storage capacity is dramatically reduced by sedimentation. The Department of Water Affairs (DWA) as a custodian of this water source will have access to the sustainable and substantial source of energy to be made available for mitigation of the negative environmental impacts and gradual restoration of critical water storage. The increase in optimal utilization of the Hartbeespoort Dam is one of the main aims of the DWA. To extend the scheme’s lifespan, the DWA introduced the restoration process named Metsi-A-Me (My Water) programme with the main objective in upgrading the dam reservoir’s water quality and capacity. The removal (dredging) and management of a top sediments layer from Hartbeespoort Dam is one of the tasks in the dam facility restoration.

A study carried out for the DWA in 2008 concluded that the sediments in the impoundment should be considered as a resource and not a waste. Beneficiary use of dredged sediments (e.g. land conditioning, compost production, building blocks manufacturing, rehabilitation of mine tailings dams, etc.) could provide substantial savings in dredging operational costs. To achieve all that, the energy in the form of electricity and oils is needed for at least the next five years. The original estimate of energy annually required to recover and to dry dredged sediments is about 10 to 12 GWh. The national electricity supplier indicated to the stakeholders in the Metsi-A-Me project that for the next several years there will be no energy available for this type of consumption. Subsequently the interest in the potential of hydroelectricity generation at the Dam came into consideration.

INFRASTRUCTURE AND OPERATION OF THE DAM FACILITY The Hartbeespoort Dam facility is situated on the Crocodile River some 40 km west of the Tshwane Metro urban area in the DWA’s Limpopo Water Management Area (No. 1). The Dam was constructed after the Great War between 1918 and 1923 making the dam wall and storage reservoir some 90 years old. In 1924 the micro hydropower unit was installed at the right flank of the dam wall providing hydroelectricity to the close vicinity of the Dam until the mid 1960s. The wall of Hartbeespoort Dam is connecting the right (east) and left (west) banks of the Crocodile River by a single vehicle all year round weather road. A short roadway tunnel is situated on the right flank just upstream of the dam wall. The dam wall has on its left flank a side channel spillway equipped by a system of ten radial gates measuring 10 m wide by 2,7 m high each. The gates were installed in 1969 resulting in the dam capacity increase. There are no direct river outlets other than the side channel spillway gates. Two outlet systems, one at each flank of the wall are providing for the water releases into the right (east) and left (west) irrigation canal systems. The combined maximum outflow through these outlets at FSL is approximately 12,0 m3/sec. i. Right (east) bank main canal system is 48 km long and serves irrigation areas around Brits and as far as the Roodekopjes Dam; ii. Left (west) bank main canal system is 56 km long and serves large irrigation areas situated west of Brits. The right (east) bank outlet works comprise a free standing tower equipped with four 760 mm diameter wall mounted sluice valves arranged in pairs opposite one another providing for a flow of water into the outlet tunnel approximately 110 m long. The tunnel opens into the free flow canal supplying right (east) bank canal irrigation system. The tunnel is inclined from RL 1138,49 m at the upstream intake tower floor to RL 1155,00 m at the exit of the tunnel. There is a bend of about 90o in the half length of inclined tunnel. A disused micro hydroelectric installation has its penstock off-take of unknown size connected into the tunnel not far from the exit of the tunnel. The maximum carrying capacity of the existing tunnel is estimated by the DWA at 9,3 m3/sec. The right (east) outlet works control the flows for irrigation as well as releasing water into the Crocodile River. The left (west) bank outlet works comprise a single 1,5 m diameter cast iron pipe leading from a wet-well tower fixed against the dam wall. The water is discharged via the isolating valves control room to the sleeve control room into the irrigation canal stilling basin and from there to the left (west) irrigation canal system.

To evaluate the hydroelectric potential of the existing but disused installation situated on the right bank downstream of the dam wall, information only from the site visits and limited historical records was used. According to the historical and limited technical information available on this installation, the plant started operation in 1924 and was equipped with a turbine unit of the Francis type manufactured in the UK in 1923. The conversion calculation based on the original imperial parameters indicated that one turbine unit of 37 kW in capacity output was originally installed with the structural provision for another similar unit. The penstock off-take connected to the right bank control works tunnel has a head of 22 m and the flow to the turbine is determined at 0,22 m3/sec from available data. The purpose of this micro hydroelectric installation was to electrify the dam facility and the nearby residential dwellings. The 37 kW plant supplied electricity to the Hartbeespoort Dam for about 40 years and was finally decommissioned in the mid 1960s. By the field assessment the whole mechanical installation is in a moderate state and can be easily refurbished. The old electrical equipment is, however, in a very poor physical state. The civil housing structure and a steel penstock are in a good general condition. Although the renovation and upgrade of this micro plant will be relatively non-expensive and even with a second turbine added, it certain that the new capacity estimated at 80 kW will not be sufficient for the purpose of the dam impoundment restoration.

HYDROELECTRICITY POTENTIAL AT THE HARTBEESPOORT DAM

The configuration of water outlet works at both banks downstream of the Hartbeespoort Dam offers several viable small scale hydropower alternatives. Each viable alternative has its advantages and disadvantages. The analysis of dam flow balance data shown that although total inflow and outflow are increasing, the gross evaporation of the dam is on a steady decrease, despite rainfall patterns showing a decreasing trend. In all likelihood this trend is due to sediment build up and eutrophication allowing less surface water area to be subject to evaporation, highlighting the importance of the sediment removal.

About 90% of the natural and artificial inflows reaching the Hartbeespoort Dam are collected by the Crocodile River with the Hennops, Jukskei, Magalies and Upper Crocodile as major tributaries. The extent of urbanization taking place in the upper reaches of the dam catchment and around the immediate banks of the dam impoundment are main reasons for the increases in surface runoff. The visible negative tradeoffs being experienced over the years from increasing urban runoff into the dam impoundment are manifesting in the serious eutrophication, salination and sedimentation problems. The water volume of dam storage capacity of some 205 million m3 is allocated to residential (12%) and irrigation (82%) water uses with a small volume proportion (6%) released for the river compensation. The water from the dam allocated for irrigation and river compensation has to be released through dam outlets situated on the right and left flanks of the dam. Theoretically that is water which can be used for hydropower generation.

From the records available on the dam water releases through the right (east) outlet works via existing tunnel almost doubled since 1971 from average flow of 4 m3/sec up to almost 8 m3/sec at present. However, the right bank outlet tunnel’s maximum flow capacity is limited to 9,43 m3/sec. Despite lower average annual rainfall expected, the flow of water through the right bank canal is increasing. This is due to increasing return flows from the Johannesburg’s Northern Wastewater Works. The flows likely available for hydropower generation at the Hartbeespoort Dam are available from DWA’s recorded data from the gauging stations situated at the right and left banks. These were extended for another 20 years into the future (with irrigation releases and right canal releases forecasted separately), using the historical trend line of the inflows into the dam. The period of 20 years is generally the minimum life span of a conventional hydropower station. Using the frequency function on Microsoft Excel the flow duration curves were generated for every year showing the frequency of certain flows that could be expected throughout the duration of the year (Hall, 2011).

During the original investigation to identify potential for hydroelectric development at the Hartbeespoort Dam facility, which took place in 2008 during the DWA’s Sakhile Asset Register project, a first order estimate of a moderate capacity of 600 kW has been mooted. Since then a focus has been aimed to the dam outlet works situated on the right bank of the Crocodile River. It was obvious from the onset of initial investigation that the energy of flowing water can be harnessed conveniently from the irrigation canal situated on the right bank downstream of the dam wall. The irrigation canal has favorable geometric configuration, sustainable water flows as well as existing outlet assembly, which might be incorporated into the proposed small scale hydroelectric scheme. The further investigation concluded that if a small scale conventional installation would be considered (i.e. an offtake from a free flow irrigation canal) there are a few possible locations suitable along the right (east) bank canal with a hydropower housing to be located in the Crocodile River bed. When the opportunity for a more serious investigation of the hydroelectricity potential at the Dam manifested at the University of Johannesburg, three possible alternatives were outlined for a pre-feasibility investigation and defined in the following descriptions.

Alternative 1: Rehabilitation and upgrade of existing hydropower plant equipment and adding another new electromechanical unit to already existing unit in a need of upgrade.

This alternative is attractive enough, however, it is envisaged that the capacity which can be installed without large investments and efforts is in order of only 80 kW. Estimated annual power production from the refurbished and upgraded installation is about 0,672 GWh which is a marginal output of the energy needed in the restoration programme. Based on the economy of scale principles this alternative has not been considered for further attention.

Alternative 2: Design and build a new conventional hydropower plant situated in the Crocodile River bed to be fed with the free flow water from the right (east) irrigation canal elevated 35 m above the river bed. This alternative will require a short steel penstock pipe diverting most of irrigation canal flows into the newly build hydropower station housing from which it will be released into the Crocodile River. The schematic illustration of Alternative 2 is given below.

Figure 1: Schematic illustration of Alternative 2 – New hydropower plant on right bank.

The location of an intake for this plant can be anywhere along some 100 m distance after the water is released from the right (east) bank outlet works tunnel, preferably before the river discharge waterfall where irrigation flows are measured (i.e. before stilling basin and Parshall flume) and the compensation flows released into the Crocodile River. The kinetic energy of moving water available in the irrigation canal flowing under certain fall toward a turbine/generator assembly located in a power house can be converted into the electricity. The power capacity output (kW) is proportional to the flow (m3/sec) and a height (m) of a fall. The small scale hydroelectricity generation is entirely non-consumptive use in contrary to any other water uses. Using essential parameters determined for this alternative as Qmax = 4,8 3 m /sec; Hgross = 30 m and assuming 5 percent losses manifesting within the penstock and valves assembly as well as applying the overall plant efficiency of 90 percent the hydroelectric capacity is determined at 1200 kW. The subsequent estimated annual power output is in order of 10 GWh which is very close to the annual energy requirements of the DWA’s Metsi- A-Me programme.

A similar most suitable example of the typical small scale conventional hydroelectric installation in South Africa is the Friedenheim Hydropower Plant (2,5 MW) situated in Nelspruit, Mpumalanga Province. The scheme cosists 5 km of earth irrigation canal, diversion weir and inlet sluice gates inlet works and two (2) 900 mm dia 460 m long penstocks, two (2) 11 kV at 1000 kW asynchronous generators powered by two (2) Francis turbines with full automatic operation. Transmission is over 2,7 km by 11 kW overhead power line. Annual maintenance expenses are on average about R300 000 with some 45% spent on the power house equipment.

Alternative 3: Design and build new plant as an extension of existing disused hydro plant situated at the right bank of the Crocodile River at the toe of Hartbeespoort Dam. This alternative will require a short penstock pipe to be connected directly to the right bank outlet works tunnel, which will have to be pressurized to take advantage of all available water head between the FSL and the river bed estimated at some 37,5 m).

Figure 2: Schematic illustration of Alternative 3 – New hydropower plant as extension on existing plant.

The schematic arrangement of Alternative 3 as given above illustrates the short penstock pipe connected directly into the pressurized tunnel. In order to utilize the full hydroelectric potential of the right (east) bank dam outlet works infrastructure and available flows from the dam, the outlet works tunnel will have to be pressurized at the tunnel exit. The extra hydrostatic head of about 7 m will be resulting in a capacity gain of about 300 kW against Alternative 2 over above conventional 1 200 kW. The operational advantage of this arrangement is that the irrigation flows will be controlled as per daily demand at the pressurized tunnel exit providing at the same time for the constant flows through the new hydroelectric installation.

3 Using essential parameters determined for this alternative as Qmax = 4,8 m /sec; Hgross = 37,3 m and assuming 5 percent losses manifesting within the penstock and valves assembly as well as applying the overall plant efficiency of 90 percent the hydroelectric capacity is determined at 1500 kW. The subsequent estimated annual power output is in order of 12,6 GWh which is more than the annual energy requirements of the DWA’s Metsi-A-Me programme. This alternative is worthwhile of consideration for implementation.

At present the costs of one MW small scale hydroelectric plant (<10 MW) installed in South Africa will be ranging between Rand 15 and 25 million depending on the location (the hydro plants added to existing water supply infrastructure will be cheaper), size and type of installation. The costs of electromechanical equipment (i.e. turbine and generator) will presently amount to at least 30% of the total cost of installation. The local market for new hydropower electromechanical equipment practically does not exists in South Africa and all equipment is designed and manufactured abroad mainly in Europe and North America. To obtain relevant design and costs for the electromechanical equipment several manufacturers are typically approached to provide this type of hydroelectric installation project.

TURBINE AND GENERATOR SELECTION

The rated flow and net head determine the set of turbine types applicable to the site and flow characteristics. The suitable turbines are those for which the given rated flow and net head plot within the operational envelope s illustrated in the graph below.

Figure 3: Turbine selection using net Head (m) and rated flow Q (m3/sec)

As a turbine can only accept discharges between maximal and the practical minimum, it might be advisable to install several smaller turbines instead one large turbine. It is necessary to seek advice from the manufacturers to select most appropriate turbine type and size. The investment costs and the annual production will allow for the final choice.

Turbine efficiency The small hydropower turbines at design flow can range from 80% to 90%.

Table 1: Maximum efficiency values for small turbines Turbine type Maximum efficiency Specific speed Single regulated Kaplan 0,91 0,19 – 1,55 1/2 5/4 Double regulated kaplan 0,93 Ns = (rev/min)*(kW) /(m) Francis 0,94 0,05 – 0,33 Pelton with one nozzle 0,90 0,005 – 0,025 1/2 5/4 Turgo 0,85 Ns = (rev/min)*(kW) /(m) Souce: European Small Hydropower Association (2004)

Generator type and efficiency There are two types of generators (i) synchronous and (ii) asynchronous. The efficiencies of generators can range from 93 to 97 percent. It is important that the transmission assembly between a turbine and generator allows to match the rotational speed of each.

Other essential components The other essential mechanical and electrical components of a small hydropower plant may include: (i) water shut-off valve(s) for turbines, (ii) river by-pass gate and controls, (iii) hydraulic control system for turbines, (iv) electrical switchgear, (v) transformer(s).

ENVIRONMENTAL AND SOCIAL IMPACTS EVALUATION AND COSTING

The environmental viability of a proposal is commonly related to the magnitude of environmental impacts that can be associated with the hydroelectric development. These impacts can vary significantly and are depending mainly on the location and overall configuration of proposed development. The key environmental issues typically refer to the status of environment around proposed site:  a development within a pristine environment (usually a conservation area or national park, etc.);  a development in altered environment (i.e. already affected environment by extensive or limited human activities, etc.); and  a development in a completely altered/adjusted environment (e.g. urban/metropolitan area).

The environmental issues generally associated with the development of the small hydropower installations are principally two folds: short term (i.e. typically during the construction period) and medium to long term (i.e. the impacts related to the operation of a scheme). The currently applicable regulations to be consulted are as follows:  Regulations in terms of Chapter 5 of the National Environmental Management Act, 1996 (Government Gazette No. 28753. No. R. 385. 21 April 2006);  List of Activities and Competent Authorities Identified in Terms of Sections 24 and 24D of the National Environmental Management Act, 1996 (Government Gazette No. 28753. No. R. 386. 21 April 2006); and  List of Activities and Competent Authorities Identified in Terms of Section 24 and 24D of the National Environmental Management Act, 1996 (Government Gazette No. 28753. No. R. 387. 21 April 2006).

REGIONAL BENEFITS FROM UTILIZATION OF HYDROPOWER AT THE HARTBEESPOORT DAM

The regional benefits which can be gained from implementation of hydropower are as follows:  can be built on a wide diversity of scales (e.g. pico, micro, mini, small and macro) ;  sustains the multiple use of water in non-consumptive manner;  technology is robust, high-efficiency and long lifetime up to 30 years;  allows for peak load energy optimising the base load generation;  enables meeting fluctuations in energy demand ;  requires low energy demand in its creation – produces 200 times more energy that is needed ;  has the highest energy payback “ratio”;  can provide indispensable back-up for other energy sources (i.e. wind and solar sources);  reduces fossil fuel prices;  decreases greenhouse gas emissions;  optimises utilisation of available water resource;  aids existing electricity grid stability, and be easily synchronised with the national grid; and  creates direct and auxiliary jobs.

Exemplifying 1MW small scale hydroelectric plant which will be attached to existing dam facility can utilise legislated ecological flow released annually for 95% of theoretical time downstream of the dam. Some 8,322 GWh of electricity can be generated in an average year. If it is assumed that the utilisation lifespan of a plant is 20 years before major refurbishment will have to take place, the gross energy output over a plant lifespan is about 166,4 GWh. The 1 MW plant can thus offset about 148 132 tons of CO2 if the World Bank baseline conversion rate of 890 tons CO2 per GWh is applied. The same size plant can also replace in 20 years about 6 000 tons of fossil fuel while supplying some 1000 sub-urban households with electricity.

CONCLUSION AND RECOMMENDATIONS

South Africa as one of the signatories of the Kyoto Protocol (1997) committed itself in reducing greenhouse emissions by 34% in 2020 below projected emissions level. The load of greenhouse gas emissions from various sources in South Africa as whole, is currently estimated at about 500 million tons of carbon dioxide equivalent (CO2e) per annum. If the South Africa is to achieve its estimated target the process of extensive implementation of renewable energy technologies has to be facilitated.

To provide suitable enabling environment for emissions reduction and reliable energy supply for the SA economy the Department of Energy (DoE) with the endorsement from the National Energy Regulator of SA (NERSA) introduced the Integrated Electricity Resource Plan (IRP) for South Africa 2010 – 2030. The DoE subsequently allocated different capacities across various renewable energy technologies from the total development capacity of 3725 MW. The hydropower sector has been allocated overall capacity of 75 MW to be commercially operational by June 2016. One of the critical qualification requirements is that only the small scale hydropower installations above 1 MW are to be included in the forthcoming selection process. The new REBID requirements exclude the renewable energy projects including the hydropower projects with capacity below 1 MW meaning that the proposed hydropower development at the Hartbeespoort Dam will qualify for inclusion in the REBID process.

In view of the international pressures on South Africa and internally rising positive sentiments about speedily implementation of the renewable energy producing projects the Hartbeespoort Hydro appears as most suitable and sustainable option. The desk top investigation, summarized in this paper indicates that there is a good hydroelectricity capacity potential at the Hartbeespoort Dam. The preliminary calculation and basic field surveys indicate that it might be possible to develop potential hydroelectric capacity up to 1,5 MW described in the Alternative 3.

It is recommended that the preliminary investigation study of hydroelectric potential at the Hartbeespoort Dam will be based in principle on the methodology illustrated in this report, taking in consideration and be stimulated by the requirements of REBID. In this way the proposal on development of renewable energy potential from existing infrastructure will comply with regulatory framework on renewable energy currently observed in South Africa.

It is also recommended that all further investigation will observe principles and methodology as prepared and defined by the International Hydropower Association (IHA)’s Hydropower Sustainability Assessment Protocol (June 2011). The protocol represents significant advancement toward achieving sustainability in the hydropower sector and it is a complementary tool enabling the users in practical evaluation and assessment of social, economic and environmental issues.

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