HYDRO POWER PLANTS ON THE MIDDLE RIVER SECTION

Andrej Kryžanowski, Anja Horvat, Mitja Brilly University of , Faculty of civil and geodetic engineering Chair of hydrology and hydraulic engineering Hajdrihova 28, Ljubljana, Andrej.Kryž[email protected], [email protected], [email protected]

Abstract

Construction of a chain of hydro power plants is planned on the Sava River from Medvode to the Slovenian-Croatian border which will, apart from the chain of HPPs on the River, represent the linchpin of renewable energy production within the Slovenian power system. The mentioned chain of HPPs will also be one of the country's main renewable energy sources that can still be developed for power generation.

Three hydro power plants, , Mavčiče and Medvode, are already operating on the Upper Sava River section. Construction of the chain is underway in the lower part of the stream where Vrhovo and Boštanj HPPs are already in operation; HPP Blanca is under construction and the site planning procedures are taking place for Krško, Brežice and Mokrice HPPs. The planned HPPs on the Middle Sava River section between Medvode and will connect the HPPs on the Upper and Lower Sava River into a closed chain which will operate on the principle of run-of-river type power plants with daily storage. Completion of all stages will enable optimal development of available hydro potential. Apart from the energy effects, also other beneficial effects of hydro power plant construction in the region can be expected: flood protection; better water supply; waste water treatment; development of transport and energy networks as well as positive economic and social effects.

Keywords: hydro power, hydro power plants, renewable energy production, power generation.

1 GENERAL

Water is the only indigenous natural resource that is in abundant supply in the resources impoverished Slovenia. The highlands in the north-western part of the country prevent penetration of warm and moist air masses from the Mediterranean. The rivers in the western and central Slovenia receive water from the rainfall in the Sub-alpine region. The most important among them is the Sava River with its tributaries which receives water from a catchment area of 10,872 square kilometers, i. e. 53.6 per cent of the country's total area. The Sava River is characterized by changing flow regime: in the upper part the snowy-rainy regime prevails which turns into rainy-snowy regime in the lower stretch. Maximum annual discharges appear during the spring and autumn months and the minimum discharges during the summer and winter months. The Sava River with its tributaries has a torrential character along the entire river stretch. The ratio between minimum and maximum discharges amounts to 1:30; in exceptional circumstances even to 1:100. Unlike other main streams, the Sava river basin is the least exploited for power generation. Of the available hydro potential of the Sava River with its tributary , 18.5 per cent have been utilized so far, i.e. 12.5 per cent of the total hydro energy generated in Slovenia (Table 1). The available hydro potential has been fully utilized on the Drava River and 34 per cent on the Soča River with its tributary Idrijca.

From the point of view of energy, the Sava river basin is divided into three sections: Upper, Middle and Lower Sava River (Figure 1). The idea of utilizing the Sava for hydro power generation dates back to early last century when the first studies about the chain of hydro power plants have been undertaken. A closed chain of hydro power plants along the entire stream has already been proposed. Head storage reservoirs (HPP Moste and HPP Radovljica) were planned in the upper stream with seasonal storage capacity to enable operational capability of the chain during the dry season of the year. Construction of hydro power plants on the Upper Sava River: HPP Moste (1952) and HPP Medvode (1953) started immediately after the War. Later the works were abandoned for some time and only in 1986, the construction of Mavčiče hydro power plant started. The plans to construct storage reservoirs on the Upper Sava River (Radovljica, Radovna) were completely abandoned at that time for nature protection reasons.

UTILIZED GROSS AVAILABLE POTENTIAL RIVER ENERGETICAL POTENTIAL POTENTIAL SHARE POTENTIAL (GWh/year) (GWh/year) (GWh/year) (%) SAVA 4.134 2.794 517 18,5 DRAVA 4.301 2.896 2.833 97,8 SOČA & IDRIJCA 2.417 1.442 491 34,0 MURA 928 690 5 0,7 KOLPA 310 209 0 0,0 REST 7.350 1.114 284 25,5 TOTALLING 19.440 9.145 4.130 45,2 Table 1: Hydro Potential of Slovenian Rivers (Porenta, 2002 with supplements)

By abandoning the head storage reservoirs in the Upper Sava River section, the chain of 15 run-of-river plants from Medvode to the Slovenian-Croatian border at Mokrice is given consideration again. The role of the head storage reservoirs is taken over by the storage reservoirs of Mavčiče and Medvode HPPs, and the role of compensating reservoirs by the storage reservoir of Brežice and Mokrice HPPs. The storage reservoir of the Vrhovo HPP and partly of Jevnica, and Ponoviče HPPs are foreseen for intermediate compensation of flows. Construction of the chain of HPPs on the Sava River has started on the Lower Sava River section as the most promising sites due to the aquiferous tributaries: the and the . Vrhovo HPP, the first in the chain has been operational since 1993, Boštanj HPP has started operation this year, the third in the chain, Blanca HPP, is under construction and it is due to be completed in 2008. Site planning procedures for the construction of Krško, Brežice and Mokrice HPPs have already started. The chain on the Lower Sava River is scheduled for completion by the year 2018 at the latest.

Figure 1: Planned Hydro Power Plants on the Sava River (Source: Savske elektrarne)

2 HYDRO POWER PLANTS ON THE MIDDLE SAVA RIVER

The hydro power plants on the Middle Sava River will connect the stages on the Upper and Lower Sava River into a closed chain. Nine power plants are envisaged on the section which is from the geographical point of view divided into two parts: the Ljubljana Basin with Ježica, Šentjakob, Zalog, Jevnica, Kresnice and Ponoviče HPPs; and the canyon part of Zasavje with , Trbovlje and Suhadol HPPs. The conditions for hydro development of the Middle Sava River section are regulated by Concession Acts; concession has been granted to Holding Slovenske elektrarne. Construction of the chain of HPPs on the Middle Sava River will probably last 20 years, its completion is expected in 2030. The project is now in the stage of preparing expert groundwork to start the site planning procedures.

The rated discharge of the chain on the Middle Sava River has been determined in a manner which enables power generation in time with the hydro power plants on the Upper and Lower Sava River. It amounts to 260 m3/s on the river section to Zalog, and to 400 m3/s at the confluence with the Ljubljanica River. The gross potential of the Middle Sava River section amounts to 1,185 GWh/year. It has been assessed on the basis of the hydrological data from the period between 1961-90 by taking into consideration the average annual flows and a gross head of 117 meters which represents the difference between the downstream water level at Medvode and the upstream water level at Vrhovo. The average annual output of the chain on the Middle Sava River is estimated at: 994 GWh/year; net electrical output: 295.4 MW. Characteristic data on individual power plants are shown in Table 2. RESERVOIR ANNUAL NET HEAD INPUT VOLUME INPUT (106 m3) (m) (MW) (GWh) HE JEŽICA 0,4 26,0 54,5 151,4 HE ŠENTJAKOB 1,9 7,3 15,9 52,4 HE ZALOG 1,1 7,2 15,7 52,8 HE JEVNICA 2,4 6,9 22,9 101,4 HE KRESNICE 1,5 8,3 27,7 96,4 HE PONOVIČE 1,4 18,8 63,0 190,7 HE RENKE 0,4 8,5 28,6 97,9 HE TRBOVLJE 0,3 8,3 27,8 97,6 HE SUHADOL 0,7 11,7 39,3 153,4 TOTALLING 295,4 994,0 Table 2: Hydro Potential of Slovenian Rivers (Source: Regulation on the Concession, 2004)

In the conceptual design of the chain on the Middle Sava River section, two types of hydro power plants are proposed: run-of-river plants and head water storage reservoirs. With run-of-river type plants, the powerhouse is a constituent part of the dam structure, and the hydraulic head is represented by the geodetic altitude between the water level in the storage reservoir and the tail water. With head water storage reservoirs, the hydraulic head is realized by a headrace tunnel connecting the dam with the powerhouse located downstream. According to the description of the power plants from the Concession Act, Ježica and Ponoviče hydro power plants are foreseen as head water storage reservoirs and the other plants as run-of-river type plants. At present, conceptual designs are being prepared and, in order to reduce the construction costs and to minimize the environmental impacts, it is foreseen to replace the head water storage reservoirs by run-of-river plants.

3 THE ROLE OF THE HYDRO POWER PLANTS ON THE SAVA RIVER WITHIN THE SLOVENIAN ELECTRICAL POWER SYSTEM

By changing the structure of electrical energy supply (a constant increase in percentage of thermal energy) and attainment of independence of the Slovenian electrical power system within the European interconnection system, also the role of the hydro power plants within the system has changed. Their role in covering the constant part of daily load diagram has been minimized and the need for peak energy generation and the capability of taking over primary and secondary regulation have increased. The ratio between peak load and base load has been increasing together with the restructuring of the country's industry due to the smaller number of large consumers. Thermal or imported energy are intended to cover the needs of base load, while the energy in the peak portion of the diagram (particularly the power) has to be produced as near to the major consumers as possible.

It is also necessary to underline the conditions that have to be fulfilled for the incorporation of the Slovenian electrical power system into the European interconnection of networks: activation of the required reserve power in the event of failure of the largest generator unit (more than 300 MW) and availability of power reserve for frequency and power regulation (more than 180 MW). Association with the European Union and the introduction of energy market require from the Slovenian power system to provide ancillary services from its own production capacities, such as: own spare peak power, a power reserve during selected periods of the day for secondary regulation, fast activating of reserve peak power, reserve reactive power for voltage compensation and black start in the event of system breakdown. In respect to the envisaged role within the Slovenian electrical power system, the chain of hydro power plants on the Sava River will operate on the principle of run-of-river power plants with daily storage, with Mavčiče and Medvode HPPs taking over the role of the head storage reservoir, and Jevnica, Kresnice, and Ponoviče HPPs on the Middle Sava River and Vrhovo HPP on the Lower Sava River, serving as afterbay and compensation reservoirs as long as the whole chain has not been completed. Upon completion of the chain, the mentioned storage reservoirs will function as intermediate afterbay reservoirs on individual sections, while Brežice and Mokrice storage reservoirs will take over the role of compensation reservoirs for the whole chain.

The possibility to operate as a run-of-river power plant will increase with the degree of completion of the chain. At the beginning, the power plants will mostly operate as run-of-river plants. By increasing the number of stages, the capability of the chain for peak energy production and power regulation will also increase. The operation will depend on the conditions and operational limits required by other beneficiaries.

4 EFFECTS OF POWER PLANT CONSTRUCTION ON THE SAVA RIVER

Apart from the energy benefits, also other useful effects of hydro power plant construction on the Middle Sava River can be expected: greater protection against detrimental action of water; improved water supply; waste water treatment; regulation of transport and energy networks; economic and social effects; opportunities for development of recreational facilities and so on.

It is also necessary to emphasize the role of the hydro power plants in ensuring stable run-off conditions of the watercourse. Encroachments on the environment may not in any way deteriorate the conditions of the watercourse and every solution selected shall represent a new value of higher quality in space. In planning the power plants on the Middle Sava River, special protective measures have to be taken, such as: flood protection, protection of groundwater quality and quantity, protection of water regime and river dynamics. At the same time, it is also necessary to ensure passage of aquatic organisms, biodiversity conservation and autochthonous habitats as well as preservation and protection of valuable natural features in the area.

Construction of hydro power plants undoubtedly means positive national economic effects, as the Slovenian industry is capable to ensure the greatest part of the investment, both by equipment supply and project implementation as well. Investments into the construction of hydro power plants will help to maintain the competitiveness and the capabilities, particularly in the field of civil engineering and building industry, mechanical industry and, last but not least, electromechanical equipment industry. Investments into the hydro power plants on the Middle Sava River will enhance the conditions for creation of jobs in the region and industrial production for a period of more than ten years. Storage reservoirs as multipurpose projects will solve the problems of drinking water supply to the population, irrigation of agricultural land, protection against detrimental actions of water and provision of aquatic and riparian environments for tourism and recreational activities. The arrangements in the influential area of the storage reservoirs also include the development of public and local infrastructure in the region, such as: regulation of tributaries, development of transport infrastructure, passage of aquatic organisms through dam structures, development of public utility and energy infrastructures as well as construction of waste water treatment plants.

References

Kryžanowski, A. (1999): Presentation of Hydro Power Plant Construction on the Sava River, Hydrotechnical Concretes – Proceedings, IRMA, Ljubljana, 77-82. Porenta, M. (2002): Hydro energy, Expert Groundwork, Ljubljana. Concession areas and conditions under which the concession for utilization of hydro potential on the Middle Sava River section has been awarded – Attachments, Official Gazette of the RS 121/04, Ljubljana 2004. Concession Act for the use of water for power generation on the Sava River section from Ježica to Suhadol, Official Gazette of the RS 121/04, Ljubljana 2004.