With wind power capacity growing, Austrian utility Vorarlberger Illwerke needed to ensure the reliability of the network. The solution was to construct Kops II, one of the world’s most advanced pumped storage hydropower plants.

Siân Green

In November 2004, an official ceremony marked the start of construction of a new pumped storage hydropower plant in Austria. By 2008, construction will be complete and Kops II – a 450 MW high-head plant – will be supplying power to the grid to meet peak load and ensure network stability.

Situated in the Montafon area in Vorarlberg, Austria, Kops II is being constructed by Austrian utility Vorarlberger Illwerke AG. Kops II will use some of the infrastructure of the existing, 35-year old Kops pumped storage hydropower plant, yet will be one of the most advanced power stations of its kind and will be instrumental in ensuring the reliability of the region’s power grid for years to come.

Figure 1. Kops II will use the upper reservoir of the existing Kops pumped storage hydropower plant
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When operational in 2008, Kops II will mainly be used to regulate the power network belonging to Energie Baden Württemburg (EnBW), which manages the control centre of Illwerke’s power plants. The key project challenges were to design a highly flexible plant capable of ensuring grid reliability, and to construct it within a fierce schedule of just 42 months.

Network challenge

In Europe, there is an increasing demand for pumped storage hydropower plants and other energy storage solutions in order to meet peak demand requirements and to ensure network stability. In particular, the rapid growth of wind energy has made network reinforcement a priority.

In Germany, over 15 000 MW of wind capacity has been installed. These wind farms operate well in good wind conditions. However, if the wind suddenly becomes too strong, many of the rotors go into the ‘sail’ position simultaneously and the network falls short. This power must be rapidly replaced to avoid fluctuations in network frequency.

Figure 2. New headrace, pressure and tailrace tunnels are being constructed
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Kops II has therefore been designed to be extremely flexible in terms of network regulation, and it is this feature which will make it one of the most modern and technically advanced pumped storage power plants in the world. Due to its quickly adjustable generating sets, Kops II can rapidly feed large amounts of power into the network, or take power from it. The plant can switch quickly between turbine mode, pump operation or back-to-back hydraulic mode, depending on network needs, to safeguard the grid against power failures and fluctuations.

New and old

Kops II will partner the existing 247 MW Kops pumped storage hydropower plant, which was commissioned in 1969. Like Kops I, Kops II has been designed as a high-head, underground cavern power plant.

Kops I utilizes a head of 780 m, drawing water from the Kopsee storage reservoir and passing it through a pressure gallery, surge tank and pressure shaft to the cavern of the power station. Kops II will use the existing Kops storage reservoirs. The upper reservoir (Kopsee) sits at 1800 m above sea level, and Kops II will operate under a net head of around 800 m. The existing Rifa reservoir will serve as the lower reservoir and has been expanded for Kops II.

Figure 3. The power plant cavern consists of three pump turbine generators, with the Pelton turbines placed on top of the motor generators
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A new headrace tunnel will be built to take water from Kopsee to the Tafamunt surge tank. It will be a 5500 m-long tunnel with a diameter of 4.6-5.3 m. From the surge tank, it leads to a 1380 m-long, 3.8 m-diameter pressure tunnel to the distributing conduit and pumped storage cavern. The power plant cavern will be connected to the Rifa reservoir by a new 360 m-long tailrace tunnel. On completion, the Kops II pressure shaft will be one of the most heavily stressed in the world, with a P x D (pressure in m column of water x diameter in m) value of over 4000.

Turbine arrangement

The power plant cavern will consist of three vertical-shaft, 150 MW pump-turbine generators. Each unit will comprise a six-jet Pelton turbine, motor generator, hydraulic torque converter and three-stage storage pump. Under a €100 million ($120 million) contract, VA Tech Hydro is to supply the three Pelton turbines and the generators as well as 6000 t of penstocks and the steel tunnel linings. Under its €60 million contract, Voith Siemens Hydro Austria is to supply three vertical shaft, three-stage storage pumps, three hydraulic torque converters and the pump and converter control units, six 1500 mm pump and turbine spherical valves (design pressure 115 bar), accessories and control units, and two pipe-break valves for the Kops II valve chamber.

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The design of the plant is unusual due to the installation depth of the storage pumps, i.e., they are arranged beneath the tailwater level of the Rifa reservoir. In a normal arrangement of the generator, turbine, torque converter and pump, the total length of the shaft reaches 37 m. However, to fulfil the cavitation criteria for the pump and at the same time a free flow of the turbine tailwater, the shaft needed to be up to 60 m. This solution was not technically nor economically feasible, however, and so the setting of the Pelton turbine was fixed at 25 m below the tailwater level. This means that during operation, the water must permanently be suppressed by compressed air to avoid the submergence of the runner.

Therefore at Kops II, the turbine outlets will be equipped with a tailwater compressed air surge tank. This provides the required exposed head for operation of the Pelton turbines. It also achieves the tailwater level reduction required in the turbine downstream pit in each case – by means of using atmospheric pressure – as a result of the required injection of compressed air via the turbine housing in the downstream pit. To ensure the feasibility of this design, the Pelton turbines, which can accommodate partial loads, were spatially arranged over the 210 MVA motor generators.

In order to support the power grid, the Kops II machines are capable of operating in back-to-back mode, during which a motor generator, storage pump and a turbine are operated simultaneously. The units can be switched from pump to turbine mode – or vice versa – in a matter of seconds depending on network needs. It is possible to both feed peak load energy into the network from +130 MW to -150 MW by switching to turbine operation as well as use energy from the network by switching to pump operation.

Minimal impact

The plant cavern will be built within the mountain between the tourist resorts of Gaschum and Partenen. Except for a new building for the SF6 switchgear and the entrance tunnel gate to the power cavern, located opposite the Rifa reservoir, the new Kops II project will be invisible from the outside. Impact on the surrounding area will therefore be minimised.

Completion of Kops II is scheduled to take place between the end of 2007 and the beginning of 2008.

Figure 4. Kops II will be on-line by 2008
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