Bringing a hybrid system to life

Owner and operator Slovenské Elektrárne is developing an ambitious plan to modernise the pumped storage facility to improve flexibility and performance to meet the needs of the Slovak and European power grids as well as extend the serviceable life of the 40-year-old plant. Proposals include refurbishing all six generating units, with the possible conversion of two 115MW units from fixed to variable speed along with incorporating a 70MW lithium-Ion LFP battery energy storage system (BESS).

Frequency regulation markets, such as the primary, secondary, and tertiary reserves enable network operators to maintain the balance between the supply of and demand for electricity. Čierny Váh’s proposed hybrid model will feed into the primary reserve market, which is the fastest response service, providing power within seconds. It is a storage solution that also offers Slovenské Elektrárne the most significant profit potential.

A BESS is well-suited for the primary reserve market because its lack of mechanical inertia or ‘rotating parts’ coupled with its reliance on power electronics and cell chemistry to determine ramping speed, gives rapid ramping capabilities. Leveraging these together the large energy capacity provided by pumped storage is a promising development for energy companies.

We are supporting Slovenské Elektrárne to develop this one-of-a-kind project by conducting an optioneering exercise to define the most suitable solution ahead of a pre-feasibility assessment of the preferred option.

Refurbishing to generate a world first

Combining variable-speed units and BESS at Čierny Váh will deliver:

• Additional grid regulation capacity, with a regulating range between 670MW and 730MW
• More capacity to deliver voltage control and more energy volume for black starting
• Higher storage efficiency of the storage – 77% rather than 73% for PSH; 85 % for BESS
• Faster and continuous ramping, up and down, to deliver FCR (frequency containment or primary reserves), aFRR (automatic frequency restoration reserves) and mFRR (manual frequency restoration reserves) from the same plant
• About 100MW of additional generation capacity for generation adequacy ¬– most from the BESS and the remainder from uprating the turbines – to help balance electricity supply and demand

The proposed refurbishment is complex.

Converting the ternary pumped storage units from fixed to variable speed by installing potentially the largest full-size static frequency converters (SFC) ever manufactured for hydro applications raises number of technical challenges, while integrating large-scale battery capacity with the operation of a pumped storage scheme is relatively uncommon.

Variable speed turbines that adjust their speed automatically according to grid conditions and reservoir water levels considerably improve the efficiency of the pumped storage process compared with running constantly at a fixed speed. SFCs are required to continuously vary the rotating speed of the pump turbine and adjust the output power to synchronise with the grid.

At Čierny Váh, two 125MVA SFCs would be required, making them the largest in the world.

Adding a battery system

Battery storage is a game-changing technology, and with the ongoing development of electric vehicles, the cost of batteries is expected to continue falling, making a grid-scale BESS even more attractive.

PSH and BESS are complementary. Pumped storage absorbs redundant power during off-peak periods and produces power during when demand rises, but it cannot ramp up quickly. This is where battery storage comes in, as it can respond to fast ramps, helping to mitigate fluctuations in frequency within seconds and allow the hydraulic units time to synchronise and take over.

The Čierny Váh BESS plant will require electrical power conversion – from direct current (DC) from the battery alternating current (AC) for grid connection – and balance of plant (BOP) systems. Our assessment covered the costs and equipment required to co-locate the BESS plant, including the inverters, converters, transformers and power electronics for the power conversion system, as well as the electrical infrastructure (switchgear, cabling, circuit breakers) and cooling systems.

Our engineers have demonstrated that the proposed project is technically viable and recommend studies continue to confirm the feasibility and to develop the tender design.

See also: www.seas.sk/en/about-us/our-power-plants/se-integrator/