Pumped storage power stations
What is the purpose of pumped storage plants?
Pumped storage is a tried and tested means of balancing supply and demand in an electricity network, both in terms of cost and environmental impact.
There is no way of storing large volumes of energy in the form of electricity. Therefore, power stations have to be able to produce as much electricity at any given time as is being used within the network. When someone switches on their oven, a power station has to generate the additional amount of electricity required for this at that precise moment. Electricity demands within a supply network fluctuate greatly over the course of a day. Consumption is at its lowest at night, whereas it peaks at lunchtime and in the evening.
In contrast to nuclear and river-flow power stations, storage power plants can adapt very quickly to changing requirements. Optimized interaction between the different types of power stations ensures that reliable, cost-efficient power supply is guaranteed around the clock. Pumped storage plants are also able to convert surplus energy which is generated during off-peak times into valuable peak-load energy. This involves pumping water back into the reservoir which is located on a higher level, which can then be used again later on to produce electricity.
How a pumped storage plant works
A pumped storage plant has a reservoir on an upper level and a water basin lower down. It is generally equipped with three types of machinery: a turbine, a generator/motor and a pump.
When electricity is being generated, water flows from the upper basin, i.e. the reservoir, into the pressure tunnel. The water drives the turbine which, in turn, drives the motor/generator; this functions as a generator in this case. The electricity produced is fed into the grid. Downstream of the turbine, the water flows into the lower basin. The motor/generator acts as a motor in pump mode. It is powered from the grid and drives the pump. It takes water from the lower basin and pumps its back up to the reservoir.
Losses are always inevitable with pumped storage. About three quarters of the electricity supplied for pump mode can be recovered in generator mode. That said, the process still makes sense to use because it enables the surplus electricity which is generated to off-peak times to be converted into valuable peak-load energy.
