Pumped storage Hydro power plant: These power plants are used when quantity of water available for generation of electricity is limited or insufficient. The water from upper reservoir is passed through penstock to turbine, where its energy is converted in to electricity and then it is released in lower reservoir. The generation of electricity is usually carried out during peak load periods. During off-peak periods, the same water from lower reservoir is pumped back to upper reservoir. Thus the limited quantity of water can be reused again and again. For this kind of operation two types of arrangements are used:
i) Separate arrangement for pumping back the water: In this case separate pumps are used to lift the water from lower reservoir to upper reservoir.
ii) Reversible Turbine Pump Unit: In this arrangement, the machine is operated as turbine during electricity generation and the same machine is operated as pump to lift the water from lower reservoir to upper reservoir through penstock. The figure shows this arrangement. The energy utilized for pumping during off-peak period is recovered by electricitiy generation during peak period and same water is reused.