Hydropower Information >> General Information >> Pumped Storage Database An introduction to Pumped Storage Power Generation, by Hiroshi Tanaka. The evolution of Pumped Storage Technology Development of reversible pumped storage units Reversible pumped storage unit consists of a reversible pump-turbine and a generator-motor, which may work as a generating unit or as a pumping unit by reversing the direction of the shaft rotation. Small capacity reversible units had been introduced earlier in 1920s. However, due to the difficulty in designing reversible machines, no large capacity unit had been put into use before one with a unit capacity of 59,500 kW was commissioned at Hiwasee Power Station in the United States in 1956. The evolution of reversible machines reduced the capital cost of pumped storage power plants remarkably. Since then, in the United States and in Japan, many pumped storage power plants equipped with reversible units have been constructed. Other than reversible units, tandem type units are still in use although they are very few. A tandem type unit consists of a generator-motor, turbine and pump coupled with a common shaft. This inevitably increases the cost of the machine but it has an excellent feature which reversible units can not compete with. Since the unit operates as turbine or as pump with the same direction of shaft rotation, it need not reverse its rotation when the operating mode is changed from generating to pumping or vice versa. Therefore, the time required for the mode change is about 3 minutes-much shorter than the time for reversible units which is approximately 7 to 12 minutes. Notwithstanding this advantage, more than 95 % of pumped storage power plants at present are equipped with reversible units for economic reasons. Figure 4 shows a reversible and a tandem type unit. Recent advances in pumped storage technology