Hydropower Information >> General Information >> Pumped Storage Database An introduction to Pumped Storage Power Generation, by Hiroshi Tanaka. Why have pumped storage? Electrical energy cannot be stored. Therefore the energy taken from an electrical power supply grid must always be equal to the energy being delivered by the electrical power plants. If this were not the case, the frequency and voltage of the supply grid would deviate from standard values. Following severe disturbances of the supply/load balance, the supply system could collapse. Pumped storage plants solve this problem by storing electrical energy as potential energy: They pump water to an upper reservoir at times of surplus energy on an electrical supply grid-typically, at night. This potential energy is then released through a hydro-electrical generator at times of high demand. Figures 1a and 1b show a typical pumped-storage scheme configuration. A basic function of pumped storage power stations is load leveling. It absorbs the surplus power from the supply grid during the off-peak period to pump water to an upper reservoir. Then in a peak period, it generates power using the water and thus it levels the load for other generating plants as shown in Figure 2. Historically, this was the first reason why pumped storage plants were built. Pumped storage power plants were first introduced onto power generating systems in Europe in the 1910s for the purpose of load leveling in daily operation or seasonal adjustment of generating capacity of the system. (These early pumped storage power plants were mostly equipped with separate generating units and pumping units. Later they were equipped with a generator-motor, turbine and pump all connected to a common shaft). However, conventional hydropower plants can store energy, and release it when the supply grid needs it. Why bother with pumped storage plants, why not just build conventional hydro-power plants? This observation is quite correct. It's quite common for hydro-power plants to be used only at times when energy demand is greatest. They are very good for this purpose, because they are capable of being started and stopped quickly, easily (in many cases, by remote-control) and at little cost. But to do this dependably, they would usually require considerable water storage and thus a large upper reservoir. And they have no ability to absorb power from the grid like pumped storage. This feature is vital to nuclear generation plants because they can only change load very slowly and pumped storage plants can be used to absorb their output at night. The maintenance costs of certain types of gas -fired power stations increase sharply if they are forced to reduce load at night, and so they also benefit from storage plant attached to the grid. Even oil and coal-fired stations can be operated closer to their optimum efficiency (and thus reduced gaseous emissions) if the supply system includes pumped storage plant. Next >>