Hydropower Information >> General Information >> Pumped Storage Database An introduction to Pumped Storage Power Generation, by Hiroshi Tanaka. Recent advances in pumped storage technologies Development of high head pumped storage The storage capacity of the energy of pumped storage is given by the product of the operating head (geodetic height between upper and lower reservoirs) and the storage capacity of the reservoirs. Therefore, by utilizing higher operating head, the size of reservoirs can be reduced and initial construction cost can be saved. In 1973, the operating head of single stage reversible pump-turbines installed in Numappara Pumped Storage Power Station in Japan first exceeded 500 m. Since then, developmental studies for higher head machines have been carried out actively and, recently in 1999, the highest head pump-turbines with the maximum pump head of 778 m were commissioned at Kazunogawa Pumped Storage Power Station in Japan. Adjustable speed pumped storage One of the recent remarkable developments in pumped storage technology is evolution of adjustable speed units. Conventionally pumped storage units use synchronous generator-motor and their operating speed is constant in both generating and pumping operations. However, in 1990, a technology was developed to operate the synchronous machine at an arbitrary speed within a speed range of plus/minus several percent of synchronous speed. This became possible by exciting the rotor with low frequency alternative current instead of direct current. The adjustable speed pumped storage units have the following advantages. Pump input power can be changed by adjusting the rotational speed. This enables to frequency control operation in pumping operation as well as in generating operation. Efficiency of pump-turbines in turbine operation is improved by 2 - 3 percent especially at part load and the operable load range in turbine operation can be widened. Conventional constant speed units can be operated normally from 50 to 100 % output but adjustable speed units can be operated from 25 to 100 %. For very rapid fluctuation of the load in the network, adjustable speed units can follow the fluctuation quite rapidly. When system disturbance arises, they change their speed rapidly and, thus, discharge or charge the energy from/to the inertia of the rotor like a flywheel generator. The power output can be changed rapidly and is very effective for stabilizing high frequency load fluctuations or system disturbances. Technology developments envisaged in the future In the 21st century, the technology relating to pumped storage will be developed further. The following developments are envisaged in the near future. Underground seawater pumped storage: To save the cost of reservoir construction, the idea of using the ocean as the upper reservoir and an underground cavern as lower reservoir is proposed. Such pumped storage can minimize the impact on environment. Also it becomes possible to construct pumped storage power plants in coastal urban area near the area of power consumption. Two stage reversible pump-turbine: Since the centrifugal stress of the rotating parts of the machine becomes higher proportional to the operating head, the maximum operating head, under which a single stage pump-turbine is operable is thought to be somewhere around 800 or 900 m. Beyond the limit, it is thought that two stage pump-turbines need to be introduced which are equipped with two runners installed in the machine in series to share the operating head by half with two runners. Multistage reversible pump-turbines were already in use but all of them were not equipped with adjustable guide vanes and their turbine output was not controllable. What is required today are two stage machines with adjustable guide vanes. The problems with this type of machines are complicated structure and difficulty in control of operation. At present, large capacity machines of this type are being installed at Yang Yang Pumped Storage Power Station in Korea. Development of power electronics for the reduction of the cost of adjustable speed units: Excellent performance of adjustable speed units contributes to improve the reliability of the power system and may enhance the competitiveness of pumped storage in deregulated power market. However, the adjustable speed system is still expensive as power electronic devices used for the low frequency alternative current excitation system are expensive. This hinders the wide use of adjustable speed units despite of their excellent performance. It is hoped to reduce its cost by adopting new power electronic devices and by refining the system design. What is the sustainable energy to be conveyed to the next generation