Pumped storage generation is mostly large-scale. For example, the plant that Tokyo Electric is expanding in Gunma Prefecture will have the world’s largest generation capacity of 2,820,000 million kW, a drastic increase from the current 470,000 kW. This plant will start operations after 2020. However, as the pondage of the upper reservoir means the maximum generation capacity, the new plant can operate for nine hours per day at the longest. Because of the limit of the pumpage volume, it is impossible to operate the system to 100%. Before the Fukushima disaster, most power used to pump up water during nighttime was nuclear power, but power from thermal power generation is currently used instead.
The energy efficiency of pumped storage generation is about 70%. That is, 30% of thermal power used to pump up water in nighttime is wasted, and the amount of carbon dioxide emitted by thermal generation increases. In addition, power generated by old and inefficient thermal power facilities is currently used for pumped storage generation. Despite these facts, however, there is no suitable and promising technology to replace pumped storage generation. It seems the best way to operate pumped storage generation as an emergency measure in extreme hot days when power supply is rather tight.