Using these models, the efficiencies and the storage costs of the lithium-ion secondary battery (LIB), sodium-sulfur battery (NAS), redox flow battery (RF) systems were calculated based on the operation data of the large-scale demonstration experiments and information gained from direct interviews.
　The power-storage efficiency of the NAS battery system was 70%, and the storage cost was the lowest at 24-30 JPY/kWh. However, because NAS batteries need to be kept at around 300℃ at all times during operation, the cost was low only when the ideal charge / discharge pattern was used. The mode of operation greatly affects the cost. The power-storage efficiency of RF battery system was 70%, and the storage cost was the second lowest of 24-40 JPY/kWh next to NAS batteries by taking advantage of its long life. LIB has a high power-storage efficiency of 80-85%, but the storage cost is estimated to be 40-70 JPY/kWh, which is higher than others. Although the charge / discharge efficiency of LIB itself is high, the power efficiency is reduced due to the power consumption of the control system and wiring because a huge number of single cells are assembled into a system. LIB is available in a long-life type and a high energy type. The high energy type is considerably cheaper due to the mass production, but the cycle characteristics are inferior to other batteries, and the long-life type is expensive.
　Along with the above topics, future issues are discussed.