Case Study of Photovoltaic Energy Surplus Absorption by Charging Lithium-Ion Batteries Considering Charge–Discharge Energy Efficiency

In Autumn 2018, reductive control of photovoltaic surplus energy was performed in Kyushu, Japan. This was a lost opportunity to utilize renewable energy, which could have been otherwise stored in rechargeable batteries. In this study, we simulated the absorption of the photovoltaic energy surplus by charging battery systems of a working electric vehicle considering the energy efficiency of lithium-ion battery systems to reduce the operation cost. This study clarified the quantitative effects of a real-time energy efficiency diagnosis of lithium-ion battery systems. From the simulation results, we discovered that the improvements of the charge-discharge energy loss and the economic gain of the photovoltaic energy surplus absorption depended on the input-output power of the bidirectional vehicle-to-grid charger and the amount of surplus energy. When the input-output power is sufficiently high and the ratio of surplus energy to total capacity of the battery systems is sufficiently low, the improvement in surplus energy absorption profit is reasonable. The real-time degradation diagnosis of the battery systems and selection considering charge-discharge energy efficiency is necessary to improve the profit from surplus energy absorption.