Renewable energy sources (REs), hybrid vehicles (HEVs), and electric vehicles (EVs) are expected to be more widespread to achieve carbon neutrality. One solution for the problem of REs' fluctuating power output is to install a microgrid in a small region and combine REs with batteries to maintain a power supply-demand balance in the microgrid. However, battery installation in a small microgrid on a kW scale generally requires high costs.

The authors have researched the reuse of used nickel-metal hydride batteries (Ni-MHs) from hybrid vehicles and used lithium-ion batteries (LIBs) from EVs as an alternative to new batteries for power supply-demand balance in microgrid. They have also proposed a control method for the used batteries and a V2X control method for EVs.

To design the V2X control method, data on how EVs are driven and parked must be investigated. The authors have used publicly available statistical data from government-conducted interviews with vehicle owners. However, this type of statistical data is inadequate for estimating EVs' operation in a specific area.

This paper describes a future scenario in which a microgrid equipped with REs and batteries is installed in a specific small town on Hokkaido Island in Japan. On-vehicle GPS-equipped loggers measured the operation data of actual petrol vehicles and EVs, including how the vehicles are driven and parked. The V2X control method was designed assuming all measured data represent EV operation.

The effects on the power supply-demand balance in the microgrid were confirmed through MATLAB/Simulink simulations in three cases: (1) when EVs are charged normally without discharging to the microgrid, (2) when EVs are V2X-controlled, and (3) when EVs are V2X-controlled and LIB is charge-discharge controlled cordinatedly.

The microgrid loads are three public facilities: a hospital, a welfare center, and a town hall. These facilities are assumed to be connected to the town's own distribution lines. It is also supposed that each EV battery has a rated capacity of 62 kWh, five or six EVs are V2X-controlled, and on-ground LIB has a rated capacity of 59 kWh.

A photovoltaic power plant and a biogas power plant are assumed to be installed in the microgrid. Ni-MH with a rated capacity of 8.7 kWh are supposed to be installed to smooth out short-term output fluctuations from solar power plant. The V2X-controlled EVs and on-ground LIB are charged or discharged to keep the power demand-supply balance.

Simulation studies covering five weekdays were conducted to investigate the total energy purchased from a power company grid outside the microgrid and the ratio of total RE energy to total energy consumption.The simulation results revealed that using V2X-controlled EVs for the power supply-demand balance reduces purchased energy and increases the utilisation factor of REs. The proposed coordinated charging and discharging control method for the LIB and V2X-controlled EVs shows the most improved power supply-demand balance.