Renewable energy sources (REs) and electric vehicles (EVs) are expected to become more widespread to achieve decarbonisation. Microgrids, the energy systems that best utilise REs, are also expected to be widely installed. Since REs have fluctuating output power, batteries should be installed with REs in the microgrid. However, the installation of batteries in a microgrid requires high costs.

The authors have researched the reuse of used batteries exhausted from electric vehicles (EVs) as on-ground batteries to substitute for new batteries and maintain the power demand-supply balance in the microgrid. The authors proposed a battery charge/discharge control method for coordinating REs, batteries and loads in a regional microgrid in a rural area. They also confirmed the proposed method through Model-IN-the-Loop (MIL) simulations.

This paper proposes a method for designing the required REs' power output, the required capacity of used batteries, and a battery charge/discharge control method to keep the power demand-supply balance in a rural microgrid. The paper also shows the results of one-year-long MIL simulations to confirm the proposed control method through MIL simulations.

The power consumption of dairy farms, such as milking machines, ventilation fans, and air conditioning units, was assumed to be loads in a rural area. The rural area's power demand was classified into the middle and summer terms. As a renewable energy source, a biogas power plant using livestock manure was assumed, and the biogas power plant's power output was supposed to be almost constant. A used lithium-ion battery (LIB) with a relatively large capacity was presumed to be installed.

The basic principle of the LIB charge/discharge control method is that if the load-consuming power is greater than the biogas plant's output power, LIB is discharged. On the other hand, if the biogas plant's output power is greater than the load-consuming power, LIB is charged.

The required output power of the biogas power plant was calculated assuming that the energy load-consuming power plant and the energy generated by the biogas power plant in a day coincide. The required capacity of the battery was calculated assuming that the charged energy and the discharged energy in a day coincide.

Since the battery state of charge (SOC) may reach its upper or lower limit in the summer term, the authors proposed another control method to slightly vary the biogas plant output power according to the LIB's SOC.

To confirm the proposed method, MIL simulations were conducted using OPAL-RT, a real-time simulator running on the CAD base of MATLAB/Simulink. The real-time simulator was set to a calculation mode that allows simulations to run faster than real time, and simulations were conducted using one-year-long data at 100 microsecond increments.

The simulation results showed that the ratio of energy purchased monthly from a power company grid outside the microgrid to the annual load consuming energy was about 2.4 % at maximum.

These results confirm that the rural microgrid with a load, a biogas power plant, and a battery can maintain a good　power demand-supply balance.