Revealing temporal and spatial variations in CO2 emission factor of electricity generation in Japan

The electricity generation sector in many countries is undergoing drastic changes toward decarbonization. A major driver of this change is the rapid integration of renewable energy, the availability of which varies temporally and spatially. To evaluate how such variations affect CO₂ emissions from electricity generation, this study measured the average emission factor across 10 different electricity service areas in Japan. Using hourly generation data spanning seven years from each area, our study revealed substantial temporal and spatial variations in the emission factor. Specifically, the results show (1) significant intraday variation with smaller values during daytime hours, (2) yearly variation with decreasing values over time, and (3) substantial spatial variation in both temporal variation and magnitude. We also used real consumption data of heat pump water heaters to simulate the extent to which CO₂ emissions can be mitigated by shifting their operation schedules from midnight to daytime, finding that households can reduce CO₂ emissions by 158 kg annually on average.