Are net zero energy buildings necessarily also net zero emission buildings? Time-integrated analysis using dynamic grid emission factors

The built environment significantly contributes to greenhouse gas emissions, with a considerable amount released during the building operation. With warming climates, the building industry has been developing net zero energy buildings to improve energy efficiency and achieve carbon neutrality. However, further research is needed to evaluate if net zero energy buildings are also net zero in terms of operational emissions. Moreover, the impact of electricity mix variations in the electricity grid on operational emissions from net zero energy buildings still needs further exploration. This study explored the ability of low-carbon energy sources to meet the energy demand of net zero energy buildings in various study locations across the Group of Seven countries. The 24/7 carbon-free energy analysis indicated a minimum value of 63.6% in Tokyo considering energy use met by low-carbon energy sources, highlighting the necessity of battery energy storage systems for peak shaving and supplementing electricity when the renewable energy sources are intermittent. Annual operational emission intensity analysis using static grid emission factors for each city indicated an underestimation of 111% in Paris and an overestimation of 120% in New York, compared to hourly dynamic grid emission factors considering electricity mix variations in the electricity grid. The misestimations in operational emission intensity could lead to flawed design practices like over-insulated building envelopes. The study outcomes indicate that net zero energy buildings are not necessarily net zero emission buildings when hourly electricity mix variations in the utility grid are factored in.