Environmental impacts of bidirectional charging for electric school buses: a cross-regional assessment

Electric school buses can interact bidirectionally with the grid during overnight idle periods, but their environmental consequences are poorly understood. This study develops a multi-objective overnight scheduling framework for electric school bus fleets that jointly optimizes charging costs, grid export, and age-dependent battery degradation under discrete charger-assignment constraints. A cross-regional assessment across six US grid subregions shows the net CO 2 effect is region-dependent, yielding reductions where the peak-to-off-peak marginal emission factor ratio is high and increases where it is low; coal-dominated grids produce net increases under current generation mixes. Benchmarking against smart unidirectional charging isolates what export itself contributes: deeper CO 2 and NO x reductions, but higher SO 2 and a reversal of the PM 2.5 benefit that unidirectional charging delivers. These outcomes are robust across five fleet scales, six utility tariffs, and five climate zones, though their magnitude depends on regional grid composition, arguing against blanket environmental credits and in favor of region-specific policy design.