What is the emissions intensity of electricity discharged from an electric energy storage system that produces no emissions itself? One method to answer this question is to track discrete packages, or "slices", of energy as they move through the storage system. The emissions associated with an energy slice is calculated using the emission associated with the electricity used to charge the slice. These emissions carry through with the slice to the corresponding discharge energy. Because energy is lost on charging, during storage, and on discharging, the energy content of an energy slice shrinks as it moves through the storage. This can concentrate the total emissions from generation (if any) into a smaller amount of storage discharge energy. Or, if the storage is charged from a zeroemissions source, the quantity of zero-emissions energy decreases as it moves through the storage. The main challenges arise when a storage system is charged from both emitting and zero-emissions electricity, requiring that energy losses are allocated appropriately to energy slices. This paper discusses the challenges with using models to allocate energy losses to energy slices, discusses the challenges in relying on metrics from battery management systems, and provides empirical demonstration of the challenges using data from a fielded energy storage system.
This method of tracking energy slices is analogous to tracking electrons on the grid - it provides a compelling narrative but relies on a conceptual energy slice with little physical meaning. The utility of this approach is eroded by error and uncertainty in state of charge and state of health estimates, complex and variable efficiency, and standby losses that are influenced by external factors (e.g. ambient temperature). These factors may mean that, in some cases, an emissions tracking scheme built on this kind of energy tracking will rely heavily on ex post facto true-ups as modeled energy losses, used to allocate losses to energy slices, diverge from measured energy losses and as state of energy estimates are updated with no measured power into or out of the storage system.