Batteries and thermal energy storage: an analytical method for sizing and analyzing potential synergies in hybrid systems

Abstract

Electric-grid generation capacity is sized to meet the peak load, typically in the summer when building cooling loads are high and is underutilized for much of the year. Electric utilities often pass on the costs associated with these peaking generators to building owners through demand charges. Building owners can minimize these demand charges by shifting energy use away from peak periods with behind the meter storage. This storage can include batteries, which can directly shift the metered load, or thermal energy storage, which can shift thermal-driven electric loads like air conditioning. However, there is a lack of research on how best to combine battery and thermal energy storage. In this study, an analytical sizing method is developed for a hybrid system, calculating the potential demand reduction and annualized cost savings for different combinations of thermal and battery energy storage sizes. It is shown that adding batteries to a thermal energy storage system increases the total system’s load shaving potential. This is particularly true when the building has onsite PV generation or electric vehicle charging, which add significant variability to the load shape. It is also shown that for a given total storage size, selecting a higher fraction of thermal energy storage can significantly lower the cycling of the battery, and therefore extend the battery life. This, combined with the expected lower first cost of thermal energy storage materials compared to batteries, shows that hybrid energy storage systems can outperform a standalone battery or standalone thermal storage system.