Field testing and simulation of load shifting for grid-interactive heat pump systems integrated with cold thermal energy storage

Abstract

Integrating thermal energy storage (TES) into grid-interactive heat pump (HP) systems offers significant potential to enhance grid resilience and reduce building operating costs through load shifting. This study investigates load shifting strategies and quantifies their impacts on electricity cost and peak load reduction. The TES unit is integrated with the HP system via a secondary loop, allowing cooling energy to be stored in phase change material (PCM) and heat transfer fluid (HTF). The stored cooling can later be discharged to the building, decoupling cooling generation from demand and enabling flexible operation. A field study compares the TES-HP system with a conventional nonTES-HP system to demonstrate the benefits of TES integration. A comprehensive set of evaluation metrics is developed to compare TES-HP and nonTES-HP systems. The metrics include instantaneous, cumulative, and unit-value indices from both building and equipment perspectives, as well as peak-hour-specific indices. A rule-based control strategy that accounts for electricity pricing and ambient temperature is implemented to maintain thermal comfort, improve charging efficiency, and achieve load shifting. To complement the field study, simulation models are developed to analyze the impact of TES storage capacity and heat transfer capability (UA, between HTF and PCM) on load shifting performance. Field results indicate that, under the same thermostat setting, the TES-HP system reduces hourly electrical load by 92%, 58%, and 36% during peak hours and lowers 24-hour electricity costs by 39% compared with the nonTES-HP system. After accounting for cooling load differences, TES-HP increases cooling capacity per cost (QpC) by 6.6 times during the first peak hour and by 27% over the full 24-hour period. Simulation results indicate that doubling TES storage capacity reduces peak-hour cost and power consumption by up to 84.8% and reduces 24-hour electricity costs by 38% compared with the original TES design. Increasing UA by ten times reduces daily electricity costs by more than 25% due to improved system efficiency. Overall, both field experiments and simulations demonstrate that TES-HP systems significantly enhance energy flexibility and cost efficiency in grid-interactive buildings.