Refrigeration Battery

The goal of this Emerging Technologies study is to determine the load shifting potential of salt-water ice storage thermal energy storage systems installed as a retrofit on an existing grocery storage refrigeration system. TECHNOLOGY DESCRIPTION This report presents findings from the field test of a new refrigeration ice storage system at a 46,000 square foot grocery store. At this store, the ice storage system was installed as a retrofit on the existing medium temperature refrigeration system, which consumes approximately 20% of the store’s annual electric energy consumption. The store’s medium temperature refrigeration system serves a combination of walk-in coolers, reach-in and coffin refrigerated cases, and a low temperature rack sub-cooler. The thermal energy storage (TES) system was sized to completely offset the store’s medium temperature compressor rack loads during the utility’s On-Peak hours and charge the system during the early morning Super Off-Peak hours. However, unlike traditional load-shifting TES systems, this TES control system uses the whole-building meter demand as a trigger for charge/discharge optimization of the system. The TES control system modulates the output of the stored refrigeration energy from the ice storage tanks in order to reduce or increase the power consumed by the refrigeration compressors, which decreases or increases, respectively, the power provided to the store by the utility grid. In order to minimize the store’s total energy costs, the TES control system monitors the grocery store’s whole building load, as measured at the utility service meter, in order to optimize the TES discharge rate in a manner which reduces the utility demand charges while minimizing ice generation and storage losses. The TES system directly shifts refrigeration loads on the store’s medium temperature refrigeration system, by tying directly into the store’s medium temperature refrigerant discharge and suction headers. Therefore, as originally designed, the maximum load shifting capacity is limited to the compressor load on the medium temperature compressor rack at any particular time, minus the power consumed by the TES unit itself (including refrigerant pumps, water loop pumps, and control hardware). During the installation of the TES system the system was modified by replacing the existing low-temperature sub-cooler with a unit with a larger capacity. Theoretically, this could allow the TES system to offset additional compressor loads on the low temperature refrigeration system, allowing the system to offset additional load. PROJECT FINDINGS This Emerging Technology reports describes the data collection and analysis done to evaluate the peak demand reduction and energy impacts associated with a salt-water ice thermal energy storage (TES) system installed as a retrofit on a grocery store refrigeration system. As part of the project, a Normalized Metered Energy Consumption (NMEC) analysis was performed on the grocery store TES installation in SDG&E’s service territory. The NMEC analysis consisted of analyzing whole-building power consumption through the utility interval meter for a period of 12 months prior to the retrofit and 9 months after the retrofit. Additionally, the walk-in cooler temperatures and the TES system amp draw were measured to confirm consistency in operations of the refrigeration system, and once installed, to monitor the status of the TES system. This data was used, along with weather data from local weather stations, to develop a regression model of the baseline building operation and the building operation after the TES system was installed. These models estimate the annual whole-building energy consumption before and after the project implementation to estimate the demand savings and efficiency losses associated with the TES system. Uncertainty analyses were run on both models to ensure that the models provided reasonable estimates of the refrigeration system operation and to determine the validity of the resulting demand impacts and energy cost savings. The following table summarizes the demand savings and energy use and cost impacts associated with the TES operating at the grocery store in this study. Project cost effectiveness of the technology is reflected in the simple payback. PROJECT RECOMMENDATIONS At the singular site analyzed in this project, demand savings were achieved that can be directly correlated to the thermal energy storage system. However, the demand savings potential for this technology is limited by maximum load on the store’s medium temperature refrigeration during peak hours, and the project site has a relatively small refrigeration system. Therefore, the limitations of this technology prevent making any conclusive statements regarding the load shifting potential of this system across other grocery stores, climate zones, and refrigeration system types. Since this system demonstrated the potential for load shifting in a technology proof-of-concept test, it has the potential to be a successful measure through statewide customized incentive programs. However, continued development of the technology will be required to improve the load shifting efficiency in order to make it competitive with other energy storage systems currently in the marketplace, including battery storage systems. Insufficient data has been gathered to generate any conclusions that could be extrapolated to thermal energy storage systems installed at other locations. Further testing across a wider range of grocery store refrigeration systems and climate zones would be required to determine if the demand savings could be predicted reliably without conducting the same level of M&V as was conducted in this study.