ReBound Thermal Energy Storage

Refrigeration is an essential process for preserving perishable food. In the United States, refrigeration loads in commercial and residential sectors add up to more than 400 terawatthours (TWh) of electrical energy usage. In addition to large energy impacts, refrigeration systems can have environmental impacts when refrigerant is released into the atmosphere. The typical leakage rate of refrigerants, in some industries, is reported to be as high as 25%. Finally, refrigeration represents a high electrical demand for utilities during peak hours. The growing need for energy efficiency, minimized environmental impact, and improved load shifting capabilities has resulted in research that addresses each issue independently. Research addressing energy efficiency has focused on component improvements and increased cycle design improvement. Natural and transitional refrigerants have been investigated to address atmospheric impacts. Distributed energy storage research has been undertaken to address load shifting. That is, there are no (or limited) refrigeration technologies that simultaneously provide increased energy efficiency, decreased environmental impact, and energy storage. Recently, Freeze Point Suppression (FPS) cooling systems have emerged that can potentially improve energy efficiency by using waste heat while simultaneously storing energy for load shifting. In their current form, they are used as secondary cooling cycles in refrigeration applications. The working fluid used by FPS systems is a mixture of ice and a suppressant agent. This laboratory study evaluates an FPS cooling system that is capable of cooling and storing energy simultaneously, for later use. The FPS unit under test is a prototype IcePoint™ from Rebound Technologies. The study is completed at Southern California Edison’s Technology Test Centers (TTC) controlled-environment test chambers. Cooling is provided by cold liquid freeze point suppressant that absorbs heat from refrigerated space via a circulating liquid loop. Since ice is required for the system operation, the prototype was built using off-theshelf commercial ice makers. As such, the prototype is not anticipated to achieve high electrical efficiencies. In field installations, on the other hand, the FPS prototype leverages existing Medium-Temperature (MT) refrigeration systems for ice generation. Obviously, that is not feasible in a laboratory setting. A series of tests was designed to evaluate the performance of the prototype. The study entailed testing the prototype using typical summer Dry-Bulb Temperature (DBT) profiles for California climate zones (CTZs) 6, 10, and Honolulu. The project objective was to capture and substantiate the key features of the technology. Performance consistency was evaluated by operating the prototype continuously over a three-day period for each test condition. To test the energy storage capabilities of the prototype, an on-peak profile was input for six hours each day (noon to 6:00 p.m.) via the prototype’s user interface. For all nine days of testing, the data collection involved monitoring and recording 20 data channels with a five-second sampling interval. Key parameters used in this study are electrical power demand, amount of cooling provided, and electrical and thermal efficiencies. The results indicated that the unit was able to continuously provide cooling during both offand on-peak hours over a three-day period, under all three test conditions. This showed performance consistency and capability of the prototype to generate and store enough energy for cooling at prescribed time periods. The daily average cooling provided by the unit ranged between 2,200 British thermal unit per hour (Btu/hr) and 2,600 Btu/hr. Total power demand of the unit during off-peak hours was around 2,900 watts (W) and about 300W during on-peak, which was attributed to the icemaker’s operation. Results demonstrated the system’s ability to take advantage of waste heat at relatively-low temperatures, between 163 degrees Fahrenheit (°F) and 173°F, which is typically unused as a resource to provide cooling. The thermal Coefficient of Performance (COP) of the prototype ranged between 0.1 and 0.2. Additionally, there was a tremendous increase in the electrical COP and accordingly, the amount of cooling provided per unit of power during onpeak hours compared to off-peak hours. The electrical COP was around 0.25, or equivalently 0.8 Btu/hr-W during off-peak hours. However, when icemakers were turned off during onpeak hours, the electrical COP reached up to 2.8 or equivalently up to 9.5 Btu/hr-W. The expected and measured parameters are summarized in Table-ES 1.