Thermal Energy Storage for Vacuum Precoolers

Abstract

Vacuum tubes are used to precool leaf vegetables prior to storing or shipping. During the summer months 350,000 cartons of lettuce are cooled and shipped daily from PG&E's service territory. Vacuum tubes are roughly the size of a rail box car, use 274 hp of refrigeration equipment and 225 hp of vacuum pumps. The equipment cycles radically creating high peak demands and low load factors. An ice bank thermal energy storage (TES) and ice water vapor condenser were installed. The existing equipment and TES system were computer monitored to determine energy consumption and potential for load shifting. Using thermal energy storage results in a load shift of 174.5 kw. With TES, total energy consumption and operating costs are reduced by $13,400 per season. This is a result of low off-peak energy rates and increased compressor efficiency at night. The ice bank thermal energy storage system has a 4.4 year simple payback. While building ice, the refrigeration system operated at a 6.26 Coefficient of Performance (COP). The refrigeration system operated more efficiently at night because the compressors ran fully loaded, had a higher average suction temperature, a lower average discharge pressure, and benefitted from increased subcooling. The average on-peak COP was 4.36. On-peak head pressures as high as 204.2 psig were observed. The night-time ice building theoretical average COP is 6.9, only 9.5% better than the on-peak period. However, because the compressors don't idle during the off-peak period, the actual on-peak COP is 24% better then the off-peak period. Product temperature of 37-38°F are attainable using 35-36°F chilled water. This was possible by over-surfacing the vapor condensor, and slightly increasing cycle times.