Conference Year

2016

Keywords

energy storage, subcooling, demand reduction, refrigeration cycle, chillers

Abstract

Integrated Thermal Energy Storage (ITES) is a novel concept in improving cooling performance of air-conditioning systems at peak-load conditions. In contrast to conventional chilled-water or ice storage, it uses stored chilled water to subcool condenser refrigerant liquid instead of supplying cooling directly to a cooling load. For typical R-134a and R-410A systems, subcooling increases capacity by approximately .5 to .7%/°F (~.9 to 1.3 %/K) without increasing compressor input power. Even larger performance improvements are possible with transcritical carbon dioxide systems. The subcooler is preferably a high-effectiveness, counterflow heat exchanger with approximately equal temperature change on both the water and refrigerant sides. This configuration allows warm water to return to the tank at a temperature that approaches the entering refrigerant liquid temperature. For air-cooled systems the water temperature change can be 60 to 80°F (33 to 44 K) or even greater. The large temperature change greatly reduces the required tank size compared to conventional chilled-water storage, which is typically limited to a temperature change of about 10 to 20°F (5 to 11 K). The high temperatures of warm water in the tank combined with lower nighttime air temperatures reduce energy required to cool the tank and improve overall system efficiency in addition to providing a large reduction in peak electric demand. Laboratory demonstration with a nominal 30-ton (105 kW) air-cooled scroll chiller confirmed large performance improvements during subcooler operation. At an ambient temperature of 115°F (46°C), the measured cooling capacity increased almost 50% with a slight reduction in compressor input power.

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