Different from normal refrigeration and air-conditioning systems, air-source heat pump water heater (HPWH) is a special refrigeration system involving higher condensing temperatures and a wide range of evaporating temperatures. The capability to reliably operate under high compression ratio, high discharge temperature, and high load has been the main challenge for the compressor design. The scroll compressor has been proved for its superior reliability in both refrigeration and air conditioning applications. This paper presents the advantages of the scroll compressor in heat pump water heater applications. Enhanced Vapor Injection (EVI) technology improves the system performance across the entire operating envelope and the compressor reliability at low ambient temperatures. The major contributions from the EVI technology are discussed in details. A prototype system with a flash tank as the economizer has been tested at various conditions. The performance optimization of the flash tank in the heat pump water heater system is discussed and the system testing data is presented.
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