Vapor compression air source heat pump (VCASHP) systems have been widely applied due to their simple structure and low initial cost. However

there are a series of problems when a VCASHP system is operated for space or water heating in cold regions. For example

with a decline of ambient temperature

the decreased heating capacity of the VCASHP system may be insufficient with respect to the consumer's requirement. Furthermore

its rising compression ratio may lead to an extremely high discharge temperature

resulting in shutting down. Therefore

a great deal of previous research into VCASHP systems has been carried out to address such problems. Nevertheless

the articles concentrated on VCASHP units with direct current (DC) speed regulation are scarce. In the present work

an air source heat pump water heater (5 HP) integrating flash-injection cooling

two-stage throttling

and DC speed regulation (DCAHP) that can be applied to cold and/or severely cold regions over wide-range temperature conditions

is proposed and studied experimentally. The results indicate that the proposed DCAHP performs well over a wide range of temperature conditions and reaches the heating capacity of 16548 W and 11278 W when the ambient dry bulb temperature is 7.0 and -20.0 oC

respectively. Furthermore

the heating energy efficiency ratio (EER) of the proposed DCAHP reached 3.0 and 1.6 individually. Specifically

the compressor input power tends to be reduced by 19% if the dimensionless distance between the thermometer of the expansion valve and the inlet of compressor is greater than 7/200 m/m; however

the valve capacity decline will be inevitable. While the area of evaporator was doubled

the heating capacity of the DCAHP rose by 18% at low ambient temperatures. In addition

the energy efficiency ratio of the DCAHP can be maximally enhanced by 50% with a subtle regulation of refrigeration oil volume of 3/20 kg/kg.