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ABSTRACT

This paper describes a single-circuit hot gas bypass defrost strategy, similar to those used in commercial refrigeration applications, for a residential heat pump. Conventional heat pump systems defrost the outdoor coil by temporarily reversing the direction of the cycle such that the indoor unit acts as an evaporator and the outdoor unit acts as a condenser, which dissipates heat to melt frost from the surface of the coil. This requires that the heat pump temporarily ceases to heat the home and actually uses the indoor coil to extract heat during defrosting. The hot gas bypass strategy does not require a reversal of the cycle, but instead bypasses some hot gas from the compressor discharge line through some or all of the circuits of the evaporator (outdoor unit) coil to remove frost. Similar defrosting strategies are successfully utilized in large-scale refrigeration systems such as those used in supermarkets, but these methods are not commonly implemented in smaller systems for the residential or light-commercial markets. A prototype system was developed for investigation consisting of a manifold system that allows hot gas to be bypassed from the compressor discharge line through any or all of the five circuits on the outdoor unit coil. Following the construction of the prototype system, proof-of-concept laboratory testing of the heat pump was carried out. Experimentation confirmed the capability of the hot gas defrosting strategy and provided initial quantitative results of the impact on system performance and energy consumption. The prototype has the capability to maintain partial heating capacity in the conditioned space while simultaneously defrosting the outdoor unit; this can be a substantial advantage over conventional systems. Hot gas bypass defrosting configurations of this type could make heat pumps a more attractive option to many users deterred by the limitations of conventional reverse-cycle defrosting.