heat pump, defrost, hot gas bypass, control strategies, coil coatings
Heat pumps are an energy efficient way to provide adequate heating to an indoor space. In contrast to electric or gas heating, a heat pump works to draw the "free" heat from the cold outdoor ambient and transfer thermal energy to the heated space. When the outdoor ambient is cold and humidity conditions are right, however, frost will start to develop on the heat pump’s outdoor coil. Frost on the coil surface blocks air flow through the coil thus reducing the heat pump’s efficiency and overall performance. Control of frosting and defrosting is particularly important to the successful use of heat pumps in cold climates. To restore the unit’s heating capacity and efficiency, a defrost cycle is needed to remove the accumulated frost developed on the coil. While defrosting restores heat pump efficiency, this period of operation itself requires additional power and ultimately results in an energy penalty. Improving defrost performance and/or reducing the number and duration of required defrost periods would significantly improve heat pump operation. Heat pump researchers and manufacturers have spent a great amount of effort on the topic of defrosting. A comprehensive review of efforts within the last 15 years was conducted to summarize existing defrosting technologies and identify those solutions that may be more feasible or readily commercialized to reduce the defrost penalty. This summary highlights technologies that may reduce or eliminate the negative impacts of the defrosting period including approaches using hot gas bypass, coil coatings and advanced control strategies. The potential impact of the identified technologies is energy and demand savings, improved performance for comfort, and removal of a significant barrier to widespread adoption of air-source heat pumps in all climates.