Development of a Cyclic Vehicle Heat Pump Frosting and Defrosting Operation Strategy

Heat pumps are used in BEV for energy-efficient heating and to increase range in winter. At low temperatures, frost forms on the exterior heat exchanger surface, which leads to a decline in the system efficiency. The heat capacity is no longer able to be covered by the heat pump and the heat exchanger has to be defrosted. Subsequently, the heat pump operation can start again, which is referred as cyclic frosting / defrosting. Up to now, heat exchanger frosting is prevented in vehicle heat pumps series production by limiting the suction pressure of the heat pump depending on the dew point of the air. A cyclic frosting / defrosting strategy offers a large potential for a more energy-efficient heating and higher winter range. The presentation provides an overview of the three-year research project �cyclic frosting / defrosting of vehicle heat pumps�. In the beginning, the frosting behavior of different micro port extruded tube heat exchanger geometries and coatings is evaluated. It is found out, that heat exchangers with low specific performances or a hydrophilic coating absorb more heat until defrosting is necessary. The work focuses on the development of a method to compute the cyclic frosting / defrosting of a vehicle heat pump. The simulation model is compared with experimental data. Optimized heat pump operating parameters result from the simulation, e.g. the fan speed and electrical auxiliary heating. The defrosting process has a significant impact on the overall efficiency of the heat pump operation and is experimentally investigated beforehand. Defrosting by reverse cycle is preferred. The influence of the operating parameters on the efficiency was determined and optimized by simulation. High compressor speeds and high expansion valve opening achieved the lowest energy consumption. An energetic consideration of the two operating strategies, frosting prevention and cyclic frosting/defrosting, is carried out using the simulation model. It is demonstrated that a cyclic frosting / defrosting strategy with a R-744 heat pump is able to achieve up to 23% more range in city traffic in contrast to a frosting prevention strategy.