CO2 heat pumps in controlled ventilation systems

Low-heating-energy houses are characterized by small transmission losses compared to the ventilation losses. For a further reduction of the heat consumption of such buildings it is obvious to reduce the dominating losses by using a controlled ventilation system combined with heat recovery. One concept for the heat recovery system is to use the combination of a heat exchanger and a heat pump. With such a system the heat load can be covered down to outside temperatures of -5 degrees C or even lower, but in cold climates it is impossible to cover also the winter peak loads. However, by preheating the fresh air in a ground collector (EHX)-concrete tubes buried in the ground around the basement of the building-even the peak loads can be covered. This means a monovalent flesh air heating system. The heat pump unit suitable for such an application is a heat pump using CO2 as working fluid. CO2 is a safety refrigerant and direct condensation as well as direct evaporation in the air stream can be realized. In order to have a proper tool for predicting the efficiency and for optimizing the components used in such systems a computer model is being developed at the Institute of Thermal Engineering. First simulation results have shown that such a heat pump exhaust air heat recovery system in combination with an EHX offers overall system COPs between 5 and 10 depending on the outside temperature. The expected overall SPF for the Graz climate is in the range of 6.1 to 6.5.