Dynamic exergy analysis of an air source heat pump

Exergy flows related to heating and cooling in buildings are very sensitive to the choice of the reference state, since climatisation systems operate very close to the dead state. Some authors recently proposed to adopt the time dependent outdoor conditions as reference state. Since dynamic exergy calculations would be more time consuming than steady state, comparing the results of the two approaches could help identifying the cases when the dynamic methodology should be preferred. Therefore in this paper the yearly behaviour of a reversible air source heat pump (ASHP) providing heating, cooling and dehumidification to a simple building was simulated by means of a dynamic energy simulation software (TRNSYS), taking into account different climates. At every time step, the exergy of the delivered and consumed energy was calculated. Moreover the instantaneous, the monthly and the seasonal exergy efficiency of the ASHP were obtained. Dynamic exergy efficiencies were finally compared with the steady state values and some useful considerations regarding the differences were carried out.