EXPERIMENTAL AND NUMERICAL STUDY OF EVAPORATORS FOR MULTI-SOURCE HEAT PUMPS

The present thesis aims at describing the operation of innovative evaporators employed in multi-source heat pumps. The exploitation of multiple sources at the low-pressure side of a heat pump is recently considered to be a good solution to air source heat pumps, whose performance is limited to the external air temperature and to the necessity of periodic defrosting cycles. In order to study the advantage of using multi-source heat pumps, it is necessary to investigate the performance of the selected evaporators at the desired operative conditions: to accomplish this purpose, the results of both experimental activities and simulations realized with ad hoc developed numerical tools are presented in this work. First, a 16 kW air/ground dual-source heat pump working with R32 for heating, cooling and DHW production has been experimentally characterized. The advantage of such configuration is that it is possible to extract or dissipate heat with the most convenient thermal source and the possibility to undersize the ground loop compared to a conventional ground-coupled system. In the framework of this activity, a novel minichannels air-to-refrigerant heat exchanger has been mounted on the heat pump and used as the evaporator. A 5 kW air/solar dual-source heat pump for water heating has also been experimentally studied in this work. When operating in solar-assisted mode, the evaporator consists of three PV-T (photovoltaic-thermal) solar collectors connected in parallel, where the refrigerant (CO2) is directly sent after the throttling process. The main advantage of using a solar evaporator is that, since the solar radiation can be considered as a forced heat flux, the evaporation temperature can be higher than that obtained in an air-to-refrigerant heat exchanger when the solar irradiance is high. Furthermore, the use of a PV-T device can reduce the global power consumption, due to PV electricity production. The presented heat pump prototype has been also designed to work in an innovative flooded-evaporation configuration: in this case, liquid CO2 enters the collectors and the flow is ensured by a natural circulation loop. The main advantage is the possibility to eliminate the maldistribution issues occurring in heat pump evaporators. All the heat exchangers used analyzed experimentally have been numerically modelled in this work. A model of a brazed plate heat exchange, of a conventional finned coil heat exchanger, of an air-to-refrigerant minichannels heat exchanger and of a PV-T evaporator have been realized. The models have been developed in Matlab environment, following a distributed parameters approach: each discretized element is treated as an independent heat exchanger where continuity, momentum and energy equations are solved; in this sense, a “physical” approach has been used. This managed to investigate the operation of the studied heat pumps in non-tested conditions to further optimize the design and functioning of their components. However, all the developed evaporators models are partly based on empirical correlations, in particular for the determination of the heat transfer coefficients on the refrigerant side. Empirical correlations are not always accurate and a well-established predictive model is not available in the literature. For this reason, a section of the present manuscript has been dedicated to presenting the description and the first results of a new CFD method for modelling the evaporation phenomenon in small channels during annular flow with refrigerants. The simulations have been realized in OpenFOAM environment and are based on the application of the Volume of Fluid method.