Development of a Detailed Model of Hybrid System Composed by Air-to-Water Heat Pump and Boiler

Air-to-water heat pumps are one of the most promising and increasingly widespread solutions, despite some intrinsic drawbacks, such as their poor efficiency at low ambient temperatures and at high sink temperatures, e.g., in domestic hot water production. In this context, hybrid heat pump systems combining air-to-water heat pumps and boilers (HSs) have been proposed on the market, especially for the renovation of existing buildings, where high supply water temperatures are typically required. Even though HSs are off-the-shelf technology, the topic has recently gained interest in research. HSs consist of two generators, which must be designed with an integrated approach from the start. However, the performance improvement hinges on the availability of a detailed model able to accurately predict the HS performance. Most of the studies available in the literature use models based on performance maps that are not suitable for HS design. This study presents a new detailed model of a hybrid system, developed in the MATLAB environment. The model adopts a quasi-physical representation of the heat pump cycle and condensing boiler. The boiler model thermodynamically simulates the combustion process, using the Cantera solver and the Gri-Mech properties. The heat pump model simulates the thermodynamic cycle, using refrigerant properties obtained from Cool-Prop libraries. A detailed model for each main component of the system is developed. Component models are combined, thus allowing the user to consider the influence of single components or construction parameters on the overall HS performance. Individual component models were validated against software or performance data provided by manufacturers. The validation proved that the models of the single components can reproduce performance with high accuracy. Therefore, the model can be used for future studies involving HS design, to analyze the influence of construction choices on overall system efficiency.