Investigation of compressors and dual-source heat pumps working with natural refrigerants

This thesis focuses on the investigation of the performance different types of compressors and the study of dual-source heat pumps using natural refrigerants. Compressor efficiency is a crucial factor in improving system performance and is influenced by the operating conditions. Therefore, experimental studies were conducted on three different compressors. The first was a rotary compressor using CO₂ as refrigerant, where the effects of compressor speed, high pressure, and evaporation temperature were analyzed, leading to the development of new performance correlations. The second was a large swept volume scroll compressor working with low-GWP refrigerants, with experimental data compared to existing numerical correlations. Lastly, a reciprocating compressor operating with propane was tested to investigate the impact of suction superheating. Additionally, a preliminary 3D CFD model of a reciprocating compressor was developed to investigate the heat transfer across the wall separating the suction and discharge chambers. The first type of compressor was tested in a dual-source heat pump. Utilizing dual-source systems presents an effective solution for air-source heat pumps, which are typically limited by low outdoor air temperatures. An experimental study was conducted on a solar-air dual-source heat pump using CO₂ as the refrigerant, designed to exploit air and solar radiation as thermal sources. This system features two evaporators: a finned coil heat exchanger and three PV-T collectors, which can operate individually or simultaneously. The configuration allowing the simultaneous use of both evaporators is entirely novel and enhances system performance by eliminating the need to manage a switch between the two sources. Performance comparisons were made between the three different evaporation modes: air-mode, solar-mode, and an innovative simultaneous-mode. A numerical model of the solar-air dual-source heat pump was developed and validated against experimental data. The model can evaluate performances in different operating conditions. Each component was modelled using a physical approach, which enabled the creation of a new model for heat pumps using CO₂ and propane for hot water production in the building sector. This comprehensive model facilitates the design of both the heat pump and the water circuit, while also enabling the simulation of the system's seasonal performance under various thermal demands and in different locations.