Heat pumps can drastically reduce energy requirements in industry. Operating a compression resorption heat pump with an NH₃-CO₂-H₂O mixture has been identified as a promising option that can have an increased performance compared to only NH₃-H₂O. In this paper an important process of the heat pump cycle is investigated: The absorption process. A mini-channel heat exchanger with 116 tubes of inside diameter of 0.5 mm is used for this purpose. For the NH₃-H₂O experiments overall heat transfer coefficients of 2.7–6 kW/(m²K) were reached for mixture mass flows of 0.71–2.5 kg/h. For the NH₃-CO₂-H₂O mixture pumping instabilities limited the operating range which resulted in higher pressures and higher mixture mass flows compared to NH₃-H₂O. The overall heat transfer coefficients were lower in the case of the added CO₂, with the maximum of 3 kW/(m²K) corresponding to a mixture mass flow of 4.2 kg/h. However, an increase in heat transfer of approximately 5% was reached with the added CO₂ which is beneficial for heat pump applications. Additionally, limited research has been conducted on absorption in upward versus downward flow and, therefore, these two configurations have also been tested in the mini-channel heat exchanger. Even though the pumping instabilities vanished with absorption in upward flow it was confirmed that absorption in downward flow with the mixture on the tube side is the most beneficial configuration for absorption of ammonia in NH₃-CO₂-H₂O or NH₃-H₂O in a mini-channel heat exchanger. The performance increased by approximately 10% with absorption in downward flow.

CO2 is a promising refrigerant compared to traditional HFCs due to its insignificant global warming potential and nonthreatening to the ozone layer. It has been used as refrigerant in industrial and commercial refrigeration in recent years. With high compactness and the ability to recover heat, CO2 booster systems have been widely installed in newly constructed supermarkets in the Netherlands. One remarkable advantage of this system is that great amount of heat can be recovered from the gas cooler for heating use due to high temperature driving force from CO2. Sometimes the COP is sacrificed to fully satisfy the heating demand. Within the present work, a quasi-steady-state computer model has been developed to study the performance of the system based on a typical Dutch supermarket. The model has been validated using experimental data. By altering condensing pressure and gas cooler capacity using different methods, various control strategies to satisfy both cooling and heating demand have been proposed and compared. The results from the simulation illustrate that some control strategies have lower energy consumption and easier operation compared to the others. The study also shows that CO2 booster refrigeration system has a good potential for heat recovery. It has the potential to save 13% primary energy compared to conventional heating method in supermarkets.

Compression resorption heat pumps (CRHP) are a promising option to upgrade waste heat from industry. Alternative working fluids can further improve the efficiency of CRHP. The ternary mixture NH3-CO2-H2O has been identified as a promising working fluid for CRHP and has the potential to further enhance the coefficient of performance (COP) of the cycle compared to the traditionally used ammonia water mixture. So far the studies on the NH3-CO2-H2O mixture have focused mainly on carbon capture applications. But the desired operating conditions are different than for CRHP applications, e.g. the NH3 concentration. Additionally the absorption process with the mixture in tubular absorbers has not yet been reported. The focus of this study is therefore to investigate experimentally the absorption process of a CRHP with this ternary mixture. To reach this goal a model is developed for ammonia-water that takes into account the kinetics and mass transfer during the absorption process. To validate the model, experiments were performed for an absorption process in a mini channel heat exchanger with NH3 concentration of 35 wt%. The results show a good match between the model and the experiments. Additionally CO2 has been added to the solution and the experimental performance was compared with the experimental performance of the NH3-H2O mixture. A concentration of 2 wt% CO2 resulted in a performance increase of up to 5% however the working fluid flow became limited by pumping instabilities.