Improvements on the effiency of an autonomous commercial refrigeration system that uses low GWP fluids by the development of a thermoelectric subcooling system
Authors:   , Casi Satrústegui Álvaro 
Published: October 2022

The refrigeration sector plays a critical role in the development and well-being of society by aiding in the conservation of perishables and providing thermal comfort in vehicles and buildings. Due to its undeniable importance and necessity, the vast volume of this sector is linked to a considerable impact on the environment. The demand of refrigeration is expected to grow over the next decades and according to the most recent estimations, the volume of the sector could double by 2050. This fact, alongside the current environmental problematic, reveals the importance of reducing the environmental impact of refrigeration systems. The environmental impact of a refrigeration system is produced by the emission of greenhouse effect gases onto the atmosphere which contributes to the global warming of the planet. The emissions are related to the electrical consumption of the refrigeration system and the leakage of refrigerants with high global warming potential. Therefore, the two main lines of action are the use of natural refrigerants with almost negligible global warming potential, such as carbon dioxide, and the development of more efficient systems in order to reduce the electrical consumption of refrigeration facilities. The present Ph. D. dissertation focuses on the development of a thermoelectric subcooling system to improve the performance of a vapour compression refrigeration cycle that uses transcritical carbon dioxide as a refrigerant. This combination of technologies presents itself as a novel and promising solution, possible due to the robustness, scalability, controlability and versatility that thermoelectric systems offer. Firstly, a computationl model has been developed. It simulates the behaviour of a transcritical carbon dioxide vapour compression cycle working with a thermoelectric subcooler. The model has been experimentally validated and the results show that it is capable of representing a real system with deviations for the main outputs between the +/-7% interval of error. Then, the computational model is used to aid in the design and to study the working principles of thermoelectric systems in order to maximize the performance of the global refrigeration system. A thermal characterization of the heat exchangers used in the thermoelectric subcooler has been performed and with the aid of the computational model developed, the impact that those heat exchangers have in the global refrigeration system has been quantified. The results demonstrate that when optimized heat exchangers are included in the thermoelectric subcooler, the enhancement in the cooling capacity of the system rises from 21.4% to 26.3%. Likewise, the improvement obtained in the coeficient of performance of the cycle rises from 11.96% to 14.75%. These results display that the heat exchangers of the thermoelectric subcooling system have a critical impact on the performance of the global system. Considering the information obtained with the computational model, a thermoelectric subcooling system is designed, built and included in an experimental vapour compression facility that uses carbon dioxide in transcritical state as a refrigerant. The experimental plant is tested for multiple climatic and operation conditions to prove through experimental results the effect of including a thermoelectric subcooling system in the global performance of the refrigeration facility. In addition, the results with the thermoelectric subcooler are compared with the inclusion of an internal heat exchanger, a commonly used technology to boost the performance of vapour compression refrigeration cycles. The results show that, in terms of cooling capacity and coeficient of performance, the improvements reported with the thermoelectric subcooler surpass the ones obtained through the inclusion of the internal heat exchanger. Using the thermoelectric subcooler the cooling capacity of the refrigeration system is enhanced by 20.8% and the coeficient of performance is boosted by 16.2%. Moreover, considering the versatility and controllability of the thermoelectric subcooler, this system is used in combination with the internal heat exchanger to test the performance of the cycle working with both technologies at the same time. The inclusion of the thermoelectric subcooler in combination with the internal heat exchanger reported a maximum improvement in the cooling capacity of 22.5% and and increase in the coeficient of performance of 22.4%. These results showcase that improvements obtained with both technologies working together surpass the ones obtained with each of the technologies on their own. The results of the present Ph. D. dissertation demonstrate that the inclusion of a thermoelectric subcooling system is a viable solution to boost the performance of vapour compression refrigeration systems that use natural refrigerants, reducing the emission of greenhouse gases from refrigeration systems and contributing to the production of cold in a sustainable way.