Experimental analysis of advanced R744 refrigeration system

Carbon dioxide is one of the oldest refrigerants, as it was used widely at the end of the nineteenth century. However, around 1931 it started its decline when synthetic refrigerants came into the market showing a higher efficiency and a cheaper implementation in the refrigeration systems, in terms of equipment. Nonetheless, carbon dioxide as a refrigerant is gaining more and more space over the last years due to its thermodynamic properties, the capability to be an optimal refrigerant for many applications and its usage reduces the emissions of greenhouse gases (GHG). Its wide implementation is confirmed by the fact that the energy demand of supermarkets can be fully satisfied by subcritical or transcritical cycles, depending on whether the climate is cold or hot respectively. In this thesis, the background of the description of refrigeration cycles using R744, both with ejector and not, are introduced at the beginning of the first chapter. The chapter continues with the description of Cold Thermal Energy Storages, i.e. CTES, which are currently investigated as an optimal solution that grants an enhancement in terms of energy saving. This technology allows energy production and storage when the electricity prices are at their lowest (when charged during the night), in order to provide further cooling capacity when the electricity prices are at their highest (during the day, when people come back from work) and to smooth the usage peak which normally happens over the afternoon. This Master Thesis project is focused on two different tasks. The first part of the Master Thesis is dealing with the investigation on the pivoting technology which is capable to reduce the number of compressors installed in the rack by using at the suction port of some compressors two valves that switch according to their position and the need. The second part is dealing with the design of a CTES located on the top of a supermarket’s display cabinet. The first task was planned to start from a numerical simulation to simulate the performance of a simplified refrigeration system using the polynomial equations for each compressor, ending with an experimental campaign that has been carried out at the SuperSmart-Rack at NTNU, Trondheim. The investigation of the pivoting technology has been done considering two different high-pressure (HP) control devices, as high-pressure valve and high pressure lift multi–ejector block, and many different temperatures at the outlet of the gas cooler to reproduce different ambient conditions. As it has been seen during the experimental campaign the most critical situation to predict accurately is when the HP multi-ejector is in operation because of the performance of the block itself. This allowed also to highlight the need to improve the ejector, since it is an expensive technology and can be used only over a certain period of the year, depending on the geographical area. From the obtained results, the pivoting enhances the flexibility of the rack lowering the number of compressors installed by one, and when the multi-ejector block is used to control the high pressure the number can be reduced further by another compressor. However, the implementation of the ejector without pivoting technology would be unsatisfactory as the number of compressors needed would raise. Because of the ejector, a lot of capacity is shifted to the parallel side. A common practice will be to include a new compressor to cover the load, that it will be used for few operating conditions. Moreover, no degradation has been seen when pivoting are working, leading to the same power consumption and COP. The second task of this Master Thesis is the modeling of a phase change thermal storage, that uses R744 as refrigerant, and water as the phase change material. The task required the design and the numerical simulation of the phase change process that occurs inside the CTES during the discharging stage. As reported widely in literature, the key factor of a two-phase thermosyphon loop is the investigation of the pressure losses over all the circuits. The cabinet available at the NTNU/SINTEF laboratories in Trondheim was used as a reference for the installation of the CTES on the top of the cabinet itself, considering the evaporator installed with its own geometric and construction features. The CTES has been modeled to match the duty of this evaporator, as well as the liquid and vapor line that linked the inlet-outlet of the evaporator with outlet-inlet of the CTES. It was concluded that, because of the low heat transfer in the CTES, stable and effective use of the thermosyphon principle can be used for a higher temperature application, for instance, the conservation of vegetables. It has been proven that a very small temperature difference between CTES and the evaporator leads to a huge heat transfer area. The cold storage was designed like a tank with circular finned over smooth tubes and is used to condensate the vapor coming from the evaporator. A liquid head of approximately 1.8 meters is used to overcome all the pressure losses in the system. The design was followed by numerical simulation of the implemented system.