| Abstract [eng] |
The aim of the Master's thesis is to investigate waste heat recovery in refrigeration systems operating with CO2 refrigerant. The project focuses on the analysis of the energy efficiency of refrigeration processes in a grocery store. The refrigeration at the store level consumes the largest share of electricity of all the systems installed in the store. At EU level, heating energy needs account for about 50% of the total energy resources. The aim of this work was to show the still under-utilised waste heat located in the urban surroundings close to residential buildings and CHP networks. Using the unique thermodynamic properties of the CO2 refrigerant, where pressure and temperature can be controlled independently in a supercritical cycle, it was found by mathematical modelling that the refrigeration system can be utilised much more efficiently in winter by switching to heat pump mode, where, despite the reduction in refrigeration efficiency, the heating efficiency can be in excess of COP = 5.7, and the recovered residual heat contributes significantly to the shop heating system. The cost of the recovered waste heat in this case is lower than that of the CHP supply. The simulation data was confirmed by analysis of real grocery store monitoring data, which showed that waste heat can be recovered during the cold season up to 236% more than the amount of electricity used to produce the cold. This heat recovery is insufficient to cover all the store's heat needs, with a shortfall of 132% in the coldest month of the year, and potentially even higher needs when ambient temperatures drop. The analysis found that the refrigeration systems installed in the 6 stores were operating at full capacity, on average only 18% of the time during the winter period, and only 21% of the refrigeration load was recorded on average during the year. The analysis showed that by reducing the downtime of the refrigeration system during the winter, most of its heating demand could be met, and by connecting the shop to the CHP to fully balance this shortfall, the shop could additionally supply excess waste heat to the CHP in the summer to serve the city needs. |
