| The application of renewable energy in the field of refrigeration corresponds to achieving China’s carbon peaking and carbon neutrality goals.The growing demand for cooling requires sustainable solutions based on renewable energy.To fill the gap between the enormous demand for refrigerated preservation of fruits and vegetables and the research progress of photovoltaic refrigeration and fresh-keeping,a new solar refrigerated warehouse system was constructed.The system adopted the cold energy storage working medium as the energy storage unit,which provided continuous and stable cooling for the solar refrigerated warehouse through the cooperative work of the direct cooling supply and cold energy release and cooling.In this thesis,research work has been carried out on the dynamic energy conversion mechanism of the constructed solar refrigerated warehouse system and the coupling characteristics in the process of cold energy storage and cooling supply.The purpose is to provide methods and references for the construction and engineering implementation of independent photovoltaic refrigeration and fresh-keeping systems.The main research work includes:1.To objectively evaluate and scientifically predict the energy efficiency performance of the solar photovoltaic refrigeration with cold energy storage and continuous cooling supply fresh-keeping system,the solar-electricity-cold energy conversion during the operation of the system was analyzed.Then,the dynamic energy efficiency model of an independent photovoltaic energy system coupled with the vapour compression refrigeration cycle was established,and the key factors affecting the system performance were studied.The results showed that the solar radiation intensity,refrigerant flow rate,compressor frequency,and the dynamic value of the physical parameters of the refrigerant had a linkage influence on the solar-to-cold conversion efficiency of the system.The power consumption of the compressor is stable when the solar radiation intensity is a certain value,and the solar-to-cold conversion efficiency increases by about 1.3%for every 1 Hz increase within the compressor operating frequency range.When photovoltaic direct-drive vapour compression refrigeration,the solar-to-cold conversion efficiency decreases with the increase of solar radiation intensity;an increase of 1 Hz in the operating frequency range of the compressor can offset the negative effect of increasing solar radiation intensity by 5 W/m2on the solar-to-cooling conversion efficiency.The solar-to-cold conversion efficiency of the system is smaller near solar noon and the compressor frequency is larger in the corresponding period,and the maximum cooling capacity can be obtained during this period.When the evaporation temperature is constant and the solar radiation intensity is less than 700 W/m2,the solar-cold conversion efficiency decreases faster with the increase of solar radiation intensity;when the solar radiation intensity is greater than 700 W/m2,it shows an approximate linear negative correlation.When the solar radiation intensity is 1000 W/m2,the solar-to-cold conversion efficiency of the system decreases by 0.45%for every 1℃decrease in evaporation temperature.When the system is operating in real weather conditions,the compressor operates in the range of 1/3 to 4/5 of its fundamental frequency most of the time,and the solar-to-cold conversion efficiency decreases by about 7%for every 100 W/m2increase in solar radiation intensity.The total cooling capacity obtained in the experiment under sunny and partly cloudy weather was 128.83 and122.00 MJ,respectively,and the average solar-to-cooling conversion efficiencies were0.30 and 0.31,respectively.2.The cold energy supply-storage optimization of the photovoltaic direct-drive refrigerated warehouse was proposed,and the control strategies for the photovoltaic refrigerated warehouse to be applied in different scenarios were proposed.Studies show that the necessary conditions for maintaining a constant temperature in a photovoltaic direct-drive solar refrigerated warehouse system are:the maximum value of the cooling capacity should be greater than or equal to the transient cooling loss of the solar refrigerated warehouse.The maximum cooling capacity utilization rate control allows intermittent periods of cooling capacity supply,and the intervention time of direct cooling capacity supply,cold energy storage,and cold release lags behind that of constant temperature control.Comparing the operating results of the constant temperature control and the maximum cooling capacity utilization control mode on sunny days,the difference between the total amount of direct supply cooling capacity and the total amount of cold storage energy was less than 1%,and the total amount of cold supply through release cooling was about 7%less,and the daily total net cold energy storage was 24%more.The ratio of the total amount of net cold energy storage on a sunny day for the three control methods of maximum cooling duration control,maximum cooling capacity utilization control and constant temperature control was about 1:0.93:0.76.3.The theoretical model of the unsteady process of the photovoltaic refrigerated warehouse was established.By analyzing the critical cooling loss coefficient of the refrigerated warehouse system,the temperature variation trend of the refrigerated warehouse can be predicted.During the unsteady cooling process of starting the refrigerated warehouse,the temperature in the refrigerated warehouse shows an exponential decay trend.The unsteady process of the refrigerated warehouse is predicted by segmenting time,and the prediction accuracy is higher.The experimental results of sunny days showed that the correlation increased by 9.1%and the coincidence increased by 3.6%.Under the same ambient temperature,to make the photovoltaic refrigerated warehouse achieve a lower storage temperature,it is appropriate to choose materials with a lower heat loss coefficient for the enclosure structure of the refrigerated warehouse.For the completed photovoltaic refrigerated warehouse,increasing the capacity of the photovoltaic array can make the system have a larger cooling capacity,and also can achieve a lower limit low temperature in the refrigerated warehouse.When the cooling capacity is increased by 25%,the temperature in the warehouse should theoretically decrease by 1/(4Ahlos).Theoretically,for every 10%increase in cooling capacity,the temperature in the refrigerated warehouse decreased by 0.276%,and the measured temperature in the refrigerated warehouse decreased by 0.289%.The established extreme low-temperature model has a good guiding significance for engineering practice in the design and construction of the refrigerated warehouse.The relaxation time of the unsteady heating process is determined by the heat loss coefficient of the refrigerated warehouse and the load capacity;for a two-ton fruit and vegetable refrigerated warehouse,reducing the relaxation time by unit time can be achieved by reducing the load capacity by 116.70 kg or improving the refrigerated warehouse to reduce the heat loss coefficient by 0.0286.4.The energy flow and energy efficiency characteristics of independent photovoltaic refrigerated warehouses with electricity storage and cold energy storage were compared and analyzed.Based on the refrigeration demand,refrigeration capacity,meteorological data of the independent photovoltaic refrigerated warehouse,the proportion of electricity storage and cold energy storage,energy flow,energy efficiency,cost budget,and energy-saving and emission reduction benefits of the two energy storage methods were analyzed.For the independent photovoltaic cold storage with power storage,the instantaneous equivalent cooling capacity obtained by the cold storage has a positive correlation trend with the increase of the direct supply ratio of photovoltaic power generation and the increase of solar radiation intensity;the degree of influence on the equivalent cooling capacity when the supply ratio increases by 8%to 9%.For the independent photovoltaic refrigerated storage with power storage,the instantaneous equivalent cooling capacity obtained by the refrigerated storage has a positive correlation with the increase of the direct supply ratio of photovoltaic power generation and solar radiation intensity.When the solar radiation intensity increases by 10 W/m2,it is equivalent to the effect of the direct supply ratio of photovoltaic power generation increasing by 8%～9%on the equivalent cooling capacity.A decrease of 1 in the refrigeration coefficient of the photovoltaic refrigerated storage system is equivalent to the increase in battery capacity required for an additional 1/4 day of rainy weather.For the independent photovoltaic refrigerated warehouse system with cold energy storage,the effect of solar radiation intensity increasing by 10 W/m2on equivalent cooling capacity is equivalent to that of the direct cooling capacity increasing by 12%～15%.If the refrigeration coefficient decreases by 1,the mass of the cold energy storage medium needs to be increased considering about 3.3 more rainy days.5.When cloudy and rainy days are out of consideration,the initial investment costs of an independent photovoltaic refrigerated warehouse system with electricity storage or cold energy storage are similar.When the number of consecutive cloudy and rainy days N is taken into account,the cost increases;for each additional day of continuous cloudy and rainy days considered,the cost of the independent photovoltaic refrigerated warehouse system of electricity storage increases by about 13.3%;the cost of cold energy storage increases by about 7.1%. |
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