| Concerntrated solar thermal technology in high temperature is widely used in many industrial and agricultural processes, including solar thermal power generation. As solar energy changes with geography, day and night, seasons, clouds and rain, and other random factors, a variety of solar thermal systems are usually equipped with a thermal storage system, in order to ensure its stability and continuity of heating, which is particularly important to light concerntrated solar thermal systems of high temperature. In present the technology of high temperature solar collector has become more mature, so thermal storage technology plays an important impact on the performance of high temperature solar thermal system, and there is a certain room for its development.This article first proposed the concept of Al-Si alloy heat storage body which is of solar energy absorbing, thermal storage and heating outside, used in the light concerntrated high-temperature solar heat utilization system. Al-Si alloy as phase change material (PCM), can directly absorb solar radiation reflected by the condenser and save them to achieve the purpose of endothermic and thermal storage (in the back the heat absorbing and heat storage process of the heat storage body are unified as the process of heat charge). There is no heat transfer fluid (HTF) between the collector and heat storage body, and piping systems can be greatly simplified, and the reliability can be improved too. HTF is used to heat outside of heat storage body in need.Heat storage body is the main equipment in the high-temperature solar heat utilization system which has the functions of solar absorbing, thermal storage and heating, so it is the focus of this study. The research included the temperature changes and heat transfer temperature difference of Al-Si alloy and its shell, HTF and its pipe in heat charge and heat discharge process at different power of charging and discharging, and also including melting and solidification speed of Al-Si alloy. The above contents were studied in three aspects of theoretical analysis, numerical simulation and physical test.In order to obtain the characteristics of this type of heat storage body in heat transfer process, at first the paper used the basic theory and knowledge of heat transfer to analysis heat transfer mechanism, including thermal conductivity and absorbing of the heat-absorbing surface, natural convective heat transfer of melted liquid Al-Si alloy, pumped convective heat transfer in tube or boiling heat transfer of HTF, thermal conductivity of the wall and solid Al-Si alloy, etc. The differential equations and boundary conditions were obtained, and some instructive conclusions were drawn out:1) In the process of heat charge the main thermal resistance is the thermal resistance of the thermal conductivity at the heat-absorbing surface; 2) The intensity of natural convection of liquid Al-Si alloy depends on the size of the Grashof number in the both process; 3) In the process of heat discharge the main thermal resistance is the resistance of wall of the HTF tube and thermal conductivity of the solid Al-Si alloy.The numerical simulation can be more efficient and save a lot of working hours. In this paper, a variety of Al-Si alloy thermal storage structure were simulated for used in the light concerntrated high-temperature solar energy utilization system, including:1) Upright cylindrical Al-Si alloy heat storage body were simulated with its physical and mathematical models which were suitable for dish concentrating solar thermal system applications, the temperature at the bottom and top were calculated while it was heated at the bottom, cooled at the top, the relationship of temperature with the diameter and height of the heat storage body were discovered; 2) Upright cylindrical Al-Si alloy heat storage body were simulated with its physical and mathematical models which were suitable for tower concentrating solar thermal system applications, the temperature of Al-Si alloy were calculated while it was heated in half of the circumference, cooled at center tube, the relationship of temperature with the diameter and height of the heat storage body were discovered; 3) Horizontal cylindrical Al-Si alloy heat storage body were simulated with its physical and mathematical models which were suitable for trough concentrating solar thermal system applications, the temperature of Al-Si alloy were calculated while it was heated in bottom half of the circumference, cooled at center tube, the relationship of temperature with the diameter and height of the heat storage body were discovered; 4) Tube in Al-Si alloy heat storage body were simulated with its physical and mathematical models which can be flowed by HTF such as air. The air temperature along the tube and convective heat transfer coefficient in the tube were calculated with different velocity.The experimental studies included two types. One is simulation test, the other is prototype test. Simulation test used electric heating device instead of high heat flux of solar radiation in the laboratory to heat Al-Si alloy heat storage body in order to obtain the temperature changes of Al-Si alloy heat storage body and HTF in heat charge and discharge process. The results can be compared with the numerical simulation data and calculated the thermal storage efficiency of the heat storage body, provided the basis for the design of Al-Si alloy thermal storage body. These included:1) Vertical cylindrical Al-Si alloy heat storage body was experimental studied of disc high-temperature solar thermal systems, the size of experimental body were φ300×300mm,φ300 × 200mm,φ200 x 200mm, the thickness of the heat-absorbing surface of 5mm, respectively, at 100kW/m2 and 200 kW/m2 heat charging and discharging heat flux density, it was found that the experimental result was close to the numerical calculation.2) Horizontal cylindrical heat storage body was experimental studied of trough high temperature solar thermal system with tubular resistance furnace. The size of heat storage body is φ89 (32) x 900mm (tube diameter 89mm, internal tube diameter 32mm, tube length 900 mm), wall thickness of 3 mm, respectively, in the 20kW/m2 and 50kW/m2 heat charging and discharging heat flux density, it was found that the Al-Si alloy temperature and its melting, solidification situation were in line with the need of heat charging and discharging.3) By the use of a tubular resistance furnace to simulate the thermal storage temperature of Al-Si alloy, the tube convective heat transfer with large temperature difference was experimental studied of three HTF such as air, water, steam and thermal oil. Experiments were done in two diameters, four flow rates and two wall temperatures. The results were sorted into the experimental correlations of convective heat transfer within the tube of large temperature difference which can be used to the heat pipe design in Al-Si alloy heat storage bodies.4) By the use of two electric phase change energy storage heating device with the heat storage capacity of 200 kWh and 50 kWh respectively, the heat discharging process were experimental studied.The experiments show that large device is mostly above 650K external heating temperature with the heating power of 8.1-15.5kW, and the total thermal storage efficiency is 91.7%; while small device is mostly above 540K external heating temperature with the heating power of 2.15-3.3kW, and the total thermal storage efficiency is 88.3%.Prototype test was carried out with a steam boilers of Al-Si alloy thermal storage by the use of a dishes solar concentrator in the outdoor, including development, testing structures, multi-disc thermal efficiency of solar concentrators test, heat transfer performance test of high temperature heat pipe with the Al-Si alloy and Al-Si alloy thermal storage steam boiler performance tests, to get the performance parameters of Al-Si alloy heat storage body in the actual case runtime, to found the basis for the Al-Si alloy heat storage body in high-temperature solar thermal system. As follows:1) According to solar radiation data in Guangzhou area, combined with the performance parameters of dishes solar concentrator, a steam boiler of Al-Si alloy thermal storage was designed, manufactured and installed, then a test system was built for the Al-Si alloy thermal storage steam boiler. The boiler’s design parameters is 10 kWh thermal storage capacities,3.5Mpa/415℃ superheated steam.2) By the use of simple receiver, the taotal thermal efficiency was tested of the multi-dish solar concentrator including simple receiver. The receiver static average thermal efficiency is 45.6% in 0-600W/m2 direct solar radiation. The efficiency can be used as the basis of the boiler design and its thermal efficiency evaluation. Receiver efficiency equation in the non-steady-state is3) By the use of heat transfer test platform of high temperature heat pipe with Al-Si alloy, its heat transfer performance was tested. The result shows that:the heat transfer density of evaporator section is 54.4kJ/m2 while the heat transport power of heat pipe is 0.75kW. The result provides a basis for the application of high temperature heat pipe in Al-Si alloy thermal storage steam boiler.4) Al-Si alloy thermal storage steam boiler was tested particularly and totally. The facula temperature of the boiler, Al-Si alloy temperature, water vapor pressure and temperature, the boiler thermal efficiency and other parameters over time (solar radiation) were recorded in the process of an average solar radiation of 445-765 W/m2, instantaneous radiation 200-900W/m2. The first-hand information was gotten about this type of boiler runnig in the actual operating conditions which lays the design foundation of the boiler.Al-Si alloy has a larger thermal conductivity and heat storage density, so the Al-Si alloy heat storage body with the function of absorbing, thermal storage and heating outside will greatly simplify the thermal heat storage system of the high temperature solar system, reducing its initial investment and operating costs, improving its operational efficiency and the ratio of income-cost of the entire system. This Al-Si alloy heat storage body for high temperature solar thermal system provides a new direction of heat storage technology development. The result shows that Al-Si alloy heat storage body has potentially enormous economic value and broad application prospects in the field of high temperature solar thermal utilization. |
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