A Research For The Thermal Performance Of Solar Collector

The factors affecting the thermal performance of solar collectors are varied, including the external impact conditions and the influence of its running state. The effect of dust accumulation on the performance of solar collector is very important, should not be neglected. The water flow rate through the solar collector is not con-stant always, when the system is running. Changes of the water flow rate make an important impact on the thermal performance of solar collectors. The steady-state test method and dynamic test method for solar collector are under the conditions that the flow rate changes little when testing the thermal performance of solar collector. So, the relation between steady-state test method and the water flow rate is analyzed, and the hot losses caused by the changes of water flow rate are added to the dynamic test method. It can better predict the thermal performance of solar collector.In this paper, the appearance and the amount of the dust accumulation on the surface of flat-plate collector, vacuum tube collector, Heat pipe vacuum tube collec-tor and CPC reflex plate collector is analyzed. The difference of the dust accumu-lation on the performance of these four kinds of solar collectors is analyzed. The influence of the dust accumulation on the performance of flat-plate solar collector is studied detailed. The effect of dust on the instantaneous efficiency, stagnation temperature and temperature rise of flat-plate solar collectors is analyzed. The efficiency of solar collectors with and without dust in steady-state condition is measured. The stagnation test is carried out. The stagnation temperature of solar collectors is measured. Furthermore, daily test is conducted. The efficiency, dust shading coefficient and temperature rise of solar collectors are calculated by using experimental data. The results show that the collection efficiency of flat-plate solar collectors, solar vacuum tube collectors, heat pipe vacuum tube solar collectors and CPC reflex plate solar collectors decrease respectively by9.69%、15.45%、20.95%、35.21%. The effect of dust on Flat-plate solar collector is major in transmittance of glass. The dust shading coefficient is0.867. The dust heat loss coefficient factor has little effect on collector efficiency, generally less than2%o. Dust accumulation has effect on the stagnation temperature, that is, it influences the heat-collecting quality in certain environmental conditions. The stagnation temperature of solar collectors without dust is7.2℃higher than that with dust in one time. The difference between the temperature rise of collectors without dust and that of collectors with dust also varies between0and1.0℃. The temperature rise difference for both collectors with and without dust is the highest at noon during the whole day.The effect of flow rate on the performance of flat-plate solar collector is researched. The fluid inlet and outlet temperature of solar collector is measured, and the efficiency of solar collector is calculated in different water flow rate, heat removal factor and overall heat loss coefficient are also calculated, the trend of solar collector efficiency、heat removal factor and overall heat loss coefficient changed with mass flow rate is observed. Based on the fitted regression equations about heat removal factor and mass flow rate、overall heat loss coefficient and mass flow rate, the relationship of thermal efficiency and mass flow rate is built. The results show that the new test method for solar collector can reflect the efficiency of solar collectors clearly when the mass flow rate changed, and the best flow rate can be calculated clearly.From the structural characteristics of flat-plate solar collector itself, its heat transfer mechanism is analyzed. The hot losses caused by the changes of water flow rate are considered. The hot losses caused by the changes of water flow rate are added on the basis of the existing dynamic test method, and the coefficient is determined according to the experimental data. The heat which the flat-plat solar collectors gain under different working condition is calculated. Steady-state condition, dynamic test condition which flow rate is constant, dynamic test condition which the difference between import and export temperature of solar collectors is constant, dynamic test condition which the export temperature of solar collectors is constant, dynamic test condition which the dust accumulation on the surface of flat-plate solar collector, these five conditions are considered. The predicted results compared with the actual value. The result shows that adopting this method into the test for the thermal performance of solar collectors dose not set the limitation to the values and deviations of measured parameters such as solar irradiance, surrounding air temperature and fluid temperature at the collector inlet during a measurement period, the thermal performance of solar collector change with water flow rate also can be described clearly. By mathematical reasoning the dynamic test model proposed in this paper owns backwards comparability to long agreed steady model. Especially in the case that the input variables of the system are not steady, therefore, both mathematical reasoning and the data of the experiments justify the validation and reliability.