槽式太阳能聚光集热器传热特性分析

Analysis of heat transfer characteristics for parabolic trough solar collector

为了研究槽式太阳能集热器的传热特性及为槽式太阳能集热器的设计提供理论依据,该文分析了槽式太阳能集热器的传热特点,建立了槽式太阳能集热器传热过程一维数学模型:利用该数学模型,计算分析了槽式太阳能集热器的传热特性。选取了2014年9月21日、10月25日的太阳直接辐照数据进行计算分析,10月25日太阳直接辐照数据均值比9月21日高37.5894 W/m^2,9月21日集热器吸收的太阳辐射热能计算均值比10月25日高196.644.W/m:接受管内外壁导热量随内外壁面温差升高而增加,接受管外径与内径的比值大于1.05时导热热阻增加到0.0004679 K/(W·m);接受管和玻璃管之问传热主要是辐射换热,辐射换热量随玻璃管内壁面温度升高而增加:对流换热量数值上可以忽略不计,且与接受管和玻璃管之间的环形空间残存气体类型有关,环形空间为氢气的对流换热量大于空气,空气大于氩气:玻璃管对外界的传热主要是辐射换热和对流换热,环境温度每下降lO℃,玻璃管对环境的辐射放热量增加约105 W/m:玻璃外管壁温度为50℃时,风速为6 m/s比0.5 m/s时的对流换热量增加约116W/m,玻璃外管壁温为80℃时,该值增加约为340 W/m;集热器的瞬时热效率随传热工质温度的升高而下降,随太阳直接辐照增加而升高;利用该文建立的数学模型计算的瞬时效率与美国可再生能源实验室的试验数据最大偏差约为3%。

Parabolic trough solar collector （PTC） is one of the most mature technologies in the medium and high temperature solar thermal utilization field, and PTC is the key component which transforms solar radiation into heat. PTC performance directly affects the performance of solar energy heat utilization system. In order to improve the thermal efficiency and provide the theoretical basis for PTC design, this paper analyzes the heat transfer characteristics of PTC. One-dimensional heat transfer mathematical model of PTC is established, and using this model, heat transfer characteristics for PTR70 2008 type PTC are analyzed. The result shows solar radiation heat absorbed by PTC is significantly affected by the solar incident angle. Heat absorbed by PTC is calculated by direct normal irradiance data in September 21st and October 25th, and the mean direct solar radiation data of October 25th is higher than the data in September 21st by 37.5894 W/m2, solar radiation heat absorbed by PTC in September 21st is higher than the data in October 25th by 196.644 W/re. The heat transferred from the outer absorber surface to the inner absorber surface increases with the increase of temperature difference of the outer and inner absorber surface, decreases with the increase of the ratio of outer absorber diameter to inner absorber diameter. When the ratio of outer absorber diameter to inner absorber diameter is grcater than 1.05, the thermal resistance increases to 0.00046-0.00047 K/（W＇m）. Convection and radiation heat transfer occur between the absorber and the glass envelope. The heat transferred across the evacuated annulus from the outer absorber surface to the inner glass surface through radiation increases with the increase of the temperature of the outer absorber surface. The convection heat transfer between the absorber and glass envelope is very small and plays a negligible role. The heat is associated with annulus gas type, and heat transfer of annulus hydrogen is greater than annulus air and heat transfer of annulus air is greater than annulus argon. The heat transfers from the glass envelope to the atmosphere by convection and radiation. The convection will either be forced or natural, depending on whether there is wind. Radiation heat loss occurs due to the temperature difference between the glass envelope and the sky. Radiation heat loss increases by 105 W when ambient temperature drops by 10℃. The convection heat increases by 116 W when wind speed increases from 0.5 to 6 m/s under the glass envelope outer surface temperature of 50℃. The value reaches 340 W when the glass envelope outer surface temperature is 80℃. The transient thermal efficiency of PTC is significantly affected by heat transfer fluid （HTF） temperature. The transient thermal efficiency decreases with the increase of the temperature of HTF, and increases with the increase of the direct solar radiation. PTC thermal efficiency is calculated by using the mathematical model established in this paper, and compared with the experimental data of the national renewable energy laboratory （NREL） in America, and the results show that the maximum deviation is about 3%. It shows that the mathematical model can reflect the heat transfer law of PTC. The characteristics of heat transfer mathematical model can provide theoretical basis for PTC design and system operation.