Study On The Performance Of Medium Temperature Air CPC Solar Collector

The global energy crisis and environmental pollution are becoming more and more serious,and the demand for practical,efficient and clean energy in the industrial field is increasing.Renewable energy sources that can replace traditional fossil energy are urgently needed.As the most widely distributed renewable energy source,solar energy can be converted into thermal energy through heat collecting devices for industrial production.Compound Parabolic Concentrator(CPC)Is an ideal concentrator designed according to the theory of edge light.It has the advantages of acceptable partial scattered light and low cost,and has broad application prospects.The uneven distribution of energy flow on the receiving surface results in low photothermal conversion efficiency of the system,uneven temperature distribution of the conversion device,thermal stress damage,and reduced receiver life.In view of the above analysis,this paper proposes a new type of medium temperature air solar concentrator(referred to as segmented CPC),and studies its photothermal characteristics.The specific contents are as follows:Firstly,aiming at the uniform distribution of energy flow density on the receiving surface,the structure of traditional CPC is optimized by using parabolic optical theory,infinitesimal theory and geometric construction method,and two new segmented CPC concentrators(type Ⅰ and type Ⅱ)are designed.The segmented CPC profile is deduced and calculated,and the expressions of type Ⅰ and Ⅱ CPC concentrator profiles under different parameters(segmented number,geometric concentration ratio,receiving surface width)are given.Secondly,the optical characteristics of the segmented CPC concentrator are simulated and analyzed by Trace Pro optical software.The influence of the number of segments and the incident angle on the energy flux density distribution on the receiving surface of the new concentrator is studied.The results show that the inhomogeneity S of the energy flow density of the segmented CPC receiving surface decreases with the increase of the number of segments.The inhomogeneity S = 20.08 when the number of segments of the type Ⅰ concentrator is 10,which is significantly smaller than the traditional CPC inhomogeneity S= 51.93.When the number of segments of the type Ⅱ concentrator is 20,the inhomogeneity S(S = 21.93)is smaller than the traditional CPC inhomogeneity S.It shows that the segmented design makes the energy flow density distribution of CPC receiving surface have good uniformity.The energy flux density gradient Max value of the segmented CPC receiving surface decreases with the increase of the number of segments.The energy flux density gradient Max = 112032.36 when the type Ⅰ concentrator has 10 segments,which is significantly smaller than the energy flux density gradient Max value of the traditional CPC.The segmented CPC effectively reduces the thermal stress damage of the receiving surface.Under the same number of segments,the uniformity of energy flow density on the receiving surface of type Ⅰ concentrator is better than that of type Ⅱ concentrator.Considering the factors such as economy and stability of the collector system,this paper selects type Ⅰ 10 segment concentrator as the specific research object to carry out subsequent experimental research.Based on the analysis results of optical characteristics,a segmented CPC prototype was designed,and a segmented CPC concentrating heat collection test system was built.The temperature field distribution test of the receiving surface and the influence of irradiance,air velocity,inlet temperature and other parameters on the heat collection efficiency were carried out.The experimental results show that the average value of the unevenness of the temperature field of the segmented CPC is 0.06,which is significantly smaller than the average value of the unevenness of the temperature field of the traditional CPC 1.67,which indirectly verifies the above optical simulation results.Under different air flow rates,the outlet temperature of the collector is consistent with the change trend of solar irradiance.The average heat collection efficiency of the collector increases with the increase of air flow rate.When the flow rate is 6.5m / s,the maximum value is 71.5 %(segmented CPC)and 59.9 %(traditional CPC),which is 10.8 %,1.6 %(segmented CPC)and 7.9 %,3.4 %(traditional CPC)higher than that of 5.5m / s and 6m / s.At different inlet air temperatures,the average heat collection efficiency of the collector decreases with the increase of the inlet temperature,and the heat collection efficiency of the segmented CPC is greater than that of the traditional CPC.When the inlet air temperature is 80 ℃,the heat collection efficiency of the collector is 43.3 %(segmented CPC)and 33.1 %(traditional CPC),which is 11.6 %,7.2 %(segmented CPC)and 10.9 %,6.5 %(traditional CPC)lower than the inlet air temperature of 60 ℃ and 70 ℃.The simulation and experimental results of optical properties show that the new segmented CPC concentrating system has better heat collection performance.This study provides theoretical support and experimental reference for the application of solar medium temperature air heating in industrial production.