Study On Concentrating Characteristics And Collecting Performance Of Small Scale CPC Microchannel Solar Medium Temperature Collector

The Compound Parabolic Concentrator（CPC）is a kind of non-imaging concentrator without tracking.However,with the increase of the concentration ratio,the maximum receiving half Angle decreases and the effective working time decreases.A tracking CPC microchannel solar collector（TCPCMSC）was proposed to realize heat collection at medium temperature and improve the effective working time of CPC at small scale.The new tracking system drives the two CPC concentrator surfaces to independently track and rotate around their respective base edges as the sun’s altitude angle changes.The optimal tracking rotation angle of the CPC concentrator surface is derived based on the CPC profile and the change in solar altitude angle,allowing the CPC to obtain the theoretical maximum concentration.A two-dimensional CPC concentration numerical calculation model is established based on ray tracing,and the concentration characteristics of the CPC under different operating conditions using the new tracking method are calculated.Based on this,a three-dimensional solar collector unit non-steady-state numerical model is established to simulate the thermal performance changes of the solar collector during all-day operation,and the effects of inlet temperature,flow rate,and absorber length on the overall thermal performance of the system are studied.A solar collector performance test experimental system was built to test the all-day operating characteristics of the solar collector and study its heat collection characteristics under different inlet water temperatures and flow rates.The main content and research conclusions are as follows:（1）The new tracking system enables the CPC to break through the maximum acceptance half-angle limitation.Under tracking mode,a CPC with a concentration ratio of 8 can still obtain an average heat flux density of 5.02 k W?m^-2 when the incident angle is 35°;when the incident angle of light is less than the maximum acceptance half-angle,CPC can obtain a larger aperture width and a larger concentration ratio than when not tracked.The tracking system has low accuracy requirements,and even with large tracking errors,CPC can still maintain high normalized optical efficiency,which helps reduce the cost of solar collectors.（2）The simulation results of the thermal performance of the entire solar collector unit show that high-temperature microchannel absorbers cause natural convection in the air layer,and hot air directly rises to the glass cover plate for heat dissipation to the external environment due to low thermal resistance and fast heat dissipation rate,which is the main reason affecting collector efficiency.Environmental temperature is another factor affecting collector efficiency.The lower the environmental temperature,the greater the heat dissipation temperature difference,and thus heat dissipation increases.Under summer working conditions,collection efficiency can reach up to 87.8%,while under winter environmental temperatures,collection efficiency drops to 82.4%.Therefore,improving collector efficiency should involve weakening internal air natural convection and strengthening glass cover plate insulation performance.（3）The all-day operating experimental test of the entire solar collector unit shows that its thermal performance is good.During all-day operation from 08:30-16:00,most of the time it can maintain a collection efficiency of over 60%.Affected by solar radiation intensity,when solar radiation intensity is high at noon,collection efficiency exceeds 80%.When solar radiation intensity is low,heat gain is less than heat dissipation,resulting in collection efficiency less than 0%.During heat collection process,attention should be paid to changes in solar radiation intensity.When solar radiation intensity is too low,stop running solar collectors to avoid energy loss.（4）Comparing simulation calculations with experimental results shows that reducing working fluid flow rate or increasing inlet temperature will increase outlet temperature of microchannels absorber,resulting in increased average thermal resistance temperature difference and increased heat dissipation rate leading to decreased collector efficiency.Experimental results show that under different operating conditions,non-uniformity degree of outlet temperature distribution for each flow channel of absorber is less than 0.029.Under combined action of new tracking method and microchannel absorber,high-concentration ratio CPC collectors can also obtain more uniform outlet temperature.