Design And Performance Of Full Glass Vacuum Tube And CPC Collector

All glass evacuated solar tube (EST, in short), popular in solar market of China,cocnsits of two concentric tubes sealed at one end and the absorber is located withinthe cover tube. But compound parabolic concentrators (CPC, in short), designedbased on the edge ray principle and identical optical length principle, extend theirreflectors all the way to the absorber, thus, gaps must be left between the reflectorand absorber to accommodate EST. In this work, six CPCs were designed forconcentrating radiation on ESTs with the inner tube of47mm and cove tube of58mm in dianmeter (47/58tubes). These designs are: CPC-1, designed based thecover tube; CPC-2, designed based on the inner tube with the EST beingpurposefully move up R-r; CPC-3, designed based on the inner tube with thereflectors near the absorber being truncated; CPC-4, designed based on “ice-cream”shaped virtual absorber with the actual absorber of inner tube; CPC-5, designedbased on the hat-shaped virtual absorber with the actual absorber of inner tube;CPC-6, the same as CPC-5but with a V-groove at the bottom of reflectors. Theoptical performance of these designs was investigated in terms of optical lossesthrough gaps, optical officieicy and annual radiation on ESTs by theoretical analysisand ray tracing simulations. A prototype of solar heater based on CPC-5and ESTswas fabricated and experimentally tested in outdoor conditions. The objective of thiswork is to find the optimal design of CPC suitable for EST and develop a CPC basedsolar heater with the EST as the receiver operating at medium temperature.Theoretical calculations showed that the CPC-6has no optical losses throughgaps thus is the best, the next best is the CPC-4, and the CPC-1is the worest in termsof optical losses averaged over the acceptance angle of full CPCs.Ray-tracing results showed that, the optical efficiency of CPC-6was highest andthat of CPC-1was lowerest within the acceptance angle of CPCs, but as theprojection incidencen angle of solar rays on the aperture of truncated CPCs beyondthe accpeancen angle, the optical efficiency of CPC-1was highest and that of CPC-6 was lowerest. In terms of optical efficiency averaged over the acceptance angle ofCPCs, CPC-6was the best and CPC-1worst, the nest best one was the CPC-3for sixfull CPCs and the CPC-4for six truncated CPCs with identical geometricalconcentration.In terms of annual collectibe radiation on ESTs, in the case of east-west orientedCPCs with the tilt-angle of aperture being yearly fixed, for full CPCs, the CPC-4wasthe bset and CPC-5worst, and for trauncated CPCs with identical Ct, the CPC-2wasthe best and CPC-1worst. Whereas in the case of tilt-angle of CPCs’ aperture beingyearly adjusted three tilts, for six full CPCs with identical acceptance angle, theCPC-1was the best, the next best was CPC-4and the CPC-5was the worst; but forsix truncated CPCs with identical Ct, the CPC-4annual concentrated the most solarradiation on ESTs thus was the best, the nest best was the CPC-3and the CPC-1worst. In practial applications, CPCs are usually truncated due to less contribution toradiation concentration, thus, CPC-4was the best design.Ray-tracing simulations also revealed that, for CPCs with a given acceptanceangle (a), the average number of reflections within CPCs before arriving ESTsdecreased with the decrease in geometrical concentarion factor (Ct), for CPCswith a=20o, no more than two reflections occurred as Ct<2.0, and for CPCswith a=26o, no multiple reflection took place as Ct<1.8, therefore, the one-refectionmodel can be used to predict the optical performance of CPCs in this case.From the optical operformcen comparison of six CPCs, it is concluded thatCPC-4was the best superior solution, the nest best was the CPC-3and CPC-1wasthe worst. The CPC-6, the one widely regarded to be the most superior designs forconcentrating radiation on ESTs in the past, have a small edge ray angle aftertruncation, a lower optical efficiency as the incidencen angle being out of aandlower annual radiation collection, thus was not an optimal design, although it has ahigh optical efficicency over the acceptancen angle.From the point of manufacturing CPCs, CPC-4has crusp reflectors at the bottomwhich is difficult to manufacture and clean due to dust deposition, but CPC-5is easyto manufacture and clean due to considerable flat reflectors and gaps at the bottom.Further more, the optical efficiency of CPC-5is very high as incidencen angle within a, and the annual radiation concentrating on ESTs is very close to that ofCPC-4in the case of the tilt-angle of aperture of truncated CPCs being yearlyadjusted four times at three tilts. For this reason, a prototype of a solar heater basedon CPC-5and EST was designed and fabricated for testing in this work.In experimental tests of thermal performace of the solar heater, water was usedas the heat transfer fluid. Test results showed that the optical efficienc of the systemwere0.376,0.4014,0.4344,0.4113and0.3654at the mass flow-rate of0.02kg/s,0.029kg/s,0.052kg/s,0.061kg/s and0.078kg/s, respectively, and a maximum valueof0.434were observed at the mass flow-rate of0.052kg/s. It was also found that thetemperature difference between inlet and outlet of the collector slightly decreasedwith the increase in the mass flow-rate and temperature of water inside the waterstorage tank. A fureture test on the thermal performance of the syetm operating athigh temperature is needed.