توسعه و تجزیه و تحلیل عملکرد متمرکزکننده های خورشیدی سهموی مرکب با تلفات کاهش یافته شکاف - بازتابنده بزرگ

A system has been developed to use compound parabolic concentrators to collect solar energy and to generate steam. A CPC reflector profile with a V groove at the bottom of the reflector to reduce the gap losses was designed with a half acceptance angle of 23.5° for a tubular absorber of OD 30 mm. Five troughs fabricated with fiberglass substrate pasted over with UV stabilized self-adhesive aluminized polyester foil having high specular reflectivity joined together side by side comprise the CPC module with an aperture area of 2.04 m2. Copper tubes coated with NALSUN selective coatings and enclosed by borosilicate glass envelope act as absorbers. The reflector absorber assembly housed in a single glass wool insulated wooden box forms the CPC collector. Using water as the heat transfer fluid efficiency tests were carried out with different inlet temperatures. In situ steam generation testing and possible application to steam cooking were also carried out. A theoretical modeling was developed by setting up different heat balancing equations and a reasonable agreement between theoretical computed values and the experimental values was observed.

Compound parabolic concentrators for a flat absorber, which consists of curved segments, is one which forms parts of two parabolas. Many improvements in the design and performance of the CPC collector have been made since its invention in 1974 [1]. The CPC reflector profile for a tubular absorber is such that the reflector touches the absorber at the cusp region. This results in conductive heat losses. So a gap between the tubular absorber and the reflector has to be created to prevent conductor heat losses from absorber to metallic reflector, and also for providing a glass envelope around the absorber, which will improve the thermal efficiency of the CPC module at high temperatures. However, the gap between the absorber and the envelope leads to losses of the incident light on the absorber called ‘gap losses’. However, with a tubular absorber one can do even better by avoiding gap losses altogether; a design that preserved the ideal flux concentration on the absorber at the expense of slightly over-sizing the reflector, was developed by Winston [2]. The design thus developed, though reduces the gap losses, does not eliminate them completely. Hence McIntire [3] modified a small portion of the reflector below the absorber to create a design without gap losses. The design proposed by Winston [2] maintains maximum concentration at the cost of optical losses. The design proposed by McIntire [4] eliminates optical losses at the cost of concentration. The most practical design for an optimized non-imaging solar concentrator as proposed by McIntire and Winston [5] is a compromise between a no gap design and a design with a gap. Recent work in the area of CPC systems includes a simulation model of a CPC collector with temperature-dependent heat loss coefficient by Fraidenraich et al. [6] and the design, construction and test results of a non-evacuated stationary CPC solar collector with flat bifacial absorbers by Tripanagnostopoulos et al. [7]. In the present work, an attempt has been made to design and fabricate a V groove CPC with reduced gap losses. A detailed performance analysis of this CPC and the steam generation capabilities were also carried out. A potential capacity of CPC is to reach an operating temperature higher than the boiling point of water. It enables CPC modules to be used as low-pressure steam generators. Acharya et al. [8] reported the use of a CPC collector system to generate electric power. It has been shown that some refrigerants can be satisfactorily used as working fluids in the CPC collector system. The non-industrial applications for solar low-pressure steam, which are of relevance in the Indian context, are steam cooking, sterilization of liquid foods and sterilization of hospital tools. At a time when attention is finally focused world-wide on the control and prevention of pollution, efficient use of energy and more reliance on renewable energy sources, this low-pressure solar steam generator is a welcome addition to the fight against environmental pollution and it is packed with energy saving and environmentally friendly features.

The optical and thermal performance of the prototype model of CPC with reduced gap losses is quite encouraging and the experimental results are comparable with the prediction made by theoretical modeling. Fig. 11 gives a comparison between the experimental and theoretically predicted performance curves of CPC. We find that the instantaneous efficiency of the CPC module is fairly high even at higher operating temperatures, when compared to flat plate collectors. The result of in situ steam generation testing lends support to this conclusion. This type of CPC collector can be used for steam cooking, sterilization and even power generation. Full-size image (6 K) Fig. 11. Comparative performance curves—CPC.