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

A severe energy shortage already exists in many parts of the developing world. In an attempt to find a technical solution, several solar energy collection technologies have been developed. A system has been designed to use compound parabolic concentrating collectors to collect solar energy and to generate steam. A compound parabolic concentrator (CPC) profile with oversized reflector and thereby reduced gap losses was designed with a half-acceptance angle of 23.5o for a tubular absorber of OD 19 mm. Five troughs fabricated with a fiberglass substrate pasted over with UV stabilized self-adhesive aluminized polyester foil having high specular reflectivity joined together side by side make the CPC module with aperture area of 0.72 m2. Copper tubes coated with NALSUN selective coatings and enclosed by borosilicate glass envelopes act as absorbers. The reflector–absorber assembly placed in a single glazed glass wool insulated wooden box forms the CPC collector. Using water as heat transfer fluid, efficiency tests were conducted with different inlet temperatures. Even at high temperature, the system operates with a reasonably high efficiency of 50%. In situ steam generation testing was also conducted. The fabricated CPC collector was used for steam cooking by connecting it to a pressure cooker. Cooking tests were conducted and the results are compared with earlier works. This cooker unites the characteristics of reflector cookers, steam cookers, pressure cookers and heat accumulating solar cookers. The fabricated CPC can be of immense and wide spread use for rural applications, such as water heating, steam cooking and sterilization.

A CPC for a flat absorber is one which consists of curved segments, which are parts of two parabolas. Many improvements in the design and performance of the CPC collector have been made since its invention in 1974. 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 this conduction heat loss 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’. So, a compromise between optical and thermal performance must be made. Several modifications of the basic CPC design were suggested for the provision of gap. Winston [1] proposed a reflector design which preserved the ideal flux concentration on the absorber of radius r1, surrounded by a glass envelope of radius r2, at the expense of slightly over sizing the reflector. This design maintains maximal concentration at the cost of optical losses. There have been a number of reports of the CPC’s optical and thermal characteristics. Based on the experience with earlier designs developed in our lab [2], an attempt has been made to fabricate a CPC with reduced gap losses and study its performance and also to assess the suitability of fiberglass as substrate materials for the reflector and to study its performance. The CPC reflector profile for an oversized reflector has been designed for a half-acceptance angle of 23.5° and a cylindrical absorber tube of OD 19 mm and with minimum gap losses. A set of detailed experiments with the collector incorporated in the fluid loop and with water as the heat transfer fluid were performed for the CPC. A theoretical model was developed by setting up different heat balance equations, and a reasonable agreement between experimental and theoretical computed values has been observed. A potential capacity of a CPC is to obtain operating temperatures higher than the boiling point of water. It enables CPC modules to be used as low pressure steam generators. The non-industrial applications for solar low pressure steam, which are relevant to 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 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.

Fabrication of CPC modules with fiberglass substrate has the main advantage that it could be easily mass produced. Gap lossless design of the CPC profile has sharp corners in the reflector parts below the receiver. Such parts could be easily made with fiberglass. The use of ferrule joints in the absorber assembly facilitates easy assembling of the absorber tubes with glass envelopes and also dismantling. The optical and thermal performance of the prototype model of CPC with reduced gap losses (oversized reflector) is quite encouraging and the experimental results are comparable with the prediction made by theoretical modeling. Fig. 13gives a comparison between the experimental and theoretically predicted performance curves of the CPC. We find that the instantaneous efficiency of the CPC module is fairly high, even at higher operating temperatures, when compared to a flat plate collector. The potential capacity of the CPC to attain operating temperatures higher than the boiling point of water enables it to be used as a low power steam generator. Full-size image (7 K) Fig. 13. Comparative performance curves – CPC. Figure options This solar based pressure cooker has the advantage of being able to cook in the shade or indoors without diurnal tracking. By keeping the CPC module outside, the generated steam can be brought into the kitchen by insulated pipes and connected to the well insulated pressure cooker. This new concentrating solar pressure cooker can be used for cooking foodstuff at households, community kitchen and noon meal centres. Compared to most solar cookers, this solar steam cooker is a relatively sophisticated device that unites some of the characteristics of reflector cookers, steam cookers, pressure cookers and heat accumulating solar cookers. Other possible applications of the CPC powered solar steam generators are sterilization and generation of electric power.