تجزیه و تحلیل عملکرد و بهینه سازی صفحات جاذب هندسی مختلف برای کلکتور خورشیدی صفحه تخت: یک مطالعه مقایسه

This paper presents a comparative study on the performance and optimization of several profile shapes namely, rectangular, trapezoidal and rectangular profile with a step change in local thickness (RPSLT). This analysis concentrates on the performance and optimization of RPSLT. A modification has been suggested for the analysis of RPSLT absorber plate that was observed by Hollands and Stedman [Solar Energy 49 (1992) 493]. The result indicates that there is optimum fin efficiency of trapezoidal profile for constant plate volume. The RPSLT profile of absorber plate is superior to other profiles because of higher performance and less difficulties in fabrication.

A flat-plate solar collector is a special kind of heat exchanger that transforms solar radiant energy to internal energy of the transport medium in the tubes to be carried out as usable energy. It is widely used for supplying thermal energy at moderate temperatures. The common applications of the flat-plate collectors are mostly found in domestic hot water and space heating, industrial processes, vapor absorption refrigeration and air conditioning system. Therefore, due to their various applications, there is a continuing endeavor of a designer to determine thermal performance of flat-plate solar collectors. Many authors [1] and [2] have concentrated on the development of effective design methods for solar collectors. For their analysis, the cross-sectional area of the absorber plate has been taken constant. However, the collector receives energy from the sun that is absorbed by the plate and is then transferred to the fluid. On this basis, energy transferred increases in the direction of flow of energy in a plate. It is well known fact that for effective design, the profile shape of the absorber plate should be divergent in nature in the direction of energy transfer. Among the different profile shapes, concave parabolic profile [3] is more efficient for transfer of energy than other profiles with identical volume. On the other hand, fabrication of such profile shape needs some special manufacturing technique and is bound to be expensive. Though the concave parabolic shape promises the highest rate of heat transfer, it is only marginally better than a triangular profile or trapezoidal profile with a small tip thickness [3]. Moreover, due to zero tip thickness manufacturing the triangular profile for the absorber plate in solar collectors is not possible. Therefore, the trapezoidal profile of an absorber plate which requires least material for a given energy transfer rate is preferable. Hottel and Woertz [4] have determined the performance of flat-plate solar heat collector. They have also examined the non-uniform temperature distribution between pipes, causing a reduction of heat transfer rate. Economic analysis has been applied for the design and operation of solar energy collector by several researchers [5] and [6]. Levinskii et al. [7] have investigated optimization of the geometry of a plastic absorbing panel of sheet tube design taken selectivity and non-selectivity into consideration of the radiation absorbing plate. They have established the optimum thickness of the radiation-absorbing plate and the distance between the tubes. The two-dimensional temperature field in the absorber plate has been considered by Lund [8] for the analysis of parallel flow flat-plate solar collector. Hollands and Stedman [9] have proposed a new profile shape of the absorber plate. They have investigated the thermal analysis of the absorber plate with a step change in thickness. They have also determined the optimum dimensions of the absorber plate for constant plate efficiency. Hollands and Stedman [9] have demonstrated the thermal analysis of the absorber plate with a step change in thickness assuming a continuity of heat flow at the junction of the two parts of the plate. The continuity of heat flow at the junction satisfies the same energy carried by conduction for both the parts of the plate. However, continuity of energy transfer at the junction section is not fulfilled due to the energy interaction to the excess thickness. Therefore, for determination of plate performance, an energy balance at the junction section is essentially needed. It may be considered as follows: energy flow due to conduction through smaller thickness and energy received from Sun to the excess thickness is equal to the energy flow due to conduction through the larger thickness and energy lost from the excess thickness. Therefore, the analysis of Hollands and Stedman [9] is essentially needed for a suitable modification. On the other hand, they described a methodology for determination of the optimum absorber plate for constraint plate efficiency. In general, constraint for the optimum is that either plate volume requires the least profile area for the transfer of a given amount of energy or the maximum quantity of energy transferred through plate for a given plate volume. So, as per specification of a designer, a generalized method of analysis is also required for the determination of the optimum absorber plate with a constraint either plate volume or energy transferred through the plate. In this study, a systematic analysis of different profile shapes namely rectangular, trapezoidal and RPSLT of absorber plates has been carried out. In case of RPSLT, the above suitable modification has been performed on the analysis of Hollands and Stedman [9]. The optimum dimensions of absorber plate have been estimated for the above type of profile shapes in such a way that, either the rate of energy transfer is maximum for a given plate volume or a specified rate of energy transfer is achieved using the lease amount of plate volume. Comparative studies on the plate performance and optimization for the above type of profiles have been done. From the results, it can clearly be observed that the trapezoidal profile shape transfers more energy than rectangular profile with identical thermo-physical parameters and plate volume. Finally, a rigorous study on the different optimum plate geometries has been made for suitable thermo-geometric parameters.

Present study concentrates on the performance and optimization of rectangular, RPSLT and trapezoidal profiles of absorber plates of a solar collector. From the analysis, it can be concluded that: 1. The analysis of Hollands and Stedman [9] of RPSLT has been significantly modified. 2. Optimization has been done in a generalized way so that either the energy transfer or the plate volume may be taken as the constraint condition. 3. Optimum fin efficiency exists for the trapezoidal profile when the plate volume is taken constant. 4. The optimum energy transferred by the RPSLT is significantly closer to the trapezoidal profile whereas plate efficiency of the RPSLT is marginally smaller than the rectangular profile. 5. A comparative study of different profile shapes has thoroughly been investigated. 6. In spite of certain manufacturing difficulties compared to the rectangular profile, the RPSLT may be recommended for selection of geometry of absorber plates. 7. The rectangular profile absorber plate transfers least amount of energy than other profiles with identical plate volume. 8. The trapezoidal profile is a better choice for transferring energy of a solar collector, although, this profile is rarely found in practical design due to inherent difficulties in manufacturing.