J.K. Nayak

Experimental and theoretical evaluation of dynamic test procedures for solar flat-plate collectors

Abstract This communication presents a critical evaluation of nine dynamic test methods for solar flat-plate collectors. The theoretical basis, the technique of parameter estimation and the test procedure of each method have been reviewed and compared. Extensive experimental studies have been carried out under a wide range of weather and operating conditions. Two commercially available collectors (from two different manufacturers) have been used in the investigation. The tests were carried out at the same location using a common test-rig, measuring transducers and controlling and data-acquiring facilities. The characteristic parameters of the collectors have been obtained on the basis of each procedure and compared with those based on the steady-state ASHRAE 93-86 standard. Further, for the methods which prescribe similar test sequences, the collector parameters have been extracted from the same data sets according to their procedures for providing a direct and very clear comparison between the methods. A sensitivity study has also been carried out in order to examine the effect of uncertainties in measurements on the values of the estimated parameters from different methods. Also investigated is the error propagation wherever applicable. Among the methods evaluated, the new dynamic method (NDM) seems to be quite reliable. The quick dynamic test (QDT) method is the most simple method and could be adopted by manufacturers as an effective tool for the purpose of quality control of their products. From the point of view of theoretical completeness, Perers’ method accounts for almost all effects.

Comparison of Cavity Receivers with and without Mouth-Blockage of Different Shapes and Sizes Used in Paraboloid Dish Applications

Numerical three-dimensional studies of the natural convection and radiative heat loss from cavity receiver of different shapes with and without mouth-blockage have been investigated under isothermal wall condition. Convective heat loss is found to decrease for cavities having mouth blockage created by reducing aperture area (case I) whereas it enhances when mouth blockages are introduced by increasing the cavity dimensions and keeping the same aperture area (case II). Convective loss is characterized by using the convective zone area (Acb ). Conical cavity yields the lowest convective loss whereas hetro-conical cavity gives the highest convective loss among different shapes investigated. Radiative loss is independent of cavity inclination and is found to be nearly constant for all cavity shapes and cavity configurations (with or without mouth blockage) so long as the aperture area remains the same; it is proportional to the aperture area. However, investigations on decrease in heat loss of mouth-blocked cavities needed to be coupled with the estimation of concentrated flux.