Experimental Investigations of Stagnation Temperature and Overall Heat Transfer Coefficient of Flat Receiver for Solar Parabolic Dish Concentrator System

Abstract

The parabolic dish concentrator is one of the most efficient technologies to convert direct beam radiation into thermal energy for steam and power generation. The parabolic dish concentrates direct beam radiation from the sun on to a receiver at the focal point. The receiver plays a major role to transform the reflected solar radiation into thermal energy. The conversion efficiency of the dish is severely influenced by imperfection of the dish collector such as the contour of parabola, size of the facets aligned, positioning of the receiver and tracking of the system. These imperfections are mainly involved in the design, manufacturing, construction and operation of a parabolic dish collector. To overcome this imperfection, secondary reflector are normally deployed at the focal region of the receiver. The function of the secondary reflector reradiates the deviated rays from the primary concentrator onto the receiver. In this aspect, flat receiver is initially fabricated to evaluate the performance of the flat receiver without secondary reflector. In this paper, the experimental investigations on flat receiver for 12.6 m2 area of solar parabolic dish concentrator system to estimate the receiver temperature and overall heat transfer coefficient of the flat receiver. In order to estimate this, rectangular box type aluminium receiver is fabricated and placed at the focal point of 2.42 m from the base of the dish. The aperture area of dish concentrator system is 12.6 m2 area and it consists of 12 petals and in each petals 128 flat mirrors of size 7.5 cm × 7.5 cm with reflectivity of 0.95 are pasted on 1.2 mm thickness MS plate to form a parabolic dish concentrator. The azimuth and elevation manual tracking arrangements are made to track the dish continuously for different orientation of the sun. K-type thermocouples are used to measure the temperatures of the top and bottom of the receiver. To measure the maximum temperature of the receiver, experiments are carried out for stagnation conditions (without heat retrieval from the receiver). Experiments are carried out on 5th, 6th and 7th March 2018 from 10.00 am to 3.00 pm. For different direct beam solar radiation, the top and bottom temperatures, ambient temperatures are measured. The maximum temperature of 399°C is achieved at the bottom surface of the flat receiver for the beam radiation of 955 W/m2, and the corresponding top surface temperature of 58°C is achieved for the same flux. Based on the measured bottom surfaces temperatures, the overall heat transfer coefficient of bottom surface are estimated as 145.56 W/m2 K. Based on this study, further heat transfer analysis will be carried out for the developed flat receiver.