Experimental calculation of the mean temperature of flat plate thermal solar collectors

Abstract

The mean temperature of flat plate solar thermal collectors (FPSTC) is used to calculate collector efficiency and other related parameters. This temperature is a key aspect for the determination of thermal losses or the inertial behavior of a collector. Consequently, the accurate determination of this parameter is of major interest to provide many collector performance indicators. in an apparently simple way, EN 12975 standard stablishes that the mean temperature of a FPSTC is given by the arithmetic mean of the inlet and outlet temperatures of the collector fluid. In order to know whether this result is close to the real mean temperature of the collector, calculations were made using data obtained in thermo-solar experimental facility of the University of Murcia.One representative tube-fin “channel” of each of the two kinds of solar collector belonging to this facility was provided with a set of 28 thermocouples, divided into four sections. This arrangement allows the continuous measurement of the collector temperature field, as well as of fluid temperatures and other data related with collector performances. A weighted average of the collector temperatures, bearing in mind the different surfaces that the measurement of each thermocouple represents, was calculated, yielding the real mean temperature of the collector plate absorber. Also, a representative point of the temperature “channel” was determined.The different curves representing the weighted average temperature of both collectors during the daylight period are depicted and compared to the mean collector temperature mentioned in the standard. In most cases only a slight difference of about 5% between these temperatures was obtained, leading to the conclusion that for general purposes the EN 12975 standard definition of collector mean temperature is sufficiently accurate. Keywords: Solar thermal energy, Mean absorber plate temperature, Temperature centroid, Collector efficiency, Experimental measurement