Measured Performance and Modeling of an Evacuated-Tube, Integral-Collector-Storage Solar Water Heater

Abstract

An experimental study of an evacuated-tube, integral-collector-storage water heater was conducted in an indoor solar simulator. Useful collected energy, radiation-induced stratification and draw-induced mixing are characterized in eight trials in which test duration, initial tank water temperature, flow rate during withdrawal of heated water from the collector, withdrawal pattern and reflectance of the back-plane were varied. All tests were performed at nominal irradiance of 900 W/m2, normal incidence, and collector slope of 45 degrees. The capability of the TRNSYS integral collector storage model to predict performance for operating conditions for which data are assumed to be unavailable was assessed. Using data from one experimental trial, optical efficiency (τα) of the collector was determined by matching predicted useful energy gain to measured gain. Effectiveness of the calibrated model was based on a comparison of predicted gains to measured values obtained in the remaining seven trials. For five of seven trials, predicted performance is within nine percent of measured performance and less than experimental error. For the other two trials, predicted performance is within 15 percent of measured performance. The higher discrepancies may be partially due to experimental conditions not modeled such as heating of the piping connecting the collector to the test facility and inadequate characterization of back-plane reflectivity. The model does not predict radiation-induced stratification. Although the effect of mixing during draws can be approximated by specifying the number of fully mixed volume segments in the tanks, selection of number of nodes requires knowledge of the behavior of the system. The number of nodes selected has minimal impact on total energy gain, but does affect the temperature of water delivered to the load.