Abstract
In this experimental work, the effects of the inclination angle β and the (Ti − Ta)/G on the efficiency and the UL-value were investigated on a medium-temperature flat plate solar collector. The experiments were based on steady-state energy balance, by heat flow calorimetry at indoor conditions and considering the standard American National Standard Institute/American Society of Heating Refrigerating and Air Conditioning Engineers (ANSI/ASHRAE) 93-2010. The solar radiation was emulated by the Joule effect using a proportional integral derivative (PID) control considering two conditions of the absorber temperature, Case 1: (To − Ti) > 0, and Case 2: (To − Ti) = 0. The inclination angles were 0°⁻90° and the (Ti − Ta)/G were 0.044⁻0.083 m²·°C/W and 0.124⁻0.235 for Case 1 and Case 2, respectively. The variations of β and (Ti − Ta)/G cause efficiency changes up to 0.37⁻0.45 (21.6%) and 0.31⁻0.45 (45.0%), respectively, for Case 1. Also, the UL(β) reached changes up to 10.1⁻12.0 W/m²·°C (19.2%) and 8.4⁻12.0 W/m²·°C (41.7%), respectively, for Case 1. The most significant changes of UL(β)/UL(90°) vs. β were 8.0% at the horizontal position for Case 1, while for Case 2, the maximum change was 1.8% only. Therefore, the changes of the inclination angle cause significant variations of the convective flow patterns within the collector, which leads to considerable variation of the collector efficiency and its UL value.