The effect of the wind conditions on the thermal performance of an unglazed solar thermal collector

Abstract

The paper presents a theoretical analysis performed for unglazed liquid solar thermal collectors (USTCs) to demonstrate the magnitude of the thermal performance uncertainty caused by the different wind conditions under the same wind speed. The literature currently comprises more than 90 combined convection heat transfer correlations. The difference between them is primarily caused by the difference in wind characteristics during the experiment (wind turbulence level, wind direction, etc.). A detailed validated theoretical model of a USTC and forty applicable combined convection heat transfer correlations were used to perform the analysis to demonstrate the magnitude of the thermal performance uncertainty caused by the different wind conditions. It was found that, for the low operating temperature, the uncertainties caused by the different wind conditions are ± 4 %, ± 6 %, and ± 8.5 % under wind speeds of 0 m/s, 1.5 m/s, and 3 m/s, respectively. These values are relatively small compared to the obtained average thermal performance values of 78 %, 73 %, and 65 %. In contrast, the uncertainty values are significant for relatively high operation temperatures: ± 11.5 %, ± 16.5 %, and ± 22 %, while the modelled efficiency values for these operating conditions are 15 %, 30 %, and 44 %. The analysis demonstrated that the difference in wind characteristics under the same wind speed significantly affects a USTC's thermal performance. From a practical standpoint, the USTC's thermal performance in real wind conditions may exhibit significant disparities compared to its performance tested in controlled laboratory conditions under the same wind speed. Furthermore, notably varied thermal performance can be expected for identical USTCs installed in different locations under the same wind speed due to the different wind characteristics (wind turbulence level, wind direction, etc.).