Wind heat transfer coefficient for glass cover of solar collectors in Indoor Testing

For designing and thermal performance evaluation of solar thermal collecting systems, the wind heat transfer coefficient is a major concern. For estimating the wind heat transfer coefficient (hw) of a flat plate solar collector or solar box cooker, the glass cover temperature is needed. The measurement of glass cover temperature of flat plate solar collectors is not so easy, especially in outdoor conditions. In outdoor conditions the glass cover temperature is measured using a thermocouple which yields an error as it's junction is directly exposed to the solar radiation. The other way of estimating the wind heat transfer coefficient is to use the available correlations as a function of wind velocity. There is considerable difference in the value of hw obtained from different correlations at the same wind velocity. Arbitrary selection of a correlation will cause a substantial uncertainty in the thermal analysis.

In the present work a method is proposed to estimate the wind heat transfer coefficient from the outer surface of the glass cover of a flat plate solar thermal collector. An unglazed heated plate, almost of same size of collector or cooker aperture, can be used to estimate the wind heat transfer coefficient of a flat plate solar collector. The unglazed heated plate can be placed near the collector for the measurement of wind heat transfer coefficient, exactly in the same conditions. Some indoor experiments were performed on a single glazed flat plate collector and unglazed heated plate, both are heated electrically from underside. Similar wind conditions were maintained on the collector as well as on heated plate by a large fan. Under similar wind conditions the values of wind heat transfer coefficient from the unglazed plate and collector are within 3% rms error.