The glazing temperature measurement in solar stills – Errors and implications on performance evaluation

Although the brine temperature measurement in solar stills would be more or less conventional, the precise condensing surface temperature measurement, appears to present a significant difficulty to measure. In the present investigation, an analysis is developed aiming to underline the parameters affecting the glazing surface temperature measurement and a series of field measurements are presented, aiming to identify and evaluate the errors associated to the measurement of this crucial physical quantity. It is derived that among other reasons the surface temperature measurement accuracy may strongly be degraded owing to the sensor tip overheating due to the radiation absorption, as well as to the development of poor bond conductance between sensor bead and glazing surface. It may also be degraded owing to the temperature drop across the glazing thickness and the non-uniform temperature distribution over the entire condensing surface area, something that makes the selection of the appropriate location of the particular temperature transducer necessary. Based on the derived measurements, an order of magnitude analysis is employed for the approximate evaluation of error range in the condensing surface temperature measurement. This, which depending on specific conditions was found to vary between about 1 and 2°C, was employed to demonstrate the implications and approximate conditions under which its effect could become excessively high in ordinary solar still investigations.