Investigation of Measurement Accuracy of Water Temperature-type Simple Pyranometer by Considering Heat Exchange between Ambient Air and the Instrument

Abstract

A simple pyranometer can be a useful instrument in science education. Its measurement principle is based on the correlation between the amount of solar radiation absorbed and the subsequent increase in water temperature within the device over a short period of time. In this study, the accuracy of the measurement hourly-accumulated solar radiation was assessed over a five month period. All the data was collected between May and September 2015 at Miyazaki University, Japan. Thus, the relationship between the hourly-accumulated solar radiation and water temperature of the simple pyranometer was derived and approximating expressions were formulated. Satisfactory accuracy was obtained using these expressions when the water temperature was normalized. This normalization was divided into two processes, namely, upward and downward, when the ascending vertical angle of the equipment surface was set to 45° given its operation at northern latitude of 31.8°. Moreover, the accuracy got higher by considering heat exchange between open air and the equipment. Under fine weather, the simple pyranometer can measure the hourly-accumulated solar radiation at error of 0.26 MJm^-2h^-1 and measure the changes in the solar radiation over time.