Temperature measurement in solar thermal applications

Abstract

While measuring the temperature of a fluid such as air or water which is exposed to solar radiation, the temperature measuring device, such as a thermocouple, should be shielded from incident irradiance. The probe should be well ventilated in order to increase heat transfer by convection and thus get a reading closer to the true temperature. The popular Stevenson screen which is used for meteorological air temperature measurement consists of a cage with horizontal louvers. This makes it too bulky to use in enclosures such as solar collectors. In this study, various shields have been analysed under laboratory conditions using a halogen lamp as a source of infrared radiation. The standard on Guidance for Solar Radiation Testing (60068-2-9:2000) suggests to use a thermocouple freely mounted in a radiation shield comprising a cupro-nickel tube. However experimental results show that the thermocouple reading resulted in 9°C higher than the true air temperature. Other radiation shields consisting of white vertical PVC tubes, varying in length and diameter, resulted in a temperature increase of 3-5°C. The closest temperature reading was achieved by using a longitudinally slit tube as the radiation screen. This resulted in a discrepancy of around 1-3°C higher than the true temperature. It was observed that the difference between the true temperature and the reading from the shielded thermocouple decreases with increasing both length and diameter of the slit tube.