A novel approach on measuring solar transmittance of glazings

Due to the increased interest of reducing building energy demand, innovative glazings are being developed and tested worldwide. Among other properties, solar transmittance should be carefully measured in order to be used in the building thermal simulation studies. Currently, the standard measuring procedure relies on the application of the ASTM E424-71 method by the use of a spectrophotometer and artificial light sources or a pyranometer and the sun. In this work, an alternative method was developed based on an in-house designed and built wind tunnel of controllable conditions of air RH, temperature and wind flow, simulated solar radiation by a 1000 W Xe source and a pyranometer of class A. The system was calibrated by an electrochromic glazing of 10x10 cm2 that was biased at different time intervals in order to scan all the transmittance range. Prior to the solar simulator measurements, electrochromic transmittance values were determined by an UV/VIS/NIR spectrophotometer with an 150mm integrated sphere. The method was applied in the solar transmittance measurement of innovative glazings like semi-transparent PV and insulating aerogels. The results of solar transmittance that were produced from this proposed method were very close to the corresponding produced results after the reduction of the observed transmittance spectrum with the ASTM G173 spectrum. Also, the results were in good agreement with the values of solar transmittance that were observed using the method of measuring the ratio of solar radiation with and without a glazing material. So, the method can be used as a fast and accurate procedure for solar transmittance measurement, while in the same measurements, the reflected radiation can be monitored.