Investigating an accurate method for measuring the outdoor mean radiation temperature

Abstract

Accurate measurement of mean radiation temperature (MRT), a main parameter affecting outdoor thermal comfort under sunlight conditions, is immensely important in improving the thermal environment of urban outdoor spaces and enhancing outdoor thermal comfort. In this study, four different surfaces (limestone, grass, concrete, and asphalt pavement) were selected and the experiments were set up under shading and no-shading conditions on four different surfaces. Based on the above conditions to analyze and compare three different MRT estimation methods, these include the black sphere thermometer measurement method under different wind speed measurement frequencies (0.1 and 10 Hz) and the six-way integrated radiation measurement method, and the outdoor wind speed (Va) of 10 Hz is the three-dimensional wind speed of the combined x-y-z three directions. The results showed that MRT was influenced by the long- and short-waves more in the vertical direction than in the horizontal direction, and the different solar radiation storage capacities of the lower cushion surfaces during the daytime led to different temperature change delays after sunset. The results shows that at the thermal comfort requirement of ±2 °C, and the acceptable error probability were 65.3% and 74.6% at 0.1 and 10 Hz, respectively. While at the thermal comfort requirement of ±5 °C, the acceptable error probability were 83.4% and 89.3% at 0.1 and 10 Hz, respectively. Noted that ±2 °C and ±5 °C as acceptable thresholds for MRT measurement accuracy are specified in ISO standard 7726:1985. Therefore, 10 Hz exhibited significantly better performance than that of 0.1 Hz for MRT estimation. Under the unshaded conditions, the MRT measurements obtained by black-sphere thermometer method is in most cases lower than obtained by six-way radiometry method, where the MRT obtained at 0.1HZ and 10HZ wind speed measurement frequencies are lower by about 5.8 and 4.2, respectively, compared to those obtained by six-way radiometry. However, due to the variation of the solar altitude angle, there is also a slightly higher phenomenon present. In addition, the results show that the RMSE of the estimated errors after correction for different cushion surfaces are lower than those before correction. Limestone is reduced by 22.15%. Concrete is reduced by 23.47%, Grassland by 28.38% and Asphalt road by 25.28%. And the correction effect is more obvious in the lower cushion surface with higher absorption rate. The conducting test that used an inexpensive black-sphere thermometer to measure outdoor MRT accurately improved the distinction between MRT estimation measurements under different conditions. It also refined the feasibility of changes in the wind speed measurement frequency to improve the accuracy of the traditional black-sphere thermometer measurements, thus, highlighting a new methodology for accurate outdoor MRT measurements that can in turn advance the improvement of outdoor thermal comfort.