Comparative analysis between constant and variable solar radiation reflectivity for exterior walls in the hot-summer and cold-winter zone

The thermal and optical performance in coating material of exterior walls affected the thermal action from the outdoor thermal environment indirectly, but there was the contrary thermal requirement in winter and summer, which could not be met by the constant-reflectivity coating. To overcome this drawback, the variable-reflectivity coating was analyzed and evaluated by taking three constant-reflectivity coatings as the references. The thermal transfer model with dynamic radiation reflectivity was built to simulate the thermal process of exterior walls in the whole year. Numerical results showed that the constant-reflectivity coating had the poor seasonal adaptability with the best performance in some certain months and the poorest behavior in other months, while the variable-reflectivity coating had the better seasonal adaptability to dampen the heat gain in summer and promote the heat gain in winter. Although the variable-reflectivity coating could not show the optimal in all months, its overall performance played the best in the whole year. And compared with constant-reflectivity coating with radiation reflectivity of 90%, 50% and 10%, the variable-reflectivity coating could reduce the annual heating and cooling loads by 2.45%, 16.67% and 46.46%, respectively, in the studied Nanjing city of China.