Theoretical evaluation of night sky cooling in the Czech Republic

Abstract Energy necessary for cooling of building in summer season makes a big part of energy required for building operation. Redundant heat removal in summer can be realized in various ways. One of possible methods, mainly in desert areas, is using thermal radiation against night sky. The paper deals with theoretical efficiency evaluation of night sky cooling in climatic condition of Czech Republic. The numerical simulations of non-steady heat transfer and internal thermal comfort evaluation in an office room has been performed. We have considered a cooling system consists of radiating panels placed on the roof, connected by pipes with the cooling ceiling in an office room. The radiating panel is used to cool down the cooling water by convection and heat radiation against the night sky. At first we processed the real hourly climatic data for city Brno and by using the extra-terrestrial solar irradiation we calculated night sky temperature during cloudy conditions and other necessary climate parameters for the long-term simulations. From this file of data, we selected seven typical combinations of equivalent sky temperatures, external air temperatures and wind velocities in order to cover the external conditions during the summer nights. Next we assumed a variable mean temperature of water flowing through the radiating panel in order to be able of computing its thermal performance. Then we carried out seven simulations of chosen radiating panel with these boundary conditions by means of computational fluid dynamics method (CFD) in the software Fluent. The data obtained were processed in the own software so we set the appropriate regression functions describing the thermal behaviour of radiating panel, which was then used for long-term thermal performance simulations in software TRNSYS. The multi zone model in TRNSYS links the building and its equipment with thermal the cooling ceiling model as well as with the water loop containing the radiating panels. This computer model is capable of simulating the night sky cooling impact on the building and its internal environment. Since the investigation we carried out at the chosen office building, we used the typical office operation inclusive the schedules of occupancy, natural ventilation, heat gains from lightning, equipment etc. Then have performed two long-term simulations with and without night sky cooling. The simulations have shown the outlet temperature of the radiant panel below the external air temperature in a few cases. This finding confirms the positive effect of radiation transfer against the night sky in the climate of Czech Republic. The thermal comfort was evaluated according the standard ISO 7730 and has been found the significant improvement of internal thermal comfort by using the night sky cooling in the testing office building.