Nocturnal Radiation: New Opportunity in Building Cooling

Free cooling systems are gaining increased attention as air conditioners are responsible for a remarkable amount of the electric peak load in summer. Several studies have shown that night ventilation of buildings reduces peak loads of the following day. However, their effect can be increased by a free cooling system based on nocturnal long wave radiation, which cools the air below ambient temperature. A perforated, corrugated metal plate which is used in transpired solar air heaters, functions as a radiating shield in this case. The air can reach temperatures below ambient as it passes the plate further increasing the free cooling's potential. A mathematical model has been set up to describe the cooling process. Based on governing equations diagrams are developed to show the correlation between the transpired radiant plate's temperature drop, as well as its cooling rate as a function of the long wave radiant heat flux and the air flow rate. Four European capitals are chosen to illustrate the cooling potential of nocturnal radiative air cooling. Building simulation is carried out using RETScreen 4 Clean Energy Project Analysis software. Results show that locations with drier climate reach higher nocturnal cooling performance as the moisture content of the atmosphere influences the radiant heat transfer.