3 rd International Conference on Contemporary Problems of Thermal Engineering CPOTE 2012, 18-20 September 2012, Gliwice, Poland Institute of Thermal Technology 1 THERMAL RADIATION HEAT TRANSFER: INCLUDING WAVELENGTH DEPENDENCIES INTO MODELLING Ron Zevenhoven 1* , Martin Fält 1 , Luis Pedro Gomes 1,2 1 Åbo Akademi University Thermal and Flow Engineering Laboratory Piispankatu 8, 20500 Turku Finland e-mail: ron.zevenhoven@abo.fi e-mail: martin.falt@abo.fi 2 visiting from University of Coimbra Department of Mechanical Engineering Coimbra, Portugal Keywords: thermal radiation, wavelength dependence, radiative cooling Abstract Thermal radiation heat transfer is receiving increased attention from the field of energy-efficient building, where air conditioning and cooling are equally important as heating or electricity production using thermal or photovoltaic solar energy systems. One feature that can be made use of is that an unbalance between thermal radiation in short-wave (SW, <4μm) and long-wave (LW, >4μm) bands leads to a net heating or cooling effect. Well known is the enhanced greenhouse effect that results from an influx of short-wave thermal radiation from the sun that nowadays is not balanced by long-wave outgoing radiation. Likewise, passive cooling can be achieved if outgoing radiation in certain wavelength bands cannot be balanced by incoming thermal radiation through another wavelength band. In this paper we will report on simulating heat transfer, primarily as SW and LW thermal radiation, from a building space using an optimized double-glazed window with the option of having a participating gas like CO 2 filling the inter-glazing space. For this purpose the infrared spectrum is sectioned into four sections: 1) SW, 2) 4 μm - 8 μm LW, 3) 8 μm - 14 μm LW (the atmospheric window) and 4) > 14 μm LW, respectively, and the relative importance of the thermal radiation in the various bandwidths is assessed. Simulation for either day-time or night-time the effect of changing the transmittance of the double-glazed gas-filled window on heat transfer is analysed. For distinguishing SW and LW thermal radiation through the atmosphere (and determining the temperature of the sky) measurement data obtained with a pyranometer and a pyrgeometer is used. The results demonstrate that the simulation tool allows for designing optimised double-glass window systems towards minimisation of energy requirements for air conditioning and cooling.