Thermal radiation heat transfer: Including wavelength dependence into modelling

Thermal radiation heat transfer is receiving increased attention from the field of energy-efficient building. Well known is the enhanced greenhouse effect that results from an imbalance between incoming short-wave (SW) solar radiation and outgoing long-wave (LW) radiation from earth. Likewise, passive cooling can be achieved if outgoing radiation is not balanced by incoming radiation through another wavelength band. This paper reports on simulating thermal radiation from a building space using a double-glazed window with the option of having a participating gas like CO2 filling the inter-glazing space. The infrared spectrum is sectioned into four bands: SW < 4 μm, and three LW bands 4-8 μm, 8-14 μm (the atmospheric window) and >14 μm, respectively, and the relative importance of thermal radiation in the various bandwidths is assessed. The effect of changing the transmittance of the double-glazed gas-filled window on heat transfer is analysed for both day-time and night-time for air, CO2 and HFC-125. For distinguishing SW and LW thermal radiation through the atmosphere measurement data obtained with a pyranometer and a pyrgeometer is used. The results demonstrate that the calculation tool allows for designing double-glass window systems towards the minimisation of energy requirements for cooling or heating. It is found that expanding the wavelength range for window material transmittance increases heat fluxes through the system while using participating gases gives an insulation effect. For HFC-125 the effect is significant while for CO2 it is much smaller.