Recovery of heat from thermal infra-red radiation: The role of heat conduction and convection, window material and system lay-out

The thermal infra-red radiation, TIR, from earth to space has only very recently been recognised as a possible energy source. It exploits the fact that this TIR interferes with so-called participating gases, more widely known as greenhouse gases (GHGs), primarily CO₂, which as it occurs in the atmosphere results in global warming and climate change. In technical systems these gases can be confined at high concentrations and elevated pressures, allowing for generating temperature differences that can be used in heat recovery, cooling (saving air conditioning power costs) or even power generation systems. Such a system can be, for example, a double-glass set-up with a participating gas, in our work primarily CO₂, between the sheets glass used in housing or office buildings, or on a larger scale as a double-glass roof for a public building like a railway station or a sports arena. It should however not be forgotten that when (gaseous) material temperature differences and gradients are created as a result of such radiation heat exchange, the other two heat transfer mechanisms, being conduction and convection, will come into action as well. This will have an effect on the final temperatures of the system parts. Besides this, the selection of the window material will be of great importance since most glass and plastic-type materials are transparent to TIR only for a certain wavelength band. Moreover, the geometry and orientation of the set-up, considering window surface area and the angles with respect to incoming TIR are of importance as well. In this paper the relative importance of the abovementioned is assessed for the temperatures that are finally obtained in a system that makes use of the earth-to-space TIR. It is shown that the geometry and orientation (with respected to TIR) of the system have the most important influence, followed by the effect of heat transfer by convection, and still less important the influence of window material transmittance. Overall, this results in temperature changes up to 5°C for the CO₂ (-containing) gas. Also, a simple experiment is reported that gives evidence that this TIR indeed gives rise to significant temperature differences.