Heat Reflection

The aim of the activity is to investigate metal oxide coatings and solar paints which potentially could have a beneficial effect in a fire scenario if put on a façade element by reflecting infrared (IR) radiation.

Radiative heat transfer accounts for around one third of the heat released from fires, and this is the most important mode of heat transfer for example at long distances and from a hot smoke gas layer to lower objects, such as to a floor for example. The possibility for reducing the absorptivity of surfaces in the infrared part of the spectrum has been discussed for several decades, mainly for energy saving purposes. Such surfaces are called low emissivity surfaces, or low emissivity coatings, and much focus has been on the spectral absorptivity up to wavelengths around 2.5 m, e.g for solar reflective paints. In this task the focus has been on radiation with wavelengths between 2 and 10 µm, which is typical for a fire.

At long distances radiative heat transfer dominates over convective heat transfer.

A material coated with a zinc oxide (ZnO) or indium tin oxide (ITO) layer will leave the spectral properties for visible light almost completely unaffected but have a significant impact on the reflectivity for radiation with wavelengths longer than a few µm, which is of great interest for preventing fire spreads. This has been confirmed by fire test of the ignition time in the cone calorimeter (ISO 5660-1) of coated and uncoated PMMA samples, where the time to ignition for an ITO coated sample in comparison to an uncoated one was more than twice as long. A challenge with different transparent conducting oxides (TCO) is to find a viable and cheap large scale solution to apply the coating to a façade.

Spectral absorptivities (or more exactly 1-R_λ) for untreated PMMA, ZnO-coated PMMA, and ITO-coated PMMA presented by solid lines. Normalized spectral exitance for blackbody radiation corresponding to 860 K is represented by the red dashed curve. This spectrum corresponds to the spectrum from the cone calorimeter with an irradiation level of 25 kW/m2.