Geometric activation of spectrally selective coatings for passive solar control in transparent systems

This work investigates a transparent WO₃/Au/WO₃ hot-mirror as a passive solar-control concept, with special emphasis on the angular blue shift of its optical cutoff and the way this effect can be exploited through roof geometry. Three cases were compared under the same clear-sky Dhahran forcing: a reference clear-glass flat roof, a hot-mirror flat roof, and a hot-mirror tent roof with face angles from 10^∘to 70^∘. The measured transmittance and reflectance of the fabricated hot mirror over 350–2000 nm and up to 70^∘incidence were used directly in a transient optical-thermal model. The results show that the hot mirror alone already provides a substantial thermal benefit relative to clear glass, reducing the peak plate temperature by about 13% and the daily absorbed solar energy by about 61%. When the same coating is combined with tent geometry, the improvement becomes much stronger. The best-performing case, a 70^∘hot-mirror tent roof, lowers the peak plate temperature by approximately 46% relative to the clear-glass flat roof and by approximately 38% relative to the hot-mirror flat roof. The optical origin of this behavior is the angular blue shift of the coating: the reflectance edge moves to shorter wavelengths as incidence angle increases, so the roof rejects a larger fraction of heat-carrying near-infrared radiation under oblique sunlight. Across the investigated tent-angle range, increasing the roof angle from 10^∘to 70^∘decreases peak plate temperature by approximately 36%, while the removed daily plate load increases from roughly 6.8×10³to 7.4×10³kJ/m²day. These findings show that the thermal performance of transparent hot-mirror roofs depends not only on multilayer spectral selectivity, but also on how roof shape activates a more favorable angular optical regime.