Design and experimental optimisation of a novel flat plate solar thermal collector with trapezoidal shape for facades integration

For developing solar–thermal facades, new efficient flat plate solar thermal collectors are required, fulfilling the prerequisites of efficiency and architectural acceptance, with new shapes and/or new colours. A new flat plate solar–thermal collector, with isosceles trapeze shape was developed and stepwise optimised focussing on the insulation, the bonding between the tubes and the absorber plate, the meander tube length and diameter, and the glazing. The results show that the improved contact between the tubes and the absorber plate has a significant effect on the experimental conversion efficiency, as also the tubes with larger diameters have. A self-cleaning coating applied on the outer side of the glazing, deposited through a simple cold-spraying process has a beneficial effect on the collector's functionality, without significant efficiency losses. To support architectural integration, a concept of a multi-coloured absorber plate is formulated and tested. Eight types of solar thermal collectors were tested and the optimised version reached a 60.7% efficiency on an indoor testing rig, corresponding to a 62.38% maximum conversion efficiency estimated based on steady-state experimental conditions. Outdoor testing of the optimal collector is reported for a sunny day that includes 2 h with solar radiation in the same range as on the indoor testing rig when efficiencies of 61.85% were obtained. - Highlights: • A new solar thermal collector is developed and optimised for facades integration. • The collector has isosceles trapeze shape and allows multi-coloured absorber plate. • Indoor testing in steady state conditions outlines a nominal efficiency of 62.38%. • Outdoor preliminary testing gives a collector efficiency of 62%. • Solar thermal arrays for various facades geometries can be further developed.