Heat recovery versus solar collection in a ventilated double window

The ventilated double window, as a passive heating system, acts as a heat reclaiming device. Part of the heat loss from inside through the window is returned back to the room by the air flow, acting as a heat recoverer. Incident solar radiation upon the window warms its components being part of that heat removed by the air flow delivering it into the room, acting as a solar collector. The effect of these two functions were analysed in this study, through numerical simulation based on outdoor tests under real weather conditions. It was found that solar collector function plays a small role in the pre-heating of the air. First of all this is due to the system's transparency, which allows most of the solar radiation to enter directly to the indoor space. Secondly, in a 24 h period there are only some hours of sunshine. Instead, heat recovery works all the time, the conclusion being that this passive heating device can be used on any facade orientation. - Highlights: ► A ventilated double window provides pre-warmed air for winter ventilation. ► It recovers part of the lost heat delivering it again to the inside. ► A ventilated double window acts as a vertical solar air collector. ► The heat transfer conditions make it suitable on any facade orientation.