Reverse solar walls for passive cooling: Prototype experiments and simulation study

This paper investigates the potential of exploiting the greenhouse effect through a solar wall from indoors to outdoors in a sun-exposed wall within a room, to reduce indoor temperature, rather than, as it commonly does, from outdoors to indoors, to increase indoor temperature. This requires installing a Trombe wall — a glazed massive wall transmitting heat by conduction and convection — on the indoor side of a sun-exposed wall, so that heat gained in the glazed cavity is transferred from the interior to the exterior, rather than into the room, as in conventional Trombe walls. This strategy entails applying a localized greenhouse effect on the concerned wall to transfer a portion of the gained heat from inside to outside and thus remove it from the room. The described configuration can lower indoor temperatures under passive solar gain conditions, making it possible to achieve higher daylight levels with reduced risk of overheating. The cooling effect observed in trials and simulations ranged between 50 and 300 cooling degree hours yearly and between 1 and 5 ◦C in terms of absolute maximum air temperatures, depending on the composition of the reverse solar wall: reductions in cooling degree hours, and even more pronouncedly, maximum temperatures were greater in lightweight walls. Also, as a consequence of the overheating reduction effect of the system, the models allowed approximately a doubling of the size of transparent openings in rooms, and greater illumination levels, while cooling degree hours remained equal. By demonstrating this effect through both monitored prototypes and simulations, this study establishes the principle that, by exploiting the greenhouse effect taking place in a solar wall along the outward direction rather than along the inward direction, solar-driven thermal gradients can be exploited for overheating avoidance rather than for heating, both in the case of massive and lightweight building envelopes, to allow for greater illumination levels in rooms while reducing the thermal effect of indoor solar gains.