Solar Air Heating

Solar power is the most common solution proposed for alternative energy, but is seldom a feasible economic solution, given the baseline assumptions of the average energy inefficient house, and the solar proposal taking the form of PV panels.  But what if we change up the assumptions, starting with a ZEH or PassivHaus, and open up the possibibilities for solar energy capture? With a lower demand for heating and cooling, and the assistance of passive solar heating and cooling techniques, are there any other non-PV approaches for solar heat capture?

Let’s look again at my favorite vehicle – air. Clearly, my paradigm is based on the benign role of air in my home stomping grounds of the western desert.  The air can become very cold or hot, but is rarely damp. This is a climate in which objects dry out and mummify, but typically don’t rot. So forgive my bias for a moment, and then we’ll add the moisture factor back in as an additional consideration.

Passive Solar house design from the 70’s invariably included some greenhouse space as pre-heat areas. The problem with this strategy was the very high requirement of occupant participation in securing thermally insulated shutters at night to prevent this same area from losing all the heat gained during the day. And, of course, these spaces overheated in the summer. The concept of capturing the air heat is worthwhile, but perhaps requires a decoupling from the interior space.

One interesting approach to an independent solar air heater is the Solar Wall. This transpired solar wall has a perforated metal plate which serves as a preheat mechanism for intake air.  When heated air is NOT needed, a switching mechanism pull opens this loop to both pull air from a cooler intake air. The integration of these panels into the indoor air environment could be as simple as connecting this fresh air intake into the return side of an HVAC system.  It could also be designed for specific applications, such as a “dryer room” in the basement – for air drying clothes - or food! - year round. Think organic.

There is perhaps a greater ROI when the solar wall concept is installed as a mechanism to remove excess heat from the backside of solar panels, thus improving their efficiency. These SolarDucts can also capture the preheated air to add heat (dry) to a un-conditioned attic in winter months, or be vented as surplus heat in the summertime.

Another approach is to transfer solar heat to a mass, which can radiate back to the interior at a later time. The Trombe wall was such a vehicle developed in the 70's solar heyday, but it was sometimes clumsy in the location in interior space, and it couldn’t be “switched off” to prevent overheating.  Heat sinks designs work better when decoupled from the living space.   This might be a contained rock bed area, or an interior pool of water. One ingenious engineer created an insulated area under his porch which was filled with barrels of water. The air preheat was pushed into this room, where the water absorbed the heat. Cheap and cheerful.

A similar principle is behind a “cold roof” application, or a rainscreen application. The airflow underneath a concrete tile roof, or behind wood siding flows due to a convective current, caused by the hotter air flowing upward and pulling in more air behind it. In this case the air is not capture for indoor use  – but it serves the purpose of distributing the captured heat evenly, pulling out moisture as it flows. In the case of a Bartile Roof, for example, this air movement serves to dissipate the capture heat in the summer, while providing an even air floor to prevent ice damming in the winter.

Sun and air are two very powerful forces,  when appropriately harnessed.