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SOLAR AIR HEATING

In this technology concept air is preheated with a novel unglazed solar collector where heat transfer is accomplished by drawing air through perforations in a false outside wall. The perforations create turbulence which improves heat transfer. This partrially warmed air is ducted into the building, pre-heating the incoming air. No heat is added to recirculating air. This is an important consideration. As with Hot Sun's unglazed technology for pool heating, its all about operating at low temperatures where expensive insulation and glazing just get in the way.

Unglazed collector performance vs high temperature collectors. These curves should represent unglazed solar air heating within reason.

. As with any solar energy or alternate energy system of any kind the correct application of the technology is the key to success. The potential of solar air heating systems with this unglazed concept is equivalent to the basic Hot Sun concept that put us in business in 1986. Low cost per area combined with high performance per area equals cost effective. Cost effective by a long shot compared to the high tech glazed insulated and evacuated tube hot water solar thermal technologies that create solar magic in the eyes of policy makers. The key point that gets overlooked is that when heating air or water at close to or the same temperature as the ambient air temperature unglazed technology collects a large portion of what is available, a larger portion than an expensive collector would even at the same low temperatures. Its counter intuitive but it is basic physics. You can't collect more than you see. Unglazed collectors collect more per area in their low temperature operating regime and they do so at a fraction of the cost per area meaning solar can compete with fossil fuels, easily and without subsidies.

Buildings that offer the best solar air preheat opportunities include aircraft hangars in Eastern Canada where large volumes of exhaust must be ventilated to outside. The Ford battery plant in Oshawa Ontario was the first major Solarwall installation. Fresh air has to replace the exhaust air and if its cold like it is in Eastern Canada in the winter then this can be a significant heating load. The Solarwall will clearly be a very good choice in these cases if there is a south facing wall available that can be superclad and if the region sees reasonable solar radiation in winter and there isn't another building shading the wall from low winter sun. The success or failure of these projects is in the details.

In high load cases in regions with cold winter air and reasonable solar availability (Eastern Canada) the payback period can be less than 10 years. The potential for this to be the most viable solar application is there. The only competition for that spot would be a Powerstrip solar water pre-heater applied to a large water heating load. This study by NREL documents many appropriate applications of perforated wall solar air pre-heaters. The Solarwall web site documents many others.

The costing of Solarwall installations per square meter seems to fall right in line with the costing of Powerstrip water heating installations. Both are unglazed applications at similar temperatures meaning efficiencies are similar. We'd expect Solarwall viability to be very similar to Powerstrip viability and it seems like they are according to reports. In both applications its very important to apply the technology correctly.

Its very important to not overlook the fact that often a large portion of the incoming air heating can be accomplished with heat recovery ventilation. These are simply gigantic heat exchangers made of lightweight and inexpensive signboard. The Solarwall requires all the ventilation air be passed thru it for effectiveness as does a HRV but an HRV uses the exhaust air at room temperature to heat the incoming air. HRV's don't require that the incoming air flow rate be as high. The appropriate applications for HRV's are vast compared to Solarwall but HRV's are perhaps more difficult to retrofit into existing buildings. HRV's work day and night. Solarwalls work when they see solar radiation. The HRV to solar air heating is like the gas fired boiler is to solar water heating. The two mechanical systems together must be considered in their entirety. Proper solar design means looking at the bigger energy picture and embracing all the possibilities, rather trying to quantify the possibilities so that good energy engineering decisions can be made.

John Hollick , the president of Solarwal has asked me to point out the following in this article...

SolarWall actually works in cloudy as well as sunny periods as cloud cover still can have energy of 100 to 400 w/m2 and will heat ambient a few degrees which is free heat when heating fresh air for a building.

Unglazed panels do have a lot of advantages which need to be articulated – no costly glazings and support structures etc

Another major factor for unglazed panels both air and liquid is that in the absence of glazing, our systems receive 100% of the sun energy falling on the surface; whereas all glazed panels only transmit 80 to 90% of the sun’s energy to the solar absorber. If the glazing is dirty, then this number will be reduced. When dust accumulates on unglazed panels, the dust does not reduce the solar gain as it becomes part of the solar absorbing surface.

Glazing can also reflect more light in the morning and afternoon and solar tests require an incident angle modifier for predicting solar gain when the sun angle is not perpendicular on the panels. Unglazed panels still receive the sun at all angles and do not require this additional test.

The trade off between glazed and unglazed is the desired temperature rise of the air or liquid which we have found for air to be up to 30 C rise over ambient. If the application needs higher temperatures, then some glazing may be warranted and we now use a 2-stage solar heating system which can reach 50C over ambient.

Thank you Mr Hollick for your contribution.