The idea is simple. Take a black metal plate. Drill holes in it. Attach it to the sunny side of a building. Voila! Solar collector.

It was a simple idea, but it took scientists at the National Renewable Energy Lab (NREL) to figure out the details, such as how many holes, how big, how far apart, etc. They used wind tunnel tests and computer simulations to answer these and other design questions. The result of NREL's research is a solar collector that is about 70 percent efficient compared to 30 to 40 percent for conventional collectors. Now, it's a commercial product called Solarwall from Conserval Systems, Inc.

Unlike most solar collectors, there's no glazing. Solarwall is a solar preheater for ventilation air. The heat captured by the collector is drawn into the house by a ventilation fan. Preheating reduces the energy needed to bring the incoming air up to comfortable temperatures. Passing through the collector boosts air temperature by as much as 54°F. As air flows through the panels, it picks up heat that would normally escape through the wall to the outside. This boosts building performance even more.

Solarwall is ideal for commercial and industrial buildings with large ventilation requirements. Residential applications can also be practical in colder climates where ventilation increases space heating loads. For homes, the panels can be sized to the amount of ventilation air needed. One to two sq. ft. of panel would be needed for each cubic foot per minute (cfm) of ventilation requirement. For example, an 1800 sq. ft. house might need 84 cfm to maintain 0.35 air changes per hour. That would require an 84-sq.-ft. to 168-sq.-ft. Solarwall.

Each panel is 2-1/2 ft. x 8 ft. For residential jobs, the panels are mounted about four inches off the weathertight exterior wall with dimensional lumber. An axial fan pulls air through the panel and blows it into the house. Air can flow directly into the living space or be connected to a forced air heating system for distribution. One control option is a differential temperature control that turns the solar fan on when the air in the panel is higher than outside. Panel cost is about $5 per sq. ft.
Aesthetics could limit Solarwall for residential applications. Color selection is limited to dark blue, dark brown, dark green and black. Some homeowners may not want large areas of their house covered with dark, perforated aluminum.

Solarwall systems offer an alternative to heat recovery ventilators for sunny building sites in cold climates.

Contact: Conserval Systems, Inc.

The unglazed Solarwall collector preheats air that feeds a fresh air venitlation system.