Solarwall--Simple and Effective Solar Heating

"Many solar heating devices are handicapped by the high cost and maintenance of glazing systems. Solarwall? is a deceptively simple yet amazingly efficient system for preheating incoming air that doesn't require glazing or any other high-maintenance parts. Generically termed a transpired solar collector, Solarwall has initially been targeted towards large commercial and industrial buildings, especially those with high-volume ventilation requirements. As the system has proven itself in field-tests, however, it is looking attractive for more and more building types.

Solarwall is a thin, dark-colored aluminum or galvanized steel cladding perforated by tiny holes, enclosing a plenum between the cladding and the building skin (see figure). Air drawn through the 1/32" (0.8 mm) holes is heated by the metal before entering the ductwork of the building's ventilation system. On a sunny day, a Solarwall can preheat air by 30deg.F to 54deg.F (17ˇăC to 30ˇăC). The same system is now being used for crop drying in the Far East.

Interestingly, the system's efficiency improves slightly at colder temperatures, because less collected heat is radiated away. Adjacent snow cover also improves performance by reflecting radiant energy onto the wall. The plenum also captures heat escaping through the building envelope, improving system performance. On cloudy days diffuse radiation is collected, albeit at much lower efficiency than direct solar radiation. Even in summer the system is beneficial, as it shades the south wall, keeping it cooler than if it were exposed. With the fresh-air intake for the building seasonally diverted to another location, air enters the plenum through the holes near the bottom and, due to stack-effect pressures, exits near the top of the wall.

Versions of the product were already being installed by Conserval Engineering, Inc. of Downsview, Ontario, Canada and Buffalo, New York, when researchers at the U.S. National Renewable Energy Lab (NREL) came up with the same idea. Advanced research on both laboratory-scale and full-size models at NREL has helped Conserval refine the product and optimize its efficiency.

Several large industrial buildings have been clad with Solarwall since the early 1990s. The first of these, a Ford manufacturing facility in Oakville, Ontario, began as a glazed air collector in 1986 and was converted in 1989 to the unglazed version. The glazed collector had been performing at 50% seasonal efficiency. "We expected the efficiency to drop when the glazing was removed," says Doug McClenahan, R&D project manager for active solar at Natural Resources Canada, "but instead it went up." Monitoring by Natural Resources Canada on a 1992 installation reveals an average seasonal efficiency of 72%. "It's the highest efficiency for a heating season in an air collector that I've ever seen," says McClenahan.

The largest installation to date, 110,000 ft2 (10,200 m2) of Solarwall on a Canadair building in Montreal, Quebec, is now 50% complete, and the first large U.S. installation, on the Fort Carson, Colorado U.S. Army base, has just been completed. McClenahan reports that capital costs for the Canadair installation were about the same as the cost for a gas-fired ventilation air preheat system, so the Solarwall had an immediate payback.

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Solarwall retrofit on a high-rise apartment buiding in Windsor, Ontario (photo: Conserval Engineering, Inc.)

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The estimated payback for most systems, accounting for the incremental cost of Solarwall over an inexpensive cladding system, is up to three years (depending on solar exposure, ventilation rates, local climate, fuel cost, and other factors). In retrofits, where the full cost of the system must be considered, paybacks can be as long as six to seven years. Conserval has found an even more attractive retrofit market in high-rise buildings built during the 1960s with cladding systems that are now failing. They have now clad the south face of five such buildings with Solarwall, in some cases over additional insulation.

On industrial buildings the Solarwall surface fits in with other metal cladding. On commercial and residential buildings, however, the large, uniform, dark surface may create an aesthetic challenge for designers.

At 0.032" (0.8 mm) thick, the cladding material weighs about 0.46 lb/ft2 (2.2 kg/m2) for aluminum and about 1.3 lb/ft2 (6.3 kg/m2) for steel. EBN's best data indicate that about 25% of the aluminum is likely recycled material, and about 22% of the steel. Solarwall is typically coated with a polyvinyl fluoride coating, such as the Kynar 500reg. coating used on premium metal roofs. In some installations a less expensive siliconized polyester coating is used.

Embodied energy of the aluminum is about 37,000 Btu/ft2 (420 MJ/m2). Given an average sunny-winter-day solar insolation on a south-facing wall of about 1,500 Btu/ft2 (17 MJ/m2) for the northern U.S. and southern Canada, it takes about 34 sunny winter days to recoup the energy used to make the aluminum in a Solarwall panel (not including coatings).

Cost of a Solarwall panel is about $2.50/ft2 ($25/m2) for the steel and $4/ft2 ($40/m2) for aluminum. With the blowers, ducts, and other components the materials for a system run about $5/ft2 to $7/ft2 ($50/m2 to $70/m2), according to Hollick. Installed systems cost on the order of $10/ft2 ($100/m2).

For more information:

John Hollick, President

Conserval Engineering, Inc.

200 Wildcat Road

Downsview, ON M3J 2N5 Canada

416/661-7057

416/661-7146 (fax)

Conserval Systems, Inc.

4242 Ridge Lea Road Suite 1

Buffalo, NY 14226

716/835-4903

716/835-4903 (fax)

Doug McClenahan

R&D Project Manager for Active Solar

Natural Resources Canada

580 Booth Street

Ottawa, ON K1A 0E4 Canada

613/996-6078

613/996-9416 (fax)

doug.mcclenahan@cc2smtp.nrcan.gc.ca

Charles Kutscher

National Renewable Energy Laboratory

1617 Cole Boulevard

Golden, CO 80401-3393

303/275-6024