Transpired Solar Collector Walls
By Charlene Riegger, ATAS International
As green building practices become more important — not only for environmental reasons, but also for cost savings and tax credits — new products supporting this philosophy are popping up everywhere. This wealth of product information and “greenwash” can be tedious for the building professional to sort through. The transpired solar collector, however, is one that deserves the attention of the savvy building envelope consultant. The U.S. Department of Energy (DOE) has called transpired collectors “the most reliable, best-performing, and lowest-cost solar heating for commercial and industrial buildings available on the market today.” (DOE/GO-10098-558, 1998.) The concept has received numerous honors and awards from DOE, the American Society of Heating, Refrigeration, & Air Conditioning Engineers, Inc. (ASHRAE), R&D Magazine, and many others worldwide.
The concept is simple: perforated metal wall cladding is attached approximately 4 to 8 inches from a south-facing wall to a support grid of vertical and horizontal channels. The system may be applied vertically or horizontally over any noncombustible wall substrate, over or around existing wall openings. The grid system vertical channels are attached to the building wall, the horizontal channels are attached to the vertical channels, and the perforated metal sheets are through-fastened to the horizontal channels.
The transpired solar collector wall may be mounted to the wall in several different ways, depending on the volume of air required. In some cases, only a portion of the south wall is needed, or even a penthouse wall may be suitable. The sun heats the metal panel and the heated air is drawn through tiny holes into the cavity between the panel and the wall by fans mounted at the top of the wall. The fans then distribute the heated air into the building through flexible ducts mounted from the ceiling, or through standard ducts connected to the heating and ventilating system. In the winter, the heated air removes a substantial load from the building’s conventional heating system, thus saving considerable energy and money. In the summer, the panel is shading the inner wall, thus reducing the cooling demands of the building. When heating is not required, a controlled damper can be opened to allow air to bypass the solar collector, providing a continual supply of fresh air into the building. The transpired solar collector wall is also effective on cloudy days, although at a reduced level.
In addition, the use of this preheated fresh air system eliminates stratification of the air inside industrial buildings, where hot air rises to the ceiling and is lost through the roof or drawn out with exhaust fans. Since the air is constantly being replaced, the system is ideal for vehicle repair shops, machine shops, chemical storage plants, and industrial applications where fumes are present. The system also provides positive pressure for the building. When a door or window is opened, the heat from indoors exits, but the cold outside air does not rush in.
New ventilation codes specify minimum ventilation rates and indoor air quality, depending on the type of building and the number of occupants. Inadequate fresh air may lead to “Sick Building Syndrome,” which results in headaches; eye, nose and throat irritations; and fatigue and/or difficulty in concentration for its inhabitants. This system replaces the air constantly and is ideal for manufacturing plants, hazardous waste storage buildings, gymnasiums, airplane hangars, schools, office buildings, apartment buildings and warehouses requiring ventilation. The system also provides process heat for agricultural or industrial purposes.
Environmentally, the transpired solar collector wall is using natural energy that is clean, thus lowering the need for fossil fuel heat and reducing production of greenhouse gases. Payback of the system is relatively short, and state and federal grants, as well as tax credits, are given as incentives for solar energy use. The installation may qualify for Leadership in Energy and Environmental Design (LEED®) points and credits. The cost of conventional energy can vary greatly, depending on the area and the season of the year. With soaring energy prices, the use of free solar heat will reduce the need for conventional energy sources.
Any existing non-combustible south-facing exterior wall in need of repair could be covered with the energy-saving transpired solar collector wall panel. For a uniform appearance, the other exterior walls can be covered with a similar panel system in the same color. For variety and aesthetics, coordinating or contrasting colors may be used. Use of the perforated panels on a vertical, southern exposure wall is recommended to collect the most solar energy. A vertical surface will give more reflected radiation, with no snow build-up and low wind loads.
Research has proven that each square foot of panel will supply 150 to 200 BTUs per hour. Intake air is preheated by up to 65°F (36°C) above ambient air temperature, reducing annual heating costs by $1 to $5 per square foot of collector wall, depending on the type of fuel displaced. U.S., Canadian, and German governments have independently monitored installations of this type. The International Energy Agency (IEA) Solar Heating and Cooling Program is reporting efficiencies of over 70%, according to its report, SHC.T14.Air.1 9/99.
The National Renewable Energy Laboratory, a division of the Department of Energy (DOE), is currently monitoring some projects and is continually researching the transpired solar collector system.
Transpired solar collectors are typically manufactured from aluminum and, to a lesser extent, zinc, in two-wide, ribbed configurations. Thickness varies from .032-in. aluminum to .028-in. zinc. Panels are approximately 40 in. wide and are available in many colors and with a PVDF (polyvinylidene fluoride) finish. Darker colors with high solar absorption rates are recommended for maximum effectiveness. The systems are virtually maintenance free, since there are no liquids or moving parts other than the fan system or optional filters. The paint finish is warranted for 30 years.
Transpired air collector systems are successfully being used by both private and governmental sectors. Installations in automotive, manufacturing, aviation, education, and distribution facilities have shown substantial energy savings. Transpired solar collector systems have been installed on many buildings throughout the United States.
Pennsylvania Manufacturing Facility
One installation, on a new 60,000 sq. ft. manufacturing facility in Allentown, PA, was completed in 2006. Its design objectives were to provide reduced heating costs, year-round comfortable air ventilation, and positive air pressure.
A 3,600 sq. ft. Classic Bronze collector (solar absorptivity 0.91) faces 16 degrees west of due south. The total airflow rate is 18,000 cubic feet per minute (cfm), which is delivered by two 30-in diameter fans. For each square foot of collector wall, the flow rate is 5 cfm. The annual renewal energy delivered is 650 million BTUs. The average air temperature rise is 13°F at 5 cfm/sq ft during daylight hours for the 9-month use season. The rise of the temperature over ambient air on a sunny afternoon is typically 55°. The annual greenhouse gas reduction for this installation was 98,000 pounds per year. The annual projected energy savings for this project was $16,500, with an actual savings of $22,086, translating to a savings of $4.60 per sq ft of collector wall. The total projected energy savings for this installation over a 30-year project life is approximately $700,000. The simple payback period, with accelerated MACRS depreciation, is less than 3½ years.
Greenwood Elementary School
The Greenwood Elementary School in Millerstown, PA, was recently renovated to include 730 sq. ft. of black-colored collector wall. The annual greenhouse gas reduction is 19,000 pounds. The annual heating cost savings is $2,553, with a simple payback of five years (based on 2008 natural gas costs). Expected savings over the 30-year project life are $115,000. Schools are not eligible for tax credits. However, when using on-site renewable energy, there is a possibility of additional grants from state governments.
The transpired solar collector wall is available from ATAS International, Inc. who manufactures the product under the trade name InSpire™ Wall.
About ATAS International
Founded in 1963, ATAS International, Inc., a leading manufacturer of metal roofing, wall and ceiling panels, produces systems for commercial and residential buildings. The ATAS product line offers more than 20 profiles available in aluminum, steel, zinc and copper. These profiles include: standing and batten seams; corrugated; curved; tapered; shingles; tiles; shakes; and narrow or wide ribbed panels with hidden or exposed fasteners. Rainware, crown moulding and perimeter edge trim are also available with the same finish to match or accent a roofing or wall system.
ATAS offers energy efficient building envelope technology, such as cool roofing products that are ENERGY STAR® qualified and InSpire, the transpired solar collector, which utilizes precision perforated metal panels to preheat ventilation air via solar energy. ATA-Solar, a building integrated photovoltaic product complements ATAS’ sustainable product offerings. Profiles are coated with a Kynar 500® PVDF or Hylar 5000® PVDF finish in 30 standard colors. ATAS has four manufacturing facilities in the United States: two facilities, including the headquarters, in Allentown, Pa.; one in Maryville, Tenn.; and one in Mesa, Ariz. All ATAS locations are certified to ISO 9001:2008 with design. For more information, go to www.atas.com.