The 16,000 square foot roof of a Federal office building at the Army Research Lab had reached the end of its useful life. After ongoing leaks and years of patching, the building managers decided the building needed a new roof.
In reviewing solutions, the facility manager uncovered an intriguing option, presented by American Solar, Inc., a solar-thermal engineering firm. American Solar proposed covering a portion of the roof with a metal solar air-heating roof. The Lab discovered that a metal air-heating roof would last longer than a standard flat roof, deliver heat to the building, reduce its operating costs and help the facility meet its sustainability goals.
This system gives metal roofers new business opportunities, especially as energy prices rise. Customers can now consider a metal roof (if it is an air-heating metal roof) an energy efficiency investment and not merely an expense.
The Project Team
For this project, American Solar, Inc. designed the system and hired two metal roofing contractors to install standing seam metal roof panels over a metal framing system. Merit Builders installed the metal framing system and Pioneer Roofing installed the metal panels. Englert, Inc. supplied the metal coils. Other team members included a duct installation firm, a systems integrator (to connect the project’s fans and controls to the building’s systems) and a structural engineer to verify the new system’s compatibility with the existing roof structure.
How a Metal Air-Heating Roof Works
Contractors installing a solar air-heating roof use a standard metal roof installation process with just a few extra steps.
Prior to installation, a solar-thermal engineer determines how the building can best use the heat from the solar-air roof.
The project team then installs:
- A support structure below the new roof to create a solar-heated airspace
- A flexible radiant barrier under the support structure to raise the air temperature in the airspace and to keep the building cool
- Fans to pull the heated-air through the system at the appropriate flow-rate
- Ducts to direct the heated-air to the appropriate place or equipment
- A thermostat to turn the fans on and off based on pre-set temperatures.
A metal solar-thermal roof uses standard metal panels and framing. All of the fans, ducts, and controls use standard HVAC components. Experienced HVAC contractors can install the components based on the solar-thermal engineer’s specifications.
The metal roof can be any color to match the aesthetics of the building, though darker colors work best. Black metal roofs heat air to up to 70 degrees F above outdoor temperatures. Other dark colors provide slightly less heating. For the Lab, the facility selected a matte black panel color. In early tests, the Lab’s metal roof has reached temperatures of 65 degrees F above outside temperatures.
Useful Life of a Metal Air-Heating Roof
With routine replacement of the moving parts, and long life of the roof panels, the systems can easily provide 40 years of service. Many metal solar-air roofs can receive long-term warranties of 20+ years.
During nighttime or cloudy days, buildings must rely on standard heating processes for heating. While a solar air-heating roof can dramatically improve a building’s heating operations, it may not meet all of the building’s heating needs. If the system provides more heat than the building needs, it does not turn on the fans and the atmosphere absorbs the excess heat. The radiant barrier keeps the excess heat out of the building.
“While many roof designers worry about minimizing a roof’s heat gain, a solar-thermal engineer looks at how to economically use the heat to improve a building’s energy efficiency,” explains John Archibald, President of American Solar, Inc. “In many cases, the recovered solar heat is worth much more than the reduced cooling value of a reflective roof. That’s especially true for buildings with year round heating loads, like auto body shops with paint booths, for example,” he added.
How do the Economics Work?
Buildings can use the heat generated by a metal solar-thermal roof in many ways. This system can heat air or water. In some climates, buildings can use the heat to dehumidify air or to re-heat conditioned air.
The Lab will use the heat from its new solar air-heating roof to pre-heat fresh outside air going into the building, to heat water with an air-to-water heat exchanger and to pre-heat air for a new heat pump, which will re-heat conditioned-air.
To improve the economics of large roofs, roofers install the metal solar air-heating roof over just the roof portion with the best orientation. The rest of the roof can be a membrane roof or an identical metal roof, without the solar heat recovery. In this case, the Lab needed to replace a 16,000 sq ft roof of which 11,000 is now a new metal solar air-heating roof and 5,000 is a new membrane roof that surrounds the solar roof.
The Army Corps of Engineers managed this project as a design-build contract. The installed system will produce more than 170 million BTUs (British Thermal Units) annually and save the facility approximately 30 barrels of oil each year (or $3,500 based on the current oil price).
Most solar-thermal projects have a three to five year payback, with solar features costing about $5/sq foot. Buildings that need low-temperature daytime heat day have the shortest payback. Many types of buildings can use solar air-heating roofs including low-rise office buildings, laboratories (which need to heat a steady-stream of outside air), schools, hotels/motels, light-industrial facilities, etc.
A New Market for Metal Roofers
Many people think of photovoltaic (electric) panels when they consider solar. Consequently, some are surprised to discover that solar-thermal (heat) applications are actually a much larger market.
According to the US Energy Information Administration, more than 60% of a building’s energy need is for heat (not for electricity). Most of a building’s heat-energy need is for low temperature heat (below 120 degrees F). For example, water heating for domestic and kitchen use is typically about 110 degrees F. Buildings need to heat air in occupied space to about 72 degrees F. Many warehouses use heat to maintain stored materials just above freezing, 35-40 degrees F. A solar air-heating roof can help meet all of these low-temperature heating needs.
With a much longer life, building owners reduce the number of roof replacements their buildings need, when they select a metal roof. When the metal roof is a solar-thermal roof, the building also reduces its heating expenses and taxes. In some retrofits, the building owner can qualify for tax credits and accelerated depreciation, which further improve the project’s economics.
The future for metal solar-thermal roofing is very bright and opportunities exist for many metal roofing firms. Since the roof uses standard metal roofing techniques, any experienced metal roofing firm can install it. As energy prices increase, the metal solar-thermal roof offers a growing market.