Combining a Cool Roof & Photovoltaics Can Enhance System Performance & Save Even More Energy
by Russell Giess, vice president, Coat’n’Cool
(Editor’s Note: Russell Giess is vice president of Coat‘n’Cool a Yorba Linda, California–based manufacturer of proprietary cool roof coating products. He is a member of the U.S. Green Building Council.)
From Western Roofing Nov/Dec ’13
Solar panels and cool roofs… it is a natural melding of sustainable technologies. Tests show that combining a photovoltaic system with a cool roof, assuming optimum operating conditions, can improve the performance of a solar-power system by as much as 10%.
One solar installation company that has embraced the marriage is Advanced Powering Services, Inc., of Rancho Santa Margarita, Calif., which has installed a cool roof/solar panel beta test site on the roof of a local industrial building. Tim Scharf, chief operating officer, says that although it’s too soon for the beta system to generate any long-term data, he reports that based on initial data the cool roof does increase the energy output of the solar panels.
“Based on the system we are using, we expected the energy output to be 10.5 KWh and in fact it is 11.8 KWh,” Scharf notes. “We attribute this increase to the fact that the cool roof maintains a lower, more optimal roof temperature, which benefits the performance of the solar panels.” Scharf points out that solar systems work best at temperatures below 90° and that if temperatures exceed 110°, the solar power output can be reduced by as much as 50%.
Bill Conley, CFM, CFMJ, LEED AP, an Aliso Viejo, Calif.-based facility management and sustainability consultant who is a board member of International Facilities Management Association, is also a proponent of marrying cool roofs with solar. He points out that as a facilities manager, his goal is to achieve the best possible operating configuration that saves money and energy, optimizes sustainability, and ensures the longest possible usage of a building and its environs.
According to Conley, who is a long-time advocate of sustainability, a cool roof combined with solar can improve the performance and thus economics of a PV system and at the same time can be an especially important factor for commercial property owners in California complying with the state’s Title 24 energy efficiency regulations. Echoing Conley, Ian McLaughlin with Lineside Electric, a San Juan Capistrano, Calif.-based installer of solar systems, states, “A cool roof as an integral component of a solar installation enhances reflectivity and other benefits such as cooling a building’s interior that can significantly improve the operation and efficiency of the photovoltaic system.”
One of the more prominent buildings, which combines a cool roof with solar panels, is the Environmental Protection Agency’s 101,000 sq.ft. National Computer Center in the Research Triangle of Raleigh-Durham, North Carolina. The Computer Center, which achieved a LEED-NC Silver rating in early 2005, handles a massive quantity of data processing equipment. To achieve the Silver rating and reduce the building’s substantial power load, 15,000 sq.ft. of the roof is covered with solar panels on top of a highly-reflective, Energy Star compliant, cool-roof white membrane (reinforced thermoplastic polyolefin) that reduces unwanted heat during the cooling season.
The solar power system consists of a 94-kilowatt (peak) photovoltaic array made up of 2,185 individual tiles. Each tile is a stacked composite made up of a layer of rigid polystyrene foam insulation board, a wiring chase and airspace, and a PV module. Each tile interlocks with adjacent tiles and rests on top of the membrane-covered roof deck with no mechanical penetrations. The output of the PV array offsets approximately 5% of the building’s electricity consumption, which is estimated to be approximately twice that of a conventional office building of equivalent size, due to the large demand from data-processing equipment.
Greg Eades, energy manager for the EPA Research Triangle Park campus, says that while he has no specific data on the effect that the cool roof has on the performance of the PV system, it does appear that the roof is beneficial. Specifically, he points out that the energy output of the system has increased over the past three years from 85,000 KWh to 105,000 KWh while the “insolation” factor has decreased (insolation is a measure of solar radiation energy received on a given surface area in a given time).
“One would expect that if the solar output is increasing, that the insolation is also increasing,” Eades explains. “However, for this system, the opposite is happening. The energy has increased while the insolation has decreased, which is counter-intuitive. Although we don’t have the data to prove it, one could surmise that the reason for the inverse relationship is the presence of the cool roof. If that is the case, then the cool roof is definitely beneficial.”
The Importance of a Cool Roof
Because of their lighter color, cool roofs reflect sunlight (i.e. solar reflectance) and efficiently emit thermal radiation (i.e. thermal emittance). This has many benefits including cutting energy costs by keeping attics and ducts cooler, improving occupant comfort, cutting maintenance costs, increasing the life-cycle of the roof, and reducing urban heat-islands along with associated smog.
White reflective coatings used in a cool roof contain transparent polymeric materials, such as acrylic, and a white pigment, such as titanium dioxide (rutile), to make them opaque and reflective. These coatings typically reflect 70% to 80% of the sun’s energy. Thus, the pigments help protect the polymer material and the substrate underneath from UV damage. As long as the coating is white or light-colored, the roof will have high reflectance and emittance levels.
Although the actual benefits of a cool roof on a particular building will depend on many factors, including building type, load, season, and climate zone, a cool roof can significantly reduce summer electrical energy usage, and at a much lower cost. A reasonable annual energy-savings expectation for a typical low-rise retail or other commercial building is 10% to 30% of the electricity usage for air-conditioning.
Based on our experience at Coat’n’Cool, a reflective roof can lower interior temperature of a commercial or industrial building by 8-12° during the hottest four hours of a summer day, noon to four in the afternoon. Not only does the lower interior temperature help reduce energy costs, it also improves worker productivity, especially in a non-air conditioned space, by creating a working environment that is more comfortable.
If you are considering installing both a cool roof and PV system, it’s important to do your homework. Not all PV systems are the same, so selecting the right system is critical to achieving optimum operation and expected energy savings at a reasonable installation cost. The best way to ensure you are installing the right system is to retain an energy consultant who has experience with both photovoltaics and cool roofs. By conducting an energy audit, the consultant will be able to recommend the right system based on the size, location, use, and age of the building(s).
It’s also important to ensure that the roof system is in good shape before installing solar panels. If the roof needs to be replaced or patched after solar has been deployed and operational, the lost revenue to the owner can be substantial. That’s why, according to David Montross, president of Montross Roofing, an Orange County, Calif.-based firm that specializes in roofing, decking, and construction services, a careful inspection of the roof system is absolutely necessary. He points out that if the roof shows signs of weathering or wear and tear such as blistering and splitting, then it’s probably best to reroof with a cool roof before installing the solar panels.
What to do during a roof inspection:
- Make sure the roof is clean of any loose items such as boards, blocks, old satellite equipment, old wiring, or even small items of debris. This will speed up the inspection and possibly lower the quote price as the roofing contractor will not need to charge for existing debris removal.
- Walk the entire roof surface looking for any areas where the roof may be torn, or a visible puncture, or an existing patch, and document it.
- Check all the corners and parapet wall (if applicable) to make sure there are no cracks or tearing in the parapet walls.
What not to do during your personal inspection:
- Do not attempt to make any repairs yourself. Making an error during a roof repair can be dangerous and costly. Call a professional.
- Do not remove anything that you are not sure what it is or its function.
Even if the roof is fairly new, a close inspection is necessary to ensure the best possible performance. Of particular concern should be depressed areas that could become ponding points for standing water, and areas where the roof covering and under-structure have been penetrated by a skyline, vent, or other intrusion that could allow water to migrate into the structure and eventually undermine its integrity. Make sure low areas are filled in and points where water could infiltrate the roof structure are sealed with mastic cement or a comparable product. For the optimum performance of the cool roof and/or solar investment, a roof should be able to last at least 20 years from the time of the installation.
“If considering a new roof, seriously consider installing a cool roof and, if possible, a solar system on that roof all at the same time,” Montross explains. “A cool roof reduces building cooling requirements by lowering the temperature of the roof and the building underneath. This means cooling equipment savings and in many cases the ability to run less air conditioning or purchase smaller air conditioning units. A cool roof will also increase the life of a roof. By lowering the roof temperature, roofing products may last longer due to less thermal stress over time.”
Cool Roof/PV Tax Benefits
Combining photovoltaics with a cool roof has also attracted attention, in a positive way, from the Internal Revenue Service. In an IRS “private letter ruling” (P.L.R. 200947027), the IRS determined that the cost of a “highly-reflective” roof installed in connection with a rooftop solar installation qualifies for the federal investment tax credit. The IRS ruled that the cost of improvements to a roof can qualify for the investment tax credit because the highly-reflective roof surface “meaningfully increased” the amount of electricity generated by the PV panels.
An important note is that private letter rulings are solely addressed to the individual taxpayer requesting the ruling. Nonetheless, private letter rulings are generally accepted as a statement of the IRS view of the law as applied to the facts in the ruling. However, each situation is different and either the contractor or the customer should consult their own tax advisor concerning the federal tax implications of an investment in PV panels in connection with a cool roof.