It seems like a natural marriage of sustainable technologies . . . solar panels and cool roofs. Why not combine the energy-saving logic behind cool roofs with the energy producing value of a rooftop solar system? One company reports that the output of its solar power system when combined with a cool roof increases 20 percent due to the improved collection of reflected and diffuse light. How do cool roofs designed to reflect sunlight work with solar panels, which are designed to absorb it? Rather than working against the panels, the reflectivity of cool roofs sends more light to the panels from all directions.
There are many types of sustainable roofs including white roofs, green roofs, and roofs with solar photovoltaic (PV) panels and/or solar hot water systems. The performance of sustainable roof technology can often be optimized if it is integrated with a different, complementary sustainable roof technology. For example, the efficiency of a PV system frequently improves when it is placed above 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”). By cooling the roof and lessening heat transfer into the building, cool roofs reduce the cooling load of the facility’s air conditioning system, thereby saving energy and money while improving sustainability by minimizing greenhouse gas emissions. It’s a win-win from every viewpoint.
Solar panels and cool roofs . . . it is a natural marriage of sustainable technologies. One solar installation company that has embraced the marriage is Advanced Powering Services, Inc., of Rancho Santa Margarita, CA, which has installed a cool roof/solar panel beta test site on the roof of a local industrial building. Tim Scharf, Chief Operating Officer, said that although it’s too soon for the beta system to generate any long-term data, he can report that the cool roof does seem to 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, whose company is currently working with several multifamily property owners in Southern California to install cool roofs and PV systems, points out that solar systems work best at temperatures below 90 degrees and that if temperatures exceed 110 degrees, the solar power output can be reduced by as much as 50%.
THE IMPORTANCE OF A COOL ROOF
Energy-efficient, cool roofing systems can significantly reduce roof temperatures during the summer, which does in fact improve the performance of the photovoltaic system. Another benefit of a cool roof is that it can significantly enhance the performance of solar panels by lowering a building’s interior temperature, thus reducing the power load necessary for air conditioning during hot summer days. 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.
Solar reflectance and thermal emittance are the two measures used to determine the «coolness» of a roof. Solar Reflectance Index (SRI) of a roofing product is a method for determining the radiative properties of roofing materials. SRI is defined by ASTM Standard E1980-01; the EPA summarizes SRI as “the relative steady-state surface temperature with respect to the standard white (SRI=100) and standard black (SRI=0) under the standard solar and ambient conditions.”
White reflective coatings contain transparent polymeric materials, such as acrylic, and a white pigment, such as titantium dioxide (rutile), to make them opaque and reflective. These coatings typically reflect 70% to 80% of the sun’s energy. Despite the white appearance, these pigments absorb the 5% or so of the sun’s energy that falls in the ultraviolet spectrum. 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.
ACTUAL BENEFITS DEPEND ON MANY FACTORS
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, cool roofs can significantly reduce summer electrical energy usage. 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 coating can lower interior temperature of a commercial or industrial building by 8-12 degrees 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.
A 2001 study by the Lawrence Berkeley National Laboratory in cooperation with the U.S. Environmental Protection Agency underscores the value of cool roofs for reducing energy usage and related costs. The study focused on a 100,000 square-foot building owned by a major retailer in Austin, TX, built with a traditional roofing system featuring an exposed black rubber EPDM membrane.
After 13 years of operation, the black roof was replaced with a white thermoplastic roofing system. The result of replacing the black roof with a white cool roof was significant. Direct benefits included daily savings from reduced demand for operation of the air conditioning system as well as present value of future savings coupled with the benefits from energy conservation programs sponsored by the local utility as well as state and federal programs.
Specific benefits from the cool roof based on 2001 factors such as electricity rates included the average summertime temperature of the cool roof compared to the black roof was 42% less, dropping from 168 degrees F to 126 degrees F, thus increasing the useful life of the roof; and reduced summertime A/C usage by 14% resulting in average monthly savings of $490.
CALIFORNIA’S NEW TITLE 24 COOL ROOF REQUIREMENT
The growing number of cool roof installations in California has been helped by the support of the U.S. Department of Energy’s ENERGY STAR Program and by the requirements of the California’s new Title 24 “Cal Green” Energy Standard, which prescribes cool roofs to be employed whenever low-slope commercial roofs are constructed or replaced.
Bill Conley, CFM, CFMJ, LEED AP, a Aliso Viejo, CA – based facility management and sustainability consultant who is a board member of International Facilities Management Association, is 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 be an important factor for California commercial property owners in complying with Title 24.
Echoing Conley, Ian McLaughlin with Lineside Electric, a San Juan Capistrano, CA – based installer of solar systems, states, “A cool roof as an integral part 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.”
Willard Young with Solyndra, Inc., a Fremont, CA–based producer of innovative photovoltaics for commercial rooftop installations, reports that based on the company’s data, the output of its unique solar power system – which consists of cylindrical collectors instead of flat panels– when combined with a cool roof increases up to 20% due to the improved collection of reflected and diffuse light.
Consequently, Solyndra is a major proponent of marrying solar power with a cool roof. Three East Coast companies that combine cool roofs with Solyndra’s system are Public Service Electric and Gas Company, Norkus Foodtown and LPS Industries, all based in New Jersey. Six of Norkus’ grocery stores have white, reflective roofs that significantly increase the energy power generation and reduce cooling costs by as much as 20%. LPS Industries, which manufactures flexible packaging that protects items such as foods, medical tests and devices, and electronic products, has experienced about the same level of energy savings. PSE&G recently installed a 0.9-megawatt solar system/cool roof combination is on its Central Division Headquarters in Somerset, NJ, and it will produce enough electricity to power 155 average size homes.
THE EPA NATIONAL COMPUTER CENTER
One of the more prominent buildings that combines a cool roof with solar panels is the Environmental Protection Agency’s 101,000 square-foot National Computer Center in the Research Triangle of Raleigh-Durham, NC. 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 square feet of the roof is covered with solar panels on top of a highly reflective, Energy Star compliant, white membrane (reinforced thermoplastic polyolefin) that reduces unwanted heat during the cooling season. In other words, a cool roof.
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.”
To enhance the EPA building’s energy efficiency, a digitally controlled, fully-automated Building Automation System (BAS) monitors and controls various aspects of the building, including temperature, pressure, humidity, electrical systems, computer room cooling units, cooling and heating equipment, maintenance indicators, lighting, and security. Electronic sensors placed throughout the facility communicate to the BAS when temperature, humidity, fresh air ventilation rates, and other environmental conditions need to be adjusted, further increasing the facility’s energy efficiency by reducing energy waste. The EPA also took advantage of numerous opportunities to optimize the design for efficiency in its mechanical specifications.
CONDITION OF ROOF IS IMPORTANT
Getting down to basics, if a 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, CA, based firm that specializes in roofing, decking and construction services, a careful inspection of the roof system is not only necessary, but critical. 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 re-roof with a cool roof before installing the solar panels. For the optimum performance of the investment, a roof should be able to last at least 20 years from the time of the PV installation.
“If you are considering a new roof, you should 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.”
Combining photovoltaics with a cool roof is also attracting attention from the Federal Government, and particularly the Internal Revenue Service in a positive way. In a recently issued “private letter ruling” (P.L.R. 200947027) requested by Solyndra, 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.
About the Author
Michael Magallanes is the Vice President of Coat‘N’Cool (www.coatncool.com), a Yorba Linda, CA – based manufacturer of a proprietary cool roof coating product.
Source / Fuente: http://www.greenbuildingpro.com
Author / Autor: Michael Magallanes
Date / Fecha: 25/07/11
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