Recent fossil fuel cost hikes and the ‘green’ movement have renewed interest in photovoltaic (PV) or solar panels. PVs have come a long way over the years, with more efficient, cost-effective systems. The rooftop of any type of building is the ideal place to mount a PV system, but doing so creates some unique challenges for building owners, roofers and membrane manufacturers alike. Roofing manufacturers, whose primary role has been membrane supply and the issuance of a comprehensive systems warranty, are entering into a complex role by incorporating a sophisticated overburden (PV panels and their attachment) system covering most of the roof assembly and warranting much more than an obligation to repair leaks.

A Brief History of Photovoltaics
The first silicon PV panel was built in 1953 at Bell Laboratories. Too expensive for commercial use, NASA incorporated them as a power source into early satellites. Oil disruptions in the 1970’s motivated the US government to begin investing in solar energy. Exxon Corporation was first to develop a cheaper PV panel that reached limited commercial applications. PV systems were still not cost-effective, with an average installation cost of $50 per watt. Pay back periods could not justify installations other than ‘show piece’ installations for trade shows and magazine articles.

PV prices did decrease in the 1980s, but so did fossil fuels, reducing urgency in product development. Tax incentives were cut or eliminated, killing solar business ventures. At the opposite end of the scale, Germany and Japan invested in programs, creating a mainstream ‘alternative energy’ industry. By the 1990’s, more efficient PV products, developed outside the United States, dropped installation costs to about $7 per watt.

Since the early part of this decade, PV installations have seen 25-35% annual growth. New PV technology and a larger supply of PV components have lowered equipment prices even further. Federal and State governments have re-instituted tax credits encouraging more installations. For example, as part of the current Federal stimulus package a tax credit of up to $2,000 is offered for residential PV systems. The cost of current PV installation now averages around $4 per watt.

What Types of PV Systems are Available?
PV systems consist of solar cells packaged in photovoltaic modules, often electrically connected in multiples creating solar photovoltaic arrays. Photons from sunlight are absorbed within the semiconductor, usually made of silicon. This process knocks negatively-charged electrons loose from their atoms, producing electricity. PV cells have electric fields that force the freed electrons to flow in a single direction. Metal contacts on the top and bottom of the PV cell draw the current off to be converted and used externally.

Applied Photovoltaics on a Roof

Four materials are currently used to make solar cells: monocrystalline silicon, polycrystalline silicon, amorphous silicon, and cadmium telluride (CdTe). Currently, the most common are crystalline cells (mono and poly) which are highly efficient at converting sunlight to electricity. These cells are expensive and require direct sunlight. Amorphous silicon cells, while less efficient, are less costly to produce and have excellent low light sensitivity. CdTe cells, a recent innovation, are more efficient than amorphous silicon and less expensive than crystalline panels. The downside is a heavy metal content that creates a disposal issue.

PV system configurations include both applied rigid module arrays and building integrated photovoltaics. The common notion of solar panels are applied modules, usually made using crystalline silicon, encased in glass and metal. These modules are arranged in groups, mounted on racks and attached to the roof assembly. When mounted facing south at a 20-30˚ angle facing direct sunlight, applied systems are the most efficient at converting sunlight into electricity. While initially seen as a novelty, the panels are eventually perceived as an eyesore.

The alternate approach is building-integrated PV systems (BIPVs), composed of flexible thin film strips of amorphous silicon (less often, crystalline silicon or CdTe) attached directly to the roof membranes or shingles. These systems are designed to ‘blend in’ by laying flat against the roof system. Not as efficient as applied systems, BIPVS often have better low light sensitivity, producing electricity for longer periods during sunlight.

Building-Integrated PV Panels

PVs on the Roof
Roof-mounted photovoltaic systems are the most common building applied systems with maximum exposure to the sun, utilizing an unused surface that is often viewed as a liability. PV systems turn the roof plane into a productive building component, making this liability into an asset.

The asset does not come without challenges. First, the roof with a PV system must receive adequate sunlight. This might seem obvious; however, this requires careful planning. No obstructions can block sunlight from reaching the PV cells. Developers and designers must examine future development plans for surrounding structures to research potential sun barriers in the planning stages, such as tall buildings that can block solar rays. Crystalline systems require direct sunlight for maximum efficiency. If direct sunlight is not available, or could be blocked, crystalline PVs probably are not the best choice. Amorphous silicon panels, while not as efficient, can produce electricity without direct sunlight on cloudy, even rainy, days, making them well suited for roofs that don’t receive large amounts of direct sunlight.

How the system will be mounted must also be considered. For applied systems, the racks that support the array are affixed directly to the roof framing, resulting in numerous penetrations in the roof membrane. These penetrations can be a primary cause of leaks through the membrane layer. Rooftop PV systems are evolving to reduce penetrations and minimize job specific engineering by providing load tables that can be used with current windload design data. Good examples are clamping systems designed to attach to standing seam metal roofs, and attachment devices for roof deck attachment that are accompanied with fasteners and installation data to meet a variety of windload requirements. For membrane systems, the panels must be directly bonded to the waterproofing layer. Uplift values must be developed for both the membrane and the panel secured to the membrane. The combined assembly must also be tested for fire resistance.

Close-up of PV Mounting Rack

Maintaining a roof with PVs is clearly more difficult and costly than maintaining a fully exposed membrane assembly. The majority of the membrane is covered with dark PV panels, covering any potential latent defects in the membrane, exposing the membrane to higher surface temperatures and potentially impeding water flow. The very application of PV panels is counterintuitive to good roofing practices, yet the clear benefit of power generation pushes the industry to overcome the negatives.

PV panels typically have a useful life of 20-25 years. Since they are predominately applied to the roof membrane layer, they must have an equal service life to achieve maximum cost efficiency. PV systems over existing roof membranes or roof assemblies that do not have a proven equal service life with elevated service temperatures can be a poor investment. In addition, proper drainage is critical to optimal performance of both the roof and the PV system. Sole source warranties, incorporating the PV system and the roof, have clear benefits, providing the warranty addresses drainage, attachment and removal of overburden to repair leaks and penetration seals as a specific obligation of the warranty.

Another design issue that must be considered is the dead weight of the entire assembly. Crystalline applied systems can weigh 5-8 poundes per square foot, yet thin film panels only weigh about 12 ounces per square foot. Heavier systems can create structural issues, such as weight, seismic load resistance and wind uplift issues for panels supported on brackets above the roof membrane layer. Once attachment systems are developed with tables tied to windloading data developed for ASCE-7, attachment can be as simple as it currently is for safety connections for lanyard tie-offs.

As noted, dark PV panels become hot during sunlight hours, causing elevated surface temperatures. The higher temperatures can potentially accelerate the aging of the membrane. Membranes must be tested to ensure the additional surface heat does not prematurely age the waterproofing layer, reducing its service life. In addition, membrane manufacturers should test PV roofing systems for fire resistance in accordance with UL 790: Standard Test Methods for Fire Tests of Roof Coverings and the National Electrical Code (NEC) 690, published by the National Fire Protection Association, to reduce the risk of fires caused by the PV system wiring. Insulation systems should be capable of withstanding anticipated surface temperatures without degradation or reduction in R-value.

Many roof membrane manufacturers have taken on the added responsibility of the power generating overburden, sometimes without a clear understanding of the impact on the waterproofing layer and the overall service life of the warranted system. It is essential that a potential purchaser ask the ‘hard questions’ about combined performance and warranty exclusions.

Case Study: Hawaii
Hawaii, with its abundant sunshine, isolation, and dearth of fossil fuels, is the perfect test lab for photovoltaic technology. Energy prices throughout the islands are well above mainland values. However, homes in Hawaii receive the energy equivalent of 15 gallons of gas in the form of sunlight on every roof each day. In order to ensure adequate access to sunlight, the Hawaiian state government has enacted air density laws to regulate construction of high-rise buildings that might shade neighboring buildings, maximizing solar energy to every structure.

The biggest barrier to the development of additional solar power is the cost of each PV installation. In response, the Hawaiian government is offering large tax credits for PV systems to supplement existing Federal tax credits. As of 2006, the state offers a tax break of 35% (up to $500,000 for commercial and $5,000 for single-family residential) of the actual system cost. In addition, some roofing companies will now cover the cost of PV installation over a roof assembly and sell the resulting electricity to the building tenant on a monthly basis, bypassing traditional power companies.

Cost reductions have resulted in increased innovation in the Hawaiian photovoltaic market. For example, highly efficient crystalline thin film PVs are now being incorporated into cool roof designs. Thus the building requires less energy for cooling and its roof produces energy to augment other forms of energy.

The days of burning vast amounts of fossil fuels to heat and cool buildings are ending. PVs are gaining a legitimate place in power generation for both residential and commercial structures anywhere in the United States. States with high fossil fuel costs are likely the incubator grounds for other states, with the cost and availability of fossil fuels driving the timelines for implementation. Solar, wind and heat exchange technology will eventually take a bite out of fossil fuel use. The roof plane, membrane manufacturers and installers will all play a dominant role in putting to use a dormant plane of the building. Roof manufacturers must carefully plan for systems that will maximize the aspects of waterproofing with the added benefit of power generation at the rooftop level. As solar power becomes part of mainstream building practice, understanding the benefits and potential drawbacks of PVs and finding innovative ways to incorporate them into building design is increasingly important.

June 2009
Issue Three, Volume Four



This article by Colin Murphy.

Colin Murphy is a founder and managing partner of Trinity | ERD.

 


"Pushing the Envelope: A Monthly Journal of Issues Concerning Building Design, Science, and Litigation" is a monthly publication of Trinity | ERD. This newsletter is intended as a thoughtful look into the issues of building construction.

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