Photo courtesy of Dan Perkins Construction Inc., Ishpeming, Mich.
Photo courtesy of Dan Perkins Construction Inc., Ishpeming, Mich.
Photo courtesy of Dan Perkins Construction Inc., Ishpeming, Mich.
Photo courtesy of Dan Perkins Construction Inc., Ishpeming, Mich.
Now that environmental and energy issues have become popular topics and the roofing industry continues to become more environmentally aware, many roofing contractors are trying to find ways to integrate energy efficiency into roof systems. And one way to do this is by using photovoltaic roofing materials.
Short history
Most photovoltaic systems are not roofing products; they are stand-alone photovoltaic arrays mounted onto racks on rooftops. A photovoltaic system converts solar energy into electricity by turning the direct current power generated by the arrays into the alternating current power used by standard electrical appliances and provided from the electrical grid.
During the late 1970s, the installed cost of a photovoltaic system was more than $50 per watt of rated capacity, or about $2 per kilowatt hour (kWh). A photovoltaic system's rated capacity is its ability to generate a specified number of watts during one hour of full sunlight. During the 1980s, several thin-film photovoltaic technologies were developed that promised cost reductions as demand for high-volume automated manufacturing increased.
By the late 1990s, demand was increasing and photovoltaic system cost had dropped to about $10 per watt, or 30 cents per kWh. At the time, almost all photovoltaic systems were installed in remote areas where connecting to a utility grid was cost-prohibitive. The first net metering law was passed in California during the late 1990s, allowing people to connect their photovoltaic systems to the electrical grid and sell power back to the utility for the same price the utility charged them on a time-of-use basis. Now, there is time-of-use net metering that allows photovoltaic system owners to be compensated at rates as high as 36 cents per kWh when peak summer demand from afternoon air-conditioning loads coincides with maximum photovoltaic performance.
According to the U.S. Department of Energy, the number of yearly installations of grid-connected photovoltaic systems has been growing 30 to 40 percent each year since 2000, and the average cost of installation has dropped to about $7 per watt, or 25 cents per kWh, for crystalline and thin-film photovoltaic systems. State and utility incentive programs and federal tax credits now bring the installed cost to less than $5 per watt for residential customers and less than $3 per watt for commercial customers. Most states now have net metering and incentive programs to promote using renewable energy.
Photovoltaic systems
There are three basic types of photovoltaic systems: centralized large-scale generators, off-grid systems and distributed grid-tied systems.
Large-scale centralized systems are highly visible and generate significant power. It is more difficult to make them economically competitive because the power they generate is diminished by transmission loss and bulk power rates are significantly lower than retail rates.
Off-grid systems are installed in remote areas that do not have ready access to a power grid. They represent the sector of the photovoltaic economy that currently works without incentives because off-grid systems can cost less than buying, fueling and maintaining mechanical generators. They are more expensive than grid-tied systems per rated watt because of the additional cost of the power storage system.
Distributed grid-tied systems generate power at the source of use. They redirect any excess power into the grid and essentially sell the power back to the utility. When they do not produce enough power to service the homes or businesses they feed, the grid provides the additional power required. The retail cost of power at the source is three to five times the value of wholesale power generated at a centralized facility. With distributed grid-tied systems, the grid essentially has become the battery for the photovoltaic array.
Ninety-five percent of the current photovoltaic market consists of polycrystalline or monocrystalline modules. They are time-proven components but expensive to manufacture. It takes a temperature of 2,732 F to grow a silicon crystal and up to five years to generate enough power with a finished crystalline module to equal the energy consumed in making it. Thin films—the next generation of photovoltaic products—are made much more efficiently.
Amorphous film, which refers to silicon-based thin films, is made by applying micron-thick layers of silicon and doping agents onto a thin stainless-steel sheet or glass. These modules are made with one-third the amount of energy and 1 percent of the industrial-grade silicon required to make a similarly rated crystalline module. Amorphous film is the only type of thin film that has had any commercial success (commanding 11 percent of the current U.S. photovoltaic market), but other thin films, such as copper indium gallium selenide, are being developed and have potential.
One advantage with thin films is they consume less energy and material during the manufacturing process. They also lend themselves well to building integration. Thin films can be applied to many existing roofing and building materials and may redefine what we know as standard building skins.
It is possible that in the future, finished roofs will be covered completely with photovoltaic skins. The primary work that still needs to be done to give photovoltaic products this type of versatility involves "back end" manufacturing, or the portion of the manufacturing process when the photovoltaic cells are wired and encapsulated. Creating final product lines that will allow for incremental cell placement over an entire roof area while maintaining the roof's functional and aesthetic integrity is the work of a future generation of photovoltaic designers, engineers and roofing contractors.
Roof system integration
The standard photovoltaic modules to which most roofing contractors are accustomed are rectangular glass-faced modules with aluminum frames. Silicon wafers with doping agents and small wires etched into their surfaces lie under the glass surface, harvesting photons (particles of light) and propelling valence electrons (electrons in the outer rings of silicon atoms) into a circuit. These modules can be mounted on free-standing frames or directly on rooftops.
As photovoltaic modules have become more common, manufacturers, architects and roofing contractors have struggled to integrate them in a functional, efficient and aesthetically pleasing manner.
In the recent past, photovoltaic modules were covered with glass, framed with aluminum and mounted on rooftops with visible wiring harnesses and conduit running across roof surfaces. Now, roofing contractors are providing accessible ridge caps for wiring channels and sleek installations that eliminate bulky racks. It is important to note these improvements come directly from the roofing community, and as more roofing contractors integrate photovoltaic products into their roof systems, this type of evolution only will accelerate.
In addition, there are a number of crystalline photovoltaic modules now available that integrate into concrete or clay tile roofs, which is a clear indication of photovoltaic systems evolving into photovoltaic roof systems.
Installing a photovoltaic array on a rooftop is relatively easy for a roofing contractor and the least desired task in the photovoltaic community. Alternative energy contractors, solar architects and manufacturers tend to be more than willing to allow a roofing contractor to perform the job.
A roofing contractor's job scope regarding a photovoltaic installation typically is installing the modules into or onto a roofing product, plugging the rooftop connections together, and feeding the connections through the ridge cap and down a conduit under the careful guidance of a licensed electrician. The alternative energy contractor or electrician then installs the disconnects, inverter and all other components (called the "balance of system") before tying the array into the distribution box or battery bank.
On the rise
Solar energy technologies are coming of age just as fossil fuels are becoming less viable as a sustainable energy source. The photovoltaic industry has grown considerably during the past five years, and during the next few years, all major manufacturers of photovoltaic products are expecting to increase their output at an even more accelerated rate. As these output projections emerge, so will the possibility of photovoltaic products producing electric power for less than it costs to produce it from fossil fuel.
Government money from throughout the world has been a primary driver in the growth of photovoltaic markets. However, government incentives and programs are exerting mixed pressures on the industry at this time. For example, when government money for incorporating photovoltaic products in buildings is more plentiful than the supply of photovoltaic products in the world market, it is tempting for manufacturers to sell to the highest bidder, build more factories to make exactly the same product they currently make, and cut research and development budgets to increase short-term profits.
Many photovoltaic manufacturers also tend to believe their role in this industry should be to develop, improve and warranty only the electronic portions of their products, leaving the assembly and warranty of photovoltaic integrated roofing products to others. Although this leaves opportunities open for roofing manufacturers, it also stands to reason a photovoltaic manufacturer that does not make research and development a priority suddenly could find itself obsolete.
The economic landscape soon will change as government incentives no longer are necessary to make photovoltaic roof systems economical. It will take photovoltaic and roofing manufacturing companies to allocate research and development funds properly and carefully formulate interindustry agreements to establish viable photovoltaic roofing product lines.
And as roofing contractors begin installing photovoltaic arrays with roof systems, they will develop better application methods. The details and approaches to photovoltaic product installation previously described are solutions achieved within the roofing community and indicative of the insight roofing contractors bring to the photovoltaic industry.
Steve Heckeroth is director of building integrated photovoltaic products for ECD Ovonics, Rochester Hills, Mich., and Dan Perkins is owner of Dan Perkins Construction Inc., Ishpeming, Mich.
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