A number of solar-electric roofing products have entered the us market in the last few years. Roofing with large-area glass photovoltaic (pv) panels is now joined by electricity, producing metal roofing, cement slates, even a commercial flat-roof system. The new products have improved aesthetics and in some cases better economics. Some utilise new thin-film technology pvs, although their longevity, particularly on a non-glass backing, is not yet proven.
The leader in a variety of off-the-shelf pv roofing systems is United Solar Systems Corporation, a joint venture of Canon (office equipment, cameras) and Energy Conversion Devices. In addition to shingles, the Unisolar line has both architectural and structural standing-seam metal roofing, with panels available in lengths from 2.8m to 7.3m. Wind loading and allowable spans remain the same as the non-solar version. Making the pvs integral to the roofing, rather than conventionally mounted on a separate array, won the company at least one project where concerns over vandalism or theft existed.
Another innovative product is the Sunslate from Atlantis, a Swiss company with manufacturing facilities in Virginia and California. Designed to be similar to slate, blue pv modules are fixed to cement-fibre roofing tiles made by Eternit. The Sunslates are installed by nailing them onto battens, with a simple - and proprietary - connection between each slate. The company claims that no extra engineering or subcontractors are needed for installation. The Sunslates are typically integrated into a roof with non-pv slates. They can resist winds in excess of 200kph and even meet hurricane-region building standards; electrical output levels are guaranteed for 10 years. The only drawback of individual pv slates is an increased number of electrical connections which can go wrong.
A different niche is the PowerGuard Solar Tile system for flat or moderately sloping roofs. This system consists of a large glass pv module stuck to concrete-skinned extruded-polystyrene roofing panels made by Lightguard. Lightguard has a 20-year track record of providing a covering for built-up roofs. The added insulation increases roofing membrane life by decreasing thermal shock. The PowerGuard 'tiles' are laid over a new or existing roof system, adding insulation and providing mounted photovoltaics which require no roof penetrations.
A hybrid system for both walls and roofs is the pv version of Solarwall. Solarwall is a Canadian product which consists of a dark-coloured metal building siding perforated with tiny holes. A plenum is left between the siding and the wall surface. Ventilation air for the building is drawn through the holes, collecting solar energy more efficiently than any system yet measured (up to 80 per cent). In the pv version, the incoming air cools pv panels mounted on the Solarwall, increasing their efficiency and reducing the system payback. With Solarwall alone paying for itself in as little as one year, the combined system can cut the payback on the pvs by more than half, according to the manufacturer.
pvs are increasingly being used as architectural elements. The clocktower on Sun Microsystems, corporate campus in Palo Alto, California has a pv facade. So do the top few floors of a new skyscraper at 4 Times Square in New York City. Costs are cut when large-area pv panels are used in place of expensive finishes such as polished stone.
Government support for using pvs varies, with state governments taking the lead. In California, a tax credit equal to nearly half the cost of a pv system is expected to generate 10,000 new installations. More interesting still is a proposed New York state tax credit of 25 per cent for standard pv arrays - or a full 100 per cent of incremental cost for building-integrated pv systems.
There is also new interest in pvs at the federal level. It may well prove more serious and show more staying power than the efforts undertaken during the energy crisis of the 1970s. Back then, President Carter put a solar system on the White House (removed under Reagan). In June, the Clinton administration commissioned a major pv installation on the Pentagon.
National Renewable Energy Laboratory, the us Department of Energy's laboratory for renewable energy and energy efficient research, has a website at www.nrel.gov. The Solar Energy Industries Association, a US-based trade group, has a website at www.seia.org. David Kaufman is president of Energy Solutions consultants in Portland, Maine, usa