Designing Buildings for Photovoltaic Systems

PV system arrays are complete connected sets of modules mounted and ready to deliver electricity. Building mounted arrays are stationary and usually consist of flat plates mounted at an angle. Tracking arrays follow the motion of the sun, providing more contact with the solar cells.

A building with good access to the sun and a roof that faces south is ideal for installation of a PV system. Roofs that face east or west may also be acceptable. Flat roofs also work well for solar systems, because the PV array can be mounted flat on the roof facing the sky or can be mounted on frames that are tilted toward the south at the optimal angle. All or most of the sun's path should be clear and not obscured by trees, roof gables, chimneys, buildings, and other features of the building and surrounding landscape. Shade falling over part of the PV array for part of the day can substantially reduce the amount of electricity that the system will produce.

The amount of mounting space needed for the solar system is based on the size of the system. Most residential systems require between 4.6 and 19 square meters (50-200 square ft), depending on the type of PV module used and its efficiency. Composition-shingle roofs are the easiest type to work with, and slate roofs are the most difficult.

The decision to install a PV system involves several economic considerations. The connection to the electrical grid and the cost of power from the grid are basic criteria. The cost of the system components over the life of the whole installation must be added to the costs of maintenance and financing. The PV system's battery can double as an emergency source for computers and peripherals to cover grid power interruptions.

Photovoltaic panels can substitute for other construction materials, providing a cost savings. New solar electric technology has made possible a number of products that serve another building function while acting as photovoltaic cells. Building integrated PV (BIPV) elements are structures that combine PV modules into roof panels, roofing tiles, wall panels, skylights, and other building materials, replacing traditional building elements. Companies in the United States, Japan, and Europe are actively pursuing new module designs. Solar roof shingles, structural metal roofing, and architectural metal roofing are now available, along with window glass. These products use flexible, lightweight panels designed to emulate conventional roofing materials in design, construction, function, and installation. Structural metal panels are used for PV-covered parking, charging stations for electric vehicles, park shelters and other covered outdoor spaces, and for commercial buildings. PV shingles can be used in combination with conventional shingles. Custom-color crystalline solar cells, including gold, violet, and green, are becoming available. Other architectural module designs have space between the cells and opaque backings to provide diffuse daylighting along with electric production.

A single-residence PV system costs about $10 per watt of rated system capacity, including installation and all system components. A 1000-W system that would supply about one-third of the electricity for an energy-efficient home would cost approximately $10,000. With larger systems and projects where costs can be shared, the cost per watt could be reduced significantly. It currently costs from $10,000 to $40,000 to install a full solar system in a home, but rebates for up to one-half of that are currently offered in about 30 states, with more considering doing so. When purchased in quantity by a builder, solar panel systems add about $50 per month to the cost of the house, while saving from $50 to $100 in monthly electric bills.

From a long-term perspective, it does not have to cost more to build with solar. The smartest building designs will specify a tight building envelope and high-efficiency lighting and HVAC equipment. The savings from these energy-efficient measures can be used to pay for a solar investment over time. While it is common for builders and architects to focus on current equipment costs, it is critical to approach projects with a focus on the cost of both constructing and operating a building over its lifetime. States offer residential tax credits for solar applications, and there are both state and federal tax incentives available for corporations, making commercial solar applications highly attractive.

As mentioned previously, the 1978 Public Utility Regulatory Policy Act (PURPA) requires that electric utilities buy electrical power from small suppliers. The price is established at a price equal to the cost the utility avoids by not having to produce that power. This has been interpreted as the cost of the fuel alone, without consideration of the cost of additional plant construction and related expenses. Under PURPA, energy has been purchased at around three cents per kWh by the same utilities that sell energy at eight to fourteen cents per kWh. This policy has discouraged development of grid-connected PV installations.

More recently, states have adopted net-metering laws that require the utility to pay PV providers at the same rate at which it sells the electricity during PV generating hours. The energy that the customer generates and uses is credited at the rate the utility would otherwise charge that customer. Only when the customer is producing more energy than he or she uses does the utility pay at the avoided cost rate. This means that small producers get fair credit for the energy they supply themselves, and are able to sell any excess to the utility, even if at a low rate. When the PV user buys from the utility, they pay at the conventional utility rate. Thirty states offer net-metering as of 2001.

Net-metering benefits both the customer and the utility. Some utilities have instituted PV installation programs primarily for residences. The utility installs and maintains the PV system on the customer's property (usually the roof), and the customer pays a small surcharge to the utility bill. The result is an environmentally beneficial power supply.

With the metering systems currently in use and the relatively high initial product costs, PV grid-connected systems can seldom justify the cost of installation on economic grounds only, but this is changing. Some utilities allow the installation of small individual PV modules in existing conventional buildings. These PV modules plug into conventional outlets in the building and supply power to the building. The excess not used in the building is fed back to the utility via a reversible energy meter. The system can be expanded gradually without centralized installation expenses.

Two large PV installations were completed in 2001. The 49-kW system on the Field Museum of Natural History in Chicago is connected to the local utility's electricity grid, reducing the amount of power from nonre-newable, high-emissions sources during peak periods. The 200-kW system mounted on the Neutrogena Cor poration headquarters in Los Angeles covers 2230 square meters (24,000 square ft) of roof area and will help reduce the company's energy consumption by about 20 percent.

At the DOE's headquarters in Washington, DC, a blank south-facing wall presents three-quarters of an acre of poured-in-place concrete to views from the National Mall. This eyesore is scheduled to become one of the largest solar installations in the world. The DOE, with the National Renewable Energy Laboratory, the American Institute of Architects, and the Architectural Engineering Institute, organized a design competition for a clean, renewable energy design. The winning design, proposed by architects at Solomon Cordwell Buenz & Associates in Chicago and engineers Ove Arup & Partners in New York, is an elegant, sweeping wall of ten-sioned cables, struts, and glass. The wall is a light, rigid structure that supports a PV collection system to turn solar energy into electricity, plus a solar thermal system that generates heat.

Many small projects are cropping up, such as the renovation of the Porter Square Shopping Center in Cambridge, Massachusetts, where roof-mounted PV panels provide almost all the energy needed for lighting. Another project is the Conde Nast skyscraper in New York City, where PV panels wrap the upper floors.

The National Fire Protection Association (NFPA) publishes NFPA 70, Article 690, Solar Photovoltaic Systems, which sets standards for PV systems. If the system is connected to the electrical grid, the local utility will have additional interconnection requirements. The electric utility will also know about the option of offering net-metering. Some homeowners' associations require residents to gain approval for a solar installation from an architectural committee, which in turn may require a system plan and agreement from neighbors. In most locations, building and/or electrical permits are required from city or county building departments. After the PV system is installed, it must be inspected and approved by the local permitting agency (usually the building or electrical inspector) and often by the electric utility as well.

More than 500,000 homes worldwide use PV to supply or supplement their electricity requirements, though all but about 10,000 are rural or remote off-grid applications. Residential and commercial BIPV are the most likely large-scale markets for PV in the developed countries. With participation of architects and building engineers, the technology is taking a progressively more sophisticated, elegant, and appropriate role in building design, putting energy-producing buildings within our reach.

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