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Photovoltaic Systems

Photovoltaic (PV) technology is the direct conversion of sunlight to electricity using semiconductor devices called solar cells. Photovoltaics are almost maintenance-free and seem to have a long life span. The photoelectric conversion process produces no pollution and can make use of free solar energy. Overall, the longevity, simplicity, and minimal resources used to produce electricity via PV systems make this a highly sustainable technology.

PV is currently cost-effective in small, off-grid applications such as microwave repeaters, remote water pumping, and remote buildings. While the cost is high for typical applications in buildings connected to the electric power grid, the integration of PV into commercial buildings is projected to greatly increase over time. In fact, worldwide PV manufacturing is growing at a healthy annual rate of more than 20 percent, and the focus of research is to reduce the cost of PV systems, and to integrate PV into building design.

The most common technology in use today is single-crystal PV, which use wafers of silicon wired together and attached to a module substrate. Thin-film PV, such as amorphous silicon technology, is based on depositing silicon and other chemicals directly on a substrate such as glass or flexible stainless steel. Thin-film PV materials can look almost like tinted glass. They can be designed to generate electricity from a portion of the incoming light while still allowing some light to pass through for daylighting and view. Thin films promise lower cost per square foot, but also have lower efficiency and produce less electricity per square foot compared to single-crystal PVs.

PV panels produce direct current, not the alternating current used to power most building equipment. Direct current is easily stored in batteries; a device called an inverter is required to transform the direct current to alternating current. The cost of reliable batteries to store electricity, and the cost of an inverter, increase the overall cost of a system.

With an inverter creating alternating current, it is possible to transfer excess electricity generated by a photovoltaic system back into the utility grid rather than into batteries for off-grid systems. In this case, the utility grid becomes a virtual storage system. Most utilities are required to buy such excess site-generated electricity back from the customer. In many states, public utility commissions or state legislatures have mandated "net-metering," which means that utilities pay and charge equal rates regardless of which way the electricity flows. Building owners in such states will find PV more economically attractive. Visit the DSIRE Database to learn more about opportunities for net metering or other incentives in your state.