Photo courtesy REGrid Power

In 2006 California made a major commitment to solar power by adopting the California Solar Initiative, a ten-year incentive program with the goal of installing 3,000 megawatts of solar power on the equivalent of one-million rooftops. This program continues the solar incentives started in 1998, but the long-term commitment to supporting solar will have a profound effect on the amount of solar installed.

PV is used extensively in rural areas that are not serviced by the utility grid. These are called “off grid” systems. This article applies primarily to on-grid or “grid-tied” systems that are receiving power from an electric utility. For a wealth of information about both types of systems we recommend Home Power magazine, http://www.homepower.com,and the Florida Solar Energy Center.

Basic System Operation: When sunlight hits the PV cells, direct current (DC) flows through the inverter, which converts it to alternating current (AC). The AC power then flows directly into the building (if there is demand), or into backup batteries if the system has them, or to the utility. When the power is flowing back to the utility grid, the electricity meter turns backward.

The Components: PV cells are the core of the system. They are made up of at least two layers of semiconductor material (usually pure silicon infused with boron and phosphorous). One layer has a positive charge, the other a negative charge. When sunlight strikes the cell, photons from the light are absorbed by the semiconductor atoms, which then release electrons. The electrons, freed from the negative layer of semiconductor, flow to the positive layer—thereby producing an electrical current.Since the electric current flows in one direction (like a battery), the electricity generated is called direct current (DC). Many individual cells are wired together in a sealed weatherproof unit called a module.

There are three types of PV modules: single crystal, “multi” or “poly” crystalline, and amorphous silicon. Each of these PV types is estimated to last at least twenty-five years. Some estimate that forty years is a reasonable expectation. The longevity rating of a module refers to the number of years before the unit starts producing only 80 percent of its original power rating. For instance, some modules are warranted to produce at least 80 percent of their full-rated power after twenty-five years. Instead of stopping production completely, a PV module will gradually produce less and less power over decades. Single-crystal modules are currently the most efficient type available, meaning that they produce the most power per square foot of module. The cells are fragile so they must be mounted in a rigid frame, and the modules usually have a polka dot or checkered pattern.

Multicrystalline modules are made of cells cut from multiple crystals that are grown together in an ingot. They are similar to single crystal cells in module structure but slightly less efficient since they require a bit more surface area to produce the same amount of electricity.

Amorphous silicon modules (e.g. thin film) are made from cells created by depositing a micro-thin layer of silicon directly onto a sheet of glass, plastic, or other substrate. Although they are less efficient and require up to 50 percent more space, they can be mounted on a flexible backing, making them easier to transport and ideal for building-integrated uses, such as roofing tiles or shingles.

For more information on system design see http://www.energyalternatives.ca/SystemDesign/pv1.html

Modules are wired together into a PV array, and the electricity they produce is fed through an inverter that changes the direct current (DC) into alternating current (AC), making it suitable for homes and business, and compatible with the electric grid. The inverter is the major electronic appliance associated with a grid-tied PV system.