Photovoltaic Cells

Photovoltaic (PV) technology converts sunlight directly into electricity. It works any time the sun is shining, but the more intense the light and the more direct the angle of the light, the more electricity will be produced. Unlike solar systems for heating water, PV technology does not use the sun's heat to make electricity. Instead, it produces electricity directly from electrons freed by the interaction of sunlight with certain semiconductor materials in the PV array.

Photovoltaic cells were developed in 1954 as an energy source for the space program. Until the 1970s, the manufacture and installation of solar energy panels was not regulated, and poor quality systems and unreliable dealers combined with lower fossil fuel prices to limit solar construction.

You've probably seen small versions of PV collectors on calculators and watches. The same type of silicon wafers that are used to make computer chips can be used to create electricity when the sun is shining on them. Photovoltaic cells are made from a very pure form of silicon, an abundant element in the earth's crust that is not very difficult to mine.

Photovoltaic cells provide direct electrical current. When enough heat or light strikes a cell connected to a circuit, the difference in voltage causes current to flow. No voltage difference is produced in the dark, so the cell only provides energy when exposed to light. A cell can be connected to a battery, to provide continuous power.

Even the best PV cells turn less than a quarter of the solar energy that strikes them into electricity, with the rest given off as heat. Commercially available cells are currently only about 10 to 12 percent efficient. New designs currently being researched are up to 18 percent efficient.

Individual PV cells are wired together to produce a PV module, the smallest PV component sold commercially, and these modules range in power output from about 10 to 300 W. Usually, individual modules are mounted onto an existing roof. Some modules can be designed directly into the roof, acting as both a roofing material and an electricity generator. To connect a PV system to a utility grid, one or more PV modules is connected to an inverter that converts the modules' DC electricity to AC electricity. The AC power is compatible with the electric grid and can be used by lights, appliances, computers, televisions, and many other devices. Some systems include batteries to provide backup power in case the utility suffers a power outage.

Small commercial and industrial PV applications include lighting, traffic counters, signaling, and fence charging. Larger systems provide electricity for residential, office, educational, and mobile electrical needs. Systems are not limited to sunny tropical areas. A solar electric system in Boston, Massachusetts, will produce over 90 percent of the energy generated by the same system in Miami, Florida. In areas with low-sun winter seasons, like New England, these systems are frequently paired with a generator or other backup systems for extra power.

Photovoltaic energy is a clean, reliable alternative for providing electrical power. It minimizes dependence on fossil fuels and reduces vulnerability to fuel price spikes. Solar energy can decrease utility bills and increase the resale value of real estate.

When the PV system generates more electricity than is needed at the site, excess energy can be fed directly onto electric lines for use by other electric customers (Fig. 27-3). Through a net-metering agreement with the electric utility, PV system owners are compensated for the excess power they produce. The PV system contractor installs an inverter that ensures that the electricity coming from the PV system is compatible with electricity coming from the power lines.

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Figure 27-3 Grid connected PV system.
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