Today, solar-generated electricity serves people living in the most isolated spots on earth as well as in the center of our biggest cities. First used in the space program, PV systems are now both generating electricity to pump water, light up the night, activate switches, charge batteries, supply the electric utility grid, and more. Whether you are a homeowner, farmer, planner, architect, or just someone who pays electric utility bills, PV may already touch your life in some way.

PV cells and modules are very reliable in space and on the earth. The Hubble space telescope (pictured here) and virtually all communications satellites are powered by photovoltaic technology. The DOE program over the last two decades has worked to bring this technology to earth.

The same sunny days that dry out plants, make animals thirsty, and heat up buildings and cars are also good days for generating electricity with photovoltaics. This electricity can be used to power water pumps for irrigation and drinking wells, and ventilation fans for air cooling. For this reason, the most simple PV systems use the dc electricity as soon as it is generated to run water pumps or fans.

This woman in India is collecting water from a pump powered by PV.

These basic PV systems have several advantages for the special jobs they do. The energy is produced where and when it is needed, so complex wiring, storage, and control systems are unnecessary. Small systems, under 500 watts (W), weigh less than 68 kilograms (150 pounds), making them easy to transport and install. Most installations take only a few hours. And, although pumps and fans require regular maintenance, the PV modules require only an occasional inspection and cleaning.

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Storing electrical energy makes PV systems a reliable source of electric power day and night, rain or shine. PV systems with battery storage are being used all over the world to power lights, sensors, recording equipment, switches, appliances, telephones, televisions, and even power tools.

One of the most simple PV/battery systems is this path light. The PV panel generates electricity during the day and stores it in the battery for use at night.

PV systems with batteries can be designed to power dc or ac equipment. People who want to run conventional ac equipment add a power conditioning device called an "inverter" between the batteries and the load. Although a small amount of energy is lost in converting dc to ac, an inverter makes PV-generated electricity behave like utility power to operate everyday ac appliances, lights, and even computers.

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When power must always be available or when larger amounts of electricity than a PV system alone can supply are occasionally needed, an electric generator can work effectively with a PV system to supply the load. During the daytime, the PV modules quietly supply daytime energy needs and charge batteries. If the batteries run low, the engine generator runs at full power—its most cost- and fuel-efficient mode of operation—until they are charged. And, in some systems, the generator makes up the difference when electrical demand exceeds the combined output of the PV modules and the batteries.

A portable PV/propane system provides electricity for California State University's Desert Research Center in Southern California. The facility is far from utility power lines, yet it includes a commercial kitchen, machine shop, classrooms, laboratory, and dormitories that sleep 75 people.

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Where utility power is available, a grid-connected PV system can supply some of the energy needed and use the utility in place of batteries.

This electric vehicle recharging station in southern Florida is powered by a grid-connected PV array mounted on the roof. When no vehicles need charging, power from the modules is transferred to the utility line.

Some homeowners, considered pioneers in the energy field, are using PV systems connected to the utility grid. They are doing so because they like that the system reduces the amount of electricity they purchase from the utility each month. They also like the fact that PV consumes no fuel and generates no pollution.

The owner of a grid-connected PV system can not only buy, but can also sell, electricity each month. This is because electricity generated by the PV system can be used on site or fed through a meter into the utility grid. When a home or business requires more electricity than the PV array is generating (for example, in the evening), the need is automatically met by power from the utility grid. When the home or business requires less electricity than the PV array is generating, the excess is fed (or sold) back to the utility. Used this way, the utility backs up the PV like batteries do in stand-alone systems. At the end of the month, a credit for electricity sold gets deducted from charges for electricity purchased.

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Large-scale photovoltaic power plants, consisting of many PV arrays installed together, can prove useful to utilities. Utilities can build PV plants much more quickly than they can build conventional power plants because the arrays themselves are easy to install and connect together electrically. Utilities can locate PV plants where they are most needed in the grid because siting PV arrays is much easier than siting a conventional power plant. And, unlike conventional power plants, PV plants can be expanded incrementally as demand increases. Finally, PV power plants consume no fuel and produce no air or water pollution while they silently generate electricity.

Sacramento Municipal Utility District's (SMUD) 2-MW plant produces enough power to serve 660 Sacramento-area homes. The 1600 modules are spread across an 8094-m2 field in this very sunny region of California. Incidentally, SMUD opted to close down the nuclear reactors in favor of "cleaner" energy technology.

Unfortunately, PV generation plants have several characteristics that have slowed their use by utilities. Under current utility accounting, PV-generated electricity still costs considerably more than electricity generated by conventional plants, and regulatory agencies require most utilities to supply electricity for the lowest cash cost. Furthermore, photovoltaic systems produce power only during daylight hours and their output varies with the weather. Utility planners must therefore treat a PV power plant differently than they would treat a conventional plant.

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Hybrid systems combine a number of electricity production and storage pieces to meet the energy demand of a given facility or community. In addition to PV, engine generators, wind generators, small hydro plants, and any other source of electrical energy can be added as needed to meet energy demands and fit the local geographical and temporal characteristics. These systems are ideal for remote applications such as communications stations, military installations, and rural villages.

Hybrid power systems combine a number of electricity production and storage pieces to meet the energy demand of a given facility or community. In a system such as this, PV arrays, wind turbines, and generators can be added as needed to meet growing energy demands and fit the local geographical and temporal constraints.

Essential to developing a hybrid electric system is knowing the energy demand to be met and the resources available. Energy planners therefore must study the solar energy, wind, and other potential resources at a certain location, in addition to the planned energy use. This will allow them to design a hybrid system that best meets the demands of the facility or community.

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