Electricity from the Sun
The Sun is our greatest energy source: powerful, lasting, reliable, and available all over the world. Every hour the sunlight that reaches the earth is greater than the amount of energy used by every person on the planet in an entire year.
Photovoltaics, a versatile and flexible technology, can be designed to meet almost any need in practically any location. In space and in air, on land and at sea, photovoltaics produce energy when and where it is needed.
The term photovoltaic is derived by combining the Greek word for light, photos, with voltaic, named for Alessandro Volta, a pioneer in the study of electricity. Thus, a solar photovoltaic (PV) cell is one that converts sunlight – as opposed to sun heat – to electricity.
PV cells perform this conversion without any moving parts, noise, pollution, radiation, or maintenance. PV cells are made of a semiconducter material, typically silicon (from beach sand, an abundant resource), which is treated chemically to create a positive charge layer and a negative charge layer. When sunlight strikes a PV cell, an electron is dislodged. These loose electrons are gathered by wires attached to the cell, forming an electrical current. The more cells, the greater the current and voltage. A number of PV cells laid side-by-side form a rectangular module; several modules together form anarray. PV modules sold commercially range in power output from about 5 watts to 300 watts, and produce a direct current (DC) like the current from a car’s battery.
PV systems are already part of our lives. The smallest systems power calculators and wrist watches. Larger systems provide electricity for water pumps, highway signs, communications equipment, satellites, mobile homes, medical purposes (to power medical equipment, water purifiers, and refrigerators holding vaccines), navigation buoys, streetlights, and even for lighting homes and running appliances.
The National Oceanic and Atmospheric Administration (NOAA) uses this solar-powered buoy to collect weather data off the coast of the United States.
STAND ALONE APPLICATIONS
Stand-alone photovoltaic applications, whether for remote buildings, service applications or island power supplies, represent about 90% of installed PV power today. Stand Alone PV systems provide power when and where it is needed, without the need for a utility grid. Batteries are included to provide energy storage to operate loads during the night and during cloudy or stormy weather. The solar module(s) recharge the batteries when average or good weather returns. PV modules and batteries are sized to meet the power requirements of the particular load. If Alternating Current (AC) power is desired, as in an off-grid remote home, a DC-AC inverter is required. For most Stand Alone applications, a PV system is the most cost-effective alternative to providing power. There are over 25,000 homes in California today that are off-grid and powered primarily by photovoltaics.
Direct Connected Systems get the job done without the need for batteries. Solar module(s) produces DC current that is immediately used by a DC motor load. As sunlight rises and falls, current and voltage rise and fall, and the motor speeds up and slows down proportionately. There is no storage, the motor does not operate at night, and operates slowly during cloudy or stormy weather. Solar water pumps used in the Sahara are an example of this type of application.
A Littleton, Colorado police officer controls traffic along a busy thoroughfare using a PV-powered speed monitor. This PV-powered "smart sign" records and displays the speed of approaching cars. (S. Thornton/ NRELPIX 03313)
Rural Electrification – In Cacimbas, Brazil, 50-watt PV systems provide homes with electricity for fluorescent lighting. This approach bypasses the need for creating utility grid, providing reliable power where it is needed. (R. Taylor/NRELPIX 01270)
9.6 kW Photovoltaic System at Pinnacles National Monument, California. The system provides power for three employee residences, a ranger station, visitor center, campground, comfort station, well pump, and two wastewater effluent pumps. It eliminates a $20,000 annual fuel bill for a diesel generator that produced 143 tons of carbon dioxide each year. (National Park Service/NRELPIX 04924)
Prototype of a high-altitude, long-endurance, solar-powered aircraft. These aircraft could potentially provide the means to creating a stratospheric satellite system. In contrast to traditional satellites, such a system concentrates capacity directly over populated regions, allows for routine maintenance & technology upgrades, and can provide an economic solution in parts of the world lacking existing telecommunications infrastructure. (photo courtesy of NASA/Dryden, info: www.aerovironment.com)
PV Cells and modules are very reliable in space. The Hubble space telescope (pictured here) and virtually all communication satellites are powered by photovoltaic technology. (NREL PIX 03602)
This portable, extreme cold tolerant runway lighting system helps ensure safe landing for cargo planes on the Antarctic ice.(NREL PIX 08962) file: pvcallbox2.jpg Roadside emergency phone systems are powered by the sun. These systems have become the standard throughout the U.S.(Tor Allen/Rahus)
Soldier with fabric-like foldable photovoltaic module. (United Solar Systems Corp.)
GRID TIED APPLICATIONS
Grid-tied photovoltaics are the new frontier. Buildings consume vast amounts of energy: more than two-thirds of the energy used in the United States goes to heating, cooling, and lighting buildings. Photovoltaic materials can be integrated into every building, new or old, to harness the Sun’s energy to generate electricity. Grid tied PV systems are wired into buildings that are connected to the Utility Grid, hence the term grid tied. Energy produced by your PV system can be used directly in your home or business, or if there is an excess, flow out through your meter, providing power to your neighbors. In many States, you receive a credit for this excess power, to be used later, at night for example, when you need power from the grid. This is an arrangement called Net Metering and helps to improve the economics of a grid-tied PV system. Batteries are generally not required since the utility grid acts as a huge ‘battery’ in this application. An exception would be to provide power for critical loads, during utility outages, which would require a small onsite battery bank.
This 4.8 kW School Lunch Shelter incorporates a flexible thin-film photovoltaic material that integrates with the metal roof. Photovoltaics installed at schools create a living laboratory for the students, teachers and the community. (Solar Utility Co / United Solar Systems Corp)
The World’s First Solar Powered Ferris Wheel – a 50 kW rooftop system in Santa Monica, California (Solar Utility Co.) Thin-Film Silicon photovoltaics integrated into rooftop of a parking shade structure at Santa Monica Civic Center. (Solar Utility Co.)
100 kW PV PowerGuard system provides both power and added insulation to the Mauna Lani Bay Hotel in Hawaii. The unique system requires no roof penetrations for mounting. (Powerlight/NRELPIX)
A 4 kW Photovoltaic array provides enough electricity for an average California home. Systems like this are commonly installed on top of existing rooftops. (Walpert/Pacific Solar)
SunSlate PV roofing tiles – high efficiency photovoltaic cells are integrated into the roofing material of this San Luis Obispo home, appropriate for reroofing situation o