Notes

Solar Basics
Energy from the sun
The sun has produced energy for billions of years. The energy in the sun's rays that reaches the earth (solar radiation) can be converted into heat and electricity.

Radiant energy from the sun has powered life on earth for many millions of years.
The sun
Source: NASA

In the 1830s, the British astronomer John Herschel famously used a solar thermal collector box (a device that absorbs sunlight to collect heat) to cook food during an expedition to Africa. Today, people use the sun's energy for a variety of purposes.

Solar energy can be used for heat and electricity
When converted to thermal energy, solar energy can be used to heat water for use in homes, buildings, or swimming pools; to heat spaces inside homes, greenhouses, and other buildings; and to heat fluids to high temperatures to operate turbines that generate electricity.

Solar energy can be converted into electricity in two ways:

Photovoltaic (PV devices) or solar cells change sunlight directly into electricity. Individual PV cells are grouped into panels and arrays of panels that can be used in a variety of applications ranging from single small cells that charge calculator and watch batteries, to systems that power single homes, to large power plants covering many acres.
Solar thermal/electric power plants generate electricity by concentrating solar energy to heat a fluid and produce steam that is then used to power a generator.

There are two main benefits of solar energy:

Solar energy systems do not produce air pollutants or carbon dioxide.
When located on buildings, solar energy systems have minimal impact on the environment.

There are two main limitations of solar energy:

The amount of sunlight that arrives at the earth's surface is not constant. The amount of sunlight varies depending on location, time of day, time of year, and weather conditions.
Because the sun doesn't deliver that much energy to any one place at any one time, a large surface area is required to collect the energy at a useful rate.
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Where Solar is Found
Solar energy is sunshine
Map of Concentrating Solar Resources in the United States showing greatest concentration mostly in the areas from western Texas westward to California and northward to Washington, Idaho, Montana and the western parts of the Dakotas; with moderate concentrations in the lower southern tier of the country from Florida through Maryland.
Source: National Renewable Energy Laboratory, U.S. Department of Energy

Map of Photovoltaic Solar Resources in the United States showing greatest concentration mostly in the areas from western Texas westward to California and northward to Oregon, Idaho, and Wyoming
Source: National Renewable Energy Laboratory, U.S. Department of Energy

World map of solar resources
World Map of Solar Resources showing greatest concentration in the southern portion of the Northern Hemisphere, South America, Africa, the Middle East, southern Eurasia, the South Pacific, and Australia
Source: United Nations Environment Programme (UNEP), NASA Surface meteorology and Solar Energy (SSE), 2008.

The amount of solar energy that the earth receives each day is many times greater than the total amount of energy consumed around the world. However, solar energy is a variable and intermittent energy source. The amount and intensity of sunlight varies by location, and weather and climate conditions affect its availability on a daily and seasonal basis. The type and size of a solar energy collection and conversion system determines how much of available solar energy can be converted into useful energy.

Solar thermal collectors
Low-temperature solar thermal collectors absorb the sun's heat energy to heat water or to heat air for heating in homes, offices, and other buildings.

Concentrating collectors
Concentrating solar energy technologies use mirrors to reflect and concentrate sunlight onto receivers that collect solar energy and convert it to heat. This thermal energy can then be used to produce heat or electricity with a steam turbine or heat engine driving a generator.

Photovoltaic systems
Photovoltaic (PV) cells convert sunlight directly into electricity. PV systems can range from systems that provide tiny amounts of electricity for watches and calculators to systems that provide the amount of electricity used by hundreds of homes.

Hundreds of thousands of houses and buildings around the world have PV systems on their roofs. Many multi-megawatt (MW) PV power plants have also been built. Covering 4% of the world's desert areas with photovoltaics could supply the equivalent of all of the world's electricity. The Gobi Desert alone could supply almost all of the world's total electricity demand.

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Solar Photovoltaic
Photovoltaic cells convert sunlight into electricity
A photovoltaic (PV) cell, commonly called a solar cell, is a nonmechanical device that converts sunlight directly into electricity. Some PV cells can convert artificial light into electricity.

Image of how a photovoltaic cell works.
Source: National Energy Education Development Project (public domain)

Photons carry solar energy
Sunlight is composed of photons, or particles of solar energy. These photons contain varying amounts of energy that correspond to the different wavelengths of the solar spectrum.

A PV cell is made of a semiconductor material. When photons strike a PV cell, they may be reflected, pass right through, or be absorbed by the semiconductor material. Only the absorbed photons provide energy to generate electricity. When enough sunlight (solar energy) is absorbed by the material, electrons are dislodged from the material's atoms. Special treatment of the material surface during manufacturing makes the front surface of the cell more receptive to the dislodged or free electrons, so the electrons naturally migrate to the surface of the cell.

The flow of electricity
When the electrons leave their position, holes are formed. When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of electrical charge between the cell's front and back surfaces creates a voltage potential like the negative and positive terminals of a battery. Electrical conductors are placed on the cell to absorb the electrons. When the conductors are connected in an electrical circuit to an external load, such as an appliance, electricity flows in the circuit.

The efficiency of photovoltaic systems varies by the type of photovoltaic technology
The efficiency at which PV cells convert sunlight to electricity varies by the type of semiconductor material and PV cell technology. The efficiency of most commercially available PV modules ranges from 5% to 15%. Researchers around the world are trying to achieve higher efficiencies.

How photovoltaic systems operate
The PV cell is the basic building block of a PV system. Individual cells can vary in size from about 0.5 inches to about 4 inches across. However, one cell only produces 1 or 2 Watts, which is only enough electricity for small uses.

PV cells are electrically connected together in a packaged, weather-tight PV module or panel. PV modules vary in size and vary in the amount of electricity they can produce. PV module electricity generation capacity increases with the number of cells in the module or in the surface area of the module. PV modules can be connected in groups to form a PV array. A PV array can be composed of two or several thousand PV modules. The number of PV modules connected together in a PV array determines the total amount of electricity that the array can generate.

Photovoltaic cells generate direct current (DC) electricity. This DC electricity can be used to charge batteries that, in turn, power devices that use direct current electricity. Nearly all electricity is supplied as alternating current in electricity transmission and distribution systems. Devices called inverters are used on PV modules or in arrays to convert the DC electricity to alternating current (AC) electricity.

PV cells and modules will produce the largest amount of electricity when they are directly facing the sun. Tracking systems can be used to move PV modules to constantly face the sun, but these systems are expensive. Most PV systems have modules in fixed positions with the modules facing directly south and at an angle that optimizes the physical and economic performance of the system at the location where it is installed.

Applications of photovoltaic systems
The simplest photovoltaic systems are solar-powered calculators and wrist watches. Larger systems can provide electricity to pump water, to power communications equipment, to supply electricity for a single home or business, or to form large arrays that supply electricity to thousands of electricity consumers.

Some advantages of PV systems are:

PV systems can supply electricity in locations where electricity distribution systems do not exist, and they can also supply electricity to an electric power grid.
PV arrays can be installed quickly and can be any size.
The environmental impact of PV systems is minimal.
History of photovoltaic
The first practical PV cell was developed in 1954 by Bell Telephone researchers who were examining the sensitivity of a properly prepared silicon wafer to sunlight. Beginning in the late 1950s, PV cells were used to power U.S. space satellites. PV cells were next widely used for small consumer electronics like calculators and watches and were used to provide electricity in remote or off-grid locations where there were no electric power lines. Since 2004, most of the PV panels installed in the United States have been in grid-connected systems on homes, buildings, and central-station power facilities. Technology advances, lower costs for PV systems, and various financial incentives and government policies helped to greatly expand PV use since the mid-1990s. There are now hundreds of thousands of grid-connected PV systems installed in the United States.

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Solar Thermal Power Plants
Solar thermal power uses solar energy instead of combustion
Solar thermal power plants use the sun's rays to heat a fluid to high temperatures. The fluid is then circulated through pipes so that it can transfer its heat to water and produce steam. The steam is converted into mechanical energy in a turbine which is then converted into electricity by a conventional generator.

Parabolic trough power plant
Picture of a parabolic trough power plant.
Source: Stock photography (copyrighted)

Solar thermal power generation works essentially the same as power generation using fossil fuels, but instead of using steam produced from the combustion of fossil fuels, the steam is produced by heat collected from sunlight. Solar thermal technologies use concentrator systems to achieve the high temperatures needed to heat fluid.

There are three main types of solar thermal power systems:

Parabolic trough
Solar dish
Solar power tower
Types of solar thermal power plants
Parabolic troughs

Parabolic troughs are used in the largest solar power facility in the world, which is located in the Mojave Desert at Kramer Junction, California. This facility has operated since the 1980s, and it accounts for most of the solar electricity produced by the electric power sector today.

A parabolic trough collector has a long parabolic-shaped reflector that focuses the sun's rays on a receiver pipe located at the focus of the parabola. The collector tilts with the sun as the sun moves from east to west during the day to ensure that the sun is continuously focused on the receiver.

Because of its parabolic shape, a trough can focus the sun at 30 times to 100 times its normal intensity (concentration ratio) on the receiver pipe located along the focal line of the trough, achieving operating temperatures higher than 750°F.

Solar dish
Image of a solar dish collector.
Source: Stock photography (copyrighted)

The solar field has many parallel rows of solar parabolic trough collectors aligned on a north-south horizontal axis. A working (heat transfer) fluid is heated as it circulates through the receiver pipes and returns to a series of heat exchangers at a central location. Here, the fluid circulates through pipes so it can transfer its heat to water to generate high-pressure, superheated steam. The steam is then fed to a conventional steam turbine and generator to produce electricity. When the hot fluid passes through the heat exchangers, it cools down, and is then recirculated through the solar field to heat up again.

The plant is usually designed to operate at full power using solar energy alone, given sufficient solar energy. However, all parabolic trough power plants can use fossil fuel combustion to supplement the solar output during periods of low solar energy.

Solar dish

A solar dish/engine system uses concentrating solar collectors that track the sun, so they always point straight at the sun and concentrate the solar energy at the focal point of the dish. A solar dish's concentration ratio is much higher than a solar trough's concentration ratio, and it has a working fluid temperature higher than 1380°F. The power-generating equipment used with a solar dish can be mounted at the focal point of the dish, making it well suited for remote operations or, as with the solar trough, the energy may be collected from a number of installations and converted to electricity at a central point.

The engine in a solar dish/engine system converts heat to mechanical power by compressing the working fluid when it is cold, heating the compressed working fluid, and then expanding the fluid through a turbine or with a piston to produce work. The engine is coupled to an electric generator to convert the mechanical power to electric power.

Solar power tower
Image of a solar power tower.
Source: National Renewable Energy Laboratory (NREL)

Solar power tower

A solar power tower, or central receiver, generates electricity from sunlight by focusing concentrated solar energy on a tower-mounted heat exchanger (receiver). This system uses hundreds to thousands of flat, sun-tracking mirrors called heliostats to reflect and concentrate the sun's energy onto a central receiver tower. The energy can be concentrated as much as 1,500 times that of the energy coming in from the sun.

Energy losses from thermal-energy transport are minimized because solar energy is being directly transferred by reflection from the heliostats to a single receiver, rather than being moved through a transfer medium to one central location, as with parabolic troughs.

Power towers must be large to be economical. This is a promising technology for large-scale grid-connected power plants. Power tower technology is in the early stages of development compared to parabolic trough technology.

The U.S. Department of Energy, along with a number of electric utilities, built and operated a demonstration solar power tower near Barstow, California, during the 1980s and 1990s. Learn more about the history of solar power in the Solar Timeline.

There are additional private company power tower projects that include:

A 5-Megawatt, two-tower project, built in the Mojave Desert in southern California in 2009
A 390-Megawatt, three-tower project being built in the Mojave Desert
A 110-Megawatt project located in Nevada
A 392-Megawatt project located in Ivanpah Dry Lake, California

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Solar Thermal Collectors
Heating with the sun's energy
Image of a house with solar cells on the roof.
Source: Adapted from National Energy Education Development Project (public domain)

Solar thermal energy can be used to heat water or air. It is most often used for heating water in buildings and in swimming pools. Solar thermal energy is also used to heat the insides of buildings. Solar heating systems can be classified as passive or active.

Passive solar space heating happens in a car on a hot summer day. The sun's rays pass through the windows and heat up the inside of the car. In passive solar heated buildings, air is circulated past a solar heat-absorbing surface and through the building by natural convection. No mechanical equipment is used for passive solar heating.

Active solar heating systems use a collector and a fluid to collect and absorb solar radiation. Fans or pumps circulate air or heat-absorbing liquids through collectors and then transfer the heated fluid directly to a room or to a heat storage system. Active water heating systems usually include a tank for storing water heated by the system.

Solar collectors are either nonconcentrating or concentrating

Nonconcentrating collectors—The collector area (the area that intercepts the solar radiation) is the same as the absorber area (the area absorbing the radiation). Flat-plate collectors are the most common type of nonconcentrating collectors and are used when temperatures lower than 200°F are sufficient. Nonconcentrating collectors are often used for heating water or air for space heating in buildings an