Table of Contents
- What is solar energy?
- How is solar energy created?
- How beneficial is solar energy to the environment?
- Solar Thermal Power Systems
- Solar Thermal Collectors
1. What is solar energy?
Solar energy is the sun’s rays (solar radiation) that reach the Earth. This energy can be converted into other forms of energy, such as heat and electricity.
Solar energy can be converted to 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 wide range 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 used to power a generator. In 2011, solar thermal-power generating units were the main source of electricity at 13 power plants in the United States: 11 in California, one in Arizona, one in Nevada.
Solar energy can also be collected (“solar thermal collectors”) to heat water and air inside of buildings.
The main benefits of solar energy are:
- Solar energy systems do not produce greenhouse gases
- It requires little maintenance
Two limitations of solar energy are:
- Sunlight is intermittent, meaning one cannot control how much solar energy one will receive because of weather changes
- Large amounts of surface area is required
2. How is solar energy created?
A photovoltaic cell, commonly called a solar cell or PV, is the technology used to convert solar energy directly into electrical power. A photovoltaic cell is a nonmechanical device usually made from silicon alloys.
PV cells are made of semiconductors, such as crystalline silicon or various thin-film materials. Photovoltaics can provide tiny amounts of power for watches, large amounts for the electric grid, and everything in between.
Sunlight is composed of photons, or particles of solar energy. These photons contain various amounts of energy corresponding to the different wavelengths of the solar spectrum.
When photons strike a photovoltaic cell, they may be reflected, pass right through, or be absorbed. Only the absorbed photons provide energy to generate electricity. When enough sunlight (energy) is absorbed by the material (a semiconductor), 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 free electrons, so the electrons naturally migrate to the surface.
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 charge between the cell’s front and back surfaces creates a voltage potential like the negative and positive terminals of a battery. When the two surfaces are connected through an external load, such as an appliance, electricity flows.
How Photovoltaic Systems Operate
The photovoltaic cell is the basic building block of a photovoltaic 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 isn’t enough power for most applications.
To increase power output, cells are electrically connected into a packaged weather-tight module. Modules can be further connected to form an array. The term array refers to the entire generating plant, whether it is made up of one or several thousand modules. The number of modules connected together in an array depends on the amount of power output needed.
Weather Affects Photovoltaics
The performance of a photovoltaic array is dependent upon sunlight. Climate conditions (such as clouds or fog) have a significant effect on the amount of solar energy received by a photovoltaic array and, in turn, its performance. The efficiency of most commercially available photovoltaic modules in converting sunlight to electricity ranges from 5% to 15%. Researchers around the world are trying to achieve efficiencies up to 30%.
Commercial Applications of Photovoltaic Systems
The success of PV in outer space first generated commercial applications for this technology. The simplest photovoltaic systems power many of the small calculators and wrist watches used every day. More complicated systems provide electricity to pump water, power communications equipment, and even provide electricity to our homes.
Some advantages of photovoltaic systems are:
- Conversion from sunlight to electricity is direct, so that bulky mechanical generator systems are unnecessary.
- PV arrays can be installed quickly and in any size.
- The environmental impact is minimal, requiring no water for system cooling and generating no by-products.
Photovoltaic cells, like batteries, generate direct current (DC), which is generally used for small loads (electronic equipment). When DC from photovoltaic cells is used for commercial applications or sold to electric utilities using the electric grid, it must be converted to alternating current (AC) using inverters, solid state devices that convert DC power to AC.
History of the Photovoltaic Cell
The first practical photovoltaic (PV) cell was developed in 1954 by Bell Telephone researchers 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 to provide electricity in remote or “off-grid” locations were there were no electric power lines. Technology advances and government financial incentives have helped to greatly expand PV use since the mid-1990s.
B. Solar Thermal Power Systems
Concentrating solar power technologies use mirrors to reflect and concentrate sunlight onto receivers that collect the solar energy and convert it to heat. This thermal energy can then be used to produce electricity via a steam turbine or heat engine driving a generator.
Solar thermal power plants use the sun’s rays to heat a fluid to very high temperatures. The fluid is then circulated through pipes so it can transfer its heat to water to produce steam. The steam, in turn, is converted into mechanical energy in a turbine and into electricity by a conventional generator coupled to the turbine.
The three main types of solar thermal power systems are:
Types of solar thermal power plants:
1. Parabolic troughs
Parabolic troughs are used in the largest solar power facility in the world located in the Mojave Desert at Kramer Junction, California. This facility has operated since the 1980s and accounts for the majority of 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 to 100 times its normal intensity (concentration ratio) on the receiver pipe located along the focal line of the trough, achieving operating temperatures over 750°F.
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, such as on cloudy days.
2. 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, typically over 2,000, with a working fluid temperature over 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.
3. 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 towers are in the early stages of development compared with 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. Projects from private companies 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
C. Solar Thermal Collectors
Solar thermal (heat) energy is often used for heating water used in homes and swimming pools and for heating the insides of buildings (“space heating”). Solar space heating systems can be classified as passive or active.
Passive space heating is what happens to your car on a hot summer day. The sun’s rays heat up the inside of your car. In buildings, the air is circulated past a solar heat surface and through the building by convection (meaning that less dense warm air tends to rise while denser cool air moves downward). No mechanical equipment is needed for passive solar heating.
Active heating systems require a collector to absorb and collect solar radiation. Fans or pumps are used to circulate the heated air or heat absorbing fluid. Active systems often include some type of energy storage 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 collector and are used when temperatures below about 200°F are sufficient. They are often used for heating buildings.
There are many flat-plate collector designs but generally all consist of:
- A flat-plate absorber that intercepts and absorbs the solar energy
- A transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber
- A heat-transport fluid (air or water) flowing through tubes to remove heat from the absorber, and a heat insulating backing
Concentrating collectors— The area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area.
3. How beneficial is solar energy to the environment?
Using solar energy produces no air or water pollution and no greenhouse gases, but may have some indirect negative impacts on the environment. For example, there are some toxic materials and chemicals that are used in the manufacturing process of photovoltaic cells (PV), which convert sunlight into electricity. Some solar thermal systems use potentially hazardous fluids to transfer heat. U.S. environmental laws regulate the use and disposal of these types of materials.
As with any type of power plant, large solar power plants can affect the environment where they are located. Clearing land for construction of the power plant may have long term impacts on plant and animal life. They may require water for cleaning solar collectors or concentrators and for cooling turbine-generators. Using ground water from wells may affect the ecosystem in some arid locations. Birds and insects can be killed if they fly into a concentrated beam of sunlight created by a “solar power tower.”
**The information on this website was obtained from various pages of the U.S. Department of Energy website (www.energy.gov) and the U.S. Environmental Protection Agency website (www.epa.gov). The data on this website was obtained from various pages of the U.S. Energy Information Administration website (www.eia.gov.). Please consult those websites for further information.