Solar power

BACKGROUND

Solar power refers to energy from sunlight that has been converted into thermal or electrical energy.

Solar energy is the source of much of the energy on the planet. Sunlight powers photosynthesis, the process by which plants convert water and carbon dioxide into glucose, the sugar that plants need to grow; over millions of years, plants decompose and turn into coal and oil. Temperature differences on the surface of the Earth caused by solar energy create wind. Wind power can be harnessed to generate electricity. Solar energy also powers the water cycle, which is the heating and cooling of water that constantly replenishes bodies of water. The water cycle fuels hydropower, which is the generation of electricity by harnessing the energy of moving water. Solar energy can also be directly used for lighting, heat, and electricity production.

The amount of solar energy that reaches the Earth depends on location, time of day, weather conditions, and time of year. On average, each meter of land on the planet receives 4.2 kilowatt-hours of energy daily, which almost equals the energy in a barrel of oil. Deserts collect the most sun, at about 6 kilowatt-hours daily. On the other hand, some northern cities can get as little as 0.7 kilowatt-hours each day, less in the winter.

Humans have used solar power for centuries to light and heat buildings, heat water, and cook food. Mirrors were used to concentrate the sun's light as early as 213 B.C.

The modern solar power industry was born out of the Industrial Revolution and fears of the eventual diminishing of fossil fuels as an energy source. In the early 1900s, an American engineer built a solar-powered plant in Egypt. In 1910, English engineer Aubrey Eaneas constructed an enormous concentrating dish that powered an engine called the Solar Motor.

In 1839, French scientist Edmond Becquerel discovered the photovoltaic (PV) effect, wherein certain materials create electricity when exposed to light. Charles Fritts constructed the first primitive solar cell, a device that captures solar energy and directly converts it into electricity. The cell was made of selenium coated with gold, and it was only one percent efficient. In other words, only one percent of the sunlight hitting the cell generated electricity. Bell Labs improved the technology in the 1950s, using silicon instead of selenium and raising the efficiency to four percent. After the energy crisis of the 1970s, the PV technology field grew, as scientists searched for more efficient and cheaper ways to create solar cells.

In the 1970s, interest in solar collectors (containers of water that collect the heat from solar radiation) increased with the increase of federal tax credits. Most solar collectors were used to heat pools or hot tubs. Sixteen million square feet of solar collectors were sold in the United States in 1984. This plummeted to four million in 1987 after prices for fossil fuels dropped and the tax credits expired. As of early 2009, fewer than one percent of households and businesses were using solar collectors. Solar collectors are more common in other countries, such as Cyprus, where grants, tax incentives, and rebates are available to the consumer.

More recently, higher fossil fuel prices and increased interest in protecting the environment has driven the development and steady growth of new solar technologies and the industry in general.

Passive building design and lighting use light and heat harnessed directly from the sun without the use of technology.

Solar collectors store heat that can power space heating, cooling, and water heating. Solar water heating and electricity generation are well-established government-supported applications of solar power.

Concentrating solar power systems (CSP) are large-scale energy producers used to power industries or generate electricity.

Solar cells are materials that directly produce electricity. They can power homes, small devices (such as watches), or larger-scale processes when used in combination with concentrating technologies, such as parabolic dishes.

More experimental uses include solar-powered vehicles like cars and balloons. Some scientists, such as John C. Mankins, have suggested capturing solar energy in space.

Solar devices can be combined to maximize efficiency. Building design is one example where multiple solar technologies can be used in combination. Skylights and south-facing windows passively collect solar energy, while solar collectors power solar heating, cooling, and water heating systems. Panels of solar cells can drive fans that distribute heat throughout the building.

There are also some disadvantages to solar power. Sunlight varies depending on the time of day, season, and location, making its efficiency erratic. The materials that compose some solar technologies, such as semiconductors (used in electronics as conductors of electricity) needed to manufacture solar cells, may also be expensive. Many solar power technologies, such as parabolic dish concentrating power systems (that use mirrors to concentrate solar energy), are not as well established and are cost-prohibitive. Although incentives are available, strict building codes and local ordinances may prevent builders from installing solar power systems, such as solar hot water heaters. The success of solar power on a larger scale rests, in part, on cooperation from utility companies to employ solar technologies to supply power to electric grids, some of which may not support the use of solar power. Some oil companies support the expansion of traditional fossil fuel industries while clean energy technologies are being developed.

In 2007, solar energy represented one percent of the United States' energy consumption. Solar energy consumption rose from 0.064 quadrillion British thermal units (BTUs) in 2003 to 0.080 quadrillion BTUs in 2007. Since 2003, residential use of solar power has increased, and electric use has remained steady. Shipments of photovoltaic (PV) cells have increased steadily since 2000. A total of 886,193 PV modules have been shipped in the United States since 1998. Shipments of solar thermal devices, or concentrating power systems (CPS), have decreased in the United States from 2006 to 2007 (from 20,744 to 15,153), although exports of them increased. The number of companies manufacturing solar thermal collectors was at an all-time high in 2007, at 60.

TECHNIQUE

Solar-powered buildings: Buildings can be renovated to maximize the use of passive solar energy (light and heat harnessed directly from the sun) without the use of technology. The combination of better building design to maximize solar energy usage, proper insulation, and efficient appliances can reduce energy use by 60-80%. The ceilings, walls, and floors of a building or home may be designed to maximize the collection, storage, and distribution of solar energy. The local climate determines how to maximize solar energy. In warmer climates, for example, cooling the house during summer months is as important a consideration as heating during the winter. Most solar design incorporates shading trees and awnings, or overhangs, which provide shade to reduce heat in the summer. Buildings or homes can be partially or entirely heated by solar energy, depending on the design. This is most easily achieved when building a new home, but older homes can be renovated to incorporate more solar design techniques.

Lighting: Proper orientation of windows and skylights may also maximize the replacement of indoor lighting with natural sunlight. Certain design techniques and materials maximize natural lighting. Clerestory windows are a band of windows located near the top of a wall, which can brighten a north-facing or darker room. Also, an open floor plan allows light to bounce through the room.

Solar design overview: Solar design techniques encompass five elements: aperture, absorber, thermal mass, distribution, and control. Building design can take the most advantage of solar energy when considering all of these elements. Sunlight enters the building through the aperture, which is usually a window, which should be facing within 30 degrees of true south to capture the most light. The absorber absorbs and stores heat from sunlight. It can be a dark wall or floor (black absorbs the most light) or a container of water. The material underneath the absorber should also retain heat, and it is called the thermal mass. Once the sunlight is collected and trapped as heat, it must be distributed to the rest of the house. The last element in solar design is control, which ensures that a building is not underheated or overheated. Overhangs can reduce the amount of light entering a window during the summer, or thermostats can detect when a room is too hot or cold.

Movement of heat: Solar design takes into account how heat travels through a building by considering conduction, convection, and radiation. Conduction is the transfer of heat through objects, wherein the molecules of a warmer object vibrate and these vibrations are transferred to cooler molecules. Convection applies to liquids and gases. Warmer air or liquid tends to rise, while colder air or liquids will sink. This explains why a basement is usually cooler than the rest of a house. Radiation explains how heat is absorbed, reflected, or distributed by materials. For example, dark colors absorb more radiation than lighter or opaque colors. Active, as opposed to passive, solar design can also use other means of distributing heat, like fans or blowers.

Energy performance: Understanding specific properties of objects involved in solar energy transfer (for example, a window), while also considering the local climate and building's overall solar design, may improve the energy efficiency of a building. Energy performance takes into account how an object gains or loses heat and is measured in several different ways.

Measurement of performance: The U-factor measures the rate at which the object conducts heat flow. A lower U-factor indicates that the object is more energy efficient. Another measure of energy performance is the solar heat gain coefficient (SHGC), which considers how heat is absorbed and distributed by a material. A low SHGC window, for example, blocks heat, while letting in sunlight. Air leakage can also affect an object's energy performance. A lower rating means the window is better insulated. Visible transmittance (VT) rates how much of the visible spectrum of light is transmitted through a window or skylight; the higher the rating, the more light is transmitted. Light-to-solar gain (LSG) indicates how much light an object or material transmits in relation to how much heat it blocks. A high LSG means that a material transmits more light without increasing the amount of heat.

Energy Star©: Energy Star© is a program sponsored by the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Energy (DOE) that rates the energy efficiency of many household items and building materials. Energy Star© rates products, including windows, doors, and skylights, based on their SHGC and U-factor.

Gain: Direct, indirect, and isolated gain are solar design techniques that optimize the distribution of heat in a building. Direct gain attempts to control the amount of solar radiation by directly heating the living space. Heat distribution begins with sunlight entering a window and hitting an absorber. Different types of absorbers include dark walls, which radiate heat as the temperature of the room decreases. Containers filled with water may be used instead of a wall, since water stores twice as much heat per cubic foot. The absorber should be insulated to prevent unwanted heat loss.

Trombe wall: Indirect gain attempts to heat a room adjacent to the living space with solar radiation. A Trombe wall, an 8-16-inch dark-colored wall located on the house's south side, is often used in indirect gain. A glass plate is mounted less than an inch in front of the Trombe wall, which stores heat. Throughout the day, heat travels through the wall itself at a rate of about 1 inch per hour. By nightfall, heat reaches the other side of the wall and begins to warm the cool room.

Sunspaces: Isolated gain distributes heat from or to the living space. Sunspaces, a room with many vertical windows, are usually incorporated for isolated gain. When a thermal mass is added to the sunspace, the room can absorb and store heat, which can be distributed to the rest of the house.

Windows and skylights: A window frame conducts heat depending on the material of which it is made. Fiberglass, vinyl, wood, or a composite (made from more than one material) will conduct the least amount of heat, unlike metal. Windows may be coated with a substance that may absorb or reflect heat, insulate the window, or block heat while allowing light to pass through. Skylights provide natural lighting and heat. The U.S. Department of Energy's Energy Efficiency and Renewable Energy (EERE) program suggests that in rooms with few windows, skylights should not exceed 15% of the area of the room's floor, which balances the levels of illumination and temperature in the room. In rooms with more windows, the skylight should be no more than five percent of the floor area. The skylight's position also affects the room's illumination and warmth. For example, skylights facing north provide illumination but little heat.

Doors: Energy-efficient doors fit properly in the door's frame so as not to allow air leakage. Weatherstripping (applying a rubber strip at the base of the door) also prevents air leakage. Some doors have a foam core and steel skin for better insulation. Glass doors are especially susceptible to heat transfer. Glazing or multiple panes can provide better insulation.

Air space heating: Air heating systems use solar radiation to heat air as a means to store solar energy. They are usually placed on a roof or in the south-facing wall of a building. Room air heaters utilize a solar air collector, an airtight container with a black plate for absorbing solar radiation and heating the air inside a collector. A fan or blower pulls air from the room into the collector, which heats the air by as much as 90 degrees Fahrenheit, and blows it back into the room. Transpired air collectors are a series of dark, metal plates placed over an existing, south-facing wall. The plates are perforated to allow air to be drawn into the collector by a fan and heated by as much as 40 degrees Fahrenheit. Air heating systems are advantageous over liquid systems because they provide more usable heat throughout the cooler months, do not freeze, and remain functional in spite of minor leaks. Air, though, is not as efficient a heat collector as liquid.

Liquid space heating: Liquid heating systems use solar radiation to heat a liquid, which stores solar energy and distributes this energy to either space or water heating systems.

Solar collectors for liquid systems: There are three types of solar collectors for liquid heating systems: flat-plate, evacuated tube, and integral.

Flat-plate collectors are the most commonly used. The collector is a liquid-filled, insulated box filled with flow tubes and an absorbent dark plate and covered by a glazed glass, both of which maximize heat absorption.

Evacuated tube collectors are rows of glass tubes with a metal tube inside to absorb heat.

Integral collector-storage systems (ICS), also known as batch systems, combine liquid-filled tubes and insulated boxes to absorb heat. Solar collectors may be connected to storage tanks filled with water, antifreeze, or another liquid. Pumps rapidly circulate the water in the storage tank to prevent heat loss. The water in the tank may heat up by 10-20 degrees Fahrenheit or more. The heated water is distributed to a heat exchanger or another storage container for later use.

Heat exchangers for liquid systems: A radiant floor heat exchanger is often used in liquid heating systems. It is difficult to heat a cold room with a radiant floor, but it may help maintain a constant temperature in an already warm room. Heated water travels from the solar collector or storage tank to pipes embedded in a concrete floor covered in tile. The water heats the floor, which in turn radiates heat to the room. Other heat exchanges are available, such as radiators and hot water baseboards, though they may require the water to be heated to a higher temperature.

Solar water heating: Solar water heating systems are similar to and often employ the same technologies as liquid space heating systems. Solar water heating is active or passive, depending on whether or not the pumps circulate the water. Active systems are more expensive and efficient, but less reliable and durable than passive systems.

Active water heating: Active systems include direct circulating systems (best for warmer climates) and indirect circulating systems (for colder climates). Direct systems pump water from a collector to the home, whereas in indirect systems, which are more common, water first travels to an exchanger. In indirect systems, the solar collector is filled with a liquid that does not freeze. The heated liquid travels to an exchanger, which heats water that is pumped to the home.

Passive water heating: Integral or batch and thermosyphon are two types of passive water heating systems. In integral water heating systems, solar collectors preheat the water, which then travels to a storage tank or a conventional water heater before entering the house. In the thermosyphon system, the solar collector is located beneath the storage tank. Warm water rises as it is heated by the sun and is then distributed throughout the building.

Supplemental heating: Most solar heating systems have conventional backup systems to supplement the space or water heating. A solar water heater can preheat water, which then is distributed to a conventional water heater, reducing the amount of conventional energy needed to heat water. The solar and conventional heater can also be combined in the same tank.

Solar space cooling: With a few additions, the same system that employs solar energy to heat air and water can also cool a building or home. Desiccant evaporators use radiant heat of a solar collector to attract water molecules in the air. The humidity is vented outside. Another option is an absorption chiller, which functions like an air conditioner, heating refrigerant under pressure, releasing the pressure, and cooling the room. An absorption chiller uses solar radiation to heat the refrigerant.

Solar ponds: Solar ponds are large areas of saltwater that collect solar energy. Heat collects in the lower layers of the pond, which contain more salt. Water containing more salt is denser and therefore sinks below the less salty water closer to the surface. This difference in salt concentrations traps heat toward the bottom of the pond. The hotter saltwater at the bottom of the pond can be used like water from other solar collectors. It can heat water, heat or cool air, or drive a turbine to generate electricity.

Solar concentrating systems: Concentrating solar power systems (also called concentrating geometries) are large-scale energy producers used to power industries or generate electricity. Mirrors or lenses concentrate solar energy and are typically found in the southwestern United States, which receives the most solar energy in the nation. Concentrating systems fall into three categories: parabolic-trough, dish/engine, and power towers.

Parabolic troughs: Parabolic troughs are the most commonly used, most reliable, and least expensive concentrating systems. Long, curved mirrors are tilted toward the sun and focus the reflected sunlight on an oil-filled pipe that runs along the trough. The hot oil heats water into steam. (Flat mirrors, such as Fresnel mirrors, can also be used to concentrate sunlight.)

Like a conventional fossil fuel power plant, this steam is used to rotate turbines, which activates a generator that produces electricity. This system intensifies the sun's energy by 30-60 times.

Dish/engine: Dish systems use curved dishes to focus sunlight on a single point, so that, in theory, this system can produce higher temperatures than trough systems. The dish focuses light onto a power conversion unit that generates electricity (as much as 25 kilowatts) or that may be connected to a Stirling engine, which converts heat energy to mechanical energy by heating and cooling the gas inside the engine. The dish focuses sunlight onto a receiver, which transfers heat to a liquid. The heated liquid expands against a turbine or piston, which runs a generator that produces electricity. Stirling engines can produce 30 kilowatts of power and replace diesel generators. A Scheffler reflector is a type of dish that can track the sun's rays and change its shape to maximize the amount of light it focuses.

Power towers: In the power tower system, a field of mirrors concentrates light onto a tower. The tower contains a receiver filled with a heat-exchange liquid or molten salt, which retains heat very efficiently. The heated liquid or salt runs a conventional steam generator that produces electricity, which can be stored for later use. Another type of tower, a solar updraft tower, is a large, upright greenhouse. Sunlight heats air inside the tower, which moves up the tower. This movement can be used to turn turbines and generate electricity.

Solar cookers: Solar energy can also be used to power cooking. A reflector cooker can be used in place of a traditional electricity, gas, or coal-fueled stove to cook food. Reflector cookers can be made by hand or commercially fabricated. Their designs vary. Mirrors can be used to focus sunlight on a darkly painted container that holds heat well. Cooking such as frying and browning is done directly on this container. Another type of reflector cooker, called a box cooker, is an insulated box lined with black walls. A glass top allows sunlight to enter the box and heat the interior, where baking and boiling occur. Mirrors can also be used to concentrate sunlight on the box cooker, which retains heat well enough to be used on cloudy days.

Photovoltaics: Photovoltaics (PV) take advantage of the natural property of some materials to release electrons when exposed to light. Using sunlight to drive chemical reactions may also be called solar chemical technology. The free electrons travel through a circuit as electricity. The bottom of a photovoltaic (PV) cell, called the n-type layer, is usually made of boron bonded to silicon crystals and is negatively charged. The top of the cell, or the p-type layer, is a positively charged material of phosphorus bonded to silicon. The cell's P-N junction is the space between these two layers. Individual PV cells are usually square, four inches on each side, and produce about two watts of power. Sunlight hits the PV cell, releasing electrons in the silicon crystals. The electrons want to flow from the negative (n-type) to positive (p-type) layers, but an electric field generated by the P-N junction prevents this. Instead, the electrons travel to thin wires connected to the n-type layer. The wires are part of an electric circuit and provide a path for the free electrons, thus allowing the PV cell to power the circuit. Because individual cells produce little power, multiple PV cells (typically 40) are grouped together as modules for greater energy production. Ten such modules are then placed in glass or plastic panels or arrays, which face south or turn to track the sun. Ten to twenty arrays can power a home.

Silicon PV cells: There are three types of silicon PV cells: single-crystal, polycrystalline, and amorphous crystalline. Single-crystal cells are made into cylinders that are then sliced into rounds. These cells, which make up about a third of the PV market, are comparatively efficient (as much as 25%), but expensive to make. They are more efficient when used in combination with concentrating systems. Polycrystalline cells are sheets of silicon sliced into squares. They make up 62% of the PV market and are cheaper to make, but are less efficient (about 15%). Amorphous silicon (a-Si) cells are glass or metal sheets sprayed with silicon. These cells are inefficient (about five percent) but cheap to produce.

Other solar-powered devices: Solar balloons are large balloons fabricated from a black material that heats up in the sunlight and may be used for transportation (like hot-air balloons) if they are large enough.

THEORY/EVIDENCE

General: Solar energy or power refers to harnessing the sun's energy to produce electricity, lighting, and heat. Since the energy crisis of the 1970s, individuals, agencies, and companies have searched for viable alternatives to nonrenewable, polluting energy sources. Energy from the sun costs nothing and cannot be exhausted. Solar power technologies heat water, heat and cool air, and generate electricity. These technologies can be used in combination with other sources of energy as well as with each other to increase efficiency. Thermogenerators, for example, can combine solar cells and solar collectors to produce both electricity and heat. Some areas, like the southwestern United States, have greater solar potential than other areas. On the other hand, large-scale solar power systems are less effective in Seattle, for example, which receives only 0.7 kilowatt-hours of sunlight per day in December.

Heating homes: More homeowners and companies are turning to renewable energy as an alternative to traditional fuels. Heating a building or home with solar energy is cost effective when a solar heating system replaces expensive fuels such as heating oil, is used for the majority of the year, and provides 40-80% of the building's heating requirements. These systems reduce greenhouse gases and air pollution. Local building codes may require a building permit to install a solar heating system. Mortgage lenders or building codes may require that the builder install a backup heating system, even if the solar heating system provides 100% of the building's heating requirements.

Energy efficiency: Windows, doors, and skylights can help or hinder a building's energy efficiency. The National Fenestration Rating Council (NFRC) rates a window, door, or skylight's energy performance based on measures such as U-factor and solar heat gain coefficient (SHGC). The NRFC's program is voluntary and provides a useful means of comparing the energy efficiency of different products. Windows are a key component to creating a comprehensive solar design scheme. A window's frame, glazing or glass, and operation affect the window's energy performance.

Hot water: Solar energy is also a cost-effective means of providing hot water to homes and buildings. Fourteen percent of household energy use is for heating water. There are 300,000 solar water heating units in the United States, and these units usually meet 50-80% of the hot water needs of a household.

Concentrating technologies: Properly oriented mirrors and lenses can greatly intensify solar energy. By increasing solar energy efficiency, concentrating solar power (CSP) technologies have enabled the solar power industry to compete with other types of energy on a larger scale, even fueling a conventional power plant. Parabolic trough systems are the most widely used and tested of CSP technologies.

CSP projects in the United States: Solar One was a power tower system operated in California from 1982 to 1988, featuring a 17-acre field of mirrors and an 80-meter tower. The second generation of this system, Solar Two, has more successfully collected and stored solar energy than the first. Like dish/engine systems, power tower technology, as of yet, has not been used commercially in the United States. Between 1985 and 1991, nine solar electric generating stations (SEGS) utilizing parabolic trough technology were built in southern California. These SEGSs produce more than 350 megawatts of electricity, which may meet the yearly electricity needs of as many as 900 homes. One SEGS plant is currently being constructed in Arizona and another one in Nevada. The California Public Utility Commission approved the construction of a 500-megawatt parabolic dish plant in the Mojave Desert in 2005.

CSP projects in Europe: Power tower systems are more established in Spain, where several projects are completed or under construction. The Planta 10 water/steam system has a capacity of 11 megawatts. Planta 20 has a 20-megawatt capacity. A third project currently under construction is the Planta Tres project. It will have a 15-watt capacity and use molten nitrate salt for heat-transfer and storage.

Photovoltaics (PV): The photovoltaic (PV) effect was discovered in the 1800s. The first solar cell device, which harnesses the PV effect, was constructed in the late 1800s. PV technology boomed after the energy crisis of the 1970s increased interest in renewable energy sources. Solar cells are relatively inefficient, however, converting as little as 5% of solar energy to electricity. This is why, traditionally, most PV-cell-powered buildings or devices were not connected to a power grid with low energy requirements. PV cells are useful in lower energy applications, as studies have demonstrated.

Other applications: One study compared the effectiveness of solar cells at removing metal (in this case cadmium) from contaminated soil to a traditional direct current (DC) power supply. Although the output potential of the solar cell depended on the time of day and weather conditions (cloudy versus sunny), it worked as effectively as the DC power supply, but also greatly reduced energy expenditure. Another study tested the effectiveness of solar cells to power a reaction and remove dye from textile wastewater. The solar cell fueled the production of a reagent (at 50-70%) when wastewater was saturated with oxygen. The reagent simultaneously decolorized the wastewater.

Fabrication: Solar cells are expensive to fabricate due to the high cost of semiconductors. There are several approaches to creating cheaper PV cells: increase the cells' efficiency, decrease the cost of production, or decrease the cost of the overall project, all of which are being employed in PV manufacturing. Costs for these systems have decreased dramatically, in part due to improvements in the manufacturing processes and equipment and cost-sharing partnerships. Government subsidies reduce the cost of solar cells to consumers, but electricity produced by fossil fuel-powered plants is still less expensive to generate. The National Renewable Energy Laboratory reports that a survey of 14 PV subcontractors showed a 54% drop in production costs between 1992 and 2005 and an annual growth rate in production capacity of 26%.

Materials: The PV industry is also moving beyond silicon-based cells and is investigating the use of other materials that are more efficient. Semiconducting materials are expensive and therefore, to make PV technology more cost effective, newer devices use less of this material while collecting as much light as possible. Materials currently being investigated include gallium arsenide (Ga-As) and cadmium-telluride (CdTe).

One study tested a new approach to PV technology: integrating silicon-based solar cells and newer, more efficient semiconductors to increase overall efficiency. A hybrid cell of lead sulfide (PbS), silicon, and nanocrystals was found to be 50% efficient in visible light and 7% efficient in infrared light.

Power grids: As the technology improves and costs continue to fall, it is more feasible for large-scale PV systems to work in conjunction with utility companies to provide electricity to power grids. A 64-megawatt Nevada Solar One power plant in Boulder City opened in 2007. It is the largest CSP facility to be built since 1991. The 760 mirror arrays covering 300 acres making up Nevada Solar One will supply more than 15,000 homes with electricity. There are several CSP facilities operating in California and one in Arizona.

One study tested the potential of PV systems connected to a power grid to reduce fossil fuel use and its associated emissions. A utility production cost model simulated the effects of PV producing 10% of the electricity in different parts of the western United States. In California, the model demonstrated that PV displaced the use of gas. In Colorado, PV mostly offset gas, as well as coal in non-summer months.

HEALTH IMPACT/SAFETY

General: Overall, solar power technologies impact the environment less than traditional energy systems driven by fossil fuels. They emit little or no pollution during operation and use less water than traditional systems. Typically, they do not require the construction of electrical wires, a process that can lead to tree harvesting and ecosystem disruption, both of which impact the survival of native plants and animals.

Air pollution: Solar technologies can heat, cool, and light homes and heat water without emitting pollution, therefore helping to improve air quality. Solar power's commercial application also displaces electricity generation from more traditional fuels such as coal, natural gas, and oil power plants. This can reduce greenhouse gases like carbon dioxide, mercury, nitrogen oxides, and sulfur dioxide. A U.S. National Renewable Energy Laboratory study demonstrated that 300 tons of nitrogen oxide, 180 tons of carbon monoxide, and 7.6 million tons of carbon dioxide emissions were offset by the 4,000 megawatts of electricity generated by concentrating solar power (CSP) plants in California.

Scalding: Solar water heating systems often use solar energy collectors to store heat that is then distributed to the home. Collectors are often dark-colored tanks located on the rooftops. The water in the tanks can reach very high temperatures, especially on sunny days. During the summer, low hot water use and hot air temperatures can increase the water temperature in the system to dangerous levels. In 2006, over 23,000 children under four years old were scalded in the shower or bath under normal conditions. Solar heating systems can add 10-50 degrees Fahrenheit to the set temperature. Third-degree burns can occur when exposed to water at 140 degrees Fahrenheit for as little as three seconds. Anti-scald valves can be installed at points along the solar heating system to respond to pressure or temperature changes and ensure that the water temperature does not exceed dangerous limits. Stagnation of water in the collector can also cause high water temperatures and potential scalding injuries. A valve installed at the collector can drain the hot water off to a safe area. Also, solar panels are usually made of metal, which, when heated by sunlight, can exceed 80 degrees Fahrenheit and cause injury.

Toxic liquids: Some solar heating systems use heat-transfer liquids to store heat without freezing in colder temperatures. Some liquids, such as ethylene glycol, are toxic. Building codes may require that, if the fluid is toxic, it be kept in a double-walled heat exchanger to protect potable water from leaks. In some areas, food grade (nontoxic) propylene glycol is required for single-walled exchangers, as long as no dyes are added to the solution.

Fire: Fire is a potential hazard to consider with any electrical system, even if that system is solar-powered. Proper installation and maintenance of a solar-powered electrical system reduces the risk of a fire hazard. The size of electrical wires is related to the power capacity of a wire. A thicker, more insulated wire can withstand more heat. Fire is a potential hazard when wiring is inadequate and cannot withstand electricity generation. Protection against lighting strikes and proper grounding of the electrical system can also reduce the risk of fire.

Land usage: Concentrating solar power (CSP) technologies use mirrors to concentrate solar energy and boost electricity generation of solar cell or photovoltaic (PV) systems. CSP systems can require several hundred acres of open, nearly leveled land for construction of solar fields.

Parabolic trough power plants: One type of CSP technology is a parabolic trough power plant. This power plant's solar field consists of parallel rows of solar collectors (mirrors, reflectors, heat collection elements, and other structures), typically aligned on a north-south horizontal axis. This type of power plant generally uses 5-10 acres of land per megawatt of electric capacity. Land requirements are greater for plants that also utilize thermal energy storage structures. Nevada Solar One is the largest CSP facility to be built since 1991. Its 760 mirror arrays cover 300 acres. Flat areas with increased solar radiation, such as the Mojave Desert or Imperial Valley in California, are the best areas for solar power plants. The construction of solar plants poses a risk to desert lands and may compete with agricultural businesses.

Manufacturing risks: Some hazards are associated with the manufacturing of photovoltaic (PV) modules, depending on their components, for the PV or solar cell industry. Occupational, environmental, and public health risks from PV manufacturing are usually associated with the use of toxic chemicals during production, particularly highly flammable gasses such as silane, corrosive liquids, and suspected carcinogens. Risk of exposure is minimized by constant monitoring, protective equipment, and good work practices.

Toxic gases: A 2003 report prepared by the Electric Power Research Institute for the California Energy Commission found that the greatest environmental risk posed by silicon PV cell production is the use of toxic gases. Phosphorus pentoxide and gaseous chlorine are by-products of the production of crystalline silicon-based PV cells and can cause respiratory tract burns and lung damage. Cleaning agents used in production can cause chemical burns. Amorphous silicon-based solar cells are produced using flammable gases, such as silane, which can spontaneously ignite. Sophisticated safety systems, however, designed to reduce this risk, are often employed by manufacturers. Hydrogen selenide, which is used in the production of copper indium diselenide PV modules, is highly toxic and poses an occupational risk and risk to surrounding communities. It has an Immediately Dangerous to Life and Health (IDLH) (the limit of exposure to a substance that can cause death or permanent injury defined by the U.S. National Institute for Occupational Safety and Health) concentration of one part per million. The risks associated with some materials used in the production of some PV modules, such as cadmium telluride, are unknown.

Operation of PV modules: Risks associated with the operation of PV cells are the accidental ingestion of flakes or dust containing semiconductor compounds that make up the cells. The PV modules, however, are enclosed by thick layers of glass or plastic. Unless these components are ground into a dust, the risk of inhalation is minimal.

FUTURE RESEARCH OR APPLICATIONS

General: Solar energy is a significant and growing source of renewable energy. Although the solar resource is free and inexhaustible, the technology that harnesses it has, in the past, been cost-prohibitive. Government incentives, tax credits, loans, grants, rebates, higher demand, and cheaper materials are reducing costs. The California Solar Initiative, for example, provides 3.2 billion dollars over 11 years to develop 3,000 megawatts of solar electricity. The solar energy market grew 48% in 2007, and its price is expected to be competitive with fossil fuels in 2015.

Incentives: Demand for heating and cooling systems fueled by renewable sources of energy will continue to increase. Historically, government incentives have driven increased demand in renewable energy technology, including solar power, and will continue to do so. The Database of State Incentives for Renewables and Efficiency (DSIRE) provides comprehensive information on state, federal, local, and utility incentives. The U.S. Department of Energy designated 25 cities as Solar America Cities to help lay the foundation for the solar power industry. The program supports local government agencies, companies, and individuals with solar power information and incentives. Participating cities include Boston, San Diego, and Minneapolis.

Resources: Many resources are available to assist builders and homeowners in choosing the appropriate technology to make their residences more energy efficient. The U.S. Department of Energy's Energy Efficiency and Renewable Energy program offers a comprehensive guide to choosing, installing, and maintaining solar power systems. The National Renewable Energy Laboratory's PV Manufacturing and Research Project offers information and support for PV contractors looking to optimize production and reduce manufacturing costs. The Solar Energy Industries Association also provides industry support and guidance for starting a solar business.

Technologies: According to the Solar Energy Industries Association, photovoltaic (PV) manufacturing grew 74% in 2007, and installations of PV units grew 45%. The United States is a leader in the manufacturing of some photovoltaic components. Germany, Japan, Spain, and the United States are the four largest markets for PV systems. In the future, PV systems connected to the power grid will continue to outpace off-grid systems. More efficient materials such as gallium arsenide, copper-indium-dielenide, and cadmium-telluride are currently being investigated as replacements for silicon-based PV cells. Amorphous silicon (a-Si) cells, being efficient and cheap to produce, are a promising technology. Windows sprayed with a-Si and PV roof tiles are new applications of solar cell technology. Increased demand for silicon, however, has increased PV costs. This has not deterred the PV industry from its goal of providing half of new electricity generation in the United States by 2025.

Government funding: The Obama administration's Energy Efficiency and Conservation Block Grant, funded by the American Recovery and Reinvestment Act (ARRA), will invest $3.2 billion in energy efficiency and conservation programs that reduce energy use and fossil fuel emissions and increase energy efficiency in the United States. Potential projects to be funded include renewable energy installations on government buildings. States will receive $770 million, cities and counties $1.9 billion, and tribal governments $54 million in federal funds. Recently, Solyndra, Inc. received a $535 million loan guarantee from the U.S. Department of Energy, as part of the ARRA, for the construction of a PV manufacturing plant.

Solar cars: Solar cars are typically electric vehicles powered by solar cell or PV technology placed on the vehicle's roof. Their commercial applications are at the moment limited due to engineering challenges. Solar panels can be large and heavy. PV technology may be used to augment the power requirements of electric vehicles that use traditional batteries.

AUTHOR INFORMATION

This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

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