The sun has produced energy for billions of years and is the ultimate source for all of the energy sources and fuels that we use today. People have used the sun's rays (solar radiation) for thousands of years for warmth and to dry meat, fruit, and grains. Over time, people developed technologies to collect solar energy for heat and to convert it into electricity.
Radiant energy from the sun has powered life on earth for many millions of years.
An example of an early solar energy collection device is the solar oven (a box for collecting and absorbing sunlight). In the 1830s, British astronomer John Herschel used a solar oven to cook food during an expedition to Africa. People now use many different technologies for collecting and converting solar radiation into heat energy for a many uses.
We use solar thermal energy systems to heat
Solar photovoltaic (PV) devices, or solar cells, change sunlight directly into electricity. Small PV cells can power calculators, watches, and other small electronic devices. Arrangements of many solar cells in PV panels and arrangements of multiple PV panels in PV arrays can produce electricity for an entire house. Some PV power plants have large arrays that cover many acres to produce electricity for thousands of homes.
Using solar energy has two main benefits:
Solar energy also has some limitations:
World map of solar resources
Click to enlarge
Sunshine is radiant energy from the sun. The amount of solar radiation, or solar energy, that the earth receives each day is many times greater than the total amount of all energy that people consume each day. Use of solar energy, especially for electricity generation, has increased a lot in the United States and around the world in the past 30 years.
Latitude, climate, and weather patterns are major factors that affect insolation—the amount of solar radiation received on a given surface area during a specific amount of time. Locations nearer the equator and in arid climates generally receive higher amounts of insolation than other locations. Clouds, dust, volcanic ash, and pollution in the atmosphere affect insolation levels at the surface. Buildings, trees, and mountains may shade a location during different times of the day in different months of the year. Seasonal (monthly) variations in solar resources increase with increasing distance from the earth’s equator.
The type of solar collector also determines the type of solar radiation and level of insolation that a solar collector receives. Concentrating solar collector systems, such as those used in solar thermal-electric power plants, require direct solar radiation, which is generally greater in arid regions with few cloudy days. Flat-plate solar thermal and photovoltaic (PV) collectors are able to use global solar radiation, which includes diffuse (scattered) and direct solar radiation. Learn more about solar radiation.
Low-temperature solar thermal collectors absorb the sun's heat energy to heat water for washing and bathing or for swimming pools, or to heat air inside buildings.
Concentrating solar energy technologies use mirrors to reflect and concentrate sunlight onto receivers that absorb solar energy and convert it to heat. We can use this thermal energy for heating buildings or to produce electricity with a steam turbine or a heat engine that drives a generator.
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 that hundreds or thousands of homes use. Millions of houses and buildings around the world have PV systems on their roofs and there are many large PV power plants.
The U.S. Energy Information Administration (EIA) estimates that total solar energy use in the United States increased from about 0.06 trillion British thermal units (Btu) in 1984 to about 1,870 trillion Btu in 2022. Solar electricity generation accounted for about 97% of total solar energy use in 2022 and direct use of solar energy for space and water heating accounted for about 3%.
Total U.S. solar electricity generation increased from about 5 million kWh in 1984 (nearly all from utility-scale, solar thermal-electric power plants) to about 204 billion kWh in 2022. In 2022, utility-scale PV power plants accounted for 70% of total solar electricity generation, small-scale PV systems accounted for 29%, and utility-scale solar thermal-electric power plants accounted for 1%. Utility-scale power plants have at least 1,000 kilowatts (kW) (or one megawatt [MW]) of electricity generation capacity. Small-scale PV systems have less than one MW generation capacity.
According to EIA’s International Energy Statistics, total world solar electricity generation grew from 0.4 billion kWh in 1990 to about 1.306 billion (about 1.3 trillion) kWh in 2021. The top five producers of solar electricity and their percentage shares of world total solar electricity generation in 2021 were:
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.
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 semiconductor material. When photons strike a PV cell, they may reflect off the cell, pass through the cell, or be absorbed by the semiconductor material. Only the absorbed photons provide energy to generate electricity. When the semiconductor material absorbs enough sunlight (solar energy), 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 that the electrons naturally migrate to the surface of the cell.
The movement of electrons, each carrying a negative charge, toward the front surface of the cell creates an imbalance of electrical charge between the cell's front and back surfaces. This imbalance, in turn, creates a voltage potential like the negative and positive terminals of a battery. Electrical conductors on the cell absorb the electrons. When the conductors are connected in an electrical circuit to an external load, such as a battery, electricity flows in the circuit.
The efficiency at which PV cells convert sunlight to electricity varies by the type of cell material and technology. The efficiency of PV modules averaged less than 10% in the mid-1980s, increased to around 15% by 2015, and is now approaching 20% for state-of-the art modules. Experimental PV cells and PV cells for space satellites are nearly 50% efficient.
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, such as for powering calculators or wristwatches.
PV cells are electrically connected in a packaged, weather-tight PV module or panel. PV modules vary in size and in the amount of electricity they can produce. PV module electricity generating 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 hundreds of PV modules. The number of PV modules connected in a PV array determines the total amount of electricity 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 (AC) in electric power lines. Devices called inverters are used on PV modules or in arrays to convert the DC electricity to AC electricity.
PV cells and modules will produce the largest amount of electricity when they are directly facing the sun. PV modules and arrays can use tracking systems that move the modules to constantly face the sun, but these systems are expensive and are mostly used in large PV power plants. Most PV systems on buildings have modules in a fixed position with the modules facing directly south (in the northern hemisphere—directly north in the southern hemisphere). Learn more about PV collector tilt angles and PV collector tracking systems.
Solar photovoltaic cells are grouped in panels (modules), and panels can be grouped into arrays of different sizes to produce small to large amounts of electricity, such as for powering water pumps for livestock water, for providing electricity for homes, or for utility-scale electricity generation.
The smallest photovoltaic systems power calculators and wristwatches. 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
The first practical PV cell was developed in 1954 by Bell Telephone researchers. Beginning in the late 1950s, PV cells were used to power U.S. space satellites. By the late 1970s, PV panels were providing electricity in remote, or off-grid, locations that did not have 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. Technical advances, lower costs for PV systems, and financial incentives and government policies have helped to greatly expand PV use since the mid-1990s. Hundreds of thousands of grid-connected PV systems are now installed in the United States.
EIA estimates that electricity generated at utility-scale PV power plants increased from 6 million kilowatthours (kWh) in 2004 to 143 billion kWh in 2022. Utility-scale power plants have at least 1,000 kilowatts (or one megawatt) of electricity generating capacity. EIA estimates that 59 billion kWh were generated by small-scale grid-connected PV systems in 2022, up from 11 billion kWh in 2014. Small-scale PV systems are systems that have less than 1,000 kilowatts of electricity generation capacity. Most small-scale PV systems are located on buildings and are sometimes called rooftop PV systems.
Solar thermal power (electricity) generation systems collect and concentrate sunlight to produce the high temperature heat needed to generate electricity. All solar thermal power systems have solar energy collectors with two main components: reflectors (mirrors) that capture and focus sunlight onto a receiver. In most types of systems, a heat-transfer fluid is heated and circulated in the receiver and used to produce steam. The steam is converted into mechanical energy in a turbine, which powers a generator to produce electricity. Solar thermal power systems have tracking systems that keep sunlight focused onto the receiver during the day as the sun changes position in the sky.
Solar thermal power systems may also have a thermal energy storage system component that allows the solar collector system to heat an energy storage system during the day, and the heat from the storage system is used to produce electricity in the evening or during cloudy weather. Solar thermal power plants may also be hybrid systems that use other fuels (usually natural gas) to supplement energy from the sun during periods of low solar radiation.
There are three main types of concentrating solar thermal power systems:
Linear concentrating systems collect the sun's energy using long, rectangular, curved (U-shaped) mirrors. The mirrors focus sunlight onto receivers (tubes) that run the length of the mirrors. The concentrated sunlight heats a fluid flowing through the tubes. The fluid is sent to a heat exchanger to boil water in a conventional steam-turbine generator to produce electricity. There are two major types of linear concentrator systems: parabolic trough systems, where receiver tubes are positioned along the focal line of each parabolic mirror, and linear Fresnel reflector systems, where one receiver tube is above several mirrors to allow the mirrors greater mobility in tracking the sun.
A linear concentrating collector power plant has a large number, or field, of collectors in parallel rows with a north-south orientation so that the mirrors track the sun from east to west and concentrate sunlight continuously onto the receiver tubes during the day.
Parabolic trough power plant
A parabolic trough collector has a long parabolic (curved-shaped) reflector that focuses the sun's rays on a receiver pipe located at the focus of the parabola. The collector moves to focus sunlight on the receiver as the sun moves through the sky during the day.
Because of its parabolic shape, a trough can focus the sunlight from 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.
Parabolic trough linear concentrating systems are used in one of the longest operating solar thermal power facilities in the world, the Solar Energy Generating System (SEGS) located in the Mojave Desert in California. The facility has had nine separate plants over time, with the first plant in the system, SEGS I, operating from 1984 to 2015, and the second, SEGS II, operating from 1985 to 2015. SEGS III–VII (3–7), each with net summer electric generation capacities of 36 megawatts (MW), came online in 1986, 1987, and 1988. SEGS VIII (8) and IX (9), each with a net summer electric generation capacity of 88 MW, began operation in 1989 and 1990, respectively. SEGS 3, 4, 5, 6, 7, and 8 all ceased operation in 2021, leaving only SEGS 9 in operation as of December 31, 2021.
In addition to the SEGS, many other parabolic trough solar power projects operate in the United States and around the world. The four largest projects in the United States after SEGS are:
Linear Fresnel reflector (LFR) systems are similar to parabolic trough systems in that mirrors (reflectors) concentrate sunlight onto a receiver located above the mirrors. These reflectors use the Fresnel lens effect, which allows for a concentrating mirror with a large aperture and short focal length. These systems are capable of concentrating the sun's energy to about 30 times its normal intensity. Compact linear Fresnel reflector (CLFR)—also referred to as a concentrating linear Fresnel reflector—are a type of LFR technology with multiple absorbers within the vicinity of the mirrors. Multiple receivers allow the mirrors to change their position so they do not shade other reflectors. This improves system efficiency and reduces costs. A demonstration CLFR solar power plant was built near Bakersfield, California, in 2008, but it is currently not operating.
Solar power tower
A solar power tower system uses a large field of flat, sun-tracking mirrors called heliostats to reflect and focus sunlight onto a receiver on the top of a tower. Sunlight can be concentrated as much as 1,500 times. Some power towers use water as the heat-transfer fluid. Advanced designs are experimenting with molten nitrate salt because of its superior heat transfer and energy storage capabilities. The thermal energy-storage allows the system to produce electricity during cloudy weather or at night.
The U.S. Department of Energy and several electric utilities built and operated the worlds's first demonstration solar power tower near Barstow, California, during the 1980s and 1990s. Two solar power tower projects now operate in the United States:
Solar dish/engines
Solar dish/engine systems use a mirrored dish similar to a very large satellite dish. To reduce costs, the mirrored dish usually has many small flat mirrors formed into a dish shape. The dish-shaped surface directs and focuses sunlight onto a thermal receiver, which absorbs and collects the heat and transfers it to an engine generator. The most common type of heat engine used in dish/engine systems is the Stirling engine. This system uses the fluid heated by the receiver to move pistons and create mechanical power. The mechanical power runs a generator or alternator to produce electricity.
Solar dish/engine systems always point straight at the sun and concentrate solar energy at the focal point of the dish. A solar dish's concentration ratio is much higher than linear concentrating systems, and it has a working fluid temperature higher than 1,380°F. The power-generating equipment of a solar dish can be placed at the focal point of the dish, making it well suited for remote locations, or the energy may be collected from many dishes and converted into electricity at a central point.
There are no utility-scale solar dish/engine projects in commercial operation in the United States.
People use solar thermal energy to heat water and air. The two general types of solar heating systems are passive systems and active systems.
Passive solar space heating happens when the sun shines through the windows of a building and warms the interior. Building designs that use passive solar heating usually have south-facing windows that allow the sun to shine on solar heat-absorbing walls or floors during the day in the winter. The absorbed solar energy heats the building by natural radiation and convection at night. Window overhangs or shades block the sun from entering the windows during the summer to keep the building cool.
Active solar heating systems use a collector and a fluid that absorbs 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 solar water heating systems usually have a tank for storing solar heated water.
Nonconcentrating collectors—The collector area (the area that intercepts the solar radiation) is the same as the absorber area (the area absorbing the radiation). Solar systems for heating water or air usually have nonconcentrating collectors. Flat-plate collectors are the most common type of nonconcentrating collectors for water and space heating in buildings and are used when temperatures lower than 200°F are sufficient.
Flat-plate solar collectors usually have three main components:
Solar water heating collectors have metal tubes attached to the absorber. A heat-transfer fluid is pumped through the absorber tubes to remove heat from the absorber and transfer the heat to water in a storage tank. Solar systems for heating swimming pool water in warm climates usually do not have covers or insulation for the absorber, and pool water is circulated from the pool through the collectors and back to the pool.
Solar air heating systems use fans to move air through flat-plate collectors and into the interior of buildings.
Concentrating collectors—The area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area. The collector focuses, or concentrates, solar energy onto an absorber. The collector usually moves so that it keeps sunlight focused on the absorber. Solar thermal power plants use concentrating solar collector systems because they can produce high temperature heat.
An array of solar photovoltaic panels supplies electricity for use at Marine Corps Air Ground Combat Center in Twentynine Palms, California
Solar energy systems/power plants do not produce air pollution, water pollution, or greenhouse gases. Using solar energy can have a positive, indirect effect on the environment when solar energy replaces or reduces the use of other energy sources that have larger effects on the environment.
Some toxic materials and chemicals are used to make the photovoltaic (PV) cells that convert sunlight into electricity. Some solar thermal systems use potentially hazardous fluids to transfer heat. Leaks of these materials could be harmful to the environment. 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 near their locations. Clearing land for construction and the placement of the power plant may have long-term effects on the habitats of native plants and animals. Some solar power plants may require water for cleaning solar collectors and concentrators or for cooling turbine generators. Using large volumes of ground water or surface water in some arid locations may affect the ecosystems that depend on these water resources. In addition, the beam of concentrated sunlight a solar power tower creates can kill birds and insects that fly into the beam.
Solar energy is any type of energy generated by the sun.
Solar energy is created by nuclear fusion that takes place in the sun. Fusion occurs when protons of hydrogen atoms violently collide in the sun’s core and fuse to create a helium atom.
This process, known as a PP (proton-proton) chain reaction, emits an enormous amount of energy. In its core, the sun fuses about 620 million metric tons of hydrogen every second. The PP chain reaction occurs in other stars that are about the size of our sun, and provides them with continuous energy and heat. The temperature for these stars is around 4 million degrees on the Kelvin scale (about 4 million degrees Celsius, 7 million degrees Fahrenheit).
In stars that are about 1.3 times bigger than the sun, the CNO cycle drives the creation of energy. The CNO cycle also converts hydrogen to helium, but relies on carbon, nitrogen, and oxygen (C, N, and O) to do so. Currently, less than two percent of the sun’s energy is created by the CNO cycle.
Nuclear fusion by the PP chain reaction or CNO cycle releases tremendous amounts of energy in the form of waves and particles. Solar energy is constantly flowing away from the sun and throughout the solar system. Solar energy warms Earth, causes wind and weather, and sustains plant and animal life.
The energy, heat, and light from the sun flow away in the form of electromagnetic radiation (EMR).
The electromagnetic spectrum exists as waves of different frequencies and wavelengths. The frequency of a wave represents how many times the wave repeats itself in a certain unit of time. Waves with very short wavelengths repeat themselves several times in a given unit of time, so they are high-frequency. In contrast, low-frequency waves have much longer wavelengths.
The vast majority of electromagnetic waves are invisible to us. The most high-frequency waves emitted by the sun are gamma rays, X-rays, and ultraviolet radiation (UV rays). The most harmful UV rays are almost completely absorbed by Earth’s atmosphere. Less potent UV rays travel through the atmosphere, and can cause sunburn.
The sun also emits infrared radiation, whose waves are much lower-frequency. Most heat from the sun arrives as infrared energy.
Sandwiched between infrared and UV is the visible spectrum, which contains all the colors we see on Earth. The color red has the longest wavelengths (closest to infrared), and violet (closest to UV) the shortest.
Natural Solar Energy
Greenhouse Effect
The infrared, visible, and UV waves that reach Earth take part in a process of warming the planet and making life possible—the so-called “greenhouse effect.”
About 30 percent of the solar energy that reaches Earth is reflected back into space. The rest is absorbed into Earth’s atmosphere. The radiation warms Earth’s surface, and the surface radiates some of the energy back out in the form of infrared waves. As they rise through the atmosphere, they are intercepted by greenhouse gases, such as water vapor and carbon dioxide.
Greenhouse gases trap the heat that reflects back up into the atmosphere. In this way, they act like the glass walls of a greenhouse. This greenhouse effect keeps Earth warm enough to sustain life.
Photosynthesis
Almost all life on Earth relies on solar energy for food, either directly or indirectly.
Producers rely directly on solar energy. They absorb sunlight and convert it into nutrients through a process called photosynthesis. Producers, also called autotrophs, include plants, algae, bacteria, and fungi. Autotrophs are the foundation of the food web.
Consumers rely on producers for nutrients. Herbivores, carnivores, omnivores, and detritivores rely on solar energy indirectly. Herbivores eat plants and other producers. Carnivores and omnivores eat both producers and herbivores. Detritivores decompose plant and animal matter by consuming it.
Fossil Fuels
Photosynthesis is also responsible for all of the fossil fuels on Earth. Scientists estimate that about three billion years ago, the first autotrophs evolved in aquatic settings. Sunlight allowed plant life to thrive and evolve. After the autotrophs died, they decomposed and shifted deeper into the Earth, sometimes thousands of meters. This process continued for millions of years.
Under intense pressure and high temperatures, these remains became what we know as fossil fuels. Microorganisms became petroleum, natural gas, and coal.
People have developed processes for extracting these fossil fuels and using them for energy. However, fossil fuels are a nonrenewable resource. They take millions of years to form.
Harnessing Solar Energy
Solar energy is a renewable resource, and many technologies can harvest it directly for use in homes, businesses, schools, and hospitals. Some solar energy technologies include photovoltaic cells and panels, concentrated solar energy, and solar architecture.
There are different ways of capturing solar radiation and converting it into usable energy. The methods use either active solar energy or passive solar energy.
Active solar technologies use electrical or mechanical devices to actively convert solar energy into another form of energy, most often heat or electricity. Passive solar technologies do not use any external devices. Instead, they take advantage of the local climate to heat structures during the winter, and reflect heat during the summer.
Photovoltaics
Photovoltaics is a form of active solar technology that was discovered in 1839 by 19-year-old French physicist Alexandre-Edmond Becquerel. Becquerel discovered that when he placed silver-chloride in an acidic solution and exposed it to sunlight, the platinum electrodes attached to it generated an electric current. This process of generating electricity directly from solar radiation is called the photovoltaic effect, or photovoltaics.
Today, photovoltaics is probably the most familiar way to harness solar energy. Photovoltaic arrays usually involve solar panels, a collection of dozens or even hundreds of solar cells.
Each solar cell contains a semiconductor, usually made of silicon. When the semiconductor absorbs sunlight, it knocks electrons loose. An electrical field directs these loose electrons into an electric current, flowing in one direction. Metal contacts at the top and bottom of a solar cell direct that current to an external object. The external object can be as small as a solar-powered calculator or as large as a power station.
Photovoltaics was first widely used on spacecraft. Many satellites, including the International Space Station (ISS), feature wide, reflective “wings” of solar panels. The ISS has two solar array wings (SAWs), each using about 33,000 solar cells. These photovoltaic cells supply all electricity to the ISS, allowing astronauts to operate the station, safely live in space for months at a time, and conduct scientific and engineering experiments.
Photovoltaic power stations have been built all over the world. The largest stations are in the United States, India, and China. These power stations emit hundreds of megawatts of electricity, used to supply homes, businesses, schools, and hospitals.
Photovoltaic technology can also be installed on a smaller scale. Solar panels and cells can be fixed to the roofs or exterior walls of buildings, supplying electricity for the structure. They can be placed along roads to light highways. Solar cells are small enough to power even smaller devices, such as calculators, parking meters, trash compactors, and water pumps.
Concentrated Solar Energy
Another type of active solar technology is concentrated solar energy or concentrated solar power (CSP). CSP technology uses lenses and mirrors to focus (concentrate) sunlight from a large area into a much smaller area. This intense area of radiation heats a fluid, which in turn generates electricity or fuels another process.
Solar furnaces are an example of concentrated solar power. There are many different types of solar furnaces, including solar power towers, parabolic troughs, and Fresnel reflectors. They use the same general method to capture and convert energy.
Solar power towers use heliostats, flat mirrors that turn to follow the sun’s arc through the sky. The mirrors are arranged around a central “collector tower,” and reflect sunlight into a concentrated ray of light that shines on a focal point on the tower.
In previous designs of solar power towers, the concentrated sunlight heated a container of water, which produced steam that powered a turbine. More recently, some solar power towers use liquid sodium, which has a higher heat capacity and retains heat for a longer period of time. This means that the fluid not only reaches temperatures of 773 to 1,273K (500° to 1,000° C or 932° to 1,832° F), but it can continue to boil water and generate power even when the sun is not shining.
Parabolic troughs and Fresnel reflectors also use CSP, but their mirrors are shaped differently. Parabolic mirrors are curved, with a shape similar to a saddle. Fresnel reflectors use flat, thin strips of mirror to capture sunlight and direct it onto a tube of liquid. Fresnel reflectors have more surface area than parabolic troughs and can concentrate the sun’s energy to about 30 times its normal intensity.
Concentrated solar power plants were first developed in the 1980s. The largest facility in the world is a series of plants in Mojave Desert in the U.S. state of California. This Solar Energy Generating System (SEGS) generates more than 650 gigawatt-hours of electricity every year. Other large and effective plants have been developed in Spain and India.
Concentrated solar power can also be used on a smaller scale. It can generate heat for solar cookers, for instance. People in villages all over the world use solar cookers to boil water for sanitation and to cook food.
Solar cookers provide many advantages over wood-burning stoves: They are not a fire hazard, do not produce smoke, do not require fuel, and reduce habitat loss in forests where trees would be harvested for fuel. Solar cookers also allow villagers to pursue time for education, business, health, or family during time that was previously used for gathering firewood. Solar cookers are used in areas as diverse as Chad, Israel, India, and Peru.
Solar Architecture
Throughout the course of a day, solar energy is part of the process of thermal convection, or the movement of heat from a warmer space to a cooler one. When the sun rises, it begins to warm objects and material on Earth. Throughout the day, these materials absorb heat from solar radiation. At night, when the sun sets and the atmosphere has cooled, the materials release their heat back into the atmosphere.
Passive solar energy techniques take advantage of this natural heating and cooling process.
Homes and other buildings use passive solar energy to distribute heat efficiently and inexpensively. Calculating a building’s “thermal mass” is an example of this. A building’s thermal mass is the bulk of material heated throughout the day. Examples of a building’s thermal mass are wood, metal, concrete, clay, stone, or mud. At night, the thermal mass releases its heat back into the room. Effective ventilation systems—hallways, windows, and air ducts—distribute the warmed air and maintain a moderate, consistent indoor temperature.
Passive solar technology is often involved in the design of a building. For example, in the planning stage of construction, the engineer or architect may align the building with the sun’s daily path to receive desirable amounts of sunlight. This method takes into account the latitude, altitude, and typical cloud cover of a specific area. In addition, buildings can be constructed or retrofitted to have thermal insulation, thermal mass, or extra shading.
Other examples of passive solar architecture are cool roofs, radiant barriers, and green roofs. Cool roofs are painted white, and reflect the sun’s radiation instead of absorbing it. The white surface reduces the amount of heat that reaches the interior of the building, which in turn reduces the amount of energy that is needed to cool the building.
Radiant barriers work similarly to cool roofs. They provide insulation with highly reflective materials, such as aluminum foil. The foil reflects, instead of absorbs, heat, and can reduce cooling costs up to 10 percent. In addition to roofs and attics, radiant barriers may also be installed beneath floors.
Green roofs are roofs that are completely covered with vegetation. They require soil and irrigation to support the plants, and a waterproof layer beneath. Green roofs not only reduce the amount of heat that is absorbed or lost, but also provide vegetation. Through photosynthesis, the plants on green roofs absorb carbon dioxide and emit oxygen. They filter pollutants out of rainwater and air, and offset some of the effects of energy use in that space.
Green roofs have been a tradition in Scandinavia for centuries, and have recently become popular in Australia, Western Europe, Canada, and the United States. For example, the Ford Motor Company covered 42,000 square meters (450,000 square feet) of its assembly plant roofs in Dearborn, Michigan, with vegetation. In addition to reducing greenhouse gas emissions, the roofs reduce stormwater runoff by absorbing several centimeters of rainfall.
Green roofs and cool roofs can also counteract the “urban heat island” effect. In busy cities, the temperature can be consistently higher than the surrounding areas. Many factors contribute to this: Cities are constructed of materials such as asphalt and concrete that absorb heat; tall buildings block wind and its cooling effects; and high amounts of waste heat is generated by industry, traffic, and high populations. Using the available space on the roof to plant trees, or reflecting heat with white roofs, can partially alleviate local temperature increases in urban areas.
Solar Energy and People
Since sunlight only shines for about half of the day in most parts of the world, solar energy technologies have to include methods of storing the energy during dark hours.
Thermal mass systems use paraffin wax or various forms of salt to store the energy in the form of heat. Photovoltaic systems can send excess electricity to the local power grid, or store the energy in rechargeable batteries.
There are many pros and cons to using solar energy.
Advantages
A major advantage to using solar energy is that it is a renewable resource. We will have a steady, limitless supply of sunlight for another five billion years. In one hour, Earth’s atmosphere receives enough sunlight to power the electricity needs of every human being on Earth for a year.
Solar energy is clean. After the solar technology equipment is constructed and put in place, solar energy does not need fuel to work. It also does not emit greenhouse gases or toxic materials. Using solar energy can drastically reduce the impact we have on the environment.
There are locations where solar energy is practical. Homes and buildings in areas with high amounts of sunlight and low cloud cover have the opportunity to harness the sun’s abundant energy.
Solar cookers provide an excellent alternative to cooking with wood-fired stoves—on which two billion people still rely. Solar cookers provide a cleaner and safer way to sanitize water and cook food.
Solar energy complements other renewable sources of energy, such as wind or hydroelectric energy.
Homes or businesses that install successful solar panels can actually produce excess electricity. These homeowners or businessowners can sell energy back to the electric provider, reducing or even eliminating power bills.
Disadvantages
The main deterrent to using solar energy is the required equipment. Solar technology equipment is expensive. Purchasing and installing the equipment can cost tens of thousands of dollars for individual homes. Although the government often offers reduced taxes to people and businesses using solar energy, and the technology can eliminate electricity bills, the initial cost is too steep for many to consider.
Solar energy equipment is also heavy. In order to retrofit or install solar panels on the roof of a building, the roof must be strong, large, and oriented toward the sun’s path.
Both active and passive solar technology depend on factors that are out of our control, such as climate and cloud cover. Local areas must be studied to determine whether or not solar power would be effective in that area.
Sunlight must be abundant and consistent for solar energy to be an efficient choice. In most places on Earth, sunlight’s variability makes it difficult to implement as the only source of energy.
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