Solar energy

Solar energy is the radiant light and heat from the Sun that we can capture using many different technologies. We can use solar energy to make electricity, heat water, or even design buildings that use the Sun's energy more effectively. There are two main ways to use solar energy: passive and active. Passive solar methods include designing buildings to let in natural light and using materials that store heat. Active solar methods use special systems like photovoltaic panels or mirrors to turn sunlight directly into power.

Solar energy is a very important kind of renewable energy because it comes from the Sun, which will always shine. In 2011, the International Energy Agency explained that using solar energy has many big benefits. It can help countries feel safer about their energy because they won’t need to rely on other places for power. It also helps the environment by reducing pollution and helping to mitigate global warming. These good effects are felt all around the world.

Potential

Further information: Solar radiation

About half of the energy from the Sun reaches the Earth. The Earth gets a lot of energy from the Sun—enough to power the whole world if we use it right.

The Earth gets a huge amount of energy from the Sun, about 174 petawatts, at the top of our atmosphere. Some of this energy bounces back into space, but most of it is taken in by clouds, oceans, and land. This energy keeps Earth warm and helps drive weather patterns like wind and rain. Plants also use sunlight to make food and grow, which is important for all living things.

We can use solar energy in two main ways: active and passive. Active ways include special panels and machines that turn sunlight directly into electricity or heat. Passive ways are simpler, like building designs that use natural light and warmth from the Sun. Both help us use clean energy from our biggest light source—the Sun.

Yearly solar fluxes & human consumption¹
¹ Energy given in Exajoule (EJ) = 10¹⁸ J = 278 TWh ² Consumption as of year 2019
Solar	3,850,000
Wind	2,250
Biomass potential	~200
Primary energy use²	633
Electricity²	~86

Annual solar energy potential by region (Exajoules)
Region	North America	Latin America and Caribbean	Western Europe	Central and Eastern Europe	Former Soviet Union	Middle East and North Africa	Sub-Saharan Africa	Pacific Asia	South Asia	Centrally planned Asia	Pacific OECD
Minimum	181.1	112.6	25.1	4.5	199.3	412.4	371.9	41.0	38.8	115.5	72.6
Maximum	7,410	3,385	914	154	8,655	11,060	9,528	994	1,339	4,135	2,263
Notes: Total global annual solar energy potential amounts to 1,575 EJ (minimum) to 49,837 EJ (maximum) Data reflects assumptions of annual clear sky irradiance, annual average sky clearance, and available land area. All figures given in Exajoules. Quantitative relation of global solar potential vs. the world's primary energy consumption: Ratio of potential vs. current consumption (402 EJ) as of year: 3.9 (minimum) to 124 (maximum) Ratio of potential vs. projected consumption by 2050 (590–1,050 EJ): 1.5–2.7 (minimum) to 47–84 (maximum) Ratio of potential vs. projected consumption by 2100 (880–1,900 EJ): 0.8–1.8 (minimum) to 26–57 (maximum) Source: United Nations Development Programme – World Energy Assessment (2000)

Thermal energy

Main article: Solar thermal energy

Solar thermal technologies help heat water, warm buildings, cool spaces, and provide heat for industrial processes.

Early commercial adaptation

In 1878, at a big show in Paris, Augustin Mouchot showed a solar steam engine. But he could not keep developing it because coal was cheap and there were other problems.

In 1897, Frank Shuman, a US inventor, built a small solar engine. It used mirrors to focus sunlight onto boxes filled with a special liquid, powering a steam engine. By 1912, he had built the world’s first solar power station in Maadi, Egypt. It used mirrors to power an engine that pumped water from the Nile River. Even though World War I and finding cheap oil stopped solar energy growth, people became interested again in the 1970s.

Water heating

Main articles: Solar hot water and Solar combisystem

Solar hot water systems heat water using sunlight. In places between 40 degrees north and south, they can provide most of a home’s hot water up to 60 °C (140 °F). Common types include special tube collectors and flat plate collectors for homes, and plastic collectors for swimming pools.

Heating, cooling and ventilation

Main articles: Solar heating, Thermal mass, Solar chimney, and Solar air conditioning

Solar energy can help heat and cool buildings. It can be used with or without special tools that follow the sun’s path. Materials like stone and water can store heat from the sun during the day to keep buildings warm or cool at night.

Cooking

Main article: Solar cooker

Solar cookers use sunlight to cook food, dry things, and clean water. There are different kinds, like box cookers, panel cookers, and reflector cookers. Box cookers are simple and can reach temperatures up to 150 °C (302 °F). Reflector cookers can get even hotter but need direct sunlight.

Process heat

Main articles: Solar pond, Salt evaporation pond, and Solar furnace

Special mirrors can focus sunlight to provide heat for factories. One early example was a system in Georgia, US, that made clothes and provided heat and cooling. Evaporation ponds use the sun to concentrate salts from seawater. Clothes dry using just the sun and wind, saving energy.

Water treatment

Main articles: Solar still, Solar water disinfection, Solar desalination, and Solar Powered Desalination Unit

Solar energy can clean salty or dirty water. One old method used sunlight in special containers to make water safe to drink. Today, people in many countries use sunlight to clean water in bottles left in the sun for hours. Solar energy can also help treat wastewater in special ponds.

Molten salt technology

Molten salt can store heat from solar power plants so they can make electricity even when the sun isn’t shining. This was tested in a project from 1995 to 1999. The salt stays liquid and can keep heat for days. When electricity is needed, the hot salt makes steam to run a turbine, just like in other power plants. Some plants in Spain and Chile use this method to provide steady power.

Electricity production

Concentrated solar power

Main article: Concentrated solar power

Concentrated solar power systems use mirrors or lenses to focus sunlight into a small, powerful beam. This creates a lot of heat, which can be used to make electricity like in regular power plants. Different kinds of these systems exist, such as curved mirrors called parabolic troughs, tower collectors, flat mirrors called Fresnel reflectors, and special dishes called Stirling dishes. These systems follow the sun’s path to keep the light focused. The heat is used to warm a special liquid, which helps make electricity or store energy. Builders must protect these systems from weather like dust storms or hail that could break the glass mirrors. Metal grills can let in sunlight while keeping most of the mirrors safe.

Architecture and urban planning

Main articles: Passive solar building design and Urban heat island

Sunlight has shaped how buildings are designed for a very long time. Ancient Greeks and Chinese people were among the first to design buildings facing south to get natural light and warmth.

Passive solar architecture uses simple ideas like facing the sun, having the right shape, shading, and using materials that keep temperatures steady. When done right for a place’s weather, these buildings stay bright and comfortable. One famous old example is Socrates' Megaron House. Today, computers help design buildings that work well with sunlight for lighting, heating, and air flow. Active solar tools like pumps and special windows can also help make these buildings even better.

Big cities can get hotter than the areas around them because of things like dark roads and buildings that soak up heat. One way to cool cities down is to paint buildings and roads lighter colors and plant more trees. These ideas could help lower temperatures in places like Los Angeles.

Agriculture and horticulture

Agriculture and horticulture aim to make the most of sunlight to help plants grow better. Farmers use tricks like planting at the right time, arranging rows in special directions, and mixing different plants to get better results. Even when there isn’t much sunlight, like during the Little Ice Age, people found ways to keep plants warm. For example, French and English farmers built walls to trap the Sun’s heat, helping fruits ripen faster. Some even thought of moving walls to follow the Sun’s path.

Greenhouses are special buildings that trap heat from the Sun, letting plants grow all year, even in cold weather. The Romans used early greenhouses to grow cucumbers for their leaders. Later, people in Europe built better greenhouses to grow rare plants brought from faraway places. Today, greenhouses and other covers like polytunnels and row covers are still very useful for growing special plants.

Transport

Main articles: Solar vehicle, Solar-charged vehicle, Electric boat, Solar balloon, and Solar aircraft

People have been trying to make cars that run on sunlight since the 1980s. One big race for solar cars is the World Solar Challenge, where teams race across Australia from Darwin to Adelaide. These cars use sunlight to power their journey.

Solar power is also used in boats and airplanes. Some boats have solar panels to help keep things cool inside. In the air, there have been many solar-powered flights, including a plane that flew around the whole world using only sunlight.

A solar balloon is a special balloon that gets heated by the sun, making it rise into the air like a hot air balloon.

Squad Solar vehicle

The Squad Solar is a small electric vehicle with a solar roof that can also be plugged into a normal outlet to charge.

Fuel production

Main articles: Solar chemical, Solar fuel, and Artificial photosynthesis

Solar chemical processes use energy from the Sun to create chemical reactions. These reactions can change sunlight into fuels that can be stored and moved easily. One important fuel that scientists are working on is hydrogen, which can be made by splitting water using heat or light from the Sun. These fuels help us use less energy from fossil fuels and can be a clean way to power our world in the future.

Energy storage methods

Main articles: Energy storage, Seasonal thermal energy storage, Phase change material, Grid energy storage, and Vehicle-to-grid

Thermal mass systems can store solar energy as heat at useful temperatures for daily or longer periods. These systems often use common materials like water, earth, and stone, which can hold a lot of heat. Good designs can reduce the need for heating and cooling and shift energy use to times when it is cheaper.

Phase change materials, such as paraffin wax and Glauber's salt, are also used to store heat. These materials are cheap and easy to find, and they can reach temperatures useful for homes. One of the first homes to use this kind of system was the "Dover House" in Dover, Massachusetts, in 1948. Solar energy can also be stored at very high temperatures using molten salts. These salts are good for storage because they are cheap, hold a lot of heat, and can reach temperatures that work well with power systems. The Solar Two project used this method to store a large amount of energy with very high efficiency.

Off-grid solar systems often use rechargeable batteries to store extra electricity. When connected to the grid, extra electricity can be sent back to it, and standard grid electricity can be used when needed. Net metering programs give homes credit for sending electricity back to the grid, which can be used later. Some systems use two meters to track electricity used and produced, but this is less common because most meters can track both ways already.

Pumped-storage hydroelectricity stores energy by moving water from a lower reservoir to a higher one when energy is available. When energy is needed, the water is released, and the pump becomes a generator.

Development, deployment and economics

Further information: Deployment of solar power to energy grids

Solar energy use began growing in the 1860s because people thought coal might run out. But then coal and petroleum became cheaper and easier to use, so solar energy development slowed down.

When there were big energy problems in 1973 and 1979, many countries started focusing more on solar energy again. They made special programs to help use solar power, and set up research places in the US, Japan, and Germany.

Solar water heaters were used in the United States a long time ago, but they were replaced by cheaper fuels. After the energy problems in the 1970s, people looked at solar water heating again. It has been growing steadily since the 1990s.

The International Energy Agency says solar energy can help solve big world problems. It can make energy safer, cleaner, and cheaper. In 2011, they said solar could provide a third of the world's energy by 2060 if leaders work to change to renewable energy.