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What is Solar Energy? (Definition, Pros, Cons and Examples)


Solar energy is heat and radiant light from the Sun that can be harnessed with technologies such as solar power (which is used to generate electricity) and solar thermal energy (which is used for applications such as water heating).

As a renewable and clean energy resource, solar can be used as a replacement for fossil fuels, producing heat, creating chemical reactions and generating electricity. Solar energy technology can be built flexibly at scale and allows the energy that is collected to be stored for later use.

The amount of potential solar energy that reaches the Earth each day far exceeds the current and expected future energy requirements of the world. The question is just how to harness the potential and, in order to understand this, we need to look at how solar energy works…


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TWI provides our Industrial Members with support for a range of services related to renewable energy sources, including solar power. Among the projects we have worked on are the development of a coating to improve the performance of solar cells and defect detection methods for solar panels. In addition, we can provide solar reflectometry services, measuring solar reflectance and absorbance on surfaces to help mitigate climate change.

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How Does Solar Energy Work?

Sunlight is the largest energy source to reach the Earth but, despite this, the intensity of the energy that reaches the Earth’s surface is relatively low due to the radial spreading of solar radiation as it travels from the distant Sun. More of this sunlight is lost in the Earth’s atmosphere and due to clouds, which between them scatter as much as 54% of the incoming light. As a result, the sunlight that reaches the ground is around 50% visible light and 45% infrared radiation with the rest being made up of small amounts of ultraviolet and other types of electromagnetic radiation. Although much of the solar energy is lost as it travels to the surface of the Earth, this energy still equals around 200,000 times the daily total electricity generating capacity of the world. Exploiting this renewable resource can be difficult however, with collection, conversion and storage still being quite costly.

When harnessed, solar radiation can be converted into electrical energy with solar panels, for example, or converted into thermal (heat) energy, which is easier to achieve.

Electricity Generation

Electricity can be generated from sunlight using solar cells (also called photovoltaic cells). When sunlight hits a solar cell, a small electric voltage is generated as the photovoltaic effect takes place between a metal and semiconductor like silicon, or two different semiconductors. The photovoltaic effect frees electrons, which flow through an external circuit as the semiconductors present a natural difference in electric potential (voltage). This supplies a small amount power to the load, with each photovoltaic cell producing around two watts. Creating solar panel arrays from large numbers of solar cells allows for thousands of kilowatts of energy to be generated. Most modern photovoltaic cells are around 15-22% energy efficient (although improvements are being made), which mean that large assemblies may be required to generate moderate amounts of power.

Common smaller scale uses of solar cells include providing a power source for pocket calculators and watches, while homeowners and businesses use larger arrays to replace or augment their conventional electric supplies and solar power plants can offer even greater levels of electricity generation by concentrating or focusing the light with aligned mirrors or lenses. This focusing can heat a target to 2,000 °C (3,600 °F) or more, operating a boiler that creates steam, which turns turbines and powers an electricity generator. One method for this is to focus the heat upon blackened pipes, through which water is passed, to warm it up.

Thermal Energy

There are several techniques that are used to capture solar energy and convert it into thermal energy. One of the more common methods is the use of large flat plate collectors, which are commonly made of blackened metal plate covered in sheets of glass. These plates, which can have surface areas of around 40 square metres, heat up as sunlight falls upon them. Carrier fluids such as water flow past the back of the plate, warming up so the thermal energy can be used directly or stored as a different medium. Flat plate collectors are used for solar water or house heating, with the heated water being gathered on sunny days and stored in insulated tanks so it can be used at night or on cloudy days. The hot water can either be drawn directly from the storage tank or passed through pipes in floors and ceilings to heat homes or business premises as space heating. These carrier fluids are typically heated to between 66 and 93°C. This method of collection has an efficiency of 20-80% depending on the design of the collector.

Thermal energy conversion can also be achieved with solar ponds. These are bodies of salt water that collect and store solar energy which can be used to produce products like chemicals, food and textiles, as well as being used to warm greenhouses, swimming pools and livestock buildings. Solar ponds can also be used to produce electricity using the organic Rankine cycle engine, but they are fairly expensive to install and their use is generally limited to warm rural locations.

Another use of thermal solar energy is to cook food in portable solar ovens, which typically concentrate solar energy from the sun gathered across a wide area to a central point, where a black-surfaced vessel converts the sunlight into heat.


Solar energy has a number of inherent advantages:

  • Renewable: Solar energy is a fully renewable energy resource
  • No Fuel Costs: There are no fuel costs associated with solar energy, which will save money
  • Environmentally Friendly: Unlike with other energy sources, such as fossil fuels, solar energy doesn’t release any harmful natural gases or hazardous by-products


Despite the inherent advantages, there are also some disadvantages associated with solar energy:

  • Reliability: Solar energy is dependent on the weather and how many hours of sunlight there are. This means that it is better suited to some parts of the world than others
  • Cost: Although the costs are reducing, solar energy technology such as solar panels can be expensive to install

Examples and Applications

Solar energy has already found a wealth of applications, whether based upon thermal or electrical energy generation, or even on more passive uses of the light and heat that reaches the Earth from the Sun each day…

Thermal Energy Applications

Thermal energy has a variety of applications including heating water, cooling or warming buildings, generating process heat, cooking, water treatment and molten salt technologies.

1. Water Heating

Sunlight can be used to heat water using evacuated tube collectors, glazed flat plate collectors and unglazed plastic collectors. Solar hot water systems are widely deployed in China and nations such as Israel and Cyprus lead the world in per capita use, while Australia, Canada and the United States mainly use solar water heating to heat swimming pools.

2. Cooling, Heating and Ventilation

Solar energy can be used or heating, cooling and ventilation. Solar heating is divided into sctive and passive solar concepts according to whether active elements like solar concentrating optics or sun tracking are used. Thermal mass, which is any material that can be used to store heat, is also used in arid or warm temperate climates to keep buildings cool. These materials include stone, cement or water, which absorbs solar energy during the day and then radiates the warmth during the cooler night-time hours. The size and location of thermal mass depends on factors like climate, daylight hours and shade but can reduce the need for auxiliary cooling or heating equipment. Solar or thermal chimneys can also be used for ventilation, allowing air to heat up inside to create an updraft that pulls air through a building, creating ventilation. Another passive form of solar heating control involves the planting of deciduous trees. When planted on the southern side of a building in the northern hemisphere or the northern side in the southern hemisphere, the leaves provide shade in the summer and the bare tree limbs allow light to pass through to the building in the winter.

3. Cooking

Solar cookers have been used for applications such as cooking, drying and pasteurisation for centuries. The first box cooker, which is an insulated container with a transparent lid that can reach temperatures of 90-150°C, was built by Horace de Saussure in 1767. Panel cookers also use an insulated container but include a reflective panel to direct the sunlight and can reach similar temperatures to box cookers. Reflector cookers use concentrating geometries such as dishes, troughs or Fresnel mirrors to focus the sunlight and can reach temperatures of 315°C, but require direct light to work correctly and must be repositioned to track the movement of the Sun.

4. Process Heat

Technology to concentrate sunlight, such as parabolic dishes, troughs and reflectors, are used to provide process heat for a range of industrial and commercial applications. This includes heating, air conditioning, ventilation and electricity generation.

5. Water Treatment

Solar energy has been used for distillation since at least the 16th Century to make brackish or saline water potable. Along with desalination, sunlight is used for disinfecting water. Solar water disinfection (SODIS) is used by over two million people in developing countries each day to make drinking water. The process involves exposing polyethylene terephthalate (PET) bottles of water to sunlight for several hours. Using sunlight to evaporate water from shallow ponds is also a traditional method of obtaining salt from seawater and can be used to concentrate brine solutions or removing dissolved solids from waste streams.

Electricity Generation Examples

The Sun can be used to generate electricity using photovoltaics (PV) and concentrated solar power. Photovoltaic cells, commonly known as solar cells, turn light into an electric current while concentrated solar power involves the use of mirrors, lenses and solar tracking systems to focus sunlight into a small beam.

1. Photovoltaics (PV)

Originally used for small and medium-sized applications like calculators and providing power for a home, solar PV has grown in scale over the years with the development of   commercial solar power plants and solar farms. The use of solar power is growing each year, generating 3.5% of the world’s electricity in 2020. This growth looks set to continue as nations seek clean, pollution-free energy sources to power our future. PV systems use solar modules made up of a large number of solar cells that each generate a small amount of electrical power. Solar installations can be on the ground, on rooftops, walls or even floated on bodies of water. They can be fixed in place or use a solar tracker to follow the movement of the Sun across the sky. Solar panels are placed facing south in the Northern Hemisphere and facing north in the Southern Hemisphere.

2. Concentrated Solar Power

Concentrated solar power (CSP) involves the use of lenses or mirrors to focus sunlight into a small beam and tracking systems to follow the movement of the Sun. The heat of this beam is then used as a heat source to heat a fluid to generate electricity (as with a conventional power plant, where water is heated to create steam that turns turbines, which power an electricity generator). Technologies to concentrate the sunlight include Fresnel reflectors, parabolic troughs, Stirling dishes and solar power towers. CSP designs often need to be protected against weather events such as dust storms that can damage the fine mirrored glass surfaces used at some solar power plants. This can be done by installing metal grills that offer a level of protection while allowing sunlight to reach the mirrors.

Passive Solar Energy Use

As well as the active uses of solar energy in electricity generation or as a thermal power source, sunlight is also used passively for a range of applications.

1. Architecture and Urban Planning

Urban planning and architecture have taken account of sunlight since ancient Greek and Chinese civilisations built their buildings facing south to provide the best light and warmth. Passive solar architecture works by taking account of a structure’s surface area and volume, its orientation, thermal mass and shade. When these factors are combined to the best effect they can deliver well-lit spaces with comfortable temperature ranges. Modern solar architecture uses these passive methods alongside computer modelling, pumps, fans, solar lighting, solar heating and solar ventilation technologies. Urban areas with high temperatures due to materials with high levels of solar absorption like asphalt can be cooled down by planting trees and panting buildings and roads white. These passive cooling techniques reduce air-conditioning costs and can even provide healthcare savings.

2. Agriculture and Horticulture

Optimising solar energy has been an important part of agriculture and horticulture for thousands of years. This has informed planting cycles, the orientation of plant rows, the use of different row heights and mixing plant varieties to improve exposure to sunlight and crop yields. Other techniques include the use of fruit walls as thermal masses to accelerate ripening by keeping plants warm. These fruit walls were widely used in England and France during a period of regional cooling in the North Atlantic region known as ‘The Little Ice Age’ (1303-1860). Greenhouses have been used to convert solar light to heat and allow crops not suited to a local climate to be grown as well as year-round production since Roman times, when primitive greenhouses were used to grow cucumbers all year for Emperor Tiberius. Modern greenhouses grew in use across Europe during the 16th Century where they were used to keep exotic plants that were brought back from around the world.

Solar Energy and Transport

Solar energy has been investigated for transport uses for decades, including the biannual World Solar Challenge to develop and race a solar powered car over 1,877 miles between Darwin and Adelaide in central Australia. Solar panels are also used to provide auxiliary power for applications like air conditioning, which reduces fuel consumption.

The first solar boat was built in England in 1975 and, since then, developments in the technology has seen solar powered boats successfully crossing the Pacific and Atlantic oceans.

Solar energy has also been used for manned and unmanned flight. Manned solar flight saw a crossing of the English Channel in 1981, a 21-stage flight from California to North Carolina in the United States in 1990 and the circumnavigation of the globe in 2016. However, solar flight has tended to focus on unmanned aerial vehicles (UAV).

Solar balloons are black balloons filled with normal air, which expands as the sunlight heats it up, creating an upward buoyancy. While some solar balloons are large enough for human flight, the surface area to payload weight means that most solar balloons are used for smaller applications, such as in the toy market.

Solar Powered Fuel Production

Solar energy can be used to drive chemical reactions, offsetting the use of fossil fuel sources to create storable and transportable fuels. These techniques can be thermochemical or photochemical and can produce a variety of fuels.

Thermochemical processes and the use of photovoltaic cells have both been explored for the production of hydrogen, which can be used as a fuel. This can be achieved by using concentrators to split water into oxygen and hydrogen at high temperatures, using the heat from solar concentrators to drive steam reformation, or simply using clean electricity from solar energy to split water into hydrogen and oxygen. You can find out more about hydrogen production and the environmental benefits here.


What is Solar Energy in Simple Words?

Solar energy the heat and light energy that is produced by the Sun. This energy can be used actively or passively by humans through activities like the installing of solar panels or designing buildings to take account of the natural effects of sunlight.

What is Solar Energy used for?

Solar energy has a wide range of applications, form heating, ventilation and cooling to generating electricity, cooking, water treatment, hydrogen production, transport, and more.

Is Solar Energy Renewable?

Solar energy is a renewable and green energy source that has the potential to power the planet. Energy generated from solar panels has no emissions or hazardous by-products. 

Is Solar Energy Efficient?

Most modern solar panels process 15-22% of solar energy into usable power, depending on factors such as positioning, orientation and weather conditions. The efficiency of a solar panel to convert sunlight into energy is known as performance.

Can Solar Energy be Stored?

Solar energy can be stored using thermal mass systems using materials with specific heat capacities such as stone, molten salts, paraffin wax, earth or water. Solar energy can also be sent directly to a grid, used to produce storable hydrogen fuel or used to pump water to a higher elevation so that it can then be recovered by releasing the water down through a hydroelectric power generator. The power stored from solar energy systems using these methods can be used at times of demand.

Can Solar Energy Replace Fossil Fuels?

Solar energy has the potential to replace fossil fuels entirely, but it is more likely to be used as part of a wider renewable energy mix including other resources like wind power. 

Can Solar Energy Power the World?

While solar power is likely to be just part of a larger group of clean energy production methods in the future, it does have the potential to meet all of the world’s energy needs. A study found that to produce the 17.3 terrawatts of power that the world consumed in 2015 would require solar panels to be installed in an area 335 by 335 kilometres (43,000 square miles). This is equivalent to just 1.2% of the Sahara Desert, which is ideal for solar power generation as it receives over 12 hours of sunlight per day. Of course, as energy consumption increases so will demand for power, with estimates saying that world energy consumption will rise to 715 exajoules by 2030. However, this rise could easily be accommodated with more solar installations. Predictions say that such a project would cost $5 trillion, which equals the world’s expenditure on the military and weapons over three years.

Can Solar Energy be used in Transportation?

Solar energy can be used for transportation by using solar panels to convert sunlight into electricity as with solar boast, solar planes and ground vehicles such as a solar rickshaws. Solar cars have been in development for years, yet none have reached production as of yet. Solar energy can also be used to produce alternative fuels such as electricity, biodiesel and hydrogen.


Humans have harnessed the natural power of the Sun for thousands of years to optimise crop growth, treat water and provide light and warmth. The energy that reaches the surface of the Earth each day is around 200,000 times the amount of daily electricity generating capacity of the world.

Using solar power to generate electricity and solar thermal energy for a range of different applications provides a renewable and clean energy resource that could potentially provide enough power for the whole world.

As the technology and efficiency of solar technologies improves and the cost reduces, solar will become an increasingly important part of the world clean energy mix to meet the future goals of Net Zero.

Whether used directly, passively, or as a resource to produce other fuels such as hydrogen, solar energy is a free and natural resource that continues to develop across a range of applications.

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