Solar energy

Solar energy is the radiant energy emitted by the Sun in the form of light and heat. It can be harnessed through a wide range of technologies, including photovoltaic systems, solar thermal systems, and architectural design strategies. As a fundamentally renewable and abundant resource, solar energy plays a central role in global strategies to transition away from fossil fuels, enhance energy security, and reduce pollution and greenhouse-gas emissions. Modern solar technologies are broadly divided into active and passive systems depending on how they collect, convert, and distribute solar radiation.

Characteristics and Types of Solar Technologies

Active solar systems convert sunlight into usable energy through mechanical or electrical means. These include photovoltaic panels that generate electricity, concentrated solar power systems that use mirrors to direct heat into power-generating devices, and solar water heating systems. Passive solar systems, by contrast, make use of the orientation, materials, and layout of buildings to optimise the absorption and distribution of sunlight. Examples include daylighting techniques, thermal mass materials that store heat, and architectural designs that enhance natural ventilation.
The International Energy Agency has emphasised the long-term advantages of solar technologies, highlighting their capacity to deliver clean, inexhaustible energy that can reduce dependence on imported fuels, increase sustainability, and contribute significantly to climate-change mitigation.

Solar Potential and Earth’s Energy Balance

The Earth receives approximately 174 petawatts of solar radiation at the top of the atmosphere. Around 30 per cent of this is reflected back into space, while the remainder—about 122 petawatts—is absorbed by the atmosphere, oceans, and land surface. Solar radiation at ground level spans primarily the visible and near-infrared bands, with a small ultraviolet component. Most of the planet’s population lives in regions receiving between 150 and 300 watts per square metre of solar irradiance, equivalent to about 35–70 kilowatt-hours per square metre per day.
Absorbed sunlight drives major Earth systems. It warms the surface, sustains photosynthesis in plants, fuels atmospheric convection, and powers the water cycle. Photosynthesis alone captures around 3,000 exajoules of energy annually in biomass. The total solar energy absorbed each year—about 3.85 million exajoules—exceeds global primary energy consumption by many orders of magnitude.
Although theoretical solar potential is vast, practical potential is shaped by geography, land availability, cloud cover, and diurnal variation. A 2021 study by the Carbon Tracker Initiative estimated that generating all global energy demand from solar power would require roughly 450,000 square kilometres of land, about 0.3 per cent of Earth’s land surface.

Active and Passive Solar Techniques

Active solar technologies encompass devices that convert sunlight into electricity or heat. Photovoltaic systems use semiconductor materials to produce electrical power directly from solar radiation. Concentrated solar power plants use arrays of mirrors or lenses to concentrate sunlight onto receivers, generating high temperatures that drive turbines. Solar thermal collectors—both flat-plate and evacuated-tube designs—are widely used for domestic water heating and industrial process heat.
Passive solar technologies reduce the need for mechanical energy by capitalising on the movement of the Sun. Building orientation, window placement, insulation, and choice of materials affect a structure’s ability to retain or dissipate heat. Thermal mass materials such as stone, water, and concrete absorb heat during the day and release it at night, helping to stabilise indoor temperatures in warm climates.

Global Solar Energy Potential

In 2000, the United Nations Development Programme and its partners estimated the annual global solar energy potential at between 1,575 and 49,837 exajoules, depending on assumptions regarding cloud cover, land availability, and average irradiance. Even at the lower end, the potential far exceeds global energy needs. Ratios comparing potential with projected energy demand indicate that solar resources can provide many times the energy required in 2050 or 2100.
These assessments highlight solar energy as one of the most scalable and geographically flexible renewable resources. Although most effective in regions closer to the equator, solar systems can be used in virtually all inhabited areas.

Solar Thermal Energy

Solar thermal technologies convert sunlight into heat for water heating, space heating, cooling, and industrial processes. Early innovations date back to the nineteenth century. At the 1878 Universal Exposition in Paris, Augustin Mouchot demonstrated a solar-powered steam engine. Frank Shuman later advanced the field by developing mirrored collectors that concentrated solar radiation sufficiently to boil water. In 1912–13 Shuman built one of the first solar-driven irrigation plants in Maadi, Egypt, using parabolic troughs to pump water from the Nile.
Solar water heating systems remain among the most widespread solar technologies. Evacuated-tube and flat-plate collectors are commonly used for domestic hot water, while unglazed plastic collectors are employed for heating swimming pools. By 2015, global installed capacity of solar hot-water systems reached around 436 gigawatts thermal, with China accounting for the majority of installations.
Solar energy also plays a role in reducing the significant energy consumption associated with heating, ventilation, and air conditioning systems in buildings. Through passive solar design and thermal energy storage, buildings can reduce reliance on mechanical heating and cooling, contributing to lower energy demand.

Thermal Mass and Building Integration

Thermal mass materials store solar heat and moderate indoor temperature fluctuations. Regions with arid or warm temperate climates have historically used thick stone or earthen walls to absorb daytime heat and radiate it at night. Modern sustainable architecture continues to apply these principles, incorporating advanced materials and design strategies to maximise solar gain in winter and reduce overheating in summer.
Seasonal thermal energy storage, demonstrated in early projects such as Solar House 1 in 1939, illustrates the potential of storing summer solar heat for winter use. Innovations in insulation, glazing, and building orientation have extended the viability of integrated solar design in both residential and commercial contexts.