Solar Energy
Solar radiation is the energy emitted by the Sun that reaches the Earth. This energy is critical to life, as it drives photosynthesis, regulates the planet's temperature, generates wind and climate, and is the basis for many renewable energy technologies.
Importance of solar radiation in obtaining energy
The relationship between solar radiation and solar energy is direct from a physical and technological point of view. Solar radiation is the flow of electromagnetic energy emitted by the Sun that reaches the Earth in the form of ultraviolet, visible and infrared radiation.
This incident energy is the primary resource that makes solar energy use possible. In other words, solar radiation represents the natural source of energy, while solar energy is the result of its capture, transformation and use through different technologies.
The technical use of this radiation depends on the ability to convert it into useful energy.
In photovoltaic systems, solar radiation is directly transformed into electricity by the photovoltaic effect generated in semiconductor materials. In solar thermal and solar thermal systems, radiation is absorbed and converted into thermal energy to produce heat or generate electricity.
Therefore, parameters such as irradiance, duration of sun exposure, angle of incidence of lightning and atmospheric conditions influence both the amount of radiation received by a surface and the final energy yield of any solar installation.
Origin of solar radiation
Nuclear fusion reactions take place in the Sun's core, where hydrogen atoms combine to form helium, releasing large amounts of energy. This energy is transformed into electromagnetic radiation, which propagates through space at the speed of light (approximately 299,792 km/s).
The solar constant represents the average power of solar radiation received per unit area at the upper limit of the Earth's atmosphere, on a plane perpendicular to the sun's rays. Its average accepted value is approximately 1361 W/m², although it presents small variations associated with solar activity.
Types of solar radiation
Solar radiation is not uniform and is distributed into several main types:
- Infrared (IR) rays: They account for about 49% of total radiation and are responsible for the heat we feel from the Sun.
- Visible light (VI): approximately 43%, responsible for the light we perceive and that allows human vision.
- Ultraviolet (UV) radiation: about 7%, which has important effects on health and biological processes.
- Other types of radiation: less than 1%, including X-rays and gamma radiation in very small amounts.
Subtypes of ultraviolet radiation
- UVA (315–400 nm): easily passes through the atmosphere and reaches the entire Earth's surface. It can lead to premature skin aging, eye damage, and weakening the immune system.
- UVB (280–315 nm): shorter wavelength, partially absorbed by the atmosphere. It is most intense in the equatorial zone and can cause sunburn, darkening of the skin, and skin cancer.
- UVC (100–280 nm): does not reach the earth's surface because it is absorbed by the ozone layer. It is used in a controlled manner in medicine to disinfect or treat certain skin diseases, such as psoriasis or vitiligo.
Spectrum and physical characteristics
Solar radiation has a broad spectrum, similar to that of a blackbody, which means that the energy is distributed along several wavelengths, without concentrating on a single one.
- The maximum radiation is in the visible light band, with a peak around 500 nm, which corresponds to the green-cyan color.
- Photosynthetically active radiation (PAR), used by plants for photosynthesis, ranges from 400 to 700 nm, accounting for about 41% of total radiation.
- Blue-violet: 400–490 nm
- Green: 490–560 nm
- Yellow: 560–590 nm
- Red-orange: 590–700 nm
Spatial and seasonal variation of solar radiation
The amount of solar radiation that reaches the Earth's surface is not homogeneous, but varies significantly depending on latitude, the season of the year and local atmospheric conditions. These differences are mainly due to the inclination of the Earth's axis, which modifies the angle of incidence of the sun's rays and the length of the day throughout the year, as well as factors such as cloudiness, altitude and atmospheric transparency.
As a result, each region of the planet has a characteristic irradiation regime that conditions both its climatic processes and its potential for energy use.
Below is a comparative table with approximate average values of horizontal global solar irradiance (GHI) in different geographical areas and seasons of the year.
Table: Average solar irradiance by geographical areas and stations (kWh/m²/day)
| Geographical area | Winter | Spring | Summer | Autumn | Annual average |
|---|---|---|---|---|---|
| Equatorial zone (e.g. Ecuador, Congo) | 4.5 – 5.5 | 5.0 – 6.0 | 5.0 – 6.5 | 4.5 – 5.5 | 5.0 – 5.8 |
| Dry tropical zone (e.g. Sahara, Arabia) | 5.5 – 7.0 | 6.5 – 8.0 | 6.0 – 8.5 | 5.5 – 7.5 | 6.5 – 7.5 |
| Mediterranean area (e.g. Spain, Italy, Greece) | 2.5 – 4.0 | 4.5 – 6.0 | 6.5 – 8.0 | 3.5 – 5.0 | 4.5 – 5.5 |
| Temperate zone (e.g. France, Germany, northern USA) | 1.5 – 3.0 | 3.5 – 5.5 | 5.0 – 6.5 | 2.0 – 4.0 | 3.5 – 4.5 |
| Nordic area (e.g. Sweden, Norway, Finland) | 0.5 – 2.0 | 2.5 – 4.5 | 4.5 – 6.0 | 0.5 – 3.0 | 2.5 – 3.5 |
You can view more accurate data in your country or region in the Global Solar Atlas (Solargis / World Bank), which offers global maps of horizontal solar irradiance with high spatial resolution (≈250 m).
Examples of solar radiation
Solar radiation on a clear day at noon:
In clear sky conditions and with the Sun close to the zenith, solar radiation reaches its maximum intensity on the Earth's surface. Under these conditions, the irradiance can approach 1000 W/m², which represents the maximum direct radiation available for physical processes and solar energy systems.
Diffuse solar radiation on cloudy days:
When the atmosphere is covered by clouds, much of the sun's radiation is scattered in multiple directions. Although direct radiation decreases significantly, energy still reaches the surface in the form of diffuse radiation, enough to illuminate the environment and allow, for example, the partial operation of photovoltaic systems.
Solar radiation at equatorial latitudes:
In regions near the equator, solar radiation is relatively constant throughout the year due to the low variation of the solar angle. This generates high levels of annual irradiation, favoring both biological processes and the high potential for solar energy generation.
Solar radiation in polar areas:
In polar regions, solar radiation is highly seasonal. During the polar summer there can be continuous sunlight for 24 hours, while in winter the radiation is practically zero, which causes strong energy and climatic contrasts.
Solar radiation in photovoltaic systems:
An applied example is the conversion of solar radiation into electricity by means of solar panels. The radiation incident on silicon photovoltaic cells excites electrons, generating an electrical current that can be used in homes, industries or isolated installations.
Solar radiation in photosynthesis:
Plants use solar radiation within the range of 400 to 700 nm (PAR) to transform CO₂ and water into glucose. This process converts solar energy into chemical energy, the basis of the earth's food chain.
The influence of the Earth's atmosphere
Before reaching the Earth's surface, solar radiation passes through the atmosphere, where it undergoes various physical processes that modify its intensity, spectral composition and direction of propagation. For this reason, the Earth's atmosphere acts as a natural filter that regulates the amount and type of radiation that ultimately reaches the ground.
The main atmospheric phenomena involved in this process are:
- Absorption: certain atmospheric gases absorb part of the sun's radiation. The ozone layer absorbs almost all UVC radiation and a significant fraction of UVB, while water vapor, carbon dioxide, and other gases absorb mostly radiation in the infrared range.
- Reflection and scattering: Clouds, aerosols, and suspended particles reflect and scatter some of the incident radiation in multiple directions. As a result, so-called diffuse radiation is generated, which reaches the surface even when direct radiation is partially blocked.
- Refraction: when passing through atmospheric layers with different densities, solar radiation experiences small deviations in its trajectory. This phenomenon slightly modifies the angle of incidence of the sun's rays and affects the energy distribution on the earth's surface.
The combination of these processes determines the proportion of direct, diffuse and reflected radiation that each region of the planet receives. Its intensity varies according to factors such as latitude, altitude, time of day, season of year and weather conditions, conditioning both the earth's climate and the potential for solar energy use.
Importance of solar radiation
Solar radiation is the main source of energy in the Earth system and plays an essential role in the physical, biological and climatic processes of the planet. Its influence ranges from the maintenance of life to the regulation of the climate and the functioning of the earth's energy system.
Its most relevant functions include:
Photosynthesis: solar radiation provides the energy necessary for plants, algae and certain microorganisms to transform carbon dioxide and water into organic matter through photosynthesis. This process forms the basis of the food chain and contributes to the production of oxygen in the atmosphere.
Climate and atmospheric regulation: The uneven heating of the earth's surface due to solar radiation generates thermal gradients that drive atmospheric and oceanic circulation. This mechanism gives rise to winds, ocean currents and weather systems that determine the global climate.
Water cycle: Solar energy is the main driver of the water cycle, as it causes water to evaporate from oceans, rivers, and soils. Subsequently, this vapor condenses forming clouds and precipitation, thus closing the hydrological cycle that sustains terrestrial ecosystems.
Earth's energy balance: incoming solar radiation, together with the infrared radiation emitted by the Earth, determines the planet's energy balance. This balance makes it possible to keep the global average temperature within ranges compatible with life and to understand phenomena such as the greenhouse effect.
Renewable energy source: solar radiation can be transformed into useful energy using photovoltaic, solar thermal and hybrid technologies. Its use allows electricity and heat to be generated in a sustainable way, reducing dependence on fossil fuels.
Health effects
Ultraviolet radiation can have various effects on the skin of humans depending on its intensity and the length of its waves.
UVA radiation can cause premature skin aging and skin cancer. It can also cause eye and immune system problems.
UVB radiation causes sunburn, darkening and thickening of the outer layer of the skin, and melanoma and other types of skin cancer. It can also cause eye and immune system problems.
The ozone layer prevents most UVC radiation from reaching Earth. In the medical field, UVC radiation can also arise from special lamps or a laser beam and is used to kill germs or to help heal wounds. It is also used to treat certain skin conditions such as psoriasis, vitiligo, and skin nodules that cause cutaneous T-cell lymphoma.