Basic Facts about Solar Radiation

essential information about solar activity, the effects of solar radiation, indirect and direct uses of solar energy and the advantages and disadvantages of solar energy

solar radiation

The continuous thermonuclear reactions that take place in the Sun create a temperature at is surface of about 5,500 degrees Celsius. These reactions emit huge quantities of electromagnetic radiation, including visible light, and high energy particles (collectively known as the 'solar wind'). While these high energy particles emitted by the Sun are deflected by the magnetic field of the Earth, which acts as a shield, the electromagnetic radiation can pass through this shield and enter the atmosphere. The quantity of energy that reaches the top of the atmosphere is called the solar constant and has a value of 1,353W/m2.

As seen in the figure below, about half of this electromagnetic energy is reflected back into space or is absorbed by the atmosphere. Some wavelengths of the incident radiation are absorbed more than others, for example, damaging UV rays (at the extreme left of the diagram) are almost entirely absorbed by the ozone layer which protects life on our planet.

graph of solar radiation intensity

The remaining 50% is distributed as follows: 15% (of the initial radiation of the solar constant) is reflected back to space by the surface of the Earth, 30% is finally absorbed by water and vegetation and the remaining 5% is absorbed by barren soil.

Solar radiation is not only absorbed by the atmosphere, but also dispersed when interacting with particles, so that the light rays change their direction as they are reflected by the surfaces of those particles. When large quantities of particles (for example water droplets or dust) are present in the atmosphere, a strong diffusion of the solar radiation takes place. The type of solar radiation present at a certain moment is essential for some solar devices. These devices cannot function with the diffuse radiation present on cloudy days, but only with the direct radiation of clear days.

Depending where the measurement takes place, the time of the day and the atmospheric conditions, an area of one square metre orientated towards the sun can receive up to about 1kW of power. At any point in time, our planet receives about 10,000 times more energy from the Sun than all of the energy we produced

Daily solar energy at different locations at ground level in Australia is derived by the Australian Bureau of Meteorology from data sent by the GMS-5 satellite. Here you can see the most recent map of solar energy distribution in Australia.

effects of solar radiation

As solar radiation is absorbed differently by the ground, by water or by surfaces covered with vegetation, those surfaces heat up more or less. A much stronger temperature difference exists between areas of the world where it is day compared to where it is night, and between sunny areas (tropical) and darker areas (towards the poles). Air in contact with the surface of warmer areas expands more and rises in the top part of the atmosphere, creating a depression underneath it. Air from cold areas is attracted by the depressions in warmer areas, creating wind. Therefore, wind energy originates from solar energy.

When wind blows above a body of water, it creates waves and so generates wave energy. The wind also moves the top layer of water that, through friction, helps move the lower layers as well; this process creates water currents. Large oceanic currents are also produced by other mechanisms based on solar energy, like the temperature and the salinity differences between bodies of water situated at large distances from one another.

As the water is heated by solar radiation, part of it evaporates and rises through the atmosphere, where it is partially condensed, thus, forming clouds. As the clouds travel with the wind, they reach areas with certain conditions where the water in the clouds gathers into larger drops that fall to the ground as rain. When rain falls on ground above sea level, it flows down the slope, creating rivers. Therefore, hydraulic energy also originates from solar energy.

Solar energy is vital to all plants as they use it to generate energy to live and grow. This energy travels from the lowest levels of the food chain and eventually reaches humans.

Plants use part of the solar energy they receive for growth and this plant matter can be used as fuel, for example, firewood, straw and biogas.

Coal, natural gas and oil have been produced through the decomposition of buried biomass that has directly or indirectly used solar energy for growth. Therefore, fossil fuel energy originates from solar energy that reached the Earth a long time ago.

In conclusion, all past and present energy on our planet (except nuclear, tidal and geothermal energy) has been produced from solar radiation.

indirect uses of solar energy

Most of the heat and the chemical energy that we produce are of solar energy origin because they are obtained by combining organic carbon and hydrogen from fuels with oxygen from air. Electrical and mechanical power that we use is generated mainly (about 85%) from burning fossil fuels. Less than 10% of the energy we use is generated in nuclear power plants that do not draw their energy from solar radiation.

direct uses of solar energy

As all of the biosphere draws its energy from solar radiation, interactions between living organisms and their associated products are also governed by solar energy. The forestry, agriculture, food, and much of the garment industries exist due to solar energy.

A significant part of the electric power generated (about 10%) is produced using hydraulic and wind energy, both originate from present-day solar energy.

A very small proportion of electric power is produced through direct conversion of solar radiation into electricity by photovoltaic panels.

There are also a few small power generators (mostly experimental) that concentrate solar radiation onto the heat receivers of a heat engine thus generating mechanical power (and eventually electric power).

At Manzanares, in Spain, between 1982 and 1989 a solar tower used a large solar collector to heat air. Due to its buoyancy, the hot air rose in the tall tower, creating an artificial wind, which actuated a turbine and an electric generator.

In agriculture, large quantities of solar energy are used to heat air in order to decrease its relative humidity. This dry, hot air is used to dry crops and timber.

Another significant use of solar energy is for heating water, both for domestic and commercial use.

advantages and disadvantages of solar energy

Solar energy has important advantages over the energy traditionally obtained from fossil fuels:

  • it does not pollute the environment
  • it does not introduce more heat into the atmosphere
  • it is free
  • it is practically unlimited over time

At present, solar energy is not the main source of energy used because:

  • before the introduction of our new technology the devices using solar energy were more expensive than those relying on fossil fuels
  • it has a small power density so we need large areas to collect significant quantities of energy
  • it is variable in time (night/day, the seasons, the clouds)
  • it cannot be stored in its original form