Solar Basics

A closer look at the awesome energy of the sun

When we think of energy, it is often in terms of coal, oil and gas. Yet the earth receives as much energy from sunlight in twenty days as is believed to be stored in this planet’s entire reserves of fossil fuels.

Although the sun releases ninety five per cent of its energy as visible light, it also produces infra-red and ultra-violet rays.

Each part of the solar spectrum is associated with a different energy. Within the visible portion of the solar spectrum, for example, red light is at the low-energy end and violet light is at the high-energy end, with fifty per cent more energy than red light.

Scientists often think of light as traveling in small packets, called “photons”, rather in the same way that water is transported by passing full buckets along a chain of people. Photons in the invisible ultra-violet region have more energy than those in the visible region. Likewise photons in the infra-red region, which we feel as heat, have less energy than those in the visible region.

How does the sun’s energy reach us?

Sunlight arrives at the Earth in a number of ways, given the collective title of “global radiation”. Individually, they are named:

• Direct radiation, when sunlight travels from the sun to the ground with only a slight scattering of the sun’s rays in the atmosphere. At any time of year, about eighty per cent of the Sahara desert’s total solar radiation is from direct sunlight.
• Diffuse radiation, when sunlight is scattered by clouds or haze. In Northern Europe, the proportion of diffuse light can be up to eighty per cent of the total solar radiation in winter and up to fifty per cent in the summer.
• Albedo radiation, when sunlight is reflected from the ground, for example with white surfaces such as snow which reflect the sun’s rays and stay cold. In contrast, dark surfaces absorb solar energy and become warm.

What are the best ways to harness this energy?

Most forms of life on earth have actively used the energy from the sun for millions of years.

Humans have long sought to harness solar power. Some 2,000 years ago the Greeks used mirrors to focus the sun’s rays on Roman ships, causing them to catch fire. More recently, scientists and engineers have searched for ways to convert the sun’s energy into electricity.

It used to be assumed that a large expanse of land was needed to gather solar energy. In reality small-scale applications – such as unused space on the roofs of buildings in urban and industrial areas – can be used very effectively.

Other areas of exploration include sea solar power, or setting a solar collector in space to store the sun’s energy and beam it down to earth to be converted to electricity. There is still much to learn, but hopefully it is only a matter of time before our understanding of solar power matches our need to harness it.