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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 travelling 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, sunlight travelling from the sun to the ground with only a slight scattering of the sun’s rays in the atmosphere. E.g. At any time of year, about eighty per cent of the Sahara desert’s total solar radiation is from direct sunlight.

• Diffuse radiation, 15.0 Megawatt sunlight 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, sunlight 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 has long been used to be assumed that much 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.

- The awesome energy of the sun
- Ancient Greece to the Space Age
- Active and passive solar energy
- A new energy source?
- Generating electricity
- Different types of solar cells

History from Ancient Greece to the Space Age

harnessing solar power through history

We owe our very existence to the power of the sun. Through solar technology, we can also harness its abundant energy to improve our quality of life.

In battle, the Ancient Greeks used mirrors to direct beams of sunlight onto Roman ships, which subsequently burst into flames. They also designed homes to absorb solar heat so effectively that they were protected from extremes of hot and cold, using what we now refer to as passive solar design.

How far have we come since then?

Despite the inventiveness of the Ancient Greeks, subsequent generations failed to develop solar technology until the end of the eighteenth century, when a French chemist named Anton Lavoiser built a solar furnace that achieved temperatures of 1,750 degrees Centigrade. In the late 1800s, Augustin Mouchet then devised several solar-powered steam engines.

By the early 1900's, solar power was widely used in the southern United States in water heaters. These systems fell from favour when cheap oil and gas became available in the 1920s. While the solar industry received a reprieve through the soaring energy prices of the seventies, it was not to last.

How do we use solar energy today?

We currently employ solar technology in three different ways:

• Passive, through solar buildings as pioneered by the Greeks
• Active, solar thermal concentration systems that collect the sun's heat
• Photovoltaic cells, which use light to generate electricity directly and were first developed for the United States' Space Programme.

In recent years the fortunes of the solar industry have improved greatly, as governments and businesses respond to pressure for greener energy alternatives to fossil fuel.

Photovoltaics - A new energy source?

If you possess a solar powered calculator, you’re benefiting from technology that originated from the US Space Programme, where photovoltaic cells were first developed. These cells are the most modern of the three main technologies that harness solar power today.

The term photovoltaic comes from the Greek “phos” which means light and “volt”, from the scientist Allesandro Volta. In other words, photovoltaic literally means “light-electricity”.

How do these cells turn light into electricity?

Photovoltaic cells take advantage of the fact that light dislodges electrons from atoms when it strikes. These are used to generate a potential difference between two semi-conductor materials. Closing the circuit establishes an electric current.

Individual cells generate between .6 and 1.2 volts and can be wired in parallel or in series depending on whether greater current or voltage are required, respectively. The current generated is direct current (DC), which therefore cannot be used directly in the same way as mains power, except to charge batteries and to run light bulbs. In order to be used by the mains system, it needs to be converted to alternating current (AC).

Are photovoltaics a competitive force in the energy market?

As early as 1987, the International Energy Agency (IEA) estimated the value of this power generation as being of the order of several tens of billions of dollars, and it has grown considerably since then.

Today, photovoltaic cells can be found in watches and calculators, in addition to being used as power systems for yachts, and even large scale rural power applications.

Photovoltaic cells are an exciting prospect for energy generation, because:
• they produce no noise, emissions, vibration or pollution of any kind during operation.
• they have a long operating life and low maintenance costs.
• they can be integrated into building designs.

Are there any disadvantages?

For all of these benefits, there are still some problems to overcome. Currently photovoltaic cells:

• are costly and complex to manufacture.
• need a large surface area to generate power due to their low area density.
• cannot generate electricity without light
• are vulnerable to seasonal and daily variations in energy output.

Despite these challenges, there is a huge incentive to develop the potential of this clean and renewable energy, and technological advances mean that solar cell efficiency is improving all the time.

Taken from a different perspective, solar modules can be treated as building elements that provide electricity as an “added extra”. For instance, they cost the same as reflective glass and are cheaper than granite or marble. If solar modules are valued for more than merely providing electricity, then photovoltaics become cost effective, not in the future but today.

Generating electricity directly from light

Solar cells are elegantly simple, made from special materials that are neither insulators (like plastics) nor conductors of electricity (like copper wire). When they are exposed to light and absorb photons (particles of light and other forms of electromagnetic radiation), these materials - called "semi-conductors" - allow an electrical current to be generated.

Photons contain various amounts of energy depending on the different wavelengths of the solar spectrum. This energy level determines what happens when photons strikes a photovoltaic cell, where they will either be absorbed, reflected or pass right through. Some of the absorbed photons generate electricity, others generate heat, and some never reach the external circuit.

Why do electrons move about?

The electrons in a semiconductor material live in a range of defined energy levels, each one known as a band.

The conduction band is partially filled with electrons, creating a negative charge.

The valence band has areas where electrons are missing - known as holes - equivalent to a positive charge.

In the absence of light, the positive and negative charges balance each other out. But when light energy in the form of protons strikes the semiconductor material, electrons are dislodged and the equilibrium is disturbed.

This causes electrons to move down an external circuit in the form of light-generated electricity: a phenomenon which is called the photovoltaic effect.

How much electricity is generated?

Around fifteen per cent of the energy of sunlight can be used to produce electricity using photovoltaic solar technology.

The individual solar cell's size determines the amount of current and power it is capable of producing - at most some 0.5 Volts (V). In order to generate a significant amount of electricity, several solar cells are assembled into modules.

Certain kinds of cells, such as those made from thin films, can be made directly into modules without needing to make separate cells first. Generally constructed to give an output somewhere between twenty W and 100 W, these modules can themselves be connected together to make arrays that could potentially supply several megawatts of power.

Different types of solar cells

Photovoltaic conversion, which enables the sun's rays to be converted into electricity, occurs in solar cells manufactured from silicon. Silicon is one of the most abundant materials on the planet (in the form of quartz sand) and is an environmentally friendly material.

This technology was pioneered in 1954, when Bell Lab scientists displayed the first silicon solar cell at the National Academy of Science in Washington DC.

The specially treated silicon strips, when exposed to light, powered a small FM radio that broadcast music to a packed audience. That year the New York Times boldly proclaimed:

"Vast power of the Sun is tapped by battery using sand ingredient"

The silicon solar battery, it was claimed, - "may mark the beginning of a new era, leading to the realisation of one of mankind's most cherished dreams - the harnessing of the almost limitless energy of the sun for the uses of civilisation."

That dream has yet to be realised, due largely to the high cost of solar cell technology. For example, C60 carbon atoms (Fullerene solar cells) are forty times more expensive than gold. This goes some way to explaining why solar power isn't more abundant.

Today, the range of solar cells spans many materials and molecular architectures, in the quest to extract maximum power while keeping the costs to a minimum.

What are the different types of solar cells?

Solar cells can be made of different semiconductor materials. Each solar cell also consists of an electrical contact with the semiconductor that carries the electrons away from the solar cell and into the electrical circuit where they can, for instance, power a light bulb or charge a battery.

The whole cell is then encapsulated in a transparent cover that provides environmental protection and contains an anti-reflection coating to make sure that as little light as possible is reflected away from the cells.

The different types of solar cell are:

• Crystalline silicon solar cells
• Thin film solar cells
• Non-silicon compound thin film solar cells
• Nano-crystalline solar cells
• Fullerene solar cells

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