Our star, the sun, produces a vast amount of energy every day and will continue to do so for at least another 4 billion years. This energy, in the form of heat and light, is the result of hydrogen being burned at the rate of about 700 billion tons per second. Only a tiny fraction of this energy comes into contact with Earth but it drives the winds and ocean currents, and allows plant life to prosper. Even the fossil fuels we enjoy today are derived from solar energy. They are the products of a process spanning millions of years, beginning with the sunlight received and stored by plants. However, solar power as a renewable energy resource is concerned with exploiting the sun's present day emissions of heat and light.
Solar power technologies currently available
Established solar power technologies can be grouped in one of two camps - solar thermal technologies, which utilise the sun's heat and photovoltaics, which convert sunlight directly into electricity.
Solar thermal technologies
There are three main types of solar thermal technologies, each of which is briefly described below.
1. Flat plate solar collector systems - are generally large, glass covered, flat boxes which contain dark coloured metal plates that absorb, and are heated by, sunlight. Water passes through pipes or tubes over the plate and is heated. This type of system is commonly used to provide domestic hot water supplies.
2. Concentrating solar collector systems - use mirrors in order to concentrate sunlight on to some form of receiver. The concentrated sunlight is converted into heat, which is then used to generate electricity. Concentrating solar collectors might come in the form of:
(a) Dish-engine systems - a mirrored dish concentrates sunlight on to a receiver, heating a fluid. The heated fluid expands against and moves a piston or turbine, which runs a generator or alternator to produce electricity.
(b) Parabolic-trough systems - as the name implies, consist of trough shaped mirrors that focus sunlight onto an oil filled pipe running along their centre. The heated oil is used to boil water in a steam generator to produce electricity.
(c) Power tower system - a large field of mirrors, or heliostats, concentrates sunlight on to a tower-mounted receiver, heating molten salt. The molten salt boils water in a steam generator to produce electricity. Because molten salt is capable of retaining heat for long periods, this type of system can continue to generate electricity on cloudy days or at night.
3. Passive solar heating systems utilise careful orientation of buildings and location of windows to regulate the amount of sunlight and heat that can enter, which will vary with the season. They also utilise building materials that absorb the sun's thermal energy during the day and release it slowly at night.
Photovoltaic technologies rely on the 'photovoltaic effect' - which is the process of converting photons (light) into voltage (electricity). This is most commonly accomplished through the use of a 'solar cell'. A conventional solar cell consists of two layers of silicon, sandwiched together, one with positive electrical properties and the other negative. When photons strike a layer of silicon they bump electrons free, and the freed electrons flow to the oppositely charged layer. This flow of electrons or current, is harnessed through a connection to an external circuit via metal contacts on the cell.
The efficiency of solar cells has increased significantly since the pioneering days of the 1950's when they offered around 4% efficiency. Commercially available solar cells now possess an efficiency of up to 20% or even higher, under laboratory conditions.
Hundreds or thousands of solar cells are usually arranged in modules or solar panel arrays. Many such arrays may be placed together in a 'solar farm', with tracking mechanisms that allow the arrays to follow the sun as its position changes throughout the day.
Energy derived from solar power may be stored through the use of rechargeable batteries or by more environmentally friendly methods such as photolysis. Sunlight is used in photolysis to split water molecules into hydrogen and oxygen. When the gases are recombined they release energy which can then be used to drive an electricity generator.
The usage of solar power worldwide
Worldwide, the amount of energy acquired from solar power is not significant. For example, in the United States in 1999, approximately 0.1% of all electricity produced was derived from solar power. However, countries like Japan and Israel have made substantial efforts to utilise solar power. For example 83% of households in Israel were using solar collectors by 1994 and 4.5 million buildings in Japan were using solar hot water systems by 1992. Japan also leads the world in demand for photovoltaic systems, accounting for 40% of the global market share in 2000. The annual growth rate for the photovoltaic industry has been around 25%.
The major reason why solar power technologies are not more widely used is economic. Fossil fuels and other non-renewable energy sources are usually readily available at a low cost, thanks to past and current subsidies, but similar subsidisation does not exist for solar power. However, while solar power is currently more expensive than other methods of producing electricity, this will change as supplies of fossil fuels continue to diminish and the efficiency of solar technologies improves.
The benefits and drawbacks of solar power
In operation, photovoltaic and solar thermal technologies produce no air pollution and little or no noise. Solar power does not require the transportation of fuel and therefore has a particular niche in providing energy in remote locations, where it would be difficult and costly to either connect to grid power or transport fuel.
It has been estimated that there is enough roof space on an average home to harness sufficient solar power to meet all of its energy needs. Depending on the local climate, a relatively inexpensive solar hot water system can meet most or all of a household's hot water needs, resulting in substantial long-term financial savings.
There are some environmental problems associated with solar power technologies, primarily:
The future of solar power
There are a number of exciting developments in solar power technology such as 'electric' windows and roof tiles with embedded solar cells. In addition, solar power is being harnessed in different ways.