Harnessing direct solar energy – a progress report

alternative energy sources. Energy sources different from those in widespread use at the moment (which are referred to as conventional). Alternative energy usually includes solar, wind, wave, tidal, hydroelectric and geothermal energy. Although they each have their own drawbacks, none of these energy sources produces significant air pollution, unlike conventional sources.

fossil fuels. Carbon or hydrocarbon fuels, derived from what was living material, and found underground or beneath the sea. The most common forms are coal, oil and natural gas. They take millions of years to form. Their energy is only released upon burning, when the carbon and hydrogen within them combine with the oxygen in air to form carbon dioxide (CO2 ), or carbon monoxide (CO) and water (H2O). Other elements within the fuels (such as sulfur or nitrogen) are also released into the air after combining with oxygen, causing further pollution with SO2 and nitrogen oxide gases. In the case of coal, ash particles are also a problem.

non-renewable energy. Used to describe energy sources that exist in a limited amount on Earth. Thus all available material could eventually be completely used up. Coal, oil and gas (see fossil fuels) are considered as non-renewable energy sources because the rate of their formation is so slow on human timescales that they we are using them without them being replaced. Uranium (used in nuclear power) is also non-renewable, although its reserves are very large compared to its rate of use. Compare renewable energy.

passive solar heating. The use of the sun to heat buildings. Careful design and positioning of buildings can ensure that sunlight in the winter months will warm them by day, with much of the warmth remaining during the night. Summer sunlight is usually kept out. This does not involve the conversion or harnessing of solar energy.

photovoltaic (PV) cells. Also known as solar cells. A photovoltaic cell is made of thin wafers of two slightly different types of silicon. One, doped with tiny quantities of boron, is called P-type (P for positive) and contains positively charged 'holes', which are missing electrons. (Electrons are negatively charged particles that orbit the nuclei of atoms.) The other type of silicon is doped with small amounts of phosphorus and is called N-type (N for negative). It contains extra electrons. Putting these two thin P and N materials together produces a junction which, when exposed to light, will produce a movement of electrons – and that constitutes an electric current. Photovoltaic cells thus convert light energy into electrical energy.

http://www.science.org.au/nova/005/005glo.htm

Can society meet its energy needs without pollution?

http://www.science.org.au/nova/005/005key.htm

Compared to fossil fuels, sunlight is a weak energy source
Capturing sunlight is not as easy as it sounds. It is a dilute energy source, spread out over time and space. Earth receives 5.6 x 10^18 megajoules of solar radiation each year, but to make it worthwhile we need to collect it over many hours and across many square metres of ground. We then need to concentrate it so as to make available the sort of power that modern society needs. Sunlight is not as ‘energy-dense’ as oil but this is made up for by the fact that it is present over such a large area.

By careful design and positioning of houses we can use sunlight to warm our homes and our domestic water. This passive solar heating can help us reduce fossil fuel use (and save money) but it’s not enough to replace those traditional fuels entirely.