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# Introduction

Energy from sources other than fossil or nuclear fuels is to a large extent free of the concerns about environmental effects and renewability that characterise those two sources. Each alternative source supplies energy continually, whether or not we use it. Many alternative sources of energy have been used in simple ways for millennia, e.g. wind and water mills, sails, wood burning - but only in the last two centuries has their potential begun to be exploited on an industrial scale. Except for geothermal energy, all have their origins in energy generated outside the Earth, yet the potential of each is limited by its total supply set against its rate of use. Each is likely to be renewable in the sense that the available rates of supply of each exceed those at which they are used. The main concern is whether or not such alternatives can supplant fossil- and nuclear-fuel use to power social needs fast enough to avoid the likelihood of future global warming and other kinds of pollution.

One of the alternative sources to consider is solar energy.

The Sun will radiate energy until it ceases thermonuclear fusion, in around 5 billion years. About 33% of the solar power that enters the Earth's system heats the atmosphere and contributes to setting winds and waves in motion. Of that reaching the Earth's surface, 70% falls on the sea, setting in motion ocean currents and a large proportion of the circulation of water vapour in the atmosphere because of evaporation from the ocean surface. The remainder falls on the land. Solar energy is redistributed through interlinked surface systems:

- the carbon cycle based on photosynthesis;
- atmospheric circulation and the water cycle;
- winds and ocean waves; and the ocean current system.

Each of them is a potential source of useable energy. In every case, with the exception of the energy available from surface water flow, humanity comes nowhere near exploiting the Sun's potential to supply useable energy; in fact, we really do not know the practical limits. Whatever those are, they will not disappear as a resource - all are renewable. Compare this with the solar energy stored chemically by the degraded products of photosynthesis in fossil fuels. Although carbon burial adds continually to that resource, its pace of renewal (between 1 to 10 GW - Figure 1) is about 2000 times slower than we use it. Fossil fuels are non-renewable and declining extremely quickly in terms of human history.

This course explores Solar power as a source of directly useable energy.

This OpenLearn course provides a sample of level 2 study in Science.

You may also be interested in our badged course Can renewable energy sources power the world?.

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OpenLearn - Energy resources: Solar energy
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