Introduction to Renewables

The Sun: Solar heat can be used directly- absorbed by ‘solar collectors’ on roof tops for domestic or industrial hot water or space heating, cutting fuel bills by about 50%. They are used very widely across the world- there is around 80,000 megawatt (thermal) of ‘solar thermal’ generation in place. Another approach is to focus the suns rays so as to boil water to drive a generator. Systems with sun tracking mirrors, dishes or troughs have been developed for large arrays in desert areas- the technology is known as Concentrating Solar Power (‘CSP’). Solar energy can also be converted directly into electricity by photovoltaic (‘PV’) solar cells on roof tops or suitable walls. There is nearly 10,000 MW of solar PV in use globally. PV is expensive at present but costs are falling, as new cell materials emerge.

Wind: Winds are the result of differential heating of air e.g. over land and sea. Wind turbines are large modem windmills which generate electricity. Typical turbines are now able to generate 2 megawatts (MW) of energy at full power- that is the equivalent of the energy needs of 2,000 electric fires (assuming they are 1 kilowatt rated, since 1MW = 1000 kilowatts). Since the wind is intermittent, over the year a typical wind turbine in the UK will deliver about 30% of the theoretical full power maximum. The UK at present has over 3,200 megawatts (MW) of full power wind generation capacity in place, mostly located on land, but increasingly off shore, with larger turbines, of up to 5 MW in size so, emerging. By 2020 it is hoped to expand the UK wind programme to over 30,000 MW, generating around 25% of UK electricity, and in theory it could be much larger. Globally at present there is over 120,000MW of wind capacity operating, with Germany and the US more or les equal at around 23,000MW each. On-land wind power is currently the cheapest of the major new renewable energy sources.

Waves: Winds moving over the sea cause waves, and the UK has a very large potential: perhaps 20% of UK electricity could be generated from this source, using suitable floating conversion devices out to sea. It is relatively expensive since, as with offshore wind, the power has to be brought back to land using undersea cables. Although less energy is available there, devices can also be mounted on the coast-line. Many projects of various types are underway around the world, with multi-megawatt wave farms being developed.

Hydro: the energy flows in rivers can be tapped by creating a head of water behind a hydro damn, which can be used to generate electricity. The energy comes from the sun which causes water to evaporate and form clouds which then precipitate as rain, to feed the river flows. This is the largest and most economic ‘traditional’ renewable energy source currently in use- with over 740,000MW supplying about 18% of world electricity, but it can be environmentally invasive, especially with very large projects, and can be effected by droughts.

Biomass: Wood and other biological material like animal dung is already widely used for heating around the world, often very inefficiently, but specially grown energy crops, or agricultural or municipal wastes, can be used more efficiently for heating, or can be burnt in power stations to generate electricity. Biomass can also used to create liquid vehicle fuel-biofuels (biodiesel or bioethanol). The energy comes ultimately from the sun which powers biological growth. The carbon dioxide gas produced from the combustion of biomass can be more or less balanced by that absorbed when plants grow, but energy is used in harvesting, transporting and processing it, so, depending on the types of biomass and the way it is processed, there may be net carbon emissions. In addition, biomass is land-hungry, and there can therefore be land-use conflicts. Some biomass/ wastes can also be converted to biogas and used for heating or electricity generation- sewage gas and landfill gas are very economic sources of power. The good thing about all bio-sources is that they can be stored, ready for use when needed.

Tides: The tides are the result of the gravitational pull of the moon (so it’s lunar energy), and a smaller pull from the sun, which cause the seas to rise in a cyclic pattern twice roughly every 24 hours. High tides can be allowed to flow through a dam or barrage on an estuary, and then be trapped behind, creating a head of water. This can be let out through a turbine to generate electricity, like hydro. Alternatively, and less invasively, the tidal flows can be used to drive free-standing tidal current turbines fixed to the sea bed or tethered under a pontoon. Perhaps 20% of UK electricity could be generated from the tides. There are many tidal current turbine projects under test around the world, but so far only one barrage of any size- on the Rance estuary in France.

Geothermal: If you drill down into the earth high temperatures can be accessed which can be used for generating electricity. About 10,000MW is in operation around the world. Use can also be made of hot water/ steam aquifers at lower depths for heating, which is also widely practiced around the world. Unlike solar or lunar energy, it can deliver continuous power, but it’s not strictly a renewable resource since it relies on the radioactive decay of elements deep in the earth, and local heat gradients can be temporarily exhausted over time. Heat can also be extracted from the earths surface (or air), using a heat pump driven by electricity. This approach is also sometimes labeled ‘geothermal’, but strictly it is not- it uses ambient heat, ultimately from the sun. But it can be an efficient way to provide heat.

Altogether there is over 1,000,000MW of renewable electricity generating capacity in use around the world, including hydro. For comparison there is about 370,000MW of nuclear plant. In terms of total energy, in 2006 fossil fuels met 79% of total world energy consumption, nuclear 3%, and, if heat supplying renewables are included along with electricity generation, renewable energy met 18% of total world energy consumption, though much of that was from traditional biomass.

The potential for the future is large. The EU is aiming to get 20% of its total energy from renewables by 2020 and some EU countries have already achieved that and more- notably Sweden (39.8% in 2005), Latvia (34.9%), Finland (28.5%), Austria (23.3%), mostly due to their large hydro and biomass contributions. The UK was at 1.3% in 2005, one of the lowest, but it has a very large potential, with the EU’s best wind, wave and tidal resources. Longer term, most countries might expect to be able to get 50% of their electricity from renewables by 2050, and most of their heat, with more thereafter. Interestingly the wind potential in China has been put at over 2,000,000 MW, compared to the world current total generation capacity of around 13,000,000MW.