Renewables describe energy sources that do not deplete the earth’s natural resources and do not create added waste products. They are therefore sustainable in that they can be used indefinitely without degrading the environment.

The fact that renewables are the only truly sustainable forms of energy production clearly makes them desirable. The impetus for renewable energy has grown much stronger in recent years due to two related drivers: energy security and climate change.

There are renewable options for all of the three major energy forms: electricity, heat and fuel. Some renewables directly use a climatic resource; others use renewable fuels as the medium for delivering energy.

Elemental renewables

The main climatic or environmental renewable energy resources are:

• Solar radiation - Both light and heat from the sun can be harnessed for their energy, and several different conversion techniques have been developed. Solar energy can be collected directly as heat or converted to electrical power. Solar power is also the ultimate cause of winds, waves and plants, so the source of almost all the earth’s energy.

• Water power was the first source of large scale electricity generation, but was used long before that for mechanical energy in water mills. Water can also be used as a thermal source for heat pumps.

• Wind too has been used for centuries as a source of energy both for motive power (such as windmills and wind pumps) and more recently power generation. The ambient air can also be used as a thermal source for heat pumps.

• Wave and tidal energy are more recently exploited sources offering potential again mainly for electricity production.

• Geothermal heat can also be harnessed and used directly for heat production or through a steam turbine for electricity. Ground source heat pumps also use the constant temperature of the earth’s sub-soil as a source of heat.

• There are also areas of the earth’s crust with liquid or gas reserves where the high geopressure can be used to drive electricity turbines.

Any of these sources, which use heat as a means of producting electricity, such as solar furnaces and geothermal generation, can also be used for combined heat and power.

Biological renewables

Biomass is also considered a renewable source of fuel because is absorbs carbon dioxide from the atmosphere, while growing. When it is later converted to energy, the carbon dioxide released back into the atmosphere matches that originally absorbed, so the whole cycle is carbon neutral.

Of course the same can be said of fossil fuels, but there the cycle is very long (millions or years); biomass is the term applied to crops (or animal by-products) grown over no more than a few decades. The following are examples of biomass accepted as renewable fuels:

• Energy crops – agricultural and forestry products grown specifically to be used for energy production, such as short rotation coppice and miscanthus.

• Standard crops and their by-products. Many crops can be used for food or fuel, such as corn, oilseed rape, wheat and many others. Many food crops have by-products, such as straw, which can be used for energy production, while the main product is used for food.

• Forestry and forestry by-products. Again timber can be used for fuel, but more commonly the non-commercial by-products, such as sawdust, small round-wood, thinnings etc. can be diverted to renewable energy production.

• The biomass element of waste streams including ICW and MSW is another source of renewable bioenergy.

• Similarly certain animal by-products from the food chain can be used for renewable fuel production.

These sources all provide fuels or products than can be converted to fuels, of many different types: liquids, gases, pellets, chips and other solid fuels. In this form they can be used for any form of energy production: heat, electricity generation, combined heat and power or transport fuels.