BIOMASS: A broad term covering organic non-fossil material of biological origin which constitutes a renewable energy source. This plant-derived material can be converted into fuels, chemicals, materials and power. Some types are being taken up more actively by industry - eg biofuels made from grain, sugar or vegetable oil - and gradually more cars are being produced worldwide that can run on biofuels or a mix of fuels.
GEOTHERMAL: Geothermal energy uses the heat in the Earth's core - either from rocks and water near the surface or through drilling deep wells. It accounts for only 0.4% of global generating capacity. Hot geothermal water is piped directly into buildings in Iceland to provide heating. Geothermal energy is widely used (directly and indirectly) in several other countries including the US, Philippines, Italy, Mexico, Indonesia, Japan and New Zealand.
HYDROGEN: Although not a primary energy source, hydrogen is thought to hold great promise for the future. A carrier of energy, the gas is abundant and non-polluting. However at the moment it takes a lot of electricity to make - either from water or fossil fuels. It is also hard to store and transport.
OCEAN ENERGY: Can potentially generate electricity by using the temperature difference between deep ocean water and surface water which has been warmed by the Sun. One estimate says less than 0.1% of the oceans' solar energy would supply more than 20 times the daily energy consumption of the US. But using this technology lies a long way ahead.