Following are some of the main types of renewable energy and alternative fuels, which together supply about 10 percent of the world's total energy.

WHAT IS RENEWABLE ENERGY?

Energy obtained from sources that are continually replaced, unlike fossil fuels of which there is a finite supply. Most renewable sources of energy are non-polluting and come from nature.

SOLAR: The earth receives as much energy from sunlight in 20 days as the planet's entire reserves of fossil fuels, according to Shell. As demand grows and costs fall, solar cells are being increasingly used to produce electricity, particularly in remote areas not connected to national grids. Solar power is also widely used to heat water for homes and swimming pools and even for air conditioning.

Tests are being carried out on solar towers, in which a large number of mirrors focus the sun's rays on one point on top of the tower, which stores the sun's energy in molten salt, allowing the plant to generate power day and night.

HYDRO: This remains the world's top source of renewable energy. Apart from hydro-electric dams, mini-hydros using the flow of water in rivers as well as ocean waves and tidal power can also be harnessed to drive turbines.

WIND: Like solar, wind power is growing rapidly with large installations built in Europe and the United States and many planned or under construction in Asia. Small turbines can be used by home-owners and remote villages not connected to the grid.

BIOMASS: Burning wood for cooking is the oldest example of using biomass, a term for any sort of vegetation, such as grass, trees, leaves or even municipal waste that can be burned on its own, or co-fired in other power plants, to generate electricity. Biomass can also be used to produce liquid fuels, for example, ethanol from sugar cane, and gaseous fuels, such as methane from rubbish dumps.

GEOTHERMAL: This uses the heat generated in many countries at the point where tectonic plates collide, and volcanoes, shallow magma and hot springs are present. This heat and steam is used to drive turbines.

BIOFUELS: Biodiesel is made by transforming animal fat or vegetable oil with alcohol and can be directly substituted for diesel either as neat fuel or as an additive. In Europe, the largest producer and user of biodiesel, the fuel is usually made from canola oil. In the United States, the second largest producer and user, biodiesel is usually made from soybean oil or recycled restaurant grease.

The fuels produce much less pollution, and engines need no significant modifications.

HYDROGEN: Weight for weight, hydrogen has the highest energy of any gaseous fuel and is increasingly seen as a fuel for the future. The challenge for the energy industry is how to source hydrogen in an environmentally sustainable manner as well as improve the efficiency of fuel cells, which use hydrogen to make electricity. Other problems include storage of the highly flammable gas and safety of distribution.

Almost all hydrogen produced today is from splitting natural gas. The thermal energy for the process comes from the combustion of fossil fuels. Hydrogen can also be produced by splitting the water molecule through electrolysis. This process requires substantial amounts of electricity, and solar energy is being used on a small scale in Europe.