There are many forms of water energy, including hydroelectric energy from dams, tidal power, tidal stream power and wave power. The potential for hydroelectric dams is restricted in developed nations because most potential sites are either already in use or cannot be used because of environmental considerations. There is also the risk of potential disaster from the failure of large dams. Wave and tidal stream power projects exist, but currently contribute a negligible amount of the world's energy needs and would need massive capital investment before they could do so.
Another form of water power, blue energy, is currently in the research stage. Blue energy refers to the energy retrieved from the difference in salt concentration between sea water and river water. Electricity is generated by separating the positive and negative ions by ion-specific membranes. The waste product in the process is brackish water. Costs are not thought to be an issue with blue energy, though tests on the potential pollution of the membrane have not been completed. It is estimated that if all goes to plan, blue energy could contribute 33% of the Netherlands' electricity needs.
Biofuel is any fuel derived from biomass - recently living organisms or their metabolic by-products, such as manure. Like coal and petroleum, biomass is a form of stored solar energy, but unlike most other fuel types it is renewable. Worldwide, biofuels comprise only about 2% of the world's total fuel consumption. Brazil, with its 30-year-old programme for making bioethanol from sugarcane, is the only country with significant capacity. But rising oil prices have heightened interest in biofuels and many countries have announced plans to increase biofuel production.
However, last year a review of studies in Science, by researchers at the University of California, estimated that bioethanol produces just 13% less greenhouse gas than fossil fuels - and then only if process by-products are burned for electricity or used for animal feed. Another study by researchers at the University of Minnesota put the figure at 12%, though it estimated biodiesel to be much higher at 41%. And even if biofuels were proven to produce substantially less greenhouse gases than other fuels, serious questions would remain over their potential for widescale use. A Worldwatch Institute report found that the EU would need to use 72% of its agricultural land to meet 10% of its fuel needs from biofuels; the US would need 30%.
For biofuels to be taken seriously as a replacement for fossil fuels, an alternative method of production will be needed. The so-called 'second generation' of biofuels, made from cellulose-rich, non-food crops such as straw, corn stover (stalks and leaves) and wood chips, or energy crops such as miscanthus and switchgrass, make use of material that nature already provides in abundance. A report from a consortium, including the EU's Joint Research Centre, estimated that second-generation bioethanol could provide nearly 23% of the EU-27 petrol market, a figure that would save 60 million tonnes of CO2 a year.
Second-generation biodiesel is harder and more expensive to produce, but is considered by some to have even greater long-term potential. The challenge will be to make second-generation biofuels competitive with fossil fuels in the long term. Impetus has come from a surprising quarter: keen to explore options that would make the US less dependent on foreign oil, George Bush has emphasised the need to diversify energy supply, including greater investment in "new methods of producing ethanol - using everything from wood chips, to grasses, to agricultural wastes".
Geothermal energy is obtained by tapping into heat trapped kilometres deep in the earth's crust and can be used in three ways: geothermal electricity, geothermal heating through deep earth pipes and geothermal heating through a heat pump. Geothermal power stations are expensive to build, but have low running costs, and the potential usage varies in different parts of the world. Geothermal power sources exist close to the surface of the earth in Iceland, the US, New Zealand, the Philippines and Italy.
Small-scale geothermal heating can also be used to heat and cool individual buildings. "A study recently estimated that geothermal energy could provide 20% of the US' energy needs," says David Downie, a programme director at Columbia University's Earth Institute. "Is it the solution to the world's energy needs? No. Can it make a contribution? Yes."