Alternative sources of energy
As supplies of nonrenewable fossil fuels run out, much investment into sustainable sources of energy is being made. Some such sources have been used for millennia whilst others are fairly innovative and have been rendered possible by technological progress.

The table below summarises the uses of sources of energy in the USA as of 1992:
 

Source	% contribution
Biomass	31%
Natural gas	24%
Wind and solar	12%
Hydro and geothermal	8%
Coal	7%
Fuel cells	6%
Nuclear	6%
Oil	6%

 

Conservation and more efficient use of existing energy sources
One possible way of averting shortages of fossil fuels is simply to use less of them. This may be achieved by increasing the utilisation efficiency of fossil fuels. Many such fuels (e.g. coal, oil) are used in machinery with very low conversion efficiencies. This leads to much of the chemical energy present in these fuels being lost as heat rather than being converted to electricity or motion of whichever form of energy would have been required. Raising the efficiency of some common forms of machinery (e.g. automobile engines) may contribute to a significant reduction in use of fossil fuel. Popularisation of mass transport over individual mean (e.g. using buses rather than cars) would also contribute to such savings. Efficient design of buildings would save energy and money on heating and cooling bills and also reduce depletion of fossil fuels supplies. One possible solution is cogeneration, where the excess heat generated from machinery is harnessed to provide another source of energy.
 

Solar energy
The quantity of heat reaching the surface of the Earth is about 8000 times greater the total amount of commercial energy used each year. Tapping such a vast resource would significantly reduce dependence on fossil fuels. Solar energy may be gathered by passive methods, where buildings would be constructed in such a way as to maximise absorption of heat. This would lead to reduced dependence on fossil fuels for heating. Active methods depend upon the movement of a heat-absorbing fluid through a solar collector. The heat absorbed by this fluid would then be transferred to water required for domestic use. Us eof parabolic mirrors rather than flat collectors can lead to increased output of energy. However, technical difficulties are considerable and add to the cost of such units. Solar energy can be transformed into electrical energy through the use of photovoltaic cells or collectors. These are material that capture solar energy and convert it directly to electrical energy by separating electrons from their parent atoms and accelerating them across an electrostatic barrier constructed by joining two semiconductors together. Photovoltaic cells were first used commercially in 1958. Since then, their cost has plummeted and their efficiency has increased from 1% to 10%. Photovoltaic cells constructed from amorphous silicon are widely used in pocket calculators watches and small devices.
 

Energy from biomass
Biomass resources used as fuel include wood, roots and other plant and animal material. The energy content of living organisms may be harnessed through burning. biomass fuel provides less than 1% of annual energy supplies in the USA and other industrialised countries but more than 95% in less developed countries. Approximately 750 million m³ of firewood are burnt each year. Burning of wood produces pollution and depletes populations of trees. In places where wood is in short supply, dried animal manure is often burnt as a fuel. His would however lead to food shortages since this manure would have otherwise been used as a fertiliser. Animal dung is also an inefficient fuel. When burnt, more than 90% of its potential energy is lost. On the other hand, if that dung is used to manufacture methane (CH₄), the chemical energy within it is used much more efficiently. If dung is sealed in a warm container containing water, anaerobic digestion (by bacteria) of the organic matter within it would lead to formation of methane. Burning methane produced from dung generates more energy than that obtained when simply burning the dung. Moreover, the sludge left over at the end of the process is also a rich fertiliser which may be applied to agricultural land.

Geothermal energy
The internal heat of the Earth gives rise to subsurface steam fields in some regions. This geothermal energy has been widely used in baths and hot springs but has also been used for the generation of electricity. This source may be tapped by sinking pipes into the steam fields. Pressurised steam travels through the pipes, drives machinery and is subsequently redirected to the steam field. Geothermal energy sources have a fairly long lifespan (for as long as the supply of subsurface water can last) and present little or no problems of waste disposal.

Energy from flowing water
The kinetic energy of flowing water or the potential energy of falling water may be used to generate electricity. The motion of the water can turn turbines that drive generators that produce electrical energy. Much of the hydropower development in recent years has been through construction of large dams. Although these produce electricity, they can lead to displacement of people, earthquakes, loss of wildlife and catastrophic floods. Reservoirs used in conjunction with dams can be subject to rapid siltation, reducing their effectiveness. In tropical climates, the reservoirs are also subject to very high water losses.
 

Wind energy
This depends on the use of windmills to harness the kinetic energy of moving air. Since this source of energy is dilute, extraction of energy usually requires the construction of large scale windfarms where a large number of turbines are placed together in a relatively small area. Many such turbines are characterised by an efficiency of approximately 35%. The main disadvantage is that significant generation requires the occupation of a very large area of land by turbines.
 

Wave energy and tidal energy
The energy of tidal current and ocean waves may be tapped to drive turbines that generate electricity. In the case of wave energy the device used generally consists of a floating bar that moves up and down as a wave passes. When coupled to a dynamo, this kinetic energy can be converted to electrical energy.

 
Ocean thermal energy conversion (OTEC)
The temperature difference between the upper and lower layers of the ocean are also a potential source of energy. In this system warm surface water is used it evaporate a working fluid, which, when in vapour form, can drive a turbine. Cold water from the ocean depths is used to condense the fluid. This system has several disadvantages, which include elevated cost of pumping cold water from a depth of one km, vulnerability to storm damage, ecological disruption of marine ecosystems and corrosion of materials by saltwater.