4.1 Currently available sources
Fossil fuels are the principal global energy source at the start of the 21st century, but on combustion they all emit CO2, which contributes to global warming, and varying amounts of SO2, along with nitrogen oxides, which all cause acid rain. Furthermore, they are a finite resource.
Coal is the most abundant of the fossil fuels but has the lowest 'energy density' and the greatest pollution potential. Commercial coal contains 5-10% or more of non-combustible mineral impurity (ash), around 1% of sulphur, and other elements in trace amounts, such as chlorine, uranium and arsenic. So-called clean coal technologies improve combustion efficiency and reduce pollution (see Section 4.2). As we saw in Box 1, however, the UK Government moved away from coal during the late 1980s and early 1990s, despite the fact that coal remains the UK's most abundant energy raw material. On a global scale, coal represented around 22% of primary energy consumption in 2002 (Figure 2), and there is enough coal to sustain demand at present levels for a couple of centuries, even without any new reserves being found.
Oil is a more versatile fuel than coal, with a higher 'energy density' and low to zero ash content. It still produces sulphur and nitrogen oxide emissions as well as CO2 (though less than coal per unit of energy obtained). Motor vehicle exhaust gases can cause high levels of pollution in urban areas, even when reduced by catalytic exhaust systems. Global oil reserves are sufficient for several decades at current (2005) consumption levels, though finding and exploiting major new reserves will be increasingly difficult and costly.
Gas has the highest 'energy density' of all three fossil fuels and is widely held to be a 'clean' fuel, because it leaves no ash and contains virtually no sulphur; but burning gas still produces nitrogen oxides as well as CO2 (though less than coal or oil per unit of energy obtained). Global reserves of natural gas are also sufficient for several decades at current (2005) levels of demand.
Nuclear fission is by far the most concentrated energy source currently available, and the actual generation of nuclear power releases no CO2, SO2, nitrogen oxides or other chemicals. However, unless very strictly controlled, radioactive by-products can contaminate air, water and soil over wide areas for centuries to millennia. Known global reserves of uranium are sufficient to last the lifetime of reactors so far constructed, and well beyond.
Biomass, mainly in the form of fuel woods and charcoal, represented around 10% of global primary energy consumption in 2002, largely in the developing world (Figure 2). Biomass is renewable if it is consumed at the same rate as new plants are grown, in which case the CO2 released during combustion would be offset through uptake by growing plants. Biofuels may also be derived from wastes, many of which are biological in origin (e.g. dried yak dung in Tibet, sugar cane waste and rice hulls in India).
Alternative energy sources are all 'clean' as well as renewable; the 'fuel' costs nothing and contributes neither to global warming nor to acid rain. Their drawbacks include low energy density so that very large installations are needed for substantial power generation, erratic geographic distribution, intermittency of supply, and in the main they are not transportable, so power stations must be built near the energy source. There are also environmental costs of constructing the plant (e.g. the materials needed for wind turbines). Nevertheless, their combined potential is considerable, and many are adaptable to local power generation.