James Richard Fromm
In this section we begin take up the chemistry of those fuels which are found in nature or prepared by chemical processes. Fuels are divided into nuclear fuels, renewable fuels, and fossil fuels. Renewable fuels, primarily wood, waste plant products, and animal dung, are used locally as fuels for heat to warm dwellings and cook food, but with the exception of wood they are of little chemical significance. Even including wood, over 90% of the fuel consumed in the world today is one of the fossil fuels--coal, oil, or natural gas. Fossil fuels originated from plants grown in earlier geological eras whose remains were buried under the earth. Fossil fuels are not now being replenished to any significant extent and the quite finite supply of them is being consumed at an increasing rate. Nuclear fuels are taken up in a separate section. With the exception of electrical power generation, nuclear fuels do not now provide a significant fraction of human energy demand on a global basis and it is unlikely that they will do so in the near future.
Source Quantity % Input (GW x 10+5) Sun (intercepted solar flux) 1,730.0 99.98 Earth (geothermal) 0.32 >0.02 Moon (tidal) 0.03 0.03??? Usage Quantity % Input (GW x 10+5) Absorbed and re-emitted 778.0 45.0 Directly reflected 519.0 30.0 Hydrological cycle 397.9 23.0 Absorbed 34.6 2.0 Winds, waves, currents 3.7 0.2 Biosphere 0.4 0.02
Table Notes: Light which is absorbed and re-emitted is re-emitted as infrared radiation (heat). The radiation absorbed is ultraviolet radiation absorbed in the ozone layer of the atmosphere.
On a planetary or global basis, essentially all of our energy comes from the sun as shown in the above Table. On a human scale, however, the energy problem is one of a sufficiently large supply of easily controllable energy. Energy is needed for heat, mechanical work, light, and more recently for operation of electrical devices.
Although considerable efforts are being made to develop direct exploitation of solar energy so that output is obtained in the form of electricity, only a comparatively low level of economic success has been achieved. Economic direct conversion of solar energy to electricity is desired because present technology can readily convert electricity into almost any other desired form of energy. On the scale of human energy production and control, use of direct solar energy (other than passive solar heating such as the design of houses to take greater advantage of the heat and light from sunlight) is negligible. Solar energy is a diffuse resource, and is directly available only in daylight, weather permitting. Without either natural or artificial devices for energy collection, concentration, storage, and distribution--many of which are being actively studied--its place in satisfying human energy needs will remain small.
Energy is required by man for many purposes which do not involve work. The majority of the energy we require has always been, and still is, used in the form of heat. Heat is required for cooking of food, for warming of living space, and for many industrial processes in addition to the requirements of heat engines. The early development of heat engines is chronicled in other sections. Virtually all of the energy required in forms other than heat is produced by heat engines; the sole exception of consequence is hydroelectric power, produced from the energy of falling water.
Mean solar energy intercepted by the earth is 173,000 TW, that of the gross input to the biosphere is estimated to be 40 TW. The percentage given is the per cent of gross input of solar energy to biosphere.
Energy production and consumption differs radically from one area to another. By far the majority of the energy is used in heat form, but a significant quantity is used to generate electrical energy. The vast majority of the electrical energy used is produced by heat engines used to rotate electrical generators. Electricity transfer across continental boundaries is insignificant. Electricity is generally used as close to the source of generation as practical, since there are always losses in transmission over power lines.
The use of wind, water, and naturally occurring sources of geothermal heat antedates the development of heat engines as practical power sources. Current wind-powered generators of electrical power produce no more than about 200 kW in practical applications, and the contribution of wind power to current or future total power or even total electrical power demands is likely to be negligibly small. Applications are probably limited to remote locations not connected to major electrical networks which require relatively small amounts of power.
Heat produced from fossil fuel is our major source of power, but heat is also available in the interior of the earth. This geothermal energy was tapped for power generation as early as 1904 when special wells were drilled into the underground steam reservoirs at Larderello, Italy (now 390 MW); similar systems now operate in New Zealand, Iceland, Japan, the Soviet Union, and the United States (The Geysers, California, 300 MW). Production of power from geothermal heat is not without problems, however, and seems to be restricted to only a few geologically favorable locations. These problems include the highly corrosive and toxic hydrogen sulfide dissolved in heated subsurface waters and the sinking of the ground surface when subsurface water is removed without replacement. It is unlikely that geothermal heat will produce a significant fraction of power demands on a world or continental basis in the foreseeable future.
Unlike wind and geothermal heat, the energy of moving water does contribute significantly to power generation, electric power generation. Hydropower complexes are usually based upon large dams behind which a head of water accumulates; the fall of this water through turbines drives electrical generators. Modern large hydroelectric generating stations range upward from 1 GW to 5 GW, though the majority of generating stations are of lower capacity. The world output of hydropower is all in electrical form and amounts to 243 GW (1967); the maximum world capacity for hydropower is estimated to be about 2860 GW (M.K. Hubbert, 1967), which is well below the present world power output. Clearly, world power requirements cannot be met from hydropower resources.
There are other minor contributions to power generation such as tidal power, in which a head of water is created by the rise and fall of the tide. A plant on the Rance River in France now produces some 0.3 GW from the 44-foot tidal difference of the English Channel tides. The total world potential for tidal power is estimated at about 2000 GW, less than the potential for conventional hydropower, and development of tidal power will probably be slower and more difficult than the development of conventional hydropower. No large-scale plant for direct commercial production of electricity from sunlight using photoelectric cells now exists. Photoelectric cells are used to power spacecraft electrical systems, but currently they are economically impractical for significant terrestrial power generation.