Figure 1.10 The energy resources of Earth.
these trends in the foreseeable future. To investigate what can be done to resolve this difficulty, we must first inquire what energy resources are available (Section 1.8) and next (Section 1.9) how we are using the resources at present.
Figure 1.1° shows the planetary energy resources. These can be renewable or nonrenewable.
Geothermal energy has been used for a very long time in Iceland and more recently in Italy, New Zealand, and the United States. In many places, it is possible to take advantage of the stability of the ground temperature a few meters below the surface. The ground can thus be used as a source of heat in the winter and of cold in the summer.
Gravitational energy—that is, energy from tides (see Chapter 16)— has been used in France. Tides can only be harnessed in certain specific localities of which there is a limited number in the world. Gravitational energy is also important in all hydroelectric plants.
Of the renewable resources, solar energy is by far the most abundant. A small part of it has been absorbed by plants and, over the eons, has been stored as coal, oil, and gas.
Estimates of reserves, fossil or nuclear, are extremely uncertain and are sure to be greatly underestimated because of incomplete prospecting. Table 1.4 gives us only a very rough idea of our fossil fuel reserves, and Table 1.5 shows an even more uncertain estimate of reserves of fissile
Table 1.4 Known Fossil Fuel Reserves
Liquefied gas Shale
>100,000 EJ (1998) 39,000 EJ (2002) 18,900 EJ (2002) 15,700 EJ (2002) 2, 300 EJ (2002)
Table 1.5 Known Reserves of Fissionable Materials^
Does not include the USSR and China.
materials. The estimates of nuclear fuels do not include the reserves of the former Soviet Union and China. Values given in the tables are very far from precise. They probably represent a lower limit, because people who estimate these numbers tend to be conservative, as testified by the secular increase in proved reserves: proved reserves of dry natural gas, 2200 EJ in 1976, rose to 6200 EJ in January 2007, notwithstanding the substantial consumption of gas in the intervening years. A similar situation exists with respect of proved oil reserves: 7280 EJ in 2002 and 7900 EJ in 2007. For oil and gas, the table lists the sum of proved reserves, reserve growth, and undiscovered reserves.
Proved reserves are fuels that have been discovered but not yet produced. Proved reserves for oil and gas are reported periodically in the Oil and Gas Journal.
Reserve growth represents the increase in the reserves of existing fields owing to further development of these fields and to the introduction of better technology for their extraction.
Undiscovered reserves represent the best possible guess of the magnitude of plausible new discoveries.
Reserve growth and undiscovered reserves are estimated by the U.S. Geological Survey (USGS). For example, in 2002 the Oil and Gas Journal reported proved reserves of oil of 7280 EJ, and the USGS estimated a growth of 4380 EJ and undiscovered oil reserves amounting to 5630 EJ, adding up to the total of 18, 900 EJ listed in the table. For coal, the table shows only proved reserves. The total reserves for this fuel are thus substantially larger than listed.
One number that is particularly uncertain is that referring to hydrated methane. William P. Dillon, a geologist of the USGS, testified in the
U.S. House of Representatives in 1998 that "the amount of methane contained in the world's gas hydrate accumulations is enormous, but estimates of the amounts are speculative and range over three orders-of-magnitude from about 100,000 to 270,000,000 trillion cubic feet [100,000 to 270,000,000 EJ] of gas." We, being ultraconservative, listed the lower figure.
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The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.