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At the temperatures where fusion can take place, all substances are in the fourth state of matter, or plasma. That is, if solid iron (for example) is heated to 1535° C, it becomes a liquid; if it is heated further, to 3000° C, the liquid boils and becomes vapor -- literally, a gas consisting of iron atoms. If the gas is heated still further, to 10,000° C and more, the increasingly energetic collisions between the iron atoms begin to knock electrons loose. When there are enough free electrons flying around in the gas of iron atoms that the gas begins to display electrical properties (for example, becoming a good conductor of electricity), the mixture is called a plasma.

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Unfortunately, it is not quite so easy in practice. Roughly twenty-five years of effort in laboratories around the world have revealed a dismaying tendency for the plasma to leak from the magnetic bottles faster than anticipated. (The enterprise has been compared to trying to hold watery Jello in a cage of rubber bands.) There are two main ways in which the plasma escapes. One is when collisions among ions knock them across the magnetic field and eventually into the walls or otherwise out of the machine. (Remember, particles that hit the wall lose most of their energy and with it the capacity to react.)

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That fusion reactors are incapable of nuclear excursions does not make them absolutely safe against release of whatever radioactive materials are inside, because other sources of energy may be available to break reactors open. Most conceptual fusion-reactor designs embody a great deal of liquid lithium, for example, which serves both as a coolant and as the means for breeding tritium fuel. Lithium, like the sodium used to cool the fission LMFBR, is chemically reactive; a large lithium fire is probably the maximum conceivable accident for a fusion reactor. Much attention is now being given in the conceptual design of fusion reactors to the possibility of using coolants other than lithium and incorporating the lithium needed to breed tritium in molten salt or a solid.

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Environmentally, fusion-fission hybrids have some of the worst characteristics of both parents. They would have somewhat fewer neutron-activation products than a pure fusion system, but almost as large a quantity of fission products as a pure fission system. There would be less tritium than in a pure fusion reactor, but still a lot, and there would be essentially the same risk of nuclear theft as in a pure fission system. The principal advantage would seem to be a much smaller chance of an accident than in a pure fission breeder reactor, if it is shown that breeder reactors are as dangerous as some of their critics think.

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The technology of dry-steam geothermal fields is the simplest. Wells are dug into steam reservoirs, typically at depths of 200 to 3000 meters. The dry-steam electricity generating plant at The Geysers, California, requires about fourteen such wells, each supplying 150,000 pounds of steam per hour, for each of its 110-Mwe units. The steam comes up the well under its own pressure and is directed through valves and pipes to a low-pressure turbine. The steam enters the turbine at about 175° C and 7 atmospheres pressure. Those values, much lower than those for fossil-fueled or nuclear generating plants, lead to a turbine efficiency of about 22 percent.

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Hot-water technology differs from that for dry steam in requiring separation of the hot water from the associated steam before the steam enters the turbine. The hot brine, whose salinity may approach 100 times that of seawater, meets one of three fates: it may be discharged at the surface to mix with fresh water runoff (as happens in the hot-water fields in New Zealand); it may be reinjected into the ground ( Japan); or it may be treated to remove the minerals for possible sale, rendering the water usable for agricultural or other use (as planned for some future installations in southern California and elsewhere). As with dry-steam systems, much of the condensate is evaporated in cooling towers.

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In climates where air conditioning is also a significant energy-user, solar cooling can be considered, too. This is not really paradoxical -- any air conditioner requires an energy source, and there are technologies for which that source is heat, not electricity. They are called absorption air conditioners, and traditionally they have been designed to use natural gas as an energy source. Versions modified to run on solar energy require water temperatures of 80° to 95° C (170° to 200° F.) or more; therefore, they need better collectors than suffice for ordinary water and space heating.

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he first energy technologies, fire and agriculture, harnessed energy stored by photosynthesis. Agriculture and burning wood are still important energy technologies today, of course, but they by no means exhaust the potential of photosynthesis. The annual net photosynthetic production of the biosphere (the amount of solar energy stored in chemical bonds by plants and not consumed by the plants' metabolic processes) is around 10 times the annual commercial energy use of civilization. An estimated 5 percent of this amount takes place in agricultural ecosystems; the photosynthetic energy that reaches the human population as food is only a tenth of that 5 percent, or 0.5 percent of net photosynthesis

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The simplest and cheapest way to harness the energy of organic wastes is by direct combustion. This is now being done in devices ranging from the electric-utility boilers in St. Louis, which burn municipal wastes mixed with coal and gas, to open fires and simple stoves for cooking and heating in the villages of India, where cow dung, crop residues, and wood are burned. The total energy embodied in these noncommercial fuels in India is apparently considerable. According to Makhijani and Poole, it is equal to all commercial energy use in that country. Unfortunately, the efficiency with which the noncommercial energy forms are converted into useful heat and work in India and other LDCs is very low -Makhijani's estimate is 5 percent.

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Finally, there is a possibility to use photosynthesis by imitating it. More specifically, some researchers hope that, by gaining sufficiently detailed understanding of the biochemical machinery of photosynthesis, they can devise nonbiological systems that use similar reactions to convert solar energy into chemical energy (in the form of hydrogen) or electrical energy, but at higher efficiency than that attained by green plants. Part of the basis of their hope for higher efficiency is that plants must spend a substantial part of the energy they handle to maintain their own life-support systems and to propagate themselves.