おんせん

(natural hot water; or natural steam.)

natural hot water; or natural steam.

(The latter two sources are limited to those few areas where surface water seeps down)

The latter two sources are limited to those few areas where surface water seeps down

(through underground faults or fractures to reach deep rocks heated)

through underground faults or fractures to reach deep rocks heated

(by the recent activity of molten rock material.)

by the recent activity of molten rock material.

(The world's largest supply of natural steam occurs at The Geysers,)

The world's largest supply of natural steam occurs at The Geysers,

(120 kilometers north of San Francisco, California.)

120 kilometers north of San Francisco, California.

(In the 1990s enough electricity to meet about half the needs of San Francisco was being generated there.)

In the 1990s enough electricity to meet about half the needs of San Francisco was being generated there.

(This facility was then in its third decade of production and was beginning to show signs of decline,)

This facility was then in its third decade of production and was beginning to show signs of decline,

(perhaps because of over development.)

perhaps because of over development.

(By the late 1990s some 70 geothermal electric-generating plants were in operation in California,)

By the late 1990s some 70 geothermal electric-generating plants were in operation in California,

(Utah, Nevada, and Hawaii, generating enough power to supply about a million people.)

Utah, Nevada, and Hawaii, generating enough power to supply about a million people.

(Eighteen countries now generate electricity using geothermal heat.)

Eighteen countries now generate electricity using geothermal heat.

(Extracting heat from very hot, dry rocks presents a more difficult problem:)

Extracting heat from very hot, dry rocks presents a more difficult problem:

(the rocks must be fractured to permit the circulation of water, and the water must be provided artificially.)

the rocks must be fractured to permit the circulation of water, and the water must be provided artificially.

(The rocks are fractured by water pumped down at very high pressures.)

The rocks are fractured by water pumped down at very high pressures.

(Experiments are under way to develop technologies for exploiting this resource.)

Experiments are under way to develop technologies for exploiting this resource.

(Like most other energy sources, geothermal energy presents some environmental problems.)

Like most other energy sources, geothermal energy presents some environmental problems.

(The surface of the ground can sink if hot groundwater is withdrawn without being replaced.)

The surface of the ground can sink if hot groundwater is withdrawn without being replaced.

(In addition, water heated geothermally can contain salts and toxic materials dissolved from the hot rock.)

In addition, water heated geothermally can contain salts and toxic materials dissolved from the hot rock.

(These waters present a disposal problem if they are not returned to the ground from which they were removed.)

These waters present a disposal problem if they are not returned to the ground from which they were removed.

(The contribution of geothermal energy to the world's energy future is difficult to estimate.)

The contribution of geothermal energy to the world's energy future is difficult to estimate.

(Geothermal energy is in a sense not renewable,)

Geothermal energy is in a sense not renewable,

(because in most cases the heat would be drawn out of a reservoir much more rapidly)

because in most cases the heat would be drawn out of a reservoir much more rapidly

(than it would be replaced by the very slow geological processes by which heat flows)

than it would be replaced by the very slow geological processes by which heat flows

(through solid rock into a heat reservoir.)

through solid rock into a heat reservoir.

(However, in many places (for example, California, Hawaii, the Philippines, Japan, Mexico,)

However, in many places (for example, California, Hawaii, the Philippines, Japan, Mexico,

(the rift valleys of Africa) the resource is potentially)

the rift valleys of Africa) the resource is potentially

(so large that its future will depend on the economics of production.)

so large that its future will depend on the economics of production.

(At present, we can make efficient use of only naturally occurring hot water or steam deposits.)

At present, we can make efficient use of only naturally occurring hot water or steam deposits.

(Although the potential is enormous, it is likely that in the near future geothermal)

Although the potential is enormous, it is likely that in the near future geothermal

(energy can make important local contributions only where the resource is close to the user)

energy can make important local contributions only where the resource is close to the user

(and the economics are favorable, as they are in California, New Zealand, and Iceland.)

and the economics are favorable, as they are in California, New Zealand, and Iceland.

(Geothermal energy probably will not make large-scale contributions)

Geothermal energy probably will not make large-scale contributions

(to the world energy budget until well into the twenty-first century, if ever.)

to the world energy budget until well into the twenty-first century, if ever.