Environmental Engineering Reference
In-Depth Information
7.4
GEOTHERMAL ENERGY
Fossil fuel deposits, which form the bulk of the world's current energy supply, are the residue of
biomass formed millions of years ago, in which solar energy was stored in living matter. Made
accessible by industrial technology, fossil fuel is the most easily and cheaply exploitable form of
energy. But the energy stored in the hot interior of the earth is vastly greater in magnitude and is
potentially exploitable, yet is almost entirely inaccessible. This energy is what remains from the
gravitational collapse of the interplanetary material from which the earth was formed.
The earth's interior consists of a core of mostly molten material at a temperature of about
4000
◦
C, extending to a little more than half an earth radius and surrounded by a mantle of de-
formable material. The outer edge of the mantle is covered by a crust of solid material, of thickness
between 5 and 35 km. Within the crust there is an outward flow of heat from the earth's interior of
approximately 50 mW/m
2
which is accompanied by a temperature gradient of about 30 K/km.
9
No
practical use can be made of such a feeble flow of heat unless it can be substantially amplified. By
drilling wells to depths of 5-10 km and pumping from them water or steam heated to 200-300
◦
C,
enough heat may be extracted to generate hundreds of megawatts of electrical power in a single
geothermal plant.
The most economical sites at which to develop geothermal energy are those where the subsur-
face temperatures are highest and underground water and steam deposits are closest to the surface.
Such sites are found mostly at the borders of the earth's tectonic plates, near active or recently
inactive volcanos, hot springs, or geysers. Favorable sites of this type occupy only a small fraction
of the earth's land area.
The principal countries that have installed geothermal electrical power generation systems
are listed in Table 7.4, together with the amounts of installed electrical power. Of the world total
TABLE 7.4
Installed Electrical and Thermal
Power of Geothermal Systems in 1993
a
Electrical
Thermal
Country
(MW)
(MW)
United States
2,594
463
Phillipines
888
—
Mexico
752
8
Italy
637
360
New Zealand
285
258
Japan
270
3,321
Indonesia
144
—
El Salvador
105
—
Other
240
6,802
Total
5,915
11,204
a
Data from Dickson, Mary H., and Mario Fanelli, Eds., 1995.
Geothermal Energy.
Chichester: John Wiley & Sons.
9
This heat flux is tiny compared with the average solar energy flux to the earth's surface of about 500 W/m
2
.
It is the latter that determines the earth's surface temperature.
