Page 44 - Geothermal Energy Renewable Energy and The Environment

Sources of Geothermal Heat: Earth as a Heat Engine 27

the development of extensive geothermal reservoirs. Important power generation facilities exist in
northern Mexico at Cerro Prieto, where a complex of power plants has a combined installed gener-
ating capacity of 720 MW. This single location accounts for nearly 75% of Mexico’s total installed
capacity of 953 MW.
In southern California, the Salton Trough is the location of more than 530 MW of installed gen-
erating capacity. Although the size of the geothermal potential in the region is a matter of consider-
able debate, the minimum capacity for the region is estimated to be an additional 2000 MW. This
region contributes approximately a third of the total geothermal power production in California,
which ranks as the world’s largest producer of geothermally generated electricity.

subducTion Zones
The Philippines: The Philippine Islands are a complex of over 500 volcanoes formed above
the westward subducting Pacific plate. Of those 500, approximately 130 are active. Beginning
in 1977, geothermal power production was initiated on the island of Leyte. As of June 2008
that initial effort had grown to more than 1900 MW of electric power production distributed
among the islands of Luzon, Negros, Mindanao, and Leyte. Geothermal power production now
accounts for 18% of the country’s electrical needs. The national goal is to achieve 3131 MWe of
geothermal power by 2013. Worldwide, the Philippines rank second to the United States in pro-
ducing geothermic energy.

hoT spoTs
Hawaii: The Hawaiian Island chain tracks the movement of the Pacific plate over a mantle hotspot
that has persisted for more than 80 million years. The island of Hawaii is the currently active
volcano in the chain. On the eastern flank of the volcano is the East Rift Zone, a zone along which
lava has repeatedly erupted. Geothermal exploration was started in this area during the 1960s,
eventually leading to development of a small (3 MWe) power generating facility in 1981. Power
production continued until 1989 when the plant was shut down. A newer facility was built in 1993,
which has continuously produced 25 to 30 MWe of base load power. The geothermal resource in the
region is capable of providing about 200 MWe of power on a continuous basis.
Iceland: Iceland is one of the world’s largest islands. It sits directly on the mid-Atlantic ridge, and
is interpreted to represent the superposition of a hotspot on a spreading center. At that location on
the mid-Atlantic ridge, the spreading rate is about 2 cm per year. Iceland has the greatest per capita
use of geothermal energy in the world. In 2006, 26% of its electricity (322 MWe) was produced
from geothermal electric generators. In addition, hot water from power plants as well as directly
from geothermal resources accounts for 87% of the hot water usage in the country. This usage
includes hot water for space heating using district heating systems, snow melting, and domestic hot
water use.

Problems
2.1 What is the global average heat flow at the surface of the Earth? What is its range?
2.2 What controls heat flow?
2.3 Assume a well is drilled in dry sand to a depth of 2,800 m and the temperature at the
bottom of the well is measured to be 200°C. Assume, too, that the thermal conductivity
of dry sand is constant between 10°C and 250°C. Is there likely to be a geothermal
resource in the area?
2.4 For a material that has a Rayleigh number of 10, what mode of heat transfer will
dominate? For a value of 100? For a value of 10,000? Will this behavior have any
influence on heat flow measured at the surface of the Earth? If so, what?
2.5 What geological regions are most likely to have high heat flows?