Visualizing Environmental Science

Cooling
water in To cooling tower

Condenser

Steam

turbine Generator

Pump

Pump

Reinjection

well

Ground level

Deep well to hot water

(340°F or more)

Steam

Heat

exchanger

Kim Hart/© Robert Harding Picture Library Ltd/Alamy

a. Steam separated from hot water pumped from underground

turns a turbine and generates electricity.

b. A geothermal power plant in Iceland.

Other Renewable Energy Sources 457

of the gravitational pull of the moon and the sun. A dam

built across a bay can harness the energy of large tides to

generate electricity. As the tide falls, water flowing back

to the ocean over the dam’s spillway turns a turbine and

generates electricity through tidal energy.

Power plants using tidal power are in operation in

France, Russia, China, and Canada. Tidal energy can’t

become a significant resource worldwide because few ar-

eas experience large enough differences in water level

between high and low tides to make power generation

feasible. The most promising locations for tidal power in

North America include the Bay of Fundy in Nova Scotia,

Passamaquoddy Bay in Maine, Puget Sound in Washing-

ton, and Cook Inlet in Alaska.

Other problems associated with tidal energy include

the high cost of building a tidal power station and po-

tential environmental problems associated with tidal en-

ergy in estuaries, coastal areas where river currents meet

ocean tides. Fishes and countless invertebrates migrate

to estuaries to spawn. Building a dam across the mouth

of an estuary would prevent these animals from reaching

their breeding habitats.

1. What are the pros and cons of using
   geothermal energy to produce electricity, and
   what are the pros and cons of using tidal power
   to produce electricity?

Ground-Effect Heat Pumps

Ground-effect heat pumps (GHPs), also called ambient

heat systems, take advantage of the difference in temper-

ature between Earth’s surface and subsurface (at depths

from 1 m to about 100 m or about 3 to 300 ft). In an

underground arrangement of pipes containing circu-

lating fluids, GHPs extract natural heat in winter, when

Earth acts as a heat source, and transfer excess heat un-

derground in summer, when Earth acts as a heat sink.

Ground-effect heating systems can be modified to pro-

vide supplemental hot water.

Though GHPs have been available for many years, they

aren’t widely used because their installation is expensive.

However, with the growth of green architecture (see the Case

Study at the end of this chapter) and rising fuel costs, com-

mercial and residential use of GHPs is on the rise. The sys-

tem’s benefits include low operating costs—which may be

half those of conventional systems—and high efficiency.

Tidal Energy

Tides, or the rise and fall of the surface waters of the

ocean and seas that occur twice each day, are the result

ii«Ê}iœÌ…iÀ“>Ê«œÜiÀÊ«>˜ÌÊUʈ}ÕÀiÊ£n°£Ó