Environmental Engineering Reference
In-Depth Information
the three most well-developed technologies are
tidal power
,
wave power
, and
ocean
thermal energy conversion
(OTEC). Ocean thermal energy conversion is limited
to tropical regions, such as Hawaii, and to a portion of the Atlantic coast. Ocean
thermal energy can be used for many applications, including electricity genera-
tion, which utilizes either warm surface water or boiled seawater to turn a turbine,
which activates a generator. Tidal power requires large tidal differences which, in
the United States, occur only in Maine and Alaska. Wave energy has a more general
application. The western coastline of the United States has the highest wave poten-
tial; in California, the greatest potential is along the state's northern coast.
It is important to distinguish tidal energy from hydropower. Hydropower is
derived from the hydrological climate cycle, powered by solar energy, which is usu-
ally harnessed via hydroelectric dams. In contrast, tidal energy is the result of the
interaction of the gravitational pull of the moon and, to a lesser extent, the sun on the
seas. Processes that use tidal energy rely on the twice-daily tides and the resultant
upstream flows and downstream ebbs in estuaries and lower reaches of some rivers.
The conversion of tidal and wave energy into electricity usually involves mechani-
cal devices. A dam is typically used to convert tidal energy into electricity by forcing
the water through turbines that activate a generator. The mechanical power created
from these systems either directly activates a generator or transfers to a working
fluid, water, or air, which then drives a turbine or generator.
Before we discuss the thermal and mechanical energy potential of the ocean, it is
important to have a basic understanding of oceans and especially their margins; it is
at the margins of coastal regions where most, if not all, ocean energy is harnessed
using current technology. The following section provides a foundation for better
understanding the ocean energy concepts presented later in this chapter.
OCEANS AND THEIR MARGINS
*
Oceans are a principal component of the hydrosphere and the storehouse of Earth's
water. Oceans cover about 71% of Earth's surface. The average depth of the oceans is
about 3800 m, but the greatest ocean depth of 11,036 m was recorded in the Mariana
Trench. The volume of all of the oceans is about 1.35 billion cubic kilometers, rep-
resenting 96.5% of Earth's total water supply; however, the volume fluctuates with
the growth and melting of glacial ice. The composition of ocean water has remained
constant throughout geologic time, with the major constituents dissolving in ocean
water from rivers and precipitation, as well as from weathering and degassing of the
mantle by volcanic activity. Seawater is approximately 3.5% salt and 96.5% water,
by weight. The dissolved salts include chloride (55.07%), sodium (30.62%), sul-
fate (7.72%), magnesium (3.68%), calcium (1.17%), potassium (1.10%), bicarbonate
(0.40%), bromine (0.19%), and strontium (0.02%).
The most significant factor related to ocean water that everyone is familiar with
is the salinity of the water—how salty it is.
Salinity
, a measure of the amount of
dissolved ions in the water, ranges between 33 and 37 parts per thousand. The
*
Adapted from Spellman, F.R.,
Geology for Nongeologists
, Government Institutes Press, Lanham, MD,
2009.
