Environmental Engineering Reference
In-Depth Information
OCEAN THERMAL ENERGY CONVERSION
The most plentiful renewable energy source in our planet by far is solar radiation:
170,000 TW (170,000 × 10
12
W) falling on Earth. Because of its dilute and erratic
nature, however, it is difficult to harness. To capture this energy, we must employ
the use of large collecting areas and large storage capacities, requirements satis-
fied on Earth by only the tropical oceans. We are all taught at an early age that
water covers about 71% (or two-thirds) of Earth's surface. With regard to the vast
oceans covering the majority of Earth, it is fitting that Ambrose Bierce (1842-1914)
referred to them as “a body of water occupying about two-thirds of the world made
for man who has no gills.” True, we have no gills, so those who look out upon those
vast bodies of water that cover the surface might ask: “What is their purpose?” This
is a good question with several possible answers. With regard to renewable energy,
we could look out upon those vast seas and wonder how we might use this massive
storehouse of energy for our own needs; it is so vast and deep that it absorbs much
of the heat and light that come from the sun. One thing seems certain: The secret
to our origin, past, present, and future lies within those massive wet confines we
call oceans.
o
Cean
e
nergy
C
onversion
p
roCess
*
The ocean is essentially a gigantic solar collector. The energy from the sun heats
the surface water of the ocean. In tropical regions, the surface water can be 40 or
more degrees warmer than the deep water. This temperature difference can be used
to produce electricity. Ocean thermal energy conversion (OTEC) has the potential
to produce more energy than tidal, wave, and wind energy combined. The OTEC
systems can be open or closed. In a closed system, an evaporator turns warm surface
water into steam under pressure (see
Fig u r es 7.7
and
7.8
). This steam spins a turbine
generator to produce electricity. Water pumps bring deep cold water though pipes
to a condenser on the surface. The cold water condenses the steam, and the closed
cycle begins again. In an open system, the steam is turned into fresh water, and new
surface water is added to the system. A transmission cable carries the electricity to
the shore. The OTEC systems must have a temperature difference of about 25°C to
operate. This limits the use of OTEC to tropical regions where the surface waters
are very warm and there is deep cold water. Hawaii, with its tropical climate, has
experimented with OTEC systems since the 1970s. Because of the many challenges
to their widespread use, no large or major OTEC systems are in operation today, but
several experimental OTEC plants have been built. Pumping the water is a giant
engineering challenge. Because of this, OTEC systems are not very energy efficient.
It will probably be 10 to 20 years before the technology is available to produce and
transmit electricity economically from OTEC systems.
*
Adapted from USDOI,
Ocean Wave Energy
, Minerals Management Service, U.S. Department of
Interior, Washington, DC, 2014 (http://www.boem.gov/Renewable-Energy-Program/Renewable-
Energy-Guide/Ocean-Wave-Energy.aspx).
