OTEC is an energy technology that converts solar radiation to electric power. OTEC systems use the ocean's natural thermal gradient—the fact that the ocean's layers of water have different temperatures to drive a power-producing cycle. As long as the temperature between the warm surface water and the cold deep water differs by about 20°C (36°F), an OTEC system can produce a significant amount of power, with little impact on the surrounding environment.
The ocean can produce two types of energy: thermal energy
from the sun's heat, and mechanical energy from the tides and waves.
Ocean thermal energy is used for many applications, including electricity
generation. Ocean mechanical energy is quite different from ocean thermal
energy. Even though the sun affects all ocean activity, tides are driven
primarily by the gravitational pull of the moon, and waves are driven primarily
by the winds. As a result, tides and waves are sporadic sources of energy,
while ocean thermal energy is fairly constant. Also, unlike thermal energy, the
electricity conversion of both tidal and wave energy usually involves
mechanical devices.
OTEC is a process which utilizes the heat energy stored in the tropical ocean. The world's oceans serve as a huge collector of heat energy. OTEC plants utilize the difference in temperature between warm surface sea water and cold deep sea water to produce electricity.
Intensive Energy
The energy associated with OTEC
derives from the difference in temperature between two thermal reservoirs. The
top layer of the ocean is warmed by the sun to temperatures up to 20 K greater
than the seawater near the bottom of the ocean. OTEC energy is different from
geothermal energy in that one cannot assume the cold reservoir is infinite. The
physical energy of two large reservoirs of fluid at different temperatures is
in J/kg where r is the mass of
warm water divided by the mass of cold water entering the plant(1). For optimal
performance, r is approximately 0.5. It is assumed in this analysis that the
specific heat of the two fluid reservoirs is an average value over the often
small temperature difference, but varying with salinity in the case of
seawater.
Thermal
energy conversion is an energy technology that converts solar radiation to
electric power. OTEC systems use the ocean's natural thermal gradient—the fact
that the ocean's layers of water have different temperatures—to drive a
power-producing cycle. As long as the temperature between the warm surface
water and the cold deep water differs by about 20°C, an OTEC system can produce
a significant amount of power. The oceans are thus a vast renewable resource,
with the potential to help us produce billions of watts of electric power. This
potential is estimated to be about 1013 watts of base load power
generation, according to some experts. The cold, deep seawater used in the OTEC
process is also rich in nutrients, and it can be used to culture both marine
organisms and plant life near the shore or on land. OTEC produce steady,
base-load electricity, fresh water, and air-conditioning options.
OTEC requires a temperature difference of about 36 deg F (20 deg
C). This temperature difference exists between the surface and deep seawater
year round throughout the tropical regions of the world. To produce
electricity, we either use a working fluid with a low boiling point (e.g.
ammonia) or warm surface sea water, or turn it to vapor by heating it up with
warm sea water (ammonia) or de-pressurizing warm seawater. The pressure of the
expanding vapor turns a turbine and produces electricity.
TYPES OF ELECTRICITY CONVERSION SYSTEMS
There are three types of electricity conversion systems: closed-cycle,
open-cycle, and hybrid. Closed-cycle systems use the ocean's warm
surface water to vaporize a working
fluid, which has a low-boiling point, such as ammonia. The vapor
expands and turns a turbine. The turbine then activates a generator to produce
electricity. Open-cycle systems actually boil the seawater by operating at low
pressures. This produces steam that passes through a turbine/generator. And
hybrid systems combine both closed-cycle and open-cycle systems.
Closed-Cycle
OTEC
In the closed-cycle OTEC system, warm sea water vaporizes a
working fluid, such as ammonia, flowing through a heat exchanger (evaporator).
The vapor expands at moderate pressures and turns a turbine coupled to a
generator that produces electricity. The vapor is then condensed in heat
exchanger (condenser) using cold seawater pumped from the ocean's depths
through a cold-water pipe. The condensed working fluid is pumped back to the
evaporator to repeat the cycle. The working fluid remains in a closed system
and circulates continuously.
The heat exchangers (evaporator and condenser) are a large and
crucial component of the closed-cycle power plant, both in terms of actual size
and capital cost. Much of the work has been performed on alternative
materials for OTEC heat exchangers, leading to the recent conclusion that inexpensive
aluminum alloys may work as well as much more expensive titanium for this
purpose. Required condensate pump work, wC.
The major additional parasitic energy requirements in the OTEC plant are the
cold water pump work, wCT, and the warm water pump work, wHT.
Denoting all other parasitic energy requirements by wA, the
net work from the OTEC plant, wNP is
wNP = wT
+ wC + wCT + wHT + wA
The thermodynamic cycle undergone
by the working fluid can be analyzed without detailed consideration of the
parasitic energy requirements. From the first law of thermodynamics, the energy
balance for the working fluid as the system is
wN = QH
+ QC
Where wN = wT
+ wC is the net work for the thermodynamic cycle. For the
special idealized case in which there is no working fluid pressure drop in the
heat exchangers,
QH
=
|
∫
|
THds
|
H
|
and
QC
=
|
∫
|
TCds
|
C
|
so that the net thermodynamic
cycle work becomes
wN
=
|
∫
|
THds
+
|
∫
|
TCds
|
H
|
C
|
Subcooled liquid enters the
evaporator. Due to the heat exchange with warm sea water, evaporation takes
place and usually superheated vapor leaves the evaporator. This vapor drives
the turbine and 2-phase mixture enters the condenser. Usually, the subcooled
liquid leaves the condenser and finally, this liquid is pumped to the
evaporator completing a cycle.
