Page 156 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

146 Fundamentals of Ocean Renewable Energy

tend to be very cautious when looking at a process that could alter the circulation
of the major ocean currents. However, compared to other processes like climate
change, any exploitation of the ocean currents to generate electricity will be rel-
atively minor, and certainly well within the bounds of natural variability of these
systems. Therefore, the environmental impacts of this form of electricity gener-
ation are considered to be negligible and localized to the array itself (e.g. [13]).

6.3 OCEAN THERMAL ENERGY CONVERSION
OTEC technologies exploit the temperature difference between warm sea water
at the surface of the ocean, and cold sea water at around 1000 m depth,
2
to produce electricity [14]. Between the tropics of the world’s oceans, the
temperature of the surface waters is significantly higher than the temperature
deeper in the water column (Fig. 6.3). By contrast, at higher latitudes, the
vertical temperature gradient is considerably less and varies seasonally. It is
◦
generally considered that a temperature difference of at least 20 C is required
for OTEC plants to operate, and so this form of energy conversion is restricted
to the tropics.
The OTEC resource is vast—the International Renewable Energy Agency
(IRENA) has estimated that the global resource could be up to 30 TW [14],
which would make OTEC by far the ocean energy resource with the largest
potential.
The temperature gradient between the sea surface and deep water can be
harnessed using different OTEC processes that mainly differ based on the
working fluid that is used. The two main types of technology are closed cycle
OTEC, which mainly uses ammonia as the working fluid, and open cycle OTEC,
which uses the sea water itself as the working fluid.

6.3.1 Closed Cycle OTEC
Warm tropical sea water, at around 25 C, is clearly at a temperature that is
◦
insufficient to drive a steam turbine [15]. However, this warm surface water
can be used to vaporize a fluid with a low boiling temperature such as ammonia.
In a closed cycle OTEC power plant, this ‘working fluid’ flows around a closed
loop as shown in Fig. 6.4. Warm water from the sea surface flows through a heat
exchanger, causing the working fluid to boil and vaporize. This vaporized fluid
flows through a turbine, which turns a generator that converts the energy into
electricity. Upon leaving the turbine, the working fluid needs to be cooled so that
it can be reused, otherwise the efficiency of the system would drop significantly.
Cold water pumped from the deep ocean flows through a second heat exchanger
that cools the working fluid to its original temperature, ready to enter the
cycle again. The cold water from the deep ocean, now at a slightly elevated

2. 23.5 degrees S (Tropic of Capricorn) to 23.5 degrees N (Tropic of Cancer).