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Power and Energy Directly from Water
Nauru [93] in 1970. While this plant became operational in 1981, it only generated
net 30 kW power for a school and other systems [92]. Many earlier efforts made in
India and U.S. [75] were not completely successful. Only successful effort was made
in 1993 by National Energy Laboratory in Hawaii, which generated 255 kW energy
and lasted for six years [75]. Currently only operating OTEC plant is the one built
by Saga University with support of a various Japanese industries in March 2013 [75].
Ocean Thermal Energy Corporation has plans to install two 10 MW OTEC
plants in the US Virgin Islands and 5–10 MW OTEC facility in the Bahamas [75].
Numerous projects in Hawaii, Hainan, and Japan have been proposed and are
being pursued [75,92]. Basic operating principles, operating sites, other usages of
OTEC process and barriers to its implementations are well described in Ref. [75].
Sections 13.4.1 through 13.4.4 briefly summarize the descriptions presented in this
reference.
13.4.1 oPerATing PrinCiPleS
A heat engine gives a higher efficiency when it is run with a higher temperature
difference. The tropical area provides the largest temperature difference between
ocean surface and deep water around 20°C–25°C. While OTEC can in principle
provide 10–100 times more energy than wave power, its thermodynamic efficiency
of 1%–3% (with old technology) compared to the theoretical maximum of 6%–7%
for 20°C–25°C temperature difference has limited its use. Modern technologies,
however, approach to the theoretical maximum efficiency. One approach that has
worked is to pump vaporized low boiling fluid into the depths to be condensed,
which reduces pumping volumes, technical and environmental problems, and costs.
The heat engine cycle can be operated in three different ways: close, open, and
hybrid. Closed-cycle systems use fluid with a low boiling point, such as ammonia
(having a boiling point around −33°C at atmospheric pressure), to power a turbine
to generate electricity. Ammonia is used because of its superior transport properties, easy availability and low cost. Other fluids (such as CFC, HCFC, etc.) are possible but they have harmful environmental effects. The ammonia is vaporized and
condensed by warm and cool water, respectively, with the use of two separate heat
exchangers. The power is generated by the expanding vapor.
In an open-cycle OTEC process, warm surface water is converted to steam by
passing it into a low-pressure vessel. The expanding steam can drive a low-pressure
turbine. The steam is then converted to purified water by exposure to cold temperatures from deep-ocean water. This method thus produces water that is suitable for
drinking, irrigation, or aquaculture [72]. The expanding (and rising) steam can also
be used in a gas lift technique to lift water to significant heights. Depending on the
local circumstances, this technique can generate power with the use of a hydroelectric turbine [73].
A hybrid cycle combines the features of the closed- and open-cycle systems. In
a hybrid process, warm seawater enters a vacuum chamber and is flash evaporated.
The steam then vaporizes the ammonia, which like in a closed cycle drives a turbine
to produce electricity. The steam condenses within the heat exchanger and provides
desalinated water. The hybrid cycle thus serves multiple purposes at the same time.