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Water for Energy and Fuel Production
13.4.2 oPerATing SiTeS
While OTEC has the potential to produce a large amount of power and hydrogen
(jointly with electrolysis), it is an expensive technology. OTEC plants require a long,
large-diameter intake pipe, which is submerged a kilometer or more into the ocean’s
depths, to bring cold water to the surface. The operating site for OTEC can be land
based, shelf based, or floating.
Land-based and near-shore facilities can be installed in sheltered areas so that
they are relatively safe from storms and heavy seas. Electricity, desalinated water,
and cold, nutrient-rich seawater could be transmitted from near-shore facilities via
trestle bridges or causeways. In addition, land-based or near-shore sites allow plants
to operate jointly with desalination or aquaculture industries. Land-based or nearshore sites can also support chilled water agriculture. Tanks or lagoons built on shore
allow workers to monitor and control miniature marine environments and allow easy
transport of the products to the markets.
Favored locations include those with narrow shelves (volcanic islands), steep
(15°–20°) offshore slopes, and relatively smooth seafloors. These sites minimize the
length of the intake pipe. A land-based plant could be built well inland from the
shore, offering more protection from storms, or on the beach, where the pipes would
be shorter. In either case, convenient access for construction and operation helps
lower costs.
There are few disadvantages to land-based operations. The prolonged turbulent
wave action in the surf zone and storms can damage discharge pipes. In addition,
the mixed discharge of cold and warm seawater may need to be carried several
hundred meters offshore to reach the proper depth before it is released, requiring
additional expense in construction and maintenance. This can be avoided by building OTEC system just offshore in waters ranging from 10 to 30 m deep which use
shorter intake and discharge pipes. The plant itself, however, would require protection from the marine environment, and the plant output would need to be transmitted to shore [74].
To avoid the turbulent surf zone as well as to move closer to the cold-water
resource, the OTEC plants can be mounted to the continental shelf at depths up to
100 m. In general, however, the stress of open-ocean conditions, difficulty in product
delivery and higher expenses for its construction and building a power delivery
system to reach land make this approach less attractive [74].
Floating OTEC facilities operate offshore. Although potentially optimal for
large systems, floating facilities present several difficulties. The difficulty of
mooring plants in very deep water complicates power delivery. Cables attached
to floating platforms are more susceptible to damage, and for depths >1000 m
they are difficult to maintain and repair. The system needs to be connected to the
sea floor by riser cables without entanglement [74]. Both warm-water intake and
vertically suspended cold-water pipe can be damaged by major storms and heavy
seas. This problem can be alleviated with the use of flexible polyethylene materials for pipes, which can be uncoupled from the plant during storm. Surface water
can also be drawn directly into the platform. Precautions must be taken to reduce
damage and interruptions to intake flow by heavy seas [74]. Connecting a floating