Ocean Thermal Energy
• Energy is available from the ocean by
– Tapping ocean currents
– Using the ocean as a heat engine
– Tidal energy
– Wave energy
Energy from ocean currents
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Ocean currents flow at a steady velocity
Place turbines in these currents (like the gulf
stream) that operate just like wind turbines
Water is more than 800 times denser than
air, so for the same surface area, water
moving 12 miles per hour exerts about the
same amount of force as a constant 110 mph
wind.
Expensive proposition
Upkeep could be expensive and complicated
Environmental concerns
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species protection (including fish and marine
mammals) from injury from turning turbine
blades.
Consideration of shipping routes and present
recreational uses of location
Other considerations include risks from
slowing the current flow by extracting energy.
The ocean as a heat engine
• There can be a 20° difference between ocean
surface temps and the temp at 1000m
• The surface acts as the heat source, the deeper
cold water acts as a heat sink.
• Temperature differences are very steady
• Florida, Puerto Rico, Hawaii and other pacific
islands are well suited to take advantage of this
idea.
• Called OTEC (Ocean Thermal Energy Conversion)
Types of Ocean heat engines
• Closed cycle system
• Heat from warm seawater
causes a fluid like ammonia
to be evaporated in an
evaporator
• Expanding vapor rotates a
turbine connected to an
electric generator.
• Cold seawater is brought up
and cools the ammonia
vapor in a condenser. This
liquid returns to the
evaporator and the process
repeats.
Types of OTECs
• Open Cycle Systems
• Working fluid is the seawater.
• Warm seawater is brought into
a partial vacuum.
• In the vacuum, the warm
seawater boils and the steam
drives a turbine
• The steam enters a condenser,
where it is cooled by cold
seawater brought up form
below and it condenses back
into liquid and is discharged
into the ocean.
Boiling water in a vacuum
• The boiling point of any liquid depends upon
temperature and pressure.
• Boiling occurs when the molecules in the liquid
have enough energy to break free from
surrounding molecules
• If you reduce the pressure, you reduce the
amount of energy needed for the molecules to
break free.
• Creating a vacuum reduces the air pressure on
the molecules and lowers the boiling point.
OTECs
• Carnot Efficiency is low, only about 7%
• Net efficiency even lower, only about 2.5%
• Low efficiencies require large water volumes
to produce appreciable amount of electricity
• For 100 mW output, you would need 25 X 106
liters/sec of warm and cold water.
• For a 40 mW plant, a 10 meter wide intake
pipe is needed. This is the size of a traffic
tunnel.
History of OTECs
• Jacques d ‘Arsonval in 1881 first proposed the idea
• Completed by his student, Georges Claude in 1930.
(Claude also invented the neon lightbulb)
• Claude built and tested the first OTEC system
• Not much further interest until the energy crisis of the
1970s.
• In the 1970s, US DOE financed large floating OTEC
power plant to provide power to islands
• One was built in Hawaii.
• Little further support
OTEC Plant on Keahole Point, Hawaii
Other uses for OTEC plants
• Generate Hydrogen for use as a clean fuel
source
• Generate fertilizer from biological nutrients
that are drawn up from the ocean floor in the
cold water intake.
• Source of ocean water to be used as drinking
water via desalination (taking out the salt).
Tidal Energy
• Most of the energy sources we have been
discussing derived their energy from the sun
originally.
• Tides are driven by gravity
• Gravity is a force that exists between any two
objects based upon their mass and the distance
between them
• Fg = GmM/R2 where M and m are the masses of
the two objects, R is the distance between them
and G is the gravitational constant = 6.67300 ×
10-11 m3 kg-1 s-2
Tides
• So the moon and Earth exert a force of gravity
on each other. The motion of the moon
around the Earth counteracts the Earth’s pull,
so the moon does not fall into the Earth.
• The moon’s pull on the Earth causes any
material that can flow on the Earth’s surface,
like large bodies of water, to pile up
underneath the moon.
Tides
• The sun also causes tides the Earth, thought the
effect is small, unless the sun and moon line up
and work together (Spring tide) or are at right
angles to each other and work against each other
(neap tides).
• In areas where there are natural basins on the
coastline, water flows in and out of these basins.
• So there are regular, predictable motions in the
oceans which could be used as an energy source.
Capturing Tidal Power
• Dams or barrage with gates are usually built
across the mouth of basins
• This allows the current to be directed into the
turbines and enhances the effect.
Rance River Tidal Power station in
France
Current and Future tidal power
stations
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Rance River, France 240Mw
White sea, Russia 1 MW
Annapolis River, Nova Scotia, Canada, 18mW
Two most favorable sites in the US: Cook Inlet
and Bristol Bay in Alaska and Bay of Funday which
covers the Northeastern US and southeastern
Canada.
• Development of the Bay of Funday would provide
15,000mW to the northeastern US and
15,000mW to Canada
Bay of Fundy
Power
• Power: * P = Cp x 0.5 x ρ x A x V³
• Cp is the turbine coefficient of performance
• P = the power generated (in watts)
• ρ = the density of the water (seawater is
1025 kg/m³)
• A = the sweep area of the turbine (in m²)
• V³ = the velocity of the flow cubed (i.e. V x V
x V)
Environmental Issues
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alters the flow of saltwater in and out of estuaries, which changes the hydrology and salinity and
possibly negatively affects the marine mammals that use the estuaries as their habitat
Some species lost their habitat due to La Rance’s construction, but other species colonized the
abandoned space, which caused a shift in diversity.
Turbidity (the amount of matter in suspension in the water) decreases as a result of smaller volume
of water being exchanged between the basin and the sea. This lets light from the Sun to penetrate
the water further, improving conditions for the phytoplankton. The changes propagate up the food
chain, causing a general change in the ecosystem.
If the turbines are moving slowly enough, such as low velocities of 25-50 rpm, fish kill is
minimalized and silt and other nutrients are able to flow through the structures . Tidal fences block
off channels, which makes it difficult for fish and wildlife to migrate through those channels. Larger
marine mammals such as seals or dolphins can be protected from the turbines by fences or a sonar
sensor auto-breaking system that automatically shuts the turbines down when marine mammals
are detected
As a result of less water exchange with the sea, the average salinity inside the basin decreases, also
affecting the ecosystem
Estuaries often have high volume of sediments moving through them, from the rivers to the sea.
The introduction of a barrage into an estuary may result in sediment accumulation within the
barrage, affecting the ecosystem and also the operation of the barrage.
Innovative strategies
• East River in New York-tidal river
– Plans for 300 underwater turbines to tap the rivers 4
knot tidal flow and produce 10mW
– Already tested with a prototype
• Tidal Lagoons
– Artificial lagoons with high walls.
– Lagoon fills and empties through apertures, turbines
are spun and generate electricity
– doesn’t disturb current environmental conditions as
much and expands locations by only requiring large
tidal variations (as opposed to that and proper natural
landforms).
Wave Energy
• It is estimated that there is 2-3 million mW of
energy in the waves breaking on the world
coastlines, with energies derived ultimately
form the wind
• In Great Britain alone, almost twice the
current electricity demand breaks on the
countries coastlines every day.
• A vast untapped resource, but how to harness
it?
How are waves formed
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As wind blows along the surface of a
body of water, a surface wave
develops.
As the wind blows, pressure and
friction forces perturb the
equilibrium of the water surface
These forces transfer energy from
the air to the water, forming waves.
The water molecules actually move
in circular motion
When a wave can no longer support
its top, it collapses or breaks.
Usually happens when a wave
reaches shallow water, such as near a
coastline.
Harnessing the energy
• Limpet (Land Installed Marine Powered Energy
Transformer)
• Breakwater Design
• PowerBuoys
• Pelamis
LIMPET
• Limpet
• Takes the wave into a
funnel and drives air
pressure past two
turbines, each of which
turns a 250 kW
generator.
• Installed on the island
of Islay, off Scotland’s
west coast.
Breakwater
• Installed where there would
normally be a breakwater
• a series of layered
‘reservoirs’ up a carefully
calculated slope.
• This is then converted to
kinetic energy (by falling
down), and this turns the
turbine/generator.
• A 500m breakwater can
produce respectable 150
kW generator capacity
• Only in design phase, non of
these up and running yet