Ocean%20Thermal%20Energy

1
Ocean Thermal Energy

• Energy is available from the ocean by
• Tapping ocean currents
• Using the ocean as a heat engine
• Tidal energy
• Wave energy

2
Energy from ocean currents

• 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
• 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.

3
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)

4
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.

5
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.

6
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.

7
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.

8
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

9
OTEC Plant on Keahole Point, Hawaii
10
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).

11
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

12
Tides

• So the moon and Earth exert a force of gravity on
  each other. The motion of the moon around the
  Earth counteracts the Earths pull, so the moon
  does not fall into the Earth.
• The moons pull on the Earth causes any material
  that can flow on the Earths surface, like large
  bodies of water, to pile up underneath the moon.

13
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.

14
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.

15
Rance River Tidal Power station in France
16
Current and Future tidal power stations

• 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

17
Bay of Fundy
18
Power

• Power P Cp x 0.5 x ? x A x V³
• Cp is the turbine coefficient of performance
• P the power generated (in watts)
• ? 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)

19
Environmental Issues

• 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 Rances
  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.

20
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
• doesnt disturb current environmental conditions
  as much and expands locations by only requiring
  large tidal variations (as opposed to that and
  proper natural landforms).

21
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?

22
How are waves formed

• 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.

23
Harnessing the energy

• Limpet (Land Installed Marine Powered Energy
  Transformer)
• Breakwater Design
• PowerBuoys
• Pelamis

24
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 Scotlands
  west coast.

25
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

26
PowerBuoys

• In a permanent magnet linear generator buoy, an
  electric coil surrounds a magnetic shaft inside
  the buoy
• the magnetic shaft is anchored to the sea floor.
  When waves cause the coil to move up and down
  relative to the fixed magnetic shaft, voltage is
  induced and electricity is generated.
• Each buoy could potentially produce 250 kilowatts
  of power.
• A fleet of about 200 such buoys could power the
  business district of downtown Portland.

27
PELAMIS

• made of large semi-submerged sections, like a
  submarine cut into pieces
• the wave action makes the Pelamis bend between
  the sections.
• This bending action forces hydraulic pistons to
  move in the device and push fluid around
  generating a linear flow, which produces energy.
• A 1 sq kilometer farm could produce 30 Megawatts.