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• 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.

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

• 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• 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• 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