Content derived from Wikipedia article on Ocean Energy
The oceans have a tremendous amount of energy and are
close to many if not most concentrated populations. Some believe that ocean
power will provide a substantial amount of new renewable energy around the
world. Difficulties arising from marine life attaching to energy systems in
the seas require these to be easily cleanable.
Renewable ocean energy
The ocean presents a vast source of renewable energy in
the form of winds, waves and tides. In addition, there is vast quantity of
energy in the form of thermal difference which can be extracted. Several
means of extracting energy from the ocean have been tried, some with limited
Wind power (offshore)
Ocean thermal energy conversion (OTEC)
Marine current power
Non-renewable ocean energy
Oil and gas beneath the ocean floor are increasingly
important sources of energy. An ocean engineer is concerned with all phases
of discovering, producing, and delivering offshore petroleum resources, a
complex and demanding task. Also of central importance is the development of
new methods to protect marine wildlife and coastal regions against the
undesirable side effects of offshore oil production.
Content derived from Wikipedia
article on Ocean Thermal Energy Conversion
Ocean thermal energy conversion (OTEC)
Ocean thermal energy conversion, or OTEC, is a way to generate
electricity using the temperature difference of seawater at different depths.
The method involves pumping cold water from the ocean depths (as deep as 1
km) to the surface and extracting energy from the flow of heat between the
cold water and warm surface water.
OTEC utilizes the temperature difference that exists
between deep and shallow waters — within 20° of the equator in the tropics —
to run a heat engine. Because the oceans are continually heated by the sun
and cover nearly 70% of the Earth's surface, this temperature difference
contains a vast amount of Solar energy which could potentially be tapped for
human use. If this extraction could be done profitably on a large scale, it
could be a solution to some of the human population's energy problems. The
total energy available is one or two orders of magnitude higher than other
ocean energy options such as Wave power, but the small size of the
temperature difference makes energy extraction difficult and expensive.
Hence, existing OTEC systems have an overall efficiency of only 1 to 3%.
The concept of a heat engine is very common in
engineering, and nearly all energy utilized by humans uses it in some form. A
heat engine involves a device placed between a high temperature reservoir
(such as a container) and a low temperature reservoir. As heat flows from one
to the other, the engine extracts some of the heat in the form of work. This
same general principle is used in steam turbines and internal combustion
engines, while refrigerators reverse the natural flow of heat by
"spending" energy. Rather than using heat energy from the burning
of fuel, OTEC power draws on temperature differences caused by the sun's
warming of the ocean surface.
1 History of OTEC
2 How OTEC works
2.1 Depending on the location
2.2 Depending on the cycle used
3 Some proposed projects
4 Other related technologies
4.1 Air conditioning
4.2 Chilled-soil agriculture
4.5 Mineral extraction
5 Political Concerns
6 Cost and Economics
7 Technical Analysis of OTEC systems
7.1 Variation of ocean temperature with depth
7.2 The open/Claude cycle
7.3 The closed/Anderson cycle
7.3.1 Working fluids
7.4 Technical difficulties
7.4.1 Degradation of heat exchanger performance by dissolved gases
7.4.2 Improper sealing
7.4.3 Parasitic power consumption by exhaust compressor
8 Energy from temperature difference between cold air and warm water
9 See also
History of OTEC
Even though it sounds technologically sophisticated, OTEC
technology is not new. It has progressed in fits and starts since the late
1800s. In 1881, Jacques Arsene d'Arsonval, a French physicist, proposed
tapping the thermal energy of the ocean. It was d'Arsonval's student, Georges
Claude who actually built the first OTEC plant, in Cuba in 1930. The system
produced 22 kW of electricity with a low-pressure turbine.
In 1935, Claude constructed another plant, this time aboard a 10,000-ton cargo vessel moored off the coast of Brazil. Weather and waves destroyed both plants before they could become net power generators. (Net power is the amount of power generated after subtracting power needed to run the system.)
In 1956, French scientists designed another 3 megawatt
(MW) OTEC plant for Abidjan, Côte d'Ivoire. The plant was never completed,
however, because it was too expensive.
In 1962, J. Hilbert Anderson and James H. Anderson, Jr.
start designing a cycle to accomplish what Claude had not. They focused on
developing new, more efficient component designs.
The United States became involved in OTEC research in 1974, when the Natural Energy Laboratory of Hawaii Authority was established at Keahole Point on the Kona coast of Hawaii. The Laboratory has become one of the world's leading test facilities for OTEC technology.
Japan also continues to fund research and development in OTEC technology.
India piloted a 1 MW floating OTEC plant near Tamil Nadu. The government continues to sponsor various research in developing floating OTEC facilities.
How OTEC works
Some energy experts believe that if it could become cost-competitive with conventional power technologies, OTEC could produce gigawatts of electrical power. Bringing costs into line is still a huge challenge, however. All OTEC plants require an expensive, large diameter intake pipe, which is submerged a mile or more into the ocean's depths, to bring very cold water to the surface.
This cold seawater is an integral part of each of the three types of OTEC systems: closed-cycle, open-cycle, and hybrid.
Diagram of a closed cycle OTEC plantClosed-cycle systems use fluid with a low boiling point, such as ammonia, to rotate a turbine to generate electricity. Warm surface seawater is pumped through a heat exchanger where the low-boiling-point fluid is vaporized. The expanding vapor turns the turbo-generator. Then, cold, deep seawater—pumped through a second heat exchanger—condenses the vapor back into a liquid, which is then recycled through the system.
In 1979, the Natural Energy Laboratory and several private-sector partners developed the mini OTEC experiment, which achieved the first successful at-sea production net electrical power from closed-cycle OTEC. The mini OTEC vessel was moored 1.5 miles (2.4 km) off the Hawaiian coast and produced enough net electricity to illuminate the ship's light bulbs, and run its computers and televisions.
Then, the Natural Energy Laboratory in 1999 tested a 250 kW pilot OTEC closed-cycle plant, the largest such plant ever put into operation. Since then, there have been no tests of OTEC technology in the United States, largely because the economics of energy production today have delayed the financing of a permanent, continuously operating plant.
Outside the United States, the government of India has taken an active interest in OTEC technology. India has built and plans to test a 1 MW closed-cycle, floating OTEC plant.
Open-cycle OTEC uses the tropical oceans' warm surface water to make electricity. When warm seawater is placed in a low-pressure container, it boils. The expanding steam drives a low-pressure turbine attached to an electrical generator. The steam, which has left its salt behind in the low-pressure container, is almost pure fresh water. It is condensed back into a liquid by exposure to cold temperatures from deep-ocean water.
In 1984, the Solar Energy Research Institute (now the National Renewable Energy Laboratory) developed a vertical-spout evaporator to convert warm seawater into low-pressure steam for open-cycle plants. Energy conversion efficiencies as high as 97% were achieved for the seawater to steam conversion process (note: the overall efficiency of an OTEC system using a vertical-spout evaporator would still only be a few per cent). In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50,000 watts of electricity during a net power-producing experiment. This broke the record of 40,000 watts set by a Japanese system in 1982.
Hybrid systems combine the features of both the closed-cycle and open-cycle systems. In a hybrid system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, similar to the open-cycle evaporation process. The steam vaporizes a low-boiling-point fluid (in a closed-cycle loop) that drives a turbine to produce electricity.
Some proposed projects
OTEC projects on the drawing board include a small plant for the U.S. Navy base on the British island of Diego Garcia in the Indian Ocean. There, a proposed 8 MW OTEC plant, backed up by a 2 MW gas turbine, would replace an existing 15 MW gas turbine power plant. A private U.S. company also has proposed building at 10 MW OTEC plant on Guam.
Other related technologies
OTEC has important benefits other than power production.
Air conditioning can be a byproduct. Spent cold seawater from an OTEC plant can chill fresh water in a heat exchanger or flow directly into a cooling system. Simple systems of this type have air conditioned buildings at the Natural Energy Laboratory for several years.
OTEC technology also supports chilled-soil agriculture. When cold seawater flows through underground pipes, it chills the surrounding soil. The temperature difference between plant roots in the cool soil and plant leaves in the warm air allows many plants that evolved in temperate climates to be grown in the subtropics. The Natural Energy Laboratory maintains a demonstration garden near its OTEC plant with more than 100 different fruits and vegetables, many of which would not normally survive in Hawaii.
Aquaculture is perhaps the most well-known byproduct of OTEC. Cold-water delicacies, such as salmon and lobster, thrive in the nutrient-rich, deep, seawater from the OTEC process. Microalgae such as Spirulina a health food supplement, also can be cultivated in the deep-ocean water.
Desalination, the production of fresh water from seawater, is another advantage of open or hybrid-cycle OTEC plants. Theoretically, an OTEC plant that generates 2 MW of net electricity could produce about 4,300 cubic meters (151,853 cubic feet) of desalinated water each day. This is equivalent to 4.3 million litres or 1.13 million (U.S.) gallons.
OTEC may one day provide a means to mine ocean water for 57 trace elements. One element in sea water thats historically been sought after is gold. There are vast amounts of gold dissolved in sea water, but due to the current cost of extracting it, it is not done. Most economic analyses have suggested that mining the ocean for any valuble dissolved substances would be unprofitable because so much energy is required to pump the large volume of water needed and because of the expense involved in separating the minerals from seawater. But with OTEC plants already pumping the water, the only remaining economic challenge is to reduce the cost of the extraction process.
Because OTEC facilities are more-or-less stationary
surface platforms, their exact location and legal status may be affected by
the United Nations Convention on the Law of the Sea treaty (UNCLOS). This
treaty grants coastal nations 3-, 12-, and 200-mile zones of varying legal
authority from land, creating potential conflicts and regulatory barriers to
OTEC plant construction and ownership. OTEC plants and similar structures
would be considered artificial islands under the treaty, giving them no legal
authority of their own. OTEC plants could be perceived as either a threat or
potential partner to fisheries management or to future seabed mining
operations controlled by the International Seabed Authority. The United
States has not ratified the treaty as of 2006 despite strong internal
Cost and Economics
For OTEC to be viable as a power source, it must either
gain political favor (ie. favorable tax treatment and subsidies) or become competitive
with other types of power, which may themselves be subsidized. Because OTEC
systems have not yet been widely deployed, estimates of their costs are
uncertain. One study estimates power generation costs as low as $.07 USD per
kilowatt-hour, compared with $.07 for subsidized Wind systems and $.0192 for
Besides regulation and subsidies, other factors that
should be taken into account include OTEC's status as a renewable resource
(with no waste products or limited fuel supply), the limited geographical
area in which it is available, the political effects of reliance on oil, the
development of alternate forms of ocean power such as Wave Energy and methane
hydrates, and the possibility of combining it with aquaculture or filtration
for trace minerals to obtain multiple uses from a single pump system.
Technical Analysis of OTEC systems
OTEC systems can be classified as two types based on the
thermodynamic cycle (1) Closed cycle and (2) Open cycle.
Variation of ocean temperature with depth
The total insolation received by the oceans = (5.457 ×
1018 MJ/yr) × 0.7 = 1.9 × 1018 MJ/yr. (taking an average clearness index of
Only some 15% of this energy is absorbed. But this 15% is
still huge enough.
We can use Lambert's law to quantify the solar energy
absorption by water,
Where, y is the depth of water, I is intensity and μ
is the absorption coefficient. Solving the above differential equation,
The absorption coefficicent μ may range from 0.05 m−1
for very clear fresh water to 0.5 m-1 for very salty water.
Since the intensity falls exponentially with depth y, the
absorption is concentrated at the top layers. Typically in the tropics the
surface temperature values are in excess of 25 °C, while 1 km below the
temperature is about 10 °C. Contrary to the usual cooking pot situation of
heat supplied from the bottom surface, the warmer (and hence lighter) waters
at the top means that there are no thermal convection currents. Due to the
very low temperature gradients, heat transfer by conduction is too low to
cause any significant change to the scenario either. So with neither of the
major mechanisms of heat transfer operating, the top layers remain hot and
the lower layers remain cold. Thus it is like an essentially infinite heat
source and an essentially infinite heat sink between a separation of about
1000 m that has been set up naturally for us to run heat engines. This
temperature difference varies with latitude and season, with the maximum at
the tropical, subtropical and equatorial waters. Hence in general tropics are
the best choice for setting up OTEC systems.
The open/Claude cycle
In this scheme, warm surface water at around 27 °C is
admitted into an evaporator in which the pressure is maintained at a value
slightly below the saturation pressure.
Image:Otec oc schematic.jpg
Water entering the evaporator is therefore superheated.
Where Hf is enthalpy of liquid water at the inlet
This temporarily superheated water undergoes volume boiling
as opposed to pool boiling in conventional boilers where the heating surface
is in contact. Thus the water partially flashes to steam with a two phase
equilibrium prevailing. Suppose that the pressure inside the evaporator is
maintained at the saturation pressure of water at T2. This process being iso-enthalpic,
Here, x2 is the fraction of water by mass that has
vaporized. The warm water mass flow rate per unit turbine mass flow rate is
The low pressure in the evaporator is maintained by a
vacuum pump that also removes the dissolved non condensable gases from the
evaporator. The evaporator now contains a mixture of water and steam of very
low quality. The steam is separated from the water as saturated vapour. The
remaining water is saturated and is discharged back to the ocean in the open
cycle. The steam we have extracted in the process is a very low pressure,
very high specific volume working fluid. It expands in a special low pressure
Here, Hg corresponds to T2. For an ideal adiabatic
The above equation corresponds to the temperature at the
exhaust of the turbine, T5. x5,s is the mass fraction of vapour at point 5.
The enthalpy at T5 is,
This enthalpy is lower. The adiabatic reversible turbine
work = H3-H5,s .
Actual turbine work WT = (H3-H5,s) × polytropic efficiency
The condenser temperature and pressure are lower. Since
the turbine exhaust will be discharged back into the ocean anyway, a direct
contact condenser is used. Thus the exhaust is mixed with cold water from the
deep cold water pipe which results in a near saturated water. That water is
now discharged back to the ocean.
H6=Hf, at T5. T7 is the temperature of the exhaust mixed
with cold sea water, as the vapour content now is negligible,
There are the temperature differences between stages. One
between warm surface water and working steam, one between exhaust steam and
cooling water and one between cooling water reaching the condenser and deep
water. These represent external irreversibilities that reduce the overall
The cold water flow rate per unit turbine mass flow rate,
Turbine mass flow rate,
Warm water mass flow rate,
Cold water mass flow rate
The closed/Anderson cycle
Developed starting in the 1960s by J. Hilbert Anderson of Sea Solar Power, Inc. In this cycle, QH is the heat transferred in the evaporator from the warm sea water to the working fluid. The working fluid exits from the evaporator as a gas near its dew point.
The high-pressure, high-temperature gas then is expanded
in the turbine to yield turbine work, WT. The working fluid is slightly
superheated at the turbine exit and the turbine typically has an efficiency
of 90% based on reversible, adiabatic expansion.
From the turbine exit, the working fluid enters the condenser where it rejects heat, -QC, to the cold sea water. The condensate is then compressed to the highest pressure in the cycle, requiring condensate pump work, WC. Thus, the Anderson closed cycle is a Rankine-type cycle similar to the conventional power plant steam cycle except that in the Anderson cycle the working fluid is never superheated more than a few degrees Fahrenheit. It is realized that owing to viscous effects there must be working fluid pressure drops in both the evaporator and the condenser. These pressure drops, which are dependent on the types of heat exchangers used, must be considered in final design calculations but are ignored here to simplify the analysis. Thus, the parasitic condensate pump work, WC, computed here will be lower than if the heat exchanger pressure drops were included. The major additional parasitic energy requirements in the OTEC plant are the cold water pump work, WCT, and the warm water pump work, WHT. Denoting all other parasitic energy requirements by WA, the net work from the OTEC plant, WNP is
The thermodynamic cycle undergone by the working fluid can be analyzed without detailed consideration of the parasitic energy requirements. From the first law of thermodynamics, the energy balance for the working fluid as the system is
where WN = WT + WC is the net work for the thermodynamic cycle. For the special idealized case in which there is no working fluid pressure drop in the heat exchangers,and so that the net thermodynamic cycle work becomes
Subcooled liquid enters the evaporator. Due to the heat exchange with warm sea water, evaporation takes place and usually superheated vapor leaves the evaporator. This vapor drives the turbine and 2-phase mixture enters the condenser. Usually, the subcooled liquid leaves the condenser and finally, this liquid is pumped to the evaporator completing a cycle.
Various fluids have been proposed over the past decades to be used in closed OTEC cycle. A popular choice is ammonia, which has superior transport properties, easy availability, and low cost. Ammonia, however, is toxic and flammable. Fluorinated carbons such as CFCs and HCFCs would have been a better choice had it not been for their contribution to ozone layer depletion. Hydrocarbons too are good candidates. But they are highly flammable. The power plant size is dependent upon the vapor pressure of the working fluid. For fluids with high vapor pressure, the size of the turbine and heat exchangers decreases while the wall thickness of the pipe and heat exchangers should increase to endure high pressure especially on the evaporator side.
A very important technical issue pertaining to the Claude cycle is the performance of direct contact heat exchangers operating at typical OTEC boundary conditions. Many early Claude cycle designs used a surface condenser since their performance is well understood. However direct contact condensers offer significant disadvantages. As the warm sea water rises in the intake pipes, the pressure decreases to the point where gas begins to evolve. If a significant amount of gas comes out of the solution, designing a gas trap before the direct contact heat exchangers may be justified. Experiments simulating conditions in the warm water intake pipe indicated about 30% of the dissolved gas evolve in the top 8.5 m of the tube. The tradeoff between pre-deaeration of the sea water and expulsion of all the non-condensable gases from the condenser is dependent on the gas evolution dynamics, deaerator efficiency, head loss, vent compressor efficiency and parasitic power. Experimental results have indicated that vertical spout condensers performs some 30% better than the falling jet types.
The evaporator, turbine, and condenser operate in partial vacuum ranging from 3 % to 1 % atmospheric pressure. This poses a number of practical concerns that must be addressed. First, the system must be carefully sealed to prevent in-leakage of atmospheric air that can severely degrade or shut down operation. Second, the specific volume of the low-pressure steam is very large compared to that of the pressurized working fluid used in the case of a closed cycle OTEC. This means that components must have large flow areas to ensure that steam velocities do not attain excessively high values.
Parasitic power consumption by exhaust compressor
An approach for reducing the exhaust compressor parasitic power is as follows. After most of the steam has been condensed by spout condensers, the non condensable gas steam mixture is passed through a counter current region which increases the gas-steam reaction by a factor of 5. The result is an 80% reduction in the exhaust pumping power requirements.
Energy from temperature difference between cold air and warm water
In winter in coastal Arctic locations, the seawater temperature can be 40 degrees Celsius warmer than the local air temperature. Technologies based on closed-cycle OTEC systems could exploit this temperature difference. The lack of the need for long pipes to extract deep seawater might make a system based on this concept less expensive than OTEC.
Deep lake water cooling
Mist flow ocean thermal energy process - Power is generated using the temperature difference between the water at the surface of a large body of water whose temperature might be in the vicinity of 25° C. or 77° F., and water at considerable depth in the body of water whose temperature might be in the order of about 5° C. or 41° F. A floating structure is provided which extends in the order of 50 meters below the surface of the water, and input water is initially filtered and deaerated, and then drops for most of the height of the submerged structure before driving a conventional hydraulic turbine.
Ocean thermal energy conversion - From Wikipedia, the free encyclopedia - Ocean thermal energy conversion (OTEC) is a method for generating electricity which uses the temperature difference that exists between deep and shallow waters to run a heat engine. As with any heat engine, the greatest efficiency and power is produced with the largest temperature difference.
Ocean Thermal Energy Conversion - A process called Ocean Thermal Energy Conversion (OTEC) uses the heat energy stored in the Earth's oceans to generate electricity.
Ocean energy and minerals: resources for the future - The United States has sovereign rights over the exploration and development of non-living resources, including oil and gas, found in the seabed and subsoil of the continental shelf, which is defined to extend to 200 nautical miles from its coast or, where the continental margin extends beyond that limit, to the outer edge of the geological continental margin. This claim, made under the Truman Declaration of 1945, is confirmed in the 1982 Convention on the Law of the Sea. Currently, about 27 percent of the natural gas and 18 percent of the oil produced in the United States is from the federally managed Outer Continental Shelf (OCS).
Video -- Renewable Energy from the Deep Ocean - Ocean Thermal Energy Conversion (OTEC) process and an ideal location in Puerto rico (en Espanol)
OTEC - Ocean Energy for Sustainable Townsville - Townsville is perfectly positioned to become the leading international centre for the research, development and commercialisation of the ocean based renewable energy technology, OTEC.
New wave Energy Association Gets Funding, Sets Priorities - The Oregon Wave Energy Trust, or OWET, has received the first part of its $4.2 million budget approved by the 2007 state legislature, and is moving ahead with plans and activities to make Oregon a global leader in this emerging industry.
Wave Energy Companies Selected for UK Ocean Test Site - The South West of England Regional Development Agency (RDA) has just named three development partners for the first phase of the proposed GBP 15 million [USD $26 million] Wave Hub project, which will be the world's first wave energy farm.
Ocean Wave Energy Company - OWECO Ocean Wave Energy Company mission is carbon combustion and nuclear processing reduction, water climate management, and hydrogen/oxygen cycling through utility-scale wave energy conversion to electricity.
Ocean Energy - Ocean energy’s location as an Irish based company is perfectly suited to its research and development work as Ireland is centrally located in one of the world’s most favorable climates for wave energy power.
Tidal Energy - The Tidal power stations can be divided into three types according to its operation method and requirement upon equipment, which are single-reservoir and single-direction type, single-reservoir, and two-direction type and the two-reservoir and single-direction type.
Energy Resources: Wave power - The problem is that it's not easy to harness this energy and convert it into electricity in large amounts. Thus, wave power stations are rare.
Capstone Design Projects for Harnessing Ocean Energy - Development of ocean-related renewable energy technologies continues to gain more interest, especially as renewable energy types and contributions increase. While there are many advantages to harnessing energy from ocean sources, there are also many challenges, including environmental, structural, regulatory and socio-political issues.
Wave Energy & The PSP - Wave energy is, in effect, a stored and concentrated form of solar energy, since the winds that produce waves are caused by pressure differences in the atmosphere arising from solar radiation. Waves transmit this energy thousands of miles with minimal loss. Wave size is a function of the wind's speed, how long it blows, and fetch, which is the distance over which it blows.
Ocean Wave Energy - Ocean wave energy is captured directly from surface waves or from pressure fluctuations below the surface.
Harnessing ocean energy - The world’s oceans could provide a limitless source of energy, according to Indian chemist Madanjeet Singh, an international authority on the subject.
Welcome to the Foundation for Ocean Renewable - The Foundation for Ocean Renewables (FOR) is a non-profit, non-government organization founded to promote energy technologies from clean, renewable ocean resources.
Delivery of Low Cost Wave Energy - C-Wave Limited is a renewable power company and the developer of an innovative wave energy technology. C-Wave offshore power systems can provide genuinely green electricity at a cost that is competitive with traditional generating technologies and established renewable energy providers such as wind power.
Intro to Wave Energy - The Japanese “Mighty Whale” has an air channel to capture wave energy. ..... While the energy is free, it costs money to collect it. ...
- Shoreline WEC: are fixed to a suitable location on a coast-line and
generate electricity by the conversion of air trapped in a chamber which is
released through a small aperture to drive a turbine. It was envisaged that
the converter could be fixed to the legs of the installation, at sea-level,
to generate electricity from the continual movement of the waves. Further to
the review of lateral loading of offshore installations it was established
that the lateral and vertical forces could jeopardise the stability of the
offshore installation and the concept was deemed unfeasible.
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