A technique for converting difference in ocean temperature between warm surface waters and cold deeper waters into a usable energy resource. A typical OTEC plant might use tropical ocean surface water to evaporate ammonia, which would cause a turbine to generate electricity. Cold deep water would be used to condense the ammonia to start the cycle again. Part of the electrical power is used to operate pumps to bring the cold water to the ocean surface.
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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.
The concept of a heat engine is very common in engineering, and nearly all energy utilized by humans uses it in some form. 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.
History of OTEC
Even though it sounds technologically sophisticated, OTEC technology is not new.
In 1935, Claude constructed another plant, this time aboard a 10,000-ton cargo vessel moored off the coast of Brazil. (Net power is the amount of power generated after subtracting power needed to run the system.)
In 1956, French scientists designed another 3 MW OTEC plant for Abidjan, Côte d'Ivoire.
In 1962, J Hilbert Anderson and James H Anderson, Jr start designing a cycle to accomplish what Claude had not.
The USA government became involved in OTEC research in 1974, when the Natural Energy Laboratory of Hawaii Authority was established at Keahole Pointe on the Kona coast of Hawaii.
The Japanese government also continues to fund research and development in OTEC technology.
India piloted a 1 MW floating OTEC plant near Tamil Nadu.
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. 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.
Depending on the location
Land based plant Shelf based plant Floating plant Submerged plant ( conceptual )Depending on the cycle used
Open cycle Closed cycle Hybrid cycleThis cold seawater is an integral part of each of the three types of OTEC systems: closed-cycle, open-cycle, and hybrid.
Closed-cycle
Closed-cycle systems use fluid with a low boiling point, such as ammonia, to rotate a turbine to generate electricity.
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.
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.
Open-cycle
Open-cycle OTEC uses the tropical oceans' warm surface water to make electricity.
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.
Hybrid
Hybrid systems combine the features of both the closed-cycle and open-cycle systems.
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.
Other related technologies
OTEC has important benefits other than power production.
Air conditioning
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.
Chilled-soil agriculture
OTEC technology also supports chilled-soil agriculture. 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
Aquaculture is perhaps the most well-known byproduct of OTEC.
Desalination
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.
Mineral extraction
OTEC may one day provide a means to mine ocean water for 57 trace elements. But with OTEC plants already pumping the water, the only remaining economic challenge is to reduce the cost of the extraction process.
Research
OTEC rigs offer a permanent platform for long-term oceanographic research, which now relies on ships unable to remain on station for extended periods.
Tourism
OTEC rigs offer a permanent platform for recreational divers wishing to experience deep water or reef dives.
Political Concerns
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).
Cost and Economics
For OTEC to be viable as a power source, it must either gain political favor (ie.
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.
See also .
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 × 10 MJ/yr. (taking an average clearness index of 0.5)
Only some 15% of this energy is absorbed.
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 for very salty water.
Since the intensity falls exponentially with depth y, the absorption is concentrated at the top layers.
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 temperature, T1.
This temporarily superheated water undergoes volume boiling as opposed to pool boiling in conventional boilers where the heating surface is in contact. 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 1/x2.
The low pressure in the evaporator is maintained by a vacuum pump that also removes the dissolved non condensable gases from the evaporator.
Here, Hg corresponds to T2. For an ideal adiabatic reversible turbine,
The above equation corresponds to the temperature at the exhaust of the turbine, T5.
The enthalpy at T5 is,
This enthalpy is lower.
Actual turbine work WT = (H3-H5,s) × polytropic efficiency
The condenser temperature and pressure are lower. Thus the exhaust is mixed with cold water from the deep cold water pipe which results in a near saturated water.
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.
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. In this cycle, QH is the heat transferred in the evaporator from the warm sea water to the working fluid.
The high-pressure, high-temperature gas then is expanded in the turbine to yield turbine work, WT.
From the turbine exit, the working fluid enters the condenser where it rejects heat, -QC, to the cold sea water. 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. 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.
Working fluids
Various fluids have been proposed over the past decades to be used in closed OTEC cycle. 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.
Technical difficulties
Degradation of heat exchanger performance by dissolved gases
A very important technical issue pertaining to the Claude cycle is the performance of direct contact heat exchangers operating at typical OTEC boundary conditions.
Improper sealing
The evaporator, turbine, and condenser operate in partial vacuum ranging from 3 % to 1 % atmospheric pressure. 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.
Parasitic power consumption by exhaust compressor
An approach for reducing the exhaust compressor parasitic power is as follows.
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.
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