Ocean thermal energy conversion

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.

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. But it was d'Arsonval's student, Georges Claude who actually built the first OTEC plant. Claude built his 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 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 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 Laboratory has become one of the world's leading test facilities for OTEC technology.

The Japanese government 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.

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 cycle

This 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. 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

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. 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

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

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.

Chilled-soil agriculture

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

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 (kelp), a health food supplement, also can be cultivated in the deep-ocean water.

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. This is equivalent to 4.3 million litres or 1.13 million (U.S.) gallons.

Mineral extraction

OTEC may one day provide a means to mine ocean water for 57 trace elements. Most economic analyses have suggested that mining the ocean for 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.

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. They operate in deep water rarely accessed for research. The rigs also become artificial reefs.

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). 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 support.

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 ^*.

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 0.5)

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,

-\frac{dI(y)}{dy}=\mu I

Where, y is the depth of water, I is intensity and μ is the absorption coefficient. Solving the above differential equation,

I(y)=I_{0}\exp(-\mu y) \,

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.

Water entering the evaporator is therefore superheated.

H_{1}=H_{f} \,

Where H_f 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. 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,

H_{2}=H_{1}=H_{f}+x_{2}H_{fg} \,

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. 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 turbine.

H_{3}=H_{g} \,

Here, H_g corresponds to T₂. For an ideal adiabatic reversible turbine,

s_{5,s}=s_{3}=s_{f}+x_{5,s}s_{fg} \,

The above equation corresponds to the temperature at the exhaust of the turbine, T₅. x_5,s is the mass fraction of vapour at point 5.

The enthalpy at T₅ is,

H_{5,s}=H_{f}+x_{5,s}H_{fg} \,

This enthalpy is lower. The adiabatic reversible turbine work = H₃-H_5,s.

Actual turbine work W_T = (H₃-H_5,s) × polytropic efficiency

H_{5}=H_{3}-\ \mathrm{actual}\ \mathrm{work}

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=H_f, at T₅. T₇ is the temperature of the exhaust mixed with cold sea water, as the vapour content now is negligible,

H_{7}\approx H_{f}\,\  at\  T_{7} \,

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 temperature difference.

The cold water flow rate per unit turbine mass flow rate,

\dot{m_{c}=\frac{H_{5}-\  H_{6}}{H_{6}-\  H_{7}}} \,

Turbine mass flow rate, $\dot{M_{T}}=\frac{\mathrm{turbine}\ \mathrm{work}\ \mathrm{required}}{W_{T}}$

Warm water mass flow rate, $\dot{M_{w}}=\dot{M_{T}\dot{m_{w}}} \,$

Cold water mass flow rate $\dot{\dot{M_{c}}=\dot{M_{T}m_{C}}} \,$

The closed/Anderson cycle

Developed starting in the 1960s by J. Hilbert Anderson of Sea Solar Power, Inc. In this cycle, Q_H 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, W_T. 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, -Q_C, to the cold sea water. The condensate is then compressed to the highest pressure in the cycle, requiring condensate pump work, W_C. 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, W_C, 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, W_CT, and the warm water pump work, W_HT. Denoting all other parasitic energy requirements by W_A, the net work from the OTEC plant, W_NP is

W_{NP}=W_{T}+w_{C}+W_{CT}+W_{HT}+W_{A} \,

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

W_{N}=Q_{H}+Q_{C} \,

where W_N = W_T + W_C 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,

Q_{H}=\int_{H}T_{H}ds \,

and

Q_{C}=\int_{C}T_{C}ds \,

so that the net thermodynamic cycle work becomes

W_{N}=\int_{H}T_{H}ds+\int_{C}T_{C}ds \,

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.

Working fluids

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.

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. Many early Claude cycle designs used a surface condenser since their performance is well understood. However direct contact condensers offer significant advantages. 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.

Improper sealing

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.

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