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To Harness the Seas
WEB EXCLUSIVE
By Michael E. McCormick and R. Cengiz Ertekin
The ocean has been long recognized as an efficient stowage of solar energy. The portion of the
solar energy absorbed by the ocean is initially thermal in nature. We find that the surface waters
heat during the day and cool during the night. The air adjacent to the water surface is also heated
and cooled, and thermal currents occur in both the air and water. Hence, the absorbed radiant
solar energy is partially transformed to thermal energy and, then, to hydraulic and pneumatic
energies. Winds, the pneumatic energies, create waves on the water surface which further
disperse the energy. These energy transformations give rise to the fields of ocean thermal energy
conversion, ocean wave energy conversion, ocean current energy conversion, and offshore wind
energy conversion.
Another partial solar ocean energy form is the tide, which is a predictable phenomenon caused
by the gravitational attraction of the moon and the sun.
Solar energy also causes water evaporation which can result in saline bodies of water, or brines.
When brine is separated from fresh water by a semi-permeable membrane, a pressure called an
osmotic pressure occurs, which is a pressure gradient across the membrane. The osmotic
pressure can exceed 200 atmospheres if the high-saline brine is at or near saturation. For the case
of ocean salt water, the potential energy associated with the osmotic pressure is approximately
23×106 newton-meters per cubic meter of water. The exploitation of this energy is called salinity
gradient energy conversion. Compared to the other ocean-solar options, the exploitation of
salinity gradient energy conversion is still in its infancy. For a discussion of salinity gradient
energy conversion, see Jones and Finley (2003) among others.
We can classify any energy source as either concentrated or distributed. This classification is
rather important, since it relates to the physical space required for an energy conversion system.
At the extremes of these classifications are nuclear energy, which is concentrated, and solar
energy, which is distributed. None of the ocean energy sources discussed herein is truly
concentrated. The ocean thermal resource, which depends on the temperature difference between
the water at a great depth and water near the ocean surface, can be considered somewhat
concentrated.
Ocean and tidal currents are more on the distributed side. For example, if a water turbine
operates in a 2 m/s ocean current, the turbine diameter must be about 25 meters to capture 1
megawatt of hydrodynamic power.
Ocean waves and offshore winds are distributed systems. For a deep-water wave having a period
of 7.5 seconds and a wave height of 1.5 meters, the power per crest width is about 16.6 kW/m.
Hence, to have 1 MW of wave power available for conversion, the energy conversion system
would need to span 60 meters of crest. For a wind turbine operating in a 10 m/s uniform wind,
the turbine diameter would be about 140 meters to capture 1 MW of wind power, which is
impractical. The wind resource is best utilized by having a field of small wind turbines, since the
resource is distributed.
There are many threshold numbers that are used to determine whether or not an alternative
energy resource should be considered for exploitation. In the1970s, Robert Cohen of the U. S.
Department of Energy suggested that an alternative energy resource for the contiguous United
States was viable if the total amount of that alternative energy within the U. S. territory was
equal to a minimum of 0.5 percent of the total energy needs of the population. In the 1970s, an
average U. S. citizen required an electrical power of 1 kW, and an average U. S. household
required 5 kW. These numbers have varied slightly over the years.
Ocean Thermal Energy Conversion (OTEC): OTEC is the conversion of the thermal energy
caused by the temperature differences of the solar-heated warm surface waters of the ocean and
the cold deep ocean water. In every deep ocean, the temperature of the water at a depth of 1,000
meters is slightly greater than the freezing temperature of water (0oC). If the surface waters are at
least 18oC higher in temperature than the deep waters, then we have an ocean-thermal resource.
The ideal regions of the world for ocean thermal energy conversion are between 20 degrees north
latitude and 20 degrees south latitude. (Note: The center of Washington, D.C., is 38.9 degrees
north latitude.)
The average surface temperature in tropical oceans can rise above 30oC. The sites where this
occurs are most advantageous for OTEC, since the cut-off temperature difference 18oC between
the surface waters and the deep waters is well exceeded.
The operation of an OTEC system is based on a Rankine thermal cycle. That is, as illustrated in
Fig. 1, a working fluid in a closed system evaporates when passed through a warm-water
evaporator and becomes a high-pressure gas. The warm surface water passing through the coils
of the evaporator cause the change of state of the fluid.
When the gas expands, it flows through a gas turbine which drives an electrical generator. The
gas then passes through a cold-water condenser, returning to the liquid state. The gas is cooled
by the up-welled cold water passing through the condenser coils. An ideal working fluid for the
closed cycle is one with a low boiling temperature and a high condensation temperature. A
candidate fluid is NH3 (ammonia). A higher temperature difference between the surface and deep
waters increases the efficiency of the system.
Although the ideal latitude band mentioned above is well away from the contiguous United
States, we can take advantage of OTEC by creating a product other than electricity. For example,
if we build a floating aluminum plant at the OTEC site, and make the aluminum from imported
bauxite, then we have produced an energy-intensive product, relieving the U. S. power grid from
that task. We note that a recycled aluminum can requires 5 percent of the energy required to
make the same aluminum can from bauxite.
Because electricity from an OTEC power plant is rather expensive, the exploitation of this
energy form has received only mild interest when compared to the other alternative energy
resources (alternative, that is, to the fossil-fuel resource). If the length of the expensive coldwater pipe in Fig. 2 can be kept relatively short, then the economics of OTEC significantly
improve. A short cold-water pipe will result in a great savings in the cost of the materialintensive and labor-intensive OTEC structure. An excellent reference for OTEC is the book by
Avery and Wu (1994).
Fig. 2
Ocean Wave Energy Conversion: Wave energy conversion devices can be classified according to
their size and orientation as point absorbers, attenuators, and terminators, which are illustrated
in Fig. 3. Point absorbers can be floating or submerged floating bodies or oscillating water
columns (OWC). The attenuators are normally floating compliant or articulated bodies. The
terminators are aligned to the wave front, and absorb and reflect the incident waves. There is no
leeward transmission associated with terminators. The energy flow lines in Fig. 3 represent the
focusing of energy on the body due to the diffraction phenomenon. This focusing affect is
maximized in monochromatic (single frequency) waves when the body is tuned to the wave.
Fig. 3
The conversion of the energy of ocean waves into usable energy forms is normally a multi-phase
process. The first phase is capturing the energy and transforming it into mechanical, hydraulic, or
pneumatic energy. The most popular wave energy conversion technique is the heaving body,
either floating or submerged. There are several methods for the power takeoff from the waveinduced motions. These include the linear electric generator, gear-driven rotational generators,
and hydro-turbine electric generators. Many advances in linear electrical generator technology
have been made in recent years; however, the synchronous rotational electrical generator is
readily available and is considered to be robust for wave energy conversion.
There are a number of prototype wave energy conversion systems that are now deployed,
including a few point absorber systems. One such point absorber is PowerBuoy of Ocean Power
Technology which has been deployed in Hawaiian waters. That PowerBuoy system is rated at 20
kW.
Another point absorber, called WaveBob, has been deployed in Galway Bay off the coast of
Ireland. According to its manufacturer, WaveBob Ltd., by using a unique control system, the
floating system can be continually tuned to the sea. The power takeoff is a result of the out-ofphase heaving motions of the float and a fully submerged inertial body. The system shown in
Fig. 4 is actually a scale model. The prototype diameter is to be 20 meters, and is designed to
operate in seas having an average power intensity of about 40 kW/m. The design life of the
WaveBob is 25 years. The operation of WaveBob is illustrated in Fig. 5.
One of the most promising wave-powered electrical generating system is the Pelamis of Ocean
Power Delivery Ltd. in Scotland. The Pelamis, shown in Fig. 6, is an attenuator system, an
articulated-body system with an internal closed hydraulic system that is part of the power takeoff
subsystem. According to Ocean Power Delivery, the length of the Pelamis is 180 meters,
comprising four components, each 45 meters long. Three Pelamis units, each rated at 750 kW,
have been built for deployment 5 km off the coast of Portugal.
The design power intensity (power per crest width) of the sea is 40 kW/m. The projected cost of
the delivered energy is about 30 cents per kilowatt-hour. It is expected that the energy costs will
be drastically reduced in the future. The cost is already very close to the kilowatt-hour cost of
electricity in the State of Hawaii in 2008.
The European Union has sponsored two oscillating water column (OWC) projects. The first of
these is on the island of Islay, off the coast of Scotland, shown in Fig. 7. The second is on Pico
Island in the Azores. These systems are coastal OWCs. The Islay system, called the Limpet, is
rated at 500 kW, where the wave intensity varies from 15 kW/m to 25 kW/m. The Pico OWC
plant is also rated at 500 kW.
The operation of a coastal OWC can be seen in the sketch in Fig. 8, from McCormick and
Kraemer (2002). In that sketch, there is free communication between the sea and the internal
water column. The incident waves cause the water column to rise and fall, respectively driving
and drawing the air above the water column through a bi-directional air turbine. The turbine
drives a rotational electrical generator which, in turn, is connected to a transformer and then to
the local power grid. The Limpet and the Pico OWCs are terminator systems.
For a detailed discussion on the basic wave energy conversion techniques, see McCormick
(2007).
Tidal Energy Conversion: We can classify the tidal phenomena relating to energy conversion
as either quasi-static or dynamic. Tides, being a wave form, can resonate with confined
waterways. For wind waves, the phenomenon occurs in harbors, and is referred to as a “harbor
resonance.” In North American waters, the Bay of Fundy is one of the best known for the tidal
resonance phenomenon. Tidal resonance can be classified as quasi-static phenomenon, since the
changes in the tide-induced maximum water depth occurs over several hours. The average tidal
range in the Bay of Fundy is approximately 10 meters. The maximum pressure available for
energy conversion is slightly greater than one atmosphere.
Tide-induced hydrostatic pressure differences induce currents in some waterways. An example
of this phenomenon is in the East River at New York City, where the current varies up to
approximately 2.3 meters per second, depending on location.
The dynamic pressure corresponding to the velocity can be exploited to provide usable energy.
An excellent summary of the dynamic tidal energy locations is found in the Electric Power
Research Institute (EPRI) Report TP-008-NA, published in June of 2006. See Bedard (2006).
The tidal energy resource is site-specific. Although there are numerous low-capacity tidal power
plants along the coastal waters of the Chinese mainland, there are few high-capacity plants in
existence in the rest of the world. The French built a tidal power plant in the Rance estuary at St.
Malo. That power plant delivers an average of 240 MW of power (240,000 kW) at a cost of
about 1.8 cents per kilowatt-hour, which is quite inexpensive. The mean tidal range at the San
Malo site is 8.55 meters. If we were fortunate to have the power plant on the U. S. shores, we
could supply electricity to one-quarter of a million U.S. citizens.
In Canada, the Annapolis Royal Generating Station is located on the eastern end of the Bay of
Fundy. That tidal conversion facility consists of a dam and an 18 MW power house on the
Annapolis River at Annapolis Royal, Nova Scotia.
At the west end of the Bay of Fundy, President Franklin. D. Roosevelt in the late 1930s funded
the construction of a tidal power plant near his retreat at Campobello Island. The island is at the
Maine-Canadian border. Because of World War II, the project was never finished. In the 1980s,
the native-American Passamaquoddy tribe was given U.S. funds to build a tidal power plant in
the estuary of the U.S. mainland, across the straight from Campobello Island. In 2002, turbines
were delivered. At the site, the tidal range is 7 meters. The Passamaquoddy tidal power plant is
expected to supply approximately 29 MW when in full operation. Based on the 1 kW need per
person, this would satisfy a population of approximately 29,000 people. Also in the United States
is Cook Inlet in Alaska. That site has the potential of producing up to 18,000 MW of power
which, at 1 kW per U.S. citizen, is enough power for 18 million people in the contiguous United
States.
For static (resonance) tidal energy, some type of barrage is required, which is extremely
expensive because of both material and labor. Furthermore, the turbines must be bi-directional
and of high capacity. They are a large-cost item. Overall, there are few locations in the world
where tidal energy conversion would be cost-effective. As demonstrated by the plant at St. Malo,
France, the amortized cost of electricity is relatively small because the static tidal power systems
are robust and have a long operational life. For thorough discussions of static tidal energy, the
book by Charlier, and Justice (1993) and the paper by Gorlov (2001) are recommended.
More recently, attention has been focused on the dynamics of the tides in the form of tidal
currents. To convert the hydrostatic tidal energy into electricity, tidal barrages are created. For
example, in the East River at New York City, the Verdant Power Company has installed
submerged water mills. According to Verdant Power, six turbines in the East River will generate
approximately 10 megawatts. On the other side of the Atlantic, Marine Current Turbines Ltd.
(MCT) has installed a 300 kW plant in the English Channel off Cornwall. A field of the MCT
water mills would resemble that shown in Fig. 9.
Again, the tidal energy resource is both reliable and predictable. With the escalating costs of oil
and natural gas, this form of ocean energy will become a viable resource in the near future.
Ocean Current Energy Conversion: Ocean currents collectively form an energy resource that
is partly and indirectly caused by the sun. The ocean currents result from the winds and the
Earth’s rotation. A part of the Gulf Stream passes through the Florida Strait, where it is known as
the Florida Current. The surface waters of the Florida Current travel at speeds up to 1.3 m/s. The
current’s speed diminishes rapidly with depth. If the goal is to supply electricity to coastal
Florida by exploiting the Florida Current, then the system should be operating within about 10
meters of the free surface.
The turbine system for ocean current energy conversion is quite different from that in the
dynamic tidal energy conversion. The turbine housing must be buoyant and tethered to the sea
bed. For most of the systems that have been conceptualized, the power cable would be attached
to the mooring line. Earlier in the paper, it is stated that a tidal water turbine operating in a 2 m/s
ocean current would need a 25-meter diameter to capture 1 MW of hydrodynamic power. For a 1
MW power from a 1.3 m/s ocean current, the ideal turbine diameter would be about 59 meters.
Relative Potentials of Ocean Energy Resources: According to Tester et al (2005), the available
and practical power values of these are as follows:
Resource
Available Power (gigawatts) Practical (gigawatts)
Waves
2,700
500
Currents
5,000
50
Ocean Thermal 200,000
40
Tides
2,500
20
Salinity Gradients 1,000,000
?
These resources are ranked according to the values in the last column. In the Table, the
“available power” is the potential power at any time. The “practical power” is that which can be
exploited. Hopefully, one or more of the ocean energy options will achieve this potential.
References:
Avery, W. H. And C. Wu (1994), Renewable Energy from the Ocean: A Guide to
OTEC, Oxford University Press, Oxford, U.K.
Bedard, R.; M. Previsic; B. Polagye; G. Hagerman; and A. Casavant (2006), “North
American Tidal In-Stream Energy Conversion Technology Feasibility Study,”
Electric Power Research Institute, Report TP-008-NA, Palo Alto, Calif., June 11.
Charlier, R. H., and J. R. Justice (1993), Ocean Energies, Elsevier Science, Oxford, U.K.
Gorlov, A. M. (2001), “Tidal Energy,” in Tidal Energy, Academic Press, pp. 2955-2960.
Jones, A. T., and W. Finley (2003), “Recent Developments in Salinity Gradient Power,” in the
Proceedings, Oceans 2003 (IEEE), pp. 2284-2287.
McCormick, M. E. (2007), Ocean Wave Energy Conversion, Dover Publications, Mineola,
N.Y.
McCormick, M. E., and D. R. B. Kraemer (2002), “Ocean Wave Utilization,” Marine
Technology Society Journal, Winter Issue, Vol. 36, No. 4, December, 2002, pp.
52-58.
Tester, J.W.; E. M. Drake; M. W. Golay; M. J. Driscoll; and W. A. Peters (2005), Sustainable
Energy —Choosing Among Options, MIT Press, Cambridge, Mass.