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AIPG Position Statement - Domestic Energy: Ocean Energy Alternatives

AIPG Position Statement - Domestic Energy:
Ocean Energy Alternatives
(October 2009)

Dr. James F. Howard, CPG -2536

Energy resources associated with the oceans are normally divided into three categories: Tidal Energy, Wave Energy and Physico-chemical Energy sources. 

Tidal Energy Sources

All coastal areas consistently experience two high and two low tides over a period of slightly greater than 24 hours.  The methods of converting the energy associated with these tidal fluctuations can be classified into two different approaches, Barrage (dam) technology, and Tidal Fence/Tidal Turbines, each using slightly different methodology.

Barrage or dam Technology utilizes traditional permeable barriers to allow entry of flood tide water into holding basins then releasing it under controlled conditions to operate traditional turbines producing electricity. Tidal fluctuations have high and low tides differentials of at least seven meters (~23 feet) to implement this technology. Existing facilities using this methodology are the La Rance Estuary, France (240 Megawatt equivalent (MWe) production capacity; Annapolis River, Canada (20 MWe) and several smaller facilities in China (total 5 MWe).  Three other sites in Wales are presently in development for a total of approximately 500 MWe capacity.

It is estimated that about 40 sites on Earth have tidal ranges great enough to be utilized for Tidal Power generation using this technology.  Major locations are under study in the Severn River in western England (est. potential production of 12 Gigawatts), the Bay of Fundy, Canada, Cook Inlet, Alaska and the White Sea in Russia.

Implementation of Barrage Technology requires a tidal range of at least 7 meters; topographic configuration allowing construction of semi-permeable structures allowing containment and controlled release of water on a diurnal cycle; a facility for storage of energy during the low-tide portions of the cycle to permit flow of electricity on a continuing, 24-hour basis; a distribution network capable of delivering the electricity produced by the facility must be constructed to provide the power to points of need; and all structures must be able to withstand high-impact events of storm surges or waves.

Tidal Fence/Turbine Technology both utilize ocean currents to produce electrical current.  Since seawater is much denser than air, ocean currents can produce significantly more energy than wind currents of the same velocities.
Tidal Fences resemble giant turnstiles and would be best located across channels between small islands or straits between the mainland and an island. The turnstiles spin via tidal currents typical of these topographically constrained areas. Some of these currents run at 5–8 knots (5.6–9 miles per hour) and generate as much energy as winds of much higher velocity because seawater has a much higher density than air, ocean currents carry significantly more energy than air currents (wind).

Tidal turbines resemble wind turbines. They are arrayed underwater in rows, similar to some wind farms. In currents of 5.6–9 miles per hour, a 15-meter (49.2-feet) diameter tidal turbine can generate as much energy as a 60-meter (197-feet) diameter wind turbine. Ideal locations for tidal turbine farms are close to shore in water depths of 20–30 meters (65.5–98.5 feet) with constant current flow.

Tidal Power plants using Tidal Fence or Tidal Turbine technology have the following requirements:

    1. The Tidal Fence turnstiles and Tidal Turbines function best where coastal currents attain velocities ranging between 3.6 and 4.9 knots (6.45 to 8.0 kilometers per hour ((kph)) and due to structural stresses on the turbine structures 2 and 2.5 kph, respectively. These velocities require location at sites where tidal currents are topographically restricted allowing diurnal channel flow, which can attain the desired flow velocities.  Tidal Fences are best located in tidal passes while Tidal Turbines would ideally be located off-shore in water depths of 20 to 30 meters due to the potential optimal radius (15 meters) of the turbines and constancy of  adequate current velocity e.g. Gulf Stream off North America or the Agulhas off South Africa.
    2.  All methods of converting Tidal Energy to electrical energy will require the development and use of corrosion-resistant materials, e.g. stainless steel, high-strength plastics or other alloys to survive the marine environment.
    3. All structures must be designed to withstand the high-impact of storm surges or waves.
Economics

Barrage technology economics are controlled by the ratio between the length of the barrage in meters to the annual energy production in kilowatt hours (the Gibrat ratio).     The smaller the Gibrat ratio, the more favorable a site will be for development as a power source.  Ratio of the La Rance estuary site is 0.36 and the Gibrat ratio for a proposed tidal barrage site in the Bay of Fundy is 0.92.  Actual electricity production costs for the La Rance plant after payoff of the original construction cost is now 0.2 Euros/kwh.  The facility averages approximately 68 MWh on a 24 hour basis and its production comprises about 1% of the electrical power used in France.

Tidal Fence and the Tidal Turbine technology economics are highly dependent on the velocity of the current or stream in which the facility is located.  No full scale commercial installations are yet in operation although numerous facilities ranging up to 10 MWe are in various stages of planning and pilot testing.  Estimates by the Electric Power Research Institute, (Offshore Wave Power Feasibility Demonstration Project, January, 2005) estimate 2006 levelized cost at 8-12 cents/kWh for Tidal Energy generation off the Oregon and Washington Coasts with potential for dropping to 4-6 cents/kwh with broader usage of the existing technology. Preliminary investigations indicate that a 1-km line of permanent turbines in the Agulhas Current could generate 100 MWh of power per day.

B.  Wave Energy Resources

Basic Discussion

Waves are generated by wind passing over the sea and as long as the waves propagate slower than the wind speed just above the waves, there is an net energy transfer from the wind to the most energetic waves. Both air pressure differences between the upwind and the lee side of a wave crest, as well as friction on the water surface by the wind shear stress causes the growth of the waves. Wave height increases with increases in wind velocity, time duration of the wind blowing, fetch (the distance of open water that the wind has blown over), and water depth (in the case of shallow water effects, for water depths less than half the wavelength, Dean, R.G. and Dalrymple, R.A. (1991).


Technology status/requirements

Wave power methodologies are generally categorized by the method used to capture the energy of the waves. Method types are point absorber or buoy; surfacing following or attenuator; terminator, lining perpendicular to wave propagation; oscillating water column; and overtopping. They can be located either on shoreline, nearshore and offshore. They can also be categorized by location and power take-off system. Types of power take-off include: hydraulic ram, elastomeric hose pump, pump-to-shore, hydroelectric turbine, air turbine, and linear electrical generator. Some of these designs incorporate parabolic reflectors as a means of increasing the wave energy at the point of capture.  Some examples of different wave power systems include Pelamis Wave Energy Converter, Wave Dragon Energy Converter, PowerBuoy, AquaBuoy, SeaRaser, and CETO Wave Energy Converter.

Global Wave Energy Power Generation Potential

Global Wave Energy Power Generation Potential – Hagerman, Electric Power Research Institute, 2004

Locations with the most potential for wave power include the western seaboard of Europe, the northern coast of the UK and the Pacific coastlines of North and South America, Southern Africa, Australia and New Zealand. The north and south temperate zones have the best sites for capturing wave power since the prevailing westerlies in these zones blow strongest in winter.

Status of Wave Power Development

The world's first commercial wave farm opened in 2008 at the Aguçadora Wave Park near Póvoa de Varzim in Portugal using three Pelamis P-750 machines with a total installed capacity of 2.25MW. Future expansion plans will increase the installed capacity to 21MW.
A funded 3MW Wave Farm in Scotland and a Wave Hub off the north coast of Cornwall for a total of 24 Mwe to provide power for up to 7,500 households.  A pilot CETO wave farm of the coast of Western Australia is now ready for further development.

Reserves

Theoretical Deep Water Wave Power resources are estimated to be between 1 Terawatt (TW=1012 watts) and 10 TW. (Brooke, John, ed., 2003).  The usable world-wide resource has been estimated to be greater than 2 TW. (Thorpe, T., 1999) (Cruz, J., Gunnar, M. Barstow, S., and Mollison, D., 2008). 

C.  Ocean Thermal Energy Conversion (OTEC)

Basic Discussion

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 15% of this energy is retained as thermal energy. Since the solar intensity decreases exponentially with depth, solar absorption is concentrated at the top layers. In the tropics, typical surface temperature are in excess of 25 °C, while, the temperature is about 5 - 10 °C at depths of approximately one kilometer. This differential can be enhanced when the bottom waters are impacted by polar currents generated by the Thermohaline deep oceanic current system. 

Neither convection nor physical mixing mechanisms of heat transfer are operative in the oceans, resulting in a stable thermal stratification with the upper layers remaining hot and the lower layers remaining cold. This stratification results in a practically infinite heat source/heat sink system separated by approximately 1000 meters of ocean water, allowing the potential application of heat engine technology. The tropics are considered to be the best locations for development of OTEC technology as can be seen in the map of oceanic thermal differential below developed by the U.S. Department of Energy’s National Renewable Energy Laboratory.

Map of oceanic thermal differential
Designs for OTEC facilities fall into three different types, Closed, Open and a combination of the two processes called a Hybrid, Cycles based on the process used to generate power from the temperature differential at the site and dispose of final water. 

Closed/Anderson Cycle

This technology was developed starting in the 1960s by J. Hilbert Anderson of Sea Solar Power, Inc. In this cycle the heat is transferred in the evaporator from the warm sea water to the working fluid (ammonia, CHCs, petroleum products or water). 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. 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 to the cold sea water. The condensate is then compressed to the highest pressure in the cycle, requiring condensate pump work. 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.

Open/Claude cycle

The Open Cycle technology is similar to the closed cycle except that the liquid used is steam and is available for other uses after the power generation cycle is completed. 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 and contaminants behind in the low-pressure container, is pure fresh water and is condensed back into a liquid by exposure to cold temperatures from deep-ocean water. This method has the advantage of producing desalinized fresh water, suitable for drinking water or irrigation.

 

                             Diagram of a closed cycle OTEC plant
Diagram of a closed cycle OTEC plant

Technology Status

The majority of testing of the process has been conducted by the U.S. Natural Energy Laboratory of Hawaii since 1974.  Pilot plants have been constructed in Cuba in 1930 (22 kW generated), Nauru, Japan in 1970 (100 kw generated), Tamil Nadu, India Pilot study (1 MW) and Keohole Point, Hawaii (50 kW generated).  The Natural Energy Laboratory in 1999 tested a 250 kW pilot closed-cycle plant, the largest of its kind ever put into operation. Present projects in design and development include one for the U.S. Navy base at Diego Garcia, (13 MW design capacity w/1.25 Mgd fresh water as a waste product, and a 10 MW proposed plant at Guam.

Reserves

Existing estimates of global energy reserves from OTEC vary widely.  The lowest reasonable value is 3 TW/yr from modeling studies by Nihous, 2005.  The highest projection by reasonable authority is 100,000 TW/yr produced by the U.S. Department of Energy (EIA, 2006).

Selected References
  1. The Marine Power Project Welcome Page http://www.esru.strath.ac.uk/EandE/Web_sites/05-06/marine_renewables/home/welcome.htm
  2. A GLOBAL WAVE ENERGY RESOURCE ASSESSMENT
    Andrew M. Cornett, Canadian Hydraulics Centre, National Research Council, Ottawa, Ontario, Canada  ISOPE 2008-579
  3. Sims, E.H , Hydropower, Geothermal and Ocean Energy, 2008., Massey University, New Zealand, International Energy Agency, France
  4. Solar Energy Research Institute. (November 1989). Ocean Thermal Energy Conversion: An Overview. SERI/SP-220-3024. Golden, CO: Solar Energy Research Institute; 36 pp.
  5. Ocean Thermal Energy Conversion, a bibliography, National Renewable Energy Laboratory, U.S. Department of Energy, http://www.nrel.gov/otec/bibliography.html#overview
  6. “Tidal Power” University of Strathclyde  http://www.esru.strath.ac.uk/EandE/Web_sites/01-02/RE_info/Tidal%20Power.htm.
  7. "Wave Power". University of Strathclyde. http://www.esru.strath.ac.uk/EandE/Web_sites/01-02/RE_info/wave%20power.htm.
    8.         Ocean Thermal Energy (OT International Energy Agency. (2006). Review and analysis of ocean energy systems development and supporting policies. Retrieved September, 2007, from http://www.ieaoceans.org/_fich/6/Review_Policies_on_OES_2.pdf.
    9.         Renewable Energy From The Ocean - A Guide To OTEC, William H. Avery, Chih Wu, Oxford University Press, 1994. Covers the OTEC work done at the Johns Hopkins Applied Physics Laboratory from 1970–1985 in conjunction with the Department of Energy and other firms.
    10.       Engineering Committee on Oceanic Resources — Working Group on Wave Energy Conversion (2003), John Brooke, ed., Wave Energy Conversion, Elsevier, pp. 7, ISBN 0080442129, http://books.google.com/books?id=UGAXRwoLZY4C&dq=John+Brooke,+ed.,+Wave+Energy+Conversion&source=gbs_summary_s&cad=0
    11.       Tom Thorpe. "An Overview of Wave Energy Technologies: Status, Performance and Costs" (PDF). wave-energy.net. http://www.wave-energy.net/Library/An%20Overview%20of%20Wave%20Energy.pdf. Retrieved on 2008-10-13.
    12.       Cruz J.; Gunnar M., Barstow S., Mollison D. (2008), Joao Cruz, ed., Green Energy and Technology, Ocean Wave Energy, Springer Science+Business Media, pp. 93, ISBN 978-3-540-74894-6.
    13.       Cruz, Joao (2008), Ocean Wave Energy - Current Status and Future Prospects, Springer, ISBN 3540748946.
    14.       McCormick, Michael (2007), Ocean Wave Energy Conversion, Dover, ISBN 0486462455 , 256 pp.
    15.       Twidell, John; Weir, Anthony D.; Weir, Tony (2006), Renewable Energy
    16.Ocean Thermal Energy Conversion
    ISBN: 0-471-96009-8
    Publishers: John Wiley & Sons, 1996
    Authors: Patrick Takahashi, Andrew Trenka Resources, Taylor & Francis, ISBN 0419253300 , 601 pp.

 

 

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