0. B. S. E. E., M. S. Space Technology
15.0 Ocean Energy
“It is pleasant, when the sea is high and the winds are dashing the waves about, to watch from shore the struggles of another.”
Lucretius, B.C.
Frank R. Leslie,
B. S. E. E., M. S. Space Technology
3/25/2004, Rev. 1.4
(321)

1. 15.O Overview of Ocean Energy
Ocean energy is replenished by the sun and through tidal influences of the moon and sun gravitational forces
Near-surface winds induce wave action and cause wind-blown currents at about 3% of the wind speed
Tides cause strong currents into and out of coastal basins and rivers
Ocean surface heating by some 70% of the incoming sunlight adds to the surface water thermal energy, causing expansion and flow
Wind energy is stronger over the ocean due to less drag, although technically, only seabreezes are from ocean energy
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2. 15.1 Ocean Energy
Sustainable energy comes from the sun or from tidal forces of the moon and sun; usually implies not using it faster than can be replenished
The tidal gravitational forces and thermal storage of the ocean provide a major energy source
Wave action adds to the extractable surface energy, but is less than tidal energy
Major ocean currents (like the Gulf Stream) may be exploited to extract energy with underwater rotors similar to wind turbines
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3. 15.1 History Some first uses of ocean energy:
Tidal grain mills developed
Currents used to cross the Atlantic and return
Ocean winds blow boats (sometimes where desired)
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4. 15.2 Sources of Energy
Tidal motion of water up and down changes potential energy
Changes of pressure beneath the tide height
Tidal horizontal flow into basins and rivers results
Wind-driven motion of water horizontally increases kinetic energy
Changes in flow rate that produces strong currents
Solar heating of surface waters warms the ocean by conduction
Upwelling and overturning mixes and heats lower layers
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5. 15.2.1 Available Energy Potential Energy: PE = mh
Kinetic Energy: KE = ½ mv2 or ½ mu2
Wave energy is proportional to wave length times wave height squared (LH2)per wave length per unit of crest length
A four-foot (1.2 m), ten-second wave striking a coast expends more than 35, 000 HP per mile of coast [Kotch, p. 247]
Maximum Tidal Energy, E = 2HQ x 353/(778 x 3413) = 266 x 10-6 HQ kWh/yr, where H is the tidal range (ft) and Q is the tidal flow (lbs of seawater)
E = 2 HQ ft-lb/lunar day (2 tides) or E = 416 x 10-4 HV kWh, where V is cubic feet of flow
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6. 15.3 Ocean Energy: Tidal Energy
Tides are produced by gravitational forces of the moon and sun and the Earth’s rotation
Existing and possible sites:
France: 1966 La Rance river estuary 240 MW station
Tidal ranges of 8.5 m to 13.5 m; 10 reversible turbines
England: Severn River
Canada: Passamaquoddy in the Bay of Fundy (1935 attempt failed)
California: high potential along the northern coast
Environmental, economic, and esthetic aspects have delayed implementation
Lunar/solar power is asynchronous with daily load cycle
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7. 15.3 Tidal Energy
Tidal mills were used in the Tenth and Eleventh Centuries in England, France, and elsewhere
Millpond water was trapped at high tide by a gate (Difficult working hours for the miller; Why?)
Rhode Island, USA, 18th Century, 20-ton wheel 11 ft in diameter and 26 ft wide
Hamburg, Germany, 1880 “mill” pumped sewage
Slade’s Mill in Chelsea, MA founded 1734, 100HP, operated until ~1980
Deben estuary, Woodbridge, Suffolk, England has been operating since 1170 (reminiscent of “the old family axe”; only had three new handles and two new heads!)
Tidal mills were common in USA north of Cape Cod, where a 3 m range exists [Redfield, 1980]
Brooklyn NY had tidal mill in 1636 [?]
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8. 15.3 Tidal Energy (continued)
Potential energy = S integral from 0 to 2H (ρgz dz),
where S is basin area, H is tidal amplitude, ρ is water density, and g is gravitational constant yielding 2 S ρ gH2
Mean power is 2 S ρ gH2/tidal period; semidiurnal better
Tidal Pool Arrangements
Single-pool empties on ebb tide
Single-pool fills on flood tide
Single-pool fills and empties through turbine
Two-pool ebb- and flood-tide system; two ebbs per day; alternating pool use
Two-pool one-way system (high and low pools) (turbine located between pools)
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9. Tidal Water Turbines
Current flow converted to rotary motion by tidal current
Turbines placed across Rance River, France
Large Savonius rotors (J. S. Savonius, 1932?) placed across channel to rotate at slow speed but creating high torque (large current meter)
Horizontal rotors proposed for Gulf Stream placement off Miami, Florida
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10. 15.3.1.1 Tidal Flow: Rance River, France
240 MW plant with 24, 10 MW turbines operated since 1966
Average head is 28 ft
Area is approximately 8.5 square miles
Flow approx, 6.64 billion cubic feet
Maximum theoretical energy is 7734 million kWh/year; 6% extracted
Storage pumping contributes 1.7% to energy level
At neap tides, generates 80,000 kWh/day; at equinoctial spring tide, 1,450,000 kWh/day (18:1 ratio!); average ~500 million kWh/year
Produces electricity cheaper than oil, coal, or nuclear plants in France
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11. 15.3.1.2 Tidal Flow: Passamaquoddy, Lower Bay of Fundy, New Brunswick, Canada
Proposed to be located between Maine (USA) and New Brunswick
Average head is 18.1 ft
Flow is approximately 70 billion cubic feet per tidal cycle
Area is approximately 142 square miles
About 3.5 % of theoretical maximum would be extracted
Two-pool approach greatly lower maximum theoretical energy
International Commission studied it 1956 through 1961 and found project uneconomic then
Deferred until economic conditions change
[Ref.: Harder]
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12. 15.3.1.3 Other Tidal Flow Plants under Study
Annapolis River, Nova Scotia: straight-flow turbines; demonstration plant was to be completed in 1983; 20 MW; tides 29 to 15 feet; Tidal Power Corp.; ~$74M
Experimental site at Kislaya Guba on Barents Sea
French 400 kW unit operated since 1968
Plant floated into place and sunk: dikes added to close gaps
Sea of Okhotsk (former Sov. Union) under study in 1980
White Sea, Russia: 1 MW, 1969
Murmansk, Russia: 0.4 MW
Kiansghsia in China
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13. 15.3.1.3 Other Tidal Flow Plants under Study (continued)
Severn River, Great Britain: range of 47 feet (14.5 m) calculated output of 2.4 MWh annually. Proposed at $15B, but not economic.
Chansey Islands:20 miles off Saint Malo, France; 34 billion kWh per year; not economic; environmental problems; project shelved in 1980
San Jose, Argentina: potential of 75 billion kWh/year; tidal range of 20 feet (6m)
China built several plants in the 1950s
Korean potential sites (Garolim Bay)
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14. 15.4 Wave Energy
Energy of interchanging potential and kinetic energy in the wave
Cycloidal motion of wave particles carries energy forward without much current
Typical periodicities are one to thirty seconds, thus there are low-energy periods between high-energy points
In 1799, Girard & Son of Paris proposed using wave power for powering pumps and saws
California coast could generate 7 to 17 MW per mile [Smith, p. 91]
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15. 15.4 Ocean Energy: Wave Energy
Wave energy potential varies greatly worldwide
Figures in kW/m
Source: Wave Energy paper. IMechE, 1991 and European Directory of Renewable Energy (Suppliers and Services) 1991
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16. 15.4.1 Concepts of Wave Energy Conversion
Change of water level by tide or wave can move or raise a float, producing linear motion from sinusoidal motion
Water current can turn a turbine to yield rotational mechanical energy to drive a pump or generator
Slow rotation speed of approximately one revolution per second to one revolution per minute less likely to harm marine life
Turbine reduces energy downstream and could protect shoreline
Archimedes Wave Swing is a Dutch device [Smith, p. 91]
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17. 15.4.2 Water Current Equations (also applies to wind turbines)
Assume a “tube” of water the diameter, D, of the rotor
A = π D2/4
A length, L, of water moves through the turbine in t seconds
L = u·t, where u is the water speed
The tube volume is V = A·L = A·u·t
Water density, ρ, is 1000 kg/m3
Mass, m = ρ·V = ρ·A·u·t, where V is volume
Kinetic energy = KE = ½ mu2
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18. 15.4.2 Water Current Equations (continued)
Substituting ρ·A·u·t for mass, and A = π D2/4 , KE = ½·π/4·ρ·D2·u3·t
Theoretical power, Pt = ½·π/4·ρ·D2·u3·t/t = ·ρW·D2·u3, ρ (rho) is the density, D is the diameter swept by the rotor blades, and u is the speed parallel to the rotor axis
Betz Law shows 59.3% of power can be extracted
Pe = Pt·59.3%·r·t·g, where Pe is the extracted power, r is rotor efficiency, t is transmission efficiency, and g is generator efficiency
For example, 59.3%·90%·98%·80% = 42% extraction of theoretical power
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19. Salter “Ducks”
Scottish physicist Prof. Stephen Salter invented “Nodding Duck” energy converter in 1970
Salter “ducks” rock up and down as the wave passes beneath it. This oscillating mechanical energy is converted to electrical energy
Destroyed by storm
A floating two-tank version drives hydraulic rams that send pressurized oil to a hydraulic motor that drives a generator, and a cable conducts electricity to shore
Ref.: TidalPower.html ©1996 Ramage
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20. 15.4.4.1 Water-Driven Wave Turbines
Davis Hydraulic Turbines since 1981
Most tests done in Canada
4 kW turbine tested in Gulf Stream
Blue Energy of Canada developing two 250 kW turbines for British Columbia
Also proposed Brothers Island tidal fence in San Francisco Bay, California 1000 ft long by 80 ft deep to produce 15 – 25 MW
Australian Port Kembla (south of Sydney) to produce 500 kW
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21. 15.4.4.1 Water-Driven Wave Turbines
Waves can be funneled and channeled into a rising chute to charge a reservoir over a weir or through a swing-gate
Water passes through waterwheel or turbine back to the ocean
Algerian V-channel [Kotch, p.228]
Wave forces require an extremely strong structure and mechanism to preclude damage
The Ocean Power Delivery wave energy converter Pelamis has articulated sections that stream from an anchor towards the shore
Waves passing overhead produce hydraulic pressure in rams between sections
This pressure drives hydraulic motors that spin generators, and power is conducted to shore by cable
750 kW produced by a group 150m long and 3.5m diameter
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22. 15.4.4.2 Air-Driven Wave Turbines
A Wavegen™, wave-driven, air compressor or oscillating water column (OWC) spins a two-way Wells turbine to produce electricity
This British invention uses an air-driven Wells turbine with symmetrical blades
Incoming waves pressurize air within a heavy concrete box, and trapped air rushes upward through pipe connecting the turbine
Wells turbine is spun to starting speed by external electrical power and spins the same rotation regardless of air flow direction
Energy is estimated at 65 megawatts per mile
Photo by Wavegen
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23. 15.4.4.2 Air-Driven Wave Turbines (Con’t)
A floating buoy can compress trapped air similar to a whistle buoy
The oscillating water column (OWC) in a long pipe under the buoy will lag behind the buoy motion due to inertia of the water column
The compressed air spins a turbine/alternator to generate electricity at $0.09/kWh
The Japanese “Mighty Whale” has an air channel to capture wave energy. Width is 30m and length is 50 m. There are two 30 kW and one 50 kW turbine/generators
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24. 15.5 Ocean Energy: OTEC (Ocean Thermal Electric Conversion)
Hawaii has the research OTEC system [shut down in 1985?]
OTEC requires some 40°F temperature difference between the surface and deep waters to extract energy
Open-cycle plants vaporize warm water and condense it using the cold sea water, yielding potable water and electricity from turbine-driven alternators
Closed-cycle units evaporate ammonia at 78°F to drive a turbine and an alternator
Ref.:
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25. 15.6 Current Flow Turbines
Current flow turbines are essentially waterproof underwater wind turbines
The forces are much greater since water has 832 times the density of air
Turbines can turn slowly and are less likely to damage marine animals
This version is raised above the water surface for maintenance
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26. 15.7 Hydraulic Pressure Absorbers for Wave and Tide
Large synthetic rubber bags filled with water could be placed offshore where large waves pass overhead
Also respond to tides
A connecting pipe conducts hydraulic pressure to a positive displacement motor that spins a generator
The motor can turn a generator to make electricity that varies sinusoidally with the pressure
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27. 15.8 Other Issues
Biofouling can clog intake pipes or other parts of submerged equipment
Storms can tear loose moorings, leading to loss of equipment
Offshore units may pose a navigation hazard
Simple obstruction
Adrift from a storm
NIMBYs may not want to see them and loudly protest
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28. 15.C Conclusion
Renewable energy offers a long-term approach to the World’s energy needs
Economics drives the energy selection process and short-term (first cost) thinking leads to disregard of long-term, overall cost
Wave and tidal energy are more expensive than wind and solar energy, the present leaders
Increasing oil, gas, and coal prices will ensure that the transition to renewable energy occurs
Offshore and shoreline wind energy plants offer a logical approach to part of future energy supplies
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29. References: Books, etc. General:
Sørensen, Bent. Renewable Energy, Second Edition. San Diego: Academic Press, 2000, 911 pp. ISBN
Henry, J. Glenn and Gary W. Heinke. Environmental Science and Engineering. Englewood Cliffs: Prentice-Hall, 728pp., , TD146.H45, dc19
Brower, Michael. Cool Energy. Cambridge MA: The MIT Press, , TJ807.9.U6B76, ’4’0973.
Di Lavore, Philip. Energy: Insights from Physics. NY: John Wiley & Sons, 414pp., l, TJ163.2.D54,
Bowditch, Nathaniel. American Practical Navigator. Washington:USGPO, H.O. Pub. No. 9.
Harder, Edwin L. Fundamentals of Energy Production. NY: John Wiley & Sons, 368pp., , TJ163.9.H37, Tidal Energy, pp
Wind:
Patel, Mukund R. Wind and Solar Power Systems. Boca Raton: CRC Press, 1999, 351 pp. ISBN , TK1541.P , ’2136
Gipe, Paul. Wind Energy for Home & Business. White River Junction, VT: Chelsea Green Pub. Co., , TJ820.G57, 621.4’5
Johnson, Gary L, Wind Energy Systems. Englewood Cliffs NJ: Prentice-Hall, Inc. TK 1541.J ’5;
Waves:
Smith, Douglas J. “Big Plans for Ocean Power Hinges on Funding and Additional R&D”. Power Engineering, Nov. 2001, p. 91.
Kotch, William J., Rear Admiral, USN, Retired. Weather for the Mariner. Annapolis: Naval Institute Press, , QC994.K64, Chap. 11, Wind, Waves, and Swell.
Solar:
Duffie, John and William A. Beckman. Solar Engineering of Thermal Processes. NY: John Wiley & Sons, Inc., 920 pp., 1991.
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30. References: Books
Brower, Michael. Cool Energy. Cambridge MA: The MIT Press, , TJ807.9.U6B76, ’4’0973.
Duffie, John and William A. Beckman. Solar Engineering of Thermal Processes. NY: John Wiley & Sons, Inc., 920 pp., 1991
Gipe, Paul. Wind Energy for Home & Business. White River Junction, VT: Chelsea Green Pub. Co., , TJ820.G57, 621.4’5
Patel, Mukund R. Wind and Solar Power Systems. Boca Raton: CRC Press, 1999, 351 pp. ISBN , TK1541.P , ’2136
Sørensen, Bent. Renewable Energy, Second Edition. San Diego: Academic Press, 2000, 911 pp. ISBN
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31. References: Internet General: Tidal: Waves:
Federal Energy Regulatory Commission
Site devoted to the decline of energy and effects upon population
Tidal:
Waves:
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32. References: Internet Thermal: Wind: http://www.nrel.gov/otec/what.html
on OTEC systems
Wind:
Wind Energy elist
Wind energy home powersite elist
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33. References: Websites, etc.
Wind Energy elist
Wind energy home powersite elist
geothermal.marin.org/ on geothermal energy
rredc.nrel.gov/wind/pubs/atlas/maps/chap2/2-01m.html PNNL wind energy map of CONUS Elist for wind energy experimenters
Site devoted to the decline of energy and effects upon population
Federal Energy Regulatory Commission
on OTEC systems
telosnet.com/wind/20th.html
solstice.crest.org/
dataweb.usbr.gov/html/powerplant_selection.html
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34. Units and Constants Units: Power in watts (joules/second)
Energy (power x time) in watt-hours
Constants:
1 m = ft exactly by definition
1 mile = km; 1m/s = mi/h (mph)
1 mile2 = ft2 = m2
1 ft2 = m2; 1 m2 = ft2
1 ft3 = L = 7.34 gallon = m3; 1 m3 = US gallons
1 m3/s = US gallons/minute
g = 32.2 ft/s2 = 9.81 m/s2; 1 kg = 2.2 pounds
Air density, ρ (rho), is kg/m3 or pounds/ft3 at 20ºC at sea level
Solar Constant: 1368 W/m2 exoatmospheric or 342 W/m2 surface (80 to 240 W/m2)
1 HP = 550 ft-lbs/s = BTU/min = = 746 W (J/s)
1 BTU = 252 cal = Wh = kJ
1 atmosphere = psi = 33.9 ft water = kPa = 76 cm Hg = mbar
1 boe (42- gallon barrel of oil equivalent) = 1700 kWh
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35. Energy Equations Electricity: Turbines:
E=IR; P=I2 R; P=E2/R, where R is resistance in ohms, E is volts, I is current in amperes, and P is power in watts
Energy = P t, where t is time in hours
Turbines:
Pa = ½ ρ A2 u3, where ρ (rho) is the fluid density, A = rotor area in m2, and u is wind speed in m/s
P = R ρ T, where P = pressure (Nm-2 = Pascal)
Torque, T = P/ω, in Nm/rad, where P = mechanical power in watts, ω is angular velocity in rad/sec
Pumps:
Pm = gQmh/ήp W, where g=9.81 N/kg, Qm is mass capacity in kg/s, h is head in m, and ήp is pump mechanical efficiency
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