15.0 Ocean Energy

Frank R. Leslie, B. S. E. E., M. S. Space Technology

3/25/2004, Rev. 1.4

fleslie @fit.edu; (321) 674-7377

www.fit.edu/~fleslie

“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, 99-55 B.C.

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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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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15.1 History

Some first uses of ocean energy:Tidal grain mills developedCurrents used to cross the Atlantic and returnOcean winds blow boats (sometimes where

desired)

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15.2 Sources of Energy

Tidal motion of water up and down changes potential energyChanges of pressure beneath the tide heightTidal horizontal flow into basins and rivers results

Wind-driven motion of water horizontally increases kinetic energyChanges in flow rate that produces strong

currentsSolar heating of surface waters warms the ocean by

conductionUpwelling and overturning mixes and heats lower

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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 lengthA 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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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 turbinesEngland: Severn RiverCanada: 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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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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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 constantyielding 2 S ρ gH2

Mean power is 2 S ρ gH2/tidal period; semidiurnal better Tidal Pool Arrangements

Single-pool empties on ebb tideSingle-pool fills on flood tideSingle-pool fills and empties through turbineTwo-pool ebb- and flood-tide system; two ebbs per day;

alternating pool useTwo-pool one-way system (high and low pools) (turbine

located between pools)

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15.3.1 Tidal Water Turbines

Current flow converted to rotary motion by tidal current

Turbines placed across Rance River, FranceLarge 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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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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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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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 SeaFrench 400 kW unit operated since 1968Plant 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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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 1950sKorean potential sites (Garolim Bay)

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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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15.4 Ocean Energy: Wave Energy

Wave energy potential varies greatly worldwide

Source: Wave Energy paper. IMechE, 1991 and European Directory of Renewable Energy (Suppliers and Services) 1991

Figures in kW/m

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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 generatorSlow 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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15.4.2 Water Current Equations(also applies to wind turbines)

Assume a “tube” of water the diameter, D, of the rotorA = π D2/4

A length, L, of water moves through the turbine in t secondsL = u·t, where u is the water speed

The tube volume is V = A·L = A·u·tWater density, ρ, is 1000 kg/m3

Mass, m = ρ·V = ρ·A·u·t, where V is volumeKinetic energy = KE = ½ mu2

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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 = 0.3927·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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15.4.3 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.: www.fujita.com/archive-frr/ TidalPower.html©1996 Ramage

http://acre.murdoch.edu.au/ago/ocean/wave.html

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15.4.4.1 Water-Driven Wave Turbines

Davis Hydraulic Turbines since 1981Most tests done in Canada4 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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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-gateWater passes through waterwheel or turbine back to the

oceanAlgerian 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 shoreWaves passing overhead produce hydraulic pressure in rams

between sectionsThis pressure drives hydraulic motors that spin generators,

and power is conducted to shore by cable750 kW produced by a group 150m long and 3.5m diameter

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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

http://www.bfi.org/Trimtab/summer01/oceanWave.htm

Photo by Wavegen

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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

http://www.earthsci.org/esa/energy/wavpwr/wavepwr.html

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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.: www.nrel.gov/otec/achievements.html

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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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15.7 Hydraulic Pressure Absorbers for Wave and Tide

Large synthetic rubber bags filled with water could be placed offshore where large waves pass overheadAlso respond to tidesA 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

http://www.bfi.org/Trimtab/summer01/oceanWave.htm

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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 hazardSimple obstructionAdrift from a storm

NIMBYs may not want to see them and loudly protest

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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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References: Books, etc. General:

Sørensen, Bent. Renewable Energy, Second Edition. San Diego: Academic Press, 2000, 911 pp. ISBN 0-12-656152-4.

Henry, J. Glenn and Gary W. Heinke. Environmental Science and Engineering. Englewood Cliffs: Prentice-Hall, 728pp., 1989. 0-13-283177-5, TD146.H45, 620.8-dc19

Brower, Michael. Cool Energy. Cambridge MA: The MIT Press, 1992. 0-262-02349-0, TJ807.9.U6B76, 333.79’4’0973.

Di Lavore, Philip. Energy: Insights from Physics. NY: John Wiley & Sons, 414pp., 1984. 0-471-89683-7l, TJ163.2.D54, 621.042.

Bowditch, Nathaniel. American Practical Navigator. Washington:USGPO, H.O. Pub. No. 9. Harder, Edwin L. Fundamentals of Energy Production. NY: John Wiley & Sons, 368pp., 1982. 0-

471-08356-9, TJ163.9.H37, 333.79. Tidal Energy, pp. 111-129. Wind:

Patel, Mukund R. Wind and Solar Power Systems. Boca Raton: CRC Press, 1999, 351 pp. ISBN 0-8493-1605-7, TK1541.P38 1999, 621.31’2136

Gipe, Paul. Wind Energy for Home & Business. White River Junction, VT: Chelsea Green Pub. Co., 1993. 0-930031-64-4, TJ820.G57, 621.4’5

Johnson, Gary L, Wind Energy Systems. Englewood Cliffs NJ: Prentice-Hall, Inc. TK 1541.J64 1985. 621.4’5; 0-13-957754-8.

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,

1983. 551.5, 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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References: Books

Brower, Michael. Cool Energy. Cambridge MA: The MIT Press, 1992. 0-262-02349-0, TJ807.9.U6B76, 333.79’4’0973.

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., 1993. 0-930031-64-4, TJ820.G57, 621.4’5

Patel, Mukund R. Wind and Solar Power Systems. Boca Raton: CRC Press, 1999, 351 pp. ISBN 0-8493-1605-7, TK1541.P38 1999, 621.31’2136

Sørensen, Bent. Renewable Energy, Second Edition. San Diego: Academic Press, 2000, 911 pp. ISBN 0-12-656152-4.

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References: Internet

General: http://www.google.com/search?q=%22renewable+energy+course%22 http://www.ferc.gov/ Federal Energy Regulatory Commission http://solstice.crest.org/ http://dataweb.usbr.gov/html/powerplant_selection.html http://mailto:energyresources@egroups.com http://www.dieoff.org. Site devoted to the decline of energy and effects upon population

Tidal: http://www.unep.or.kr/energy/ocean/oc_intro.htm http://www.bluenergy.com/technology/prototypes.html http://www.iclei.org/efacts/tidal.htm http://zebu.uoregon.edu/1996/ph162/l17b.html http://www.bluenergy.com/public/index_2.html

Waves: http://www.env.qld.gov.au/sustainable_energy/publicat/ocean.htm http://www.bfi.org/Trimtab/summer01/oceanWave.htm http://www.oceanpd.com/ http://www.newenergy.org.cn/english/ocean/overview/status.htm http://www.energy.org.uk/EFWave.htm

http://www.earthsci.org/esa/energy/wavpwr/wavepwr.html

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References: Internet

Thermal: http://www.nrel.gov/otec/what.html http://www.hawaii.gov/dbedt/ert/otec_hi.html#anchor349152 on OTEC

systems

Wind: http://awea-windnet@yahoogroups.com. Wind Energy elist http://awea-wind-home@yahoogroups.com. Wind energy home

powersite elist http://telosnet.com/wind/20th.html

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References: Websites, etc.

awea-windnet@yahoogroups.com. Wind Energy elistawea-wind-home@yahoogroups.com. Wind energy home powersite elistgeothermal.marin.org/ on geothermal energymailto:energyresources@egroups.com rredc.nrel.gov/wind/pubs/atlas/maps/chap2/2-01m.html PNNL wind energy map of CONUS

windenergyexperimenter@yahoogroups.com. Elist for wind energy experimenterswww.dieoff.org. Site devoted to the decline of energy and effects upon populationwww.ferc.gov/ Federal Energy Regulatory Commissionwww.hawaii.gov/dbedt/ert/otec_hi.html#anchor349152 on OTEC systemstelosnet.com/wind/20th.htmlwww.google.com/search?q=%22renewable+energy+course%22solstice.crest.org/dataweb.usbr.gov/html/powerplant_selection.html

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Units and Constants

Units: Power in watts (joules/second) Energy (power x time) in watt-hours

Constants: 1 m = 0.3048 ft exactly by definition 1 mile = 1.609 km; 1m/s = 2.204 mi/h (mph) 1 mile2 = 27878400 ft2 = 2589988.11 m2

1 ft2 = 0.09290304 m2; 1 m2 = 10.76391042 ft2

1 ft3 = 28.32 L = 7.34 gallon = 0.02832 m3; 1 m3 = 264.17 US gallons 1 m3/s = 15850.32 US gallons/minute g = 32.2 ft/s2 = 9.81 m/s2; 1 kg = 2.2 pounds Air density, ρ (rho), is 1.225 kg/m3 or 0.0158 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 = 42.42 BTU/min = = 746 W (J/s) 1 BTU = 252 cal = 0.293 Wh = 1.055 kJ 1 atmosphere = 14.696 psi = 33.9 ft water = 101.325 kPa = 76 cm Hg

=1013.25 mbar 1 boe (42- gallon barrel of oil equivalent) = 1700 kWh

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Energy Equations

Electricity: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/sP = 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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