HomeMy WebLinkAboutIce Problems of Tidal Power Generation in Upper Cook Inlet, AK Brian B Yanity est 2004Ice Problems of Tidal Power Generation in Upper Cook Inlet,AlaskaBrianB.Yanity'
INTRODUCTION
Cook Inlet is a 350-km-long estuary in south central Alaska.The area surrounding
the northern end of Cook Inlet,including the city of Anchorage,is home to over 350,000
people.Cook Inlet is a well studied body of water,due to heavy navigation needs,as
well as oil and gas infrastructure.The 7.5 to 12 m mean tidal range of upper Cook Inlet
is the second greatest in the world,behind only the Bay of Fundy in eastern Canada.
Upper Cook Inlet,which for the purpose of this paper includes the "head”region of both
Knik Arm and Turnagain Arm,is next to half of Alaska's population.Due to it sub-
Arctic location,Cook Inlet has a great deal of ice every winter.The ice problems of
Cook Inlet are not so serious as to preclude all tidal power development.However,the
ice conditions certainly will have an affect on the economics,design and operation of a
tidal power plant.
Tidal energy offers promise in much of upper Cook Inlet.For example,prime
Knik Arm tidal energy sites are located just five miles north of downtown Anchorage,
near the corridor of the proposed Knik Arm Bridge.The preliminary studies conducted
for the bridge proposal have resulted in a wealth of information relevant to tidal power
studies in Knik Arm.Even if large-scale tidal energy development does not happen in
upper Cook Inlet,research done on this body of water could help with other tidal
developments in Alaska or around the world.
BACKGROUND ENERGY SCENARIO
The main reason that Cook Inlet's large tidal energy resource has not yet been
developed is the abundance of natural gas also found in the same region.These fields
have been in commercial production since the late 1950s,and are expected to continue
producing for the next decade.Natural gas power generation is a well-established
technology and the most efficient fossil fuel for power generation.Thus,gas-fired
generation has been dominant source of electricity in south central Alaska since the
1960s.Natural gas prices are expected to increase in Alaska in the years ahead,even
when taking the proposed North Slope gas pipeline construction into account.
Nationally,gas prices have already risen unexpectedly high over the past several years.
The bus price (not including transmission)of electricity produced at the Beluga power
plant,on the west shore of Cook Inlet,is presently about $0.02/kWh.
All of the generating plants serving the Anchorage area are connected to the
Railbelt transmission system,which is an energy corridor that extends as far north as
Fairbanks,and as far south as Homer.The Railbelt system,with a total generating
'Graduate Student,School of Engineering,University of Alaska Anchorage,3211
Providence Drive,Anchorage,AK 99508,E-mail:asbby@uaa.alaska.edu
capacity of 1270 Megawatts (MW),serves 80%of Alaska's population and four major
military bases.The peak electric load of the Municipality of Anchorage is around 450
MW.Aside from gas and oil-fired generation,the remainder of the railbelt system's
power comes from three hydroelectric facilities:Bradley Lake (120 MW),Eklutna (30
MW),and Cooper Lake (17 MW).The proposed 100 MW wind farm on Fire Island is
another energy source that,if developed,would most likely be completed before any
utility-scale tidal generation.Because of the variations in the tidal cycle,tidal power
could not feasibly be the Railbelt's dominant electricity source,and would always have to
be complemented by easily-controllable sources such as hydro or gas-fired plants.
However,a well-operated power grid with modern control systems can integrate tidal
power as an important part the energy mix.If oil and gas prices continue to increase,
tidal power could become an attractive alternative energy source for the Anchorage area
within a decade.
TIDAL POWER BARRAGES
The tidal energy potential of the upper parts of Cook Inlet has been studied since
the 1950s (Wilson and Swales 1970).The most detailed research program so far was a
1981 study that proposed building barrages (tidal dams)in Knik and Turnagain arms
(Acres International 1981).A tidal barrage is very similar to a conventional,low-head
hydroelectric dam,and relies on a high tide range.In addition to high capital costs,tidal
barrages completely enclose estuaries,resulting in extensive environmental impact.
Common environmental impacts of tidal barrages include negative effects on bird,fish,
and other wildlife (Muirhead 1992).A tidal barrage in upper Cook Inlet could harm
Beluga whale populations and other marine life.
The world's largest operating tidal power plant is the 240 MW La Rance tidal
barrage in northern France.The plant encloses the entire La Rance estuary,which has a
mean natural tidal range of 8.8 m.The La Rance plant consists of 24 Kaplan turbines,
each driving a 10 MW generator mounted in a bulb configuration.The La Rance barrage
has been operated continuously by Electricite de France (EDF)since 1966.During
construction,the La Rance project caused the almost complete disappearance of plant and
animal species,as the estuary was totally closed off from 1963 to 1966 (Rodier 1992).
The most recent tidal power feasibility study done in Alaska was on tidal energy
potential near Cordova (Tidal Electric 2001).The proposed installation was a 5 MW,
circular 'tidal tank'system,featuring an enclosed wall structure that fills up with
seawater during high tide.The proposed plant could not economically compete with the
6 MW Power Creek hydroelectric facility then being built by the Cordova Electric
Cooperative.Future energy needs in Cordova will determine if tidal power is an
attractive option there.
TIDAL CURRENT ENERGY IN COOK INLET
Upper Cook Inlet offers both a high tidal range and strong tidal currents.Tidal
current turbines have a lower energy density than tidal barrage schemes;though they
have a much lower environmental impact.Tidal current energy has received more
attention in recent years because of the high impacts and costs of tidal barrages.Akin to
underwater windmills,these turbines operate purely by the conversion of the kinetic
energy of the natural water current.For power production,the kinetic energy of tidal
currents is harnessed by "in-stream”or "hydrokinetic”turbines that are akin to
underwater wind turbines.The output of such turbines can be estimated by the power
equation below:
Power (kW)=0.5*n*p*A*v?
nN =mechanical turbine efficiency (%/100)p =density of seawater (kg/m?)A =area of turbine cross-section (m7)
v =water current velocity (m/sec.)
For a turbine efficiency of 30%,operating in a 3 m/sec.current,the energy conversionwouldbe4kWperm*of turbine cross section.For the same turbine operating in a 4m/sec.current,the energy conversion would be 9.5 kW per m”of turbine cross section.
In Cook Inlet,tidal currents change direction (ebb or flood)about every 6 hours and 12
minutes.Therefore,turbines with bi-directional capability are necessary.BC Hydro's
tidal current energy study (BC Hydro Engineering 2002)was the first comprehensive
regional assessment of tidal current energy,but did not discuss potential ice problems
because the coast of British Columbia has a climate milder than south central Alaska.
Central Cook Inlet
Acoustic Doppler profiles of a wide area of central Cook Inlet were conducted in
1999,primarily for the purpose of studying possible oil spill trajectories (Johnson et.al.,
1999).Maximum currents in this area tended to be between 1.50 to 2.00 m/s.In central
Cook Inlet,ice problems are slightly less severe that upper Cook Inlet,though the tidal
power potential is significantly less.
Turnagain Arm
Famous for its bore tides,Turnagain Arm provides strong tidal currents running
close to sections of the Seward Highway.Highway lighting along this section could be
powered by small tidal current turbines next to the road.The town of Girdwood could
also make use of small tidal power facilities along Turnagain Arm.However,the tidal
current velocities are not as well known as in Knik Arm,and any offshore developments
would be limited by the presence of the Anchorage Coastal Wildlife Refuge.
Knik Arm
The proposed site for the Knik Arm Bridge,shown in Figure 1 below,is attractive
for tidal energy production,not only because of "dual use”possibilities of tidal turbines
mounted on the side of the bridge piers.The bridge site is deeper than much of Knik
Arm (over 20 m),and the tidal currents are faster,leading to less sediment buildup.Near
the Port of Anchorage,NOAA predicts for maximum ebb flow a current speed of 3.7
knots (2.06 m/s)and for maximum flood flow,a current speed of 4 knots (1.90 m/s).
Near proposed bridge site,4 m/s spring tide currents are common.As part of the Knik
Arm Bridge feasibility studies,mobile acoustic Doppler current profiler (ADCP)
measurements were conducted during July and August 2004.According to these studies,
the bridge abutments would cause a major constriction of flow,directing all tidal flow
through the bridge opening.Because of this constriction,mid-channel speeds presently
on the order of 3 m/sec would increase to about 3.8 m/s (Smith 2004).No natural
settlement of fine material occurs to any significant extent in the project area,and scour is
expected to be a possible problem if the bridge is built.
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Proposed Current and Sediment
Field Data Coltection KNIK ARM
CROSSING
Figure 11.Field data collection plan for Knik Arin Bridge conceptual engineering studies.
Figure 1:Area of proposed Knik Arm crossing (Smith 2004)
EXISTING TIDAL POWER PLANTS IN COLD CLIMATES
Operational experience exists for tidal power plants in cold regions,but there is
no published record of ice effects on operation at any of these existing plants.Three
examples of existing tidal power facilities are described below.
Kislaya Guba
The Kislaya Guba [Gislogubskaya]tidal power station is located 60 km west of
Murmansk,Russia.The average January temperature range in this area is between -7°C
and -13°C (Bernshtein 1972).The Kislaya Guba estuary only has a tidal range of only 2
to 3 m,though the maximum natural tidal currents of 3 m/s to 3.8 m/s,similar to Knik
Arm.The facility featured innovative caisson construction (Bernshtein 1970,1978,
1992),and two bulb-type generators were used,similar to the La Rance plant.One of the
turbines was supplied same French firm that made the La Rance Kaplan turbines.Initial
tidal energy studies of the Kislaya Guba site began in 1938.The experimental plant came
online in 1968,and halted operation in 1994.
Annapolis Royal
The only operating commercial tidal power plant in North America is along the
Bay of Fundy at Annapolis Royal,Nova Scotia.The average January temperature range
in this area is between -3°C and -6°C.With a 20 MW capacity,the small tidal barrage
produces more than 30 GWh each year.The facility entered commercial service in 1984,
and is still operating.The Annapolis Royal plant is noted for the use of the Straflo
turbine design (Rice and Baker 1992).
Kvalsundet
Kvalsundet is a narrow straight next to the small town of Kvalsund,near
Hammerfest,on the northern cape of Norway.The average January temperature is -4°C
in Hammerfest.The turbines presently being installed in Kvalsundet are in-stream
machines that resemble large wind turbines.The pilot project,which eventually is
planned to have 20 turbines,is located well underneath any passing ice.The channel
maximum depth is 50 m,and the turbines are submerged deep enough to give a low-tide
"sailing”depth of 17 m.The average current velocity at Kvalsundet is 1.8 m/s,with
maximums of 2.5 m/s;considerably less than the velocities found in Knik Arm.
Sea of Okhotsk
The Penzhinsk and Turgur bays in the Russian Far East offer similar climate
conditions as Alaska.Several tidal barrage feasibility studies have been made in these
two areas,though no operating power plants have yet been built.Penzhinsk is possibly
the largest potential tidal power site in the world,with 87,400 to 100,000 MW of
potential.Turgur Bay could produce up to 6,800 MW,within feasible transmission
distance of the northeastern part of China (Bernshtein 1992).
THE ICE REGIME OF UPPER COOK INLET
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Figure 42.ice in Kaik Arm at the Port of 'Anchorage.
Figure 2:Knik Arm ice near the Port of Anchorage (Smith 2004)
Ice is a major factor in the development of tidal energy in upper Cook Inlet,and
may well increase the capital cost of such installations.There is ice in the upper Cook
Inlet usually 5 to 6 months of the year.Ice first appears in late fall/early winter (October
to December)and disappears completely each spring (March-May).The depth of the
surface ice is typically no more than 1 m.Cook Inlet ice conditions have been regularly
reported by the National Weather Service since the 1969-70 winter season (Smith 2004).
The best overall description of sea ice conditions is the Marine Ice Atlas for Cook Inlet,
Alaska (Mulherin et al.2001).As summarized in the conclusion of the Knik Arm Bridge
report (Smith 2004):
Historical ice conditions [in Knik Arm]range from nearly ice free all winter to 10 tenths
concentration (complete ice cover)of first-year medium ice (to 2 ft thick).Ice formed in the
turbulent brackish conditions of Knik Arm has random orientation of small grains with less
compressive strength than columnar-grained lake ice or sea ice of the high Arctic.The most
significant ice masses in Knik Arm are medium floes (to 500 m across)of floating sheet ice and
sediment-laden irregular conglomerates of beach ice formed in the upper tidelands and floated
away during extreme high tides.
Ice control structures are difficult to operate under these conditions due to the rapid tidal
currents.Below is a brief description each of the principal types of ice found in upper
Cook Inlet.
Frazil Ice
Highly turbulent,open-water conditions allow frazil ice to form in supercooled
water.Frazil accumulates into "slushy,viscous mass”(Mulherin et al.2001),and could
stick to underwater tidal turbines.This could cause hydrodynamic drag,and thus reduce
the turbine power output.USCG cold weather rules for upper Cook Inlet require vessels
to ballast down their seawater intakes to 10 ft.below the surface to prevent frazil from
entering (Smith 2004).Frazil ice can form pan-like structures,and is a major component
of new surface ice that forms in upper Cook Inlet.
Structural Ice
Structural ice forms on pilings and other structures,and has the same saline
properties of sea ice.Cold metal pilings accumulate successive layers of ice in the tidal
zone of the structure,forming large "ice collars.”Ice collars have been known to
suddenly release en masse from pilings at the Port of Anchorage,possibly causing
damage (Smith 2004).To prevent structural ice formation,any tidal turbines installed in
upper Cook Inlet must be kept below "surface ice”of the inter-tidal zone,as is done at
the Kvalsundet project.A detailed study of the history of structural ice problems at the
Port of Anchorage and Port MacKenzie should also be carried out.
Sea Ice
Sea ice is the broad category of floating ice sheets formed by the freezing of
surface waters.The strong tidal currents in upper Cook Inlet prevent continuous ice
sheets from forming.Due to the extreme turbulence,upper Cook Inlet sea ice is "small-
grained randomly oriented crystal structure”,not regular columnar ice crystals.The brine
and salts in sea ice reduce its strength,though the strength of sea ice from Cook Inlet has
not yet been tested in a laboratory.Pack Ice is any form of freely floating sea ice forming
directly from the freezing of seawater.
River and Estuarine Ice
River ice is formed on the surface and banks of the rivers and streams which
empty into the inlet.Because it has no brine pockets,river and estuarine ice is typically
stronger than sea ice.River ice usually only enters upper Cook Inlet after the spring
breakup.However,it is not known what proportion of upper Cook Inlet ice is riverine in
origin.Estuarine ice can mix with and become entrained by pack ice at any time of the
ice season.
Beach Ice
Beach ice forms on the extensive tidelands in upper Cook Inlet,by successive
tidal flooding of previously frozen layers adhered to sediment below.Beach ice is
heavily-laden with sediment,and is the most massive and destructive form of ice in Cook
Inlet.Beach ice in upper Cook Inlet can be up to 4 m thick,as pictured below in Figure 3.
Reece rere ree eenen cmccenceperegrine attests
Figure 43.Beach ice on Turnagain Arm,near Anchorage,
Alaslia,
Figure 3:(Smith 2004)
Ice Jams
Pushed along by strong currents,an ice jam could build up quickly in between
pilings or bridge piers during severe ice conditions,greatly increasing structural loads.
Laboratory flume testing may be needed to study ice jams around tidal power installation
or bridge piers,given the strong currents of upper Cook Inlet.
EXISTING OFFSHORE STRUCTURES IN UPPER COOK INLET
Port MacKenzie
Port MacKenzie,located about 2 km from the Port of Anchorage on the west
shore of Knik Arm,offers existing pilings and structures in an area of strong tidal
currents.Slightly more than 1 km from the proposed Knik Arm crossing,Port
MacKenzie is an excellent location to mount test tidal turbines as a pilot project,in order
to study the feasibility of Knik Arm Bridge turbines.The port and industrial facilities in
the immediate area are expected to continue expansion in the years ahead,creating new
loads for power demand.
Port of Anchorage
Typically,ice begins coating the steel pilings by mid-November (Perdichizzi and
Yasuda 1978).A handful of serious ice-floe incidents have occurred,as described in
Table 2 below.
Table 2:Partial record of ice problems at the Port on Anchorage (Mulherin et al.2001)
November 29,1964 Winter ice and tides tear pilings from petroleum dock,causing approx.$33,000
of damage.
March 29,1967 Anchorage city dock extension torn from pilings and demolished,causing $1.9
million of damage.
May 2,1967 Winter ice ripped out marker buoy on Knik Arm shoal,later causing a tanker
accident.
January 10,1983 523 ft.ship came lose from Port of Anchorage dock when ice severed mooring
lines.
Offshore Oil and Gas Rigs
Fourteen oil and gas and gas platforms have been installed in upper and central
Cook Inlet,some of which have had over 30 years of exposure to sea ice.Figure 4 below
shows the size of the floating ice sheets compared to a typical oil production platform.
The design standards listed in Table 1 below are very useful for tidal plant design,but
may have to be made more stringent for Knik-Turnagain conditions.
Figure 49."Phillips A”oil production platform in
upper Cook Inlet.
Figure 4:(Smith 2004)
Table 1:Ice design criteria for Cook Inlet petroleum industry platforms (Mulherin et al.2001)
Design parameter Original Design API (1988)recommended
Ice thickness,level ice (m)1.1 0.6-0.9
Ice thickness,rafter ice (m)NA 1.2-1.5
Compressive strength,unconfined (MPa)3.8 3.4-4.1
Compressive strength,confined (MPa)300 275 -330
Maximum load (MN)42 x 10°40 x 10°
TIDAL TURBINES AND ICE PROBLEMS IN KNIK ARM
Knik Arm is a more convenient area than Turnagain Arm to test tidal turbines,
because of existing dock and pile structures,and proximity to the potential electric loads
of downtown Anchorage and port activities.The bridge supports of the proposed Knik
Arm Bridge offer an attractive potential structure to mount tidal turbines for power
generation.Depending on the allowable size of the installation,these turbines could help
power the bridge lighting or contribute to the local power grid.Despite the wide
fluctuation in water level due to the strong tides,it is necessary that the position of the
tidal turbines be kept:
1.At optimal depth for tidal current power generation (strongest possible current)
2.Sufficiently below floating ice to avoid direct contact
A hydraulic lift system could support a turbine support structure mounted between two
bridge piers,for either a vertical-axis or horizontal-ice configuration.An automatic depth
control system with acoustic Doppler sensors,to sense the depth of the ice layer,would
be necessary for control of the vertical turbine position.In severe ice conditions,the
hydraulic lift system could keep the turbine carriage below the ice level minimum,
possibly up to 15 m below mean low tide depth,as depicted in the diagram below.
||Water Surface Level
4 fee on RITrat |cee wide
ant
_Hozting lee layer
Generator
"Ring"Mountings
Around Piling
Turbine and
Support Frame
Steel Piling
CONCLUSION
While they present serious challenges,ice problems alone should not preclude the
development of upper Cook Inlet's tidal energy resources.The economic impact of ice-
mitigation measures needs to be studied carefully.As with any source of energy,tidal
current energy has to be compared with other available sources of for a complete
assessment of economic feasibility.Other sources of electricity in the Anchorage area,
such as oil,gas,coal,wind,conventional hydropower or even geothermal,must be
compared with tidal with regards to future costs and availability.A Knik Arm tidal
power plant only as large as 10 MW could be useful to local utilities,or even for direct
power consumption at nearby port and military facilities.
REFERENCES
Acres International (1981).Preliminary Assessment of Cook Inlet Tidal Power:Phase I
Report,prepared for the State of Alaska,Office of the Governor,Division of Policy
Development and Planning.
BC Hydro Engineering (2002).Green Energy Study for British Columbia,Phase 2:
Mainland,Tidal Current Energy,prepared by Triton Consultants Ltd.,Vancouver,
BC.
Bernshtein,L.B.(1970)."Kislaya Guba Experimental Tidal Power Plant and Problem of
the Use of Tidal Energy,”Proceedings of an International Conference on the
Utilization of Tidal Power,Atlantic Industrial Research Institute,Halifax,Nova
Scotia.
Bernshtein,L.B.(1972).Gislogubskaya Tidal Electric Power Station.Energia,Moscow
[translated from Russian,published for the Division of Ocean Sciences,National
Science Foundation,Washington,D.C.,by the Al-Ahram Center for Scientific
Translations,Cairo (1978)].
Bernshtein,L.B.(1978)."Designing of Tidal Plants in the USSR,”Proceedings of the
Thirtieth Symposium of the Colston Research Society,University of Bristol,UK.
Bernshtein,L.B.(1992)."Tidal Power in Russia,”Tidal Power Trends andDevelopments:Proceedings of the 4”Conference on Tidal Power,Institution of Civil
Engineers,London,UK.
Johnson,M.,S.Okkonen and S.Sweet (1999)."Cook Inlet Tidal Currents and Acoustics
Measurements,”Proceeding of Cook Inlet Oceanography Workshop,Kenai,AK.
Muirhead,S.J.(1992)."The Environmental Effects of Tidal Energy,”Tidal power:Trends and Developments:Proceedings of the 4"Conference on Tidal Power,
Institution of Civil Engineers,London,UK.
Mulherin,N.D.,W.B.Tucker,O.P.Smith and W.J.Lee (2001).Marine Ice Atlas for
Cook Inlet,Alaska.Prepared for U.S.Army Corps of Engineers,Cold Regions
Research and Engineering Laboratory (CRREL),Hanover,NH.
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Perdichizzi,P.and T.Yasuda (1978)."Port of Anchorage Marine Terminal Design.”
Proceedings of the Conference on Applied Techniques for Cold Environments:
Volume IT,Anchorage,AK.
Rice,R.G.and G.C.Baker (1992)."Annapolis:The Straflo Turbine and Other OperatingExperiences,”Tidal Power Trends and Developments:Proceedings of the 4"
Conference on Tidal Power,Institution of Civil Engineers,London,UK.
Rodier,M.(1992)."The Rance Tidal Power Station:A Quarter of a Century Operation,”Tidal Power:Trends and Developments:Proceedings of the 4"Conference on Tidal
Power,Institution of Civil Engineers,London,UK.
Smith,O.P.(2004).Knik Arm Current,Sediment Transport,and Ice Studies,
prepared for PND Inc.consulting engineers,Anchorage,AK.
Tidal Electric (2001).Cordova Tidal Power Feasibility Study:Assess Cost,Benefits and
Feasibility of a 5 MW Tidal Power Facility Near Cordova,prepared by Tidal Electric
of Alaska Inc.,Anchorage,AK.
Wilson,E.M.and M.C.Swales (1970)."Tidal power from Cook Inlet,Alaska,”
Proceedings of an International Conference on the Utilization of Tidal Power,
Atlantic Industrial Research Institute,Halifax,Nova Scotia.
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