Tidal In-Stream Energy Overview
Brian Polagye
Research Assistant
University of Washington
Department of Mechanical Engineering
September 11, 2006
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Agenda
• Resource and Performance
• TISEC Devices
• Siting Arrays in Puget Sound
• UW Research
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Tidal power is different than other forms of renewable energy
Tidal Power
Resource and
Performance
- Comparison to Wind Wind
Tidal
Resource
• Driven by uneven heating of
earth’s surface by sun
• Occurs throughout the world
• Driven by gravitational pull of
moon and sun
• Highly localized - requiring
specific tidal range and bathymetry
Availability
• Intermittent
• Long-term predictions as good
as a weather forecast
• Intermittent
• Predictable centuries in advance
Proximity to
Loads
• Often distant from load centers
• Often close to load centers
• Mature technology
• Developing technology
Maturity
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There are two very different approaches to harnessing the energy of the tides
Resource and
Performance
Tidal Power
- Utilizing the Resource -
Barrage
• Dam constructed across estuary
― High cost ($ Bn)
― Long construction period (decade)
• Power produced by closing dam at
high tide and allowing water to run
through turbines once ocean has
returned to low tide
― Completely alters estuary circulation
― Power produced in twice-daily surge
― All attendant problems of hydroelectric dams
• Low-cost power production at very
large scale
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In-stream Tidal
• Turbines installed in estuary at
constrictions in groups called arrays
― Moderate unit cost ($ MM)
― Short unit construction time (weeks)
• Power produced directly from tidal
currents
― More continuous (but still
intermittent) power production
― Smart choice of turbines and layout
of arrays should avoid significant
environment impact
• Moderate-cost power production at
varying scales
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At a very basic level, tidal currents are generated by the rise and fall of the tides –
water runs downhill
Resource and
Performance
Tidal Currents
Side View
Top View
Ocean
Ocean
Water level
increasing
Flood tide
Slack water
Tidal
Basin
Tidal
Basin
Ebb tide
Water level
decreasing
Seabed
• Slack water
― Constant water height
― No velocity
• Flood Tide
• Ebb Tide
― Water level higher outside
estuary than in main basin
― Water flows into estuary
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― Water level higher in basin
than ocean
― Water flows out of basin
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Tidal currents vary primarily on a fourteen day lunar cycle
Resource and
Performance
Tidal Cycle
Spring Tides (strongest)
3
Current Velocity (m/s)
2
1
0
-1
-2
-3
-4
1-Feb
6-Feb
11-Feb
16-Feb
21-Feb
26-Feb
Date
Neap Tides (weakest)
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Flow power has a cubic dependence on velocity – small velocity changes have a
large effect on power
Device Performance
Resource and
Performance
- Resource Utilization -
Device Performance
Representative Day
9000
1400
Fluid Power
Power (kW)
1000
1
x Density x Velocity 3 x Area
2
Electric Power
800
Rated Speed
600
400
Electric Power
7000
Power (kW)

1200
Fluid Power
8000
6000
5000
4000
3000
2000
200
1000
Cut-in Speed
0
0.0
1.0
2.0
0
3.0
4.0
5.0
Current Velocity (m/s)
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0:00
4:48
9:36
14:24
19:12
0:00
Time
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Power generation varies day-to-day, but is consistent on a monthly basis and
shows no seasonal dependency
Device Performance
Resource and
Performance
- Variable Predictability -
Daily Average
Monthly Average
250
500
Average Power (kW)
Average Power (kW)
450
400
350
300
250
200
150
100
200
150
100
50
Date
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Dec
Nov
Oct
Sep
10/28 12/17
Jul
9/8
Jun
7/20
May
5/31
Apr
4/11
Mar
2/20
Feb
1/1
Jan
0
0
Aug
50
Month
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Agenda
• Resource and Performance
• TISEC Devices
• Siting Arrays in Puget Sound
• UW Research
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All turbines have a number of common components, but many variants
TISEC Devices
Turbine Overview
Gearbox
• Increase rotational speed of
shaft from turbine
• 80-95% efficient
Powertrain
or Drivetrain
Rotor
• Extracts power from flow
• Turns at low RPM
• Efficiency varies with flow
velocity (45% max)
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Generator and
Power Conditioning
• Generate electricity
• Condition electricity for
grid interconnection
• Turns at high RPM
• 95-98% efficient
Foundation
• Secure turbine to seabed
• Resist drag on support structure
and thrust on rotor
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Two basic types of rotors have been developed – horizontal axis and vertical axis
TISEC Devices
Rotor Variants
Horizontal Axis
Vertical Axis
Gearbox and
Generator
Gearbox and
Generator
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Ducted turbines have been proposed to augment power production
Power Augmentation
TISEC Devices
• Enclosing turbine in diffuser duct boosts
power
• A number of questions remain unanswered
regarding this approach
• Is it economically justified?
―Ducts were never justified for wind turbines
―Different set of circumstances for tidal
turbines
• Is there an increased hazard to marine
mammals and fish?
―Can a large fish or mammal become trapped
in the duct?
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Foundation selection is usually driven by site water depth
TISEC Devices
Foundation Types
Monopile
Hollow steel pile driven or
drilled into seabed
Gravity Base
Pros: • Deep water installation
feasible
Pros: • Small footprint
• Established technology used
in offshore wind
(10-40m)
Chain Anchors
Cons: • Large footprint
• Scour problems for some
types of seabed
• Decommissioning problems
Cons: • High cost in deep water
• Installation expensive for
some types of seabed
Chains anchored to seabed and
turbine
Pros: • Small footprint
• Deep water installation
feasible
Cons: • Problematic in practice
• Device must have high
natural buoyancy
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Heavy foundation of concrete
and low cost aggregate placed
on seabed
Tension Leg
Submerged platform held in
place by anchored cables under
high tension
Pros: • Small footprint
• Deep water installation
feasible
Cons: • Immature technology now
being considered for
offshore wind in deep water
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TISEC Devices
Maintenance Options
Divers
Divers service turbine
• Marine intervention extremely costly and
must be minimized if TISEC devices can
hope to compete economically
Pros: • Divers widely available
Cons: • Difficult to work underwater
• Very high intervention cost
• In deep water, dive time
measured in minutes per day
• All device developers pursuing lowmaintenance philosophies
Device Retrieval
Crane barge mobilized to
retrieval entire turbine
Pros: • Less costly than divers
• Deep water feasible
Cons: • High cost to mobilize
heavy-lift crane barge
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Integrated Lift
Lifting mechanism integrated
directly into turbine support
structure
Pros: • Maintenance without specialty
craft
• Deep water feasible
Cons: • Cost of lifting mechanism
• Support structure may be
surface piercing (aesthetic
and shipping concerns)
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Marine Current Turbines is furthest along in the development process
Marine Current Turbines (MCT)
Power train
TISEC Devices
Horizontal axis (2 bladed)
Planetary gearbox
Induction generator
Rated from 1.2 – 2.5 MW
Foundation
Monopile drilled or driven into seabed
Two turbines per pile
Maintenance
Lifting mechanism pulls turbine out of
water for servicing
Development
Large Scale
(18 m diameter)
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3 years of testing prototype in UK
1.5 MW demonstration planned for
installation in 2006/2007
Conceptual fully submerged units
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Verdant is positioned to install the first array of TISEC devices in the world
TISEC Devices
Verdant
Power train
Small Scale
(5 m diameter)
Horizontal axis (3 bladed)
Planetary gearbox
Induction generator
Rated at 34 kW
Foundation
Monopile drilled or driven into seabed
Maintenance
Retrieval of power train by crane barge
Divers employed during installation
Development
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Installing 6 turbines off Roosevelt
Island, NY City (Starting mid-Sept)
First permitted test project in US
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Lunar Energy has adopted a different philosophy with an emphasis on a
“bulletproof” design
Lunar Energy
Power train
Foundation
TISEC Devices
Horizontal axis (ducted)
Hydraulic gearbox
Induction generator
Rated at 2 MW
Gravity foundation using concrete and
aggregate
Maintenance
Heavy-lift crane barge recovers
“cassette” with all moving parts
Development
Tank testing
Nearing end of design for first large
scale unit
Large Scale
(21 m diameter inlet)
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Agenda
• Resource and Performance
• TISEC Devices
• Siting Arrays in Puget Sound
• UW Research
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Environmental issues are probably the biggest unknown for siting arrays of tidal
in-stream turbines
Case
Siting
Study
Environmental Issues
- Marine Life Considerations Environmental
Issue
Key Questions
Answers (so far)
Direct “impact”
of turbine on
marine life
• Will a turbine make sushi in
addition to electricity?
• No. Maximum tip velocity limited by
cavitation. (~10 RPM for large turbines)
• Will the rotor injure or harass
fish and marine mammals?
• Unknown. Considerable cost and effort being
expended by developers to prove technology
is benign. No Altamont Passes.
Indirect impacts
• Will anti-fouling paints used
on turbines and supports
degrade environment?
• Developers are testing inert, glass-based
anti-fouling paints to minimize this impact.
• Will oils and lubricants leak
from the turbine?
• Not in large quantities, but developers are
working to minimize any leakage.
• How much of the seafloor
will be disturbed during
installation?
• Depends on type of foundation and
construction techniques. Choices will be
driven by site depth and local concerns.
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Case
Siting
Study
Environmental Issues
Environmental
Issue
Effect of energy
extraction on the
environment
Key Questions
• What is the effect of energy
extraction?
Answers (so far)
• Altered circulation in estuary
• Effects complicated and counterintuitive
― Velocity increases downstream of an
array and water depth decreases
― Overall flow rates are reduced
• How much energy can be
extracted without substantially
altering circulation?
• Rough estimates. 15% of the kinetic
energy in a channel used as
placeholder in resource studies.
― Overly conservative in some cases,
overly optimistic in others.
― Question needs to be addressed on a
case-by-case basis
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In addition to environment, a number of factors need to be considered when siting
turbine arrays. Most have not yet been addressed for sites in Puget Sound.
Case
Siting
Study
Array Siting Issues
- General Issue
Key Questions
Status
Resource Size
and Quality
• How large is the extractable
resource?
• How many turbines in an
array?
• Preliminary estimates using NOAA
single-point current predictions
• Next Step: Current measurements
Electrical
Infrastructure
• Will new transmission lines
need to be built?
• What local loads exist?
• Not yet determined – requires
consultation with local utilities
Bathymetry and
Seabed Geology
• What foundation types are
suitable for water depth?
• What foundations can seabed
support?
• Not yet determined – requires
geologic survey
Port Facilities
• Are there local marine contractors
capable of performing installation
and maintenance of an array?
• Not an issue in Puget Sound for
most types of construction
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And the list goes on…
Case
Siting
Study
Array Siting Issues
- General Issue
Key Questions
Status
Shipping
Traffic
• What is the maximum draft of
shipping traffic in channel?
• Not yet determined – requires
consultations with marine exchange
and Coast Guard
Large-scale
Turbulence
• Are there local geographic
features that would give rise to
large-scale eddies?
• Not yet determined – requires
consultations with oceanographic
experts
Multiple Use
• How is the site currently used?
• Does the site overlap with
major recreation or fishing
areas?
• Not yet determined – requires
consultations with regional
stakeholders
Economics
• Will turbines produce costeffective power?
• Tacoma Narrows study predicted a
cost of energy of ~10 cents/kWh
• Next step: Feasibility study
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There are a number of prospective tidal energy sites in Puget Sound
Puget Sound Resource Study
Siting
- Overview -
Spieden
Channel
Guemes
Channel
San Juan
Channel
Deception
Pass
Admiralty
Inlet
Agate
Passage
Rich
Passage
Site
Power Density
(kW/m2)
Resource
(MW)
Depth
(m)
• Tacoma Narrows
1.7
106
40
• Admiralty Inlet
―Point Wilson
―Marrowstone
―Bush Point
0.6
0.6
0.4
167
195
132
60
71
75
• Deception Pass
―Deception Pass
―Yokeko Point
5.5
0.4
26
3
30
16
• Guemes Channel
1.5
35
14
1.5
0.9
3
9
6
15
0.6
0.6
45
56
63
69
• Bainbridge Island
―Agate Passage
―Rich Passage
• San Juan Islands
―San Juan Channel
―Spieden Channel
Tacoma Narrows estimated COE ~10 cents/kWh. Other sites?
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San Juan Channel represents a substantial resource, but the channel is quite deep
San Juan Channel
Siting
- Overview Preliminary Turbine
Layout
Preliminary Array Layout
0.8 km
(0.5 mi)
Turbine +
Lateral Spacing
San Juan
Channel Ref.
0.6 kW/m2
• 116 turbines (20 m diameter)
• Average installation depth ~95m
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Preliminary Array
Performance
• 5 MW average electric power
• 16 MW rated electric power
• 39,900 MWh annual generation
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Spieden Channel also represents a substantial resource, but is again a deep water
channel
Spieden Channel
Siting
- Overview Preliminary Turbine
Layout
Preliminary Array Layout
1 km (0.6 mi)
Turbine +
Lateral Spacing
Limestone Point
Ref.
• 168 turbines (20 m diameter)
• Average installation depth ~83m
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0.6 kW/m2
Preliminary Array
Performance
• 8 MW average electric power
• 26 MW rated electric power
• 62,700 MWh annual generation
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Agenda
• Resource and Performance
• TISEC Devices
• Siting Arrays in Puget Sound
• UW Research
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Question 1: How much tidal energy can be environmentally extracted?
Extraction Limits
- Balancing Resource Against Environmental Impact -
?
Point
Wilson
UW
Case
Research
Study
• How much kinetic energy can be extracted by
an array?
Admiralty
Head
― Current estimates are 15% of kinetic energy
in a channel (little physical reasoning)
― Probably much more site specific and
closely related to frictional losses in channel
?
• Does the construction of one array preclude
the construction of others?
Marrowstone
Point
― Can 20+ MW arrays be built at Pt. Wilson,
Marrowstone and Bush Point?
― Can an array be built at Admiralty Inlet if
one already operating in Tacoma Narrows?
Indian Island
Bush Point
?
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• Building an understanding with 1-D models
― Very interesting preliminary results
― Will be expanding to 2-D and 3-D cases
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Question 2: How tightly can turbines in an array be packed?
Array Packing
- Most Economic Use of Resource -
• Regions of high power flux may be
relatively short and narrow
Low Power
Density
San Juan
Island
Lopez
Island
High
Power
Density
Low Power
Density
• Economic reasons to site as many
turbines in high power density
regions as possible
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UW
Case
Research
Study
• How close is too close?
― Since flow is bi-directional, wind
turbine spacing rules are probably
too conservative
― Downstream turbines must be
beyond wake of upstream turbines
― Wakes degrade performance and
accelerate metal fatigue
• Approaching with a combination of
analytical and computational tools
― Little or no physical data available
(since no arrays operating)
― Plan to leverage results of CFD
modeling to suggest “engineering
rules” for array layouts
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