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B
Interim Letter Report
NRC ASSESSMENT OF MARINE AND
HYDROKINETIC ENERGY TECHNOLOGY:
INTERIM LETTER REPORT
Released on July 12, 2011
Division on Engineering and Physical Sciences 500 Fifth Street, NW
Board on Energy and Environmental Systems Ninth Floor
Washington, DC 20001
Phone: 202 334 3344
Fax: 202 334 2019
July 12, 2011
Dr. Henry Kelly
Acting Assistant Secretary
Office of Energy Efficiency and Renewable Energy
U.S. Department of Energy
1000 Independence Avenue, S.W.
Washington, DC 20585
RE: NRC Assessment of Marine and Hydrokinetic Energy Technology:
Interim Letter Report
Dear Dr. Kelly:
The Department of Energy’s (DOE’s) Wind and Water Power Pro-
gram requested that the National Research Council (NRC) provide an
evaluation of the detailed assessments being conducted by five indi-
vidual resource assessment groups for the DOE, estimating the amount of
extractable energy from U.S. marine and hydrokinetic (MHK) resources.
117
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118 APPENDIX B
In response, the NRC formed the Committee on Marine and Hydro
kinetic Energy Technology Assessment, which has begun its review of the
resource assessments.
In this letter report, the committee responds to its charge of writing
an interim report assessing the methodologies, technologies, and assump-
tions associated with the wave and tidal energy resource assessments. The
DOE specifically requested that these two MHK resource assessments be
evaluated in the interim report and that the committee’s final report also
cover the three other assessments—those on free-flowing water in rivers
and streams, on marine temperature gradients, and on ocean currents.
Attachment A contains the committee’s statement of task. Attachment B
presents biographical information on the committee members.
The committee presents this letter report, in accord with the statement
of task, as its preliminary assessment of methodologies and assump-
tions used in the estimation of wave and tidal resources. The commit-
tee’s review is based on the presentations that it received from the wave
resource assessment group (which consists of the Electric Power Research
Institute [EPRI] working with Virginia Polytechnic Institute and State
University [Virginia Tech] and National Renewable Energy Laboratory
[NREL]) and the tidal resource assessment group (consisting of Georgia
Institute of Technology [Georgia Tech] working with Oak Ridge National
Laboratory [ORNL]). These presentations were made at the committee’s
first two meetings, in November 2010 and February 2011. The committee
also received presentations from the DOE as well as written information
submitted by all five of the resource assessment groups.
Although the wave resource and tidal resource assessment groups
will eventually release final reports, their reports were not available for
the development of this interim report. Thus, the committee believes that
it is important to complete its interim letter report at this point, not only
for the letter report’s potential impact with respect to the wave resource
and tidal resource assessments, but also to provide timely feedback to
the other assessment groups. The committee will continue to review the
methodologies and assumptions that are used in all five of the assess-
ments, as it completes its study and writes its final report (currently
scheduled for completion in the spring of 2012).
In the sections that follow, the committee first describes the motivation
for and purpose of this report. It then presents the conceptual framework
of the overall MHK resource assessment process that it developed in order
to have a consistent, clear set of definitions and a framework for assessing
the approaches of the individual groups. The committee’s evaluation of the
wave resource assessment and of the tidal resource assessment is presented
in the next two sections, with conclusions and recommendations in each. A
final section on overarching conclusions completes the body of the report.
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APPENDIX B 119
As elaborated on in the sections that follow, the committee concludes
that the overall approach taken by the wave resource and tidal resource
assessment groups is a useful contribution to understanding the distribu-
tion and possible magnitude of energy sources from waves and tides in
U.S. waters. However, the committee has concerns regarding the useful-
ness of aggregating the analysis to produce a “single number” estimate
of the total national or regional theoretical and technical resource base
(defined in the section below entitled “Conceptual Framework”) for any
one of these sources. The committee also has some concerns about the
methodologies and assumptions, as detailed in the sections below. For the
wave resource assessment, the committee is particularly concerned with
the extension of the analysis into shallow depths, where the modeling is
most inaccurate. One important issue for the tidal resource assessment
is the lack of clarity on how the assessment group will incorporate any
sort of technological considerations into its resource assessment. The
committee is also concerned about the limited scope of the assessments’
validation exercises. These issues are discussed further below.
MOTIVATION FOR AND PURPOSE OF THE INTERIM REPORT
Marine and hydrokinetic resources are increasingly becoming part
of energy regulatory, planning, and marketing activities in the United
States and elsewhere. In particular, state-based renewable portfolio stan-
dards and federal production and investment tax credits have led to an
increased interest in the possible deployment of MHK technologies. This
interest is reflected in the number of requests for permits for wave, cur-
rent, tidal, and river-flow generators that have been filed recently with
the Federal Energy Regulatory Commission (FERC); at the end of 2010
FERC had issued preliminary permits for 110 projects and had another
12 preliminary permits pending. It should be noted that although permit
activity is a measure of the potential interest in MHK resource develop-
ment, it is not a reliable predictor of the future development of hydro
kinetic resources because developers apply for permits before planning
the facility or obtaining financing.
In order to assess the overall potential for U.S. MHK resources and
technologies, the DOE is funding the following: (1) detailed resource
assessments for estimating what the DOE terms the “potential extractable
energy” for each resource and (2) projects for generating the technology-
related data necessary for estimating the expected performance of the wide
variety of technology designs currently under consideration (DOE, 2010;
Battey, 2010, 2011). The objective of the DOE’s work in the area of MHK
resource assessments is to help the DOE prioritize its overall portolio
f
of future research, increase the understanding of the potential for MHK
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120 APPENDIX B
resource development, and direct MHK device and/or project develop-
ers to locations of greatest promise (Battey, 2011). In terms of resource
assessments, the Energy Policy Act of 2005 (Public Law 109-58) directed
the DOE to estimate the size of the MHK resource base. Earlier estimates
(EPRI, 2005, 2007) of the amount of energy that could be extracted from
MHK resources are based on limited and possibly inaccurate data regard-
ing the total resource size and on potentially dated assumptions related
to the amount of each resource that might ultimately prove extractable.
To improve these estimates, the DOE contracted with the five assessment
groups referred to above to conduct separate estimates of the extractable
energy from five categories of MHK resources: waves, tidal currents,
ocean currents, marine temperature gradients, and free-flowing water in
rivers and streams (DOE, 2010). Performing these assessments requires
that each group estimate the average power density of the resource base,
as well as the basic technology characteristics and spatial and temporal
constituents that convert power into electricity for that resource. Each
assessment group is using distinct methodologies and assumptions. This
NRC committee is tasked with evaluating the detailed assessments pro-
duced for the DOE, reviewing estimates of extractable energy (typically
represented as average terawatt-hours [TWh] per year)1 and technology
specifications, and accurately comparing the results across resource types.
In reviewing the initial methodologies from the five U.S. MHK
resource assessment groups contracted by the DOE, the committee
observed that the groups all employed different terminology to describe
similar results. Thus, besides providing its review comments on each
individual assessment, the committee is also taking on the role of provid-
ing a forum for comparing and contrasting the approaches taken by the
respective assessment groups. To that end, the committee developed the
conceptual framework of the overall MHK resource assessment process,
presented in the section below, in order to help develop a common set of
definitions and approaches.
1 Note that terawatt-hours per year can be translated into units of power, such as
g
igawatts, and used to represent the average power generation over the time period indi-
cated. However, a unit such as terawatt-hours per year (or, as shown in an electricity bill,
kilowatt-hours per month) is a standard unit for the electricity sector. Energy units such
as kilowatt-hours or terawatt-hours measure the commodity that is generated by power
plants and sold to consumers. For example, the Energy Information Agency’s table of total
electricity generation (see http://www.eia.doe.gov/aer/pdf/pages/sec8_8.pdf) is given in
billions of kilowatt-hours per year.
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APPENDIX B 121
CONCEPTUAL FRAMEWORK
In order to develop its approach to the study task and to review the
individual resource assessments, the committee developed a conceptual
framework (Figure 1) for visualizing the processes used to develop the
assessment results requested by the DOE. This framework establishes
a set of three terms—theoretical resource, technical resource, and practical
resource—and their definitions, provided below, to clarify elements of
the overall resource assessment process as described by each assessment
group and to allow for a comparison of different methods, terminology,
and processes among the five assessment groups. The committee recog-
nizes that communities involved with other energy types, such as wind
and fossil fuels, use different terms to describe their resource bases (i.e.,
“resources,” “proven reserves”). It has instead chosen to follow emerg-
ing trends in terminology for MHK resources as used in the European
marine energy community, including the European Marine Energy Centre
(EMEC; http://www.emec.org.uk/standards.asp). The EMEC terminol-
ogy has been submitted to the International Electrotechnical Commission
for consideration as the basis of an international standard. In addition to
employing terminology used in the European marine energy community,
the committee developed Table 1 as a common source of definitions and
units used in this report.
FIGURE 1 Conceptual framework developed by the committee for marine and
hydrokinetic resource assessments. NOTE: GIS, Geographic Information System;
TBD, to be determined.
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122 APPENDIX B
TABLE 1 Definitions Used by Department of Energy Marine and
Hydrokinetic Resource Assessment Groups and National Research
Council (NRC) Committee
Term to be
Quantified Definition Units Notes
General
Energy The capacity to do work Joules (J)
Power Energy per time Watts (W) =
joules per
second
Resource Average annual power Terawatt- Representing a
hours (TWh) potential energy
per year resource base for
(1 TWh/yr = the electricity
114 megawatts sector in
[MW]) terawatt-hours.
Waves
Wave power Power of waves per unit Watts per Horizontal
density crest length based on meter energy flux
(Mei, 1989) (power density);
Pvector = ρg Σ S(f,θ)cg df applies to a
single device.
Wave power Power of waves per unit Watts per Horizontal
density circle based on meter energy flux;
(Electric Power applies to a
Research Pscalar = ρg ΣΣ S(f,θ)cg df dθ single device.
Institute [EPRI])
Total regional Based on annual average Terawatt- Overestimates
wave resource sum of wave power density hours per the total
(EPRI) along a line defining a year resource by
region of coastline, such as a (= 114 MW) including energy
bathymetric contour. flux along the
line.
Pcoast = Σ Pscalar dl
Total regional Based on annual average Terawatt- Remains
wave resource sum of wave power density hours per approximately
(recommended crossing perpendicular to year constant as
by this a line defining a region (= 114 MW) waves travel
committee) of coastline, such as a shoreward from
bathymetric contour. deep water.
Pcoast = Σ Pvector cosθ dl
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APPENDIX B 123
TABLE 1 Continued
Tides
Tidal power Power of horizontal tidal Watts per Horizontal
density currents flowing through a square meter kinetic energy
vertical plane of unit area. (W/m2) flux; applies to a
single device.
P = ½ρu3
Total regional Based on annual average Terawatt- Maximum
tidal resource power available from a tidal hours per power
(Garrett and bay or channel year obtainable with
Cummins, 2008) (= 114 MW) a complete tidal
Pmax = 0.22ρgaQmax fence; equivalent
to a barrage.
NOTE:
List of variables:
ρ = water density
g = gravitational acceleration
S = wave spectrum (sea-surface height variance, per frequency and direction)
cg = wave group speed
f = wave frequency
θ = wave direction
l = length of coastline, depth contour, or other region
u = tidal current speed
a = tidal amplitude (half of tidal range)
Qmax = maximum horizontal tidal volume flux (over tidal cycle)
The theoretical resource, shown in the left column of the conceptual
framework in Figure 1, is defined as the average annual energy produc-
tion for each source of hydrokinetic energy. Determining the theoretical
resource requires a series of inputs (including methods, models, assump-
tions, and data and observations) for each source of hydrokinetic energy
(e.g., waves, tides). In response to the original DOE request, some, but
not all, of the assessment groups have identified paths designed to pro-
duce two key outputs for the theoretical resource: (1) overall regional
or national numbers for the U.S. theoretical resource, expressed as an
average annual energy resource (typically in terawatt-hours per year);
and (2) a Geographic Information System (GIS) database that represents
the spatial variation in average annual power density in units appropri-
ate for each source (i.e., watts per meter for waves or watts per square
meter for tides).
The technical resource (center column in Figure 1) is defined as the
portion of the theoretical resource that can be captured using a specified
technology. For each resource, there are technological constraints that
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124 APPENDIX B
represent how much of the theoretical resource can actually be extracted.
The committee conceptualizes these constraints as “extraction filters” con-
sisting of physical and technological constraints, including back effects2
and technological characteristics associated with one or more energy-
extraction devices (representing factors such as device efficiency, device
spacing requirements, and cut-in and cut-out parameters).3 Some of these
filters are resource-specific; others are applicable across all of the MHK
types. During presentations made to the committee and from its discus-
sion with the DOE and the assessment groups, it became clear that each
group offers a different interpretation of what types of constraints need
to be included among its extraction filters. However, it is clear to the com-
mittee that estimating the technical resource from the theoretical resource
requires filters that represent physical and technological constraints asso-
ciated with energy-extraction devices. Outputs related to the technical
resource include an estimate of the energy resource and a GIS represent-
ing spatial and temporal variation in the resource associated with various
technologies. In the committee’s view, the assessment groups determined
that reporting the technical resource represented the completion of their
projects.
Some of the assessment groups recognized that, beyond the extrac-
tion filters, there were additional filters influencing when and where
devices could be placed. The practical resource (right-hand column in
Figure 1), is defined as that portion of the technical resource available
after consideration of all other constraints. In the conceptual framework,
these constraints are captured in socioeconomic filters. For example, the
fi
lters involving logistical and economic considerations include costs of
raw materials and maintenance, resources associated with transmission
and distribution, electricity demand, and the cost of electricity. Environ
mental and use constraints include issues relating to a variety of impacts
on the environment (e.g., protecting threatened species or ecologically
sensitive areas), sea-space conflicts (e.g., involving shipping channels,
navigation, protected areas), and multiple- or competing-use issues (e.g.,
fisheries, viewshed impacts, recreation, national security). Such filters are,
by nature, specific to and critical at the local sites where decision making
related to marine and hydrokinetic projects will occur.
A determination of the practical resource is beyond the scope of the
resource assessment groups’ tasks as defined by the DOE. However,
2 A back effect refers to the modification of an energy resource owing to the presence of
an extraction device. In the case of turbines in a river or tidal channel, the back effect is the
modification of currents in the whole cross section of the channel, particularly the reduction
in the volume flux through the channel.
3 In some cases, such as for tidal resources or steady currents, the estimation of the theo-
retical resource requires allowance for back effects.
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APPENDIX B 125
the committee sees the constraints represented by the socioeconomic filters
as being among the most important set of considerations influencing future
investments in marine and hydrokinetic energy. The socioeconomic filters
are also the most important set of considerations if one is to develop an
assessment of what might ultimately be considered the maximum estimate
of MHK resources that could be used to generate electricity. An approach
for assessing these socioeconomic considerations might be to merge the
GIS databases resulting from the theoretical and technical resources with
existing spatial information about other economic and ecological uses of
the ocean and coast, such as shipping channels and areas associated with
critical habitats and species. Although such information would be helpful
in highlighting potential multiple-use conflicts, it will not be sufficient for
quantifying the practical resource base. The quantification of the practi-
cal resource could be done as part the planning processes for site-specific
management or for local, state, or regional management.
As discussed below, the wave and tidal resource assessment groups
employ different GIS platforms to display their results. Given that one
of the DOE’s objectives is to be able to compare the various resource
types with one another, this lack of coordination among the assessment
groups precludes the easy integration of all resource assessments into a
single database and seems counterproductive to the ultimate DOE goals.
Moreover, this same coordination and consistency would, if present, help
the five resource assessment groups develop resource assessments that
are easily comparable and that could be easily integrated into a common
platform. Given that many of the extraction and the socioeconomic filters
might be similar across the assessment groups, coordination would also
help in the development of a GIS database useful to policy makers and
developers.
The DOE requested that the assessment groups determine the “maxi
mum practicable, extractable energy.” Although maximum practicable,
extractable energy could possibly refer to the practical resource in the
conceptual framework shown in Figure 1, discussion with the DOE and
the assessment groups led the committee to conclude that the term is
instead equivalent to the technical resource in the conceptual framework.
It was also made clear that the assessment effort did not include incor-
porating site-specific information that would be required to define the
practical resource base.
Additionally, there is a lack of clarity on the geographic scope for
the estimate of maximum practicable, extractable energy. It is unclear
from discussions with the DOE and the assessment groups whether the
estimate is to be a national, regional, or local resource estimate. The
committee finds that the resource estimates, especially the resource base
aggregated to a regional or national level, have both limited utility and
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126 APPENDIX B
potential for misuse. Although such estimates might provide broad order-
of-magnitude estimates of which resources have the greatest potential, the
conceptual framework shown in Figure 1 clearly illustrates that there are
many extraction filters needed to determine the technical resource. The
assessment groups can only assess a few of these filters, and many of the
filters require assumptions about which particular MHK technologies will
be used. Moreover, a wide array and diversity of socioeconomic filters
ultimately limit only a portion of this technical resource base to be repre-
sentative of what the maximum practicable, extractable energy might be
from MHK resources.
WAVE RESOURCE ASSESSMENT
Introduction
Power in ocean waves originates as wind energy that is transferred
to the sea surface when wind blows over large areas of the ocean. The
resulting wave field consists of a collection of waves at different frequen-
cies traveling in various directions, typically characterized by a direc-
tional wave spectrum. These waves travel efficiently away from the area
of generation across the ocean to deliver their power to nearshore areas.
Wave power density is usually characterized as power per length of
wave crest; it represents all the energy crossing a vertical plane of unit
width per unit time. This vertical plane is oriented along the wave crest
and extends from the sea surface down to the seafloor. To capture this
orientation, wave power is expressed as a vector quantity, and accurate
representation of its magnitude and direction requires the consideration
of the full directional wave spectrum. Note that the wave energy conver-
sion devices currently under development are designed to operate at
different locations in the water column, and only a portion of this overall
wave power may be available to these devices (e.g., devices that respond
only to heave motions associated with the waves). As noted in the discus-
sion above of the committee’s conceptual model, the considerations of the
amount of power that can be extracted by specific wave power devices are
incorporated in the estimation of the technical resource.
Because wave energy travels in a particular direction, care must be
taken when interpreting maps that show wave power density as a func-
tion of location but do not indicate predominant wave directions. It also
must be recognized that if the energy is removed from the wave field at
one location, by definition less energy will be available in the shadow
of the extraction device. It would not be expected that a second row of
wave energy extraction devices would perform the same as the first row
of devices that the wave field encountered, because any recovery of the
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APPENDIX B 127
wave field due to additional wind input (if present) would occur over
distances much larger than the spacing between rows of wave energy
extraction devices that are currently under consideration. This shadow-
ing effect implies that it is erroneous to estimate the theoretical resource
as the sum of the wave power density over an area as one might do for
solar energy. Note that the magnitude of this shadowing effect is likely to
be highly dependent on the specific characteristics of the device (e.g., size,
efficiency). Although there are some initial publications with rigorous
analytical approaches for quantifying the effect of an arbitrary array of
point absorbers devices (e.g., Garnaud and Mei, 2010), shadowing effects
due to realistic devices are a topic of active research. The planning of any
potential large-scale deployment of wave power devices would require
sophisticated, site-specific field and modeling analysis of the devices’
interactions with the wave field.
One approach to interpreting wave power density maps correctly is
to evaluate the wave energy traveling shoreward across a line parallel to
the coastline (perhaps located on a bathymetric contour). This is shown in
Table 1 as the “total regional wave resource” assessment recommended by
the committee. Provided that the selected line is on the continental shelf,
it is reasonable to assume that the winds do not add significant energy
to the wave field after the waves cross this line. In this case, the wave
power density across such a line provides a reasonable approximation
to the theoretical resource that represents the wave energy available to
nearshore wave energy devices in a region. To do this estimate properly,
wave direction information, in addition to the wave frequency spectrum,
must be known.
Description of Wave Resource Estimate
The wave resource assessment group was tasked with producing
estimates of the theoretical and technical resource in U.S. coastal regimes.
In order to obtain estimates of the theoretical wave resource (left column
in Figure 1), the wave resource assessment group utilizes a hindcast of
wave conditions that was assembled by the National Oceanic and Atmo-
spheric Administration’s (NOAA’s) National Center for Environmental
Prediction (NCEP) using its wave-generation and -propagation model
WAVEWATCH III. The hindcast generally provides wave parameters over
a 4’ x 4’ grid, although the resolution is coarser in a few areas (Alaska, for
example, is gridded at 4’ x 8’) (Jacobson et al., 2010). Thus the resolution is
generally on the order of many kilometers, whereas the shelf bathymetry
can vary rapidly over a few hundred meters. The assessment and valida-
tion groups first resolve several potential issues related to the available
hindcast (i.e., a short data record of only 51 months, a lack of full spectral
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APPENDIX B 133
well as information about expected extreme conditions. In its final report,
the committee will also consider how information on the MHK resource
base might be overlaid on other ocean uses (e.g., fishing grounds, naviga-
tional concerns, recreation areas) to make an assessment of the practical
resource base.
Recommendations: The committee recommends that the wave resource
assessment group’s approach to estimating the theoretical resource base
acknowledge that the energy flux (power density) of waves is a direc-
tional quantity, and it recommends that the approach consider only the
component of the wave power density vector that is perpendicular to
the line of interest. Hence, as indicated in Table 1, rather than summing
over a collection of cylinders, a simple line integral should be computed.
The committee also recommends that strong caveats accompany the
estimates of the total theoretical and technical resources.
The wave resource assessment group should be very cautious in
presenting information for shallow-water environments, where its
approach is most inaccurate. There has been a recent trend to envision
wave energy extraction farther offshore, in deep water, to avoid some
ecological and other impacts. However, some potential projects are still
seeking shallow-water siting for the closer proximity to transmission and
other logistical requirements. Shallow-water sites also generally have
lower construction and maintenance costs. Given that the actual place-
ment of devices may occur in such shallow-water areas, the committee
recommends that any siting considerations be accompanied by a model
ing effort that resolves the bathymetric variability on the inner shelf
and accounts for the physical processes that dominate in shallow waters
(e.g., refraction, diffraction, shoaling, wave dissipation due to bottom fric-
tion and wave breaking). The wave resource assessment group should
provide to any potential developers and to other users guidance in the
application of this assessment in shallow-water areas. For example, some
virtual stations could be established where the full directional spectrum
would be available for potential users. A developer or coastal engineer
could then perform high-resolution simulations and the necessary field-
work to develop local fields using a shallow-water model such as SWAN
combined with an accurate bathymetry.
Additionally, the committee recommends that the wave resource
assessment group clearly define the GIS outputs. The full directional wave
energy spectrum should be included in order to retrieve the directionality
and the time series of the wave parameters, which would allow the GIS
data to be used either as input for a more detailed analysis in shallow
water or as an informative wave climate geographic tool. Simple sum-
mary plots would be convenient to give an overview of the wave climate
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134 APPENDIX B
as wave power roses (diagrams showing the distribution of wave height
and direction), probability distribution of wave parameters, wave power
monthly average time series, and Gumbel distribution of the extreme
events.
TIDAL RESOURCE ASSESSMENT
Introduction
Ocean tides are a response to gravitational forces exerted by the
Moon and the Sun. They include the rise and fall of the sea surface and
the associated horizontal currents. The potential of tidal power for human
use has traditionally led to proposals that envision a barrage across the
entrance of a bay that has a large range between low and high tides. A
simple operating scheme is to release water trapped behind the barrage
at high tide through turbines, generating power as is done in a traditional
hydropower facility.
In recent years, considerable attention has been paid to the direct
exploitation of tidal currents using in-stream turbines rather than a bar-
rage, in a manner similar to the way that wind turbines work. By way of
scale comparison, even a strong current of 3 m/s (~10 ft/s) is equivalent
to a hydraulic head of only 0.5 m (~1.6 ft), which is considerably less head
than a typical tidal range. As the power produced by a turbine is related
to the product of the head and the flow rate, it is clear that capturing tidal
currents is considerably less effective than capturing the hydraulic head
associated with a modest tidal range.
The upper bound on the power from such an in-stream turbine
is shown in Table 1 and is expressed by the Lanchester-Betz limit of
0.3rAu3, where r is water density, u is current speed, and A is the cross-
sectional area across the blades (also referred to as the swept area).4 The
L
anchester-Betz limit shows that the turbine power is related to the cube
of the current and demonstrates the advantage of deploying turbines in
regions of strong current. As an example, if the cross section area A is
100 m2 (~1,075 ft2) and the current speed u is 3 m/s, the upper bound
on the power from a turbine is 0.8 MW. The average power over a tidal
cycle is, of course, considerably less than that obtainable at the maximum
current.
4 TheLanchester-Betz limit applies to a turbine in an unbounded flow. If a turbine array
occupies a significant fraction of the channel cross section, it can create a sufficient blockage
and build up a large head, and more power can be obtained. This could ultimately approach
the power from a barrage, if the array blocks the entire channel cross section (Garrett and
Cummins, 2007).
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APPENDIX B 135
Project Description
The tidal resource assessment group conducted its tidal energy assess-
ment study by developing a set of models to simulate all U.S. coastal
regions and to estimate the maximum tidal energy based on predicted
tidal currents (Georgia Tech Research Corporation, 2010; Haas et al., 2010;
Georgia Tech Research Corporation, 2011; Haas et al., 2011). The model
used in the study was the three-dimensional Regional Ocean Model-
ing System (ROMS),5 which is often used in model studies of coastal
oceanography and tidal circulation. The model was configured with eight
l
ayers and set up for 51 domains, with grid resolutions in the range of
200 to 500 m. Each domain included a section of coast or a particular bay,
with offshore boundaries that included part of the adjacent continental
shelf. The models were forced at their offshore boundaries by predicted
tidal constituents, using the Advanced Circulation Model (ADCIRC) tidal
database6 for the East Coast and Gulf of Mexico regions and the TPXO
database7 for the West Coast region. River inflows and atmospheric forc-
ing (such as wind) were not considered, and stratification and density-
induced currents were not simulated. The landward model boundaries
and bathymetry were defined using coastline data from NOAA’s National
Ocean Service and digital sounding data from NOAA’s National Geo-
physical Data Center. The effect of tidal flats was initially evaluated but
not considered in the final model runs.
The tidal resource assessment group calibrated the tidal models by
adjusting the single friction coefficient to improve the comparison among
model results, NOAA predictions of tidal elevation and currents, and
limited observations of depth-averaged tidal currents. Model validation
performed by the Oak Ridge National Laboratory was done by comparing
model predictions with observed tidal elevations and currents at selected
stations that were not included in the calibration exercises (ORNL, 2011;
Neary et al., 2011). Model skills and error statistics were generated in this
validation.
Model output was used (1) to provide an upper bound, Pmax , of the
power available from in-stream turbines for each bay and (2) to create a
web-based GIS interface of quantities such as the local average power
density (watts per square meter) in a vertical plane perpendicular to the
average current at each model grid cell. Visualizations of average power
density could, in principle, be used to estimate the power available from
a single turbine or a few turbines (an array small enough not to have
5 ee http://www.myroms.org/; accessed June 21, 2011.
S
6 ee http://www.unc.edu/ims/ccats/tides/tides.htm; accessed June 21, 2011.
S
7 See http://www.esr.org/polar_tide_models/Model_TPXO71.html; accessed June 21,
2011.
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136 APPENDIX B
a significant back effect on the currents). The tidal resource assessment
group used ArcView GIS software. The GIS developed by the group was
well designed and executed, and it allowed for downloading of the tidal
modeling results for further analysis by a variety of knowledgeable users.
Based on the assessment group’s last presentation to the committee (Haas
et al., 2011), the committee concluded that the resource assessment will
not produce estimates of the total theoretical energy resource or incor-
porate technology characteristics to estimate the technical resource base.
Comments on Methodology
ROMS is a structured-grid, open-source coastal ocean model. It has
performed well in the prediction of coastal circulation and tides in a large
number of applications (e.g., Warner et al., 2005; Patchen, 2007; NOAA,
2011). Finer grid resolution may be needed to represent bathymetry accu-
rately in high tidal current regions. Increasing the grid resolution in local
areas of a ROMS model often results in a significant increase of the total
model grid size, owing to the structured-grid framework. In contrast,
unstructured-grid models, which have greater flexibility for high grid
resolution in complex waterways, could provide an alternative choice,
especially for areas of complex geometry with high tidal energy (see, e.g.,
Patchen, 2007). An evaluation of the effect of grid resolution in high tidal
energy regions is necessary for future studies.
The location of the offshore boundary, partway out onto the continen-
tal shelf, is adequate for this effort, assuming that only a single turbine or
a limited number of turbines is represented. Extension to the shelf edge
may be necessary in the future if models are rerun with representations
of a large turbine array that would be extensive enough to have a back
effect on offshore tides. Estimates of available power may not be accurate
without considering the effect of the locations of open boundaries. This
question could be evaluated in follow-on studies.
According to the materials provided to the committee, the model
tends to reproduce observed tidal elevations well. This is essential for
the accurate prediction of the currents, but it may not be sufficient. It is
possible for a model to reproduce tidal elevations well but still to have
incorrect current patterns. Comparisons between predicted and observed
currents indicated that errors associated with predicted currents may be
30 percent or more (Neary et al., 2011). It could be useful to consider more
conventional model evaluation skill metrics used in the ocean-modeling
field (Warner et al., 2005; Patchen, 2007; NOAA, 2011). Because power is
related to the cube of current speed, errors of 100 percent or more occur
in the prediction of tidal power density in many model regions. It is
unclear whether model calibration through the adjustment of the single
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APPENDIX B 137
friction coefficient is more appropriate than adjustment or improvement
of other factors, such as offshore boundary values, model bathymetry, or
grid resolution. As noted by the tidal resource assessment group, errors
in currents may be a consequence of inadequate model resolution rather
than a consequence of an erroneous friction coefficient or uncertain forc-
ing from the open boundary.
Comments on the Estimate of Available Tidal Power
One principal result of the tidal resource assessment is the maximum
power, Pmax, extractable from the tidal currents in a bay. Pmax is the basis
for the theoretical resource shown in the left column of Figure 1. Pmax
would result from the use of a complete “fence” of turbines across the
entrance to the bay, but it is not the horizontal kinetic energy flux 0.5ru3
times the area of the vertical cross section of the entrance to the bay (e.g.,
Garrett and Cummins, 2007, 2008). Instead, as stated in Table 1 of this
letter report, Pmax is given to a reasonable approximation by
Pmax = 0.22 gr a Qmax,
where g is gravity, a is tidal amplitude (the height of high tide above mean
sea level), and Qmax is the maximum volume flux into a bay in the natural
state without turbines. Pmax increases with the tidal amplitude, a, and
the surface area of the bay. This result is for a single tidal constituent. If
the dominant tide is the twice-a-day lunar tide, Pmax is equivalent to the
provision from each square meter of the bay’s surface of 0.3a2 W if a is in
meters. For example, a tidal amplitude of 1 m (3.28 ft) would require more
than 300 square kilometers (over 110 square miles) to produce 100 MW
as an absolute maximum. In an area with multiple tidal constituents,
the potential power is greater than that available from the dominant
tide alone (see, e.g., Garrett and Cummins, 2005). In the assessment,
Pmax was based on all constituents that were extracted for each site. The
result makes it clear why serious consideration of tidal power is generally
limited to regions with a large tidal range. As reviewed by Garrett and
C
ummins (2008), this formula for Pmax is also a reasonable approximation
for the power available from a tidal fence across a channel that connects
two large systems in which the tides are not significantly affected. In
this case, a is the amplitude of the sinusoidal difference in tidal elevation
between the two systems.
In the Pmax scenario, the fence of turbines is effectively acting as a bar-
rage, and therefore Pmax is essentially the power available when all water
entering a bay is forced to flow through the turbines. Pmax is thus likely to
be a considerable overestimate of the practical extractable resource once
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138 APPENDIX B
other considerations, such as the extraction and socioeconomic filters
shown in Figure 1, are taken into account.
Lesser but still useful amounts of power could be obtained from
turbines that are deployed in regions of strong current without greatly
impeding a bay’s overall circulation. As mentioned earlier, a single tur-
bine can extract no more than the Lanchester-Betz limit. A total power P
requires a volume flux through the cross-sectional area of the turbines of
P/(0.3ru2), so that even with a current speed of 3 m/s, the volume flux
required for a power of 100 MW is nearly 40,000 m3/s (~1.4 million ft3/s).
Delivering such a flux would require a large number of turbines (for
example, 120 turbines if each had a cross-sectional area of 100 m2, or
24 turbines with 25 m diameter if full-scale turbines were employed).
Many more turbines would be needed for more typical, smaller, average
currents. Deploying an extensive array of turbines would impact other
marine resource uses, such as other sea-space uses and ecological services,
and would necessitate extensive, site-specific planning efforts.
More importantly, a single turbine or a small number of turbines
would not significantly affect pre-existing tidal currents, but an array
large enough to generate tens of megawatts would have back effects that
reduced the current that each individual turbine experienced. In theory,
this back effect is allowed for in a complete tidal fence considered in the
calculation of Pmax. However, allowing for the back effects of an in-stream
turbine array in a confined region requires further, extensive numerical
modeling that was not undertaken in the present tidal resource assess-
ment study and is in its early stages elsewhere (see, e.g., Shapiro, 2011).
Other than for the case of a complete tidal fence, which estimates
something close to the theoretical resource base, the tidal resource group’s
assessment cannot be used to estimate directly the potential power of
strong currents in specific bays if more than a few turbines are considered.
Nonetheless, an early group presentation to the committee (Haas et al.,
2010) attempted to evaluate the technical resource based on Pk, the power
that could be obtained if turbines of a specific swept area and efficiency
were deployed at a specified spacing in regions satisfying specified mini-
mum average current and minimum water-depth criteria, while making
the assumption that any back effects on the currents would be small. This
assumption is likely to be false, particularly if Pk is a significant fraction of
Pmax. In that case, the turbines would have an effect on currents through-
out the bay, and Pk would be an overestimate of the power available from
the turbine array. If Pk is not a significant fraction of Pmax, circulation in
other areas of the bay might not be greatly impacted, but local reductions
in the currents would still be likely and could again cause Pk to be an over-
estimate. The group could consider choosing the lesser of Pk and Pmax as
an estimate of the technical resource base. However, the committee notes
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APPENDIX B 139
that the tidal resource assessment group abandoned Pk , and thus any
evaluation of the technical resource, because of the major uncertainties
inherent in specifying parameters (personal communication to the com-
mittee from Kevin Haas, Georgia Institute of Technology, March 18, 2011).
Conclusions and Recommendations
The assessment of the tidal resource assessment group is valuable for
identifying geographical regions of interest for the further study of poten-
tial tidal power. However, although Pmax may be regarded as an upper
bound to the theoretical resource, it is an overestimate of the technical
resource because one must assume a complete fence of turbines across
the entrance to a bay, a situation that is unlikely to occur. Thus, Pmax over
estimates what is realistically recoverable, and the group does not pres-
ent a methodology for including the technological and other constraints
necessary to estimate the technical resource base.
The power density maps presented by the group are primarily appli-
cable to single turbines or to a limited number of turbines that would not
result in major back effects on the currents. Additionally, errors of up to
30 percent for estimating tidal currents translate into potential errors of
a factor of more than two in the estimate of potential power. Because the
cost of energy for tidal arrays is very sensitive to resource power density,
this magnitude of error is quite significant from a project-planning stand-
point. The limited number of validation locations and the short length of
data periods used lead the committee to question whether the model was
properly validated in all 51 model domains, as well as in the vertical struc-
ture. Further, the committee is concerned about the potential for misuse
of power density maps by end users, as calculating an aggregate number
for the theoretical U.S. tidal energy resource is not possible from a grid
summation of the horizontal kinetic power densities obtained using the
model and GIS results. Summation across a single-channel cross section
also does not give a correct estimate of the available power. Moreover,
the values for the power across several channel cross sections cannot be
added together.
Recommendations: The tidal resource assessment is likely to highlight
regions of strong currents, but it includes large uncertainties in its char-
acterization of the resource. Thus, developers would have to perform
further fieldwork and modeling, even for planning small projects with
only a few turbines. The committee recommends that follow-on DOE
work for key regions should take into account site-specific studies and
existing data from other researchers. If regions are identified in which
utility-scale power (greater than 10 MW) is thought to be available, fur-
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140 APPENDIX B
ther modeling will need to include the representation of an extensive
array of turbines in order to account for changes in the tidal and current
flow regime at local and regional scales. For particularly large projects,
the model domain extent will require expansion, probably to the edge
of the continental shelf (see, e.g., Garrett and Greenberg, 1977).
As will be discussed in the committee’s final report, further DOE
work on tidal assessments might include additional filters to progress
from theoretical resource estimates to estimates of the technical and
practical resource bases. Given that the DOE’s objective for the resource
assessments is to produce estimates of the maximum practicable, extract-
able energy, it is clear that estimates of the practical resource base need to
incorporate additional filters beyond the first column of the committee’s
conceptual framework (Figure 1). As a way to investigate estimates of
maximum practicable, extractable energy, one might consider a region
of strong tidal currents in which there is also a large tidal range, such as
Cook Inlet. Such an example might consider a comparison of an in-stream
tidal power scheme with a tidal power scheme involving a barrage across
the head of a bay or involving a lagoon enclosing a coastal area. The
reasons for this include the following: (1) as noted above, even a current
of 3 m/s is equivalent to a head of only 0.5 m, much less than would be
available with a barrage or lagoon; (2) the construction of a lagoon should
be much simpler than the installation of a large number of in-stream tur-
bines in a region of strong currents; and (3) it is possible that the overall
environmental impact of a lagoon might be less than that of an array of
turbines producing the same average power.
OVERARCHING CONCLUSIONS
Use of Resource Assessments
On the basis of the information that it reviewed, the committee con-
cludes that the overall approach taken by the wave resource and tidal
resource assessment groups is a useful contribution to the understanding
of the distribution and possible magnitude of energy sources from waves
and tides in the United States. The models, data sources, and visual dis-
play technologies, provided they are conveyed with appropriate caveats
and documented assumptions, should aid planners and those interested
in potentially developing marine and hydrokinetic energy sources.
The committee has some individual concerns about the methodolo-
gies and the communication of these methodologies that are detailed
above. Moreover, the committee has a concern regarding the usefulness
of aggregating the analysis to produce a “single-number” estimate of the
total national or regional theoretical and technical resource base for any
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APPENDIX B 141
one of these energy sources. Based on the information presented to the
committee by the wave resource and tidal resource assessment groups,
the methods and level of detail in these studies will not be able to provide
a defensible estimate of the resources that might be practically extractable
from each of the resource types. The committee concludes that developing
an estimate of the practical resource would require reaching the bottom of
the third column in its conceptual framework (Figure 1). As discussed in
the sections reviewing the wave resource and tidal resource assessments,
the groups have had varying degrees of success getting to the technical
resource base, the bottom of the second column in Figure 1. Although the
DOE may desire these overall numbers for some general purposes, such
as comparing the sizes of individual MHK resources with one another
or comparing the MHK resource base with other renewable resources,
a single number is of limited value for understanding the potential con-
tribution of MHK resources to U.S. electricity generation, which must
ultimately be assessed from the bottom up on a site-by-site basis. The
tapping of wide swaths of ocean or coastal straits and embayments for
harvesting a significant portion of their tidal and/or wave energy runs
into insurmountable barriers of other ocean uses in addition to technology
and materials limits. Furthermore, attempting to develop such a national-
level assessment requires that the assessment groups expend effort and
resources in locations of lower power density that may divert the groups
from doing a thorough assessment in locations with high resource poten-
tial. However, the committee recognizes that one of the objectives of this
study could be not only to advise developers of areas of high energy,
but also to inform decision makers, within a common platform, with
an understanding of areas in which there is limited resource potential.
Therefore, the assessment groups’ confirmation of the spatial variability
for wave and tidal resources is useful for a number of interested parties.
The committee’s final report will consider types of information that
might be needed and follow-up studies that might be done to help esti-
mate the maximum practicable, extractable resource base. Included might
be the detailed assessments of specific sites, including investigations
where the deployment of MHK devices might be promising and might
possibly serve an additional purpose, as well as where the use of MHK
resources might serve remote locations with difficult access to other elec-
tricity supplies. The final report will also further consider the source and
magnitude of the uncertainties in the resource estimates.
Coordination Among GIS Products
A lesser overarching concern than those summarized above is the
inconsistency across the implementation of GIS databases for presenting
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142 APPENDIX B
power density results. Continuing the committee’s warnings on total
resource numbers, the local results and spatial distribution of power
densities are agreed to be the primary utility of the resource studies. For
this reason, it would be best to have the GIS products coordinated and
readily able to be integrated across the resource assessment groups. This
was not included in the DOE tasking of the groups and has not been done
spontaneously by them. Additionally, there is a concern that the databases
will not be maintained after the performance period of the DOE con-
tracts. Finally, the committee concludes that caveats and warnings need
to accompany the GIS products so that users are not tempted to sum over,
or extrapolate from, the power density maps.
Sincerely,
Paul Gaffney, Chair
Committee on Marine and Hydrokinetic Energy Technology Assessment
Attachments
A Statement of Task
B Biographies of the Committee Members
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