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Summary
Increasing renewable energy development, both within the United
States and abroad, has rekindled interest in the potential for marine and
hydrokinetic (MHK) resources to contribute to electricity generation.
These resources derive from ocean tides, waves, and currents; tempera-
ture gradients in the ocean; and free-flowing rivers and streams. One
measure of the interest in the possible use of these resources for electricity
generation is the increasing number of permits that have been filed with
the Federal Energy Regulatory Commission (FERC). As of December 2012,
FERC had issued 4 licenses and 84 preliminary permits, up from virtually
zero a decade ago. However, most of these permits are for developments
along the Mississippi River, and the actual benefit realized from all MHK
resources is extremely small. The first U.S. commercial grid-connected
project, a tidal project in Maine with a capacity of less than 1 megawatt
(MW), is currently delivering a fraction of that power to the grid and is
due to be fully installed in 2013.
In order to better understand MHK’s potential, the Energy Policy Act
of 2005 directed the U.S. Department of Energy (DOE) to estimate the
size of the MHK resource base. DOE funded detailed assessments of five
resources: waves, tides, ocean currents, ocean thermal energy conversion
(OTEC), and rivers and streams. Its objective was to estimate the maxi-
mum practically extractable energy for each MHK category. These assess-
ments have the potential to direct the developers of MHK devices and/or
projects to locations of greatest promise and to inform the development
of DOE’s research portfolio. Additionally, the assessments could inform
1
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2 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
policies for commercial projects, technology development, environmental
management, and funding. However, it is important to note that each of
the independent assessment groups contracted by DOE employed dif-
ferent methodologies and terminology to describe conceptually similar
results, probably because the DOE funding opportunity announcements
(Appendix A) lacked clear direction.
As part of its assessment of MHK resources, DOE asked the National
Research Council (NRC) to provide detailed evaluations. In response,
the NRC formed the Committee on Marine and Hydrokinetic Energy
Technology Assessment. As directed in its statement of task (SOT), the
committee first developed an interim report, released in June 2011, which
focused on the wave and tidal resource assessments (Appendix B). The
current report contains the committee’s evaluation of all five of the DOE
resource categories as well as the committee’s comments on the overall
MHK resource assessment process. This summary focuses on the com-
mittee’s overarching findings and conclusions regarding a conceptual
framework for developing the resource assessments, the aggregation of
results into a single number, and the consistency across and coordination
between the individual resource assessments. Critiques of the individual
resource assessments are contained in Chapters 2 through 6 of this report,
further discussion of the practical MHK resource base is in Chapter 7, and
overarching conclusions and recommendations are found in Chapter 8.
CONCEPTUAL FRAMEWORK
To shape its approach to the SOT and to review individual resource
assessments within a single context, the committee created a conceptual
framework for the overall MHK resource assessment (Figure S-1). The
conceptual framework allowed the committee and those who read its
report to conceptualize the processes used to assess the resources. It
established a set of three terms—theoretical resource, technical resource,
and practical resource—to clarify elements of the overall resource assess-
ment process as described by each assessment group and to allow for a
comparison of different methods, terminology, and processes used by
the five assessment groups. An example of the relationship between the
theoretical, technical, and practical resources is found in Box S-1.
• The theoretical resource, shown in the left column of the con-
ceptual framework in Figure S-1, is the average annual energy
available for each source of MHK energy. The resource assess-
ment groups produced two key outputs from their assessments of
the theoretical resource: (1) overall regional or national numbers
for the U.S. theoretical resource, expressed as an average annual
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SUMMARY 3
FIGURE S-1 Conceptual framework developed by the committee for MHK re-
source assessments. The asterisk in the third column denotes that the resource
assessment groups did not attempt to evaluate the practical resource.
energy resource, typically in terawatt-hours per year (TWh/yr),
and (2) a geographic information system (GIS) database that rep-
resents the spatial variation in average annual power density in
units appropriate for each source—for example, W/m for waves
or W/m2 for tides.
• The technical resource (center column in Figure S-1) is defined as
the portion of the theoretical resource that can be captured using
a specified technology. Physical and technological constraints,
conceptualized as extraction filters in Figure S-1, restrict how
much of the theoretical resource can actually be extracted. Based
on the presentations and discussions with the resource assess-
ment groups, the committee found that each group offered a
different interpretation of what types of constraints would need
to be included among its extraction filters. However, it is clear
to the committee that estimating the technical resource from the
theoretical resource requires filters that represent the general
physical and technological constraints associated with energy-
extraction devices. In the committee’s view, reporting of the tech-
nical resource represented S-11
completion of the assessment project
for each group. The committee also recognizes that there are
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4 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
BOX S-1
The Theoretical, Technical, and Practical Resource
MHK resource assessments are going to be of interest to a variety of par-
ties, including electric utilities, project developers, and public officials. However,
the orders-of-magnitude differences between theoretical, technical, and practical
resources need to be stressed, especially because some resource assessments
have been publicized in terms of a national or regional single-number estimate.
To provide a better understanding of the difference among these resources, two
scenarios are provided below.
• Scenario 1. A local official examines one of the MHK GIS databases and
notes that there is a 100 MW theoretical resource nearby. After taking into
account the efficiency of the extraction device, such as a turbine (30%),
coverage of the resource by the device(s) (20%), and the efficiency of
connecting the extracted energy to the electricity grid (90%), the technical
resource amounts to only 5.4 MW. The local official notes that 50 percent
of the remaining power would interfere with existing fisheries and navigation
routes in the area, leaving a practical resource of 2.7 MW.
• Scenario 2. A developer is interested in building a 100 MW MHK plant.
This would be considered the desired practical resource. In this case, 20
percent of the site is unavailable because it is in a Marine Protected Area.
After taking into account device efficiency, site coverage, line efficiency, and
the practical constraints posed by the use conflict, the site of interest would
have to be endowed with a theoretical resource of 2,300 MW.
filters in addition to the extraction filters that influence when and
where devices can be placed.
• The practical resource (right-hand column in Figure S-1) is that
portion of the technical resource available after consideration of
all other constraints. In the conceptual framework, these con-
straints are represented as social, economic, regulatory, and envi-
ronmental filters.
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SUMMARY 5
Although a determination of the practical resource is beyond the scope
of the tasks set for the resource assessment groups, the committee sees the
constraints represented by the socioeconomic and nvironmental filters as
e
being among the most important considerations influencing future MHK
investments. These constraints are also critical when attempting to evalu-
ate the maximum amount of U.S. MHK resources that could practically
be used to generate electricity on a utility scale (greater than 10 MW).
The regional approach used by the resource assessment groups was a
top-down evaluation that is most useful in understanding the utility-scale
potential for MHK rather than its small-scale potential (typically less than
10 MW). Compared with small-scale MHK deployments, utility-scale
projects require significant infrastructure and could have more potential
for substantial environmental impacts and conflicts with other ocean and
freshwater uses. For example, extracting 1 GW of power from waves
would likely require a row of devices at least 100 km long parallel to the
coast; extracting a similar power from tides would effectively require a
barrage. Similar examples can be envisioned for utility-scale in-stream,
OTEC, and ocean current installations. Because of infrastructure costs
and the potential for environmental impacts, MHK resources will prob-
ably be developed in only a limited number of discrete spots where the
high energy density of the resource warrants such investment or in niche,
small-scale applications where there are minimal local impacts. Such con-
straints will greatly reduce the aggregate practical resource as compared
to the theoretical and technical resource.
Continued development of U.S. MHK resources requires clear
conceptual and operational definitions and objectives. While many of
the questions that are raised regarding MHK resource development will
ultimately be decided at the local, state, and regional scale, there is an
opportunity for DOE to play a leadership role by assessing resources and
disseminating results. The committee noted that the U.S. MHK energy
community has not settled on a common set of definitions for resource
assessment and development. The committee has provided a conceptual
framework for assessment of MHK resources that is consistent with termi-
nology used by the European marine energy community. This framework
was essential for understanding the factors considered when comparing
the five MHK resource assessments.
Recommendation: DOE should develop or adopt a conceptual
framework that clearly defines the theoretical, technical, and prac-
tical MHK energy resource (Chapter 8).
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6 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
USE OF SINGLE NUMBERS FROM RESOURCE ASSESSMENTS
The committee has strong reservations about the appropriateness of
aggregating theoretical and technical resource assessments to produce
a single-number estimate for the nation or a large geographic region
(for example, the West Coast) for any one of the five MHK resources.
A single-number estimate is inadequate for a realistic discussion of the
MHK resource base that might be available for electricity generation in
the United States. The methods and level of detail in the resource assess-
ment studies do not constitute a defensible estimate of the practical
resource that might be available from each of the resource types. This
is especially true given the assessment groups’ varying degrees of success
in calculating or estimating the technical resource base.
While the DOE may want an aggregated value for its internal research
or for investment purposes—it might, for example, wish to compare the
size of individual MHK resources with each other or with other renewable
resources—a single number is of limited value for understanding the
potential contribution of MHK to U.S. electricity generation. Challeng-
ing social barriers (such as fishery grounds, shipping lanes, environmen-
tally sensitive areas) or economic barriers (such as proximity to utility
infrastructure, survivability) will undoubtedly affect the power available
from all MHK resources, but some resources may be more significantly
reduced than others. The resource with the largest theoretical resource
base may not necessarily have the largest practical resource base when
all of the filters are considered. It is not clear to the committee that a
comparison of theoretical or technical MHK resources—to each other or
to other energy resources—is of any real value for helping to determine
the potential extractable energy from MHK. Rather, it is the practical
resource that will ultimately determine the potential contribution of
an MHK resource to U.S. electricity generation. Site-specific analyses
will be needed to identify the constraints and trade-offs necessary to
reach the practical resource. Because the assessment groups were tasked
by DOE to come up with a national assessment, they by necessity did not
target their efforts on locations with high resource potential. However,
many of these areas were identified even though their exploitation was
not the sole focus of the assessment. It is these areas that most need char-
acterization for their potential contribution to the U.S. electricity supply.
COORDINATION AND CONSISTENCY
ACROSS RESOURCE ASSESSMENTS
Each of the resource assessment groups provides a useful contribu-
tion to understanding the distribution and possible magnitude of marine
and hydrokinetic energy sources in the United States. The models, data
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SUMMARY 7
sources, and visual display technologies, provided they are conveyed
with appropriate caveats and documented assumptions, can aid in plan-
ning. However, the lack of a common framework allowed for a multitude
of approaches to the individual assessments. The resource assessments
lacked coordination and consistency in terms of methodology, valida-
tion, and deliverable products. Each of the assessment groups chose its
own method of assessing the resource. While some variation between
methodologies was due to differences among the MHK resource types,
greater initial coordination among the assessors could have identified
commonalities and led to easier comparison among the assessments.
Quantifying the interaction between MHK installations and the envi-
ronment was a challenge for the assessment groups. Deployment of
MHK devices can lead to complex near-field and/or far-field feedback
effects for many of the assessed technologies. Analysis of these feed-
backs affects both the technical and practical resource assessments (and
in some cases the theoretical resource) and requires careful evaluation.
The committee noted in several instances a lack of awareness by the
assessment groups of some of the physics driving their resource assess-
ments, such as the lack of incorporation of complex near-field and/or
far-field feedback effects, which led to simplistic and sometimes flawed
approaches. The committee was further concerned about a lack of rigor-
ous validation.
A coordinated approach to validation would have provided a mecha-
nism to address some of the methodological differences among the groups
as well as provide a consistent point of reference. However, each valida-
tion group (chosen by individual assessment groups) determined its own
method, which led to results that were not easily comparable to each
other. In some instances, the committee noted that that there was a lack
of sufficient analysis to be considered a true validation. Weakness of the
validations includes using only a limited amount of observational data,
the inability to capture extreme events, inappropriate calculations for the
type of data used, and focus on validating technical specifications rather
than underlying observational data. The lack of consistent, effective vali-
dation is especially problematic given the large uncertainties described
in assessment results.
All five MHK resource assessments lack sufficient quantification
of their uncertainties. There are many sources of uncertainty in each of
the assessments, including the models, data, and methods used to gener-
ate the resource estimates and maps. Propagation of these uncertainties
into confidence intervals for the final GIS products would provide users
with an appropriate range of values, rather than the implied precision of
specific values, and thus better represent the approximate nature of the
actual results.
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8 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
The GIS database products themselves are informative individual
products for public use, but they are not able to be viewed as an aggregate
product due to a lack of coordination during project development. Given
that one of DOE’s objectives is to compare the various MHK resources
with one another and with other renewable energy resources, stronger
initial coordination among the assessment groups could have led to prod-
ucts that were developed in a common format.
As part of the evaluation of the practical resource base, there seemed
to be little analysis by the assessment groups of the MHK resources’ tem-
poral variability. This is in contrast to the spatial variability, which is com-
paratively well characterized through modeling and GIS displays. The
committee recognized that the time-dependent nature of power genera-
tion is important to utilities and would need to be taken into account in
order to integrate MHK-generated electricity into any electricity system.
Recommendation: Further evaluation of the MHK resource base
should use the theoretical and/or technical results of the DOE resource
assessments and appropriate decision support tools to identify the
constraints that affect the practical resource and to help identify
individual, highly promising sites for continued study of the practi-
cal resource. A site-specific approach to identify the practical MHK
resource could help to estimate the potential contribution of MHK to
overall U.S. electricity generation (Chapter 8).
For example, connecting and integrating the MHK resource to the
electric utility grid may alter the number of developable sites or prioritize
more easily connectable, economically viable sites. A next research step
could be to create detailed assessments of two types of sites—“hot spots”
with potential for large-scale MHK deployment and sites that might be
promising for small-scale applications (for instance, remote communities
without access to a regional transmission system).
Although DOE contracted for assessments that would provide the
extractable U.S. MHK resource, the contractors focused on the theoretical
and technical resource base at both national and regional levels. However,
they did not make it to the level of estimating the practical resource.
Recommendation: Should DOE (or any other federal agency or
regional/local decision-making body) decide to assess or support
decisions on the potential practical MHK resource for specific
regions of high potential MHK opportunity, it should include the
best available socioeconomic and environmental filters for that
region (Chapter 8). The tidal assessment group’s identification of
relevant socioeconomic factors is a good beginning.
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SUMMARY 9
Recommendation: DOE should ensure that spatial data resulting
from the MHK resource assessments are readily and publicly avail-
able for use in siting and permitting decisions (Chapter 7).
DOE has already made progress by making data on the spatial dis-
tribution of the theoretical energy resources readily available and should
continue to play an active role in the characterization of the resource base
and in developing decision support tools that can help guide consider-
ations toward areas that could be the most productive and feasible for
development. An accessible spatial database of theoretical and technologi-
cal MHK resources would provide substantial information on the location
of high-priority sites.
LIMITATIONS ON COMPARISON OF
EXTRACTABLE MHK RESOUSCES
DOE requests for proposals did not offer a unified framework for
the efforts, nor was there a requirement that the contractors coordinate
their methodologies. The differing approaches taken by the resource
assessment groups left the committee unable to provide the defensible
comparison of potential extractable energy from each of the resource
types as called for in the study task statement. To do so would require
not only an assessment of the practical resource base discussed by the
committee earlier but also an understanding of the relative performance
of the technologies that would be used to extract electricity from each
resource type. Simply comparing the individual theoretical or technical
MHK resources to each other does not aid in making such a comparison
since the resource with the largest theoretical resource base may not
necessarily have the largest practical resource base. However, some
qualitative comparisons can be made, especially with regard to the geo-
graphic extent and predictability of the various MHK resources. Both
the ocean current and OTEC resource bases are confined to narrow geo-
graphic regions in the United States, whereas the resource assessments
for waves, tides, and in-stream show a much greater number of locations
with a large resource base. As for predictability, while there is multi-day
predictability for wave and in-stream systems, especially in settings
where the wave spectrum is dominated by swells or in large hydrologic
basins, the predictability is notably poorer than for tidal, where the tim-
ing and magnitude of events are known precisely years into the future.
The OTEC resource in the United States has little day-to-day variability
but, like in-stream, is seasonally dependent. However, location and vari-
ability are but two of the many factors that will determine what MHK
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10 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
resources are capable of contributing significantly to power generation
in the United States.
RESOURCE-SPECIFIC RECOMMENDATIONS
Each of the five resource assessments provides valuable informa-
tion that can be used to identify geographic regions of interest for the
further study of potential MHK development. However, utilizing this
information to further assess the MHK resource that could be practi-
cally available for electricity generation will require improvements in
methodology and characterization. The assessment and development of
each MHK resource will face unique challenges. Overall, the committee
would like to emphasize that the practical resource for each of the indi-
vidual potential power sources is likely to be much less than the theo-
retical or technical resource. An additional criticism regarding most of
the assessments was the lack of some degree of study prioritization based
on existing knowledge, which could have led to a stronger focus on areas
with higher potential. Recommendations for future study are considered
below.
Tides
The tidal resource assessment is likely to highlight regions of strong
currents, but large uncertainties are included in its characterization of the
resource. Errors of up to 30 percent in the estimated tidal currents trans-
late into potential errors of more than a factor of two in the estimate of
potential power. Although maximum extractable power may be regarded
as an upper bound to the theoretical resource, it overestimates the techni-
cal resource because the turbine characteristics and efficiencies are not
taken into account.
Recommendation: In regions where utility-scale power may be
available, further modeling should 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 partic-
ularly large projects, the model domain extent will require expan-
sion, probably to the edge of the outer continental shelf (Chapter 2).
Waves
The theoretical wave resource assessment estimates are reasonable,
especially for mapping wave power density; however, the approach taken
by the assessment group is not suitable for shallow water and is prone to
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SUMMARY 11
overestimating the resource. The group used a “unit circle” approach to
estimate the total theoretical resource, which summed the wave energy
flux across a cylinder of unit diameter along a line of interest, such as a
depth contour. This approach has the potential to double-count a por-
tion of the wave energy if the direction of the wave energy flux is not
perpendicular to the line of interest or if there is significant wave reflec-
tion from the shore. Further, the technical resource assessment is based
on optimistic assumptions about the efficiency of conversion devices and
wave-device capacity, thus likely overestimating the available technical
resource.
Recommendation: Any future site-specific studies in shallow water
should be accompanied by a modeling effort that resolves the inner
shelf bathymetric variability and accounts for the physical pro-
cesses that dominate in shallow water (e.g., refraction, diffraction,
shoaling, and wave dissipation due to bottom friction and wave
breaking) (Chapter 3).
Ocean Currents
The ocean current resource assessment is valuable because it provides
a rough estimate of ocean current power in U.S. coastal waters. However,
less time could have been spent looking at the West Coast in order to con-
centrate more fully on the Florida Strait region of the Gulf Stream, where
the ocean current can exceed 2 m/s. This would have also allowed more
focus on the effects of meandering and seasonal variability. Additionally,
the current maps cannot be used directly to estimate the magnitude of the
resource. The deployment of large turbine farms would have a back effect
on the currents, reducing them and limiting the potential power.
Recommendation: Any follow-on work for the Florida Current
should include a thorough evaluation of back effects related to
placing turbine arrays in the strait by using detailed numerical
simulations that include the representation of extensive turbine
arrays. Such models should also be used to investigate array opti-
mization of device location and spacing. The effects of meandering
and seasonal variability within the Florida Current should also be
discussed (Chapter 4).
OTEC
The OTEC assessment group’s GIS database provides a visualiza-
tion tool to identify sites for optimal OTEC plant placement. However,
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12 DOE’S MARINE AND HYDROKINETIC RESOURCE ASSESSMENTS
assumptions about the plant model design and a limited temperature
data set impair the utility of the assessment. In addition, the committee
considers the use of deep, cold water for air conditioning to be a potential
use of this resource.
Recommendation: Any future studies of the U.S. OTEC resource
should focus on Hawaii and Puerto Rico, where there is both a
potential thermal resource and a demand for electricity (Chapter 5).
Recommendation: The OTEC GIS should be modified to display
monthly resolution over a longer time period (at least a decade) to
allow for evaluation of the thermal resource for the full seasonal
cycle as well as for special periods such as El Niño and La Niña.
I
sotherm depths (at 1°C intervals) should be included in the data-
base so other pipe lengths can be evaluated for OTEC and seawater
air conditioning (Chapter 5).
Rivers and Streams
The theoretical resource estimate from the in-stream assessment group
is based upon a reasonable approach and provides an upper bound to the
available resource; however, the estimate of technical resources is flawed
by the assessment group’s recovery factor approach (the ratio of techni-
cal to theoretical resource) and the omission of other important factors,
most importantly the omission of statistical variation of stream discharge.
Further work is required with respect to the approach to estimate the
technically recoverable resource before it will have value as an estimate
to guide in-stream hydrokinetic development.
Recommendation: Future work on the in-stream resource should
focus on a more defensible estimate of the recovery factor, includ-
ing directly calculating the technically recoverable resource by
(1) developing an estimate of channel shape for each stream seg-
ment and (2) using flow statistics for each segment and an assumed
array deployment. The five hydrologic regions that comprise the
bulk of the identified in-stream resource should be tested further
to assure the validity of the assessment methodologies. In addition,
a two- or three-dimensional computational model should be used
to evaluate the flow resistance effects of the turbine on the flow
(Chapter 6).
