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; temperature 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 maximum practically extractable energy for each MHK category. These assessments 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
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 different 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 committee’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.
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 assessment 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 conceptual framework in Figure S-1, is the average annual energy available for each source of MHK energy. The resource assessment 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
FIGURE S-1 Conceptual framework developed by the committee for MHK resource 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 represents 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 assessment 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 technical resource represented completion of the assessment project for each group. The committee also recognizes that there are
MHK resource assessments are going to be of interest to a variety of parties, 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 constraints are represented as social, economic, regulatory, and environmental filters.
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 environmental filters as being among the most important considerations influencing future MHK investments. These constraints are also critical when attempting to evaluate 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 probably 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 constraints 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 terminology 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 practical MHK energy resource (Chapter 8).
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 assessment 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. Challenging social barriers (such as fishery grounds, shipping lanes, environmentally 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 characterization 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 contribution to understanding the distribution and possible magnitude of marine and hydrokinetic energy sources in the United States. The models, data
sources, and visual display technologies, provided they are conveyed with appropriate caveats and documented assumptions, can aid in planning. 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, validation, 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 environment 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 feedbacks 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 assessments, 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 rigorous validation.
A coordinated approach to validation would have provided a mechanism to address some of the methodological differences among the groups as well as provide a consistent point of reference. However, each validation 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 validation 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 generate 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.
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 products 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’ temporal variability. This is in contrast to the spatial variability, which is comparatively well characterized through modeling and GIS displays. The committee recognized that the time-dependent nature of power generation 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 practical 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.
Recommendation: DOE should ensure that spatial data resulting from the MHK resource assessments are readily and publicly available for use in siting and permitting decisions (Chapter 7).
DOE has already made progress by making data on the spatial distribution 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 considerations toward areas that could be the most productive and feasible for development. An accessible spatial database of theoretical and technological 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 geographic extent and predictability of the various MHK resources. Both the ocean current and OTEC resource bases are confined to narrow geographic 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 timing 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 variability are but two of the many factors that will determine what MHK
resources are capable of contributing significantly to power generation in the United States.
Each of the five resource assessments provides valuable information 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 practically 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 individual potential power sources is likely to be much less than the theoretical 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.
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 translate 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 technical 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 particularly large projects, the model domain extent will require expansion, probably to the edge of the outer continental shelf (Chapter 2).
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
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 portion 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 reflection 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 processes that dominate in shallow water (e.g., refraction, diffraction, shoaling, and wave dissipation due to bottom friction and wave breaking) (Chapter 3).
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 concentrate 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 optimization of device location and spacing. The effects of meandering and seasonal variability within the Florida Current should also be discussed (Chapter 4).
The OTEC assessment group’s GIS database provides a visualization tool to identify sites for optimal OTEC plant placement. However,
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. Isotherm depths (at 1°C intervals) should be included in the database 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 technical 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, including directly calculating the technically recoverable resource by (1) developing an estimate of channel shape for each stream segment 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).