Increasing renewable energy development, both in the United States and abroad, has rekindled interest in the potential for marine and hydrokinetic (MHK) resources to contribute to electricity generation. In particular, state-based renewable portfolio standards and federal production and investment tax credits have led to increased exploration of MHK technologies. This interest is reflected in the number of requests for permits for wave, current, tidal, and river-flow generators that have been filed recently with the Federal Energy Regulatory Commission (FERC); as of December 2012, FERC had issued 4 licenses and 84 preliminary permits while an additional 42 projects are in the pre-filing stage for a license.1 Though permit activity is not a reliable predictor of the future development of MHK resources because developers apply for permits before completing project plans and financing, it does indicate increased interest in MHK resource development. However, most of these permits are for developments along the Mississippi River, and the actual deployment of all MHK resources is extremely small. The first U.S. commercial grid-connected project, a tidal project in Maine with a capacity 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 response to the rising interest in MHK energy, the Energy Policy Act of 2005 (Public Law 109-58) directed the Department of Energy (DOE)
1 Available at http://www.ferc.gov/industries/hydropower/gen-info/licensing/hydrokinetics.asp. Accessed January 3, 2013.
to estimate the size of the MHK resource base. In order to assess the overall potential for U.S. MHK resources and technologies, DOE funded detailed resource assessments for estimating what it terms the “maximum practicably extractable energy” or “maximum practical, extractable energy” for each resource (see Appendix A for the funding announcements), as well as projects for generating the technological data necessary to estimate the expected performance of several MHK device designs currently under consideration (DOE, 2008 and 2009). The objective of DOE’s MHK resource assessment work was to help prioritize its overall portfolio of future research, increase understanding of MHK’s potential for generating electricity, and steer the developers of MHK devices and/or projects to locations of greatest promise.2 Earlier estimates (EPRI, 2005 and 2007) of the potential MHK resource are based on limited, possibly inaccurate data and assumptions related to the total resource and the fraction that might prove extractable.
DOE contracted with five assessment groups to conduct separate estimates of the extractable energy from five categories of MHK resources: waves, tidal currents, ocean currents, marine temperature gradients (also known as ocean thermal energy conversion [OTEC]), and free-flowing water in rivers and streams (DOE, 2010). The resource assessment groups are listed in Table 1-1. Each group was tasked with estimating the average power density of the resource base, as well as basic technology characteristics for potential devices and spatial and/or temporal variability of the resource. DOE requests for proposals did not offer a unified framework for the efforts, nor was there a requirement that the contractors coordinate their methodologies. As a result, each assessment group used distinct methodologies and assumptions, although there is some commonality between assessments being overseen by the same groups. The DOE contracts did specify that each assessment would have a validation component; those groups are also listed in Table 1-1.
DOE asked the National Research Council (NRC) to convene a committee of experts to evaluate the detailed assessments produced by each group, review the estimates of extractable energy, typically represented as average terawatt-hours per year (TWh/yr),3 and technology specifications,
2 H. Battey, U.S. Department of Energy, “DOE Water Power Program,” Presentation to the committee on February 8, 2011.
3 Note that TWh/yr is a unit of power and may be used to represent the average power generation over the time period indicated (1 gigawatt [GW] = 8.8 TWh/yr, 1 TWh/yr = 0.114 GW). However, a unit such as TWh/yr (or, as shown in an electricity bill, kilowatt-hours [kWh] per month) is a standard unit for the electricity sector. Energy units such as kWh or TWh measure the commodity that is generated by power plants and sold to consumers. For example, the Energy Information Agency’s (EIA’s) Annual Energy Review 2011 includes a table of total electricity generation that is given in billions of kWh/yr (EIA, 2012, Table 8.2a).
|Resource Assessment||Assessment Group||Validation Group|
|Tides||Georgia Tech Research Corporation||Oak Ridge National Laboratory|
|Waves||Electric Power Research Institute, Virginia Tech||National Renewable Energy Laboratory|
|Ocean currents||Georgia Tech Research Corporation||Oak Ridge National Laboratory|
|Marine temperature gradients/OTEC||Lockheed Martin, Florida Atlantic University, University of Hawaii||National Renewable Energy Laboratory|
|Rivers and streams||Electric Power Research Institute, University of Alaska||National Renewable Energy Laboratory|
and compare the results across resource types. The committee members had expertise in oceanography, ocean engineering, hydraulics, civil engineering, electric power engineering and electric utilities, energy economics, and environmental and resource policy; their biographies can be found in Appendix C. The complete statement of task (SOT) can be found in Box 1-1. As requested in the SOT, the committee completed an interim report with initial commentary and review of the draft wave and tidal resource assessments. That report, Assessment of Marine and Hydrokinetic Energy Technology: Interim Letter Report (NRC, 2011), was released on July 12, 2011, and is reproduced in Appendix B. In it, the committee concluded that the wave and tidal assessments would be useful for determining the theoretical and technical resources, but it had concerns about the usefulness of producing a single-number estimate for the entire United States. It also noted a lack of consistency and coordination across the assessments. Each of these points will be discussed in full in this report.
The nation’s MHK community currently lacks a well-defined, consistent resource terminology. The committee observed that each of the assessment groups employed different terminology to describe similar results. This was likely due to imprecise language in the DOE funding opportunity announcements (DOE, 2008 and 2009), which called for an assessment of the “maximum practicably extractable energy” or the “maximum practical, extractable energy” without defining the terms. In addition, the NRC statement of task used language (“extractable energy,”
This committee will evaluate detailed assessments produced by the U.S. Department of Energy (DOE) of the extractable energy from U.S. marine and hydrokinetic (MHK) resources (waves, tidal currents, ocean currents, marine temperature gradients, and free-flowing water in rivers and streams); review extractable energy estimates and technology specifications; and accurately compare the results across resource types. There are five assessments that will need to be evaluated by the committee addressing: (1) wave energy resources; (2) tidal energy resources; (3) hydrokinetic energy in streams and rivers; (4) marine thermal energy; and (5) ocean current energy. In addressing its statement of task, the committee will:
(1) Interact with the principal investigators of each individual assessment developed by DOE to understand and question their approach and perhaps suggest additional information or methodological approaches to facilitate consistent comparison across the assessments;
(2) Review and assess MHK technology-related data, critically analyzing methodologies, technical robustness, reliability, and assumptions related to the performance of the various technologies under consideration;
(3) Review and assess each of the resource assessments, critically analyzing methodologies, technical robustness, and assumptions related to the resources that might be practicably available for energy conversion and potential limitations on these resources;
(4) Based on its review and critique of the assessments, provide a defensible comparison of the potential extractable energy from each of the resource types;
(5) Make recommendations, as appropriate, for improving the assessments, improving the consistency among the assessments, or for improving the methodologies for making the assessments;
(6) Write an interim report reviewing the methodologies and assumptions, and provide any recommendations associated with the first two assessments being undertaken by DOE (wave and tidal energy); and
(7) Write a final report reviewing all five of the assessments.
“potential extractable energy") different from what DOE used for its funding opportunity statement.
In order to develop its approach to the SOT and to review individual resource assessments within a single context, the committee created a conceptual framework (Figure 1-1) of the overall MHK resource assessment.
FIGURE 1-1 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.
This allowed the committee and those who read its reports to visualize the processes used to develop the assessment results requested by DOE. This framework establishes three terms—the theoretical resource, technical resource, and practical resource—to clarify 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. Each of the three terms is defined in the following sections.
The committee recognizes that communities involved with other energy types, such as wind and fossil fuels, use different terms to describe their resource bases (such as “resources” or “proven reserves"). The committee’s framework is consistent with terminology for MHK resources as used in the European marine energy community, including European Marine Energy Centre (EMEC)4 terminology incorporated in International Electrotechnical Commission (IEC) technical specification 62600-1 (IEC, 2011). In addition, the committee created Table 1-2, which contains the definitions and units used in this report.
|Term to Be Quantified||Definition||Unit||Note|
|Capacity to do work||joules (J)|
|Energy per time||watts (W) = J/s|
|Rate of flow of a property per unit area|
|Average annual power||TWh/yr (1 TWh/yr = 114 MW)||Representing a potential energy resource base for the electricity sector in TWh.|
|Tides, ocean currents, and riverine/in-stream|
Current power density
|Power of horizontal currents flowing through a vertical plane of unit area.
||W/m2||Horizontal kinetic energy flux (power density). Applies to a single device. Excludes consideration of back effects.|
Wave power density (Mei, 1989)
|Power of waves per unit crest length based on
||W/m||Horizontal energy flux. Applies to a single device. Vector quantity (has both direction and magnitude).|
Wave power density (EPRI, 2011)
|Power of waves per unit circle based on
||W/m||Horizontal energy flux. Applies to a single device. Scalar quantity (has no directional information).|
Ocean thermal power density (Nihous, 2007a)
|Net extractable power per unit flow of upwelled cold water
||W/(m3/s)||Net power from pumping 1 m3/s of cold water without consideration of back effects on the ocean.|
NOTE: Variables are as follows:
P, water density;
v, tidal/ocean/river current speed (scalar);
g, gravitational acceleration;
S, wave spectrum (sea-surface height variance, per frequency and direction);
cg, wave group velocity;
f, wave frequency;
θ, wave direction;
CP, heat capacity of seawater (J/(K kg));
TGE, OTEC turbo-generator efficiency (~0.8-0.9);
ΔT, temperature difference between warm and cold water for OTEC plant (°C);
PL, pipe loss/fractional energy loss to cold water pumping (~0.2-0.3); and
TS, warm water intake temperature for OTEC (°C)
The theoretical resource, shown in the left column of the conceptual framework in Figure 1-1, is defined as the average annual energy available from each MHK resource. Determining the theoretical resource requires a series of inputs (including methods, models, assumptions, and observational data) for each source of MHK energy (waves, tides, ocean currents, marine temperature gradients, and rivers and streams) in order to determine the physical upper limit on the total amount of available energy. For waves, the theoretical resource is effectively the power density of waves approaching the shore (see Table 1-2). For in-stream power from rivers, the theoretical resource is the power that is lost to friction as water flows from higher to lower ground.
For some of the theoretical resource assessments, it is also important to consider far-field back effects. These refer to the modification of an energy resource owing to the presence of an extraction device or devices. In particular, for tidal currents, ocean currents, and marine thermal gradients, the theoretical resource cannot be estimated without taking into account the far-field back effect. Here, the back effect refers to the reduced potential of the resource due to feedbacks from the presence of a device or device array. For tidal and other ocean currents, placement of a turbine will create drag, reducing the current velocity and therefore the potential power available for each turbine. As turbines are added to an array, at some point the extra power generated by an additional turbine will be less than the decrease in power due to the reduced current available for all the other turbines. This maximum available power is equivalent to the theoretical resource when far-field back effects are considered. Similarly, the operation of a series of OTEC plants can affect the ocean’s thermal structure, decreasing the potential power of each plant. Depending on the community, back effects are also known as feedbacks or blockage effects.
In response to the original DOE request, the assessment groups produced two key outputs from their characterization of the theoretical resources: (1) overall regional or national numbers for the U.S. theoretical resource, expressed as an average annual energy resource (typically in TWh/yr), and (2) a geographic information system (GIS) database that represents the spatial variation in average annual power density with units appropriate for each source (e.g., W/m for waves or W/m2 for tides). The committee equates the theoretical resource with the “potential extractable energy” mentioned in the SOT.
The technical resource (center column in Figure 1-1) 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 determine how much of the theoretical resource can actually be extracted. The committee conceptualizes these constraints as physical and technological extraction filters. These include physical near-field (local) back effects from turbine interactions in a river channel or wave system as well as technological characteristics associated with one or more energy-extraction devices: characteristics such as device efficiency, device spacing requirements, drag on supporting structures, and cut-in and cut-out parameters (the minimum or maximum speeds at which devices can operate). Some of these filters are resource-specific; others are applicable across all MHK resources. During presentations from DOE and the assessment groups and ensuing discussion with the committee, it became clear that each group offers 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-water or field tests would assist in the quantification of realistic extraction filters and/or device-specific conversion efficiencies, because the data obtained could be used to calibrate numerical models. Outputs related to the technical resource include an estimate of the energy resource and a GIS that sets forth 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 (rather than the practical resource) represented the completion of their projects. The committee equates the technical resource with the review of “extractable energy” charged in the SOT.
The committee also recognizes that, beyond the extraction filters, there are additional filters influencing when and where devices can be placed. The practical resource (right-hand column in Figure 1-1) is defined as 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. For example, some of the filters attempt to capture the logistical and economic considerations associated with building the MHK devices and connecting them to the electricity system, which could include costs of extraction and electricity delivery. Environmental constraints related to quantifying the practical resource include issues such as protecting threatened species or ecologically sensitive areas. Other use issues include sea-space conflicts raised by, for instance, shipping channels, navigation, and military
considerations and multiple- or competing-use issues such as fisheries or recreation. Such filters are, by nature, specific to the local sites where decisions related to MHK projects will be made. The practical filters can greatly influence the timing of the permitting process and can lead to unpredictable consequences, which in turn can affect a project’s economic viability. Box 1-2 presents two scenarios to help elucidate the differences between the theoretical, technical, and practical resource.
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.
In its funding opportunity announcements (DOE, 2008 and 2009), DOE requested that the assessment groups determine the “maximum practicably extractable energy,” which the committee originally interpreted as equivalent to the practical resource called for in the conceptual framework. After discussion with both DOE and the assessment groups, the committee concluded that the groups had interpreted “maximum practicably extractable energy” to mean the technical resource and that DOE did not expect the assessment effort to incorporate site-specific information needed to quantify the practical resource.
While a determination of the practical resource is beyond the scope of the tasks assigned by DOE, the committee sees the constraints represented by the socioeconomic and environmental filters as being among the most important considerations influencing future MHK investments. Box 1-3, which discusses these types of constraints on the development of solar energy, is presented as an example of what might be needed to assess the MHK practical resource. These filters are also central to evaluating the potential maximum contribution of MHK to U.S. electricity generation. The socioeconomic and environmental filters that need to be considered in an assessment of the MHK resource are described further in Chapter 7.
Assessing the potential for a particular renewable technology to address U.S. energy needs based on the theoretical resource alone would be inappropriate. As an example, solar power plants (which were first constructed nearly 30 years ago) currently provide less than 0.1 percent of the electricity consumed in the United States despite having a theoretical resource base that is orders of magnitude larger than current U.S. electricity consumption (EIA, 2012). While national-scale resource assessments may be useful for identifying geographic regions of interest for a particular MHK extraction technology, the practical resource will depend on a host of technical and environmental factors and may be significantly lower than what the assessments indicate is regionally or locally available. A survey of annual total energy outputs from several existing solar plants indicates that the ratio of plant outputs to the locally available theoretical resource ranges from as little as 2 percent for photovoltaics to as much as 12 percent for concentrated solar (National Renewable Energy Laboratory [NREL], available at http://www.nrel.gov/gis/solar.html; EIA, available at http://www.eia.gov/electricity/data/eia923/index.html). It is not possible to predict the practical MHK resource from national resource assessments until the constraints posed by both the technical extraction filters and the practical socioeconomic and environmental filters are better quantified for each of the specific resources.
It is also important to note the difference between utility-scale and small scale developments, as these terms are mentioned throughout the report. Utility-scale MHK developments would produce from tens to hundreds of megawatts and would require significant infrastructure and fully-proven MHK devices rather than prototypes. Utility-scale MHK deployment has the greatest potential for substantial environmental impacts as well as conflicts with other ocean and freshwater uses. In comparison, smaller-scale developments would typically produce less than 10 MW and potentially have fewer conflicts and adverse impacts. Small MHK developments could be deployed in locations with high local resource availability and low electricity demands (such as remote villages or small islands) or in locations that lack interconnection to a utility-scale electricity system. Additionally, a project developer would need to prove the feasibility of a smaller-scale pilot application before a utility would invest in building a utility-scale system. The regional- to global-scale approach used by the resource assessment groups was a top-down evaluation that is most useful in understanding the utility-scale potential for MHK.
Although each of the five MHK resource assessments is evaluated in detail in Chapters 2 through 6, here the committee draws attention to an important point that applies to the assessments both individually and for the project as a whole. The committee is concerned about the appropriateness of aggregating the results of individual MHK resource assessments to produce a national or regional single-number estimate of the theoretical and/or technical resource for any one of these energy sources. It finds that the theoretical resource assessments, especially when examined at a regional or national scale, have limited utility for developers and stakeholders and also have potential for misuse. As an example, the numbers associated with the wave and tide assessments do not accurately convey how the theoretical resources are concentrated along the coast, nor do they explain how much power would be practically available once devices are deployed. Although such estimates provide a broad order-of-magnitude idea of potential energy resources, many extraction filters are needed to determine the technical resource, and at this time the assessment groups can rigorously evaluate only a few of these filters. Most of the extraction filters require assumptions about which particular MHK technologies will be used and what their technical specifications will be; moreover, the technologies are likely to vary by resource and location—for instance, wave energy off the coast of Oregon
and ocean current energy in the Florida Straits. In addition, socioeconomic and environmental filters will ultimately limit the practical resource to only a fraction of the technical resource, so it is unlikely that the resource assessments, which at best provide only a partial assessment of the technical resource, could serve as a defensible estimate of the available practical resource. Although DOE may want overall numbers in order to compare individual MHK resources with one another or with other renewable resources, a single number is of limited value for understanding the potential contribution of MHK resources to U.S. utility-scale electricity generation. Instead, site-by-site analysis will be needed to estimate the resource that might ultimately be available for electricity generation. This number is likely to be much smaller than the numbers generated by national resource assessments.
Another issue that applies broadly to the entire DOE-funded assessment efforts was the coordination among and consistency between individual resource assessments. These efforts suffered from a lack of coordination and consistency in terms of methodology, validation, and deliverable products. Each of the assessment groups chose its own methodologies, and while the committee understands that there was likely to be variation simply because the resource types differ, greater coordination at the outset could have discerned some commonalities that would have allowed easier comparison of the assessments. In addition, each validation group chose its own method, which also led to inconsistent results. In some cases, the method appeared to be less of a validation than a spot-checking of results with varying degrees of thoroughness. The committee is also concerned about the scientific validity of some assessment conclusions; these concerns are addressed in later chapters. The lack of coordination and consistency also affected the GIS database products. While some are already integrated into GIS Web applications hosted by DOE’s National Renewable Energy Laboratory (MHK Atlas and River Atlas5), others are currently hosted on platforms operated by individual assessment groups. Given that one of DOE’s objectives is to compare the various MHK resources with one another and with other renewable energy resources, the lack of coordination and consistency between the assessment groups was counterproductive.
The committee evaluated the five assessments contracted by DOE (tides, waves, ocean currents, marine thermal gradients/OTEC, and rivers and in-stream). Each of the assessments is presented in a separate chapter, which introduces the basic resource, describes the project, comments on assessment methodology and validation, and offers conclusions and recommendations. The discussion of tides can be found in Chapter 2, waves in Chapter 3, ocean currents in Chapter 4, OTEC in Chapter 5, and riverine and in-stream flows in Chapter 6. A discussion of the practical MHK resource and constraints posed by socioeconomic and environmental filters is included in Chapter 7, and overarching conclusions and recommendations are presented in Chapter 8.
Evaluations of the resource assessments are based on presentations by the assessment groups and DOE to the committee at each of its six meetings (meetings and presentations to the committee are detailed in Appendix D). The committee also received written responses to its questions from each of the groups. Chapters 2 and 3 are based on information initially discussed in the committee’s interim report (NRC, 2011). These chapters have been updated to include information that had not been available at the time of the interim report release. For this report, the committee reviewed final assessment reports for the waves, tides, and OTEC assessment groups and a July 2012 draft final report from the riverine assessment group.6 No final report was available for review from the ocean currents resource assessment group; its report is expected to be complete by June 2013. Instead, the committee based its evaluation on presentations from and discussions with the assessment group.
6 The final report was published in December 2012 and is available at http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001026880.