Assessment of Marine and Hydrokinetic Energy Technology Interim Letter Report (2011) / Chapter Skim
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Letter Report
Letter Report
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The committee presents this letter report, in accord with the statement of task, as its preliminary assessment of methodologies and assumptions used in the estimation of wave and tidal resources. The committee'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]
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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.
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. 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.
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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. The theoretical resource, shown in the left column of the conceptual framework in Figure 1, is defined as the average annual energy production for each source of hydrokinetic energy.
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3 In some cases, such as for tidal resources or steady currents, the estimation of the theoretical resource requires allowance for back effects. NRC Assessment of Marine and Hydrokinetic Energy Technology Interim Letter Report 5
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Pcoast = Σ Pscalar dl Total regional Based on annual average sum of Terawatt- Remains wave resource wave power density crossing hours per approximately (recommended by perpendicular to a line defining year constant as waves this committee) a region of coastline, such as a (= 114 MW)
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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.
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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 representative of what the maximum practicable, extractable energy might be from MHK resources.
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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.
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. The committee therefore reviewed the work of the assessment group on the basis of these materials and identified concerns related to the suitability of the hindcast data set in shallow waters, the approach used to compute the total theoretical resource from the maps of wave power density, the technology assumptions utilized for assessment of the total technical resource, and the lack of a demonstrated GIS tool.
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Reporting the wave power density magnitude as well as direction would alleviate this concern. The total theoretical resource is estimated by the wave resource assessment group using the concept of wave energy flux impinging on unit diameter cylinders from any direction.
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Currently, the group plans only to provide maps of annual and monthly average wave conditions (i.e., wave power density, wave height, period, direction) in the GIS display.
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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)
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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.
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Project Description The tidal resource assessment group conducted its tidal energy assessment 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)
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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.
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. 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.
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The group could consider choosing the lesser of Pk and Pmax as an estimate of the technical resource base. However, the committee notes 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 committee from Kevin Haas, Georgia Institute of Technology, March 18, 2011)
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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 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.
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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 NRC Assessment of Marine and Hydrokinetic Energy Technology Interim Letter Report 20
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