An Evaluation of the U.S. Department of Energy's Marine and Hydrokinetic Resource Assessments (2013) / Chapter Skim
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B Interim Letter Report
B Interim Letter Report
Pages 117-143
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Department of Energy 1000 Independence Avenue, S.W. Washington, DC 20585 RE: NRC Assessment of Marine and Hydrokinetic Energy Technology: Interim Letter Report Dear Dr.
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Thus, the committee believes that it is important to complete its interim letter report at this point, not only for the letter report's potential impact with respect to the wave resource and tidal resource assessments, but also to provide timely feedback to the other assessment groups. The committee will continue to review the methodologies and assumptions that are used in all five of the assessments, as it completes its study and writes its final report (currently scheduled for completion in the spring of 2012)
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However, the committee has concerns regarding the usefulness of aggregating the analysis to produce a "single number" estimate of the total national or regional theoretical and technical resource base (defined in the section below entitled "Conceptual Framework") for any one of these sources.
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of the amount of energy that could be extracted from MHK resources are based on limited and possibly inaccurate data regarding the total resource size and on potentially dated assumptions related to the amount of each resource that might ultimately prove extractable. To improve these estimates, the DOE contracted with the five assessment groups referred to above to conduct separate estimates of the extractable energy from five categories of MHK resources: waves, tidal currents, ocean currents, marine temperature gradients, and free-flowing water in rivers and streams (DOE, 2010)
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This framework establishes a set of three terms -- theoretical resource, technical resource, and practical resource -- and their definitions, provided below, to clarify elements of the overall resource assessment process as described by each assessment group and to allow for a comparison of different methods, terminology, and processes among the five assessment groups. The committee recognizes that communities involved with other energy types, such as wind and fossil fuels, use different terms to describe their resource bases (i.e., "resources," "proven reserves")
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flux along the line. Pcoast = Σ Pscalar dl Total regional Based on annual average Terawatt- Remains wave resource sum of wave power density hours per approximately (recommended crossing perpendicular to year constant as by this a line defining a region (= 114 MW)
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Qmax = maximum horizontal tidal volume flux (over tidal cycle) The theoretical resource, shown in the left column of the conceptual framework in Figure 1, is defined as the average annual energy production for each source of hydrokinetic energy.
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However, it is clear to the committee that estimating the technical resource from the theoretical resource requires filters that represent physical and technological constraints associated with energy-extraction devices. Outputs related to the technical resource include an estimate of the energy resource and a GIS representing spatial and temporal variation in the resource associated with various technologies.
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Given that many of the extraction and the socioeconomic filters might be similar across the assessment groups, coordination would also help in the development of a GIS database useful to policy makers and developers. The DOE requested that the assessment groups determine the "maxi mum practicable, extractable energy." Although maximum practicable, extractable energy could possibly refer to the practical resource in the conceptual framework shown in Figure 1, discussion with the DOE and the assessment groups led the committee to conclude that the term is instead equivalent to the technical resource in the conceptual framework.
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Because wave energy travels in a particular direction, care must be taken when interpreting maps that show wave power density as a function of location but do not indicate predominant wave directions. It also must be recognized that if the energy is removed from the wave field at one location, by definition less energy will be available in the shadow of the extraction device.
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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 products of the wave resource assessment will include a database of 51-month time series at 3 hr intervals of wave parameters that can be used to reconstruct the frequency spectra, although directional spreading information is not available. In addition, the group will provide maps of annual and monthly average wave conditions (i.e., wave power density, wave height, period, direction)
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Reporting such values is highly misleading and should be avoided. The wave power density at a given location 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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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. Depending on the direction of wave approach and the orientation of the line of interest, there is a distinct possibility that waves passing through one cylinder and into the next cylinder will be counted repeatedly in the aggregate estimate of wave power, resulting in an overestimate of the total theoretical resource.
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The 51-month time series of hindcast conditions can be used to deduce information about interannual variability, including some estimate of extreme conditions that devices would have to be able to survive. Currently, the group plans only to provide maps of annual and monthly average wave conditions (i.e., wave power density, wave height, period, direction)
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Conclusions and Recommendations The maps of wave characteristics produced by the wave resource assessment group could prove useful to developers who are interested in identifying general regions for their particular wave energy conversion devices. Similarly, the approach outlined for the application of the extraction filters and subsequent estimation of the technical resource represents a defensible attempt, albeit limited by the lack of detailed information about the relevant wave energy devices.
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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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As the power produced by a turbine is related to the product of the head and the flow rate, it is clear that capturing tidal currents is considerably less effective than capturing the hydraulic head associated with a modest tidal range. The upper bound on the power from such an in-stream turbine is shown in Table 1 and is expressed by the Lanchester-Betz limit of 0.3rAu3, where r is water density, u is current speed, and A is the cross- sectional area across the blades (also referred to as the swept area)
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The effect of tidal flats was initially evaluated but not considered in the final model runs. The tidal resource assessment group calibrated the tidal models by adjusting the single friction coefficient to improve the comparison among model results, NOAA predictions of tidal elevation and currents, and limited observations of depth-averaged tidal currents.
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, the committee concluded that the resource assessment will not produce estimates of the total theoretical energy resource or incorporate technology characteristics to estimate the technical resource base. Comments on Methodology ROMS is a structured-grid, open-source coastal ocean model.
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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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Further, the committee is concerned about the potential for misuse of power density maps by end users, as calculating an aggregate number for the theoretical U.S. tidal energy resource is not possible from a grid summation of the horizontal kinetic power densities obtained using the model and GIS results.
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it is possible that the overall environmental impact of a lagoon might be less than that of an array of turbines producing the same average power. OVERARCHING CONCLUSIONS Use of Resource Assessments On the basis of the information that it reviewed, the committee concludes that the overall approach taken by the wave resource and tidal resource assessment groups is a useful contribution to the understanding of the distribution and possible magnitude of energy sources from waves and tides in the United States.
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. As discussed in the sections reviewing the wave resource and tidal resource assessments, the groups have had varying degrees of success getting to the technical resource base, the bottom of the second column in Figure 1.
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2005. Final Summary Report, Project Definition Study, Offshore Wave Power Feasibility Demonstration Project.
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Wave Energy Resource Assessments. Presentation at the Second Meeting of the Committee on Marine and Hydrokinetic Energy Technology Assessment.
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