An Evaluation of the U.S. Department of Energy's Marine and Hydrokinetic Resource Assessments (2013) / Chapter Skim
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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)
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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.
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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.
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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%)
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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.
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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.
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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)
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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 practi cal resource.
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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.
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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.
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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.
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. 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)
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