This chapter presents the committee’s overarching conclusions and recommendations that emerge from its consideration of all five of the marine and hydrokinetic (MHK) energy assessments. Based on the information reviewed, the committee concludes that the overall approach taken by the assessment groups contributed to understanding the distribution and upper bound of U.S. MHK energy sources. It notes that the models, data sets, geographic information systems (GISs), and visualizations should aid stakeholders and MHK energy developers, provided they are conveyed with appropriate caveats and include well-documented assumptions. In Chapters 2-6 of this report, the committee makes observations, outlines its concerns, and makes recommendations for each of the five MHK resource categories. Conclusions and recommendations that were originally presented in the interim report (Appendix B) have been reexamined for this report, and those that are relevant have been reiterated in this and other chapters.
As first expressed in its interim report, the committee continues to have strong concerns about the usefulness of aggregating theoretical and technical resource assessments to produce a single-number estimate for any one of the five MHK resources. This single-number estimate is inadequate for a realistic discussion of the practical 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.
While the Department of Energy (DOE) may want an aggregated value for internal research and/or investment purposes, such as comparing the relative sizes of individual MHK resources or comparing the MHK resource base with other renewable resources, a single-number estimate of each theoretical or technical MHK resource is of limited value for understanding the potential extractable energy that each resource might contribute to U.S. electricity generation.
DOE contracted for assessments of extractable MHK resource levels. The five resource assessments focused mainly on the national level and did not reach the point of estimating the practically extractable resource in regions of high interest. Both the theoretical and technical resource bases are developed by summing all the energy available over large tracts of ocean or long river stretches. However, attempts to tap wide swaths of ocean or coastal straits and embayments for harvesting energy will run into challenging social or economic barriers (e.g., entrenched uses such as fisheries and shipping lanes or environmentally sensitive areas) as well as technology, materials, and engineering issues (e.g., proximity to utility infrastructure, survivability). The tidal assessment group’s identification of relevant socioeconomic factors is a good beginning for this type of analysis.
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.
Inevitably, some of these theoretical and technical resource estimates include large areas where the energy density is so low that energy development would be impractical. Such practical limits will undoubtedly affect the power available from all MHK resources, but some resources may be more significantly reduced than others, and the resource with the largest theoretical resource base may not necessarily have the largest practical resource base. Thus, it is not apparent that comparing the theoretical or technical resources of the various MHK types is of any real value for determining the potential extractable energy from MHK. Rather, it is the practical resource that will ultimately contribute to U.S. electricity generation. To ascertain the practical MHK resource, site-specific analysis is necessary. Because the assessment groups were tasked by DOE to
create a national assessment, they by necessity did not target their efforts at locations with high resource potential. However, it is these areas that most need characterization for their potential contribution to U.S. electricity supply.
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.
For example, the ability to connect or integrate the MHK resource into the electrical grid may influence the number of realizable sites or prioritize among more easily connectable, economically viable sites. A next research step could be to create detailed assessments of two types of sites—those hot spots with potential for large-scale MHK deployment and those that might be promising for small-scale applications (for instance, remote communities without access to the nationwide transmission/ distribution system).
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. While this issue was not raised in the interim report, the committee recognized that the time-dependent nature of power generation is important to utilities and would need to be understood in order to integrate MHK-generated electricity into any electricity system.
For example, the predictability of the tidal resource would ease its integration into an electricity system. In contrast, large variations in the wave resource due to extreme weather can affect not just power availability but also a location’s desirability. For ocean currents, seasonality and meandering could limit device placement to narrow regions where flow is consistent throughout the year. While OTEC is more predictable, large interannual variations in available temperature difference that would limit power generation in some locations may be masked by monthly or seasonal averaging. Even greater seasonal and interannual variability can be expected in riverine resources. The assessment groups did very little to quantify resource variability and in some instances averaged away the precise seasonal variation that may be of most interest to developers. It should be noted that utilities are increasing their experience
with incorporating temporally varying renewable resources such as wind and solar power, and those resources are likely to show greater temporal variability than MHK resources.
Continued development of U.S. MHK resources requires clear conceptual and operational definitions and objectives. While ultimately many of the questions raised about MHK resource development will be decided at the local, state, or regional level, there is an opportunity for DOE to play a leadership role in assessing resources and disseminating results. As first discussed in Chapter 1, the U.S. MHK energy community has not converged upon a common set of definitions for resource assessment and development. The committee has provided a conceptual framework for the MHK resource that is consistent with that of the European marine energy community. This common set of definitions was essential for understanding the factors considered when developing and comparing the five MHK resource assessments, and the committee feels it would be beneficial for DOE to either develop its own framework or adopt an existing framework.
Recommendation: DOE should develop or adopt a conceptual framework that clearly defines the theoretical, technical, and practical MHK energy resource.
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. However, the absence of a common framework allowed for a multitude of approaches to the individual assessments. In its interim report, the committee noted that the assessments suffered from a lack of coordination and consistency in terms of methodology, validation, and deliverable products. Each of the assessment groups chose its own method of evaluating the resource. While some variation between methodologies was due to differences in the MHK types, greater initial coordination among the assessors could have identified commonalities and facilitated comparison among the assessments.
Quantifying the interaction between MHK installations and the environment was a challenge for the assessment groups, as described in previous chapters. Deployment of MHK devices can lead to complex 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 needs to be carefully evaluated. The committee was disappointed by the resource groups’ lack
of awareness of some of the physics driving their resource assessments, which led to simplistic and often flawed approaches. The committee was further concerned about a lack of rigorous statistics, which are essential when a project involves intensive data analysis.
A coordinated approach to validation would have provided a mechanism to address some of the methodological differences between the groups as well as 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. In some instances, the committee noted a lack of sufficient data and/or analysis to be considered a true validation. The weakness of the validations included an insufficiency of observational data, the inability to capture extreme events, inappropriate calculations for the type of data used, and a 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 lacked 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 instead of the implied precision of a specific value, thus better representing the approximate nature of the actual results.
The GIS database products themselves also reflect an apparent lack of coordination in their development, which led to duplication of effort and additional time needed to integrate the final products. At the time of this writing, the wave and OTEC databases are the only MHK resource assessments integrated into the National Renewable Energy Laboratory’s (NREL’s) MHK Atlas; the in-stream resource assessment is hosted separately in the NREL River Atlas. The tidal resource database is currently hosted by the tidal resource assessment group and will be integrated with other NREL products; however, the visualization and analysis tools developed by the assessment group will not be implemented in the MHK Atlas. 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 developed in a common format.
The different 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. Understanding the performance characteristics of the technologies that might be used to tap these different resources is either just emerging, as is the case for wave and tidal devices, or limited to modeling or sparse pilot plant demonstrations, as is the case for OTEC and ocean currents.
Some comparisons can be made based upon attributes of the different MHK resources, especially their geographical extent and predictability. Clearly, both the ocean current and OTEC resource bases have very limited geographical extent in the United States. The main potential for ocean currents is in the Florida Straits, and the coastal regions of the Hawaiian Islands and Puerto Rico are the most likely places for efficient OTEC siting. In contrast, the resource assessments for waves, tides, and in-stream show a much greater number of locations with substantial resources, though by far the largest location for tidal resources in the United States is in the Cook Inlet of Alaska. Predictability is another important characteristic to consider if a resource is to be incorporated into an electricity system. Tidal resources are highly predictable, with the timing and magnitude of tidal events being known precisely years into the future. In contrast, waves and in-stream resources are related to meteorological conditions that unfold over days and weeks. There is multiday predictability for wave and in-stream systems, especially in settings where the wave spectrum is dominated by swells or in large hydrologic basins, but the predictability is notably less than for tidal systems. 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.