represent how much of the theoretical resource can actually be extracted. The committee conceptualizes these constraints as “extraction filters” consisting of physical and technological constraints, including back effects2 and technological characteristics associated with one or more energy-extraction devices (representing factors such as device efficiency, device spacing requirements, and cut-in and cut-out parameters).3 Some of these filters are resource-specific; others are applicable across all of the MHK types. During presentations made to the committee and from its discussion with the DOE and the assessment groups, it became clear that each group offers a different interpretation of what types of constraints 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 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. In the committee’s view, the assessment groups determined that reporting the technical resource represented the completion of their projects.
Some of the assessment groups recognized that, beyond the extraction filters, there were additional filters influencing when and where devices could be placed. The practical resource (right-hand column in Figure 1), is defined as that portion of the technical resource available after consideration of all other constraints. In the conceptual framework, these constraints are captured in socioeconomic filters. For example, the filters involving logistical and economic considerations include costs of raw materials and maintenance, resources associated with transmission and distribution, electricity demand, and the cost of electricity. Environmental and use constraints include issues relating to a variety of impacts on the environment (e.g., protecting threatened species or ecologically sensitive areas), sea-space conflicts (e.g., involving shipping channels, navigation, protected areas), and multiple- or competing-use issues (e.g., fisheries, viewshed impacts, recreation, national security). Such filters are, by nature, specific to and critical at the local sites where decision making related to marine and hydrokinetic projects will occur.
A determination of the practical resource is beyond the scope of the resource assessment groups’ tasks as defined by the DOE. However,
2 A back effect refers to the modification of an energy resource owing to the presence of an extraction device. In the case of turbines in a river or tidal channel, the back effect is the modification of currents in the whole cross section of the channel, particularly the reduction in the volume flux through the channel.
3 In some cases, such as for tidal resources or steady currents, the estimation of the theoretical resource requires allowance for back effects.