Lesser but still useful amounts of power could be obtained from turbines that are deployed in regions of strong current without greatly impeding a bay’s overall circulation. As mentioned earlier, a single turbine can extract no more than the Lanchester-Betz limit. A total power P requires a volume flux through the cross-sectional area of the turbines of P/(0.3pv2), so that even with a current speed of 3 m/s, the volume flux required for a power of 100 MW is nearly 40,000 m3/s (∼1.4 million ft3/s). Delivering such a flux would require a large number of turbines (for example, 120 turbines if each had a cross-sectional area of 100 m2, or 24 turbines of 25 m diameter if full-scale turbines were employed). Many more turbines would be needed for more typical smaller average currents. Deploying an extensive array of turbines would impact other marine resource uses, such as other sea-space uses and ecological services, and would necessitate extensive site-specific planning.
More importantly, a single turbine or a small number of turbines would not significantly affect preexisting tidal currents, but an array large enough to generate tens of megawatts would have near-field back effects that reduce the current that each individual turbine experiences. In theory, this back effect is allowed for in a complete tidal fence considered in the calculation of Pmax. However, other than for the case of a complete tidal fence, which results in estimates fairly 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.
Nonetheless, an early group presentation to the committee (Haas et al., 2010) attempted to evaluate the technical resource based on Pk, the power that could be obtained if turbines of a specific swept area and efficiency were deployed at a specified spacing in regions satisfying specified minimum average current and minimum water-depth criteria, while assuming that any back effects on the currents would be small. This assumption is likely to be false, particularly if Pk is a significant fraction of Pmax. In that case, the turbines would have an effect on currents throughout the bay, and Pk would be an overestimate of the power available from the turbine array. If Pk is not a significant fraction of Pmax, circulation in other areas of the bay might not be greatly impacted, but local reductions in the currents would still be likely and could again cause Pk to be an overestimate. 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 thereby 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).