in the Mississippi, Alaska, Pacific Northwest, Ohio, and Missouri hydrologic regions. These rivers alone account for 95.3 TWh/yr, or ∼95 percent of the estimated technically available resource. Given that the largest portion of the resource is estimated within these five hydrologic regions, further testing of the approach in these areas is needed. Also, it is noteworthy that the recovery factor for the hydrologic regions varies from a few percent to nearly 24 percent for the Lower Mississippi region, raising doubt about the effectiveness of the recovery factor approach.
As a simple estimate of RF’s upper bound, one can assume a dimensionless flow depth h and a unit height D (equal to 0.8 x h), and a spacing between units of 2D. For a rectangular portion of a channel with a width of 3D, the swept area of the machine is ∼0.5, the total flow area is 2.4, and the share of flow captured is ∼20 percent. Factoring in turbine efficiency (∼30%), lost area along sloping channel edges with depth less than D, lost flow area above the depth h when flows are above the minimum flow, and energy lost to friction along the longitudinal distance, the RF approaches 2-10 percent (arguably 5% or lower). The committee is concerned that RF is not necessarily defensible based on the above arguments.
Last, there are many limiting factors to be considered that will reduce the realizable in-stream hydrokinetic energy production. These factors include but are not limited to ice flows and freeze-up conditions, transmission issues, debris flows, potential impacts to aquatic species (electromagnetic stimuli, habitat, movement and entrainment issues), potential impact to sites with endangered species, suspended and bedload sediment transport, lateral stream migration, hydrodynamic loading during high flow events, navigation, recreation, wild and scenic designations, state and national parklands, and protected archeological sites. These considerations will need to be addressed to further estimate the practical resource that may be available.
After reviewing the in-stream resource assessment report, the online information database, and additional information presented by the assessment group during committee meetings, it is the committee’s opinion that the estimate of the theoretical resource is based upon a reasonable approach and provides an upper bound to the available resource; however, the estimate of the technical resource is flawed by the assessment group’s recovery factor approach and the omission of other important factors, most importantly the statistical variation of stream discharge. A more thorough assessment of both modified Manning’s coefficient and the recovery factor used by the in-stream assessment group is needed to ascertain the usefulness of these approaches. Further work