theories and models (Bryan, 1987; Zhang et al., 1999) recognize that the thermohaline circulation is controlled by the mixing rate, not the cold water supply. Indeed, the ocean is certainly not lacking in cold water—its average temperature is ∼3.5°C (Worthington, 1981). Nevertheless, it is not unreasonable to assume that OTEC’s impact on global circulation should not be too large, and using the Nihous (2007b) limit is a plausible approach in the absence of a proper modeling study. However, the assessment group’s use of it to address plant packing density is misguided.
The committee is disappointed that the OTEC assessment group did little to address device spacing requirements, individual plant size, or the limits of the ocean thermal resource. Clearly, a key question for determining the OTEC technical resource would be how closely plants could be spaced without interfering with each other or excessively disturbing the ocean thermal structure. A related issue would be how spacing might differ in coastal and open-ocean environments. Another issue would be the size limit of a single OTEC plant, due to back effects on the ocean thermal structure such as smaller temperature gradients owing to decreased thermal stratification. While a global resource assessment is difficult to constrain, the committee had hoped the assessment group would address constraints such as plant spacing, tidal amplitudes, and anchoring in deep water or strong currents.
The group focused its validation efforts on the Lockheed Martin OTEC plant operating model while neglecting validation of the thermal resource.3 Focusing the validation process on the proprietary plant model seems inappropriate and not at all transparent to this committee. In fact, it appears rather to be a reverse engineering of their plant model, as the agreement on performance seems remarkably perfect (Lockheed Martin Mission Systems & Sensors, 2012). Assuring the accuracy of the temperature gradient in the assessment group’s database would have been a more valuable effort, especially with a focus on how well the 2-yr HYCOM run represents the available temperature difference. The seasonal and inter-annual statistics and the model representation of nearshore deep temperatures are of particular interest, especially as the group noted a problem with deep temperatures off Florida.4 The validation effort should have drawn on the many available hydrographic databases and compared surface and deep temperature contrasts between observational data and the
3 H.P. Hanson, Florida Atlantic University, “Global OTEC resource assessment,” Presentation to the committee on December 12, 2011.
4 See preceding footnote.