An Evaluation of the U.S. Department of Energy's Marine and Hydrokinetic Resource Assessments (2013) / Chapter Skim
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5 Ocean Thermal Energy Conversion Resource Assessment
5 Ocean Thermal Energy Conversion Resource Assessment
Pages 56-66
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From page 56... ...
The OTEC process absorbs thermal energy from warm surface seawater found throughout the tropical oceans and ejects a slightly smaller amount of thermal energy into cold seawater pumped from water depths of approximately 1,000 m. In the process, energy is recovered as an auxiliary fluid expands through a turbine.
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From page 57... ...
However, these designs face the issue of bringing power to shore. The earliest practical OTEC plants are likely to be based on or near tropical islands that have steep topography, which will make it easier to reach deep cold water and transmit power to shore.
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From page 58... ...
The model provides an approximately 7 km resolution, and the 2-year model run included strong El Niño and 1 H.P. Hanson, Florida Atlantic University, "Global OTEC resource assessment," Presenta tion to the committee on September 27, 2011.
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From page 59... ...
Therefore, a key imperative for the OTEC resource assessment is to evaluate global ocean surface temperatures and their seasonal fluctuations, along with temperature gradients as a function of depth and location. The committee views the use of the HYCOM model for assessment of the theoretical resource to be inadequate and also regards the application of a specific proprietary Lockheed Martin plant model with a fixed pipe length to be unnecessarily restrictive.
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From page 60... ...
The advantage of HYCOM's higher resolution over earlier estimates from coarser climatologies may vanish if HYCOM is used without appropriate boundary conditions near the coasts, resulting in inaccurate seasonal and inter nnual a statistics on thermal structure. Without these abilities, this study is not much more valuable than prior maps of global ocean temperature differences (Avery and Wu, 1994)
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From page 61... ...
The combined GDEM and MODAS data sets can be queried to find not only mean monthly SSTs over regions with subsurface temperatures below 4°C, but also average 4°C isotherm depth in regions where monthly mean SST exceeds certain thresholds (21°C, 24°C, and 27°C, for example)
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From page 62... ...
The potential impacts of these effects, such as decreased tropical surface temperatures or increased primary productivity due to an influx of nutrients from deep cold water, would require careful modeling and would remain speculative until actual plant operations commenced. Instead of looking at plant spacing issues or the size of individual plants, the OTEC assessment group focused on the supply of cold water as the resource limit.
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From page 63... ...
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. Validation 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.
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From page 64... ...
While this suggests that OTEC is a very substantial ocean energy source, the many technical and environmental obstacles to its deployment, especially the challenge of utilizing the power produced at sea, means that this concept is still quite far from such large-scale implementation. The GIS created by the OTEC assessment group was a good way to visually identify sites that might be optimal for OTEC plant placement.
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From page 65... ...
There are plentiful, widely available oceanographic databases available for comparison of the thermal resource. Because the ocean's thermal stratification is the key input for the OTEC resource assessment, it would have been more useful for the validation to have focused on its representation in the model rather than on a specific plant design.
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From page 66... ...
Recommendation: Any future studies of the U.S. OTEC resource should focus on Hawaii and Puerto Rico, where there is both a potential thermal resource and a demand for electricity.
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