Gérard C. Nihous

A Preliminary Assessment of Ocean Thermal Energy Conversion Resources

Worldwide power resources that could be extracted from Ocean Thermal Energy Conversion (OTEC) plants are estimated with a simple one-dimensional time-domain model of the thermal structure of the ocean. Recently published steady-state results are extended by partitioning the potential OTEC production region in one-degree-by-one-degree “squares” and by allowing the operational adjustment of OTEC operations. This raises the estimated maximum steady-state OTEC electrical power from about 3TW(109kW) to 5TW. The time-domain code allows a more realistic assessment of scenarios that could reflect the gradual implementation of large-scale OTEC operations. Results confirm that OTEC could supply power of the order of a few terawatts. They also reveal the scale of the perturbation that could be caused by massive OTEC seawater flow rates: a small transient cooling of the tropical mixed layer would temporarily allow heat flow into the oceanic water column. This would generate a long-term steady-state warming of deep tropical waters, and the corresponding degradation of OTEC resources at deep cold seawater flow rates per unit area of the order of the average abyssal upwelling. More importantly, such profound effects point to the need for a fully three-dimensional modeling evaluation to better understand potential modifications of the oceanic thermohaline circulation.

An Order-of-Magnitude Estimate of Ocean Thermal Energy Conversion Resources

Worldwide power resources that could be extracted from the steady-state operation of ocean thermal energy conversion (OTEC) plants are estimated using a simple model. This order-of-magnitude analysis indicates that about 3 × 10 9 kW (3 TW) may be available, at most. This value is much smaller than estimates currently suggested in the technical literature. It reflects the scale of the perturbation caused by massive OTEC seawater flow rates on the thermal structure of the ocean. Not surprisingly, maximum OTEC power nearly corresponds to deep cold seawater flow rates of the order of the average abyssal upwelling representative of the global thermohaline circulation.

Mapping available Ocean Thermal Energy Conversion resources around the main Hawaiian Islands with state-of-the-art tools

This paper aims to demonstrate how the evaluation of Ocean Thermal Energy Conversion (OTEC) resources can benefit from currently available high-resolution ocean models. The case of waters around the main Hawaiian Islands is presented because of its relevance to the future development of OTEC. OTEC resources are defined here by ocean temperature differences between water depths of 20 and 1000 m, with little loss of generality. Using state-of-the-art tools like the HYCOM+NCODA (1/12°) model affords the possibility to track changes on a daily basis over a wide area (e.g., 17 °N to 24 °N and 153 °W to 162 °W). An examination of numerical data over a time period of 2 years reveals interesting geographical patterns. It is found that average OTEC temperature differences are consistently higher (by about 1 °C) west of the islands, whereas the amplitude of the yearly cycle globally decreases from north to south as expected. Better OTEC resources in the lee of the islands are attributed to the narrow eastward-flowi...

An Assessment of Global Ocean Thermal Energy Conversion Resources With a High-Resolution Ocean General Circulation Model

Global rates of ocean thermal energy conversion (OTEC) are assessed with a highresolution (1 deg 1 deg) ocean general circulation model (OGCM). In numerically intensive simulations, the OTEC process is represented by a pair of sinks and a source of specified strengths placed at selected water depths across the oceanic region favorable for OTEC. Results broadly confirm earlier estimates obtained with a coarse (4 deg 4 deg) OGCM, but with the greater resolution and more elaborate description of key physical oceanic mechanisms in the present case, the massive deployment of OTEC systems appears to affect the global environment to a relatively greater extent. The maximum global OTEC power production drops to 14 TW, or about half of previously estimated levels, but it would be achieved with only one-third as many OTEC systems. Environmental effects at maximum OTEC power production are generally similar in both sets of simulations. The oceanic surface layer would cool down in tropical OTEC regions with a compensating warming trend elsewhere. Some heat would penetrate the ocean interior until the environment reaches a new steady state. A significant boost of the oceanic thermohaline circulation (THC) would occur. Although all simulations with given OTEC flow singularities were run for 1000 years to ensure stabilization of the system, convergence to a new equilibrium was generally achieved much faster, i.e., roughly within a century. With more limited OTEC scenarios, a global OTEC power production of the order of 7 TW could still be achieved without much effect on ocean temperatures. [DOI: 10.1115/1.4023868]

An assessment of global Ocean Thermal Energy Conversion resources under broad geographical constraints

Rates of Ocean Thermal Energy Conversion (OTEC) are assessed with a high-resolution (1° × 1°) ocean general circulation model when broad geographical restrictions are imposed on the OTEC implementation area. This may correspond to practical or legal limitations, such as the cost of long submarine power cables or the extent of Exclusive Economic Zones. Because some environmental effects predicted under large-scale OTEC scenarios exhibit a strong asymmetry among major oceanic basins, numerical experiments where the OTEC domain is restricted to such specific areas are also conducted. Results suggest that in all cases, a rate of about 0.2 TW per Sverdrup of OTEC deep cold seawater is sustained when overall OTEC net power peaks. At that juncture, temperature profiles in the OTEC implementation areas are affected in similar ways, while the strength of the Thermohaline Circulation roughly doubles. Overall geographical constraints simply defined by distance to shore, given the models 1° horizontal resolution, pr...

Stress Concentration Factors for Oblique Holes in Pressurized Thick-Walled Cylinders

Elastic stress concentration factors (SCFs) for internally pressurized thick cylindrical vessels with oblique circular crossholes are reported. Results of finite-element analyses for two wall ratios (k equal to 2.25 and 4.5) and a range of crosshole ratios (d from 0.1 to 0.5) show that SCFs sharply increase with the inclination a of the crosshole axis. These findings are consistent with earlier empirical design guidelines based on experimental investigations.

Effects of ocean thermal energy conversion systems on near and far field seawater properties—A case study for Hawaii

In light of renewed interest and efforts in the development of ocean thermal energy conversion (OTEC) systems with a vision to provide mankind with a long-lasting energy resource, the potential environmental impacts of this technology should be considered from the perspective of sustainability. As an important step toward such a goal, we examine effects of OTEC effluent discharge on the physical aspects of the ocean environment near a Hawaiian Island in the North Pacific Ocean. We use modeling tools comprised of a mixing model that predicts the near field dilution of effluent plumes and a high-resolution ocean circulation model that simulates the dispersion of effluent in the far field. Numerical experiments are conducted to explore factors that influence effluent dispersal. We find that OTEC thermal resource is favorable and stable at the chosen location for the time period experimented. For a given OTEC design, the effluent discharge settles at a depth sufficiently far from the depths of discharge or in...

Notes on the temperature dependence of carbon isotope fractionation by aerobic CH4-oxidising bacteria

While the importance of environmental analyses based on isotope discrimination has been growing, uncertainties remain about underlying phenomena. Published results on the temperature dependence of carbon isotope fractionation during methane oxidation in various media show different trends. A decrease in fractionation is generally expected with temperature, but some data for methane oxidation in aqueous media show an inverse relationship. This apparent contradiction was probed by representing the first methane oxidation step as three elementary processes: the adsorption of methane on the bacterial cell wall, the desorption of methane from the wall, and the conversion of methane into methanol mediated by methane monooxygenase (MMO) enzymes. Assuming that the proportion of vacant adsorption sites is stationary, a formula for the composite fractionation factor α was obtained. It was shown that α not only expresses the fractionation that may occur in each elementary process, but that it also depends on the ratio of the kinetic rates for conversion into methanol and desorption. This result and experimental data were used to estimate the activation energy for the desorption of methane from methanotroph cell wall in aqueous medium (≈200 kJ/mol). Simple Rosso models of bacterial maximal-specific growth rate were then used to demonstrate that α and the isotope fractionation from the MMO-mediated conversion into methanol alone could vary in opposite ways as temperature changes, but that care must be exercised when using fitted relationships across wide temperature ranges.

Droplet Number Spectra in the Jet Atomization Regime

A previous model [1] to determine the number spectra of droplets generated by the injection of a fluid into another fluid is extended to cases of instantaneous disruption (near-zero jet breakup time). Under this scenario, droplets immediately form at the injection nozzle from unstable antisymmetric jet-surface waves. Resulting droplet characteristics only depend on the initial jet-surface disturbance. Full atomization is attained as the jet fluid is consumed by a series of successive droplet formation events that peel away the jet surface. The model is applied when the initial jet-surface disturbance is represented as broad-band white noise: each peel is characterized by a selected wave amplitude and high-cutoff wavenumber. Very good agreement was observed between model results and experimental droplet number spectra [2] obtained in the atomization regime.Copyright

Simple Game Theoretic Considerations for Environmental Problems

This paper uses elementary arguments from game theory to consider the interaction between protagonists involved in environmental problems. It is argued that ‘ generic’ global problems often result in a Prisoner’ s dilemma, with the status quo as equilibrium. Also, a brief consideration of the Kyoto Protocol confirms that if developing countries were asked to join, their dominant strategy would be to refuse, and that the recent withdrawal of the United States leaves other signatories in a precarious position. Finally, the paper focuses on a specific recent project, the CO 2 Ocean Sequestration Field Experiment, to demonstrate that many of the difficulties hampering the resolution of global-scale environmental problems have to be dealt with at local levels as well.