Dossier Océan et énergie - Énergie Thermique des Mers

Sommaire IOA News Letters


By Victor D. Phillips

To my knowledge there is no ocean thermal energy conversion (OTEC)-specific language in the clean air legislation presently being considered by the U.S. Congress, which are the House of Representatives Bill No. 3030(HR 3030) and the companion Senate Bill No. 1630(S 1630). Yet, OTEC has the potential to signi-ficantly improve air quality and reduce CO2 emissions implicated in global warming by substituting fossil fuel to generate electricity. OTEC may represent a viable alternative for certain utilities in appropriate regions to help comply with a more stringent clean air law.

Meanwhile, estimates of atmospheric emissions from OTEC continue to be developed. A 1986 study conducted by the Solar Energy Research Institute sum-marized that OTEC operations could have direct atmospheric interactions involving ocean surface cooling and release of CO2 from deep ocean reservoirs (1). Both processes may be correlated with climatic change, but neither OTEC/atmosphere interaction is well-defined due to limited experimental data. OTEC alterations of sea surface temperature or atmospheric budgets of CO2 are possible, but indirect climatic influences of OTEC deployment due to displacement of existing fossil-fuel electric plants may mitigate these effects. Since OTEC plants will discharge large volumes of cold water at or near the thermocline, a slight surface temperature depression may result in localized fog. OTEC, in contrast to a fossil-fuel plant or nuclear plant, release no additional heat to the biosphere and substantially .less CO2. Outgassing of dissolved CO2 during OTEC operations can be minimized or avoided by discharge at a depth below the surface mixed layer(1).

Virtually no atmospheric emissions would result from the operation of a closed-cycle OTEC facility. Total CO2 concentrations at the surface and 1000 m are typically 20 and 2.3 m moles kg-1 respectively. Differences relative to total concentration are trivial, and no effect due to redistribution can be foreseen in a closed-cycle OTEC plant. An OC-OTEC plant design that incorporates subsurface mixed discharge is expected to result in no long-term CO2 release (1). OTEC operations will not result in the formation of criteria air pollutants ,and compliance with the projected clean air amendments, as well as possible CO2 limits, should be readily attained by engineering the OTEC effluent discharge at a sufficient depth to preclude significant CO2 atmospheric emissions.

Conflicting reports of CO2 emissions from an open-cycle OTEC (OC-OTEC) plant range from 50% greater to 2500¢M CO2 in comparison with a fossil fuel power plant generating an equivalent amount of electricity. This is perhaps not surprising because of the paucity of data that exist to date. For example, at a U.S. Department of Energy workshop conducted in 1980, it was reported that a 100 MWe OC-OTEC plant would release approximately one-half of the total CO2 to the atmosphere or roughly 0.3¡Ñ1010 g day-1. For comparison, a coal-fired 100 MWe power plant emits approximately 0.2¡Ñ1010 g day-1 (2).

In constrast to the above analysis, Quinby-Hunt et al. cited a 1981 analysis by Bailey and Vega who reported that the maximum CO2 released per unit of power produced for OC-OTEC systems is one-third of the minimum released by conventional power plants burning fossil fuel (3). More recently in 1989, Oney and Krock stated that the CO2 emission from the most conservative fossil fuel power plant, i.e. one burning natural gas, would emit nearly 25 times the CO2 than from an OC-OTEC plant of similar power capacity (4). This agrees with Green and Guenther's 1989 analysis that the immediate CO2 release from future OC-OTEC plants is projected to be 15 to 25 time smaller than that from fossil fuel power plants of equal size (5).

The Hawaii Natural Energy institute at the University of Hawaii is coordinating an international consortium's program entitled "PACIFIC RESPONESES TO GLOBAL ENVIRONMENTAL CHANGE." This research effort includes an oceanic component to characterize and model the flux and fate of CO2 and other radia-tively active trace gases in the open oceans, and to evaluate the potential of managing the open oceans for climate control via enhanced CO2 uptake and dime-thyl sulfide production by marine algae. A nutrient subsidy to stimulate marine algal productivity form artificially upwelled water derived from OTEC operations and perhaps augmented with terrestrial sources of irom and other nutrients may provide a "bluewater" solution to the greenhouse effect. In partnership with the University of Hawaii and HNEI, the Pacific International Center for High Technology Research (PICHTR) in Honolulu, Hawaii, is formulating proactive plans for utilizing OTEC technologies to help "clear the air."

References cited

  1. Solar Energy Research Institute 1986. Environmental impacts of ocean thernal energy conversion SERI/SP-271-2796.42.p
  2. U.S. Department of Energy 1981. Workshop proceedings of the potential environmental consequences of ocean thermal energy conversion (OTEC) plants. CONF-800154.January 22-24,1980,Brookhaven National Laboratory, Upton, New York,60 p.
  3. Quinby-Hunt, M.S. Wilde, P., and Dengler, A.T.1986. Potential environmental impacts of open-cycle ocean thermal energy conversion. Environ. Impact Asses. Rev.(6):77-93.
  4. Oney, S.K.and Krock, H.J.1989. Significance. Presented at dioxide release. Presented at the International Conference on Ocean Energy Recovery, Honolulu, Hawaii, November,1989.
  5. Green. H.J. and Guenther, P. R. 1989. Carbon dioxide release from OTEC cycles.

Presented at the International Conference on Ocean Energy Recovery, Honolulu, Hawaii, November, 1989.

(Victor D. Phillips, Hawaii National Energy Institute, U.S.A.)