The processes for ocean surface warming are highly nonlinear and involve mass transport, chemical adsorption, radiative heating, conduction, evaporative cooling, re-radiation to the atmosphere and ultimately to space, and localized effects of ocean currents in 3D. 98% of the earth's CO2 is trapped in the ocean, mostly below 500m depth within the thermocline and in substances lying on the ocean floor. The size of the heat sink represented by the "cold ocean mass" in the tropics needs to be more than roughly 300 times or larger resource than that of the OTEC power generation over a year so that OTEC may become a third order effect. If we estimate the total volume of water below 500m depth in the tropical oceans, roughly 500 million cubic kilometers, 5e17 cubic meters, or 5e20 liters, we arrive at an estimated 5e20 joules per degree Celsius differential in the heat sink, or 139e6 TWh, over 317 times that from 2.5 TWe of OTEC each year. The efficiency of OTEC conversion is proportional to the temperature difference (dT) between the surface layer and the mean temperature of the heat sink (~3C). If we assume very large OTEC utilization, say 2.5 TWe as shown, with an average dT of 20C, the average efficiency is roughly 70% of the Carnot efficiency (taking into account parasitic losses), or 4.73%. The amount of heat dumped by that much OTEC into the ocean's heat sink at depth is therefore just over 50 TWth, and that is also equal to the heat removed from the surface plus the power output, about 53 TW. The heat sink is replenished by cold arctic and antarctic waters sinking to the bottom at the poles. The reradiation from the world's oceans should also be enhanced by the elevated temperatures due to global warming, but the amount of water sinking to the bottom will likely remain in balance. In other words, as long as the heat sink is replenished by the arctic currents at near to or the same as is done today, the added heat from OTEC will not measurably impact the thermocline for centuries or longer, after which OTEC's cooling effect on the ocean may enhance the replenishment of cold water at the poles. The surface layers of the ocean have relatively small volume, three orders of magnitude less, compared to that of the heat sink at depth. Therefore, OTEC's impact on reducing the surface water temperature over time will be much larger, on the order of one degree F per decade at this power level.
With a slightly different design, using an ammonia heat
pipe instead of a cold water pipe, proposed by Jim Baird and Dominic Michaelis
(British Patent No. GB 2395754), no water from the bottom is released into the
upper strata of the ocean, trapping all the CO2 deep beneath the thermocline.
Little pumping energy is used to circulate the ocean water, simply enough to
pump warm surface water to flow over the evaporator end of the heat pipe. If
the condensing end of the heat pipe is exposed to a thousand feet or more of
near freezing temperatures below the thermocline, no cold water pumping is
required. The parasitic losses are cut in half. The costs for the cold water
pipe are eliminated, along with the cold water return pipe and condenser pumps,
the cleaning system for the condenser, and the overall plant efficiency
approaches 85% of Carnot vs. about 70% with a cold water pipe.
The parasitic losses could be reduced as much as 50% and the complexity, mass (and cost) of the system reduced by at least 30%. The vast reduction in operating costs and environmental impacts would be worth investigation alone.
Also, the use of Solid State Ammonia Synthesis ( SSAS, Holbrook , et.al.) will reduce the energy required to produce ammonia from roughly 12000 to 7000 kWh of electricity per MT of NH3 product. The use of Seawater Reverse Osmosis (SWRO) would be cheaper and more efficient than evaporation for production of drinking and process water. The addition of a fourth energy product, pharmaceutical and/or industrial grade oxygen, may greatly enhance system economics. If pure oxygen is used to burn or oxidize toxic and solid wastes from our cities, the effluent gasses are much easier to clean up and isolate. Further, if the oxygen is used in flash smelting or oxidation of sulfurous compounds, the resulting efficiency for the recovery of copper, platinum series elements, rare earth elements, and radioactive ores can be greatly enhanced. We found this to be the case in copper smelting in the '80's. The notion that OTEC could be used to clean up the waste dumps of the world would attract much more interest from multinational corporations and governments.