Ocean Thermal Energy Conversion (OTEC) is by far the most balanced means to face the challenge of global warming. It is also the one that requires the greatest investment to meet its potential. It is a most intriguing answer that can save us from Armageddon.
Dominic Michaelis' Energy Island Concept for OTEC (see http://www.energyisland.com/gallery/visuals/visgallery.html)
The Applied Physics Laboratory at Johns Hopkins University was one of its earliest proponents, whose team was led by Gordon Dugger (see photo below). Given modern materials and design techniques, we should be able to build grazing OTEC plants that may become economical with just a few production units, based upon anhydrous ammonia as the hydrogen carrier. The grazing OTEC plants would produce anhydrous ammonia while surfing the oceans for hot spots to curry heat for their power plants. (BTW there are ammonia pipelines in Indiana and other midwest states today for fertilizer distribution). Ammonia is the second-most predominant chemical manufactured in the world. Since the volumetric energy density of ammonia is three times that of liquid hydrogen, and ammonia combustion can be exceptionally efficient (about the same as burning diesel fuel in turbodiesels), it may be true that a hydrogen economy based upon OTEC and ammonia may be close at hand. The overall replacement of transportable carbon fuels by OTEC-based ammonia is estimated at 100 million barrels of oil per day equivalent over about 40 years if we move to a hydrogen economy. Along with other technologies, carbon fuels could be replaced in roughly 80% of all applications.
OTEC is a true triple threat against
global warming. It is the only technology that acts
to directly reduce the temperature of the ocean (it was estimated one degree
Fahrenheit reduction every twenty years for 10,000 250 MWe plants in '77),
eliminates carbon emissions, and increases carbon dioxide absorption (cooler
water absorbs more CO2) at the same time. It generates fuel that is portable
and efficient, electricity for coastal areas if it is moored, and possibly food
from the nutrients brought up from the ocean floor. It creates jobs, perhaps
millions of them, if it is the serious contender for the future
multi-trillion-dollar energy economy.
In concert with wind and solar power, OTEC will complete the conversion of the human race to a balance with Nature. We need only choose life over convenience.
Some folks know
that I've been a proponent of ocean power since the late '70s. Rummaging
through old stuff on the internet, I found this ancient photo of me in Miami in
1977, on a panel discussing OTEC. This may have been the first time that OTEC
was discussed in public in terms of global warming.
Oddly enough, the concern was that we might cause an Ice Age!
Here is the document, which describes the technology quite well at that point in time, more than 30 years ago: otec_liaison_1_613.pdf
We should be more worried about global warming upsetting the ocean currents by overheating the ocean, which is now happening at an alarming rate. The latest guess is +5C (9F) by 2100!
This technology may be deployed as a means to bring the ocean back into balance, not to upset it.
The designs for these OTEC ships have features that are quite innovative and cost effective. Estimates range from $3000 to $6000 per kWe installed in 2010 dollars, depending on the configuration and proximity to shore. The capacity factor should be close to 100%, especially with the modular designs for the power modules. This means that OTEC annual power production will average three times that of solar and wind per unit of power capacity. Gulf plants may be moored in deep water and connected directly to the grid, bypassing the ammonia step. Tropical ships may graze from site to site and perform stationkeeping to stay in place when it's advantageous to do so. One design called for neutrally buoyant hulls to allow for submerging the ship in the event of any major storm to levels below the wave action zone. The major expenses are for the heat exchangers (titanium alloys or aluminum), cold water pipe, and ammonia production/electrical generation and transmission facilities.
The heat would be dumped into the cold water stream, which cools the condenser and is ejected below the thermocline so that the water would not release its CO2 content except to the colder surrounding water at depth, where the CO2 would remain sequestered. The ocean bottom waters are at 1 to 3 degrees Celsius everywhere year round, at depths over 1000 meters, while the seas average over 4000 meters in depth worldwide. This is the source of cooling water for OTEC. CO2 is dissolved in water at cold temperatures, and the ocean depths hold over 98% of the world's CO2 sequestered in solution. It's cold below 1000 m depth everywhere, even at the equator. In Hawaii, the cold water we brought to the surface chilled our beer to 34 F. It was 90 F outside. The warmer water used for the evaporator would be ejected near the surface where it came from and would mix in the ship's wake.
Biofouling would be handled with chlorination or ozonation, probably the latter in the tropics version. Periodic flushing would be part of the routine, and automated. The cleaning technique is used on most iron ships on the high seas for over a century. If we build over 20000 OTEC plants (each about the size of the nearly 7000 oil platforms in the Gulf of Mexico) deployed in the tropics, we could generate 5000 GWe of power and reduce the surface water temperature by 1C each decade. OTEC kills two birds with one stone: It generates power for the planet and stops global warming. I was aboard OTEC-1 during its shakedown tests off the big island in Hawaii in 1979.
The pilot plant, built by DOE, performed its tests and passed with flying colors. Funding for the commercial scale demonstrator was killed by the next administration.
The Potential for OTEC
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.
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