Ocean Thermal Energy Conversion and CO2 Sequestration

Ocean Thermal Energy Conversion (OTEC) extracts solar energy through a heat engine operating across the temperature difference between warm surface water and cold deep water. In the tropics, surface waters are above 80°F, but at ocean depths of about 1,000 meters, water temperatures are just above freezing everywhere in the ocean. This provides a 45 to 50°F temperature differential that can be used to extract energy from the surface waters.

Of course, with such a low differential, the Carnot efficiencies of such a scheme are very low; for a system operating between 85°F and 35°F the maximum theoretical efficiency is only 9.2% and real efficiencies will be less. Regardless, OTEC has been demonstrated as a technically feasible method of generating energy.

There are a number of different concepts for the heat engine including low temperature difference Stirling cycle engines and direct use of water vapor derived from the surface waters that is condensed with the cold water, but most concepts have a Rankine cycle using a fluid with a low boiling point.

It works like this: Warm water is used to heat a fluid such as ammonia to vapor. The vapor then runs through a turbine to generate power and the cold water is used to condense it. Let's use ammonia as an example. Ammonia boils at 85°F and 166 psi and condenses at 35°F and 66 psi. This gives us 100 psi to run a turbine. Unfortunately this cycle only provides about 7% efficiency, though it can be boosted a bit by superheating, reheating and similar strategies used in steam cycles. However the big advantage is that OTEC is a solar power system with no collector — the ocean itself is the collector. This means it also is available constantly.

Considerations, Problems and Solutions

There are many practical issues as well. Again, with ammonia as the example, ammonia attacks copper bearing alloys, but only copper alloys resist marine fouling, and only a small amount of fouling is enough to drastically cut efficiency. Systems using ammonia have to have sophisticated waterside cleaning systems. There are also issues with the design of efficient low head turbines, very high performance heat exchangers, the long cold water pipe, and the platform, if it is floating (most OTEC designs are floating platforms, "grazing" in the open ocean).

Finally, there is the problem of using the energy. Most OTEC plants will be far at sea, because deep water in the tropics is generally far from energy markets, so the energy is "stranded."

Since the 70's a few developers have been experimenting with approaches using different fluids, with improved heat exchanger and turbine technology and innovative platform and cold water pipe designs and materials.

Other developers have been working on techniques to use the stranded energy, usually by making an energy intensive chemical at sea that can be used as a fuel or to supplant energy that would otherwise be used to make the chemical. One candidate is ammonia, which currently requires substantial energy to provide the world's need for fertilizers, and can be used as an alternative fuel as well. Another is sodium, made from salt; combining eleven pounds of sodium with water makes one pound of hydrogen. So sodium is potentially a very effective "storage medium" for hydrogen. These developments, plus the growing cost of energy, have people looking again at OTEC.

OTEC and Carbon Sequestering

However, deep cold water is laden with nutrients. In the tropics, the warm surface waters are lighter than the cold water and act as a cap to keep the nutrients in the deeps. This is why there is much less life in the tropical ocean than in coastal waters or near the poles. The tropical ocean is only fertile where there is an upwelling of cold water.

One such upwelling is off the coast of Peru, where the Peru (or Humboldt) Current brings up nutrient laden waters. In this area, with lots of solar energy and nutrients, ocean fertility is about 1800 grams of carbon uptake per square meter per year, compared to only 100 grams typically. This creates a rich fishery, but most of the carbon eventually sinks to the deeps in the form of waste products and dead microorganisms.

This process is nothing new; worldwide marine microorganisms currently sequester about forty billion metric tonnes of carbon per year. They are the major long term sink for carbon dioxide.

In a recent issue of Nature, Lovelock and Rapley suggested using wave-powered pumps to bring up water from the deeps to sequester carbon. But OTEC also brings up prodigious amounts of deep water and can do the same thing. In one design, a thousand cubic meters of water per second are required to produce 70 MW of net output power.

We can make estimates of fertility enhancement and sequestration, but a guess is that an OTEC plant designed to optimize nutrification might produce 10,000 metric tonnes of carbon dioxide sequestration per year per MW. The recent challenge by billionaire Sir Richard Branson is to sequester one billion tonnes of carbon dioxide per year in order to halt global warming, so an aggressive OTEC program, hundreds of several hundred MW plants might meet this.

In economic terms, optimistic guesses at OTEC plant costs are in the range of a million dollars per MW. Since a kilowatt-hour (kWh) of electricity generated by coal produces about a kilogram of carbon dioxide, a carbon tax of one to two cents per kWh might cover the capital costs of an OTEC plant in carbon credits alone. The equivalent in gasoline tax would be ten to twenty cents per gallon. With gasoline above three dollars per gallon and electricity above ten cents per kilowatt, these are not entirely unreasonable charges.

More Testing Is Necessary

The actual effectiveness of OTEC in raising ocean fertility and thereby sequestering carbon still has to be verified, and there has to be a careful examination of other possible harmful environmental impacts — an old saying among engineers is "it seemed like a good idea at the time."

The most important issue is that the deep water already has substantial dissolved carbon dioxide, and so an OTEC plant may actually release more carbon than it sequesters, or it might just speed up the existing cycle, sending down as much as it brings up with no net effect. This question has to be answered before OTEC is implemented.

It may also be possible to optimize sequestration by being selective about the depths that water is drawn from, or possibly by adding other trace nutrients, especially those that enhance species that sequester carbon in shells.

An OTEC plant optimized for ocean fertility will also probably be different than one optimized to generate power, so any OTEC-based carbon scheme has to include transfer payments of some sort — it won't come for free. Finally, who owns the ocean thermal resource? Most plants will be in international waters, though these waters tend to be off the coasts of the developing world.

Saving the World

There might be an additional benefit: Another saying is "we aren't trying to solve world hunger," but we may have. Increased ocean fertility may enhance fisheries substantially. In addition, by using OTEC energy to make nitrogen fertilizers, we can improve agriculture in the developing world. OTEC fertilizer could be sold to developing countries at a subsidy in exchange for using the tropic oceans.

If we can solve the challenges of OTEC, especially carbon sequestration, it would seem that the Branson Challenge is met, and we have saved the earth, plus solving world hunger. Since President Jimmy Carter originally started OTEC research in the '70's, he deserves the credit. I'm sure he will find a good use for Sir Richard's check.

Christopher D. Barry is a naval architect and co-chair of the Society of Naval Architects and Marine Engineers ad hoc panel on ocean renewable energy. He has worked in design agencies, shipyards and manufacturers in the marine industry and in offshore oil exploration and currently works for the Coast Guard, but is not associated with any OTEC program. The opinions expressed are those of the author and do not necessarily reflect the opinions or policy of SNAME or of the Coast Guard.