Carbon Dioxide Emissions Solved ? – or - Jimmy Carter Saves The World

I would like to apologize for a short diversion from fundamental thermodynamics, but this is a blog, after all. I'll get back to the second law shortly.

There are many opportunities for renewable energy but one stands out because it may inherently sequester carbon by enhancing a natural process that is already a major long term carbon sink.

Ocean Thermal Energy Conversion (OTEC) extracts solar energy through the temperature difference between warm surface water in the tropics and cold deep water: Warm water is used to heat a fluid with a relatively low boiling temperature and the vapor runs through a turbine to generate power. Below about 1,000 meters, water temperatures are just above freezing everywhere in the ocean, so the vapor can be condensed by cold water brought up from the deeps. There are many different approaches using different fluids, variations of heat exchanger and turbine technology and different platform and cold water pipe designs (most OTEC designs are floating platforms, "grazing" in the open ocean). OTEC has been demonstrated as a technically feasible method of generating energy – I worked on the design of “OTEC Early Ocean Test Platform 1” in the late 70’s, which demonstrated various parts of an OTEC system on a platform converted from a small oil tanker, nominally producing one megawatt – but it was not economical at the time.

The unique feature of OTEC is the cold water. The temperature difference between the warm and cold water is small, so OTEC extracts only a small amount of energy from each cubic meter of water and thus uses prodigious amounts of water. In one design, a thousand cubic meters of water per second are required to produce seventy megawatts of net output power.

The cold water is also 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 in the Arctic, Antarctic and North Atlantic and Pacific. The tropical ocean is only fertile where some feature such as an island or a submerged canyon causes an upwelling of cold water. One such place is off the coast of Peru, where the Peru (or Humboldt) Current creates a nutrient laden waters near the Equator. In this area, with lots of solar energy and nutrients, ocean fertility is on the order of 1800 grams of carbon uptake per square meter per year, (mostly by microscopic photosynthetic phytoplankton). In the adjacent waters, and most of the rest of the ocean, fertility is typically well below 100 grams per square meter per year. This creates a rich fishery, but most of the carbon eventually sinks to the deeps in the form of waste products and dead microorganisms.

Throughout the entire world, these microorganisms currently sequester about forty billion metric tonnes of carbon per year. They are the major long term sink for carbon dioxide. A certain amount of carbon is converted to calcium carbonate, which eventually becomes limestone or other sedimentary rock and under very special circumstances, some of the organic material even becomes oil. Algae and other microorganisms that fed on them in ancient seas are the source of today’s oil.
We can make various estimates of fertility enhancement and sequestration, but a reasonable guess is that an OTEC plant designed to optimize nutrification might result in as much as 10,000 metric tonnes of carbon dioxide sequestration per year per megawatt. 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 megawatt plants, might meet this.

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

The actual effectiveness of OTEC in raising ocean fertility and thereby sequestering carbon has to be verified, and there has to be a careful examination of possible harmful environmental impacts – an old saying among engineers is "it seemed like a good idea at the time". An OTEC plant optimized for ocean fertility will also 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.
As to this last question, there is an additional benefit: Another saying among engineers is "we aren’t trying to solve world hunger". In this case, though, we may have. Increased ocean fertility may enhance fisheries substantially. In addition, a problem of OTEC is that the energy is "stranded" far at sea, possibly on a drifting platform – a thousand-mile long extension cord is not an option. However, many OTEC advocates have suggested making nitrogen compounds for fertilizers at sea – this is an energy intensive process, now mostly using natural gas, so fertilizers are expensive. OTEC fertilizer could be sold to developing countries at a subsidy, where they would greatly enhance farm yields.

It would seem that the Branson Challenge is met, and more, but as much as I would like to claim the Branson prize, I cannot. I worked on OTEC for a contractor under the ERDA, in a program initiated by President Carter, who I am told, was an OTEC enthusiast. I’m sure Jimmy Carter will find a good use for Sir Richard’s check.