I have no personal experience of OTEC. However, it looks like a promising technology. In theory this technology could do a number of useful things.

• 1. It can produce electricity without giving off any carbon dioxide, and do it for a long period (indefinitely), being a derivative of solar energy.

• 2. Some versions of the technology can produce desalinated water for irrigation or domestic use.

• 3. It tends to cool the surface of the sea in its vicinity. In some areas, such as the Caribbean, this might mitigate the strength of Hurricanes, which gain their power from the warm surface water.

• 4. There can be an increase of fish production in fish farms and in the open ocean.

• 5. It can provide useful employment in areas bordering on tropical seas. (It might save Somalia from statelessness)

The principle of OTEC is that there is a temperature difference between water at the bottom of the sea and the water at the top. This temperature difference can be used to operate a Heat Engine.

The cold water derives from the slow circulation of the oceans. Cold water at the polar regions sinks to the bottom at about 4 degrees Celsius and flows towards the equator. The surface warm water flow in the Atlantic is from the Caribbean area towards western Europe.

At an OTEC site cold water is raised from the lower levels of the sea and used for the cooler side of a heat engine. The hotter side is supplied by the surface water.

The products of the whole process are:

• 1. Energy from the heat engine. This will usually be used to produce electricity. In tropical areas the output will be fairly constant and will continue night and day. That is, unlike direct solar converters it does not slow down at night. This means that, like a nuclear power station, OTEC can be used as base load.
• 2. Cooler water at the ocean surface. This does not mean there is less heat stored in the water. The effect is to produce a thicker layer of warm water at a cooler temperature. Some of this water can be used on land for cooling purposes.
• 3. Nutrient rich water (because the bottom water has more nutrients than the surface). This can be used to promote water plant growth and fish.

Can these plants pay for themselves? The sources of income are:

• 1. Sale of energy.
• 2. Carbon credits (paid for energy that does not emit warming gases). Two islands in the Caribbean are part of the European Union as French Overseas Territories and could therefore receive Carbon Credits from the EU system.
• 3. Profits of fishery improvements
• 4. Sale of fresh water
• 5. Air conditioning (some of the cold water can be used for cooling purposes, such as types of agriculture)
• 6. Insurance payments from potential hurricane victims.

People who have said these plants can't pay their way usually ignore every source of income except the first.

Seawifs

Like other forms of renewable energy, OTEC won't play well if that government considers only the immediate bottom line. Large OTEC plants could become cost-competitive if oil doubles from its current $18 or so a barrel, says Vega. Oil prices don't include what Vega and others call "externalities," such as money spent coping with the polluting effects of burning hydrocarbons or military defense of oil fields. Factoring in oil defense alone would make oil's "true" cost $100 a barrel, says energy guru Amory Lovins. (Among the closed- cycle test plant's funders is the Department of Defense's Advanced Research Projects Agency, which considers the development of new fuel sources to be of importance to the nation's defense.)

 A report of some activity.

"The potential of OTEC is great," says Joseph Huang, a senior scientist for the National Oceanic Atmospheric Administration and an expert on the process. "The oceans are the biggest solar collector on Earth, and there's enough energy in them to supply a thousand times the world's needs. If you want to depend on nature, the oceans are the only energy source big enough to tap."

The US Government stopped Federal Research into OTEC, presumably because of the antipathy of the second Bush regime to thinking about alternative energy or Climate Change in general, which they persisted in denying is influenced by human activities.

But here is a useful description and survey of possible sites, with a map. It mentions probable sites in Australia, Hawaii and other "developed" countries but does not advocate building plants in those countries. There is no physical reason why some these also could not benefit, but perhaps at the time the paper was written oil companies (and the US government) didn't want a viable alternative to their products. Australia could be the site for very large OTEC plants as part of a large scale carbon absorption industry.

This too is a useful survey of OTEC possibilities.

However, the State of Hawaii has continued research.

hosted by Ed Minchau at September 11, 2004 02:56 PM
I believe I have a highly practical suggestion that, if implemented on a large enough scale, might be successful in eliminating or partially effective in controlling hurricanes, if carried out to a sufficient degree.
Basically, it involves a way to cool the ocean surface, by extracting the thermal energy in the surface waters, utilising floating heat pump engines, that will convert the thermal energy into electricity. I'm referring to the OTEC, or Ocean Thermal Energy Conversion research that was started years ago, and has successfully developed working engines that actually produce a net positive output of energy on an ongoing basis. There are numerous OTEC web sites covering this concept.
OTEC requires a temperature difference of about 36 deg F (20 deg C). This temperature difference exists between the surface and deep seawater year round throughout the tropical regions of the world. To produce electricity, we either use a working fluid with a low boiling point (e.g. ammonia) or warm surface seawater, and turn it to vapor by heating it up with warm seawater (ammonia) or de-pressurizing it (warm seawater). The pressure of the expanding vapor turns a turbine and produces electricity! When using warm seawater (Open Cycle OTEC) vapor, we can also produce fresh distilled water when it is condensed back into a liquid. A closed cycle system can indirectly produce freshwater, since its deep seawater discharge is still cold because it only warms up about 8 deg F (4 deg C) in the condenser heat exchanger. When this cold seawater flows through a separate atmospheric heat exchanger, it condenses fresh water from the always humid tropical air.
If enough of these OTEC engines were to be built and placed in the tropical waters where hurricanes are spawned, they could certainly reduce temperature of surface water sufficiently to greatly moderate the generation of hurricanes.

 1. Engineering problems

What is the most effective Thermal Conversion system? Some of this research has probably been done already, but I expect the perfect design has not yet been achieved, considering the small amount of money given to research up to now. Each plant built will generate improvements to subsequent plants. Some useful information here.

A serious problem that needs to be solved is that salty oceanic water is biologically very active and deposits algae on surfaces of pipes and heat exchangers, reducing the efficiency of heat exchange. This suggests that heat exchanging surfaces in contact with sea water may need to be designed for ease of cleaning without complete disassembly.

2. Climate effects
Models are needed of the expected effects of releasing cooler water into a sea with currently high temperature surface water.

a) Where should the cooler water be released to avoid reducing the efficiency of the OTEC plant? For example, if the water is released near the warm water intake will that reduce power output?

b) How many plants are needed to affect the surface temperature of seas such as the Caribbean where Hurricanes receive their power?

c) What would be the effect on heat transport of warm ocean currents carrying Caribbean heat to western Europe?

d) How close can OTEC plants be to each other. For example, the Puerto Rican, Cuban and Haitian coasts may be good sites. How many can these north coasts support without cooling the local sea surface too much?

OTEC plants can only be expected to receive financial return from Hurricane prevention if the models show the effect likely to be real.

3. Economic effects.

These include: value of increased fish production, value of fresh water, employment.

The electricity produced has a value. Should it be used to supply the local grid or should it be used to produce hydrogen for export power? How much is likely to be available?

Reliable local power sources could be the basis of development in Haiti and the Dominican Republic. Yucatan is also a possible site.

One method of removing carbon dioxide from the atmosphere might be hydroponics, to grow blue green algae such as Spirulina on a very large scale (gigatonnes per year is needed). The Western Sahara and Cape Verde Islands might be a useful site. OTEC plants to produce freshwater could be symbiotic with spirulina farms (however, the sea surface off the western Sahara -see this site for a useful map - may not be very warm because of an upwelling of cold water, but could be warmed by making shallow ponds). OTEC may be a part of an integrated industry for producing energy and taking carbon dioxide out of the atmosphere (sequestration).

Various Projects for Andaman Islands and other islands These projects were explored but were not built, perhaps mainly because Diesel was so cheap. The much higher present day cost of oil products should make OTEC look more attractive.