OTEC

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OTEC stands for Ocean Thermal Energy Conversion. The basic idea is that you use the temperature difference between warm surface water and cold deep water to power a heat engine to generate power. Because the temperature difference between these two heat reservoirs is small, the efficiency tends to be quite low. The technology required is not at all unlike the technology in your refrigerator. Furthermore, the lower temperatures make it easier to design than traditional heat engines that operate at much higher temperatures.

OTEC designs historically have functioned by seeking to replicate what is called a Carnot Cycle heat engine. Every thermodynamic system exists in a particular state. When a system is taken through a series of different states and finally returned to its initial state, a thermodynamic cycle is said to have occurred. In the process of going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine. The Carnot cycle is a particular thermodynamic cycle proposed by Nicolas Léonard Sadi Carnot in 1824 and expanded by Benoit Paul Émile Clapeyron in the 1830s and 40s. A system undergoing a Carnot cycle is then a (hypothetical) Carnot heat engine.

A heat engine acts by transferring energy from a warm region to a cool region of space and, in the process, converting some of that energy to mechanical work. The cycle may also be reversed. The system may be worked upon by an external force, and in the process, it can transfer thermal energy from a cooler system to a warmer one, thereby acting as a heat pump rather than a heat engine.

The Carnot cycle is the most efficient existing cycle capable of converting a given amount of thermal energy into work or, conversely, for using a given amount of work for refrigeration purposes. Carnot realized that there will always be losses in every step and his idealize cycle cannot be perfectly replicated, but his theorem is that no other cycle can be more efficient than a Carnot Cycle.

Because of these losses, OTEC tends to more closely resemble the Rankine Cycle, with a low pressure turbine. These can be either closed or open cycle in design. A closed cycle OTEC system uses refrigerants as the working fluid, using heat exchangers to absorb heat from the warm water side and emit heat on the cold water side. An open cycle system uses water as the working fluid. The larger the temperature differential between the warm and cold water sides, the more power that can be generated. Since the temperature of water at the ocean surface, or heated by solar concentrators, will represent a maximum temperature, it is to the best to seek to obtain water as cold as possible for the cold side of the cycle. This typically requires an expensive large diameter pipe reaching up to 1 km deep into the sea. Normally shore based OTEC systems require more than 1km of pipe to reach this depth and thus run into cost issues, but we do not have this problem with Seasteads provided there is sufficient depth below the seastead or nearby, to reach a depth to obtain water at or around 1 Celsius.

Note that pumping cold water up from such depths can be power intensive, as this water is denser than water at the surface, and of lower salinity (at equatorial/tropical latitudes). It can be pumped with surface pumps if the pipe is solid walled, otherwise it requires a pump at the bottom end which allows a less expensive cloth-based piping, although this sort of piping is more prone to damage from rock, coral, and other fouling.

Power requirements for pumping and system operation are typically 50-75% of gross power output. One positive side effect is that some styles of open cycle OTEC can produce significant quantities of fresh water in addition to power, as a natural byproduct of the thermodynamic cycle. This obviously poses significant benefit to a seastead.

Using solar energy the needed temperature difference of 40 F can be reached than by using refrigerants and complex heat exchangers to improve the thermal head, as outside of latitudes between the tropics, producing this size thermal differential can be very difficult if not impossible. For instance, Puget Sound waters range 50-60 F, which mitigates against OTEC for seasteads in such regions unless solar concentrators are used and sufficient insolation is available.

See also

Videos

OTEC in Nauru
The world's first OTEC pilot plant was situated in Nauru.
Ocean Energy Launch
Global Marine Development, inc. documentary on OTEC.


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