|Fig. 1: This figure is a diagram of the OTEC system and also shows what the system can be used for, besides energy production. (Source: "Wikimedia Commons)|
The worlds oceans cover over 70% of the earth, making it possible to produce vast amounts of renewable energy from these oceans.  There are many different technologies that allow for energy production from the ocean, such as generating energy from waves, tides, and ocean temperatures. Ocean Thermal Energy Conversion (OTEC) is a method of generating energy that uses the temperature differences between warm surface water and colder deep seawater to produce energy. The two most common OTEC systems are: closed-cycle and open-cycle. Both of these systems require a working fluid, condenser and evaporator within the system. 
In order for the OTEC to work it needs a heat source, which is the warm surface water, and a heat sink, which is the cold deep water.  In a closed-cycle OTEC system the warm water is pumped over a closed loop pipe filled with a liquid with a low boiling point, likely ammonia. The warm surface water evaporates the liquid into a vapor and this vapor turns a turbine. After the vapor passes through the turbine, cold deep seawater is pumped over the pipe which cools the vapor and condenses it back to a liquid. This liquid is then returned back to the beginning to begin the loop again and both the warm and cold seawaters are discharged back into the ocean.
An open-cycle OTEC system is similar to that of a closed-cycle system, except the open-cycle system gets rid of the low boiling point fluid and instead just evaporates the warm seawater in a low pressurized chamber. The surface water is pumped into the low pressure chamber and the pressure in the chamber causes the seawater to evaporate and turn into water vapor, which turns the turbine. The water vapor is then condensed back into water with the cold water. The benefits of the open-cycle system are, you don't have to deal with the low boiling point fluid, which could be dangerous if it escaped into the environment. Also, because you are evaporating the warm sea water, when it is condensed back into water it is freshwater, which could be given to the local communities in need. The one disadvantage is, the pressurized chamber has to have zero leaks in order for the chamber to be able to evaporate the water, so it takes a lot of maintenance.
Two of the biggest advantages of OTEC are that it produces clean environmentally friendly renewable energy and, unlike solar plants which can't work at night and wind turbines which only work when its windy, OTEC can produce energy at all times.  Other benefits in the open-cycle system, as seen in Fig. 1, include working as a desalination plant and providing freshwater to the communities around it. Another benefit is that, once the cold water pipes are installed to pump the cold water up from the deep, the cold water can be used for other things such as, air conditioning and refrigeration.
Some of the cons for OTEC are that it is economically extremely expensive, the energy produced offshore has to be sent through underwater electrical lines to shore, it takes a lot of energy to pump the cold deep seawater to the surface, the construction and presence of the pipes and plant can disrupt sea life, and most importantly it only works in a few locations. In order for OTEC to work you need a temperature difference of 20 degrees Celsius between the warm surface water and cold deep water. This is a big temperature difference and this temperature difference is only present in tropical water, which excludes many possible locations for an OTEC plant. 
Overall, Ocean Thermal Energy Conversion is a great possible way to produce clean renewable energy without any harmful emissions. There are many benefits to OTEC, but there is also some cons. The pros are OTEC is fuel free, has a low environmental impact, can supply pure water for both drinking and agriculture, can supply refrigeration and cooling and can provide a coastal community with reliable energy.  The cons are that OTEC is economically expensive, and it only works in a few regions of the world.
© Colin Hyatt. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 K. A. Finney, "Ocean Thermal Energy Conversion," Guelph Engineering Journal 1, 17 (2008).
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 K. Galbraith, "Generating Energy From the Deep," New York Times, 29 Apr 09.