|Fig. 1: Red and Orange areas indicate parts of the ocean in which OTEC is a viable option due to temperature gradient. (Source: Wikimedia Commons)|
Oceans account for more than 70% of the Earth's surface and are the world's largest collector of solar energy.  The world oceans harness enough thermal and mechanical energy to meet and exceed global energy demands, however a relatively small amount of renewable energy research has focused on utilizing ocean energy.  Ocean Thermal Energy Conversion (OTEC) is one way in which sustainable energy can be generated from the ocean. The concept of OTEC was first explored in 1881 by French physicist, Jacques Arsene d'Arsonval, but was not explored in the United States until the 1970s.  OTEC uses the temperature gradient that exists in tropical oceans to generate electricity from thermal energy.  OTEC functions similarly to a steam engine, yet it utilizes a lesser temperature gradient.  An OTEC plant uses the ocean's warm surface water to vaporize a working fluid and drive a turbine, while cold water is pumped from the deeper parts of the ocean to re-condense the vapor. [5,6]
OTEC can be categorized as open cycle, closed cycle, or hybrid cycle, depending on the type of working fluid used by the plant.  In open cycle OTEC the working fluid is seawater.  The warm seawater is flash-evaporated in a vacuum chamber in order to produce steam, which will drive the turbine.  Closed cycle OTEC, on the other hand, does not use seawater as its working fluid. Instead, a fluid with a low boiling point (such as ammonia) is used as the working fluid and heated by the warm surface water.  The vaporized ammonia is then used to power the turbine and drive the generator.  Hybrid cycle OTEC combines elements of open cycle and closed cycle OTEC, using both seawater and ammonia as working fluids.  OTEC plants can be built either onshore or offshore, with each option providing unique benefits and drawbacks. Onshore plants can be expensive to build on valuable coastal lands, so offshore floating plants may offer more space.  However, offshore plants must spend more money on transferring the energy generated at the plant to the shore through seafloor cables. 
Unlike solar plants or wind turbines that are dependent on certain conditions in order to operate, an advantage of OTEC as a renewable energy source is its ability to generate energy at all times.  OTEC also does not emit pollutants into the atmosphere.  This has huge implications for climate change because OTEC can produce energy without burning fossil fuels, and can help reduce our global reliance on unsustainable forms of energy.  Another advantage of OTEC, particularly open cycle OTEC, is its ability to create fresh, desalinated drinking water when the warm water is recondensed.  OTEC could be a great option for developing areas that not only need domestic power, but also need fresh drinking water.  The cold-water effluent produced through OTEC can also provide useful by-products that have applications in mariculture, agriculture, ice production, hydrogen production, and even air conditioning. [1,2] These by-products alone could translate into a lucrative market for OTEC plants. 
One major drawback of OTEC is its economic feasibility. The Big Island of Hawaii wants intends to build a OTEC is a much more expensive energy source than fossil fuels, which has made many policy makers reluctant to get behind it.  Another drawback to OTEC is that it can only be performed in tropical oceans where there is a thermal gradient of at least 22°C between the surface water and the water at 1000 m (See Fig. 1 to view these areas).  In order for the United States to utilize OTEC, plants must be located in areas such as Hawaii, Puerto Rico, or the Gulf of Mexico.  Lastly, there is concern with OTEC that discharges of ammonia, changes in water temperature, and intake pipes could all pose a threat to local communities of marine life. 
OTEC holds promise as a sustainable way to meet global energy needs, while reducing climate impact. It seems that this technology will be most successful in areas that are in need of resources like clean drinking water and sustained agriculture, in addition to energy needs.  However, it is clear that more research must be done to develop a more cost-effective approach to OTEC in order to make the technology a financially feasible option.
© Zoe White. 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.
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