Time to Revisit Ocean Thermal Energy?

I am writing this blog to shed a little light on an often neglected potential form of renewable energy, Ocean Thermal Energy.  Ocean Thermal Energy Conversion (OTEC) is the common acronym used to describe the process of turning the thermal energy stored in ocean water into a usable form of energy like electricity.  The idea of tapping into the ocean thermal gradient as a power source is not a new one, and was actually first described by French scientist Jacques-Arsène d’Arsonval back in the late 1800s; he proposed using the thermal gradient to run a low temperature Rankine cycle with an ammonia working fluid.

The concept is pretty straightforward.   When water heats up, it becomes less dense, and in a sense floats on top of the colder water below.  When the sun shines onto the ocean surface, nearly all of the sunlight is absorbed by the first 100 meters of water; in places where the ocean is thousands of feet deep, the solar energy does not reach down into the depths where the water temperature can be a frigid.  Deep areas in the tropical oceans can have surface temperatures of 80-85°F, but 1000m down, it could be 40°F.  This yields a theoretical maximum efficiency of 6-7%, which in reality is probably closer to 2-3%.  The OTEC power plant makes up for this horrendous efficiency through sheer volume, and since the amount of thermal energy annually stored in the ocean by sunlight is so large (something like 4000 times our annual consumption), our fuel supply is effectively limitless.  For a Closed Cycle plant, tens of thousands of gallons per second are pumped up from about one thousand meters below, as well as from the near surface; the water is then run through heat exchangers used to boil and condense a working fluid.  The working fluid would be a refrigerant, like ammonia or R-134a, and this is because they boil and condense under easily managed pressures at the operating temperatures supplied by the ocean waters.

If it is so wonderfully, head-slapping-ly obvious then how come we aren’t all powering our homes with OTEC, you ask? As my father says, the devil is in the details.  People have tried before, and have even succeeded in producing power, but there have been complications.  The first plant an open cycle plant (it utilizes a partial vacuum to create low-pressure steam) built by a French scientist named Georges Claude, a student of d’Arsonval.  The plant was constructed in Cuba in 1930, and while it produced 22kW of electrical power, the design and small-scale limited the plant to only producing enough power to run the water pumps.  Creating net power has since been achieved with closed cycle plants, but the real problem has since proven to be building and deploying an OTEC system economically.  The cold water pipe that draws water up from the ocean depths must be huge, on the order of 800-1000 meters long and up to 11 meters in diameter; building and deploying such a structure has proven to be extremely difficult.  Another of the major hurdles yet to be cleared involves the heat exchangers between the ocean water and the power cycle working fluid.  These must be as efficient as possible, because the more efficient they are, the less water needs to be pumped to properly heat the working fluid.  Conversely, if the heat exchangers are corroded or fouled up from organic material, then the required water flow rate goes up.  Even what type of pumps and turbines to use, how to house them, and where best to place them are all questions that are still unanswered.  There are also big unknowns as to what sort of effect transferring all of this water will have on the ecosystem.  The cold water is actually very nutrient rich, and this could be anywhere from beneficial for the surface environment to extremely destructive, depending on who you ask.

The reason I am writing this is that all of these problems are engineering problems not fundamental conceptual problems; they are all potentially solvable.   Wind and solar are great renewable energy sources, but that is only because we have spent that last 30 years working on them, and only now are they beginning to compete.  OTEC is stuck in the proof of concept stage due to the fact that it requires huge capital investments, and can’t easily be proven on a  small-scale.  The government has been very hot and cold with OTEC funding as well (pardon the pun).  There was a huge wave of support for it when the oil shocks in the 1970s caused energy prices to spike, but when oil prices bottomed out in the 1980s, all funding was cut.  However, with the demand for reliable, clean base-load power on the rise, the idea of OTEC power plants sounds good once again.

I only scratched the surface on this subject, and if you want to know more, I will be writing about this for the research paper.  Or you can ask me, and I’ll do my best to answer it; either way, I think more people need to know this is out there.

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1 Comment

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One response to “Time to Revisit Ocean Thermal Energy?

  1. chrisvdh

    Another issue of OTC you might want to address is availability. Looking at this map, http://140.96.175.55/vol/images/2-6.jpg , it seems like most of North America, Europe, and Africa wouldn’t be able to gain appreciable generating capacity from OTC without a lot of investment in transmission infrastructure.

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