Ocean Thermal Energy Conversion

There are several potential sources of alternative energy that I think are quite promising, but don’t get nearly the press of wind or solar power. Geothermal power is one. There are already a number of geothermal power plants in operation around the world, but you wouldn’t know it by the amount of press coverage. Iceland derives 20% of their electricity and nearly 90% of their heating from geothermal power, but in terms of total megawatts the U.S. is the world’s largest producer of geothermal energy (covered in a previous post here).

Ocean thermal energy conversion (OTEC) is another potential energy source that gets little press. Think of OTEC as a big heat pump that operates off of the temperature differences between the surface temperature of the ocean and the cooler temperatures in deeper waters.

I recently received an e-mail on this topic from Bob Cohen, a friend of my friend Jerry Unruh and advocate of ocean thermal energy. He included an essay that he has written on the potential of the technology. I am reproducing it below, with Bob’s permission.

Status and potential of ocean thermal energy technology

Unlike most renewable energy options, ocean thermal energy conversion (OTEC) technology is a “baseload” (continuous) renewable energy source that potentially can provide a substantial portion of global energy needs. As such, it is worthy of ample national attention and R&D funding. Yet today, OTEC technology is largely unknown to the public and has become an “orphan technology” that is being widely overlooked and left out of the public discussion on energy.

In September 1973, during President Nixon’s “Operation Energy Independence”, I left my scientific career at NOAA/Boulder in favor of becoming the first ocean energy program manager within the budding U.S. federal solar energy R&D program, then headquartered at NSF/RANN. Ocean energy R&D was later moved to DOE. The initial federal ocean energy R&D program was mainly focused on OTEC technology, in view of that technology’s potentially large energy payoff. Indeed, assuming that OTEC-derived ammonia or other OTEC-derived energy carriers become competitively viable, OTEC technology could provide the largest payoff of any renewable energy source.

Soon after arriving at NSF, I initiated an RFP seeking an objective evaluation by industry of OTEC’s viability, to get their take on what four groups of OTEC advocates (three of which were in academia) were saying about OTEC. That RFP led to our funding two parallel, buffered studies, which were conducted by Lockheed and TRW starting in 1974. Both studies independently reached favorable conclusions as to the prospects for the technical and economic viability of commercial OTEC plants. Since 2006, there has been a resurgence of interest in OTEC at Lockheed Martin, which is currently investing significant internal funds in reexamining OTEC technology.

From 1972 to 1981, the U.S. ocean energy R&D program thrived and grew under the Nixon, Ford, and Carter Administrations. But in 1981 the Reagan Administration curtailed funding, just on the verge of DOE’s realizing a 40 MWe cost-shared OTEC pilot plant. All funding of the DOE ocean energy R&D program ceased in 1995, leaving the nation hitting on only five of the original six renewable energy R&D cylinders. Meanwhile, DOE has been advancing the other five renewable-energy technologies (wind, photovoltaics, biofuels, heating/cooling of buildings, and solar thermal) toward commercialization.

Thanks largely to an emerging constituency in the coastal states—groups who have been working on generating energy from ocean waves and tidal currents—Congress recently reactivated in FY08 the former DOE ocean energy R&D program (including hydropower R&D) under the name “Water Power Energy R&D”. Congress gave DOE considerable latitude on how to allocate the $10 M FY08 appropriation among hydropower and the various ocean energy technologies.

Subsequent to the U.S./DOE R&D program on OTEC technology during the 70s and 80s, which expended a total of about $250 million, the offshore oil industry has made remarkable progress in developing and advancing marine technology, much of which can be spun off to OTEC applications. So I conclude that elements within the oil industry are likely to become interested in diversifying into an OTEC industry, since they have the capital to do so and because they are likely to be comfortable investing in this sister technology.

There is a substantial U.S. market for OTEC-derived electricity that can be delivered via submarine electrical cable. For example, from locations about 60 to 100 miles off Tampa, New Orleans, and Brownsville. Also, OTEC plants situated much closer to shore can provide electricity and fresh water to a large island market, such as in Hawaii, Puerto Rico, Virgin Islands, and U.S. military bases (e.g., Guam, Hawaii, Kwajalein, and Diego García). Abroad, there are similar potential markets in developing nations that are located in tropical and sub-tropical latitudes. The reason that many such locations are especially attractive early markets for OTEC-derived electricity is that it can be supplied—often, along with fresh water as a valuable co-product—competitively with the oil-derived electricity presently being utilized.

Each baseload megawatt of OTEC power—cabled to locations like Hawaii and Guam that presently use oil to generate electricity—could replace 40 BBL of oil each day. Projections made in 1981 estimated an early OTEC market in such places at some 50,000 MWe, achievement of which would offset a total of about 2 MBBL of oil per day.

Renewable energy technologies are often lumped together and dismissed as serving only nîche or boutique markets. However, OTEC technology is likely to be an exception, assuming that baseload OTEC electricity—harvested aboard factory “plantships” grazing on the high seas—can be converted at sea to viable energy carriers that can economically and competitively deliver those products to markets ashore. Promising candidates for OTEC energy carriers include hydrogen or, more likely, ammonia (as a hydrogen carrier, fuel for combustion or use in fuel cells, or for end-uses like fertilizer). By employing such energy carriers or energy-intensive products as an “energy bridge” to shore, OTEC has the potential to become a major global source of renewable energy.

From the standpoint of national security and energy security, achieving the goal of importing substantial amounts of renewable ocean thermal energy—harvested in international waters by a fleet of domestically-owned OTEC plantships—would be in marked contrast to importing oil from foreign, often hostile, sources. At the same time, OTEC can become an attractive means for mitigating global warming.

I responded by telling Bob that I had a favorable impression of ocean thermal, and I followed up with a few questions about cost, as well as whether anyone has actually managed to produce ammonia in this method (a similar scheme has also been proposed for wind power, but not yet demonstrated) because I am concerned about the worldwide fertilizer situation. Bob’s response (in part):

Insofar as energy cost, one current capital-cost estimate for a proposed first commercial ocean thermal plant — a 75MWe plant projected for a location in Puerto Rico — is $8,000 per kWe. Assuming that that capital-cost estimate is realistic and in the ballpark, that first plant would generate electricity roughly costing about 15¢/kWh.

You mentioned being alarmed by the fertilizer situation. So I’ve just asked the NYTimes to send you a front-page article they published in April [RR edit: Here is the article] about the threat to farmers being posed by increasing fertilizer cost and shortages.

Re deriving ammonia from offshore wind, I’m in contact with George Hart, Jr., who is looking into that possibility conceptually. George is Chief Technical Officer for the Ocean Energy Institute, which was formed a year or so ago by oilman Matt Simmons. My impression is that there has yet to be anything demonstrated along those lines. George is talking about offshore wind @ something like $3.50 per installed watt, and he estimates a duty cycle of ca. 40%. One question I was asking Jerry [Unruh] about is the feasibility of intermittently powering an ammonia plant from a source of electricity like wind.

One nice feature of producing ammonia from renewables would be that — assuming there is ever a cap-and-trade system — those processes ought to warrant a CO2 credit. As contrasted with a CO2 debit when ammonia is produced from natural gas, oil, or coal, since I gather that when those fossil fuels are used to make ammonia, the resulting CO2 is dumped into the atmosphere.

I was recently asking Jerry about the prospect of avoiding the electrolysis of water to make ammonia, by instead combining nitrogen and water. You may have already heard about the technology being proposed to do that, which is being called “Solid State Ammonia Synthesis” (SSAS). A PDF file of a presentation on that subject is available here.

My understanding is that the primary obstacle right now is capital costs. The ocean is very harsh on equipment. But the potential is there for a renewable source of energy that does not have some of the negative land usage baggage that is associated with biofuels.

Like most of the stuff I do, Bob is a pro bono advocate because he thinks this is the right thing to do. If you have some useful expertise or criticisms to add, Bob can be contacted at r.cohen ‘at’ ieee.org.

10 thoughts on “Ocean Thermal Energy Conversion”

  1. I think they need the oil companies for hot rock technology too. They can go deep and sink pipe horizontally. Rock might need to be fractured from time to time. Google just sunk $10M into the technology. 2% of the heat down there would provide 2500X our energy needs. I’m surprised Shell isn’t knee deep in it.

  2. The big issue with OTEC has always been & will always be fouling.

    There are plenty things that live in the ocean (algea & photoplankton being just two) that will quickly gum up the hot side of you're equipment.

    The traditional way for ships to avoid marine growth on their hulls has been the use of toxic paint solutions applied to the hulls that constantly leach out nasty toxins into the water. There are better solutions available now such as teflon type non stick paints onto which marine organism cannot get a toe hold. The problems with these are that they are only effective one a certain threshold scrubbing speed in the water is passed. This is fine for large commercial carriers that spend all their time at sea, but its a poor solution for a yacht that might spend considerable time not moving.

    This is important for OTEC because for optimal heat exchanger (HE)performance you'll want a turbulent flow within your HE but you don't want your flow velocities too high else your HE equipment becomes too large.

    The other main deal killer I saw for OTEC was that you need ridiculously large equipment to produce meaningful power, and the bigger the kit gets (and the larger the flow volumes) the larger the flow losses become so you're net output is compimised.

    For example, to obtain your 'cold' side sea water requires you to pump immense quantities of seawater from 100+ metre depths, and the resulting flow losses eat a large part of your gross electrical output.

    Or you can reduce your flow velocities by building larger diameter pipes but then you're capital costs (and install costs) become ridiculous. The article I read on OTEC whilst an undergrad studying Mech Eng, described one OTEC plant as potentially requiring a 'boiler' the size of a cathedral. (literally, as in not a descriptive exaggeration)

    I've always liked OTEC, but its always been beset by huge capital costs & low output.

    I got the distinct impression from my reading of it, that it had died a death simply because it was such a poor solution.

    To my mind it has one major flaw. Unless you build your OTEC plant when energy is incredibly cheap (and by association so is your stainless steel, titanium, and concrete) then its one of these solutions that as oil (or any energy) becomes more expensive, then a new OTEC plant becomes even more expensive that simply continuing to burn expensive oil.

    But like nuclear plants in that regard, but I've always had my suspicions that new nuclear is expensive as much because of litigation and licensing requirements as it was simply because of the engineering costs.

    Whereas OTEC is just plane expensive.

    I'm sorry I can't reference the paper I read at Uni, but I can't find it anymore. I prefer my blog posts to be backed by facts rather than my piss poor memory….


  3. We live in a world where the Party that controls Congress refuses to consider drilling for oil in a small part of the highly-prospective Alaska National Wildlife Refuge — supposedly for environmental reasons. Same Party wants to drive fossil fuels into extinction — also supposedly for environmental reasons.

    So let’s look at Ocean Thermal Energy Conversion — thermodynamically a low efficiency process for extracting useable power from the temperature difference between warm surface water and cooler deep ocean water. That energy conversion will require cooling the surface waters and/or heating the deeper water. To put that in environmental terms, it means upsetting the ocean thermal gradient on which much marine life depends. Even if it does not cause species extinction, it will certainly interfer with important fisheries.

    Are the environmentalists who oppose a little drilling in ANWR going to stand by idle when someone proposes a large-scale OTEC scheme? We all know the answer to that.

    As in so mamy matters relating to energy, the big barriers are political rather than technical.

  4. If OTEC works, it seems a utility company would want to try it.
    Or perhaps we need an X prize: A $1 billion award to the first commercially viable OTEC plant.
    I generally like the price mechanism and free market solutions. Sometimes, the price mechanism needs goosing.
    The beauty of an X prize is taxpayers don’t spend a dime without a concrete result.
    And even a $1 billion prize is peanuts next our monthly budget for Iraq/Afghanie.
    I would like to see a whole string of X prizes: For the first EV that gets 50 miles on charge and 60 mpg; for the first algae-to-oil process that is commercial; for the first commercialized shale oil extraction etc.
    Jeez, for the price of one month of the Iraq-Afghanie effort, we could have a whole hots of meaningful incentives — and taxpayers wouldn’t lose a cent until the result ws in.

  5. Andytk has brought up the issues that concern me.

    The thermal gap between the surface of the ocean and the depths is far too small to produce useful work without very large amounts of equipment. That and the issues created by the marine environment (corrosion, fouling, storms) lead me to suspect that OTEC would be far more expensive than wind or solar, even if you include the cost of creating energy storage systems with those intermittent sources.

  6. I visited the OTEC facility on the big Island in Hawaii in 1994. I was impressed by the size of the equipment for such a modest amount of output. In addition, they seemed to be fixing this and that throughout the plant.

    I think ocean wave energy conversion (OWEC) rather than OTEC has a lot more promise. The problem with current devices is scale. I will have an illustration/pictures of my methodology, up on the web in a couple of weeks, at http://www.undulationalharvester.com that will reveal some good ideas. Try to stay away from overutilizing copper and magnets, for one. Avoid sending people offshore, for two.

    An analogy that illustrates the problems inherent in the current crop of OWEC devices would be the Colorado River watershed, which covers hundreds of square miles of land. If you attempted to get “hydropower” from that watershed, by constructing 5-meter diameter pools, then putting that water collected in the pools through one-generator-per-pool, it would appear that “hydropower is uneconomic”. Compare that to Hoover Dam, where the power is extracted, not close to the source, but only after the potential energy of rainfall/water at a high altitude, is aggregated mechanically before attempting to convert it to electricity

  7. I remember first reading about OTEC in Marshall Savage’s goofy/engaging book The Millennial Project: Colonizing the Galaxy in Eight Easy Steps. Obviously he aims high here (complete with Star Trek refs) but the first steps were admirably down-to-earth: Start up Foundation, then colonize oceans – there are three more “Earths” waiting at sea, after all. Artificial islands (with housing built free-form out of extruded concrete) would be powered by OTEC. Fun book and very out-of-the-box thinking as well.

  8. I get the idea of using cold, subsurface ocean water for refrigeration and cooling on tropical islands. This displaces a lot of very expensive electricity.

    I don’t really get the idea of adding huge, expensive heat engines to inefficiently convert this low delta-T energy source to electricity.

    Sea water air conditioning (SWAC) is used at the Bora Bora Intercontinental resort. This Environmental Impact Statement (pdf file) gives an overview of a 25,000 ton SWAC district cooling system for Honolulu using a 63 inch pipe at a 1600 foot depth. It mentions other projects, including the National Energy Lab/Keahole Point site which has used SWAC since 1986. Based mostly on Keahole they claim 75 year pipe life and minimal marine fouling at this depth.

    OTEC advocates should focus on widespread SWAC deployment. SWAC gives by far the most bang for the buck today, and creates a large base of deepwater pipe experience to serve as a foundation for future OTEC R&D. Once you have a database of SWAC capital and operating costs in various locations you can pretty easily do the math on adding low delta-T heat engines to make electricity.

  9. Knowing of your interest in OTEC as a renewable energy technology with vast potential, I invite you to check in now and then with my blog — http://HawaiiEnergyOptions.blogspot.com All signs point to an announcement that could come at any time about a small OTEC plant that would be the beginning of the OTEC revolution in the 50th state. Aloha ~ Doug Carlson

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