by Bruce Dorminey, via Renewable Energy World
The world’s largest untapped source of solar energy doesn’t lie on the vast sands of the Sahara or even atop the high chaparral of the desert Southwest. Instead, it stretches across at least 23 million square miles of earth’s tropical oceans; the uppermost layers of which make a prime natural source of thermal energy.
Regardless of time of day or cloud cover, Ocean Thermal Energy Conversion (OTEC) promises to harness this thermal sea-based resource year round.
OTEC production converts heat energy from seawater into kinetic energy using the ocean’s naturally steep temperature gradient. It’s this juxtaposition of tropical (and sometimes subtropical) subsurface seawater at temperatures typically above 80 degrees F. and below 40 degrees F. that makes OTEC possible.
An OTEC plant literally pumps the warm surface seawater through a heat exchanger connected to a closed circuit filled with several hundred tons of liquid ammonia. Since ammonia boils at lower temperatures and at lower pressures than water, once the warm seawater hits the heat exchanger, it causes the ammonia to vaporize and expand in volume. As this ammonia vaporizes, it creates pressure to run a turbine coupled to a generator. In most cases, the resulting electricity would be delivered onshore via an undersea cable.
Once this ammonia vapor exits the turbine, it flows through a second heat exchanger that is connected to a cold water pipe carrying tons of seawater pumped from depths of 3000 ft. This cold seawater, in turn, condenses the spent ammonia vapor back into liquid and the whole OTEC process begins again.
But despite the fact that the idea for the technology is more than a century old; to date, OTEC has only been successfully demonstrated on small scales of less than a quarter of a megawatt (MW) and has yet to produce utility-scale power.
“Funding certainly is the biggest obstacle for OTEC,” said Gerard Nihous, an ocean engineer at the University of Hawaii at Manoa. “While nothing we have learned in the past suggests that OTEC has major technological hurdles left to clear, OTEC cannot be considered ready for commercialization. A multi-year operational record at sea would help resolve lingering uncertainties and fix the design ‘bugs’ that are bound to be revealed.”
Such sea operations would begin aboard a stationary floating plant that would skim off a small percentage of the surface layer to use as the heat source. Auxiliary power sources would get the OTEC process and the pumps started; then the plant would generate enough energy to power itself. But even so, an OTEC plant’s real-time operating efficiency is expected to reach only a few percent.
“The heat exchanger cost-efficiency and turbine cost-efficiency tradeoff is different with OTEC than in a conventional steam power plant,” said Chris Barry, an Annapolis-based naval architect and the ocean renewable energy panel chair for the Society of Naval Architects and Marine Engineers. “We have to squeeze everything we can out of the energy we have. Each element of an OTEC plant has to be incredibly efficient, because you’ve got very little to work with.”
Even so, all U.S. territories in the tropics would be prime locations for OTEC, including, Puerto Rico, the U.S. Virgin Islands, Guam, and American Samoa.
In the continental U.S., Florida is seen as a potential prime OTEC producer. Florida Atlantic University (FAU) in Boca Raton, in collaboration with Lockheed Martin Corporation, did an ocean thermal resource assessment off the state’s southeast coast. As a result, Howard Hanson, chief scientist at the Southeast National Marine Renewable Energy center at FAU, now says he initially envisions three 100-MW plants operating just offshore feeding 300 MW of power into south Florida’s electrical grid. But not everyone is convinced that south Florida’s top ocean layer would be warm enough for year-round OTEC.
Meanwhile, the Baltimore-based OTEC International, LLC (OTI) is planning on the 2014 completion of a small 1-MW land-based plant to demonstrate the technology. To be located on the Kona coast of the Big island of Hawaii, its cost will run in the tens of millions. But thus far, all of OTI’s efforts are being funded by Baltimore’s Abell Foundation.
Although located onshore, Barry Cole, OTI executive vice president and its director of technology development in Baltimore, says that the 1-MW demonstration plant will use an existing infrastructure of more than 10,000 feet of pipes to tap into the requisite offshore warm and cold water reserves needed for OTEC production.
Lockheed Martin did not respond to requests for comment on their own OTEC initiatives, but they have been actively working on Hawaii-based OTEC plans with Makai Ocean Engineering, Inc. of Honolulu. * (see 1st comment below)
Customer end user costs for OTEC power in Hawaii are expected to be in the neighborhood of 0.25 to 0.35 cents per kWh which is in line with current residential rates now.
“Everybody’s pushing for large OTEC plants to produce electricity at an attractive rate,” said Joe Van Ryzin, a vice president at Makai Ocean Engineering, Inc. “OTEC’s potential is absolutely huge; build enough 100-MW OTEC plants in Hawaii and you could provide all of its electricity needs.”
Van Ryzin says Lockheed Martin has conceptual designs for a variety of OTEC pilot plants. One of those is a 5- to 10-MW pilot plant, which could see construction off Oahu by 2015; with a 100-MW commercial plant to follow by as early as 2020. Some cost estimates for a 5 -10 MW plant run as little as $300 million. However, a 100-MW plant may hit $1.5 billion.
From a distance of 10 miles off shore, such a floating plant would look like one of the ships that routinely bring fuel oil to Hawaii’s ports. But its generated electricity would arrive onshore via undersea cable.
“The offshore oil industry has done us a great favor by putting billions of dollars into development of these floating platforms,” said Robert Cohen, an independent OTEC consultant in Boulder, Colorado. “They’ve done all the engineering. So we don’t have to build a fancy platform; we just have to build an OTEC power module that will work.”
Once built, such floating OTEC plants would operate much like offshore oil rigs. However, Cole says the ultimate future of OTEC, for the U.S. at least, is the production of concentrated liquid energy; such as ammonia and hydrogen using electricity generated far from land in the ocean’s equatorial belt. These liquefied chemical energy carriers could then be readily shipped back to the mainland U.S. via tanker.
Hydrogen can be readily made by the electrolysis of seawater into hydrogen and oxygen. Ammonia (NH3), in turn, can be manufactured by combining atmospheric nitrogen with hydrogen from seawater.
“Ammonia is an easy high-energy chemical to make and can be used as a primary fuel potentially even in ammonia-burning hybrid vehicles,” said Barry.
OTI is currently negotiating with Caribbean Utilities Company, LLC. of the Cayman Islands about construction of OTI’s first commercial 25 MW OTEC plant at a still undisclosed location.
But Cole says the plant is expected to see construction by 2018 at a cost of several hundred million dollars. OTI is also in negotiations with the Hawaiian Electric Company for a follow-on 100-MW OTEC plant to be located off Oahu which is hoped to also be completed by 2018.
For all its potential, however, OTEC still remains largely overlooked by the larger renewable energy community.
“OTEC is the renewable energy elephant in the room; huge and hard to ignore, although many do,” said Van Ryzin. “Today, we are focused on renewable low-hanging fruit, on what we can conveniently do now. But as a nation, are we addressing our major long-range [energy] problems?”
Bruce Dorminey is an award-winning science journalist who is a former Hong Kong bureau chief for Aviation Week & Space Technology magazine and a former Paris-based technology correspondent for the Financial Times newspaper.
This piece was originally published at Renewable Energy World and was reprinted with permission.
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“Customer end user costs for OTEC power in Hawaii are expected to be in the neighborhood of 0.25 to 0.35 cents per kWh…”
Nobody is going to fall for this one. Perhaps you should adjust these numbers a bit, and put them up (X 100) into the believable range.
Is ammonia THAT easy to make? I seem to recall it takes 250 atmospheres @ 450 degrees Celsius to get a reasonable reaction rate, with large overall energy losses getting the Hydrogen and Nitrogen to and from that state.
That aside, with this approach there would be a relatively large gain for every extra degree of difference between the hot and cold water sources, just like with any other heat engine. Unfortunately the easiest way to do this in engineering terms would probably be to enclose a volume of surface water and apply some of the mechanisms used in solar-heated swimming pools, which may or may not be sustainable but which certainly wouldn’t be environmentally neutral.
Do not forget that ammonia is made from hydrogen by the Haber Process:
3H2 + N2 = 2NH3
To make hydrogen, you have 2 options:
1)electric current on water. Is expensive because CONSUMES A LOT OF ELECTRICITY, so this is not used in practice.
2)Steam reforming of NATURAL GAS, OIL or COAL. In short, the following reaction is used:
H2O(vapor) + CHx = CO2 + H2
It may be a not so good idea to produce massive quantities of ammonia…
It’s too bad that the article makes zero mention of environmental issues. As one would expect from any technique that overturns millions of gallons of deep ocean water daily, there are many possible impacts from OTEC. See journal review at
http://www.marscigrp.org/EnvImpact86.pdf
For example, OTEC may release a third as much CO2 as fossil-fuel generation per MWh generated, due to degassing of water brought up from depth to lower pressures.
Also, what about standard industrial risks of the proposed bulk ammonia manufacture at sea? (Since transporting H2 is almost certainly far more expensive, MWh-equivalent for MWh-equivalent, than transporting NH3, you would have to make the stuff on the platform.)
It might be reasonable to ask if, given the drawbacks of the various alternatives,the tropics might be better off tapping their copious solar resources than building OTEC platforms. Perhaps OTEC would come up all shiny after a fair assessment, a wise part of the energy mix, but without addressing its drawbacks we can’t think seriously about it. At the very least, possible environmental impacts should be _mentioned_!
Larry
Larry, the answer to OTEC’s cost and environmental problem is moving small volumes of vaporized working fluid to a deep water condenser and pumping the condensed fluid back to the surface in a counter-current flow pattern that recycles the bulk of the heat to the ocean’s surface.
There is no upwelling of cold water either to release CO2 or eutrophy the water column. No bio fouling of the condenser, because condensation takes place hundreds of meters below the surface, internally. And the size of the heat pipe required is between 1 and 2 meters for a 50MW unit as opposed to 10 meters for a cold pipe with conventional approaches.
> moving small volumes of vaporized working
> fluid to a deep water condenser
Hmmm. If it’s vaporized, how can it be small volume?
References?
Martin, Vega and Michaelis’ article, First Generation 50 MW OTEC Plantship, points out that to produce 50MW of power they must move 270,400 kg/s of warm water and 142,300 kg/s cold water. In the process 2,750 kg/s of ammonia is used as the working fluid.
The options are therefore pumping 2750 kgs from a depth of about 1 kilometer in a closed system with no impact on the environment or 150 times as much water with massive environmental impact.
OTEC’s low thermodynamic efficiency also means you dump 20 times more heat to the depths with conventional approaches than energy produced.
Richard Smalley said we will need 60TW by 2050 to fill the needs of the planet, which means you would have to dump 1.2 peta watts of heat into the depths and you would overturn the Thermohaline Circulation.
With Counter-current heat flow, you produce 60TW by extracting 120TW of heat from the surface and dumping 60TW to the depths.
One big hurricane pumps between 50 and 200TW worth of heat so with counter-current there is little to no effect on the Thermohaline nor is the thermodynamic efficiency of the process degraded by pumping so much heat from the surface into the cold heat sink required to produce power with the process.
OTEC burns the planet’s excess calories, 90 percent of which have been accumulating there, causing thermal expansion and icecap melting.
The planet can’t go on a diet, the sun is relentless but we can convert its excess calories to useful energy by producing all of the renewable energy we need with OTEC.
In fact it is the only way this thermodynamic problem can be addressed.
Measures like geoengineering are tantamount to hiding the planet’s expanding girth under a corset and cutting back, or out, CO2 emissions does nothing to reverse the thermal inertia already accumulated in the oceans which is believed to make four meters sea level rise inevitable over the next millennium.
The idea of getting energy from the ocean’s thermal gradients has been kicked around since people started thinking about alternate energy sources in the 1970s. While the technical problems of generating electricity can probably be worked out without too much difficulty, the relative cost of the installations, compared do developing desert-based concentrated solar power, could be a limiting factor.
(The ammonia production seems a little iffy.)
Also, there are some political issues about who has the right to which thermocline. But it seems that those could be resolved because there is plenty of energy to go around.
The good news is that between the deserts and the oceans, we have more than enough incoming solar energy to power all of civilization’s anticipated needs.
Jim Baird #4, too simple. OTEC redistributes heat, it doesn’t make it disappear. What large-scale OTEC might however do is reduce the risk of tropical cyclones by slightly cooling the tropical surface water on which these feed.
I’m happy to see that the OTEC idea is not forgotten. It is really a very old idea, dating back to the age of Jules Verne (Jacques d’Arsonval, a pioneer, wrote about how one day mankind would mine heat from the oceans in the same way as they were mining coal back then). And, it is a technique benefiting the tropics where the greatest growth in power consumption is expected.
Martin, it doesn’t make the heat disappear, it coverts it to work in accordance with first law of thermodynamics.
As above to produce 60TW you would convert 60TW of surface heat to power. An alternative like fission or fusion would produce an additional 120TW of heat which would mostly end up back in the ocean.
A recent NASA study determined the average amount of energy the ocean absorbed each year over the period 1993 to 2008 was enough to power nearly 500 100-watt light bulbs for each of the roughly 6.7 billion people on the planet.
This 330 terawatts and OTEC is the only way you can have any positive impact.
Reducing the risk of tropical cyclones by slightly cooling the tropical surface water may benefit large coastal populations mitigating the possibility of tropical cyclones hitting landfall; and, potentially a very important geo-engineering initiative as extreme climate events increase in frequency.