byChris Woodford.Last updated: September 16, 2023.
Earth? It might be better called Oceanus: most ofit is, after all, covered in water—much of itvery warmwater. The really interesting thing about the ocean is not how hot itis, but the difference in temperature between the surface (where theSun keeps the sea relatively hot) and the depths (where the water, never warmed by the Sun, isconsiderably cooler). As any engineer knows, a temperature differencelike this is very useful indeed if you're trying to make power. Sowhy not use the heat in Earth's vast oceans to generate useful energy? That'sthe basic thinking behindOTEC(ocean thermal energy conversion), first suggested in 1881, whichinvolves extracting useful energy from the heat locked in the oceans.How much energy are we talking about? According to some estimates,there's enough heat in the upper layers of the oceans to meethumankind's energy needs hundreds of times over.[1]Sounds great! So... how exactly does it work? Let's take a closer look!
Photo: There's a huge amount of energy trapped in the oceans. OTEC (ocean thermal energy conversion) makes power using thetemperature difference between warm surface water and cooler water down below. Photo courtesy ofNASA/JPL.
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Artwork: The concept of OTEC: could we really use heat differences in the ocean (left) to make electric power for things like laptops and kettles?
Most of theelectricity we use comes fromheatengines of one kind or another.
A heat engine is a machine that cycles between two different temperatures, one hot and one cold,usually extractingheat energy from a fuel of some kind. In asteam engineor asteam turbine, for example, coal heatswater to make hot,high-pressure steam, which is then allowed to expand and cool down toa lower temperature and pressure, pushing a piston and turning a wheel as it does so. Thegreater the temperature difference between the hotsteam and the cooled water vapor it becomes, the more energy can beextracted (and the more efficient the engine).
Artwork: Temperature gradients: As you can see from this NASA map of ocean temperatures, there are huge variations in ocean temperature between warm tropical areas (red, orange, and yellow) and colder polar and temperate regions (green and blue). What you can't see from this map is the variations in temperature that exist at different depths of the ocean in the same region. The tropics (the area colored orange and yellow) have the best potential for generating OTEC power. Image by NASA MODIS Ocean Group, Goddard Space Flight Center, and the University of Miami courtesy of NASA Goddard Space Flight Center (NASA-GSFC) andInternet Archive.
In OTEC, we use the temperature difference betweenthe hot surface of the ocean and the cooler, deeper layers beneath todrive a heat engine in a broadly similar way—except that no fuel is burned:we don't need to create a difference in temperature by burning fuel becauseatemperature gradient exists in the oceans naturally! Since the temperature difference is all-important, we need the biggest vertical, temperature gradient we can possibly find (at least 20° andideally more like 30–40°). In practice, that means a placewhere the surface waters are as hot as we can find and the deepwaters (perhaps 500–1000m or 1000–3000ft beneath ) are as cold aspossible. The best place to find such a combination is in the tropics(between the latitudes of about 20°N and 20°S).
Chart: How ocean temperature various with depth. In the warmest, tropical parts of the world's oceans, surface temperatures are typically 20°C (68°F) or more. In the coldest depths, they're close to freezing (around 4°C or 39°F). This huge temperature difference makes OTEC possible. Water temperature changes rapidly with depth in the middle region, which is known as the thermocline.
“... tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion barrels of oil.”
Considering how big and deep the oceans are, it comes as no surprise to find theysoak up and retain vast amounts of solar energy. Some years agoocean engineer Richard Seymour estimated that the oceansand atmosphere between them "intercept... about 80 trillion kW, orabout one thousand times as much energy as used... globally."[2]How much of that could we recover from the sea?According to the US Department of Energy'sNational Renewable Energy Laboratory(DOE/NREL), on a typical day, the tropical oceans mop up heat energyequivalent to 250 billion barrels of oil. Converting a mere 0.005percent of this into electricity would be enough to power the wholeof the United States! However, impressive-sounding estimates likethis don't take account of the tremendous practical difficultiesinvolved in harvesting ocean energy.
There are essentially two different kinds of OTEC plant, known as closed cycle and open cycle.
In closed-cycle OTEC, there is a long, closed loopof pipeline filled with a fluid such as ammonia, which has avery low-boiling point (−33°C or 28°F).(Other fluids, includingpropane and various low-boiling refrigerant chemicals, have also been successfully used for transporting heat in OTEC plants.) The ammonia never leaves the pipe: it simplycycles around the loop again and again, picking up heat from theocean, giving it up to the OTEC power plant, and returning as acooled fluid to collect some more.
How does it work? First, the pipe flows through aheat exchanger fixed in the hot surface waters of the ocean, whichmakes the ammonia boil and vaporize. The heated ammonia vapor expandsand blows through aturbine, which extracts some of its energy,driving agenerator to produce electricity. Once the ammonia hasexpanded, it passes through a second heat exchanger, where cool waterpumped up from the ocean depths condenses it back to a liquid so itcan be recycled. You can think of the ammonia working in a broadlysimilar way to the coolant in arefrigerator, which is also designedto pick up heat from one place (the chiller cabinet) and carry it elsewhere(the room outside) using a closed-loop cycle. In OTEC, the ammoniapicks up heat from the hot, surface ocean waters (just as the coolant chemicalpicks up heat from the chiller compartment), carries it to a turbine where much of its energy is extracted, andis then condensed back to a liquid so it can run round the loop formore heat (just as the coolant in a refrigerator is compressed andcooled in the fins around the back of the machine).
Here's a summary of the key steps in a closed OTEC cycle:
In open-cycle OTEC, the sea water isitselfused to generate heat without any kind of intermediate fluid. At thesurface of the ocean, hot sea water is turned to steam byreducing its pressure (remember that a liquid can be made tochange state, into a gas,either by increasing its temperature or reducing its pressure). Thesteam drives a turbine and generates electricity (as inclosed-cycle OTEC), before being condensed back to water using coldwater piped up from the ocean depths.
Photo: A model of a simple open-cycle OTEC system. The heart of it is a large turbine driven by steam, which is cooled by water pumped up from the deep ocean. Photo by Warren Gretzcourtesy ofUS DOE/NREL.
One of the very interesting byproducts of this method is that heating and condensing sea waterremoves its salt and other impurities, so the water that leaves theOTEC plant is pure and salt-free. That means open-cycle OTEC plantscan double-up asdesalination plants,purifying water either for drinking supplies or for irrigating crops. That's avery useful added benefit in hot, tropical countries that may be short of freshwater.
Open- and closed-cycle OTEC can operate either onthe shore (land-based) or out at sea (sometimes known asfloating orgrazing). Both have advantages and disadvantages, which we'llconsider in a moment. Land-based OTEC plants are constructed on theshoreline with four large hot and cold pipelines dipping down intothe sea: a hot water input, a hot water output, a cold-water input,and a cold-water output. Unfortunately, shoreline construction makesthem more susceptible to problems like coastal erosion and damagefrom hurricanes and other storms.
Photo: A prototype land-based OTEC plant in Hawaii, photographed in the early 1990s. Photo by Warren Gretzcourtesy ofUS DOE/NREL.
Sea-based OTEC plants are essentially the same but haveto be constructed on some sort of tethered, floating platform, notunlike a floating oil platform, with the four pipes running down intothe sea; early prototypes were run from converted oil tankers and barges. They also need a cable running back to land to send theelectrical power they generate ashore. Hybrid forms of OTEC are alsopossible. So, for example, you could build an OTEC platform somedistance offshore on the continental shelf, which would share some ofthe advantages of land-based OTEC (stability and durability,closeness to the shore, and so on) and floating OTEC (opportunity toexploit a greater temperature gradient, so generating power moreefficiently).
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OTEC sounds immensely attractive: it's clean,greenrenewable energy that doesn't involve burning fossil fuels,producing large amounts ofgreenhouse gases, or releasing toxicair pollution. By helping to reduce our dependence on fuels such aspetroleum, OTEC could also help to reduce the "collateral" damage the world suffers from an oil-dependent economy—including warsfought over oil andwater pollution from tanker spills. It could also provide a veryuseful source of power for tropical island states that lack their ownenergy resources, effectively making them self-sufficient. As we'vealready considered, open-cycle OTEC can play a useful part inproviding pure, usable water from ocean water. OTEC can also be usedto produce fuels such as hydrogen; the electricity it generates canbe used to power anelectrolysis plant that would split seawater intohydrogen and oxygen, which could be bottled or piped ashore and thenused to power such things asfuel cellsinelectric cars. The waste cooling waterused by an OTEC plant can also be used foraquaculture(growing fish and other marine food such as algae under controlledconditions),refrigeration, andair conditioning.
The biggest problem with OTEC is that it'srelatively inefficient. The laws of physics (in this case, theCarnot cycle) say that any practical heat engine must operate at less than 100 percent efficiency; most operate well below—and OTEC plants, which use a relatively small temperature difference between their hotand cold fluids, have among the lowest efficiency of all: typicallyjust a few percent.[3] Taken by itself, low efficiency doesn't mattertoo much because the energy in ocean water is completely free:if we capture only a small proportion of what's there, it's not a huge problem.
However, low efficiency does have other consequences.OTEC plants have to work very hard (pump huge amounts of water) to produce even modest amounts ofelectricity, which brings two problems.[4] First, it means a significant amount of the electricity generated (typically about a third) has tobe used for operating the system (pumping the water in and out).Second, it implies that OTEC plants have to be constructed on a relativelylarge scale, which makes them expensive investments. Large-scaleonshore OTEC plants could have a considerable environmental impact onshorelines, which are often home to fragile, already threatenedecosystems such asmangroves andcoral reefs.
Photo: Onshore OTEC plants can take up a lot of valuable coastal land. This is the Natural Energy Laboratory at Keahole Point, Hawaii. Photo by Warren Gretz courtesy of US DOE/NREL.
Although OTEC plants are only suitable fortropical seas with relatively large temperature gradients, that'sless of a problem than it sounds. According toDOE/NREL, OTEC couldtheoretically operate in 29 different sovereign territories(including warmer, southern parts of the United States) and 66developing nations; and temperate parts of the world that can'toperate OTEC most likely have alternative forms of ocean power theycould exploit, including offshorewind turbines, tidal barrages, and wave power.
Although OTEC produces no chemical pollution, itdoes involve a human intervention in the temperature balance of thesea, which could have localized environmental impacts that would needto be assessed. One important (and often overlooked) impact of OTECis that pumping cold water from the deep ocean to the surfacesreleases carbon dioxide, the greenhouse gas currently mostresponsible for global warming. The amount released is only afraction (perhaps 10 percent) as much as that produced by afossil-fueled power plant, however.[5]
Scientists and engineers have been trying toextract useful heat energy from the oceans for over a century, withvarying amounts of success. So far, only a few small-scaleexperimental units are operating. One is producing about 100kW ofelectricity (about 5–10 percent as much as a single 1–2MW wind turbine) inJapan, another is generating about half as much in Hawaii, anda third is now producing about 1MW in India; these are tiny amounts of energy that don't prove thelong-term commercial viability of OTEC in a world where there aremany other sources of power, other forms of renewable energy(such as wind and solar) are becoming dramatically cheaper, and the economics of energy have tobe rewritten from one day to the next.
All that could be about to change, however. After years of planning and construction, the Lockheed Martin company finally finished work on its 100kW prototype OTEC plant in Hawaii in August 2015;work began on the Global Ocean reSource and Energy Association Institute's equally tiny 100kWOTEC demonstration facility in Kumejima, Okinawa in 2013. Depending on how successful thesemodest experiments prove to be, bigger plants could follow; Lockheed has already announced plans for a 10MW offshore plant (with 100 times more generating capacity) in China,while KRISO (the Korean Research Institute of Ships and Ocean Engineering) has been developing a 1MW OTEC unit for the Pacific island of Kiribati since 2013.Under current economic conditions, OTEC plants are most likely to be constructed in or near small tropical islands like this that have little or no energy resources of their own,a high-dependence on expensive, imported oil, and perhaps a pressing shortage of freshwater as well; a combined OTEC power and desalination plant could be very attractive in thatsituation. Early customers are likely to include power-hungry USnaval bases in tropical American territories—and that's one of thereasons why the US Navy is currently investing in the technology.
All this sounds very exciting, but OTEC is still, essentially, a prototypetechnology that's harder to perfect in practice than to conceive in theory.In 2018, for example, amibitious plans to construct a 16MW offshore OTEC plant in Martinique were shelvedindefinitely following major technical difficulties with the coldwater inlet pipe.Barbados andPuerto Ricoare among the other nations still actively investigating OTEC as a future power source.
Here's a brief timeline of some key moments in thehistory of ocean thermal energy.
Chris Woodford is the author and editor of dozens of science and technology books for adults and children, including DK's worldwide bestsellingCool Stuff series andAtoms Under the Floorboards, which won the American Institute of Physics Science Writing award in 2016. You can hire him to write books, articles, scripts, corporate copy, and more via his websitechriswoodford.com.
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