

Power from the sea

Second time around…

Ocean heat may be used to generate electricity

See article

Readers' comments



Dear Sir
And about to use the natural solar energy on inhabited Deserts? Perhaps in the future thousands of square kilometers on Surface’s Deserts will be covered by Black Body Sun heat absorbing material.
By the way the State of California is mainly a Desert .Even that, it is the richest US State!
Oceans ( natural water) must be untouchable , if our civilization wishes to survive Ecological deep damage already being done , besides of geological and artificial temperature changes on the Planet in a near future .

Sid 34

Would I invest $10000 so I could run my kettle for free, probably not. But I am sure the US military will spend that.


Regarding your article about OTEC, "Power From the Sea, Second Time Around..." January 7, 2012, the long delay we have seen in implementation may have been caused by lobbies for the coal and nuclear interests. A similar situation took place in the 1980's. See "The Ecologist," Vol.20, no.3 May/June 1990, "Dirty Tricks: How the Nuclear Lobby Sabotaged Wave Power In Britain."

In the US today, there are five groups working on OTEC. In addition to the three you mentioned in your article, Lockheed-Martin, Makai Ocean Engineering and OTE Corporation, there is also OTEC International and Sea Solar Power. The US could become a big OTEC player. No other countries have as much interest in developing the resource.

The estimate of a billion dollars for the first 100-MW floating plant given in the article is probably high. What is necessary first are 10- to 25-MW floating plants which utilize different solutions for keeping the costs down. The biggest cost to control is the heat exchangers, which separate the working fluid of the power cycle from the warm and cold sea water. Because of the small temperature difference, millions of square feet of heat exchanger surfaces are required even for a small plant. These must be as efficient as possible and dirt cheap. Aluminum alloys will be used, not more expensive metals.

I believe the temperature differences in the tropical and subtropical oceans are the most valuable resource on our planet. In addition to being free like solar and wind, these are available 24/7, or all the time because so much solar heat is stored in the ocean's surface. Solar and wind have operating capacities of 30% or roughly 1/3 of OTEC's. It has been estimated that OTEC could theoretically supply 100 times our total energy use. The potential is huge. The biggest resources are found in Southeast Asia and the Caribbean.

There are valuable by-products of OTEC: advanced plants will make fresh water, the surface of the ocean can be made to support increased marine life with nutrient-rich deep water, carbon sequestration is likely as are lower ocean temperatures, which should reduce hurricane activity and save coral reefs.

If we decide not to mix the cold water with warm surface water, it will sink back down to its appropriate depth only heated a couple of degrees.


$1 billion for a first generation 100MW plant would be bloody amazing!

Ballpark: imagine it runs at 80% of its annual generating capacity (some downtime for maintenance and such). Imagine we are depreciating it over 20 years. Imagine we are paying 2% real interest (2% above inflation) for our capital. Imagine we are selling the power wholesale at 12.5 cents/ kWh (that's only 30% above the current wholesale US average - which will rise after the shale gas boom).

Then we our total capital cost (20 year bonds) would be $1.49 billion, and total revenue would be $1.55 billion.

The subsidies necessary will be modest, and these numbers would certainly make sense already for naval bases and pacific islands. If costs come down with the next generation, scale economies and outsourcing to China, then this could provide a significant share of world energy (if the numbers really did bare out in practice, this would be much more promising than wind or solar).

A renewable source with massive potential. This and other energy research need far more government investment - time to slash electric car subsidies for the rich, stop mass (government subsidized) installation of noncompetitive solar panels and cut corn ethanol subsidies!

It is right for the government to fund development of new technology (especially in strategic areas like energy), but it is very wrong for the government to subsidize the consumption of rich people.

shaun39 in reply to shaun39

And more importantly, if you could cut capital costs from $1 billion to $200 million for a 100MW plant...

It could yield a 6% real return on capital (6% above inflation) depreciated over 10 years, allowing for $10 million/ year in maintenance costs, and providing $0.06/ kWh wholesale (same as natural gas) running at 80% of annual generating capacity.

If the first generation really does turn out to be $1 billion, and if mass production/ outsourcing could cut initial costs by 80%, this would be enormously profitable. Deep sea ports would be the sites of aluminium smelters and steel works (convenient for shipping to boot).

This ain't so crazy as some people seem to think. This could be really cheap energy - well worth investing in pilot schemes, and well worth localized damage to marine ecosystems (hell, at least it would reduce the rate of ocean acidification!).


Scaling up an OTEC plant to 100MW is the WORST thing to do with it. These plants are more useful in the 1-5MW range, where they can be used as baseload plant for remote islands which would otherwise have to use diesel engine/turbine generators at much lower efficiency (30-35% at partial load) than the conventional CCGT (50% plus) in the mainland.
By the way, low efficiency of OTEC plant is almost irrelevant at small scale, because the heat source is free. Same low efficiency applies to most natural energy sources such as wind turbines and photovoltaic cells...


Solitaire first shipped with Windows 3.0 in 1990 so the first nine years of the OTEC permit office must have been very dreary indeed


@professional rationalist

My mistake:
20 TW = 20x10^12 watts
100 MW = 1x10^8 watts

If the plants ran continuously, then you would need 20x10^4, or 200,000 of these to generate 20 TW. No plant runs 100% of the time, so allowing for around 1/4 downtime, you get roughly 250,000 plants, not 25 million.

Staggering numbers either way.

Also, 1.5 million liters of water through a 10 meter diameter pipe is just about 20 meters per second, or 72 kph (roughly 45 mph). Clearly, whoever came up with this design hasn't tried controlling water moving at this speed, much less a 1 km column of it weighing over 1.5 million metric tons. Serious water hammer this.

Setting aside the cavitation and control aspects, the losses would be considerable, even if you could keep the barnacles off, so my 5-6% efficiency should be reduced. Maybe we need 350-400,000 of these things now.

Me, I'd just like to see them put the brakes on that much 70 kph water without blowing the entire thing to smithereens.

spandas in reply to PSH

@professional rationalist

Thank you for debunking this fantasy. I wish that Economist would get proper reality check before running this kind of "thumbs up' stories. On the other hand, the quality of tech reporting in cyber security and computing in general is even worse - I gave up trying to responde to all that nonsense.

shaun39 in reply to PSH


The world doesn't consume energy at an average rate of 20TW!

Rather, the world consumes 18 trillion kWh / year (18*10^9 kWh).
(https://www.cia.gov/library/publications/the-world-factbook/fields/2042.... , see entry for "world")

There are 24*365.25 = 8765 hours in the average year. So the world consumes energy at a rate of (18*10^9 ) / ( 8.77*10^3) = 2.05 * 10^6 kW.

That's 2.05 TW, or roughly the equivalent of 20,000 separate 100MW plants running at full capacity.

20TW really would be insane - the last doubling of world electricity consumption took 28 years. Even if the growth rate doubled (crazily bullish), we wouldn't see 20TW before 2065.

But such talk is trivia; the world economy is a big thing, and this won't impact on the viability or attractiveness of test plants (though it has implications for the potential regarding scale economies).

Fergus MacPhellimy in reply to PSH

Furthermore, accelerating 1.5 * 10^6 kg of water to 20 m/s would require 3*10^8W. Where did this figure of 1.5*10^6 l/s come from?

Also the cold water will be denser than the surface water, so extra energy will be needed to lift it.


Uh oh.
But still better than some.
Nuclear power gets mighty expensive if you include the 100,000 year security costs.
Coal? How do you replace a mountain?
Fracked natural gas? How do you fix a polluted aquifer?
Stirling Solar? Infinia's concentrating dish solar Stirling came in around $20/Watt. SES went under.
PV Solar? What about during the night time?
Wind? What about when it isn't windy and what about all those birds?
Oil? Just how many wars are we prepared to fight and how many beaches to clean up?

Maybe OTEC is the answer.
Solar preheating could improve the efficiency.
With low operating temperatures, why use metal?
Make it a contest and I'm sure a cold water pipe with the correct specs could be constructed for far less.
Of course, there's the ocean thermal and salinity pollution that would have to be addressed but that seems mild compared to Fukishima fission products in the ocean or a dead Gulf of Mexico or acid rain or natural gas fizzing from water faucets.

Professional Rationalist

I agree that OTEC is a fantasy for all but the most remote applications; one must assume a fossil fuel cost of roughly 5 times the current rate per BTU to make $10M per MW installed work out even Stephen over 20 years, even discounting operation and maint costs, which will be huge. However, I disagree with PSH's calculations; I come up with 23 thousand (NOT 25 million) of these giant drinking straws needed to produce the 20 THOUSAND Tera-watt Hours needed to power the whole world. As a matter of scale, a 1 GW plant is a typical large power plant and there are perhaps 4 thousand plants on this scale operating in the world, some as baseline and some as peaking plants. I recommend Brian Hayes' excellent book on infrastructure for those interested.

That said, I admire the sort of thumbnail calculations native to the Engineer that can combine money, time and thermodynamics into a coherent package. It is sad that such basic math is rarely undertaken and too-often ignored, as the ethanol idiocy shows us.


Some perspective on a 1.5 million liters per second flow rate. This is about
twice the Thames River
5% of Niagara Falls
3.5 times the Colorado River
(average flow).


More energy lunacy. Would you spend a billion dollars to grow a million dollars worth of potatoes? What is it about energy that inhibits rational thought? The world is accessing a huge new supply of natural gas - use it.


A wonderful idea at first glance that doesn't survive 10 minutes computation, has never deterred the hucksters from hype. Let's take 2 minutes to debunk most of it here.

The efficiency of any heat engine has an upper limit. Assuming a 20 degree temperature differential at 25 degrees for the hot side of a heat engine cycle yields a theoretical maximum efficiency of 8.4%. That's before flow loss/turbine/generation/transmission losses, so actual efficiency can't get much above 5-6%.

The process thus requires roughly moving 15,000 liters of water per second per megawatt. So a 100 MW plant will move 1.5 million liters per second of 20 degrees colder water to the surface. Imagine the havoc that much cold water will have on the local ecosystem, not to mention the local weather.

And that's just one 100 MW plant. The world generates over 20 TW hours of electricity per year, or roughly 25 million times this 100 MW example.

An article in Scientific American some 35+ years ago hyped this idea predicting that the US could get as much as 10% of its electricity from the Atlantic Ocean. Alas, running their numbers showed that they used the entire Gulf Stream flow lowered by 1 degree celsius to generate that power. Somehow, I don't think Ireland or UK would smile on a dewy eyed US plan that lowered their local water temperature by so much as a fraction of a degree.

Hey, maybe we've found the solution to global warming! Just build a few million of these things, and the ocean surface will cool by enough to stop the Gulf Stream in its tracks, and put us into an ice age.

More likely, the relentless Carnot equation will yet again overcome even the most ardent hype, and this idea will die the same death it did 30 years ago.

Fergus MacPhellimy in reply to PSH

Deep ocean water contains a lot more dissolved CO2 than surface water because of the greater pressure. I have read that natural upwelling releases significant amounts of carbon dioxide into the atmosphere, but I have not yet found out how much.

Can anyone estimate how much carbon dioxide would be released, and how would this compare with, say, burning natural gas?

Fergus MacPhellimy in reply to PSH

Are you sure of your figures? I suspect you have misplace a decimal point somewhere.

Give an efficiency of 5% means that for every Joule of electricity generated 20 Joules of heat must be transferred to the cold water. Assuming a specific heat capacity of 4.2kJ/kg raising the temperature by just over 2K will sink about 10kJ. So each kJ generated requires 2l of water. So 1MJ requires 2000l of water, or 1MW requires 2000l/s.


I'm skeptical about this alt energy option. Like previously stated, it seems like a lot of capital to invest in infrastructure that will soon succumb to the harsh conditions of the ocean. Also, with high storm intensity from hurricanes in areas such as the Caribbean, would the facilities be able to stand the test?

As a marine ecologist, I cannot help but wonder how this form of energy would impact the local ecosystems. Cold waters from the deep can be quite different in chemical composition than at the surface. Would the cold water be released at the surface? Or pumped back down to deeper water? If released at the surface, it could potentially induce phytoplankton blooms that normally do not occur in those areas.


After a tour of the facility in Hawaii it occurs to me that the real value is for aquaculture. Essentially the net power gain from a 20 c temperature difference is so low that it seems more likely to provide power for an aqua culture facility.

Deep ocean water is high in nutrients for phytoplankton and is cold so popular cold water species can be grown in the nutrient rich cold water. This is working well in Hawaii where, as I understand it, they raise lobster to sell to asia (and locally I presume).


if its ever up and running, then some save-the-world environmentalists will find a computer model that shows that changing the huge ocean's "delicate" temperature balance will result in catastrophe. the ensuing battle to save the earth will keep the environmentalists funded for decades, and maybe make for some more movies like "2012".


Though the first plant was in the west, it was Japan who truly pioneered it. India learned from Japan, and recently Lockheed consulted India for their plant.

Efficiency is indeed low, but if the plant could last a few decades, its worth the effort as the fuel doesn't run out. Given that offshore oil rigs survive for a long time (digging up filthy corrosive oil),
OTEC plants can potentially last long enough.

I would still favour wave and tidal energy converters as they require much lower capital, are already as competitive as offshore wind(economically), and will be easier to maintain.

Anjin-San in reply to rvsmaverick

Thank you for referring to the pioneering work by the University of Saga, whose design was used in the OTEC test vessel "Shakti Saga" that have conducted test power generation in the Indian Ocean.

Peter Sellers

Exactly the question that came to my mind as I read the article. The writer does not say how much power would be required to pump seawater up a one kilometer pipe and how it compares with the final output. Would the science correspondent care to enlighten us?

ZeFox in reply to Peter Sellers

The power required depends on the diameter of the pipe and can be quite negligible as the losses are only due to friction along the pipe. The water is not really pumped "up" it is pumped "through" 1 km. Essentially the pressure at 1 km depth is equivalent to the head needed to pump water from -1km to the surface. Another way of looking at it is that at the bottom of the 1 km pipe you have 100 bar of pressure from the 1 km of seatwre over your head to push the water in the pipe.
On land however the situation is quite different. if you pump water up a 1 km pipe, there is no surrounding pressure at the base of the pipe to push the water through the pipe.
Other expalnations are rather mathematical I'm afraid.

ZeFox in reply to Peter Sellers

Unfortunately your proposal would not work in practice. Essentially for every 10m extra water depth, the isostatic pressure rises by 1 Bar. (isostatic = compressing a volume uniformly from all directions in space). To turn the 100Bar pressure at -1km into work you would need to channel the pressurised water from -1km through a turbine to wherever there is zero water pressure and that would be at the surface. The energy needed to do that is more than the energy you would get. Nature's way of saying there is no such thing as a free lunch. Just theoretically, another way would be to find an underwater cavern at -1km free of water containing air at 1 Bar, but that would fill up at some point with all the water coming through the turbine and then then the turbine would stop.
As to the efficiency, as said by the previous poster, it is limited by the temperature difference acc. to Carnot equation which gives the upper theoretical limit . According to this equation, effciency n = (Temp hot side - Temp cold side) / Temp. hot side. Put all the temperatures in degrees Kelvin (= Temp.in degrees Celcius + 273.15) and you get:
(298.15-278.15)/298.15 = 0.067 or 6.7 %. (number in % = number x 100) Then you have to subtract from this ideal efficiency figure the losses mentioned by previous poster. Losses in the heat exchangers, pipe friction, turbine etc. which are yet to be determined and validated by the proponents.
One other point. The power P available at 100% efficiency in the water temperature difference is calculated acc.
P = rho x A x V x Cp x DT
rho = 1000 kg/m3 water density
A = section of 10 m tube = 78.5 m2
V = water velocity in m/s
Cp = water thermal capacity = 4000 J/kg
DT = temperature difference
This is for 100% efficiency. If you apply the efficiency factor n found above you cannot get P but at most you get nxP acc.:
n x P = n x rho x A x V x Cp x DT
This calculated power taking into account only the Carnot efficiency has to reach the target of 100 MW or 100'000'000 W, lets call it Pt
Pt = n x P = n x rho x A x V x Cp x DT
You want to get the velocity V
V = Pt / (n x rho x A x Cp x DT)
V = 100'000'000 / (0.067 x 1000 x 78.5 x 4000 x 20) = 0.23 m/s
This seems quite a reasonable figure which would not cause too much friction. The speed is inversely proportional to efficiency. So if half the efficiency of 6.7% gets eaten up by the system you would need to double the velocity, to about 0.45 m/s or 1.7 km/h. However...the devil is in the detail.
It is highly likely the proponents have run through the figures in fare greater detail. As long as there are enough kilowatts produced per $ invested it may be worth it but it seems to hinge on the price of the 1 km pipe. If that alone costs 1 billion dollars, added to all the other costs it may prove too much for the 100 MW net production. What is needed is a demonstration plant to weed out all the detail problems and a better idea for the pipe than standard concrete/steel construction from the offshore oil industry. Then you can get an idea of the real costs. Engineering is about paying attention to detail.

ZeFox in reply to chocerady

Sorry the previous post should have been in reply to chocerady in chocerady who asked:
Would it not be simpler to utilise this "free" water pressure to turn a turbine and produce electricity in this way?

spandas in reply to ZeFox

@ ZeFox

"To turn the 100Bar pressure at -1km into work you would need to channel the pressurized water from -1km through a turbine to wherever there is zero water pressure and that would be at the surface."

You kind of give the right answer but for a completely wrong reasons. Please, please pick up any school book on Archimedes law and see your error!!! Until than, do pity on some of us who actually studied and taught physics and stop posting nonsense.


The small temperature difference dooms OTEC to efficiencies on the order of 4%. Hardly encouraging.


Latest blog posts - All times are GMT
Car ban in the French capital: Paris in the smog
Charlemagne 1 hrs 10 mins ago
Bitcoin: New money
Free exchange March 17th, 8:18
The Economist on China: Weekly round-up
Analects March 17th, 4:34


Products & events