I must confess I’d never heard of it, but the concept is simple. Take some liquid ammonia and heat it with the warm surface water in the sea. The expanding gaseous ammonia drives a turbine, generating electricity. Then cool the ammonia back into liquid form using water pumped up from the cool depths of the ocean. Repeat ad infinitum.
(Image from New Scientist via SeaWayBLOG.)
While the concept has been around since 1881 and there have been pilot plants for some decades, the ambition seems to be to build a commercial plant of 100 megawatts or larger.
One problem seems to be in the sheer size required to scale up. A 100 MW plant would require a pipe about 30 feet across bringing water up from 3000 feet below. It seems that the Indians trying to set up a small 1 MW plant have already dropped two 800-metre pipes onto the sea floor because of winch problems.
Also the warm surface water used for heating would need to be discharged at least 100 metres below the surface to avoid mixing, whereupon being slightly denser it would sink from there.
This article suggests that a temperature differential of 20 to 24C is required, which is generally the case in the tropics:
I note that the New Scientist conceptual model suggests a differential of 10C.
That map above is courtesy of a site with many useful links arising out of a workshop to promote Townsville as an OTEC (Ocean Thermal Energy Conversion) centre.
It seems that OTEC technology has low efficiency (about 5-6%), but is nevertheless claimed to be cost competitive.
Further information:
OTEC combined with solar thermal
So what do you think? I have no idea whether the sea off SEQ is deep enough but if so perhaps Bligh should power the next desalination plant with OTEC power.
Something similar here. Might be a better use of the system for cities close to deep water as there is less loss due to no conversion into electricity.
Worth a serious investigatio. Just one doubt: does the global ecology depend (amongst a thusand other factors and effects) on the existing temperature difference to drive ocean circulation, which affects in turn local warming and cooling, transport of deep-ocean nutrients to coastlines [e.g. to anchovies feeding off Peruvian coast, highlighted in discussions of 'el Nino'], etc. Global weather and climate effects… ??
It may be that even with widespread human extraction of this energy, no discenible change to the temperature differences would occur. OK, I’ll say it: “a drop in the ocean”. If so, please proceed to cost it and plan.
The trouble is that it’s such a low-grade source of a temperature differential, it makes the process terribly ineffient. Not that there’s any shortage of warm or cold seawater, but, as you’ve noted, the infrastructure has to be gargantuan to support it.
That’s why it’s never been anything like cost-competitive, even with shipping diesel, in the past. Maybe these guys have some tricks up their sleeve – for instance using the Kalina cycle, but it’s still very hard to see how you can do this cost-competitively.
Robert, the Wiki article suggests that the costs could be comparable with wind, but no-one really knows yet.
I did see another claim that it was competitive with fossil fuels. I’ll see if I can find it tonight.
Andrew, the Toronto use of cold water for air conditioning was given in an accompanying story with the article. It seems to me to be a bit different, as it wouldn’t require such great depths and hence less distance, also less infrastructure.
Ambigulous, environmentally OTEC is supposed to be a positive as more nutrients exist at depth than near the surface, especially in the tropics, thus promoting fish growth.
Andrew, see related technologies at Wiki.
Brian: I’d be skeptical of claims of environmental benefits.
The extra fertilizer getting dumped on the Great Barrier Reef isn’t exactly doing that ecosystem any favours.
I suspect cost-competitiveness from fossil fuels varies a lot based on the cost of the fossil fuels. In some locations where OTEC is being considered, there are no locally-available fossil fuels, meaning fuel (usually diesel) has to be shipped there at enormous cost. When your competition is that or other renewables, it’s a lot less of a challenge.
Incidentally, I wonder what difference to the efficiency of a conventional steam plant (fossil fuel, solar thermal, geothermal or nuclear) it would make to be running low-temperature cooling water from the bottom, compared to high-temperature surface water.
I have no doubt that these types of ideas will become more and more viable as we turn our backs on the carbon economy.
The OTC system is not all that new and has a poor Carnot efficiency. It is a variant on the Organic Rankine Cycle (ORC) system that is used for the recovery of energy from the waste heat of industrial processes. There is a rather neat ORC in Birdsville that has been going for years and that generates power from hot bore water.
The answer to your question Rober about water from the depths for efficiency improvement in your nuke is – not much – a few percent – that would be easily swallowed up in the pumping costs etc. The thermodynamic limit of all these thermal systems (including nuclear) is the Carnot expression (T1-T2)/T1. There is no way to break this. The Kalina cycle does not defy Carnot – simply gets closer to the theoretical limit. There is no way you or any-one else can break the second law of thermodynamics I’m afraid.
The OTEC system will probably cause immense damage to the ocean system, but hey who cares we have been dumping shit into it for years. Fished it to exhaustion and filled it full of radioactive stuff from your nukes. Like the ocean system is of infinite extent babe.
If you want something really intersting check this out:
Wednesday, December 3, 2008
VIVACE (Vortex Induced Vibrations Aquatic Clean Energy)
A novel approach to extract energy from flowing water currents. It is unlike any other ocean energy or low-head hydropower concept. VIVACE is based on the extensively studied phenomenon of Vortex Induced Vibrations (VIV), which was first observed 500 years ago by Leonardo DaVinci in the form of “Aeolian Tones.” For decades, engineers have been trying to prevent VIV from damaging offshore equipment and structures. By maximizing and exploiting VIV rather than spoiling and preventing it, VIVACE takes this ‘problem’ and transforms it into a valuable resource for mankind.
Vortex Induced Vibrations (VIV) result from vortices forming and shedding on the downstream side of a bluff body in a current. Vortex shedding alternates from one side to the other, thereby creating a vibration or oscillation. The VIV phenomenon is non-linear, which means it can produce useful energy at high efficiency over a wide range of current speeds. http://www.vortexhydroenergy.com/
How about a few of these in the Brisbane river??
Huggy
The idea was promoted by Jerry Pournelle back in the late 70’s.
I would have guessed that long distance underwater electricity transmission is not the simplest thing to set up and maintain either.
One consequence which I haven’t seen discussed anywhere yet is that deep water is more “acidic” than surface waters, and one of the major concerns of ocean acidification is that the depth at which the water becomes undersaturated with carbonate will rise closer to the surface. I can’t see that pumping deep ocean water to the surface is going to do anything other than make surface pH (and carbonate levels) worse than they are already heading.
Brian, isn’t “pump” the most important word in that diagram?
How much power is needed to pump all those gigantic 30ft wide columns of water around the system?
Serious question.
I”m no engineer, but just looking at the diagram suggests that after powering that pump, you’d only be left with loose change.
Steve, exactly, and we really want to disturb all that Methane Hydrate in the deep oceans?
Fucking irony is that there is a vast resource of waste heat available in industry, from steel mills to milk processing and on and on. What do we do with it ? SFA that’s what. These sources are present within the grid itself, so no need for extra transmission lines – they work both ways.
Huggy
steve from brisbane wrote:
the deep water returns to the deep Steve. However, there are other issues (it’s warm when it’s returned to the deep, so it’ll probably set up it’s own slow thermo current as the warm water rises back to the surface.
It’s an intriguing idea, but as others have said the pumping losses will make it unfeasible – especially the pump that lifts the cold water to the condensor, plus the issues with making sure the intake and exhaust are far enough apart that the water going up the pipe is still cold enough to make it work.
It’s basically another levitating banana scheme.
Levitating bananas? What’s that for, to trip up the angels?
I have to say, when I read this New Scientist article this week over my Weetbix I was overcome with incredulity, especially after reading the story about the Indian’s failed scale-ups (that actually made me laugh out loud).
I think Huggybunny has an interesting point, in that humans have been using the ocean as a mine (fish) and a dump (waste-water) as if it were limitless. This technology seems like it’s based on that assumption, without thought to the consequences of disturbing the natural thermal gradient.
I’m much more interested in wave power, to be honest. The kind of technology currently being tested off Fremantle, WA, (CETO wave power) seems like a much better solution to getting clean, cheap, environmentally sustainable power from the ocean. It also has the great advantage of being able to produce desalinated water and power at the same time and is feasible for almost all coastal settlements in Australia.
Mercurious, the theoretical limit I saw was 6-7% efficiency. It does seem that moving all that water is a problem.
Another worry. Townsville was said to be a good site because the continental shelf was ‘only’ 100k away. Apart from the distance it puts a lot of expensive gear into areas that are somewhat cyclone prone, I would have thought.
David, I mentioned deep water at the surface because I recalled that Pournelle used to say that one of the side benefits was that deeper water being brought up would bring nutrients with it that would make the surface more productive (for sealife). So I assumed that there was to be at least some release of deep ocean water near the surface. Obviously, design ideas could have changed since the 1970’s though, and I haven’t actually read the articles on current proposals.
Anyhow, like most readers here, it just sounds all too hard to me, compared to building big structures on nice solid land. (Like that giant greenhouse power tower thingee that got some publicity a while ago.)
Re: levitating banana scheme.
Basically, it’s a method of overcoming the peak energy crisis developed right here at LP. Before you leave home, you pack a bunch of levitating bananas with you for lifting up hills, then on downhills you eat the bananas and spread the banana peel around for frictionless motion. At the same time, the slowly rotting banana peels sequester carbon and more carbon is sequestered in human poop from all the banana munching.
It can’t fail. I just have to get the bananas to levitate.
Before you all kill yourselves laughing, recall that Lovelock was proposing putting heaps of pipes into the ocean to bring up cool water to the surface through wave action, as a way of cooling the surface and buying us time.
I’d really like to hear from someone genuinely knowledgeable about ocean currents and marine ecology. My understanding is that tropical surface waters, apart from where there are coral reefs, are essentially marine deserts.
I’m also inclined to think that this technology will be a ‘drop in the bucket’ as it were and it doesn’t actually introduce any nasties into the sea.
For Australia, geothermal power is for more practical: two pipes in the ground in certain places (there is one near Brisbane etc) down 3 to 5km. Water goes down one and steam comes up the other to run a conventional steam-turbine power station, and there is already a 125 megawatt geothermal plant up and running in the Philippines.
Rex is right. Geothermal in the Bowen basin; for example, has a Carnot efficiency about 3-4 times that of the ocean thermal plant. It helps that it is on dry land too. There are no emissions and almost no impact on the environment at all, thousands of times less than the nuclear cycle that ill informed wish to inflict upon us. In the Bowen basin you bring hot water to the surface , extract some of the energy with an ORC and then let it fall down again. No energy is expended in bringing it to the surface because the water going down forms a closed syphon. Oh sure there are minor friction losses.
Huggy
The difference between geothermal and OTEC seems to be that the former works with the thermal gradient, while the latter works against it. Hence more energy is need to move the water in OTEC than in geothermal, which results in lower efficiency.
The cooling water loop is closed at the top, so no movement of nutrients, and the water released at the end of the cycle should be the same temperature as the intake water, so no warming of the deep ocean. Other than the lower efficiency, problems should all be amenable to engineered solutions. Cost will probably be the deciding factor.
Young David Rubie inscribed: “It can’t fail. I just have to get the bananas to levitate.”
OK, so let’s get these bananas straight: will they be earning Carbon Credits? Or will Saint Kevin of Poznan need to introduce separate Banana Credits? Will these be allowable only on Certified Queensland Bananas? And what do we Southerners do if another cyclone tears the Heart out of the crop, and the Soul out of Qld?
Have you thought of that, you cruel, calculating inventor? It sounds like a recipe for a very unhappy Christmas: kiddies in tears. *sob*
Why get it from bananas or ammonia when we may be able to get so much more from appreciating fishies?
Of all* the recent new cutting edge renewables I’ve read about, this one struck me as the most exciting.
* true, it’s not that many
If they can commercialise it – good luck.
I can image the hoohah trying to lay subsea cables through the great Barrier reef Marine Park.
myriad wrote:
That’s cool, but like a lot of sea based stuff, it still suffers from the inevitable degradation and buildup of sea gunk that makes submerged moving parts impractical over the longer term. How do you clean the thing? How long before it becomes a massive oyster hatchery? Mmmm, oysters.
Dunno the answers to those ones David, but they don’t strike me as particularly difficult to overcome. There’s a tonne of things we suspend happily in the water now worth squillions (oil & gas drilling equipment spring immediately to mind) that we know how to clean etc., so I’m sure we can adapt something & figure it out.
The claim I saw of cost competitiveness was at the Townsville site linked in the post. $25 for a barrel of oil was mentioned. It came from this paper dating from 1997 which goes into some detail about where the best sites might be.
There is a preference for building the plants on land, which would need deep water within 10 kilometres. Four sites are identified near the USA:
* the island of Hawaii
* Providence Island in the Bahamas (Nassau)
* St. Croix in the Virgin Islands
* Grand Cayman
If, like me, you have a few shares in Geodynamics, you have to keep an eye out for competition that might blow them out of the water, as it were. I don’t think OTEC is it. Still it could be the go in places like Hawaii and the US naval base in Diego Garcia where one is being built. As Robert said, in these places the usually ship in diesel.
I’m doubtful that it will be economic (no expertise, but it was so far off when proposed in the 70s, and its not obvious where the technological improvements would come). However, in the right location I’d imagine it would be a small environmental benefit.
It’s true that too many nutrients are damaging the reef but that’s partly because they are a concentration of certain nutrients, rather than the mix from deep water that ocean life has evolved to deal with. It’s the difference between feeding a visiting possum refined white bread and a diverse diet more similar to the one it’s used to in the wild.
And of course one of the worrying aspects of climate change is a reduction in mixing between the surface and the deep ocean, although the areas this would operate in are probably not the places where the fall off is occuring.
What you describe here requires the active pumping of water as a coolant. Unnecessary.
If instead the ammonia were in a valve controlled (to prevent backflow) closed loop inside an open top canopy/sheath, extending deep into the cooler waters, cold water confined by the sheath would be heated by descending ammonia, rising through the sheath and drawing more cold water in. The ammonia would cool and liquefy, being forced by pressure up the other arm of the loop where it would then start to boil and transition to gas, driving turbines at the top.
The nutrient rich deep ocean water drawn up through the sheath as a result of being heated would, instead of being dumped back into the depths as in your diagram, fertilise the upper ocean waters creating a localised biological oasis in the surrounding watery desert. Increased biological diversity and fishing productivity is a by-product of the design.
I think this design was actually implemented in the Caribbean sometime in the 1920’s.
If the differential temperature is 24 K and the surface temperature is 300 K, the Carnot limit – the absolute maximum efficiency – is only 8%, or as low as 3.3% for a temperature difference of 10 K. In practice, a real heat engine can’t reach that limit. I would assume that the quoted efficiency of 6-7% is based on the assumption of a temperature difference of 24 K – which is really the practical maximum you can find – and it doesn’t even begin to factor in the energy input to the pumps.
At last my little “Carnot’s Principle”, oui? Good for some principles and genuine thermodynamics is here with the debate. Congratulations, Messieurs! Au revoir, et bon chance with this glow ball warming.
Sadi Carnot
very quick thought – you have to move the cold water closer to the surface – could take a lot of energy