Laura Johnson says: I think this is a very interesting and exciting idea - I'm surprised there is nothing like it out there at the moment (surely the energy companies are be developing this kind of thing!). I have a few questions:

1) How does this compare to wind power in terms of energy produced? I imagine water currents are more consistent than wind.

2) Where can these turbines be installed (again, I imagine currents will vary significantly)? Are there obvious locations in and around the UK?

3) Other than cost, is there anything preventing these from being built (ie govements permissions/applications required)?

4) With regard to the video, how small were the models in the tank compared to the proposed real thing?

John Armstrong says: Thanks for the comment Laura - in reply to your questions:

1) Water currents being denser than air (but slower than wind), the energy is about 4 times as dense as for wind, meaning that tidal turbines have about a quarter of the footprint area. Neither wind nor tide are constant, but tides have the huge advantage that they are predictable - so you can plan your power generation with absolute certainty - something you can't do with wind power.

2) There are some very energetic channels around the UK coast - for instance in Scotland and Northern Ireland, and especially between N Scotland and Orkney. But also near France around the Channel Islands, between Italy and Sicily, around some of the Norwegian fjords and really anywhere where there are islands and constricted tidal channels.

3) The real obstacles are that the technology doesn't exist yet, and it will be expensive to develop and prove this at full sea-going scale. It is rather high risk for banks and venture capitalists, and really only Government can prime the pump financially.

4) The model in the video was 2.5m high - 1/23rd scale.

Robert Shaw says: Whilst I understand that it will require a series of these units, spread out along the seabed, to overcome the problem of slack water at high and low tides at any one location, but how can you protect them from damage, such as by deep water industrial fishing machinery.

John Armstrong says: Robert - any tidal energy device would need exclusion zones, even if totally submerged (because you can't have maintenance activities in a shipping or fishing zone) - so there shouldn't be damage from ships. However, if there were, then the great advantage of float-out systems like the one I am proposing is that you can simply disconnect and float it back to base for repairs.

David Carslaw says: How does this technology compare with other wave and tidal energy devices in terms of capital investment required, rate of return and operating costs?

John Armstrong says: David - the big advantages of this system are that float-out installation means you don't need some of the huge crane-barges needed for offshore wind energy and fixed tidal devices; and easy access for maintenance means you don't have to take the unit back to base every time a minor repair is needed - which is true of other tidal devices. A further thing that determines cost is unit capacity - it costs a lot to put a foundation on to the seabed, so you need to hang as much capacity from it as you can. At 10MW, our unit is twice as big as the largest wind turbine yet installed offshore anywhere. So we predict that the capital installed cost and operating cost will be markedly less than that for offshore wind, which at present is the main plank of the UK government's renewables policy. And rate of return should be greater than for offshore wind or more conventional tidal devices. As far as wave devices are concerned, energy capture from these looks like being much less than for tidal systems, kg for kg, so there is really no competition there.

Jonas Van Poucke says: How does you invention stand against the corosive qualities of sea water and how will they react agaist clamps and other marine life that wil come to the machine over time?

John Armstrong says: Good points, Jonas, and the need to maintain the protection against corrosion, for instance, is another reason why it's important to be able to bring the equipment to the surface at regular intervals for maintenance. Also the need to clean the blades of weed and other marine growths regularly in order to keep them working properly. Very good reasons why an accessible system like the one I am proposing is the way to go.

Mollie Green says: This is really cool! We have so much tidal energy around our shores it is criminal to ignore it. I have been a big advocate of using this energy for ages but not seen any system yet that seems to approach it in a logical way - this just may be it if you can answer the following!

You say the system can be brought back to the surface for maintenance, can you get onto it? It looks big enough. Can you do major repairs if needed without bringing it back to shore?

What if a turbine breaks down? Do you have to stop all the others for safety?

Does it swing around like a kite to capture the best flow?

John Armstrong says: Hi Mollie - yes for repairs the Triton system is very stable in its maintenance postition (because of the rigid tether to the seabed), so getting on to it is like getting on to a fixed platform. If a turbine needs replacing, you bring in a 50m workboat with a heave-compensated crane on the bow, moor it with a 3-strut system to the Triton and then you can change out an individual turbine. The good thing about tidal energy is that the rotors are much smaller than for wind power (although the loads are higher)... so handling a 1.5 MW turbine and rotor is not too hard, even at sea.

Yes, the Triton swings round to capture the best flow. This means that you don't waste energy (compared to fixed alignment turbines which are those being built at present) and you don't have the loads from off-axis flow, which shorten the life of the rotating parts. I used to design wind turbines, so I know about some of these things the hard way!

Good questions!

Arnulf Knittel says: Dear John Armstrong,

Brilliant! And a few great steps beyond the potentially very unbehaving single-rotor configuration you display on the website. I trust you are right to pursue the multi-rotor option in this, and to rely on stochastics and to keep loads levelled out...

I appreciate your in-depth answers, and I would like to hear your opinion about the prospects of this technology for regions where tidal streams are not plentyful as in the UK and around NW Europe:

(i) What role does, on your view, cavitation play, and could cavitation not be pushed to higher tip speeds by increasing the hydrostatic pressure, in other words the depth of installation? Is a certain depth required, or can you operate the array near the surface?

(ii) What can you say about deploying such a turbine in 'blue water', e.g. near the surface in a thermal current without flow reversal, say the Gulf Stream? Is the length of the tether incompatible with the catamaran concept? Can you, economically, rate the turbine down to achieve a higher capacity factor in moderate speeds? Can annual average mean speeds vary as much as for wind turbines with a more or less 'one-size-fits-all' rating?

I look forward to your answers, and I hope this idea (it seems a bit more mature than that) will receive prime attention and the fostering it doubtlessly deserves.

John Armstrong says: Arnulf, good to hear from you... cavitation will always limit the tipspeed and as you say will be more of a limitation near the surface where the pressure is less than deeper down. It is possible we could run the lower turbine rotors faster, however the tidal flows are less at greater depth, so that might not be so efficient. We would try to operate the top rotors as near as possible to the surface but below the wave dips - perhaps having the tips 5m down...
As you know only too well, the energy of a stream varies with the cube of the flow, and my impression is that blue water flows such as the Gulf Stream are much slower than the strongest tidal currents... perhaps 0.5 m/s on average compared to 2 m/s.... that means only 1/64 of the energy...! And yes, I think average mean speeds will vary over this range in a similar way to wind, so one-size-fits-all not so good... What is your impression?

Michael Todman says: Excellent!

However, can you comment on whether this device can utilise turbines currently being developed by other companies?

There may well be other developers who have sunk a lot of money into developing the turbines but who have not got such a practical method of deploying or maintaining them - it would really make this a winner if it can accommodate these as well!

John Armstrong says: Mike - what we are developing is really a support platform for tidal energy, so are fairly agnostic about the turbines themselves. There are several people developing the turbine technology - which with all our marine traditions and after all the experience with wind turbines should not be too difficult. But the system that gets the turbines out there and keeps them accessible for maintenance is crucial, and that is our niche. So with the right financial backing, we are confident that this will indeed be a winner!

Alex Robertson says: I'm pleased to see that your turbine is generating a lot of interest. I think its a magnificent design.

I am a big fan of direct drive for offshore wind turbines. I was wondering if you see your technology as suiting a direct drive of geared design?

John Armstrong says: Thanks for the nice comments, Alex. I think you are asking about direct drives of gearless design..... as I said above, we are fairly agnostic about the rotor and drive type, and there is no reason why a gearless design, of say the Open Hydro type, should not be fitted - there are certainly less parts to go wrong (which is presumably why you like them). And we actually show on one of our brochure images gearless turbines mounted to Triton.

Having said that, gearless designs tend to be rather large diameter and heavy compared to geared drives, and I do wonder whether the final cost effectiveness of a gearless drive - even allowing for potentially less maintenance - will actually allow it to compete in cost terms. After all, most ships and most marine drives use gearboxes rather than direct drives. But yes, we could certainly accommodate these on Triton.

John Armstrong says: Sorry, poor English, should have said fewer parts to go wrong...

Dan Frederiksen says: seems like a good idea and the variable float system a good way to solve some problems. as others have mentioned it's worth looking into its cost effectiveness and compare to other flow power generation systems which I don't know enough about.

it might not be a problem, but how do sea life respond to such devices? I mean do they know to stay away or will they be mammal blenders? :)
we are stupid enough with regard to ocean life as it is so it would be unfortunate if this will kill whales, dolphin, sea lion, shark etc

it might be better to just go with windmills and work on energy storage technologies for those which we will likely need anyway.

John Armstrong says: Dan, good points... the MCT experience, with a prototpye in seal country, seems to be that marine mammals stay away - most in any case are used to dealing with machinery in the form of ships and know to stay away from propellors (incidentally, the tip speed of our rotor blades is about 12 m/s - less than the hull speed of many ships and much less than the propellor speed). People at one time worried about windmill blade strikes on birds, but these didn't materialise except when they put the windmills directly in a migration route like Terrifa. And tidemill blade tips go 6 times as slowly as the tips on wind turbine rotors.

Ref your comment about just going with windmills, these are far more intrusive than tidemills would be... can you just imagine the backlash there will be in the UK to 10,000 more onshore wind turbines (today's Daily Telegraph)....? Actually we would all benefit from a mix of wind, wave and tide, as these spread the generation and you need less back-up firm generating capacity than if you have just one renewable alone. So I can't agree with your last comment!

Arnulf Knittel says: Dear John, Getting back to the issue of power rating, or rating in general for tidal vs thermal currents: from oceanographic literature I understand the Gulf Stream can attain a surface velocity up to 2 m/s over large stretches. Certainly, there are wind-induced fluctuations leading to a sort of a Weibull distro as in wind proper. However, when compared to wind and tidal currents, the thermal currents certainly have a much narrower current speed distribution, with almost all energy present in a very narrow range, and probably lower extremes. (simply due to the fact that they don't stop). The turbine can be geared to this 'site speed', and it would certainly be worth it. I guess a net capacity factor of 0.8 would not be out of the range for a marine current turbine in a thermal current. In my opinion this would well pay for some cabling, mooring ... all problems that can be overcome. What sort of a net capacity factor would you expect for tidal, say the Pentland Firth, as an example? Best regards, Arnulf

John Armstrong says: Arnulf, if you are right, then this would indeed extend the market for semi-submersible platforms of the type I am proposing, and of course the higher capacity factor would pay for being probably in deeper water, far from shore etc. (Incidentally the capacity factor for the best sites in the Pentland Firth would be just under 40%). Good point!

Norman Davison says: Dear John,

The engineering concept is expceptional, but one thing wories me . The last power turbine I was involved withstarting up, for Hong kong Electric in 2006 had generating capacity of 380 Mw..
That is equivilant to 38 Subsea turbines, providing all the turbines are in operation and not down for Maintenance.
I realise that this was guzzling tons of LPG per hour , and I further realise that LPG consumption is in direct competion with Human food production ( via the LPG->NH3-> NPK fertiliser production)
However the logistics and maintenance for any thing to do with ofshore is horrendous .
It looks to me like the viablity of this concept will be down to reliable run times . Do you expect the run times time to be 38 times better than a conventional gasturbine set ?

Jonathan Leslie says: Dear John
I think this isan excellent project and I only hope it can succeed, because it is quite clear we are not going to get anywhere the renewables target if we are depending on wind power to get us there. What spending on infrastructure will be necessary to bring the power onshore?
Jonathan Leslie

John Armstrong says: Norman, we most certainly don't expect them to be more reliable than gas turbines, and this is one reason we have adopted the strategy of making them as accessible for maintenance and repair as possible.

Gas turbines are a very mature technology and have an enviable reliability record, but as you say, they do consume vast quantities of gas, which in the main comes from natural gas resources - definitely not renewable! There are some that use gas fuel derived from biomass, which as you say, compete with food production for crop growing area – fortunately these are few and far between for good reason.

However, if a 380MW gas turbine has a problem you lose an awful lot of power in one hit. In the equivalent capacity tidal farm of 38 Triton systems there will be a total of 228, 1.7MW turbines. If a turbine goes down it will only reduce total output by about 0.5%. I agree a failure of a tidal turbine is going to be far more likely than a gas turbine (until tidal turbines get to the same stage of maturity), but it is highly unlikely all 380MW would be lost at once. I hope this helps.

John Armstrong says: Thank you Jonathan, a good question. It is essential that all practical forms of renewable energy are utilised to help get close to the overall target – as you say wind power alone will struggle to get there. However, this needs the infrastructure of the existing national grid system to be reinforced to take the power from the more remote locations. This is needed whether this is tidal, wave, or wind, and this needs the full commitment of the Government to ensure it is put in place in time.

The lines bringing the power onshore from the tidal farms to connect with the grid would be funded from the individual project cost of the tidal farm in the same way as existing off-shore wind farms. The actual cost per installed MW will vary depending on the distance from the shore and the size of the farm. Tidal farms, and ones utilising the Triton system in particular, would have an advantage over offshore wind as:
1. the units can be installed much closer together (due to the higher energy content of tidal currents), this saving cable lengths;
2. the Triton can produce 10MW from one installation – far higher than any existing offshore wind turbine – so again reducing the number of cables required;
3. most tidal farms will be sited within a few km of the shore as this is where the strongest currents can be found – again reducing the lengths of cabling back to a shore connection.

So costs should certainly be competitive with, and quite likely better than those for, the offshore wind industry.

steve clemens says: Dear John,
I think your design is excellent. It hits all the spots and I hope you get your hands on serious funding to go the next stage. Maybe you should study the Pelamis business case to help you there.
.
But guys (commenting) please don't bash the wind industry. There's nothing to gain from that. The Renewables part of our energy mix will have to consist of many technologies, even the wind execs know this. The fossil/nuclear crowd use this weakness to peddle their huge machines, polluting massively for decades in the future, as if there's nothing wrong with that.
Tidal should fight its battles against fossil and nuclear, not against other renewables.

Moreover, there's a huge opportunity many people overlook: large tidal projects need marine traffic exclusion zones and underwater hook-up to the distant on-shore grid, just like off-shore wind...
Why not combining wind with tidal projects? The offshore windfarms are coming, that is a reality, so why not use a smart business shortcut and partner up? The HAWT's submerged foot can act as anker for your tidal devices, and the transformers and grid hook-up are already there.

I really hope your design makes it to the next stage. I live in Belgiuum and I can imagine us having a few thousand off our coast and into a few estuaries too.
Good luck!

giovanna barbaro says: This idea poses a lot of ecological problems:
a) by sinking the catamarans, they will damage the seabottom , which is the source of nutrients for microplankton.
b) The turbines will reduce the water speed, and this will affect macroalgae. Macroalgae play an important role in absorbing CO2, so the project can potentially accelerate global warming if conduced to global scale.
c) The turbines will suck and chop any fish passing nearby, so the consequences for the fauna and local economy can be dammatical.
d) which material should be these turbines made of? Corrossion is inevitable, so the the particles of corroded metals will tend to cumulate in the influence zone of the local tidal currents. If made of plastic, much worse: it has been demonstrated that plastics hydrolize in sea water and the decomposed matter can carry carcinogenic compounds.
e) What happens with sediment? By intercepting a fluid current and driving energy, the fluid will reduce its speed, hence the sediments wil tend to cummulate in the influence area of the installation. Apart from the aforesaid ecologic problems, maintaining the turbines free of sediments will cost a lot of money.

Dan Frederiksen says: John, I submit to you that a huge sharp blade going 12m/s in murky water will be hard for marine life to see coming. contact with a ship's hull is a grazing, not a blade slicing. 12m/s is quite fast.

imagine you swimming in such water knowing there was a 12m/s blade but you couldn't tell where because visibility in the water was 50cm. you would panic and with good reason.
I think such turbines could well be killer blenders in the ocean. Birds can easily see windmills miles away, in a lot of ocean water you can't see your hand in front of you even if there was light.