Science Interviews


Wed, 1st Oct 2014

Harnessing wind and wave power

Dr Richard McMahon, Cambridge University

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Wind power is a widely used form of renewable energy, but improvements can stillWind Power be made to better harness the power of the wind. Wave power, however, is much more difficult as the up-and-down motion of the waves is harder to convert into useful energy. Dr Richard McMahon from Cambridge University works on improving the output of both types of technology. 

These types of power rely on simple generators. Ginny & Dave explain exactly how and why generators create energy from movement. 

Richard -   Wind power is relatively mature.  Itís making a big contribution to the generation of electricity in many countries.  The thing is to reduce cost, improve reliability.  I work on generated technology that will achieve that.

Chris -   When we say wind power, how are we trying to harness wind power?  Is that just with windmills or are there other ways of doing this?

Richard -   The way thatís emerged if you like as the sort of standard way of doing it now after a lot of years of development is the thing that I think most of us are familiar with, which is the horizontal axis 3-bladed wind turbine which you see everywhere.  People have tried other different forms of wind turbine, but the main one for the foreseeable future is the one I have described, the 3-bladed horizontal axis machine.

Chris -   How does that actually work?

Richard -   Well, you need some wind to start with and the wind blows on the blades.  The force of the wind turns the blades and if you look at these wind turbines, you see the bit behind the blades which is called an nacelle.  In there, you think it might all be electrical generator - actually, the generator is about 1/3 of it.  The biggest thing is the front bearing because youíve got all those weight of the blades on the front of the turbine and there's probably going to be a gearbox as well to increase the speed from the very slow rotation of the blades to high speed for the generator.

Chris -   How much electricity do we produce with wind in Britain at the moment?

Richard -   Somewhere around 6% to 7%.  I mean obviously, it fluctuates.  Some years are windier than others and the aim is by 2020, is to get to 1 in 7 kilowatt-hours produced from wind.

Chris -   Itís quite a lot isnít it?

Richard -   Iíd agree with you, yes.  I mean, it shows that wind is really making a significant contribution and of course, itís genuine green energy.

Chris -   Indeed, but the wind doesnít always blow.

Richard -   That is quite true.  Weíve got a couple of problems to look at.  At the moment, we can balance wind with other forms of generation - so, there's no worry about the lights going off.  In some countries like Germany and Spain where the wind penetration is getting high, weíre having to look at sort of low balancing things, and thatís where these smart grids come in, can we manage the load, so as to match the generation.

Chris -   So, this is where the wind stops blowing.  So, we need something else that can step in and fill the gap while the wind isnít blowing.

Richard -   Well, depending on the time of day.  My friends in the solar business I'm sure could help and of course, my other topic is wave power which, as another colleague points out, you can buy the tide tables for about 20 years ahead.

Chris -   Indeed.  Whoís got some questions about the future of wind power?  Who shall we start with?  Hands up.  One over here.  Let me just get to you.

Nelson -   I'm Nelson.  How much wind power is produced a year?

Richard -   To give it in its sort of form or units, they're actually called terawatt hours - its a little bit difficult But if I go back to a sort of percentage, itís about 7% of our national consumption.  We burn about an average of 40 gigawatts which is 40,000 million units of electricity. If somebody is good at sums, you can multiply 7% of 40 gigawatts by 365 times 24.  I can't do that in my head I do apologise.

Chris -   Neither can I.  So, if we already are doing this, what can a researcher like you add?  Is it just more turbines or are we trying to make these windmills better in some way?

Richard -   Well, the analogy I use is that suppose weíre in the 1930s and we bought a car, we think it was a pretty hot car.  We could get to 100 miles an hour, it would be really exciting, but look how much car technology has advanced.  I think, although itís sometimes hard to envisage, in 30 years, todayís wind turbines will seem rather basic and the ones of the future will be cheaper and more reliable, less noisy and all the good things that we want.

Chris -   Shall we find out actually how we generate electricity with a turbine, Ginny?

Ginny -   The way that wind turbines work is they have to convert that motion that the wind is turning the blades around.  Youíve got a lovely model of it there, haven't you?  So, the wind is going to hit those blades and turn it around.  But then weíve just got movement and thatís not what we want.  We want electricity.  So, how are we going to turn that movement into electricity, Dave?

Dave -   First of all, we need someone to produce some movements.  So for this, I need a volunteer.

Ginny -   What's your name?

Innes -   Innes.

Ginny -   And how old are you?

Innes -   10.

Dave -   So Innes, if you can just stand at the side of this.  What I've got here is 2 coils of wire and 2 magnets.  The coils of wire are just wired up to this meter which measures how much electricity is being produced.  At the moment, how much electricity is being produced?

Innes -   Zero.

Dave -   All weíve done is wired some wire into a meter and nothing is happening.  Now, what Iíd like you to do is to take this magnet and poke it into the middle of the coil.  If you move it forwards and backwards, can you start to see that needle moving a little bit?

Innes - Yeah.

Dave -   Try a bigger magnet.

Innes -   Itís moving a lot.

Ginny -   So, the needle is moving backwards and forwards every time you move that magnet in and out of that coil isnít it?

Innes -   Yeah.

Ginny -   So, what's going on there, Dave?

Dave -   So, if you move a magnet near a coil of wire, what you're actually doing is pushing little tiny subatomic particles which are parts of the atoms, pushing them around those coils, they're called electrons, pushing electrons around the coil, and thatís what we call electricity.  And the faster you move the magnet and the bigger the magnet, the harder they're pushed, the higher the voltage, and so, we get a bigger reading here.

Ginny -   So, why did it work better with the second magnet than the first, because the first one was a bit rubbish, wasnít it?

Dave -   So, yeah, the bigger the magnetic field, the more magnetic field you're changing inside that coil, the bigger the voltage and the molecules that you produce.

Ginny -   So, the second magnet was bigger so we got a bigger difference.  Brilliant!  So, thatís very interesting.  If you wiggle a magnet near a coil, you can make some electricity.  But thatís not really what's going on inside a wind turbine, is it?

Dave -   Pretty much, that is what's going on inside a wind turbine.  You're moving magnets past coils of wire.  Sometimes the magnets are created by putting electricity through the coils of wire which sounds a bit circular, but it works.  Engineers are very good at that sort of thing. And if you move a magnet near a coil of wire, you produce electricity.  So here, I've got a whole series of coils with the magnet in the middle.  If I turn it upside down so the magnet can roll through itÖ

Ginny - There are little lights on each of the coils and I can see them flash as the magnet falls through that particular coil.  So again, this is just a slightly more high-tech version rather than having someone there to move the magnet in and out of each coil.  You can turn it and make the magnet fall through the coil.

Dave -   Thatís right and basically, everything apart from solar power is based on this principle Ė you move magnets near coils of wire and then produce electricity.

Ginny -   That still doesnít seem very efficient.  You're having to stop every time and turn it back over.  There must be a better way of doing it inside wind power generators and that sort of thing.

Dave -   So normally, if you arrange your magnets in a circle, in the coils in the circle, you could keep on going round and round, and round - you donít have to keep starting and stopping.  You can also use gears to speed it up.  Remember the faster the magnet moves, the more power you generate.  So, you can make everything go in circles, and itís much more efficient and you can use a lot of power with quite a small device.

Ginny -   And that must be quite easy for wind turbines because they start off by going around in circles.  Is that right?

Richard -   Thatís right, Ginny.

Ginny -   But you also look at wave generation and waves donít go around in circles, do they?  They go up and down.  Do you have to do something more like what we have here where you actually have something turning over and does that make it more difficult?

Richard -   You're spot on, Ginny.  Sometimes we can use, if you like, underwater wind turbines where weíve got a tidal current, a stream of water from the tides.  But if we want to get power from the actual waves Ė the bobbing motion - thatís difficult.  People have come up with a lot of really clever things going right back to Salterís duck in the Ď70s tlil today and weíve got a lot of things on test.  But we haven't Ė I think got the right answer yet.

Chris -   What is Salterís duck?

Richard -   Essentially, itís a duck.  Itís a thing that sits on the water that bobs up and down with the waves, and uses that to convert it to oscillatory motion, you know moving motion and then you can do something quite complicated and this is the problem you can pump say, some hydraulic oil, and then you can use that to turn a hydraulic motor which is going around and then you can use what your rotary generated.  Sounds complicated, it is a bit complicated.

Chris -   So, it doesnít work then.  Is that a long answer to say, Ďdonít workí?

Richard -   No, itís not that it doesnít work.  As we know, we donít want to pay too much for our electricity.  So, weíd like to get the system simpler.  So A, they run longer without trouble and B, it doesnít cost so much money to generate the electricity power.

Chris -   Any questions on air or wind, or waves?  Letís just head this wayÖ

Jess -   My name is Jess from St. Yves.  My question is, have we crossed the threshold where we make more energy than it takes to make the wind turbine?

Richard -   Very definitely.  If you think about the embedded energy in a wind turbine, sure, there's steel and copper, and concrete in the foundations.  But on a good site, you'd expect to pay the energy back in under a year.  There's a slightly more subtle question in that as wind turbines get cheaper, you might put them in less windy sites where it takes longer to pay back.  So, maybe thatís a worry, but I'm quite comfortable that we can pay the energy back quickly.

Lowen -   Hi.  This is Lowen from Cambridge.  Do you think that in the future, weíll have bigger wind turbines to capture energy or smaller, more efficient ones?

Richard -   I think both things will happen.  On land, there doesnít seem to be a really big push to increase the size.  The plan there is generally a standard unit, you know Henry Ford kind of policy, Ďmake it cheap, put them up.í  Offshore, itís a different story because itís a lot of effort to put foundations in.  So, if you're going to put a foundation and you probably want to put the biggest wind turbine, you reasonably can.  So, weíll see I think growth in offshore size, but on land, I think itíll go for the mass production option.

Edward -   I'm Edward from a town called Swavesey. What's the biggest windmill  you have ever made?

Richard -   Me personally? I've never had the privilege of building a whole windmill because a lot of things go into a windmill, but the biggest generator I've built is a prototype, is 250 kilowatts which is about enough for 200 houses.  But the real size ones are now in the megawatts.  And actually, weíre building a prototype at the moment.  So maybe in a yearís time, if weíre on the show again, I can show you.

Frank -   Hello.  My name is Frank.  I'm from the United States.  I was just wondering, is there any kind of technology Ė you say you're doing wave technology as well - but putting these wind turbines on some kind of buoy system if they're going to be offshore to harness the wave energy in, the wind energy at the same time.

Chris -   Now, there's an interesting idea Ė a hybrid, so you can bob up and down and collect the wind.  What do you think?

Richard -   Itís an interesting point.  We like, so far, to put our wind turbines on nice solid foundations both on land, and sea.  And people are thinking, well, it would actually be quite nice to have some kind of floating system.  The only trouble is that itís quite a bit of work to make sure it all is reliable and doesnít tip over and so on.

Alisha -   I'm Alisha, I'm from the USA.  You always hear about bats and migrating birds and things flying into the wind turbines and getting killed.  Is that still a big problem and if so, is there anything being done to mitigate that?

Richard -   Well, Iíll have to be honest.  Wind turbines do kill birds.  They kill bats.  You said, ďIs it a big problem?Ē  And thatís quite a difficult question.  I mean, in terms of things that happens to birds, they're not very likely to get hit by wind turbines.  There are much worse fates for birds.  So, I'm not pleased that we kill any but I mean, you got to keep in perspective.  And I think weíre quite good.

Steven -   Steven Halliday from Cambridge.  Is there any way of storing electricity which is generated by wind turbines and not used when itís generated?  For example, a windy night, lots of energy from wind turbines, no one wants it.  Can we store it and use it later?

Richard -   Absolutely.  The difficulties with that is, we can store it as so-called 'pumped hydro' - you can pump it up to a high reservoir and let it come out.  There are other means.  You can use batteries.  The only trouble is that we would need an enormous number of batteries.  At the moment, the economics do not favour a lot of storage.  So, we just say, run a gas plant a bit harder or not so hard, or you have Norway as a neighbour.

Holly -   This is Holly, and I'm from Florida.  You said you're building a prototype.  Just exactly what is a prototype?

Richard -   When you design something new, youíve got to find out whether it works.  Weíve designed a new type of generator.  We need to build it, test it, to see whether first of all, does it generate power and does it comply with all the sort of rules and regulations that you need for a wind turbine generator.  Actually, we tested it in Norwich and we managed not to blow up the Norwich electrical supply, so weíre very happy.

Chris -   I'm not sure if thatís a slight on Norwich or a slight on your engineering.

George -   I'm George from Ely.  I'm just wondering, how do wind turbines catch the wind?

Chris -   Yeah, good question.  So how does the blade there that youíve got on your nice model, how does that actually convert the motion of the air into electricity generating motion?

Richard -   Well, I can't blow strong enough George, but the wind coming in has a certain amount of momentum and the blade shape is such that the wind is deflected off the blades and that produces the turning force.  You know how like an airplane flies Ė the wind cuts through the air and produces some lift.  Itís the same principle.

Chris -   So, what you're saying is that the air hits the blade and because the blade pushes the air in a certain direction, the air pushes back on the blade.  And so, you actually make the blade move in a certain direction.

Richard -   Thatís right, yes.

Chris -   Any other questions or weíre going to let this man off the hook?  We have one more over here.

Joe -   Hi.  My name is Joe from Caldecote.  My question is, as I've heard about wind turbines, that they cost a lot of money to maintain. I just wonder is it efficient and effective enough for the money we generate from the wind turbine to support the maintenance?

Richard -   Well, if I were an investor in a wind farm, I would be very concerned about those issues.  The general view now is that wind power on land is the cheapest form of generation including the maintenance costs.  Thatís particular so in very good sites in the American Midwest.


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A video on development of the "Salter Duck" (8 minutes):

They claim it could absorb 80% of the energy in a wave.

One problem with wave power is that there is an enormous amount of energy in the waves from a big storm, which is likely to tear apart the wave-power collection device.
evan_au, Wed, 8th Oct 2014

The problem with all these technologies is

1. The source is unreliable: it produces power when it wants to, not when you need it

2. The product is electricity, which only accounts for a fraction of our energy needs

3. Adding an unreliable source to a grid, destabilises it and reduces the economic and physical efficiency of the reliable sources

4. The lifetime cost per unit energy is absurd

5. The environmental impact is unpredictable

6. The total available power is negligible

7. The mean power is about 5 - 10% of the installed capacity, so the capital investment is uneconomic

8. Maintenance costs "climb the bathtub" very rapidly

One could go on, but as long as the public can be conned into subsidising these absurdities, people will make money out of them, so politicians will raise taxes to pay for them. alancalverd, Wed, 8th Oct 2014

One method touted to deal with unreliable generation is variable consumption (or "Demand-Side Management", in the jargon).

Given that:

almost every electronic appliance today contains at least one computer,

within a decade, most new appliances will be networked (the "Internet of Things")

Then that opens up a lot of opportunities to save power by switching to an energy-saving mode when generation capacity is marginal (some devices or organisations may even be able to switch to an energy-producing mode). 
evan_au, Thu, 9th Oct 2014

I think judicious use of a combination of flywheels, capacitors and rechargeable batteries or fuel cells can even out any variability in power generation on the scale of seconds, minutes and months (respectively).

Can it be done economically? Not with existing technology, but I think we will see major shifts in the next decade or so. Also, wide adoption will also be able to take advantage of the economies of scale, increasing the "bang per buck." chiralSPO, Thu, 9th Oct 2014

Last month (a warm September)  there was no wind above 5 knots over England for more than 10 days. As wind turbines produce power accordiing to a v3 law, this means that if , say, 20% of grid power was generated by wind, we would need a storage capacity of about 500 kWh per capita to maintain our present standard of living under perfectly normal circumstances. That's 500 car batteries per person, assuming 100% efficiency. You would need to double that to cope with average winter conditions, and at least double it again to cope with peak demand, like cooking breakfast.

So if you are an average family you will need to find space for 8000 car batteries just to cover the 20% of your electricity that comes from wind. At £40 each, that more than doubles the cost of your house. And the batteries will need replacing every 5 years or so.  alancalverd, Thu, 9th Oct 2014

The problem is private enterprise reselling, Wind power is more a problem of equipment reliability than cost for a DIY consumer, true it is expensive but it is quite possible with only mild care at choosing modern household appliances to be able to live the same life as grid electricity with 8Kw hybrid kit.
In Australia the electrical utilities are privatised government sector that own or lease back the generators and power stations e.t.c. BUT, it does not mean that if you are more than 80 meters away from a neighbour that an OFF GRID 8Kw power system cannot supply everything required for 1:1 against grid electricity!
Another great feature of off grid is a small 2500w 12v blackout power system, that also serves as a computer network attack protection by not being connected to grid power.
NOTE: To truly understand "wind charge controllers" and "generation efficiency / time unit" most wind turbine generating always occurs in the lower 10% trickle of the wind turbine wind speed rating(You are so right if you know wind turbine ratings that the wind does not always reach that speed ).

I rest my case! Not to be demeaning in front of you, and not 8000 automotive starter type, but, "deep cycles" anything from "16"x 80Kg 250ah to "80"x 25Kg 80ah for 10 to 12 years use, if you read the article they do not use "car batteries" they use either special "2v (two volt) deep cycle batteries e.g. 230Kg 3000ah" or standard "12v deep cycle" and you would need quite a few things but there is much more to know about deep cycle batteries such as lead acid , GEL , silicon, (Lithium deep cycle(*deep cycle - synonym for "industrial") is way too expensive). While "float charge" is 30% in a lead acid type, for off grid you are trying to get an average days complete use on 10% - 15% "battery bank" discharge(down to 90% full - called D.O.D. Depth Of Discharge). Other requirements , not to be UPS batteries, able to last chemically for 10 years at or above 20 degrees Celsius on float.

They are cost efficient by self import at this very time - pending exchange rate(be your own customs broker - broke the deal and fill out the form yourself all DIY).
In import deals, the more you buy(batteries(correct name "accumulator"), pv panels e.t.c.) the cheaper it gets but costs do "add up" so use your calculator from start to finish.
You need a 3 tonne hydraulic tail gate van and palette jack trolley to pick up from customs warehouse.
nicephotog, Thu, 9th Oct 2014

If I read your figures correctly, a 230 kg 2volt deep cycle unit will give you 3000 Ah. That is 6 kWh, so you need 24 of these to supply 20% of one person's energy needs  for 5 days. Add in the controller and connectors, and we are talking about 40 tonnes of battery pack per household. Most believable price today is about $500 per cell, so $50k or so per household for the batteries alone. And then there's the small problem of dealing with 4 tonnes of scrap lead and a few gallons of sulfuric acid every year. 

Compared with an oil tank (20,000 kWh, 2 tonnes) at $5000, it's a nonstarter. alancalverd, Thu, 9th Oct 2014

Car batteries are not the solution, for many obvious reasons, as Alan has laid out quite well. However, power to gas conversion might have a chance. Electrolysis of one liter of water will produce 4.5 kWh worth of hydrogen, so only about 120 L of water would be required to store that 500 kWh value put forth earlier. Of course the hydrogen takes up space too--at atmospheric pressure, 500 kWh worth of hydrogen gas would require 141 m3, or a sphere of radius of 3 meters. This is unreasonable to have in a house or apartment, but if large wind or solar farms had a substation to store and dole out energy, it would make sense to produce and store highly compressed or even liquified hydrogen (or methane).

A small town could have a substantial fraction of their energy needs backed up with a single water tower, a couple MW-scale electrolyzers, a compressor/liquifier and a liquid hydrogen tower capable of holding a few hundred m3 of liquified hydrogen. None of this is cheap, but it is all durable (MW electrolyzers require some annual maintanence, but can last well over 40 years).

Changing our energy infrastructure will be incredibly expensive and slow, but consider the amount invested in building up the oil and coal industry. The infrastructure and manpower required for prospecting, extraction, processing, transportation, distribution and use of fossil fuels isn't exactly cheap either. And we are not even considering the cost of environmental damage (direct and indirect). chiralSPO, Thu, 9th Oct 2014

The hydrogen economy can be set up at very low cost as we already have the necessary distribution network for most stationary purposes. Centralised storage isn't a problem: oldfashioned gasholders worked at low pressures, modern LPG farms can handle several atmospheres' compression, and the bigger the storage vessel, the more economical it is to fill it with liquid hydrogen.

You can burn hydrogen in a conventional gasfired power station any time you need electricity. Indeed the maintenance costs of a hydrogen furnace are much lower than for fossil fuels or even methane.

The only real difficulty is using it for transport as the losses from small liquid hydrogen tanks are significant and the specific energy of the gas is too low for aviation, though adequate for urban transport. . alancalverd, Thu, 9th Oct 2014

Agreed. So if we had a 100 MW wind or solar (or wave) farm producing hydrogen, and feeding that to a hydrogen-burning power station, would that not solve the problem of intermittency? (provided that one could store enough of the hydrogen to smooth out the seasonal variation in energy capture and demand). chiralSPO, Thu, 9th Oct 2014

Read energy or extinction by Fred Hoyle from the 1970's its a great small book.
See the power if a mile of sea  compared to the windmills.

It's a great starting point allan marsh, Thu, 9th Oct 2014

That's why several High-Voltage DC cable connections between France, Norway and Germany (and into Spain and North Africa) are recommended for efficient use of renewable sources (as well as efficient use of non-renewables).

When it is blowing a gale in the North Sea, the Spanish can turn their coal-fired generators back to "idle". evan_au, Thu, 9th Oct 2014

Yes but. The problem with a 100 MW wind farm is that it only produces, on average, 5 - 10 MW. Now we lose 60% of that when we convert the hydrogen back to electricity (40% thermal efficiency is pretty good for a small station) so we have installed 110 MW of generating capacity to yield 6 MW overall - just enough to run one locomotive (Eurostar uses 12 MW).

Still it's better than wave power, which has been in development now for 40 years and produced no energy at all.  alancalverd, Thu, 9th Oct 2014

Alas, no. Wind turbines are generally shut down when the predicted wind speed exceeds 35 - 40 kt. They are usually rated at a steady speed of about 30kt but can be damaged if they are hit by a gust, so they don't work in bad weather. Or in good weather.  alancalverd, Thu, 9th Oct 2014

Real world capacity factors
UK Wind average (2007-2012): 27.5%

" capacity factor of any offshore wind farm - 46.7%, having produced 1,278 GWh over 1.5 years".

I think making ammonia efficiently could be an alternative (better) use of excess electricity from intermittent renewables, instead of producing hydrogen. peppercorn, Fri, 10th Oct 2014

The problem with capacity factors is that they are arbitrary. If I put a 100 kW blade on a 150 kW alternator (which makes sense, because blades are a lot more expensive than alternators, and you certainly don't want to do it the other way round!) I can put a 50 kW rating plate on it and claim 20% capacity factor even though it only produces 10 kW.  alancalverd, Fri, 10th Oct 2014

Interesting. Are you thinking of creating ammonia for use as a fuel/energy storage medium, or as a useful product to make when there is excess electrical energy available, and then finding some other source of energy when there is a deficit?

I know of a company that pays industrial electro-refineries to up- and down-shift their electricity demand as the grid energy balance fluctuates--they never sell any electricity back to the grid, but because the operation is so energy intensive, and would otherwise be going 24/7, a reduction is consumption is equivalent to increasing the available energy for the grid. chiralSPO, Fri, 10th Oct 2014

I am unclear, following your logic, how you get a nameplate rating would be 50kW in your example. It could conceivably be 100kW, based on the size of the generator installed.

And I would question why such a turbine would use a 50% larger generator anyway. Any increasing in size and weight of electrical machinery in a nacelle, a couple of hundred metres in air must also scale up everything else up as well (tower rigidity etc); there will an additional increase in maintenance costs and, off-shore, this is obviously a serious overhead - thus, a generator, say, 10-15% larger than the blade's continuous rating seems somewhat more realistic. peppercorn, Mon, 13th Oct 2014

The second. The ammonia produced would be "a useful product to make when there is excess electrical energy available".  There's plenty of it needed for making fertilizers for a start, and the hydrogen needed to industrially produce NH3 comes predominately from natural gas; itself a (fossil!) fuel.  And, in addition, on the occasions when a heavily renewable reliant grid falls short, the option to use natural gas can still be there; whilst the CO2 from said gas is being effectively offset from another sector - ie. agriculture.

I imagine this company could be an aluminium smelter, for example.
Though clearly it's a little more tricky to turn a large smelter on or off, I suggest that it might be possible to throttle them to a point.
I expect this type of very large demand-side planning would get less practical with a grid heavily reliant on renewables, because of the loss of predictability involved.  However, if we have a future situation where thousands of electric vehicles are plugged in to charge/discharge as needed across the grid then some of the stability (and therefore, predictability) could return. peppercorn, Mon, 13th Oct 2014

Indeed it could, but it is up to the manufacturer to specify the plated output, and if he wants to claim a very high capacity factor, all he has to do is declare a very low power rating. Business, not physics. And the business is not making electricity (you get a subsidy for making it when it's needed, and for not making it when it's not needed), but selling windmills and getting subsidies.

It all depends on the probable peak gust output of the blade before it stalls or shuts down in a gale. Shutdown is not instantaneous so you need a generator that can absorb gusts. Just how big you make it depends on your assessment of gust risk and the cost of repairs.

There's nothing unusual about derating. There is a class of light sport aircraft that is restricted to 80 horsepower, which is fine for cruising but a bit marginal for takeoff, so the manufacturers fit a 120 or 150 HP engine and a throttle detent labelled "takeoff only": this simplifes both licensing and cooling, and the rating plate just says "80 HP continuous".

At one extreme, racing car engines are deemed overweight and undertuned if they survive more than one race (now one weekend, under Formula 1 rules). 500 HP per liter is about right for F1, and 1000 HP/liter would be a target for a dragster. At the other extreme, aero engines using exactly the same physics and chemistry rarely generate more than 50 HP/liter because reliability is essential.  Probably the most extreme examples of derating are in spacecraft and undersea cable repeaters, where maintenance is either impossible or prohibitively expensive, so many systems use "old" technology (components that have been in commercial use for at least 5 years) derated by an order of magnitude. alancalverd, Mon, 13th Oct 2014

NO not 40 tons(you don't just go throw them away), 2 to 4 tonnes "deep cycle" lead acid (GEL type only half the number "approx" e.g. system voltage(not inverter output power supply) 48v , 96v , 110v , 120v , 240v , 380v , 520v e.t.c. ...), that will last ten years or more (in Britain the climate is much milder and only medium temperature lead acid chemistry accumulators (10 years at 20 deg. Celsius float life-cycle) are needed).
e.g. 80 of 80ah "deep cycle accumulators" = 1.88 tonne (1880 Kg)

What's more , as i said it will last 10 years at least set up properly(calculated and appliances better acquired for the system).

You need to do some research to understand the battery market for that is available, 2v (two volt) 3000ah or 1500ah or 500ah e.t.c. .....
**2v lead acid "acumulators" are really for 12v and 24v storage systems (output whatever you require by obtaining the correct inverters)
**BECAUSE 12v does not allow more than 4 in parallel and not much wiser in 24v!!!!
12v deep cycle 250ah is the most common largest..."made in abundance on offer in the market"...(there are one or two slightly larger).

nicephotog, Sun, 19th Oct 2014

I have news for you. We already do it (though not with the methods you suggested)
Perhaps we  need to hollow out a few more Welsh mountains.
Or, if you prefer

Bored chemist, Sun, 19th Oct 2014

I know that we use pumped hydro and compressed air to store energy now, but it is a tiny percentage of the energy consumed, and requires enormous reservoirs. They are some of the most energy efficient methods employed these days, but in terms of space and water required, there must be something better (pumping 10000 m3 of water to a height of 100 m won't even store 3 MWh). Additionally, most of the geographic formations that allow for this are already employed in this way.

We can start hollowing out more mountains or displacing people from river valleys so they can be dammed up, or we can invest in technologies that can store and dispense hundreds of megawatt hours from a building the size of a high school gymnasium... chiralSPO, Sun, 19th Oct 2014

I'm no mathematician, so please correct me, but:

10,000 cubic meters of water = 10,000,000 Kgs. According to mgh,

(10,000,000kgs x 100mtrs x 9,81) / 3600,000 =2725 MWh.

Still not a huge amount though, but useful.
Re all the comments on batteries, there are I think some grid scale storage installations in progress, but let's not forget those already operating using vanadium redox flow batteries.
Also re: storage, rather than pursue the madness of carbon capture and storage, why not use underground spaces (old coal mine, oil wells, etc.) as air pressure reservoirs (instead of more pumped hydro).
Not great efficiency I understand because of heat losses, but there are projects in Europe and America I think to recover the heat lost in pressurising air to improve the efficiency of the process.
One huge advantage of compressed air energy storage (CAES) compared to carbon capture and storage is that leaks will be harmless. teragram, Sun, 19th Oct 2014


is indeed instructive. It says that 2.3 tonnes of batterries will supply 400 AH at 240V, and I have no argument with that. That amounts to 96 kWh. Now each person in the UK consumes 5 kW, so to supply 4 people for 5 days you need 2400 kWh, i.e. nearly 60 tonnes of batteries.

The problem is that people (well, politicians) talk about domestic electricity consumption, but that is only a tiny part of our energy use. If you wrap up warm and stay indoors, you can manage with 100W for cooking and watching television (but not at the same time), but someone else will have to grow and transport your food, pump your water and waste, and indeed light the TV studio and power the transmitter. And make your next car, television, house.... 

Pumped water storage is a lot of fun. It's very expensive both in land and capital to install, which is why there are only two significant installations in the UK. Dinorwig is the larger one. Its capacity is about 10,000 kWh, so it can supply all the UK's energy needs for about 0.12 seconds, then takes about 6 hours to recharge. Not quite the Hoover Dam, which generates 2,000,000 kW continuously, by flooding an area the size of Middlesex to a depth of 600 feet, and feeding the lake from a catchment area twice the size of the UK.  We'd need 150 Hoover Dams to supply the UK's energy needs, but where would we live, and where would we get the water from?
alancalverd, Sun, 19th Oct 2014

*** If ever true , nobody has ever bought a RAPS anywhere in the world "ever" and is not feasible to have built such to market even *as chinese PRC business!
*..actually,...well them perhaps!?!?...

First for those that it rushes past about VOLTAGE and AMPERAGE and WATTAGE relationship:
AND how it works from solar wind and particularly A.K.A. 2.3 TONNE OF BATTERIES (..."of batterries will supply 400 AH at 240V"...)
4.1 amps at 240v (for 1 hour continuous) = 1 Kwh
8.2 amps at 120v (for 1 hour continuous) = 1Kwh
80(actually 83) amps at 12v (for 1 hour continuous) = 1 Kwh
40 amps at 24v (for 1 hour continuous) = 1Kwh
20 amps at 48v (for 1 hour continuous) = 1Kwh
... ..

There-fore an 80ah "accumulator(international English trade/cargo language)" in its entire stores as good as 1Kwh of charge If discharged to 100% DOD(depth of discharge) which destroys a deep cycle battery instantly!

80 x accumulators(n) is more about "trade deal cost" the more you buy the less it costs but for domestic it is simply a must to stay above the MOQ , and, 80 x (1 Kwh battery) but is divided by "6" to gain approximate 15% DOD capacity total allowed discharge p/day to allow the battery bank to last ten or more years.
These last two points are extremely serious for the TEN YEAR PLUS ECONOMICS: "Lead acid batteries(DEEP CYCLE)" quality life-cycle is usually(see the models data-sheet specifications) 1200 discharges to 30% DOD (down to 70% full) (OR only 150 to 200 discharges known as "deep cycling"), it also involves temperature, usually at or below 20 degrees Celsius,
and, one final coupling point to this, 30% DOD and above is the "float charging" range of an accumulator type battery, this means that to properly operate the chemistry, it can have a constant uninterrupted charge applied to it, below the 30% DOD level, the charger must stop for periods or the battery will get damaged(known as deep cycling)!
(NOTE: there is a lot to be careful of in making a RAPS, the reason for so many accumulators is because a single or even a small quantity of batteries cannot discharge massive currents, to distribute it requires parallel strings/rows in series to the specified system charge/store voltage A deep cycle 12v battery should not discharge more than 0.5Kwh half a Kwh per battery(and should be 48v system or higher voltage or it will simply be being *adverb* "deep cycled") or it will seriously risk damage!!!)

The batteries in a RAPS is not all that is simply in use, the direct take-off from either PV Panels or Wind Turbine(alike P.T.O. from a tractor) is also obtainable during current draw to the output inverter. The general rule for solar is it operates around 80% of its "rating total" of its array at best for reasons of temperature and sun angle.

More again, Australia has gone through a phase of grid tie renewables acquirement-installation (mainly only solar) in domestic electricity that is part of a national tax/tariff rebate scheme, it does/did include businesses whether commercial or industrial.
A domestic house in Australia (could be any number of people) all tends to be 25Kwh p/day to 35Kwh p/day.
This lesser sum is also the standard 2 bedroom flat unit(25Kwh p/day).
All-Electric house or flat.
# 1. Electric cooking is the bulk of electricity use 80%
# 2. Running Hot Water is often touted as the biggest but their larger domestic elements are only 2.8Kw draw and 5Kwh per boil at the biggest (time/density)
# 3. Everything else generally is done under 2.5 Kw to 1.5Kw draw except the electric kettle for a cuppa which is 2.5Kw draw at largest usually 1.5 to 2.x Kw
(to reach the 2.5 Kw draw level requires something like a tradesman with a power tool or a tradesman with a hair dryer)
(other odd points are a 500w washing machine really draws at 5Kw because of continually laboring the electric motor by "surge draw")

Nyngan NSW Austrlia 2014 - Solar power plant:

List of Wind Farms in NSW Australia:

It is understandable the UK has some trouble here with large scale power generation because of size of area required , particularly for renewables methods. Maybe grow mushrooms under a solar farm?.

However, Wind appears a good solution and it appears the coastal shallows gain the edge on that problem.
So the question is not what type of renewable, but more how do you engineer economic and manageable bases in those shallows. Your looking at the wrong end of the structure , the problem you mention(what medium to draw power from) is already solved.

As for wind in a non - sunny country, 10Kw wind turbine RAPS with 1.5Kw Solar(because it's a small outlay and sun occurs enough) would suffice any medium to large domestic household.
The annoying problem is the 1.5Kw piece , like i explained before a device called a "solar controller or solar charger" holds the input float voltage steady putting in whatever amperage so with 5Kw(rated) of panels around 4.5Kw constant in good weather is available to use or allow to charge as pressure/draw occurs (automatically) after leaving the controller.
As that is, Why a 10Kw domestic turbine, because at low revs there should be around 1 -2Kw becomes available direct(bypasses accumulator bank) from the controller when it gets moving in gentle breezes.

If you are serious about wind turbines, then you need to understand the factor of obstructions and turbulence in gas not being dissimilar to that of viscosity in liquids. Again, the closer to the ground a turbine is the glue-ier the air is by pressure and hence more force loaded into more compressed gas. So it is a must to lift them at least 12m to 15 meters above the ground whether they are a 600w or a 20Kw wind turbine, but this again is because meteorology already has data proving that per 2m height gained "air speed meters per second" of wind greatly increases and is reasonably relative to height/temperature/air pressure and significant to wind turbine performance not merely the point of viscosity , turbulence and obstructions.

Percentile Layers and PBL

How about pre-loading wind power by wind power?
You simply make a huge bank of wind turbines BUT, they don't generate electricity specifically, they pump sea water(or compress air with the tanks exposed in the sun and a moveable shade board to make compression easier and release better - and sprays and gutters for before compression) up 50 - 100 meters into a giant blocked off valley entrance with a sealed rock and cement skin coating to prevent leaching.
Then let it go continuous as hydro electric back into the ocean.
In outback Australia we had a main river (Murrumbidgee) running past a few miles away and when i learned there were small domestic hydro units on the market 0.5,3,5,10Kw i thought of that one once because i had to fix the ball-cock on a water trough.
nicephotog, Mon, 20th Oct 2014

Just do the maths instead of shouting.

The two important numbers are population density and total power requirement per capita. Multiply these together and if the answer is greater than the renewable power input per unit available collecting area, it can't be done. It is possibly feasible in Australia because there is a negligible population scattered around a vast area of desert, but it won't work in the UK - we hardly have enough land to feed ourselves, let alone collect energy from it.   

My Aussie family have a domestic solar water heater which works pretty well but it won't run the car. 

Friends in the USA have a garage full of batteries driven by a windmill, and a forest for heating and cooking, but they still go to bed two hours after sunset because the (low consumption) lights won't work for more than a week in winter.  alancalverd, Mon, 20th Oct 2014

First however, i'd deal with the problem of government corruptions subversive media , derogation and demeaning against wind power capabilities, along with the generous personal support of having its own electric wind power to be two faced.

That was priceless!

Good afternoon. nicephotog, Tue, 21st Oct 2014

Corruption, certainly. Taxpayers are forced to support wind farms which only damage the stability of the grid, so some politician's brother in law must be making money out of the scam. alancalverd, Tue, 21st Oct 2014

So, people need to have the information to, decide , outlet to acquire and construct for themselves an off-grid power system individually.
Economically it is safer and more secure (deep cycle batteries are reasonably reliable).

If someone lives 80m or more away from others they can use as small as a 5Kw (3-wind 2-solar) off-grid power system for a normal domestic house with 4 people by changing the appliances and lifestyle, and use better rostering(1 job after another not simultaneous) and they can achieve more electricity usage in 10 years at less cost than the grid.

Three quarters of personal acquisition is against fossil fuel use being the corruption because its' use is by monopolisation.

............(re edited)
The billions of dollars taken from either industrial or domestic by government for power generation is unaccountable truthfully, but also a rip off because there is no true regulation except how much can be squeezed out of every-ones pay cheque.
It is extremely advisable to have as few loop-holes in lump sum dipping as possible. Allowing expansion(of operational costs by point of it adding a profit margin to be cost effective) or monopolisation encourages and preserves corruption and prevents accountability. nicephotog, Wed, 22nd Oct 2014

I understand that Dinorwig has six 300MW generators, or 1800MW total, rating 1650MW. Correct me please, but if it's capacity is only 10MWh, does this not mean that it can only run for

                          10MWh / 1650MW = 0.006hours, or about 21 seconds?

And yet the published information indicates that the station can operate for ďup to six hoursĒ. We must of course be careful of that old phrase ďup toĒ, I havenít yet found out what power output can be sustained for that period of time. 
The UK pumped storage stations (Dinorwig, Ffestiniog, Ben Cruachan, and others) were primarily intended to assist the (then) planned  fleet of nuclear stations. These have difficulty matching their output to sudden load variations, so the storage systems would have been extremely successful in helping them to cope. Fortunately it was realised that nuclear power was far to expensive, so the installation of them was abandoned. Sadly, they once again rear their heads, but judging by the tone of this discussion so far, I will not mention taxpayer subsidy, and deals with foreign energy and finance interests. 
teragram, Wed, 22nd Oct 2014

Apologies for screwing up the decimal points earlier! Dinorwig capacity is published as about 6.5 GWh (1.3 GW for 5 hours). UK energy consumption is about 300 GW, so it could run the country for 78 seconds. alancalverd, Wed, 22nd Oct 2014

Thanks Alan for clarifying that. teragram, Thu, 23rd Oct 2014

You say the problem is space for wind turbines, looking at some of the newer small turbines that have a large number of blades and mounted on a
pole inside a short tube(to that effect) and coupling what was learned in the turbo-prop passenger aircraft industry
you could always see it as get your "blade area"(sail) from a much smaller diameter by more blades which again has less stress on the structure during torque,
BUT mount these in racks of them(sets) on a base alike a skyscrapers internal super structure with a turntable on top.
Or sort out the weight and friction for using the blades to run a ring of 6 generators(look a little like the old early 20th century aircraft rotary engines)
not one on the spindle (as before a bit of modification to spindle and tower structure and sail power).

As all that is, there is what i said of "wind grid tied" for domestic(which happens to be considered a gigantic parasite because of economic returns
- only money is made from industry usage)
£185 p/turbine($350AUD) , Three of these makes 1.8Kw max but performs by 24 hour at the per year level of a 3Kw solar (it's done by trickle not the rating - using rating is a bad mistake many people make , the turbine mentioned is possibly the only one can do it!!!)!!!
So this would be great for UK and Europe.....(the solar(mentioned in the document PDF) is optional but more for Australia because of air con.)
In Australia, this is why the government(that owns the electricity system) didn't mind making a "RET" bill ( or a carbon emissions tax ) to fulfill green legislation nationally and internationally
because, solar, is quite worthless!; to most domestic connections on the grid and too huge for industry physically, with domestic they will cook and shower in the morning(95% of all domestic use drawn!!!) and night 3/4 of the year and personally get nothing from their panels, and again, mostly the people are not there at home during the day and the government owned supplier-broker companies regulate the "FIT deal-contract" (Feed In Tariff p/Kwh) payback from excess electricity produced from the grid tie panels e.g. 2 cents p/Kwh.

In Australia, the electricity charges(called "tariff") is four sections peak=2 periods, a few hours, of morning and night each (cooking and showers) and the most expensive, while everyone is asleep "off peak", the cheapest, and "shoulder period" is everything else and the tariff nearer peak but between peak and off peak value.
They allow all the solar panel dumping on the earth in this country, but as soon as they hear "domestic wind system" or "off grid" they scream blue murder foreign loss of capital.

How can we harness wind and wave power? , It's probably illegal and we not simply have a gun in our possession but an illegal firearms type!!!!!
nicephotog, Fri, 24th Oct 2014

The problem isn't the size of windmills, though there is a practical limit to hiow big you can make them, but the energy density of wind. Wind speed squared, multiplied by air density, comes out as watts per square meter, so if you want more watts, you need more square meters of wind, not just sail area. You then design the sail to optimise the power extraction from th area you have got, and it turns out that multiple baldes work well for high windspeeds and small areas, but two or three blades are best for low speeds and large areas - it's the difference between a jet engine and a helicopter!. alancalverd, Fri, 24th Oct 2014

Interesting you mention helicopters.

But that then never solves the problem of compaction (of 4 to 6 x generators on one pole head)
( e.g. the picture of the wind farm here with 1 generator size-(n) output per pole... (think of six times the output from this...) ) could be better calculated to carry four generators a head, and perhaps the friction-inertia problem can be simply reduced to a slower start-up or better auto transmission gearing (quite aware there is a limit somewhere).
My guess is the problem is the leading blade sweep interfering sonic-ally with the next blades' sweep, of which the Doppler points to density and blade shape displacement plays a role in the inefficient frequencies, a problem solved by Soviet engineers on an attack helicopter called  Kamov Hokum for its counter rotating blade set, one set sitting above the other set.

I don't see the space on a pole being utilised well enough, they should have their physical carrying(mounting) capacity and weathering stresses modified for max, extraction by carrying many more generators on the one set of blades. The compaction would be far more productive and economic finally.

nicephotog, Sat, 25th Oct 2014

You can't extract more power than the wind puts in. If your generator isn't matched  to the sail, just use a bigger generator.

Contrarotating propellors or chopper blades are very useful at low speeds as they generate no net torque, which makes slow flying and/or hovering a lot easier, but the efficiency of such a system is limited to a very narrow speed range. The Fairey Gannet was an extremely useful naval aircraft which could loiter at 20,000 ft  for several hours, travelling not much faster than the ships below it, thanks to a unique double turboprop engine. alancalverd, Sat, 25th Oct 2014

Well aware about the sail torque AND, as i said, it's a matter of matching sail power to INERTIA and some of it FRICTION of the bearings of , the latter can be almost friction-less so it leaves inertia that can simply be reduced to a longer start-up time with an extra set of generators.
The real difficulty, is loading all of that onto a structure safely.
This is about compaction of much more into one site to operate successfully, so e.g. that previous marine wind farm picture would only be a quarter or less of its view for the same output.
I was reading the main point they have trouble is mounting the spindle for the blades because of the stresses on either the hub roots of the sails or the structure housing holding all of the blades and hub. nicephotog, Sun, 26th Oct 2014

Alas, no. Friction and inertia are not the problem. It's all a matter of conservation of energy, the lack of appreciation of which seems to run through all your posts!

The maximum power you can extract from a windmill at a given air speed, depends on the swept area of the sails. Adding more or better generators will not affect the speed of the wind or the density of the air. alancalverd, Sun, 26th Oct 2014

What you said is liken-able in every metaphorical detail to "Astronauts cannot train in water tanks or air suspension for weightlessness" , However, NASA spends billions training them that way!

For example, "conserve energy" and this topic "Harness wind and wave power" with regard to friction, inertia, gearing, fly-wheeling and the effortlessness of rotation because it does not particularly respond to gravity, only inertia and friction.
So in theory, you can use three angled paddle-pop sticks to start one of those wind generators out there in the ocean farm, it will simply take much longer(because there is very little input harnessed)!
NB: Fly-wheeling conserves energy, so also free spinning(disengaging the throughput drive train) if the vacuum on the blades reverses by either wind direction or blade speed exceeding volume sweep balances.
Things that spin(for this purpose), do not really have a weight, ONLY inertia and friction. nicephotog, Wed, 29th Oct 2014

How sad that every scientist and engineer has got it wrong all these years. All you need to extract unlimited power from a gentle breeze is lots of oil on your bearings, and once it starts rotating you can get rid of the supporting mast! alancalverd, Wed, 29th Oct 2014

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