More support for merit order effect | 28 comments (28 topical, editorial, 0 hidden)

Re: Speaking of costs (none / 0)

Wind power could be much cheaper?
A new 2000W vacuum cleaner costs 50e. If we couple a vacuum cleaner in reverse, it works as a generator.

Let's assume that this generator lasts 1 year. It produces 2kW*8600h = 17 MWh. So the costs are 50e/17MWh = 3 e/MWh.

by kjr63 on Wed Mar 2nd, 2011 at 01:35:34 PM EST
[ Parent ]

Re: Speaking of costs (none / 0)

forgot to mention the €3060 for the year's continuous electricity.

"Life shrinks or expands in proportion to one's courage." - Anaïs Nin

by Crazy Horse on Wed Mar 2nd, 2011 at 01:44:35 PM EST
[ Parent ]

Re: Speaking of costs (none / 0)

Electricity for what?

by kjr63 on Thu Mar 3rd, 2011 at 07:53:59 AM EST
[ Parent ]

Re: Speaking of costs (none / 0)

I'm sure you realise this, but you would need a might big funnel in the wind to make a vacuum cleaner run in reverse.

by njh on Wed Mar 2nd, 2011 at 10:06:22 PM EST
[ Parent ]

Re: Speaking of costs (none / 0)

You mean the rotation speed of a wind mill is slower than a vacuum cleaner?

by kjr63 on Thu Mar 3rd, 2011 at 08:02:51 AM EST
[ Parent ]

Re: Speaking of costs (none / 0)

Uh, yes. This should not come as an unbearable surprise to anybody who is even casually acquainted with the physics involved.

- Jake

Friends come and go. Enemies accumulate.

by JakeS (JangoSierra 'at' gmail 'dot' com) on Thu Mar 3rd, 2011 at 12:09:42 PM EST
[ Parent ]

Re: Speaking of costs (none / 0)

Wow. Such an "obvious" knowledge? Please elaborate. Is the wind speed so much higher in a vacuum cleaner? I have seen pretty fast rotating wind mills. What about a gearbox? That would take out as much rotation speed as needed.

by kjr63 on Thu Mar 3rd, 2011 at 12:45:26 PM EST
[ Parent ]

Re: Speaking of costs (none / 0)

Is the wind speed so much higher in a vacuum cleaner?

The pressure gradient is a lot steeper (on the order of a tenth of an atmosphere per meter, while the pressure gradients generating winds are less than a hundredth of an atmosphere per kilometer). How that translates to wind speeds is a bit out of my field, but I guarantee you that you will not get a kW out of a vacuum cleaner fan if you stick it in even a gale-force wind.

What about a gearbox? That would take out as much rotation speed as needed.

Yeah. That's what windmills do. Large blades, big gearboxes. That's what makes them expensive. The generators themselves aren't, AFAIK, the main cost.

- Jake

Friends come and go. Enemies accumulate.

by JakeS (JangoSierra 'at' gmail 'dot' com) on Thu Mar 3rd, 2011 at 01:59:09 PM EST
[ Parent ]

Re: Speaking of costs (none / 0)

From wikipedia:
http://en.wikipedia.org/wiki/Airwatt

Airwatt
An airwatt or air watt is a unit of power very close to the watt which is used for various vacuum cleaning systems, such as vacuum cleaners. The airwatt is derived from English units. ASTM International defines the airwatt as 0.117354 * F * S, where F is the rate of air flow in ft3/m and S is the pressure in inches of water. This makes one airwatt equal to 0.9983 watts.
The airwatt is useful measurement of vacuum power, since the power carried by a fluid flow (in the case of a typical house vacuum the fluid is air) is equal to pressure times volumetric flow rate. This could be a more useful figure than the electrical power drawn by the vacuum system's motor, since the efficiency of motor and vacuum systems varies.

If i understand correctly, this means that electric power is directly related to air flow = air speed. So if, you put a relevant rotor in the vacuum cleaner, you get 2000W. Should not be too difficult.

That's what windmills do. Large blades, big gearboxes. That's what makes them expensive. The generators themselves aren't, AFAIK, the main cost.

Hard to believe. Can a rotor and couple of gears rise price from 3e/MW to 80e/MW? I believe generator manufacturers are being screwed.

by kjr63 on Thu Mar 3rd, 2011 at 02:30:01 PM EST
[ Parent ]

Re: Speaking of costs (none / 0)

If i understand correctly, this means that electric power is directly related to air flow = air speed. So if, you put a relevant rotor in the vacuum cleaner, you get 2000W. Should not be too difficult.

No you won't. With a pressure gradient on the order of tenths of atmosphere per meter, you get a considerable loss from friction. You'd be lucky to recover 200 W from a setup like that.

Hard to believe. Can a rotor and couple of gears rise price from 3e/MW to 80e/MW?

And a tower. And grid connection. And transformers. And installation. And output governors. And you're converting a pressure gradient into electricity, which is a lot harder than converting electricity into a pressure gradient.

- Jake

Friends come and go. Enemies accumulate.

by JakeS (JangoSierra 'at' gmail 'dot' com) on Thu Mar 3rd, 2011 at 04:11:19 PM EST
[ Parent ]

Re: Speaking of costs (none / 0)

Similarly, you assumed that your generator would survive 8600 hours of continuous operation. Assuming that your average household vacuum cleaner is used for three hours every week, that translates to a lifespan of something like fifty years.

- Jake

Friends come and go. Enemies accumulate.

by JakeS (JangoSierra 'at' gmail 'dot' com) on Thu Mar 3rd, 2011 at 04:16:55 PM EST
[ Parent ]

Re: Speaking of costs (none / 1)

The rotation of the blade tips is limited in absolute terms by the speed of sound.

Divide the speed of sound by three times the diameter of the blade to get the maximum RPM allowed (of the order of tens thousand for a vacuum cleaner, of the order of a hundred for a wind turbine).

So, in what may be my last act of "advising", I'll advise you to cut the jargon. -- My old PhD advisor, to me, 26/2/11

by Carrie (migeru at eurotrib dot com) on Thu Mar 3rd, 2011 at 12:42:31 PM EST
[ Parent ]

There are earlier limits (none / 1)

I think that speed close to 80-100m/s (Crazy Horse can confirm), so only a third of the speed of sound, are already a pretty hard limit, for reasons of acoustics as well as damage to the tips of the blades. So in practice, large wind turbines rotate at something like 20-30rpm or so.

Wind power

by Jerome a Paris (etg@eurotrib.com) on Thu Mar 3rd, 2011 at 12:53:51 PM EST
[ Parent ]

Re: There are earlier limits (none / 0)

Okay, if the blades were made of unobtainium the limit would be the speed of sound.

So, in what may be my last act of "advising", I'll advise you to cut the jargon. -- My old PhD advisor, to me, 26/2/11

by Carrie (migeru at eurotrib dot com) on Thu Mar 3rd, 2011 at 05:15:16 PM EST
[ Parent ]

Re: Speaking of costs (none / 0)

Sure. But naturally the same laws work in a vacuum cleaner. 2000W motor creates a certain wind speed related to the physical dimensions. And the generator speed is not limited to the rotor speed.

I'm just curious about the difference in costs between these two "machines."

by kjr63 on Thu Mar 3rd, 2011 at 12:57:48 PM EST
[ Parent ]

Re: Speaking of costs (none / 1)

A vacuum cleaner is not a reversible transducer, due to turbulence or something. I think it's similar to the feynmann story about sprinklers in swimming pools.

Oh, and vacuum cleaner motors are universal motors, which don't make very good generators. You generally want either an externally excited motor, a permanent magnet motor, or a synchronous AC motor for good generation capacity. These are more expensive.

And then there is synchonisation with the grid, stability, remote control, self protection in high winds, abrasion (the vacuum cleaner turbine is in a dust free environment - I once tried to use my mum's vacuum cleaner without the bag, with a second pipe from the blow hole out the window. It lasted about 5 minutes before it seized up.)

by njh on Thu Mar 3rd, 2011 at 06:49:45 PM EST
[ Parent ]

Re: Speaking of costs (none / 0)

Obviously generator and vacuum cleaner motor have a somewhat different construction. However, motors and generators are so similar by design that it is hard to believe significant cost difference between them. And naturally there are other costs than just motor. But so has a simple vacuum cleaner.

Anyway, Jerome's graph includes transfer price? That's half of the overall price, so the actual (end) production cost is in fact about 40e/MWh.

by kjr63 on Fri Mar 4th, 2011 at 02:09:18 PM EST
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Re: Speaking of costs (none / 1)

Cheap motors are cheap. But they don't make good generators.

by njh on Tue Mar 8th, 2011 at 05:02:43 PM EST
[ Parent ]

More support for merit order effect | 28 comments (28 topical, 0 editorial, 0 hidden)

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