BS: Wind turbine efficiency

Subject: RE: BS: Wind turbine efficiency
From: JohnInKansas
Date: 06 Dec 06 - 11:49 PM

While the flywheels at the Bitter Magnet Lab are 10 tons each, around 10 feet in diameter, take hours or days to spin up to speeds of a few hundred rpm and dim the lights in Cambridge while running up, they store a very large amount of energy, which is fed into the magnet coils to bring the flywheels to a halt in a few milliseconds. They "simulate" briefly directing approximately the full generating capacity of the Cambridge MA grid to one user.

There was a spurt of publicity several years ago though about flywheels for vehicle "load leveling" where they used fiberglass/composite flywheels encased in vacuum enclosures, generally with "dynamic bearings" of a couple of kinds, with peak speeds of up to 100,000 rpm or so. These flywheels were on the order of 10 to 30 pounds, and under about 100 pounds for the whole "storage assembly." A few prototype buses were built and were used in test/demonstration street use. Laboratory efficiency numbers looked good; but I don't recall ever hearing of a "final report" on the street testing; and haven't heard of any continuing advocacy for these schemes among transportation engineering companies or groups.

Someone mentioned flywheel bearing problems earlier. In either kind of flywheel storage system, there are adquate bearing designs, if intelligently used. (But good bearing designers are really scarce.)

There is a very real problem with containment in a failure for the casual experimenter, but with the lighter weight very high speed flywheels used in the vehicle tests the cases required to hold the vacuum were almost automatically provable (at least by analysis) as being adequate to contain the energy dump in case of failure. (Flywheels of this kind would be very inefficient due to "windage" without the vacuum.) The flywheel itself of course self-destructs in almost any failure, but for small amounts of energy storage fail-safe design isn't too much of a problem.

The amount of energy to be stored dictates whether the small high-speed flywheel or the massive low speed one is more efficient, and unfortunately the difference between energy storage requirements for a small bus (or single home) and storage requirements even for a local municipal power grid are like the difference between a flea-fart and a hurricane.

John

Subject: RE: BS: Wind turbine efficiency
From: The Fooles Troupe
Date: 07 Dec 06 - 05:01 AM

"pumping water uphill"

This is used in Brisbane - we have two dams ar different levels - at off-peaks the water is pumped back up, and power generated during peaks. Well, it was done when we had plenty of water in the dams before the drought...

Subject: RE: BS: Wind turbine efficiency
From: Schantieman
Date: 07 Dec 06 - 01:58 PM

Petr - what's an "Exojoule"?

I've heard of kilo- (10^3)), Mega- (10^6), Giga- (10^9) and Tera- (10^12). Is Exo- 10^15 ?   If so why do they use such a confusing prefix? - exo- normally means 'outside'

Steve

Subject: RE: BS: Wind turbine efficiency
From: JohnInKansas
Date: 07 Dec 06 - 03:52 PM

Steve -

From the NIST Special Publication 330, 2001 Edition, The International System of Units (SI)

Page 14

10²⁴ = yotta, symbol Y
10²¹ = zetta, symbol Z
10¹⁸ = exa, symbol E
10¹⁵ = peta, symbol P
10¹² = tera, symbol T
10⁹ = giga, symbol G
10⁶ = mega, symbol M
10³ = kilo, symbol k
10² = hecto, symbol h
10¹ = deka, symbol da
10^-1 = deci, symbol d
10^-2 = centi, symbol c
10^-3 = milli, symbol m
10^-6 = micro, symbol
10^-9 = nano, symbol n
10^-12 = pico, symbol p
10^-15 = femto, symbol f
10^-18 = atto, symbol a
10^-21 = zepto, symbol z
10^-24 = yocto, symbol y

For US users, at least (if I didn't make any transcription errors).

John

Subject: RE: BS: Wind turbine efficiency
From: JohnInKansas
Date: 07 Dec 06 - 03:55 PM

^#%@$!

Make that page 24.

John

Subject: RE: BS: Wind turbine efficiency
From: JohnInKansas
Date: 07 Dec 06 - 04:26 PM

Compressed looks a good solution, and you could probably replace the electric generators by compressors directly at the turbine shaft, though the pressure required to store a lot of energy sounds a bit frightening.

Unfortunately there are some "Laws of Thermodynamics" that say that compressed gas is a rather inefficient way of storing and recovering energy. Efficiency (thermodynamic) is usually better using larger volume at lower pressure, but the volumes required for "city scale" storage, at reasonable effectiveness, are immense. The volumes that can be accommodated for a vehicle, or even a single-house, dictate rather high pressures to obtain useful energy density. "Compression losses" followed by "expansion losses" make this a poor scheme in most situations.

Compressed nitrogen bottles are used for "emergency actuators" in many places, particularly in large aircraft. You can pack lots of energy into a small and relatively lightweight package; but the energy consumed to pack it is about 3 times what's contained, and the energy that is recovered (usable) when you fire the release is about one-quarter of what was "in the bottle." (From one specific, but pretty typical, design I'm somewhat familiar with.) Less "explosive" compression and recovery would do a bit better, perhaps.

A few moderate sized facilities have used "air bladder" type storage, where the energy is stored by moderate compression of air (sometimes another gas) contained in a sealed container, but the compression is done by pumping an "incompressible" fluid, often water, in and out. The thermodynamic losses are somewhat smaller for pumping liquids than for gases - usually. On a small scale, this method is used for many home well systems, with the pump putting "stored pressure" into a small reservoir, and the air pressure providing the "pumping work" to deliver the water to the tap as needed, avoiding the necessity of having the pump turn on everytime you draw a cup'a.

Larger ones may occupy an acre or so of ground, with dead-weight(often concrete) "roofs" to contain the pressure, and are used in a few places in lieu of a "water tower" to provide municipal water supply pressure, or for pumping the crud in sewage treatment processes.

John

Subject: RE: BS: Wind turbine efficiency
From: GUEST,MarkS
Date: 07 Dec 06 - 05:44 PM

This may be real blue sky, but perhaps the low power generated by small solar or wind stations could be used to crack water into hydrogen and oxygen, to be stored and then recombined (burned!) as needed for use as a fuel for transportation or heating.
Even small amounts, but generated over very long times could produce a usable amount of fuel with minor environmental impact.

Subject: RE: BS: Wind turbine efficiency
From: Cluin
Date: 07 Dec 06 - 06:11 PM

Many of us are familiar with the lasting effects of a good blow.

Subject: RE: BS: Wind turbine efficiency
From: GUEST,petr
Date: 07 Dec 06 - 08:29 PM

back in the 20s one scientist suggested building 1000s of windturbines
with electrolyzers to convert water in to hydrogen and oxygen.
so the idea isnt new. Electrolysis is probably not the best use of the energy as it is only 35%? effiecient.
WHich is why I think there may be other options to store the energy (eg. flywheel, compressed air, pumping water uphill, batteries etc.

One interesting project is the superconducting pipes which would allow use superconducting wires which would be encased in pipes containing super cooled hydrogen gas. This would allow for long distance transmission with low power loss to resistance as well as provide a storage medium for hydrogen. This kind of helps solve the hydrogen economy infrastructure problem. (Ie. the chicken or the egg of a network of hydrogen gas stations or hydrogen fuel cell ir ICE cars that run on hydrogen)

So theoretically any excess energy generated by wind turbine, solar,
tidal etc. can be converted to hydrogen with electrolysis and pumped into such a grid for later use.
They are already planning to build a test pipe.

OBviously there will be concerns about the dangers of failure of such a system - but there is a quote from an 1880 Congressional report on the invention of gasoline that puts things into perspective- it states that it would be very dangerous substance, would require all sorts of safety regulations for containment, and the subsequent move from a horse based economy would destroy Americas agriculture.

Subject: RE: BS: Wind turbine efficiency
From: JohnInKansas
Date: 07 Dec 06 - 09:03 PM

Under thorough analysis, at least at present, photovoltaic (pv) electrical generation isn't really a "low impact" method.

The fossil fuel consumption to produce current pv panels - the fuels burned/consumed directly in the manufacturing process and burned to generate electrical power used during manufacture - now exceed the fuels that would need to be burned in reasonably efficient combustion generators to produce all the electrical energy that the panel will produce in 20 years of using the panel. (Twenty years use is the current "state of the art" lifetime of a typical pv panel under good conditions, according to manufacturers. They tend to be optimistic.)

The pollution at the point of use of the pv device is low, but to get that "clean application" now requires that "somewhere else" quite a lot of pollutants were produced, and a lot of conventional fuels were consumed. The net result is that widespread use of pv devices may actually cause more "global pollution" and fossil fuel consumption than conventional on-site generation with conventional methods for the same amount of energy.

People are working very hard at improving the efficiency of the solar/electric conversion of pv devices, and at reducing manufacturing cost (mfg. cost is proportional to, if not closely equivalent to, "fuel consumed to make"); but for the present, pv devices really are useful only where the cost of transporting the fuel to remote places would be excessive, or where no suitable fuels are available at any cost.

John

Subject: RE: BS: Wind turbine efficiency
From: JohnInKansas
Date: 07 Dec 06 - 10:15 PM

The direct conversion of low-density electrical energy, from photocells, wind, or other "locally clean" sources to hydrogen for storage until the hydrogen is needed for fuel sounds pretty simple, but there are some drawbacks.

Gaseous hydrogen is incredibly difficult to store and extremely dangerous to handle.

There are several methods of storing hydrogen that various people tout as effective. A popular one has been to combine the hydrogen as a "metal hydride" in storage. Since there's no existing way to pump the metal hydrides through a pipe, this still requires handling gaseous hydrogen at least for fuelling and within the engine for combustion. This might be acceptable for a vehicle, but does nothing for the storage/transport problem.

A disadvantage of metal hydride storage especially in a vehicle is that the entire storage volume must be operated at high temperatures to release the hydrogen for burning. As a "personal opinion" either large high pressure inert gases or very high energy flywheels likely would be safer for any large-scale public use, especially for uses such as in vehicles.

The former Beech Aircraft company started a project to design a "hydrogen powered automobile" using metal hydride storage sometime ca. 1960, and the project was still running (after they became Raytheon Aircraft), without notable success, at least ca. 1998 if information overheard then was correct. A problem is that hydrogen is "very low octane" and tends to explode rather than burning smoothly, so small parts of the engine kept getting propelled into strange places, or mangled and bent inside the engine. (At least ca. 1970 - '74 when I had occasional "direct observation" contact with the project.)

The more "modern" idea for hydrogen burning engines seems to be "generating" the hydrogen directly from petroleum fuels. Lots of people are working on this concept, but few "hardware examples" have appeared outside laboratories. While this may result in lower emissions from the hydrogen burning device, there is at present no efficient way to directly produce more complex hydrogen-bearing molecules (hydrocarbons) from low density electricity, so it doesn't help the "store it as hydrogen" idea.

The "flying engine parts" that Beech/Raytheon encountered illustrates a difficulty of handling/transporting hydrogen as a gas. When compressed enough to provide useful expansion during burning, it has a tendency to detonate (engine knock). When compressed enough to be efficiently pushed through a pipe, it has a tendency to detonate, especially with very low amounts of oxygen intermixed, under any condition that can cause ignition.

Some years ago, ca 1950s(?), a few natural gas "pipeline explosions" occured that were quite spectacular. One or two destroyed more than 100 miles of pipe each.

Increasing the pressure of a gas increases the speed of sound in the gas. A detonation travels at approximately the speed of sound in the gas. A fracture under stress of the pipeline material cannot travel faster than the speed of sound in the material from which the pipe is made.

If the pipe is strong enough to contain the pressure at which burning of the gas transitions to detonation (produces a shock wave), any "ignition" will raise to pressure inside the pipe to the detonation point in the vicinity of the combustion/detonation front.

If the speed of sound in the gas (the speed at which the "explosion" travels) exceeds the speed of sound in the (usually metal) pipe, the shock wave stays ahead of the crack that would release the pressure enough to lower the burning condition below the "detonation transition pressure" so the explosion and the crack both travel to the end of the pipe, exiting potentially (it was suggested) where it blows your kitchen stove through your roof. Being a rather light gas, the speed of sound in moderately compressed hydrogen is "pretty fast."

Natural gas pipelines now have "interrupters" at frequent intervals, with valves that shut down all of the valves on the pipeline if pressure at any point on the line drops, and in some cases vent a few sections on both sides of where the drop was detected. Rupture points are built in that will allow the pipe to break and (hopefully) release the gas pressure locally before it rises high enough to allow the natural gas to produce a sonic shock. Working on natural gas pipelines is still considered a "hazardous occupation" even thought it's very much safer than any easily conceived hydrogen pipe.

There is no pressure at which gaseous hydrogen can be pumped efficiently through a long pipe that will not result in detonation if any ignition accident occurs, if even minute amounts of oxygen are available; and perfect exclusion of oxygen is extremely difficult.

John

Subject: RE: BS: Wind turbine efficiency
From: The Fooles Troupe
Date: 08 Dec 06 - 05:59 AM

Theoretical Power is much easier to achieve than Practical Power, as both Real World Engineers and Politicians eventually discover.

Subject: RE: BS: Wind turbine efficiency
From: Schantieman
Date: 08 Dec 06 - 11:15 AM

Wow, John