Discussion in 'Offtopic Discussions' started by Wally, Apr 14, 2018.
coal still sucks, so ...
A recent media report here focused on how Chinese companies are 'helping' set up coal power stations in eastern european countries like Serbia.
But yeah, Germany's dependence on coal doesn't seem to have dropped off like it should according to their decarbonisation plan. It did drop in 2004, but then started climbing again, and besides, it only dropped to mid 90s levels of fossil fuel use, which is hardly impressive. Their power from nucelar reactors has dropped off as they decomissioned old ones, but that has only been matched by energy from renewable sources, leaving the dependency on coal and gas basically unchanged.
The big advantage of gas, as I understand it, is that gas turbines are quick and easy to start and stop. If it's a windy day, you can't just turn off the coal-fired power stations and let the country run off wind. Nuclear is even worse
Better, of course, would be storage - but batteries are only barely worth it, and there aren't that many good places to build bidirectional hydro stations.
The odd thing is that the once-common "economy" tariffs in the UK are now almost unheard of. The idea was that the 'leccy company would install a second meter (billed at a cheap rate) and a time switch (set to turn on off-peak). You could then connect power-hungry, but time-ambivalent appliances, often heaters, to the cheap meter, and help even-out the demand spikes.
Nowadays, it would be possible to have a switch that came on, not at fixed times, but based on unscheduled commands from the utility company. The consumer could be guaranteed a certain on-time per 24h, but the supplier would be able to toggle a substantial part of the load on and off in small increments as required.
Of course, if people don't have that many "as and when" appliances, that doesn't work...
You can still do that apparently; the electricity company in many parts of the world (AIUI) vary the frequency of the mains depending on demand, so that it only averages 50Hz in Europe and 60Hz in America. I'm not sure if over frequency means there's more or less demand though, and presumably the opposite to whichever way round that is is what under frequency means.
At present though, they don't charge you less for using more of your electricity at low utilisation times, but companies are building water heaters and electric car chargers based on this little trick, I've heard.
I thought the varying frequency was an unintended result of imperfect load-demand matching, rather than a deliberate ploy.
Loads are organised to appear resistive - this means that as much of the current in the grid as possible does something useful
As such, it doesn't really matter what frequency you run them at; the power will be the same
A large proportion of generation is done by turbines attached to synchronous generators.
These turn at an exact fraction of grid frequency, and are electromagnetically kept in phase
That is, it is physically impossible for the generator speed and the grid frequency not to match
Sometimes, load outstrips supply
So consumers are taking electrical energy faster than generators are generating it
How can that work? Where does the extra power come from?
From then inertia of the generators!
Slowing down every big turbine in the country (by 0.3% or so) frees up a lot of energy, which makes up for the shortfall
As that happens, automatic control systems notice that everything has slowed down, and react
They increase the supply of steam/gas to the turbines, and their power increases
Now the generator power is slightly more than the load, and the generators speed up again
The whole thing is carefully controlled to make that frequency 'wobble' as small as possible
But you can never completely eliminate it, because it takes a finite time to adjust the output of each power station
there is also the concept of windgas, which sounds pretty good/green.
A slick video, though I'm not entirely sure about "Hydrogen can be stored easily (1:13) ... free of losses (1:47)"
I'm also a bit surprised that putting hydrogen into the existing (methane) gas supply pipes would work out as easily as they imply. If you have an equal volume-flow rate of hydrogen and methane at equal pressure and temperature, my arithmetic says the hydrogen will need a quarter as much air, and yield a fifth the energy. That means all your gas boilers (etc) will need redesigning.
Carrying hydrogen around in road tankers is a questionable idea, too, since:
Bottles strong enough to contain it (or chemicals that absorb it) are heavy and bulky.
The energy used getting it into the bottle (Low temperatures? High pressures? both?) is substantial, and difficult to recover at point of use.
That's not to say the whole scheme wouldn't work, but its hardly a magic solution to all our energy needs!
You may well be right. All I know is that users of significant amounts of electricity are using it to time when they use the most power.
These are electric turbines that consume the electricity I assume?
This doesn't sound like an electric turbine now. I am confuse.
Is this sentence backwards? I'm assuming a dependent 'if this then this' type construction, so it would make more sense written backwards; Now the generators speed up again, the generator power becomes slightly more than the load.
Having thought on this slightly more, I think they key might be that any significantly reactive load (those with a high power factor, it's called apparently) will shift the mains phase either forwards or backwards depending on whether it is capacitive or inductive. Apparently, those metal cabinets about three feet tall that you see in suburban streets are filled with capacitors and designed to resist the inductive load that generally comes from households I assume.
That wouldn't shift frequency though, just shift the phase. Perhaps if enough reactive loads come online in a sequence, that would present as a bump in frequency though. And in any case, you'd only be detecting loads this side of your local power factor corrector cabinet. But I guess that's what you want to do if you want to stop your local substation going down as a big puddle of melted steel and aluminium.
Stones instead of Batteries.
One Issue whit Hydrogen is: Ditnt somewhone Remember what happens to the "Hindenburg" ??
This Stuff is Quite Flamable, so its have to be quite save Tankers to transport..
Mybe a Solution, even if lots of Peaple dosnt agree whit me:
How about a Wind Energie Plant on every Neighborhood, and Photo Voltaic on every Roof ??
Then you can Make you own Electrical Power, mybe put some Chargin Station in your Garden that you use to sell your Power to other peaple whit elektrical cars:
No need for more Nuclear Plants for the Electric Car, the Power that the Sun gives to us for free want lost un used
and our cars ditnt need big batteries because you can recharge it on every House..
Hear in Germany, there is a lot of Protest against Wind Power Plants, the peaple have fear for the noise and the big shadows of this wind mills,
but isnt the living in a world whit climate change much worse than the living near such a big ventilator ??
By turbine, I mean a device that produces mechanical motion from flowing gases (or liquids). The exact inverse of a pump.
No, the steam turbines in power stations that turn the generators. The steam pushes the turbine blades round. Note that slowing them down frees up energy, not power. (Although releasing energy can be described in terms of power)
Coal fires, nuclear reactions (etc) boil water, the steam escapes through a valve into a turbine, which spins. The turbine is mechanically coupled to a generator, which is wired to the grid.
Energy is stored in the motion of the generators and turbines in the power stations - a bit like giant flywheels.
If the energy consumed (houses, factories) is less than the energy supplied (power stations), conservation of energy says the excess has to go /somewhere/ - and that somewhere is the motion of the turbines & generators themselves.
Okay, now I'm confused where you seem to be saying that because demand is outstripping supply, you slow down the turbines.
The turbines are both a converter of energy, and a store of energy
A turbine spinning quickly has lots of a energy stored as motion
A turbine going a little slower has slightly less energy.
If a turbine's speed is declining, then its output power can be more than its input power.
Agreed; anything spinning will have residual angular momentum.
The input power is irrelevant to this particular equation I think; we need the output power to match the load, largely regardless of what any input is (although thermodynamically you want that to match over time as well for obvious reasons).
I also suspect talk of speeding up and slowing down turbines is largely offtopic in the field of electricity generation. They need to keep up to speed in order to maintain synchronisation with the grid, assuming they're AC generators. I'd have thought power would be controlled by how many turbines you run in parallel, but I'm not a particular expert in this field, just someone who's read a bit of the internet.
Precisely my point!
The grid frequency slows down because every turbine-driven synchronous generator attached to it also slows down!
Earlier today it reached 49.9Hz. At that point, every big generator was running slow - at 49.9*60 = 2994rpm
Why were they running slow? Well, suppose one generator's rotor is a uniform cylinder with a mass of 250t, radius 2m, and suppose there are 40 of those in the country. Suppose 100,000 people suddenly decide to turn on their 2kW electric kettles at teatime. We need another 200MW of power to the grid instantly.
Even highly responsive power stations (hydro is pretty good) don't do 'instant' - so the only way to generate the extra power at such short notice is to tap into that reserve:
Moment of inertia of each rotor = 0.5mr² = 0.5 * 250E3 * 4 = 500E3
Total moment of inertia for all the generators = 250E3 * 40 = 10E6
At 50.0Hz = 3000rpm = 314.0rad/s
Kinetic Energy of all the rotors = 0.5 * 10E6 * 314² = 493E9
At 49.9Hz = 2994rpm = 313.5rad/s
Kinetic Energy of all the rotors = 0.5 * 10E6 * 313.5² =491E9
So the difference in kinetic energy is 2Gj
If it took 10s to slow all those generators down from 3000rpm to 2996rpm...
...that would mean an extra 2GJ / 10s = 200MW being fed into the grid.
That means that the computers in the national grid control centre have 10s to find another 200MW of generating capacity and bring it online. As soon as they do, the generators stop slowing down and the grid frequency stabilises.
There! Have I sufficiently derailed this thread from politics yet?
I think this must fall into the 'whatever' category more than direct politics, but it still fits by my reading.
I think we're nearly on the same page now. The only trouble I'm having is understanding why slowing the rotors gives extra current. The mass of the rotors spinning is all potential energy, and you only get it out when you stop feeding steam into the turbine end, and it's a diminishing power source. I suspect you might have caught when everyone was brewing up their kettles having gone off and were about to enjoy a nice hot cup of tea or coffee; they'd brought up slightly too many turbines while the load was higher, so they slowed them all slightly while they turned the extra ones off.
Another (totally different) topic:
One of my favorite Kurzgesagt videos came out recently:
For the german speakers:
thanks for thinking about solutions for saving our climate/planet! we definately need more wetware/thords from great minds as you are for creating a (better) future.
maybe we should create an own thread for this?
... next, there is this other problem (not only) here in germany, nazies.
Failed State BRD (google translated)
pretty spooky read!
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