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My question might seem a simple one, but it has been troubling me for a while. How many hours in a day do conventional electrical generators work (the ones in large power plants)?

Do they by any method conserve or store the power before it reaches the consumer?

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    $\begingroup$ Why do you think they can't operate 24/7? Most base loaded power plants run all day, every day except for scheduled maintenance periods (typically several weeks a year) and occasional unscheduled outages. $\endgroup$ – Mark May 18 '16 at 16:50
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    $\begingroup$ I've rolled back the recent edits. I wasn't an easy decision. But the edits turned it into completely different question, rendering the existing answers invalid. The later question you asked is also valid, and on-topic here, and worth asking in its own right as a separate question. $\endgroup$ – 410 gone May 18 '16 at 19:53
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There isn't currently any practical means of storing electrical energy on a large scale. There are technologies which can store energy for specific applications but certainly nothing which would have much of an impact on national level distribution.

For obvious reasons, in a distributed power grid the power demand will vary considerably throughout any given day and over the course of year and a lot of analytical effort goes into predicting this demand in great detail, even to the extent of keeping track of popular television programmes.

Different types of generating plant has different abilities to match demand depending on how quickly it can be turned on and off.

In general terms steam turbine systems (eg nuclear, coal, oil fired etc) are not easy to shut down and tend to stay on more of less permanently, only being shut down for maintenance, these provide the base load above which demand fluctuates.

Gas turbines are a bit more agile and can be switched on at relatively short notice to meet peak demands, the downside being that this can be a more expensive way to generate electricity.

Another option is for neighbouring countries to buy power off each other to help to balance supply and demand across a wider geographical area. There may also be schemes for industrial users with high energy demands to stop or reduce production at peak times in return for a discount.

It's also worth noting that effective prediction and management of energy supply and demand is a big factor in the overall efficiency of energy usage.

Similarly it is usual for countries (or equivalent geographical areas) to have integrated electrical distribution networks such that all of the major power plants are contributing to a grid system. This helps to ensure redundancy and flexibility in meeting demand so that if one generator goes offline for scheduled maintenance or unexpected failure the slack can be picked up by reserve capacity elsewhere. Obviously in very large areas this may be broken down into regions but conversely there is a lot of buying and selling of electricity throughout Europe.

Obviously there are a lot of variables here and changes in government policy in terms of taxation, subsidy and general regulation will have a significant effect as well as local conditions of geography and energy usage.

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  • $\begingroup$ Quite a few claims here are incorrect. Pumped hydro storage is a well-established grid-scale means of storing electrical energy (temporarily as gravitational potential). Electricity from gas is often cheaper than that from coal - and it changes over time, as the prices of fuels and carbon vary. Both coal and oil generators can be (and indeed are) run as peak or mid-merit as well as baseload (in yesterday's jargon). $\endgroup$ – 410 gone May 18 '16 at 20:02
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Nuclear power plants are always working, except circa one month/year for maintenance. It's mainly because stopping the nuclear reaction and cooling the reactor down is a very long and difficult process (a few weeks). So "turning on" and "turning off" a Nuclear Power Plant (NPP) is not as simple as pushing a button.

The primary circuit (containing the water heated by the nuclear reactions) is always connected to the secondary circuit (which is a classical Rankine cycle), and the steam of the secondary circuit, which is used to make the turbines turn, and therefore the alternator, thus producing electricity.

As far as I know, the processes for stocking the electricity produced by the NPPs are very limited. I know that there are some kind of big batteries, but their capacity is limited.

This is a big problem in my home country (France), where 75% of the electricity is produced by nuclear energy (we are the most "nuclearized" country in the World), and the electrical consumption is - obviously - much smaller in August than in December.

The consequence is that in August, we produce about 70 GW of electrical power and use only 55 GW of it, and therefore we are forced to sell electricity to other countries because we can't stock it.

For the renewable sources: I know that for photovoltaics (PV) systems, there are small systems of batteries. This is much simpler because the power is way smaller than nuclear power plants (about 1-10 kW by PV system). Moreover, the intermittency of photovoltaics (you don't produce electricity at night, while night is the moment when you consume the most) make it really necessary.

For windpower, you can read a bit about CAES (compressed air energy storage), which is a very promising way to stock energy.

And for thermal power plants (fuel, charcoal) and dams, as someone said previously,

the energy is stored before it reaches the consumer - but that storage happens before generation of electricity. Either in the form of fuel stores, or in a hydro reservoir.

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  • $\begingroup$ Why is France forced to overproduce and sell the excess at certain times of the year just because it's hard to turn a nuclear power plant off? Surely it's possible to turn it down by about 20% without turning it off? $\endgroup$ – Mason Wheeler May 18 '16 at 19:32
  • $\begingroup$ @MasonWheeler I don't know either, I've been wondering about the same question. Last summer (2015) I've been doing an internship at EDF (French electricity distributor), and I was at the service which manages all the nuclear power plants. One day, my tutor showed me the figures I mentioned above, but I need to precise that it was the 15th of August, so undoubtedly the least-consuming day in the whole year. The situation is less critical on other days. $\endgroup$ – R. Bourgeon May 18 '16 at 19:52
  • $\begingroup$ OK, maybe I'm missing some cultural detail here, not being French, but why is the 15th of August "undoubtedly the least-consuming day in the whole year"? $\endgroup$ – Mason Wheeler May 18 '16 at 20:11
  • $\begingroup$ Because it's a day-off (Assumption of Virgin Mary, we still have those catholic holidays in France) and it's in the middle of the summer, so people are either on the road coming back from vacation, either spending time with their family, or other things that do not consume electricity. This "holiday effect" adds up to the fact that it's in the middle of the summer, during which electric consumption is already low. $\endgroup$ – R. Bourgeon May 18 '16 at 20:17
  • $\begingroup$ Ah, that makes sense. :) $\endgroup$ – Mason Wheeler May 18 '16 at 20:18
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There isn't a single answer that covers large power plants around the world. There's way too much variation in generation plant, and way too much variation in the way that plant is used.

Some plants will operate for just a few minutes occasionally.

Some will run 24/7 except when closed for maintenance.

And yes, absolutely, the energy is stored before it reaches the consumer - but that storage happens before generation of electricity. Either in the form of fuel stores, or in a hydro reservoir.

And in general, such generators are part of a grid that might span a region of a country, or even several countries - and the grid pools both generation and consumption. So just because you've got electricity, does not mean that the generator closest to you is working: your electricity might have travelled a thousand km to get to you.

And yes, absolutely, the energy is stored before it reaches the consumer - but almost all that storage happens before generation of electricity. Either in the form of fuel stores, or in a hydro reservoir. There is some on-grid electricity storage too, but this is absolutely tiny in comparison (several orders of magnitude). It's almost always in the form of pumped hydro storage - so surplus electricity is used to lift a large volume of water a few hundred metres; the water sits in a reservoir; and is then released through turbines later to generate electricity again. This is about 80% efficient. And it's very rarely sited with a generator - it's sited where there are some suitable mountains (or, in at least one case, coastal cliffs).

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  • $\begingroup$ This is inaccurate. $\endgroup$ – Carl Witthoft May 18 '16 at 12:09
  • $\begingroup$ @CarlWitthoft Please elaborate; what part(s) and in what way(s)? $\endgroup$ – Air May 18 '16 at 16:44
  • $\begingroup$ @Air There are post-production storage methods, which is what I suspect the OP was after. A plant that "run[s] 24/7" is not well-parametrized, since it could be running at various load levels. $\endgroup$ – Carl Witthoft May 18 '16 at 17:05
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A typical conventional power plant may consist of multiple generating units rather than one big huge unit. By having multiple units, the plant is better able to respond to changes in demand during the day or to shut down generating units for maintenance.

By having power distributed by a sophisticated electrical grid, if a plant in one region of the country must go off line, power can be purchased from other operators and sent to the affected area over the power grid until the local plant can come back on line.

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  • $\begingroup$ What I understand is that when there is a shortage of energy production in a particular region, it is compensated by the other plants connected in that grid.Am I right?And how large is this electrical grid?(connected across the whole country or in a particular smaller region) $\endgroup$ – Adnan Arif Sait May 19 '16 at 5:08
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This answer is with reference to power generating plants which feed the grid, not "small" generators which just power a local building. The latter, of course, can be turned off when there's no demand.

In general, power plants which use hydropower can be 'turned up' or 'turned down' relatively easily by shunting the water flow.

Power plants which depend on local heating, especially by burning fuel, however have extremely long ramp-up and ramp-down times (many hours), so by and large they end up dumping excess electrical power at night or other times of low demand. There are a few plants which try to recover some of this by doing things like using the excess electrical power to pump water into a reservoir, and using that water to generate power during peak demand.

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    $\begingroup$ Gas turbines are frequently used in peaking applications to meet short-term high loads. Many older coal plants have been switched from baseload to cycling operation by dropping load at night. $\endgroup$ – Mark May 18 '16 at 13:44
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How many hours in a day do conventional electrical generators work (the ones in large power plants)?

Short answer; it depends on the constraints of the generators and the priorities of the system operator.

The underlying area is known as Unit Commitment:

A family of mathematical optimization problems where the production of a set of electrical generators is coordinated in order to achieve some common target, usually either match the energy demand at minimum cost or maximize revenues from energy production.

Assuming that you are referring to thermal generators, the constraints include:

  • minimum up/down time
  • ramp up/down rate
  • modulation/stability
  • start-up/shut-down ramp rate

Do they by any method conserve or store the power before it reaches the consumer?

No. At least not over the time frames that unit commitment is considered (of the order of hours/half hours).

Over much shorter timeframes (seconds) energy is stored in the rotating mass of the generators. This is described in answers to this question.

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