Normally DC sources are saved in storage device. So if it is not used, they can be used later or saved up for later use. But I have never heard of the power plants storing AC current. I suppose they produce AC instantaneously. When AC electricity is produced and if nobody used it for one hour? Is the energy spent on the production lost. At least in terms of cost.

How do they manage the production so that for example they don't waste too much coal and nobody uses it. Do they produce variable amount during different time of the day according to the requirement?

 

A generator only produces as much power as is demanded by the grid, it never produces excess power so there is no reason to save it. A generator generates voltage with the current determined by the load. If there is no load then the generator provides no current and the power output is zero.
If no power were consumed by the grid then the generator would just free-wheel with only frictional power being required from the prime mover.

 

Normally DC sources are saved in storage device. So if it is not used, they can be used later or saved up for later use. But I have never heard of the power plants storing AC current. I suppose they produce AC instantaneously. When AC electricity is produced and if nobody used it for one hour? Is the energy spent on the production lost. At least in terms of cost.

How do they manage the production so that for example they don't waste too much coal and nobody uses it. Do they produce variable amount during different time of the day according to the requirement?

Put in simpler terms, find a simple DC motor. Spin it. It spins easily. This is a generator. Add a small LED across the leads of the motor. Spin the motor again. If it spins easily, reverse the LED or spin the motor in the reverse direction. You will see that it gets hard to turn now that you have a load to turn. So, the power plant adds just enough coal to keep the turbine spinning. It doesn't take much steam pressure to turn a turbine if there is no load (everyone has their lights and air conditioners turned off). It takes a great deal of energy to spin the turbine if everyone is running their lights and air conditioner.

The only buffer in the system (buffer is a time lag), is that of the steam. It is heated and pressurized by the coal and pressure released when it drives the turbine. This buffer is generally on the short second to fraction of a second time-scale. Over a big grid, power demand ramps up and down through the day so there are usually no instantaneous changes and that fractions to a few-second time lag and the valves, baffels, expansion chambers, valves and pressure regulators are good enough for power plant operators to keep everything running at 50 or 60Hz (depending where you live).

 

For hydroelectric generating facilities they do save "excess" energy. To store AC power, they pump the water back uphill and store it in a reservoir for later use.
Why would they do this? They do this because they can generate power day or night for about the same cost. At night, when local demand is low, they can sell the excess energy for a higher price into a market that needs it. Or, they can save save it for later and sell it to their local customers during the day for a higher price.

 

the power companies are pretty good at keeping records of how much power is used during the day, and starting up "peaking" generators at peak use times. they are pretty good at running enough generating capacity to keep everything running smoothly during the day and night,. its all in the record keeping.

 

For hydroelectric generating facilities they do save "excess" energy. To store AC power, they pump the water back uphill and store it in a reservoir for later use.
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They may do that to store energy from other sources of energy, such as wind, but it makes no sense to do that for hydroelectric since they just reduce the water flow when less power is needed.

 

Put in simpler terms, find a simple DC motor. Spin it. It spins easily. This is a generator. Add a small LED across the leads of the motor. Spin the motor again. If it spins easily, reverse the LED or spin the motor in the reverse direction. You will see that it gets hard to turn now that you have a load to turn. So, the power plant adds just enough coal to keep the turbine spinning. It doesn't take much steam pressure to turn a turbine if there is no load (everyone has their lights and air conditioners turned off). It takes a great deal of energy to spin the turbine if everyone is running their lights and air conditioner.

The only buffer in the system (buffer is a time lag), is that of the steam. It is heated and pressurized by the coal and pressure released when it drives the turbine. This buffer is generally on the short second to fraction of a second time-scale. Over a big grid, power demand ramps up and down through the day so there are usually no instantaneous changes and that fractions to a few-second time lag and the valves, baffels, expansion chambers, valves and pressure regulators are good enough for power plant operators to keep everything running at 50 or 60Hz (depending where you live).

Thank you so much for your elaborate and tiny details. This answers a lot of question that followed as well. Good people on this forum.

 

They may do that to store energy from other sources of energy, such as wind, but it makes no sense to do that for hydroelectric since they just reduce the water flow when less power is needed.

This is the generating facility I was thinking of:
http://www.xcelenergy.com/Company/O...ation_Stations/Cabin_Creek_Generating_Station

More information here:
http://www.colorado.gov/cs/Satellit...goBlobs&blobwhere=1251898265069&ssbinary=true

It was built well before wind and solar were available nearby. They say that it will help load level wind power but I am a bit dubious since the unit is at 10,000 feet and I don't think there is a source of wind anywhere near by.

 

The first one I ever heard of was the Courtright/Wishon Reservoir in California.
Wikipedia's description

During times of peak demand for electricity, which is also when it is most expensive, water is drained from Courtright Reservoir, run through the 1,212 MW Helms Power Plant and empties into Wishon Reservoir. When demand and prices for electricity are low, water is pumped from Wishon Reservoir to Courtright Reservoir using the power plant's reversible turbines.