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What happens to excess electricity?
Old 12-03-2010, 06:39 PM   #1
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What happens to excess electricity?

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Originally Posted by ERD50 View Post
One of the advantages of EVs is you can charge them overnight when the utilities have excess capacity. I've read (not sure it's true) that in the 'wind down' phase they actually produce excess as you just can't slow down a thermal process that quickly without stressing everything. That is actually 'free' electricity from an environmental standpoint.
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Not wanting to hijack the other thread, I'm starting this one.

I have heard of excess capacity too. So what really happens to it? Does it turn into heat, noise or what?
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Old 12-03-2010, 07:43 PM   #2
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In any electrical system, the generator output always equals the load, up to the capacity of the generator (with some exceptions that I'm sure will be pointed out by others). So, as an example, if there are only ten light bulbs on at one time, the generator makes enough electricity to power those ten light bulbs, but not more. There is really no "excess" electricity floating around looking for a home. In fact, electricity is unlike other energy sources in that it cannot readily be stored. (There are such things as batteries and pumped storage, but they are limited in capacity). The phrase "excess capacity" really means that the generator "could" generate more electricity if there were more load.

To better understand the concept, think of the situation on a hot summer day during the week -- everyone is running their air conditioner, factories are using electricity and the generator is producing as much electricity as it possibly can. If more people wanted to run their air conditioner, you would have to pay someone to start up another generator and you would pay a high price for that additional capacity. Under that condition, electricity is expensive.

Now think of a balmy spring Sunday night. No one needs air conditioning, there is no factory usage and, hence, the generator is easily capable of supplying all the needed electricity. In fact, it has "excess capacity" because it could generate more electricity at virtually no additional cost, since it is already spinning. At such times, electricity is cheap.
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Old 12-03-2010, 08:27 PM   #3
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Gumby covered it well. I'll just add from a different angle.

As stated, the electricity itself isn't wasted. But (as I understand it), they can't just cut the flames on coal plants, they need to change slowly (hours?) to avoid thermal stresses. So the heat is going to waste (out the chimney I guess), and the generators just spin along producing what is required.

Think of it this way - that electrical socket in your house with nothing plugged in isn't wasting electricity. There is the capacity to provide it with power, but not using it doesn't waste it (maybe a little). But they still have to spin generators to provide the capacity. If everyone unplugged everything, they would still need to spin generators to provide a baseline voltage and to be ready for when someone plugged something in. But that would be minimal power if they could count on everyone having everything shut off at a given time, and then they could boost the flames if they knew more would be required at a different time.

That's pretty much off the top of my head, I might do some research later, as I get more curious the more I think abut it, but I'm pretty sure that's mostly correct. I would think (and maybe they do), that electric companies would offer really low rates at those times to industrial users. Maybe they could run freezers, heaters, air compressors, pumps or other equipment that stores energy in some form during at that time and save money (and the electric company wins by reducing peak capacity).

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Old 12-03-2010, 08:36 PM   #4
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Not wanting to hijack the other thread, I'm starting this one.
I have heard of excess capacity too. So what really happens to it? Does it turn into heat, noise or what?
The utility's electrical generator is usually cranked by a steam turbine or a gas turbine, rarely by a diesel engine, and possibly by water flowing over the dam. In all of those cases the generator turns at a speed to produce 60 Hz at some rated output (say 450 volts). Once the operator (or the controller) sets this up the system is at equilibrium-- generating as much power as everyone is using.

When someone turns on their air conditioner or refrigerator, it loads down the grid and puts more demand on the generator. The generator slows down and causes voltage/frequency to droop. The controller cranks open the throttles to speed the generator back up to its rated output. When the air conditioner turns off, the grid is using less electricity and the generator spins faster... so the throttles shut a little. The grid's bus voltage/frequency stay more or less constant as the equilibrium shifts a little.

The utility keeps a spare generator or two just idling at standby (or ready to be started up in a few seconds). When the first generator approaches max rated output then the second generator is added to the grid in parallel to share the load. If this isn't done correctly then there's a brief voltage/frequency transient. If it's done badly then you might even notice the power hit on your house lights or electrical appliances. When I was on a submarine in charge of the sensitive electronics in weapons fire control systems, I used to dread "electrical operator training". Based on what I see in my house these days, I'm convinced that HECO does a lot of this training between 6 AM and 7 AM on weekday mornings.

The Mainland electrical grid is tied into three or four main monster networks (I forget how many exactly) generating gazillions of kilowatts. Very little electricity is actually stored because most of the time the various utilities are able to share the loads among themselves to maintain voltage/frequency without too much trouble. One household is a tiny fraction of a percent of a generator's capacity while an entire town might approach 5-20%. If everyone in town starts cranking their air conditioners at 1:30 PM then the town utility is usually able to get its own extra generators spinning in time or at least to temporarily buy a few thousand kilowatts from another utility elsewhere on the monster grid. If your house voltage/frequency droop a little as this shifting is going on then you may never even notice, especially if it's less than 5%.

One option that a utility can use on a hot day (or when they have mechanical breakdowns) is euphemistically called "load shedding". They just turn stuff off-- like a building or an entire city block. This can also turn into a rolling brownout as they drop power across the grid in order to temporarily reduce loads. This is awful hard on most electronics and costs as much in damage/lawsuits as it saves in power-generation reliability.

HECO actually pays its customers to volunteer for load shedding. They attach a pager to a controller on the homeowner's water heater, and they've done this to tens of thousands of homes. If they need to shed load they'll phone a single number to all those pagers, which will simultaneously open the breakers to the water heaters. It's a distributed hit to the grid so it doesn't disrupt the generator controller too much, and it reduces load by a few percentage points to give HECO the time it needs to come up with more generators or other solutions.

The grid-control problems happen when the grid is small (like Hawaii) or the transients are large (like a bunch of wires on a transmission tower overheating, drooping, and grounding out all at once). That can produce a transient as much as 15-20% of the grid bus voltage/frequency. You'll definitely notice that at home as generator controller systems cycle wildly to keep up with the transients. If the grid design can keep up with the transient then it'll eventually dampen it out, but usually the transients occur far more quickly than controllers can handle (or extra generators can be put online). In that case everyone hopes that the grid is big enough to absorb the hit and spread it out among all the other utility's generators-- like throwing a big rock into a small pond and watching the ripples spread across the water.

Worst case is that a voltage/frequency transient causes a generator to overspeed (as the load vanishes) and trip offline. A couple years ago a lightning storm hit an Oahu transmission tower and blasted its hardware to smithereens, dropping a fairly large load in a few microseconds. The transient oversped HECO's generators and tripped them offline, so the whole island went dark.

It'd be really nice to store energy in a gargantuan flywheel or battery, but we've already discussed the shortcomings of those systems in other threads. It's inefficient & expensive, too-- there are losses (friction, heat, and mechanical) caused by stuffing the energy into these storage systems, and the storage systems have their own leaks. There are just as many more losses transferring the energy out of the storage device and turning it back into electricity. Smaller energy-storage devices include pumping water upstream into reservoirs at night and generating hydroelectric power during the day. Some commercial buildings will make ice at night and melt it into their air conditioning system during the day.

Storage is one of the big design problems with solar power, especially on a small grid like Hawaii that wants to generate 70% of its power from "alternative energy" by 2030. Right now HECO and the neighbor-island utilities are squabbling with the photovoltaic industry over raising the PV limit from 10% to 15% of the total grid size. The problem is cloudy weather. When the PV panels are generating power on a sunny day then everything is just fine, but when clouds blow across the sun then the power vanishes in a few seconds and the utility's generators have to cope with a fairly large (10-15%) transient. A lot of effort is going into designing advanced controller systems to keep up with this problem.

A local energy company, Sopogy, has designed a system that stores heat energy. Their solar arrays heat oil, not electrons, and the oil drives a steam generator turning a low-pressure turbine to generate electricity. When there's more sunshine than demand, the extra hot oil is diverted into a large (20-foot-tall) thermos flask filled with salt. The hot oil heats up the salt and melts it, and the thermos flask is heavily insulated to minimize heat loss. At sundown when the solar array isn't heating any more oil, Sopogy still gets another hour of power from the thermos flask as the molten salt heats the leftover oil that's pumped through the turbine.

Energy-storage technology is so far behind rudimentary that it's easier/cheaper to design advanced grid control systems, or even to try to build new & bigger grids. The owner of most of the island of Lanai, David Murdoch of Dole, has built one of the world's larger photovoltaic farms on old pineapple plantation land. Next he's going to spend a few billion dollars building an underwater transmission line from Lanai to Oahu to deliver PV power to HECO. In 3-4 years we'll know how well this is going to work out...

Here are a couple links on the "good" vs "evil" sides of the proposal:
http://www.sandia.gov/segis/IEEE%20P...esentation.pdf
Island battery: Is supplying 10 percent of Oahu’s power worth destroying Lanai? - The Hawaii Independent : News Culture Community
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Old 12-03-2010, 09:11 PM   #5
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Only one nit -- sometimes on a large commercial generator, the throttles are fixed and the generator rpm is fixed. As load is added, the DC excitation field is simply increased to bring the output AC voltage back up to spec. (obviously, there is a limit to the amount excitation voltage can increase and you eventually need to open the throttle)
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Old 12-03-2010, 09:20 PM   #6
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... The problem is cloudy weather. When the PV panels are generating power on a sunny day then everything is just fine, but when clouds blow across the sun then the power vanishes in a few seconds and the utility's generators have to cope with a fairly large (10-15%) transient. ...
A similar thing is happening with wind power. It fluctuates wildly, and then the other companies on the grid have to jump in to supply the difference. As you mentioned earlier, that can mean keeping some equipment running on standby so it's ready to go. This raises costs. So these energy companies are suing - the wind farms get subsidies to provide expensive uneven power, and then the cost falls on the other suppliers to make it up when the wind isn't blowing.

As that great philosopher Kermit said "It isn't easy being green".

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Old 12-04-2010, 08:31 AM   #7
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Smaller energy-storage devices include pumping water upstream into reservoirs at night and generating hydroelectric power during the day.
Even seemingly green, simple, safe energy storage schemes like this can have unintended consequences and dangers. One of these breached a few years ago in Missouri and destroyed many forested acres, one of the most beautiful parks in Missouri as well as ruining one of Missouri's best floating rivers for a few years. If this had happened on a weekend during the summer, I hundreds of people (likely including yours truly) could have been killed: Taum Sauk Hydroelectric Power Station - Wikipedia, the free encyclopedia
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Old 12-04-2010, 10:12 AM   #8
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In this part of the country TVA has a way to store electricity sort of. They just pump water up hill to a higher grade lake above the dam during low demand times and then during high demand times they release the amount of water needed through the turbines to run the appropriate number of generators and let gravity do its thing.
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Old 12-04-2010, 10:55 AM   #9
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In this part of the country TVA has a way to store electricity sort of. They just pump water up hill to a higher grade lake above the dam during low demand times and then during high demand times they release the amount of water needed through the turbines to run the appropriate number of generators and let gravity do its thing.
Here in Columbia country we just store it in the river, until some just has to be let over the spillway.

Awesome to go down into the dam power plants and see and feel those giant turbines spinning on a hot summer day.

Oh yes, in the old days we also used to store it as aluminum.

Ha
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Old 12-04-2010, 11:15 AM   #10
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Only one nit -- sometimes on a large commercial generator, the throttles are fixed and the generator rpm is fixed. As load is added, the DC excitation field is simply increased to bring the output AC voltage back up to spec. (obviously, there is a limit to the amount excitation voltage can increase and you eventually need to open the throttle)
I love this stuff.

Way back in 1982 after Hurricane Iwa (and, notably, after ADM Rickover had retired) Kauai's electric plant was completely shut down by the damage. After initial repairs, when the time came to restart the generators they discovered that they had no source of power to flash the excitation fields, so no output voltage. (Something to do with battery or generator damage.) USS INDIANAPOLIS had been sent over there for some reason (Probably the first naval vessel available to get underway? Already on the range for an exercise?) and rumors flew that the submarine was going to power the island from its reactor. There was a spirited discussion about using the submarine to power the field-flashing equipment but portable generators were flown in before the technical questions could be resolved, so the utility was able to bootstrap itself without the nukes.

I don't know what Naval Reactors feedback rippled down the chain of command in the submarine force, but it must've been traumatically emphatic. I was at COMSUBPAC in 1992 when Iniki pretty much repeated Iwa's rampage. As the damage reports came in, we had already sortied the submarines and of course a couple were over by Kauai. I didn't know the INDIANAPOLIS history at the time but during the admiral's brief the next morning some helpful (non-nuclear-trained) staffer suggested sending a conveniently-positioned submarine over to Nawiliwili Harbor to "assist with the electric plant repairs". I thought the nuclear guy was going to burst a blood vessel with his "NO!!" response...
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Old 12-04-2010, 11:30 AM   #11
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Not wanting to hijack the other thread, I'm starting this one.

I have heard of excess capacity too. So what really happens to it? Does it turn into heat, noise or what?
It's stored in giant flux capacitors located under Al Gore's house in Tennessee.
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Old 12-04-2010, 11:40 AM   #12
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I have heard of excess capacity too. So what really happens to it? Does it turn into heat, noise or what?
Excess electrical capacity is only one part of alien terraforming. People like Michael Faraday and Edison were aliens sent to begin the process we currently call global warming to prepare the Earth for eventual colonization. The aliens can currently get a small ship to the Earth but not enough ships until enough power is stored here on Earth. The excess capacity is being stored by the aliens underground for eventual use to provide the energy they will need during their initial invasion.

Al Gore is part of the alien political party that opposes the invasion. He came to earth to warn us of the terraforming. Similar, the the US's Republican party, it has some good ideas but is poor on presentation.
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Old 12-04-2010, 02:59 PM   #13
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One of the other sources for extra electricity is to call up companies that have their own back up generators... when I worked at mega... we had a lot of buildings with enough generating capacity to run the whole building with some to spare... it is not the cheapest electricity, but at least they were able to check to make sure everything was working as intended if there was a black out...
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Old 12-04-2010, 03:30 PM   #14
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Not wanting to hijack the other thread, I'm starting this one.

I have heard of excess capacity too. So what really happens to it? Does it turn into heat, noise or what?
There is excess capacity but not all is turned on. Different generators have different cost of generation, so turn on the generators, first cheapest and go on to higher cost till demand is met. When there is less power than load, voltage as well as frequency go lower. As already noted, gas pedal of generators can not be used like a car's, some require hours. Lastly it is generally better to run a power plant at full capacity, because it is most efficient at full load. So with cars being charged at night, no need to throttle down power plants except high cost ones (eg open cycle gas turbines).
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Old 12-04-2010, 07:45 PM   #15
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I've worked in power plant and electrical system operation for most of my adult life. Nords and Gumby gave great explanations.

Still, I enjoyed the "flux capacitors under Al Gore's house" theory too
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