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 Pr...June 2010 IEEE PVSC addendum presentation.pdf
Island battery: Is supplying 10 percent of Oahu’s power worth destroying Lanai? - The Hawaii Independent :€“ News · Culture · Community