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If they could've collocated near a landfill, they could burn methane at night. Still environmentally friendly/neutral (all landfills have to flare the methane they generate), and you don't need as large of a methane supply as you're just burning it when the sun is down.

Sounds right. Just as a point of reference, power plants that turn on and off are known as "dispatchable:" http://en.wikipedia.org/wiki/Dispatchable_generation

There might be one reason they let this spin up and down.

We mostly power the grid through a bunch of constantly humming systems, but our usage has all kinds of peaks (diurnal, seasonal... the British grid has a massive peak right around the time Eastenders ends and everyone puts on a kettle[1]).

If you are adding some power to the baseline, that's helpful. If you're promising the grid you can add some power during one of the daily or seasonal peaks, that's almost as valuable as adding a new source of always-on power. By providing energy for even just six hours a day, you might save the grid from requiring a new always-on plant. So in terms of opportunity costs, even by being twice as expensive moment to moment, you're still cutting costs in half (unless everyone followed your strategy).

So there's probably some number of hours of shutdown where it's worth the coal, some number where it's not, all contingent on what plants are nearby, and how close your customers typically are.

Related but tangential, one of the unintended benefits of electric cars would be adding a bunch of consumption at night, smoothing the diurnal cycle, letting us pursue more lower-cost always-on generation. (This is before you even consider distributed storage proposals that let the grid draw from parked cars in emergencies.)

Predictability is almost as good as a natural resource.

[1] http://www.bbc.co.uk/britainfromabove/stories/people/teatime...

There's also a given that 6hrs after sunset there isn't a need for as much power... most of the population is asleep, lights off, TVs off, etc. There are other traditional power plants (Hydro and Nuclear) in AZ that provide overflow service for off hours. It actually takes a combination of plants to offer the best load distribution.

In parts of AZ there are serious brown outs and rolling blackouts in the summers as A/C kicks in everywhere, which is probably the biggest draw.

I'm opposed to coal altogether for pollution reasons alone. It's bad enough between the cars and airport traffic, don't need more.

Do you have any kind of source on that? I've lived in Arizona all my life, and the only rolling brownouts I've experienced and heard about have been in California. Who I believe already buy surplus power from AZ.

I lived in Prescott Valley, and several years in a row saw brown/blackouts in mid-summer. Also saw them a number of times in the Peoria & I-17 area in phx (closer to cactus)

I've seen outages lasting a couple hours from components like transformers breaking, but that's a different issue from the grid itself not producing enough power to meet demand. Can't speak for Prescott Valley though.

Interestingly, the only article on a Phoenix area blackout due to high load I was able to source happened in the winter... probably caught SRP totally off guard:

http://www.azfamily.com/news/local/Rolling-blackouts-sparked...

> six hours after sunset there isn't enough heat to keep the turbines going

Note that's when running at "maximum power". You don't have to extract the heat that fast. Given a hypothetical and utterly irresponsible grid supplied solely by this design, you'd overbuild to the point that you'd have enough stored heat to last all night.

> Any power plant that uses steam turbines is going to have a long ramp-up time to boil a huge quantity of water needed to produce the steam.

This is a matter of tradition and convenience. It is quite possible to use a just-in-time steam generator.

In fact many steam power systems are vapor phase only (non-condensing) to avoid corrosion and droplet impact damage. Steam is used because it is cheap and nontoxic.

Nuclear reactors are easy to turn on and off once a day. Naval reactors do it as a matter of course. There's just no point. If you build a grid scale reactor, you might as well leave it running all the time, at which point you optimize away the rapid start capability.