This doesn’t worry me. In fact I like the idea of coal power plants being a backup and ready to go. The more solar we have the less backup we’ll need, but it’s always nice to have a backup.

Coal generators don't work this way. It can take 12-36 hours to ramp from a cold start. Natural gas (combined cycle turbines) can throttle up in minutes, which is what pushed coal out of the generation mix (natural gas is a perfect compliment to renewables until battery storage is cost competitive, you have to be able to fill in generation intermittency gaps with little response latency).

Here's a great story [1] from the Tampa Bay Times about what happened when TECO (the local utility) didn't want to power down a coal fired boiler to take the generator out of service to clear slag from the bottom of the boiler (TL;DR several people died a horrible death from the equivalent of flowing magma).

Coal is almost dead, just a bit more of a way to go.

[1] http://www.tampabay.com/projects/2017/investigations/tampa-e...

Coal is dead in Britain, there will be zero generation in the next few years. It was the deregulated energy industry that killed it, starting in the 80's, out-competed on price and flexibility by gas.

I don’t want to sound like I’m a fan of coal, but coal died in the UK before it became uneconomical: the coal miners went on strike for long enough that the government made it a strategic goal to never let striking workers be politically relevant again.

A much better plan is to use an excess of solar energy to power processes that produce ethanol-based fuels and then keep a reserve of those (which by the by, is carbon sequestration at best and neutral at worst).

Coal power plants are awkward and inefficient and require a lot of work just to move the fuel around. There's a reason the world worked hard to develop better fuel technologies as quickly as possible.

That's kind of ironic, considering that solar plants can keep an effectively unlimited number of "days of fuel" on hand.

They can't though -- the next day could always be cloudy and there's a very strong chance that one day in 90 will be.

So solar panels can't stock a reliable X day supply, even if over any (moderate) period of time, they have a very guaranteed supply.

They can with batteries, but that is a slightly different situation.

In answer to the question below, this isn't practical with modern battery technology.

Is there a battery technology that can store 90 days worth of full-plant production (which doesn't require a second generator consuming a fuel later)?

All the ways I know of on that scale actually require a second plant/turbine -- in which case, you didn't store 90 days of solar production capacity, you actually stored 90 days of the second plant's fuel, which you produced intermitently with solar capacity. (Which can still be a very good and environmentally friendly thing to do! But isn't what we're talking about as a technical point.)

Ways of the second variety:

- Hydro pumping

- Hydrocarbon synthesis

- Hydrogen separation

- Compressing gasses

And there's probably more that I'm forgetting.