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Comment by Lagged2Death | Hacker News Reader

I've been puzzled for some time: Is anyone / why isn't anyone working on more flexible and responsive coal and nuclear plants? Instead of storing renewable energy, it would be more efficient, less complex, and less capital-intensive (one would think) to consume the renewable energy immediately and make up the difference between supply and demand with flexible traditionally-fueled plants.

I'm sure there's a good reason. I just don't know what it is.

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By flexible, do you mean "able to ramp up to meet demand and ramp down when not needed"?

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. Nuclear plants do this, and I imagine most/all coal plants do as well. Hydroelectric plants don't need to because the falling water drives the turbines directly.

The first thing I thought of when I read the article is that this power plant can't be very efficient. It's using steam turbines, but six hours after sunset there isn't enough heat to keep the turbines going. That means they need to reheat every morning. It seems like it would have been better for them to include a coal plant for nighttime operations to keep the steam boilers hot and keep the turbines going all night. That would be flexible, because the coal burners could start up quickly to keep the already-hot water boiling. (Can't use nuclear this way because you can't really turn a nuclear reactor on and off.)

I'm totally guessing here, but I suspect they didn't include an alternate nighttime operations fuel because this is a "100%-Green" project that either wouldn't have gotten funding if there was a coal plant involved or it was just intended as a demonstration of the solar capability and didn't need to be practically efficient.

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.

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...

> 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.

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...

> 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.

As an uninformed guess: you want to minimize the number of heating and cooling cycles because that causes joints to crack. They used to keep steam trains running for days at a time (meaning they were kept fired overnight even when not used) to avoid cooling cycles.

Actually I would also be interested to know the real answer to this too. There was an interesting article in the Economist this week about the problems of the constant power output of nuclear and coal plants.

Reminds me of a story I heard from a guy in mining. Some sort of furnace they used took $1m of power to heat from a cold start. So they were reluctant to ever power down.

Seems like this paper answers some of those questions: http://www.energy.siemens.com/us/pool/hq/power-generation/po...

tldr: natural-gas combined-cycle power stations can achieve ~30 min startup times, and are being used to provide flexibility in the grid.

For the nukes there are a few so called SMR (small modular reactors) being worked on, and some of the are in process of licensing with NRC. The main selling pint is that these units with a capacity of about 200MWe can be deployed under 1bil dollars when compared to 15bil dollars for a large plant (>1000MWe). Most of these small plants can operate in a load follow mode, similar to gas plants. Also their footprint is similar to that of a gas plant rather than to that of a large nuke.

See for example:

BW mPower http://en.wikipedia.org/wiki/B%26W_mPower

Nuscale http://en.wikipedia.org/wiki/NuScale

SMR-160 http://en.wikipedia.org/wiki/SMR-160

Westinghouse SMR http://en.wikipedia.org/wiki/Small_modular_reactor#Westingho...

Pretty sure coal plants are already used in that way, but on a 'grid-wide' level; based on predictions based on the time of day, time of year, and other factors, traditional power plants get scaled up / down or powered up / down, preferably with minimal waste. Wasting fuel and powering up/down isn't profitable for the energy companies either. I can imagine that the fast-responding power plants are the hydroelectric ones, since their power output can be adjusted by just closing off some valves. Of course, if their reservoir is empty or low they can't scale up easily.

There are already 'peaker plants' which are generally gas fired turbines which can be activated to supply power on demand, but they are more expensive to operate as start/stop systems (startup consumes a lot of energy in its own right).

So generally 'peakers' aren't good fits for the other side of the energy generation equation.

Pumped-storage Hydro is a solution for storing power that is generation-method agnostic; it works with electricity from nuclear just as well as it works with electricity from wind. Basically the PSH station buys cheap electricity, and uses it to pump water up a hill into a reservoir. When electricity becomes expensive, it reverses the flow, generates electricity with the falling water, and sells it to the grid for a profit. These setups can switch to power generation rather quickly.

The only problem is that you need suitable geography. It's a bit more forgiving than finding proper geography for regular hydroelectric, but it is still a restriction.

http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity

The problem with that approach is that Gila Bend has more sunlight than water.

you don't need the source of the electricity and the storage to be in the same location though.

HVDC transmission is reasonably efficient.

I remember reading a typically opinionated article on the The Register where the claim was made that pumped storage doesn't scale very well. Or rather, it scales fine, but your country will look very different when you've finished the necessary building work.

Yeah, it can be very good, but there's a limit.

There's a very interesting project in Finland to build a pumped storage facility that uses a 1400m deep abandoned gold mine as the storage location. The idea is to use a relatively small amount of water but at very high pressure provided by the depth of the mine -- the high potential difference makes generation more efficient and reduces capital costs. If this thing turns out to be profitable, this will probably be done to all sufficiently deep mine shafts and other deep enough holes.

PDF: http://laguna.ethz.ch/indico/getFile.py/access?contribId=2&s...

So do the turbines/generators go at the bottom, in the mine, in this setup? That seems like it would have some interesting engineering challenges.

France uses nuclear plants for load following. It more or less has to; its only other significant source of power is hydro. It works, but it not as economical as it might be.

That just means running the plants below capacity, which is certainly done when electricity price falls below running costs. But since fixes costs are high, it is rarely a good investment to build a lot of overcapacity for these. Gas plants are low fixed / high running cost and quick to ramp up so are better for peak production.

Such nuclear designs are being worked on. In fact they're essentially ready now, but one does not simply get approval to spool up a nuclear plant. There's no inherent reason why either type cannot supply variable power needs instead of baseload, but it tends to be more inefficient overall to use a design with a wide variance of generation capacity, which is likely why utilities try to stray away from that.

I'm sure there's other problems though that an actual power distribution engineer could describe. E.g. the wide variance of wind/solar production from second to second would require a lot of energy storage and filtering to allow it to be placed on the grid (especially the rickety U.S. grid) without a series of transients accidentally opening circuit breakers and causing a blackout.

This is already the case. Gas plants take very little time to start up and shutdown. Combining thermal storage with the solar plant allows it to act as more of a base load power system. I would guess the goal is allowing utility companies to decommission more costly intermittent plants with solar replacements.

While not exciting this weekend the link shows a graph of Ontario's electricity supply mix. During the week you can see the variations in gas plant performance, from no work output to full work output within an hour.

http://media.cns-snc.ca/ontarioelectricity/ontarioelectricit...

The fuel costs are a small portion of the running costs of a nuclear plant, and the other costs don't go down when you shut it down, so there is very little reason to ever produce at less than maximum.

I assume that responsiveness isn't the issue but that the economics don't work for Arizona Public Service to maintain 750MW worth of coal/nuclear waiting offline for intermittent use only.

You don't build a coal power plant speccing in how much variable renewable energy will be produced. You spec for over the max power - in the middle of summer, with all the air conditioners on, and then you throttle down from there. As a utility, you wouldn't depend on someone else's renewable energy if your job is ensure that an area gets power.

That said - the best time for renewable energies to provide power is when the sun is shining the hottest, which for solar based power, coincides nicely with the maximum power use.

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