The economics of flywheels for this kind of application versus just using another battery tend to rest on the purported "unlimited cycle life" of the flywheel system compared to, say, Li-ion batteries that have a very well documented finite cycle capacity that degrades even further when doing sub-optimal cycling. To a lesser extent you can also bank on lower parasitic loads during standby as the environmental requirements for a flywheel aren't as stringent as batteries that need to be either heated or cooled almost all the time in many climates.
The problem is that, by and large, "unlimited cycles" is not true. You still have huge, very high speed bearings. Motors that require routine electrical testing and can fail. And now all this stuff is sitting below ground under a massive concrete lid for containment so it's not as easy to do maintenance on compared with a similarly-sized battery system. You also need uninterruptible power supply to maintain safety and control systems when grid power is unavailable since you've still gotten a huge spinning mass that you can't slow down without somewhere to send the energy (it's possible to use braking resistors, but it's another cost).
Batteries also benefit from massive economics of scale (both on the actual cells and the power electronics) that are getting better with time and driving costs down, while flywheels have been "1 year from commercialization" for the last 25 years.
I remain skeptical of the commercial benefits vs. increasingly commoditized and readily available battery systems.
I love this place sometimes.
However, if you have a fast enough control and communications system, you can provide the desired response for both frequency and voltage control and that's a big part of what we were trying to do. You can ramp from minimum to maximum active or reactive power very, very fast (like 5 AC cycles), which is much faster than any traditional generator. Due to the fast ramping capability, we also researched the possibility of using flywheels for large unit trip contingencies in small power systems (e.g. Hawaii) where operating reserve is very expensive.
At the end of the day, the biggest problem is that anything flywheels can do, batteries can do too - and batteries are getting cheaper every year.
You might also be interested in this:
“In the first four months of operations of the Hornsdale Power Reserve (the official name of the Tesla big battery, owned and operated by Neoen), the frequency ancillary services prices went down by 90 per cent, so that’s 9-0 per cent. And the 100MW battery has achieved over 55 per cent of the FCAS revenues in South Australia. So it’s 2 per cent of the capacity in South Australia achieving 55 per cent of the revenues in South Australia.”
Since it seems like the main source of stability in that system is the inertia in the rotor, would it be fair to describe it as a kind of flywheel? I didn't see anything about connecting an actual wheel to such a system, but it seems like it would be the same thing with more inertia, right?
There are FERC rate schedules for this service that make it a profitable application for flywheels.
With modern power electronic converters, it's probably cheaper and easier to just use the main AC grid as the sink for the regen braking power.
Why? If it's safe in it's vault when powered, why is it unsafe when in it's vault unpowered? Hell, you could use one flywheel to power the rest, and as they lose too much velocity the next one becomes the generator, until either they're all spun down, or power comes back, then you have at least some of the flywheels ready to go. Now, let's say you just don't do anything, they're spinning away slowly slowing, then power comes back and they don't have to spin back up from a dead stop. Why is that not true?
As for using the flywheel(s) themselves as the source of backup power, that was our original design and definitely feasible at a conceptual level, but there's a lot of engineering in getting that to work properly while maintaining grid code compliance. You need your grid-tie inverter (which also provides the 60 Hz AC used by the support systems) to disconnect from the grid and transition to island mode /without interruption/ very, very fast (since utilities have standards on how fast generators need to disconnect during a system fault) and basically it required us to write our own firmware for the VFDs we were using which in turn invalidated their safety certifications. So definitely a solvable problem but we just didn't get there.
I can only make a (barely) educated guess at the difference between Li-ion battery cycle life (single digit thousands of cycles to 80% capacity seems to be what I see everywhere?) compared to bearing replacement schedules and motor/controller maintenance (and I don't have even best guess anecdotal data for this? A little Googling suggests some Rolls Royce airliner jet engines have 15,000 hours between overhauls, but that at least one has made 42,000 hors without an overhaul).
I'd _guess_ you probably don't dump energy back into the flywheel as fast as you pull it out? (I base this on calculating a Tesla 100KWhr battery requires 600(+)KW to recharge in 10 mins, and if you could pull that off the grid easily, you'd just do that. They seem to get enough grid power to charge a Tesla in ~1hr, so they've got ~100KW available I guess?) For back of the envelope calculations I'm gonna use 1 hour as "one cycle" (discharge in 10 mins, recharge in 50 mins seems a reasonable/conservative estimate) - that'd implie a flywheel with similar bearing longevity to a 747 engine bearings would last about 10 times as long (15k - 40k hours) as a Li-ion battery takes to drop to 80% capacity (say 1.5k to 4k cycles?).
The big difference would be a flywheel with new bearings is "as good as new", whereas there's nothing besides replacing the Li-ion battery that gets it back to new.
Pretty sure "charge directly off the grid" is the optimal option for "supercharger like charging stations" (perhaps not for the grid operator), but if you want 600+KW per charging station, and the grid cannot deliver that (economically) where you need it, I'd be surprised to find flywheels would come out something like an order of magnitude cheaper to operate long term than Li-ion battery storage.
(But I'm certainly not a "Former flywheel energy storage startup engineer" - I'd love to know where I screwed up my calculations to indicate and order-of-magnitude benefit that _probably_ doesn't exist???)
The other problem we had was that we were making 10s of flywheels per year and competing against Samsung and LG's battery manufacturing efficiencies. And buying an ultra-low-volume product from a startup that might not be around to maintain it in 20 years is also a tough sell in the risk-averse power industry.
As usual - the actual problem isn't reflected in "assume a perfectly spherical cow of uniform density" physics, or in the "Hey, I know a tiny bit about a related problem to this, I'll just extrapolate from there, WCPGW?" analysis.
I reach towards both those oversimplifications way to often.
Glad to see the guestimates/calculations I did bares at least an order-of-magnitude level of correctness. I _mostly_ do these quick calculations so I can rule out thinking harder about things that are 3 or 4 orders of magnitude away from possible.
"Yeah boss, we can do that, it'll take <scrible scrible, estimate, google, calculate, double check> something like $800k of Amazon resource per month, maybe only half a mil if we commit to 12 months up front. How much did you say we could sell this for? To how many customers?"
The braking resistors are however a much lower cost, which for this (infrequently used) application is mostly what matters.
It seems like spending >10x more on a contingency system just to save a few hundred kWh (costing a few tens of dollars) every few years seems suboptimal. Especially since those batteries could be in a daily cycling installation, so the opportunity cost (compared to using those batteries elsewhere) is very high.
Why buy X kWh of batteries to sit idle 24/7/364, plus X kWh of flywheel storage? Why not A) eliminate the flywheel, use the battery, and be done with it? Or B) use the braking resistor? It seems like either A or B should always be preferable to a "hybrid" given reasonable assumptions.
Favoring energy recapture over resistor heat dump seems like very suboptimal high-level design coming from a flywheel storage engineer, so what am I missing here?
Do you know any companies doing in the 15kWh ballpark type flywheel batteries? It's an idea I really like - I wanted to pair it with a small hydropower installation.
Consumers are bad at maintaining mechanical things.
Think about this for a moment...
Now Google "flywheel explosion".