5 ms·

Former flywheel energy storage startup engineer here. The economics of flywheels for this kind of application versus just using another battery tend to rest on

Former flywheel energy storage startup engineer here. 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.

greglindahl 8y ago
I mostly hear about flywheels used in datacenters, to bridge the time from a utility outage to diesel generator startup. That doesn't happen very often at the typical datacenter -- monthly testing and maybe a real outage or two per year.
- jacquesm 8y ago
  Hospitals as well. That's called 'ride through'.
  - robotresearcher 8y ago
    Another cool use is in railway stations to store the energy of a train as it slows into the station, then return it when it accelerates away.
    - andrei_says_ 8y ago
      How does this work? If the flywheel is in the station, how does it “catch” the train’s energy?
      - robotresearcher 8y ago
        They are on a common electrical system. The train uses regenerative braking to put power into the 'grid'. The flywheel senses this and draws power to spin up. When the train tries to draw a huge current to speed up, the flywheel senses this and dumps power to meet the demand.
        andrei_says_ 8y ago
        Makes sense. I was imagining some kind of steampunkish mechanical contraption.
        taneq 8y ago
        There was something a bit like that for trucks a few years ago, that captured pressurised exhaust while the truck was engine braking and then used it (not sure exactly how) to help the truck accelerate again. Apparently it saved significant amounts of fuel in stop-start situations (rubbish trucks etc.), not sure what happened to it though.
        db48x 8y ago
        Giant gears that mesh with the sides of the train would be pretty awesome.
rwmj 8y ago
Some kind of frictionless magnetic bearing is not possible?
- jacquesm 8y ago
  It is, and there is actually a really neat passive and stable solution to this (circular Halbach array with compensation coils running from one side to the other).
  - murbard2 8y ago
    "Electrodynamic bearings" is the keyword.
- eigenvector 8y ago
  It's possible and it exists in other applications but my understanding is that the costs were prohibitive for our product (~500 kWh flywheels).
  - bachbach 8y ago
    I saw a startup, Velkess with a flexible lasso type flywheel (safety), I think 15 kWh. It folded though. 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.
schiffern 8y ago
>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). 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?
- eigenvector 8y ago
  It's not X kWh of batteries for X kWh of flywheels, it's 0.01 kWh of batteries to provide enough power to run the flywheel's protection and control systems while it 'freewheels'. Ideally, you don't want to brake the flywheel while the grid is down, because then you're losing all the storage energy and need several minutes to spin back up when the grid is back.
  - robotresearcher 8y ago
    Surely you can power the control electronics from the flywheel, right? That should always work except when the thing is stopped, which is a safe state.
    - hyperbole 8y ago
      in failsafe systems it's best not to depend on the thing you expect to fail to behave in a particular way when it... fails, redundancy is the key.
JohnJamesRambo 8y ago
How did you feel working on a project you obviously have a lot of knowledge in but don’t believe in long term?
- eigenvector 8y ago
  Happy that I was learning a lot by getting to do hands-on product design but constantly checking in with my professional contacts to see if there would be somewhere for me to land if it all went belly-up. ;)
anitil 8y ago
> Former flywheel energy storage startup engineer here I love this place sometimes.
- mlvljr 8y ago
  Yr n0t 'lone :v
drumttocs8 8y ago
Power utility engineer here. I keep imagining a giant flywheel located at every substation- could be very useful for load balancing, voltage regulation, even frequency control. I guess what I mean is, a large spinning mass could potentially have other benefits than just storage, right? Any research into substation application at that startup?
- sgentle 8y ago
  I was recently reading about power factor correction and found out that "synchronous condensers", basically just large synchronous motors with no load, are sometimes used at utility scale for PFC and frequency stabilisation. Eg, here: http://www.think-grid.org/synchronous-condensers-better-grid-stability http://www.think-grid.org/synchronous-condensers-better-grid... 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?
  - eigenvector 8y ago
    Where "flywheels" in common parlance differ from synchronous condensers is that condensers run at zero torque - so they provide no active power to the system. They provide reactive power, which is needed to regulate and maintain the stability of the power system, but not active power which is used to match generation and demand or shift load. There's no actual source of energy being fed into a synchronous condenser. In a flywheel, you're drawing energy from the grid to spin up a really big mass and then storing it in rotational inertia so you can output it later very quickly. Flywheels can provide reactive power too, through their DC/AC power converters, but since you don't actually need any rotating mass to do that (recall, reactive power requires no torque), you can use a STATCOM - which is functionally like a synchronous condenser just without any moving parts.
- bigiain 8y ago
  They do this in datacenters - mostly I think to bridge the gap between a power outage and diesel generator startup time. You might also be interested in this: https://electrek.co/2018/05/11/tesla-giant-battery-australia-reduced-grid-service-cost/ https://electrek.co/2018/05/11/tesla-giant-battery-australia... “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.”
bigiain 8y ago
Gut feel suggests flywheels would last longer, be less expensive to recondition. 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???)
- eigenvector 8y ago
  Your numbers are reasonable. 5-10x as many cycles on a flywheel versus best-available Li-ion tech was what we figured, too. The initial construction cost per kWh is much higher for a flywheel, though, basically enough to wipe out that advantage. Also, even if a flywheel is cheaper in the long run, it's a tradeoff of upfront capital cost vs long-term maintenance costs - and when you refurbish your battery in 10 years, cells are probably going to cost 20-30% less than they do today. 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.
burnte 8y ago
> 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 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?
- eigenvector 8y ago
  The rotor spins in a vacuum to reduce standby friction losses, and it requires power to maintain that vacuum. The bearings also need active cooling. Without these support systems you won't have a catastrophic failure, but you'll be damaging your equipment and reducing its lifespan. 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.
  - burnte 8y ago
    I would think that a momentary loss of power wouldn't be a big deal, even with active cooling and a vacuum pump. As long as your disconnect is fast, if it takes 500-1000ms for the pump and cooling to come back online from flywheel power, that seems like a much easier solution than worrying about five nines. The wear in that second can't be significant.
walrus01 8y ago
I am a member of a forum for professional car mechanics. You would not believe the number of people who abuse their car by driving with brake rotors down to ridiculous thicknesses, or only changing their oil once in 40,000 miles. Consumers are bad at maintaining mechanical things. Think about this for a moment... Now Google "flywheel explosion".
- eigenvector 8y ago
  We now have government-mandated maintenance standards for some types of power utility equipment (mostly protection systems) in North America because it turns out even big companies with hundreds of engineers on staff sometimes aren't very good at maintaining stuff.
jve 8y ago
You seem to be the correct person to ask what are the do's and dont's regarding lithium battery maintenance and to increase device (say mobile phone) battery capacity?

greglindahl 8y ago

I mostly hear about flywheels used in datacenters, to bridge the time from a utility outage to diesel generator startup. That doesn't happen very often at the typical datacenter -- monthly testing and maybe a real outage or two per year.

jacquesm 8y ago
Hospitals as well. That's called 'ride through'.
- robotresearcher 8y ago
  Another cool use is in railway stations to store the energy of a train as it slows into the station, then return it when it accelerates away.
  - andrei_says_ 8y ago
    How does this work? If the flywheel is in the station, how does it “catch” the train’s energy?
    - robotresearcher 8y ago
      They are on a common electrical system. The train uses regenerative braking to put power into the 'grid'. The flywheel senses this and draws power to spin up. When the train tries to draw a huge current to speed up, the flywheel senses this and dumps power to meet the demand.
      - andrei_says_ 8y ago
        Makes sense. I was imagining some kind of steampunkish mechanical contraption.
        taneq 8y ago
        There was something a bit like that for trucks a few years ago, that captured pressurised exhaust while the truck was engine braking and then used it (not sure exactly how) to help the truck accelerate again. Apparently it saved significant amounts of fuel in stop-start situations (rubbish trucks etc.), not sure what happened to it though.
        db48x 8y ago
        Giant gears that mesh with the sides of the train would be pretty awesome.

rwmj 8y ago

Some kind of frictionless magnetic bearing is not possible?

jacquesm 8y ago
It is, and there is actually a really neat passive and stable solution to this (circular Halbach array with compensation coils running from one side to the other).
- murbard2 8y ago
  "Electrodynamic bearings" is the keyword.
eigenvector 8y ago
It's possible and it exists in other applications but my understanding is that the costs were prohibitive for our product (~500 kWh flywheels).
- bachbach 8y ago
  I saw a startup, Velkess with a flexible lasso type flywheel (safety), I think 15 kWh. It folded though. 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.

schiffern 8y ago

>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). 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?

eigenvector 8y ago
It's not X kWh of batteries for X kWh of flywheels, it's 0.01 kWh of batteries to provide enough power to run the flywheel's protection and control systems while it 'freewheels'. Ideally, you don't want to brake the flywheel while the grid is down, because then you're losing all the storage energy and need several minutes to spin back up when the grid is back.
- robotresearcher 8y ago
  Surely you can power the control electronics from the flywheel, right? That should always work except when the thing is stopped, which is a safe state.
  - hyperbole 8y ago
    in failsafe systems it's best not to depend on the thing you expect to fail to behave in a particular way when it... fails, redundancy is the key.

JohnJamesRambo 8y ago

How did you feel working on a project you obviously have a lot of knowledge in but don’t believe in long term?

eigenvector 8y ago
Happy that I was learning a lot by getting to do hands-on product design but constantly checking in with my professional contacts to see if there would be somewhere for me to land if it all went belly-up. ;)

anitil 8y ago

> Former flywheel energy storage startup engineer here I love this place sometimes.

mlvljr 8y ago
Yr n0t 'lone :v

drumttocs8 8y ago

Power utility engineer here. I keep imagining a giant flywheel located at every substation- could be very useful for load balancing, voltage regulation, even frequency control. I guess what I mean is, a large spinning mass could potentially have other benefits than just storage, right? Any research into substation application at that startup?

sgentle 8y ago
I was recently reading about power factor correction and found out that "synchronous condensers", basically just large synchronous motors with no load, are sometimes used at utility scale for PFC and frequency stabilisation. Eg, here: http://www.think-grid.org/synchronous-condensers-better-grid-stability http://www.think-grid.org/synchronous-condensers-better-grid... 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?
- eigenvector 8y ago
  Where "flywheels" in common parlance differ from synchronous condensers is that condensers run at zero torque - so they provide no active power to the system. They provide reactive power, which is needed to regulate and maintain the stability of the power system, but not active power which is used to match generation and demand or shift load. There's no actual source of energy being fed into a synchronous condenser. In a flywheel, you're drawing energy from the grid to spin up a really big mass and then storing it in rotational inertia so you can output it later very quickly. Flywheels can provide reactive power too, through their DC/AC power converters, but since you don't actually need any rotating mass to do that (recall, reactive power requires no torque), you can use a STATCOM - which is functionally like a synchronous condenser just without any moving parts.
bigiain 8y ago
They do this in datacenters - mostly I think to bridge the gap between a power outage and diesel generator startup time. You might also be interested in this: https://electrek.co/2018/05/11/tesla-giant-battery-australia-reduced-grid-service-cost/ https://electrek.co/2018/05/11/tesla-giant-battery-australia... “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.”

bigiain 8y ago

Gut feel suggests flywheels would last longer, be less expensive to recondition. 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???)

eigenvector 8y ago
Your numbers are reasonable. 5-10x as many cycles on a flywheel versus best-available Li-ion tech was what we figured, too. The initial construction cost per kWh is much higher for a flywheel, though, basically enough to wipe out that advantage. Also, even if a flywheel is cheaper in the long run, it's a tradeoff of upfront capital cost vs long-term maintenance costs - and when you refurbish your battery in 10 years, cells are probably going to cost 20-30% less than they do today. 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.

burnte 8y ago

> 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 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?

eigenvector 8y ago
The rotor spins in a vacuum to reduce standby friction losses, and it requires power to maintain that vacuum. The bearings also need active cooling. Without these support systems you won't have a catastrophic failure, but you'll be damaging your equipment and reducing its lifespan. 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.
- burnte 8y ago
  I would think that a momentary loss of power wouldn't be a big deal, even with active cooling and a vacuum pump. As long as your disconnect is fast, if it takes 500-1000ms for the pump and cooling to come back online from flywheel power, that seems like a much easier solution than worrying about five nines. The wear in that second can't be significant.

walrus01 8y ago

I am a member of a forum for professional car mechanics. You would not believe the number of people who abuse their car by driving with brake rotors down to ridiculous thicknesses, or only changing their oil once in 40,000 miles. Consumers are bad at maintaining mechanical things. Think about this for a moment... Now Google "flywheel explosion".

eigenvector 8y ago
We now have government-mandated maintenance standards for some types of power utility equipment (mostly protection systems) in North America because it turns out even big companies with hundreds of engineers on staff sometimes aren't very good at maintaining stuff.

jve 8y ago

You seem to be the correct person to ask what are the do's and dont's regarding lithium battery maintenance and to increase device (say mobile phone) battery capacity?