4 ms·

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 bat

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 9y 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.
- bigiain 9y ago
  Thanks! 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?"

eigenvector 9y 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.

bigiain 9y ago
Thanks! 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?"