Ditch the Batteries: Off-Grid Compressed Air Energy Storage (2018) • Hacker News Board

Re: Ditch the Batteries: Off-Grid Compressed Air Energy Storage (2018)

#91

Post by infogulch » Fri, Oct 16, 2020, 3:59 AM UTC

The efficiency loss compressed air suffers is mostly at the hands of the ideal gas law. Storing energy by pressurizing the container heats up the gas which increases the pressure requiring even more energy to store additional gas. When retrieving energy it depressurizes the container which cools the gas down which reduces the pressure which gives you even less energy. What if they could cancel out?

Say you have two separate pressure vessels where to charge the system you pump air out of one into the other. Well the depressurizing vessel will cool down, but the pressurizing vessel will heat up. If they came to thermal equilibrium with each other their net temperature change would cancel out somewhat. This is great because it would restore the pressure on both sides to have more favorable conditions for energy storage and extraction.

So I wonder if you could design a pressure battery that would exhibit no change in temperature on the exterior throughout the charge/discharge cycle.

Re: Ditch the Batteries: Off-Grid Compressed Air Energy Storage (2018)

#92

Post by jillesvangurp » Fri, Oct 16, 2020, 7:45 AM UTC

jacknews wrote: post #75

jillesvangurp wrote: post #46

Earlier quoted context omitted.

You stopped reading too soon: e.g. " By discharging the cylinders sequentially, the discharge time can be greatly increased, making the system comparable to lead-acid batteries in terms of energy density. Based on their experimental set-up, the researchers calculated the efficiencies for different starting pressures and numbers of cylinders. They found that 57 interconnected cylinders of 10 litre each, operating at 5…

This is by far the most efficient example from the article, and seems to make this kind of system viable for domestic use, but I question whether the numbers are true, as I've seem no follow-up studies.

Too early for that. There are barely any products in the market for this. The ultimate proof will be people installing this in their homes for a certain amount of $ and then getting some measurable ROI (also in $).

But do you have reason to be skeptical about the physics? The premise here is brutally simple centuries old physics and engineering that any high school kid should be able to understand. Simple pressurized vessels holding compressed air. It sounds plausible to me that that ought to be dirt cheap to implement with very low tech solutions for pumping and compressing air and storing it in some kind of vessel.

You see people arguing here for an alternative based on Tesla batteries costing in the order of thousands of dollars even for a modest/small setups. It's kind of easy to see that this could potentially undercut that by orders of magnitudes because there are no special/hard to get materials and it's all based on dirt cheap commodities and technology. Pumps, valves, steel, etc.

Re: Ditch the Batteries: Off-Grid Compressed Air Energy Storage (2018)

#93

Post by Doxin » Fri, Oct 16, 2020, 12:44 PM UTC

jandrese wrote: post #81

atrus wrote: post #66

Earlier quoted context omitted.

I mean, doesn't any big energy storage device become essentially a bomb? Batteries have some pretty impressive failure modes. As do large tanks of fuel. Even piped in energy sources can be dangerous as well. How many houses have literally exploded over a leaking gas line? Or burnt down due to faulty wiring? Large amounts of energy stored in an area is inherently dangerous, no matter the form.

People talk about batteries "exploding", but really they're just burning fast. An air tank explosion however will decimate everything nearby. Even small SCUBA tanks can release a tremendous amount of energy when they go, which is why they are typically refilled inside of water baths.

Scuba tanks operate at wildly higher pressures than your average air compressor. Scuba tanks are at anywhere from 160 bar to 240 bar. The usual sort of compressor you'd use for running your tools tops out at 8 bar.

Make no mistake, 8 bar escaping from a compressor is no joke. It's the equivalent of two elephants worth of air suddenly entering the room. It'll rupture eardrums and shatter windows but the building will still be standing. I wouldn't be so sure about that when a scuba tank decides to explode though.

Re: Ditch the Batteries: Off-Grid Compressed Air Energy Storage (2018)

#94

Post by notmadnomad » Mon, Oct 19, 2020, 6:47 AM UTC

PaulDavisThe1st wrote: post #8

notmadnomad wrote: post #3

For fixed-site applications of sufficient land area and scale, it's difficult to beat pumped-storage hydroelectricity (PSH). The round-trip efficiency is around or above 70%, the same or better than CAES. CAES is probably best used in applications similar to flywheel (FES), away from occupied areas.

TFA already says that large scale CAES is probably a dead end and that small scale CAES is where we might/should focus our attention.

CAES also has awful energy density of ~100 kJ/m^3 or ~100 J/L. It's not worth it. Small CAES implies closer to people and more likely to experience heat and fires, more so than say 200 psi / 13 atm air compressors, and so inherently more dangerous than all existing residential and commercial products including water heaters with failed/blocked relief valves. Perhaps almost on-par with LPG tanks, which are not supposed to be stored inside building either but in a remote pit or shed.

Large amounts of reactive lithium metal in NMC (0.90–2.43 MJ/L) "powerwall" reserve banks in or on a building also seems like a major fire risk. LiFePO4 (1.2 MJ/L) is safer due to much less lithium.

By comparison with fuels:

- Al thermite is ~18 MJ/L

- Cu thermite is ~21 MJ/L

- hydrogen is ~28 MJ/L @ 700 psi

- gasoline is ~32 MJ/L

- Jet A is ~35 MJ/L

- diesel is ~36 MJ/L

In the long-term, a reliable, reversible fuel cell combined with H2 tanks and H2 generation and distribution infrastructure seems like the best hybrid on- / off-grid way to go, at deployment cost though, but it could eliminate the increasingly-dangerous electrical grid and natural gas grid. Short-term, LiFePO4 looks better and better.

Re: Ditch the Batteries: Off-Grid Compressed Air Energy Storage (2018)

#95

Post by jacknews » Sun, Oct 25, 2020, 10:21 AM UTC

jillesvangurp wrote: post #92

jacknews wrote: post #75

Earlier quoted context omitted.

This is by far the most efficient example from the article, and seems to make this kind of system viable for domestic use, but I question whether the numbers are true, as I've seem no follow-up studies.

Too early for that. There are barely any products in the market for this. The ultimate proof will be people installing this in their homes for a certain amount of $ and then getting some measurable ROI (also in $). But do you have reason to be skeptical about the physics? The premise here is brutally simple centuries old physics and engineering that any high school kid should be able to understand. Simple pressurized…

Yes, I question the physics. The last example, where they use multiple fire-extinguishers as the storage vessel appears to me to be able to store far more energy per volume than the other examples cited.

If these numbers are accurate, then such a system seems quite feasible for domestic use. The others, much less so, as the storage vessels to store just a few KW/h are so massive.