

The most awesome startup I have ever seen. - danshapiro
http://www.danshapiro.com/blog/2012/03/the-most-awesome-startup-i-have-ever-seen/

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drostie
It's a cool building, but I was a little less thrilled when I read about their
tech. It's true that an isothermal process is a reversible process, and
isothermal air compression is therefore a great way to get high efficiencies,
but compressed air just doesn't seem like it will scale well. The work formula
for isothermal compression of an ideal gas is:

    
    
        W = - ∫ P dV = - n R T ∫ dV / V =  P₀ V₀ ln(P₁ / P₀).
    

A shipping container holds 38.5 m³ according to the Wiki. At atmospheric
pressure this would mean P₀ V₀ ~= 1 kWh. Cheap hardware might get you to 10
atm pressure or so -- but let's go crazy and suggest that you could get 100
atm, since it's only logarithmic in P₁ anyway. You would still store less than
5 kWh per shipping container, no? That could run a window-unit air conditioner
for about a day, only. If you pay 15 cents per kilowatt hour, you could fill
up the shipping container with under $1 of electricity, only. My cell phone
battery stores an amp-hour at around 3.5 V, so if the above calculation is
right, to store the same amount of energy in 100-atm compressed air you'd need
27 liters of air -- two backpacks or so. I'm saying this to guesstimate that
it's about a factor of 1,000 less energy density than modern battery
technology -- and that that's a fundamental limitation to the medium.

Given all that, I'm really interested to see how they'll scale compressed air
up to handle the sheer amount of energy that they want to store.

(On the other hand, the factor of 1000 might not matter too much: it means
that if they can build a shipping container air storage unit cheaper than
electronics companies can build a battery the size of three backpacks, they
could indeed be cheaper to store energy en masse.)

~~~
DaniFong
Dude, I like your analysis and eager attempts to calibrate but you messed up
your calculation somehow.

I'm pretty sure your problem is that your formula misestimates the amount of
gas stored. It is compressed, you know. You of off by about a factor of 200 -
we use 200 atm air. It's 1.1 MWHr per high-boy shipping container

The energy stored is

efficiencyOutgoing * P_top * V_tank * (ln pressure ratio - 1 + 1/pressure
ratio) -- you can look at our patents for more detail.

~~~
drostie
Sorry, you're pretty much right. The equation I wrote is correct, but I
naively plugged in a bad number: yes, P₀ V₀ = n R T = P₁ V₁, but I used the
known number for V₁ (the final air in the container) as V₀. The proper formula
is indeed P₁ V₁ ln(P₁ / P₀).

You have my apologies for my confusion.

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petepete
_"middle school-dropout"_

Not the words that I'd use to describe someone who starts studying for a
degree at the age of 12.

~~~
danshapiro
Me either, but that's how she describes herself.

<http://daniellefong.com/about-the-author/>

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carlesfe
"Danielle started her PhD at 17" -- Thanks for ruining my monday!

Just kidding, awesome idea. We're so used to IT startups that we often forget
about other engineering. Very cool stuff.

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blaireaug
Wow. These guys rock.

~~~
DaniFong
Thx :-)

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ableal
Found additional details here: [http://daniellefong.com/2012/01/22/green-
dreams-life-in-the-...](http://daniellefong.com/2012/01/22/green-dreams-life-
in-the-year-of-the-rabbit/)

