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:<p><pre><code> W = - ∫ P dV = - n R T ∫ dV / V = P₀ V₀ ln(P₁ / P₀).
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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.<p>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.<p>(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.)