The part at the end where she is against top-down thinking, and instead promotes bottom-up inductive thinking where you first figure out how to make airplane parts well before tackling the whole aircraft, is a lesson I've learned the hard way an almost unhealthy number of times. She also seems to understand the difference between leadership and management.

Other than that, the 90% figure seems pretty believable to me. If you are playing with containers of gas, you can do stuff that is quite close to the physics textbooks' chapters on thermodynamics. The water is probably so they can get to 750 kwh per tank.

Are there any other processes where the engineered efficiency is that close to ideal? Pumped-storage hydroelectric is close.

It says 90% thermal efficiency, which makes me wonder what the electrical efficiency is. Also, "500 hours reliable operation" doesn't look great.

Combined heat and power (CHP) might be indicative for that - nevertheless 90% roundtrip what they say on their website is very impressive.

Compressed Air storage has been around for some time and one of the limits is of course the low energy density. They mention 750 kWh modules containerize - how many containers will that be at what pressure - 20 ? 50 ? 100 ? (so that they can still match safety standards while being closer than e.g. 1km to residential areas)

Hi there,

This is Danielle Fong -- I'm also a longtime HN'er.

1 container is 750 kWh at 200 bar.

We adhere to the ASME codes which have an impressive set of tests you need to pass -- for example you need to cycle 60000 times in an oven, cycle to 1.5x design pressure with two deep gashes in the outwear walls (40% of the depth of the wall), and survive without bursting a 50 caliber armor piercing round shot from an anti-tank rifle.

You are right that 90% thermal efficiency doesn't account for electrical or mechanical losses.

Unfortunately it is a little confusing and we need to fix that. The gold standard is electrical to electrical efficiency, but we had not optimized or demonstrated that at the time. (We are very close!)

What we had done (2011 - 2012) is demonstrated the thermodynamic, scientific viability of the process. The 90% one-way thermal efficiency compared to what a typical adiabatic process would have of about 65%. The one way efficiency of a typical compressor is further reduced by friction, etc. to about 59%, and still further by electrical losses to about 55%.

At the moment we can achieve a bit above 71% one way in our best runs (including electrical) and are trying to drive to 80% this year -- that's a 50%+ or 64% electric to electric efficiency. We want to be near to 60% across a whole operating cycle.

Note that with 55% one way efficiency from the best previous compressors, that's only about 30% efficiency. So the promise of our tech is that we double efficiency overall for energy storage -- and that's enough to make it viable.

Many thanks for the explanations Danielle - this sound like some quite substantial improvements over numbers I know from large scale CHP solutions e.g. Turboden (turboden.eu) that are providing about 19% as gross electric efficiency. They have been building large scale CHP solutions for more than 15 years in Europe but of course they are using turbines and their applications (heat recovery, biomass energy production, Rankine Cycle / ORC) are seemingly quite different from what you're currently working on.

"The part at the end where she is against top-down thinking, and instead promotes bottom-up inductive thinking where you first figure out how to make airplane parts well before tackling the whole aircraft"

I initially thought this line was a parody and the article would be full of buzzwords, not the case!

The LightSail website has a nice comprehensive overview of how the technology works. Well worth a read: http://www.lightsail.com/

Particularly impressive: 90% thermal efficiency.

I want to invest...

What I'd like to see a matrix comparing all of the different systems using compression or stored energy via non-battery means.

They mention low cow air storage tanks - comparable to what though?

The DOE has some great resources, I can't the one I'm thinking of, but this document:

http://energy.gov/sites/prod/files/2013/12/f5/Grid%20Energy%...

On page 30 has a matrix of the different attributes of different energy storage systems (but unfortunately not cost or capacity).