Q&A: Danielle Fong, Chief Scientist, LightSail Energy

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.