More LightSail Frequently Asked Questions…

by Danielle Fong

Hello everyone, missed you!

Firstly, I wanted to encourage the readers who visit this blog to follow my twitter. I’m fair bit more active there these days, possibly because it’s less of a time investment than blogging and it’s very busy here at LightSail!

Secondly, I’m posting a short note because some of the articles online and some of the things on our website confused some people. Particular here, at ValueWalk.

So, we’re going to try to make things clearer. But for now, here’s a short list of answers to frequently asked questions, and *particular* misapprehensions we weren’t expecting. I’ve posted this here, and to the comments page on that article. Enjoy.

Hello everyone, Danielle Fong (from the article) here.

Thanks for the coverage and interest. This article slipped my notice, and it seems that there are some misconceptions I should clear up.

1. Why is energy storage so important now?

First, here’s the main point. The objective is to make solar and wind cheaper than power from fossil fuels, and available whenever it is needed. To do that, you need energy storage. Only since about 2004-2005 has it been broadly true that solar ad wind energy are cheaper sources than energy from oil. Now, economical energy storage is needed to make it available whenever people need it most.

2. What’s new about our approach? 

Second, we know that Compressed air energy storage is not new!

But we’ve made several significant improvements to the state of the art that significant reduce the cost and increase the efficiency, and we calculate that this allows us to make stored solar energy cheaper than power from diesel fuel. That’s a very big deal, if we can do it.

Probably the most important advance is the reduction in cost. We’ve kept relatively quite about this, but we’ve developed the worlds lowest cost air tanks using composite technology. Though the fibers we purchase are more expensive than steel per pound, they are vastly stronger, and thus about 2-3x cheaper for the strength you get than steel. That’s an advance by our cofounder and CEO Steve Crane, and spearheaded by Dr. Neel Sirosh.

We’ve also reduced the number of cylinders needed. Previously, to compressed and expand to and from 200 atmospheres of pressure, you’d need at least 5-6 compression stages, and 5-6 expansion stages, and 4-5 intercooler heat exchangers in between the compression stages, and 4-5 interheaters in between the expansion stages. That’s a lot of equipment.

With our technology, we only need two compression stages, because we avoid many thermal problems by using water spray (more on that later).

The same compression cylinder stages can be used for expansion — the compression cylinders are also used to expand the air. This is what we mean by reversible.

3. What do we mean by “Our system is fully reversible.”

This refers to slide 3 in our main page / technology page.

Our System is Fully Reversible

To store energy, an electric motor drives an air compressor. To deliver energy, we reverse the process–the air compressor becomes an expander, and the electric motor becomes a generator.

“Heat from compression is stored or routed to nearby buildings, providing heating. During expansion, heat is extracted from storage, or buildings providing air conditioning. This dramatically increases building energy efficiency.”

Unfortunately, it has been confusing to people that we use the term “Reversible” in this context. Sorry about that. There is a specific meaning for reversible in thermodynamics, but we thought people would know that is not what we meant, since a thermodynamically, perfectly reversible system is impossible. We thought it would be clear that what we meant is that we use the compression cylinders as expansion cylinders, and the electric motor as an electric generator — that’s what the animation shows (see here, on the “Our system is fully reversible” slide, click the arrows).

Apparently, we have been confusing people! Sorry about that. Our mistake.

4. We figured out how to increase the efficiency dramatically

In previous attempts at compressed air energy storage, air got hot when you compressed it. Very hot. So hot that people had not figured out how to capture it practically. To store the air, they had to cool it, and they just rejected the heat to the atmosphere — turning it into waste heat.

Hot air is at a higher pressure * volume, all else being equal. So you have to expend more energy compressing it. Rejecting the heat means that the air decreases in pressure * volume, and you get less energy out when expanding. This effect was responsible for more than 50% of all of the efficiency losses of previous compressed air energy storage systems.

Our approach is:

a. injecting a dense, cool water mist in during compression to cool the air, keeping the energy needed for compression low,
b. holding onto the warm water in an insulated container,
c. and injecting the warm water mist back into the air, keeping the air pressure higher longer and increasing the energy you get back out

This has been very successfully demonstrated, and we’re getting better and better at it. Many people thought it was not practical (would break the compressor), or wouldn’t work (some thought the heat transfer would be too slow), but we’ve proven that it does. It’s still hard work ahead to optimize this and launch the product, but that’s the approach we’re using and it really works.

5. There’s more information online!

Lastly, there’s more information available.

Our website: gives more details.

This talk also should help. https://www.nantucketproject.c…

Also, there’s a lot of patents, though the material is pretty dense.

I’m not going to go through all of the comments here, but if anyone has further questions they can ask me here in this thread or on twitter at @DanielleFong

Danielle Fong, Cofounder and Chief Science Officer, LightSail Energy