Right now Lightsail is attempting to cross the startup death valley by first selling its pressure vessel technology before tackling the more demanding energy market. They recently announced a tractor trailer sized compressed natural gas module. The idea is to get their costs down to where they can be competitive in energy storage, its an open question on whether that is possible.

Btw, their vessels are carbon fiber, not steel. And deriving heat is a problem not a benefit. Every bit of heat that leaves the system is lost efficiency.

http://www.marketwired.com/press-release/lightsail-energy-la...

http://www.wsj.com/articles/energy-storage-startup-lightsail...

Some groups like LightSail think that compressed air storage is going to have the lowest lifetime costs. Others think that flow batteries will win. Others think that thermal storage will win. Others think that big banks of lithium ion batteries will win.

I don't know who's going to win. All of the above approaches are technically viable. The winners and losers are going to be shaped by things other than simple technical viability: business plan execution, manufacturing scalability, path dependency... I say that storage is not "solved" because there's no default good-enough solution proven yet. We have to wait for some years after plans leave PowerPoint and inhabit the real world to see what works well in practice.

Electrical engineering is far more advanced technologically than compressed gas thermodynamics and the gap continues to widen.

You can make a very efficient turbine jet that runs at one pressure and one power output at very high expense and ongoing maintenance of moving parts. But it won't run or only run as dismal efficiency at half pressure or quarter pressure.

Meanwhile you can very efficiently move electrical watts around in various formats, cheaply, and scalably, and no moving parts.

As an example of what I'm talking about, liquid fuel has insane amounts of energy stored in it and only recently can we make turbines efficient enough to keep spinning while having fuel poured into them, only at certain constrained power outputs. Meanwhile its very easy with modern technology to run an electric motor from 0.1% to 100% speed using electronic speed control VFDs. You might have trouble cooling it but you won't have trouble making it spin.

Why air? If you are using solar to compress the air...why not take it a step further. Use the underground storage you propose to store hydrogen. Solar energy drives electrolysis, leaving you with hydrogen and oxygen, both of which can be safely stored and have vastly greater energy potential(when burned) than the stored energy of air.

I love your idea of using solar to build a store of energy, but you could refine the idea a bit and really do something fun. Having a large quantity of oxygen(liquify it!) could be a boon to the backyard rocket engine enthusiasts :)

Without knowing too much about LightSail, the answer is likely due to cost. It is probably much cheaper to buy a bunch of lead-acid or li-ion batteries and put those in your backyard. It is not trivial to put huge steel tanks in the ground and keep air compressors and generators running smoothly over many years. This can change with time though.