It is not super clear what the innovation is here as I am sure someone must have thought of storing compressors heat with water before.
I think maybe the novel thing is to pass the water as a mist through the compressor cylinders where it can transfer energy from air much faster than if, for example there was just a heat transfer closed circuit around the cylinders.
Their website states:
"We have achieved these high thermodynamic efficiencies at higher RPMs than many thought possible. This is crucial to achieving low cost: the higher the RPM, the higher the power of the same machine and the lower the cost per kW."
So basically you need fewer compressors and heat transfer systems for the same amount of power. I guess as long as the added complexity and maintenance doesn't add too much cost it could be more economical than using the higher number of compressors.
They do mention in the WSJ article that one challenge is preventing 'hydrolock' which, if I understand correctly, would happen if you accidentally injected too much water in a cylinder. Since water is not compressible, you could bend or break your piston rod, crank shaft or 'cause the cylinder to explode'.
The main thing is that the mist process is higher efficiency.
If you compress the air, let it heat up, and THEN cool it by mixing the air with water, the pressure will be high during the compression process, which will take a lot of energy to compress it, and then cool off and reduce in pressure. That's bad.
What you want is to keep the temperature as low as possible during compression, and to keep it as high as possible during expansion. We do this.
When you cool the air by mixing it with water spray, a good portion of the energy is now spent producing low temperature water vapour. So... how do you recover that energy? You'd need to condense the water vapour to get at the latent heat.
And if you do recover the heat by condensing this vapour, it is low grade heat, which CANNOT be efficiently converted back to mechanical power or electricity.
Your web site claims 90% of the "grid" energy goes to heat storage. AFAICT This is NOT POSSIBLE if the heat comes from air compression. Is this an error in presentation? A fundamental error in your concept? Or am I mistaken... please explain.
Sorry! This is a very subtle process. If you analyze it superficially, it makes sense, dig deeper and it's confusing, and then dig still deeper and it makes sense again.
When you cool the air by mixing it with water spray, a good portion of the energy is now spent producing low temperature water vapour. So... how do you recover that energy? You'd need to condense the water vapour to get at the latent heat.
You're right in direction but not in magnitude. There isn't much vapor produced, because the saturation vapor density is very low. Initially it evaporates, this cools the air before compression, and then it saturates. Any additional vaporization is recovered, because it condenses on expansion.
And if you do recover the heat by condensing this vapour, it is low grade heat, which CANNOT be efficiently converted back to mechanical power or electricity.
Also, interestingly, low grade heat can be converted into energy when you have a source of compressed air. This is not a full thermodynamic cycle because at the end of the expansion, you've also expanded air.
One of the best ways to see this is to imagine an energy storage system that's a giant Carnot cycle. The energy out/energy in is T_exp/T_comp. This is higher than the Carnot efficiency -- because it's not accounting for the energy in! The Carnot efficiency is E_out - E_in/Q_in which is 1 - T_c/T_h, the familiar expression.
Your web site claims 90% of the "grid" energy goes to heat storage. AFAICT This is NOT POSSIBLE if the heat comes from air compression. Is this an error in presentation? A fundamental error in your concept? Or am I mistaken... please explain.
Actually, if the compression is isothermal (and it's an ideal gas), 100% of the energy from the grid is turned into heat, and the energy state of the air is constant. U = 5/2 NRT.
Likewise, upon expansion, 100% of the energy comes from the heat.
The state of the air changes, but not in energy -- in entropy. As the air is compressed, work is added at teh same rate as heat -- and entropy, is removed.
From a technological point of view, is there any reason this is only possible now? Or would it have been possible decades ago, if only there had been enough interest in it or someone had had the idea earlier?
This seems like a really smart feat of thermodynamical engineering, but it does not reference explicitly any technology that would not have been available 30 years ago. I could imagine that being hidden in the subtitles of getting the process efficient enough - e.g. in computer-based component design and CFD simulations.
I think maybe the novel thing is to pass the water as a mist through the compressor cylinders where it can transfer energy from air much faster than if, for example there was just a heat transfer closed circuit around the cylinders.
Their website states: "We have achieved these high thermodynamic efficiencies at higher RPMs than many thought possible. This is crucial to achieving low cost: the higher the RPM, the higher the power of the same machine and the lower the cost per kW."
So basically you need fewer compressors and heat transfer systems for the same amount of power. I guess as long as the added complexity and maintenance doesn't add too much cost it could be more economical than using the higher number of compressors.
They do mention in the WSJ article that one challenge is preventing 'hydrolock' which, if I understand correctly, would happen if you accidentally injected too much water in a cylinder. Since water is not compressible, you could bend or break your piston rod, crank shaft or 'cause the cylinder to explode'.