Can you explain in detail what problem this solves and how it works to someone with a basic background in physics?
Below is what my understanding:
Current energy storage mechanisms (batteries) are inefficient. In theory one way to store energy that would be more efficient (why?) is to use the energy to compress air (with a conventional air compressor?). The air is stored in a container until it is converted to electrical energy through some inverse process (powering rotary screws?). Unfortunately compressed air is very hot and difficult to store. Instead water droplets can be injected into the compressed air container. These droplets will absorb most of the energy of the compressed air. The vapour is separated from the compressed air and stored in other containers (still as vapour?). How is the vapour then converted back to electrical energy? Isn't the vapour just as hot as the compressed air? Is it easier to deal with because it can be stored in a larger volume?
Energy Storage is important because the supply of electric power has to match the demand in real time. Renewable power has problems doing this which creates demand for energy storage.
Many powerplants such as Nuclear or Coal can take hours to startup and shutdown. The power markets and grid management orgs spend a lot of time scheduling power production and balancing supply and demand. Plants tell grid operators, we're running from 2-6pm on Tuesday - and face fines if the power isn't delivered. Some plants get paid to be on "standby" to deal with surges in demand.
The price of power can at times become negative, because for some plants it is cheaper for the operator to get zero or even negative revenue for a short period vs. shutting down. Wind power Tax credits and carbon offsets at times mean windfarms can make money with a negative powerprice.
Wind power can have problems meshing with the grid and scheduling/dispach of power. If you're producing power when you're not scheduled and demand is low the price a plant gets can be very low, even negative, because it isn't needed. Part of this is because of market rules, and part of it is actual operations.
So if the wind is blowing during a period of time that people are not using much power the windfarm operator makes very little or nothing. If you can store energy for even a few hours it can be moved from periods of low demand to high demand which makes a lot of money and ensures you are not fined for failure to deliver power.
The big economic impact here is that there are a lot of powerplants called peakers that only run durring periods of peak demand. Peakers tend to be the least efficent plants out there because they are run so little, like burning 1.5-2 times more natural gas per unit of output. If you can store energy then you can build efficent power plants that produce all the time and use a lot less fuel.
Yes, I was an Energy Economist and part of the Enron Power Trading desk years ago.
Stupid question: why can't you store all the energy in warm water, eliminating the problematic (big, dangerous) pressure vessel altogether?
I envision a compressor, a heat exchanger (mist or otherwise) and an expander. The expander and compressor are connected mechanically (they can be the same device like in a piston engine or can be rotating machinery connected via shaft like in a turbine).
During storing, or water warming, the expander produces less power than the compressor so you need electrical energy to spin the system.
During energy release, or water cooling, the expander produces more power than the compressor needs and you can use a generator to extract that.
This is pretty much a standard heat pump or refrigerator arrangement.
I assume this is not efficient because of some not at first sight obvious quality of thermodynamics. It'd be cool to get a little bit insight into that.
By the way, your diagram's first picture with the piston is off: either the shaft should be thicker or it should depict sealing between piston edge and cylinder, now the volume of the pressure vessel changes very little when the piston moves.
The trouble with your arrangement is that if there ∆T is low, there must be very many cycles between mechanical and heat energy into order to store an equivalent unit of energy. This gives very many opportunities to lose efficiency.
There's a company out there, Isentropic Systems, that's trying this. A steep mountain to climb.
> Unfortunately compressed air is very hot and difficult to store.
Not OP but a few comments on what you just said. If you ever tried to manually compress air in a metal pump (say when inflating a bicycle tire), you probably noticed that the outside of the pump gets very hot when you press down. That is because gases have a property of increasing their temperature when pressure increases. So now in order to maximize your ability to store energy efficiently, you are facing two problems: (1) how to prevent gas from escaping, (2) how to prevent heat from dissipating through pipes/pistons. It looks like OP had managed to solve (2) by efficiently capturing the heat using water vapor, though I'm not sure about specifics.
Water actually has the highest volumetric sensible (no phase change!) heat capacity of any substance on earth.
We don't rely on water vapor -- the problem is that at low temperatures there's very little vaporization that can even take place. This limits the heat transfer. Though there are alternative approaches that use this effect more fully.
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.
Below is what my understanding: Current energy storage mechanisms (batteries) are inefficient. In theory one way to store energy that would be more efficient (why?) is to use the energy to compress air (with a conventional air compressor?). The air is stored in a container until it is converted to electrical energy through some inverse process (powering rotary screws?). Unfortunately compressed air is very hot and difficult to store. Instead water droplets can be injected into the compressed air container. These droplets will absorb most of the energy of the compressed air. The vapour is separated from the compressed air and stored in other containers (still as vapour?). How is the vapour then converted back to electrical energy? Isn't the vapour just as hot as the compressed air? Is it easier to deal with because it can be stored in a larger volume?