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Design compressed air energy storage for PV plant 3

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Ameen1985

Ameen1985

Mechanical
Feb 10, 2021
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Hi All
For a PV project of 5 kW, we will use a CAES.
The preliminary design will consist of a compressor - 2 heat exchanger - Air receiver - air motor - generator - 2 water tanks as a thermal storage units to have an adiabatic systems.
For a 5 kW PV plant, an energy of 20 to 26 kWh is expected.
At this stage I don't have enough information about the components size and I want to start with the air tank size.
I want to study different sizes and different tank pressures which will accommodate a certain amount of energy.
Any help please?
 
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You need to build simple models of each section of the plant, then you can test different configurations.
There are so many issues with storage, pressure, size, number of vessels, and cost to build.
These variable often work against each other so you need to find a balance.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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I saw this and thought how to answer.

As usual it depends on what you're trying to achieve, space and money available and energy to be stored and for how long

As an electric to electric storage it is terrible in terms of efficiency even with your impractical heat storage idea.

For pressure you either go high pressure low volume and take the hit on efficiency or low pressure huge volume and the attached showed a volume of 18 m3 to store 350Wh!!

There's a reason most people go for battery storage....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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What is the business case for this?

Notwithstanding the dubious website provided by LittleInch, the efficiency of electric to air to electric will be awful.

The heat will of low quality, and of no possible use to make electricity. Maybe you will be able to make a few pots of luke-warm tea.
 
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Littleinch is correct....

Storage of PV electrical energy through the use of compressed air is a bad idea because of massive thermodynamic losses both in the compression of the gas and in the expansion of the pressurized gas.

Additionally rotating machinery always requires lubrication and maintenance costs

Storage of energy in batteries makes much more sense. Batteries are solid state, have reasonable lifetimes and are becoming less costly

MJCronin
Sr. Process Engineer
 
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in 2000-2001, my employer obtained a large CAES project (2700MW total) which eventually was not built simply from the standpoint of economics. the facility could not sell the power generated, including profit, for more than what it consumed for compression. it was a grand plan/design, but not to be built.

fyi, page 3 at this link provides a good summary.

i just do not think the economics are there yet.
 
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Many many thanks for your kind replys and help.

I just wanted some help in terms of thermodynamics to test different pressure ratio for the same amount of energy to see its effect on the expected air bottle size.

If the air motor works at a certain pressure, what should be the minimum and maximum pressures in the air bottle for good operation?

 
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It appears that you have selected CAES.
Given all the debates CAES regarding lower efficiencies etc. it is still a valid option for energy storage because:
1. It has more life than batteries
2. It could be cheaper than batteries, though batteries are fast catching up.
3. More than 10 large scale CAES have been installed.
(Refer Wikipedia)
Regarding the tank volume requirement please see the approximate calculation below:
Since you have considered heat exchangers it is presumed that you envisage an isothermal system
image_zg8nt3.png

Caution: Even though P_A has been considered atmospheric but you need to take P_A as minimum operating pressure of your air turbine. So in actuality the storage pressure and/or the storage volume may be much higher than above.

Engineers, think what we have done to the environment !
 
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Many thanks goutam_freelance for your help.

we will have the heat exchanger to store the heat generated durning compression in a water tank to be used during expansion. Meanwhile the cold temperature resulted from expansion will be used to cool the air during compression to have an adiabatic system as I mentioned in my post.

So, our system is not Isothermal I guess. Should I still use these equations ? Or should I add an isentropic efficiency term to the equations ?

Many thanks in advance
 
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@Ameen1985.
What I have calculated is isothermal compressed air energy stored in tank(s). The temperature of air inside the tank will reduce due to expansion(theoretically isentropic). But if you keep the tank at the same temperature using hot water reservoir then theoretically the tank is in isothermal condition. But if you do not maintain the tank in isothermal condition then you need to consider reduction in temperature of air inside tank (theoretically isentropic).

Engineers, think what we have done to the environment !
 
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"what should be the minimum and maximum pressures in the air bottle for good operation?"

This is the root of the issue and only you can decide.

There are two points to note here:

The power consumed by the compressor will rise as the pressure in the tank increase if you have a single simple piston type pump. This will probably not match the power profile coming from the PV panels over the day which will peak at mid day. Therefore you need to figure out what to do with the spare power or how you will vary it. so either some sort of variable speed device or a number of fixed speed small compressors you can turn on and off as required.

Same with the power output which will either need throttling and hence some losses to a fixed pressure or a very variable output.

So you can choose either basically a minimum I would say of 5 bar up to whatever you want and you can vary the parameters in goutams spread formula to be very high low volume PV or vice versa.

If you're made of money then you could either go for a min of say 5 bar and a max of 10 or min of 50 and max of say 70 bar to operate in but your vessel size gets big. The smaller the pressure range, the easier it is to have a fixed energy input and output, but the bigger the PV.

BTW you call what we are talking about as Pressure Vessels. "Air bottle" people think of as 65 or so litre bottles you use for scuba diving or similar. You're talking at least 30 or 40,000 litres at low pressures. That's a lot of air bottles.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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Ok let's say I have an available energy of 26 kWh per day and about half of it will be stored during the day. Taking into consideration the compression efficiency of 85% so I will have about 11 kWh to be stored.
There will be losses in terms of heat (I want to store this heat to be used during expansion). I will also cool the air during compression but it won't be Isothermal process of course. How many kWh of the remaining 11 will be available to be stored ??

Also, let's say I have an air motor with air supply pressure of 6.3 bar. Should this be minimum pressure inside the pressure vessel and replace the atmospheric pressure in the equations with this value?
 
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Good luck in storing this relatively low grade heat. It has a tendency to leak away very rapidly.

My guess is a max of about 8kWh.

Yes if your motor needs 6.3 bar (~90psi) then that becomes you minimum storage pressure and then you need to regulate higher pressures down to that supply pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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Ameen1985 said:
How many kWh of the remaining 11 will be available to be stored ??
Click to expand...
Your question is not clear. Theoretically whole 11 kWh is available for storage. But when you are cooling the compressed air the heat removed is not available for compresses air storage. Part of it may be stored in hot water storage. These depend on system details.
Ameen1985 said:
Also, let's say I have an air motor with air supply pressure of 6.3 bar. Should this be minimum pressure inside the pressure vessel and replace the atmospheric pressure in the equations with this value?
Click to expand...
Throttling of air involves wastage of recoverable energy. Your turbine should be designed for variable pressure for best system efficiency. If your turbine becomes very inefficient below 6.3 bar then you should stop operating the turbine below 6.3 bar. Then your P_A is 6.3 bar in formula above. But the required volume V_B increases.

Engineers, think what we have done to the environment !
 
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What if you used a low boiling organic, rather than air as the working medium. Like butane or pentane in an ORC?

Then you only need to compress (with PV generated power) butane up to a press that will enable air or water cooling. Then the rest of the pressure boosting can be with a much more efficient liquid pump. And you get more efficient recovery of compression heat. So instead of storing compressed air, you store compressed butane.

It may be possible to further refine this by working with a binary C4/C5 mix to account for ambient temp variations between summer and winter.

 
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But then he would need to store a huge volume of gaseous butane to use on the next cycle.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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