Water injection improves or reduces gas turbine power?

[Sorade]

Hello,

I am currently working on a storage project which allows me to decouple the compressor from the expander of a gas turbine. (see image link below)

The reason I use a gas turbine is to reheat the air which was cooled after being compressed. As you can imagine there is a efficiency loss associated with that.

When thinking of ways to improve the efficiency during the power generation phase (expansion) I was wondering if adding "small amounts" of water in the combustion chamber would increase or reduce my efficiency... By "small amounts" I mean amounts higher than atmospheric air, but that would not lead to condensation.

I can think of arguments for both but don't really know which one would be dominant:

1) Improves Efficiency: Adding liquid water into the combustor and converting it to steam would greatly increase it's volume and therefore increase the inlet pressure to the expander and hence produce more work out of the expansion.

2) Decreases Efficiency: Converting the liquid water to steam would drop the temperature of the gas entering the expander and therefore reduce the work of expansion.

 

[Ketch22]

It is indeed a common thing to inject water on industrial Gas generators. There is an observable increase in power although it tends to be small. It also affects combustion temperature enough to have an impact on oxides of nitrogen and thus it is more economical to scrub the exhaust.
My initial thoughts are that in a system such as you picture it would not be helpful. By storing the gas in an earthen chamber you are also adding a heat sink. The gains from water injection would be lost to condensation once sequestered. You may be able to add one more compressor and achieve some gains. I am thinking a Gas generator with water injection (water injection adding to the mass flow) leading to a power turbine of conventional design leading to an additional Axial or Centrifugal compressor ( this would benefit from the increased power output of the turbine. It would be difficult to overcome the huge buffer introduced by the chamber though.
Lots of stuff for you to consider and calculate.

 

[Rx7man]

How about injecting water immediately after the compressor to turn it into steam? Ketch22 has a point, condensation may be big enough problem that it makes water injection useless.

This cavern better be MASSIVE in order to supply air for turbines of any size for any length of time

 

[Sorade]

Thank you both for your replies.

@Ketch22 , so do you suggest adding a compressor just before the turbine ? Wouldn't that defeat the purpose of decoupling the air compression and expansion ? At the moment I have ~100% of my power output which can be sent to the grid ... if I was to add a compressor during in the expansion phase wouldn't that consume too much of my power output (up to ~40% if I'm not mistaken) leaving me just ~60% for the grid ?

@Rx7man , I don't think injecting before the cavern would work since the cavern temperature is about 30 or less degrees Celsius leading to condensation. Current CAES plants have more than one cavern each in the order of 300 000m3.

 

[Rx7man]

If you're going to have a condensation problem, you're going to have a condensation problem whether you inject it before or after the compressor

 

[Ketch22]

I looked over your drawing again and I think I allowed myself to be mislead. The drawing you posted showed a HP and LP turbine and you referenced a Gas turbine. This is common for Electric generation to use a Gas turbine to drive a generator. The Core of a Gas turbine engine is a Gas generator, this produces a consistant medium pressure high velocity gas. That Gas is used to drive a separate turbine which drives the gear reduction/generator. This is what I thought I saw in you drawing. With your additional input I think I am seeing two turbines that run of compressed air from storage. Although this would technically be a "gas" turbine. it is a gas turbine in the fashion I was thinking of.
The high temperature system of external combustion used in a gas turbine engine is what benefits from the temperature reduction for emissions. There is an additional benefit of increased mass to the exhaust flow which allows for more power to be harvested. (as a side note, jet airliners in the right conditions of heavy rain can actually experience an increase in effective thrust.
For application the water is usually injected alongside or concurrent with the fuel. It is sometimes injected shortly before combustion. This allows for the water to flash off into vapor.
For the application you are looking into I do not see a good path of flashing the water off to get an effective increase. If they atruly are compressed ait turbines the expansion of the air wil cause a drop in temp. The water then becomes an impeadence as it is less likely to vaporize in the higher pressure environment.

If I am misunderstanding your drawing please elaborate a little further. If you are in fact using a Gas Turbine engine I can go further into what my idea was

 

[Rx7man]

@Ketch22 I don't think you're misunderstanding it that much.. I see a natural gas (or other fuel) line going to the turbines, it looks as it's just getting fed pre-compressed air from the cavern, and it's not JUST the compressed air doing the work

 

[Sorade]

@Ketch22 as @Rx7man says, the air is pre-compressed (say at night time) and then, re-heated by adding gas and combusting the mixture, before being expanded. I do agree that me using the term "gas turbine" is misleading, since usually the compressor is part of it. I guess you could see it as being better described as an expander. But to avoid the air temperature dropping to much in the expander it has to be re-heated (otherwise expanding air at 50 + bar and 30 degrees C to atmospheric pressure would cause severe issues). At the moment I am actually looking at design which over-heat the gas by quite a lot (say 900 + degrees K) which prevents condensations in the expander as the exhaust temperature remains high. And to avoid too many losses by sending a lot of heat to the stack I use a heat recuperator.

So to sum it up, my cool air coming from the cavern to my expander needs to be heated up. So far I use:
- heat from recuperator
- Natural gas combustion

Would adding some water into the combustion process be beneficial for me ?

PS : I should probably have said this before but I am not an engineer, but a geologist. So it is a pretty steep learning curve for me !

 

[Nidum]

1) Improves Efficiency: Adding liquid water into the combustor and converting it to steam would greatly increase it's volume and therefore increase the inlet pressure to the expander and hence produce more work out of the expansion.

The inlet pressure to the turbine can never be higher than the inlet pressure to the combustor in a standard design gas turbine .

To get any significant benefits from water injection the water has to be introduced at entry to the compressor .

There have been experiments with waste heat steam turbine energy recovery systems .

You cannot guess your way through the design of a system such as you are proposing .

Look at :

The fundamental thermodynamics of the system .

The mass flow , pressures and temperatures of air needed to generate useful amounts of power .

 

[Rx7man]

From what I see, and no, I'm not an expert, but adding water after the high pressure turbine would probably be most beneficial.. LOTS of heat there to vaporize a lot of water, making much more volume for the low pressure turbine to work with.. You may be able to add some before the high pressure turbine, depending on how hot the intake air is after getting heated by the recuperator

 

[Nidum]

Even if injecting water into the turbine was a useful thing to do there would be considerable practical difficulties in actually doing it .