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I'm wondering, if you put an air compressor pipe into a micro turbine, lets say something like that attached to a motor, does air that get out of the turbine have the same energy as when it gets in ?

Because i get that the compressor might have to push harder, but the air itself still has to get out of the pipe, basically i don't get why you can't put like 10 of those turbines attached together and somehow get free energy, i know that it's impossible but i don't get why..

Is there a way to calculate the energy of that thing?

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  • $\begingroup$ i've just watched a youtube video about that, i clearly get that energy comming from the air is lossed when the turbine works, but what i don't get is that, ( assuming there is no leaks ) energy is generated by the air that gets in, so that air got energy, but the exact same amount of air gets out of the turbine, certainly at the exact same pressure too since there is no leak, so how could the air that come out have less energy ? $\endgroup$
    – airturbineguy
    Oct 27, 2017 at 17:46
  • $\begingroup$ i'm talking about a video on energy balance in turbine... $\endgroup$
    – airturbineguy
    Oct 27, 2017 at 18:28
  • $\begingroup$ I don't believe in free energy at all i'm just wondering where do the energy get lossed knowing that the same amount of air at the same pressure gets out... the informations i found are all about complicated maths that i can't understand that is why i asked the question. $\endgroup$
    – airturbineguy
    Oct 27, 2017 at 18:51
  • $\begingroup$ Total energy - Work = Change in internal energy, so check out enthalpy and now it's goodbye from me $\endgroup$
    – Solar Mike
    Oct 27, 2017 at 19:25
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    $\begingroup$ @airturbineguy - I have edited one of your earlier comments to remove the unnecessary language. Please keep in mind StackExchange's "Be Nice" policy. $\endgroup$
    – user16
    Oct 28, 2017 at 12:47

2 Answers 2

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When you compress air it gives off heat per the ideal gas law. When the air re-expands it doesn't get that heat back. There has been some research in using large salt caverns to store compressed air and using water or molten salt do store the thermal energy and return it to the air as it expands in the turbine. Even then, there is still loss because heat transfer requires a differential. This results in a lower expansion temperature and a lower energy extracted.

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  • $\begingroup$ More precisely, when you compress air (adiabatic compression), you simultaneously heat it up (input work needed both to increase pressure and temperature). During storage it cools down - loses heat. When decompressed, it can give out only the pressure-derived part of energy, the heat-derived part is already lost. $\endgroup$
    – SF.
    Nov 29, 2017 at 12:35
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No, the energy coming out of the turbine does not have the same energy as when it goes in. The compressed air does work on the turbine by decompressing: this means that the pressure downstream from the turbine is less than the pressure upstream from it.

The reason you don't get more energy when you put them in series, is that you've still only got the same energy being input - the energy in the compressed air. You can't get more energy out than you put in.

The mechanism that prevents the energy out being proportional to the number of turbines in series, is as follows.

If you've got just one turbine, then the upstream pressure is the pressure from the compressor or compressed storage, and the downstream pressure is about equal to ambient air pressure.

Now, if you've got two turbines in series, then the upstream pressure on the first turbine is still equal to the compressor pressure; and the downstream pressure from the last turbine is still about equal to ambient air pressure. And that means that the air pressure between the two turbines is somewhere between the two, which means that the air has done less work on the first turbine, and less work on the second turbine.

All in all, the two turbines combined had the potential to do no more work than one turbine on its own, because the pressure gradient from start to end is the same: except now, that work is spread across two turbines instead of one, so you've got twice the losses, and hence in reality less work is done by two turbines than one.

Note that this applies because the two turbines have same kind of work applied to them by the same kind of energy. Combined-cycle gas turbines get more work out of two turbines in series than one, by burning fuel to create an expansionary force that drives the first turbines; the heat in that exhaust air is then used to heat water, creating steam, which then drives a second turbine. This means that the second turbine does not have to compromise the pressure downstream from the first turbine.

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