Pneumatics versus stirling engine versus electrical engines

A pneumatic motor , or compressed-air engine, is a type of motor which does mechanical work by expanding compressed air. Pneumatic machines work well on stationery devices that can be powered by a compressor.When the compressor itself can be made to run on a renewable source like wind or solar or stored heat then pneumatics are simpler and cheaper than electrical machines

A Stirling engine is an externally heated engine. Heat is supplied from outside the engine leading to expansion of the working fluid and generation of work.

Stirling engines have higher effective power density (storage + transducer) approaching that of electrical machines .They have been demonstrated to work with vehicles (land). The thermal battery technology can easily be scaled to megawatts making stirling engines very attractive for extracting energy out of stored heat. Heat exchangers can ensure operating temperature of up to 600 degree Celsius on the hotside.

https://youtu.be/KbnGlcQiL1c?si=slQE5c0E-EY2he5m

This makes them good substitutes for land and water use.

(More heat approaching 1000 degree Celsius will be necessary for aircraft propellers. Thermal batteries can handle the load but can heat exchangers be designed at a thousand degrees?)

[Where do pneumatics shine? Where does the stirling engine shine? The big problem of pneumatic vehicles – effective power density of pneumatics]

It is true that pneumatic machines are lighter than their electrical counterparts but it needs to be supported with a constant compressed air source which is consumed quickly.

Now imagine what would happen if an electrical generator had to be hooked up to an EV for it to run? Or what would happen if a ceiling fan had to bear the weight of a power station to turn?

This is why pneumatic machines will not work with vehicles, the total weight of the air tank, motor, and compressor is too large. Air is not being burnt, just decompressed and it's quickly consumed on a kilowatt scale motor .5l/sec. Unless this compressed air is replenished quickly the engine would stop and the vehicle would not even reach the next fuel station.

[Compressor for pneumatics]

Oil and gas pipeline infrastructure operates megawatt scale compressors that can transmit compressed air 60-100 Km range at a time.

The choice of pneumatics versus electricity depends upon how far generated energy is to be transmitted. Pneumatics are good at mid range while electrical energy can be transmitted across continents. On the other hand it's easier to generate compressed air than it is to generate electricity.

How big of a problem will heat generation be? Pneumatics are cool under operating conditions, don't really require heat management and can be made from polymers. They are a good choice for heavy but stationary machinery.

For the same reason that pneumatics are not good for vehicles they are less desirable sources of electrical energy generation as compared to stored heat.

It makes no sense to run a compressor on the back of work done by Stirling engine and then use compressed air to generate electricity. Might as well generate electricity directly from stored heat. Heat can remain stored without losses for months.

It is however a much more effective solution than pumped hydro and is a good storage strategy for wind power systems mechanical energy can be stored as compressed air later used to power electrical generators. Because the wind may stop blowing at any time.

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Notes by Akshat Jiwan Sharma

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