Electrical Car Consept – Feasibility

[Pommie]

The heat pump example has convinced me that the basic concept is correct, however, should Carnot's rule be applied to steam or IC engines. As I can run a steam engine on compressed air and do useful work with no temperature differential, doesn't this show that Carnot's rule cannot always be relied upon to predict efficiency?

Mike.

 

[Hero999]

Carnot's rule only applies to converting thermal energy to another form of energy (be it mechanical, electrical or chemical) and to doing work to move heat from one place to another.

Carnot's rule doesn't apply to other energy conversion devices such as electric/pneumatic motors, transformers, hydrogen fuel cells and LEDs..

If you use compressed air to power a steam engine (in which case you could no longer call it a steam engine), the maximum theoretical efficiency will depend in the pressure differential between the input to the exhaust pressure.

All Carnot's rule says is that in order to convert heat to another form of energy you need a temperature difference, just like you need a pressure difference to power your 'steam engine' from compressed air. It's actually quite obvious when you think about it; it's just not always easy to visualise and that's why it isn't always easy to understand.

 

[Oznog]

Well you need to look up not just the "energy" and "efficiency" content, but "entropy" and "enthalpy".

For example, a Stirling engine can take a mass of hot liquid or gas and
a mass of cold liquid or gas and make work out of it. However, if it has no cold mass, it cannot do any work. If I place it in a sealed tank of 300F fluid it can't work.

That's not a limitation of only the Stirling design. It's a limitation of ALL machines and processes. Think of it like having a tank of 100PSI air and an air turbine to generate electricity. If I lived on the surface at 14PSI, I could let the air blow through the turbine. It raises the pressure of the environment (though the environment being so huge it's an inconsequential amount). But if I try to install it in undersea research sub and the pressure outside is 100PSI, the pressure is the same and the air won't flow. We can only extract useful energy (work) through lessening the energy difference between two mediums. Sad too- I mean a refrigerator should MAKE electricity otherwise, shouldn't it? You're trying to take energy out of a box. But no, there's like 200 years of solid science on this one and it's as solid as the "energy is neither created nor destroyed" principle.