Ice expansion to drive motor?

[brainzlol]

When water turns to ice it expands by approx 9%.

This expansion does exert a very large force...don't know how much?

Can an engine be made to use this force?

The motor needs to amplify the small/slow "travel", at the expense of the large force.

I know we have to spend energy to cool the water, (but we then dump the ice, not reheat it)

Any ideas please?

Thanks

brainzlol (on twitter)

[Enthalpy]

To check if a medium is interesting in a thermal engine, you can compare its P*dV with the heat used in the operation.

 

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In a diatomic gas, 20.79+8.31J/mol of heat (=cost) give 1K which changes PV by 8.31J/mol (=benefit, potential and overestimated). Usually the strongest candidate.

 

Take ice: 334kJ/kg to freeze or melt it, and 8.3%*183MPa=15kJ/kg (or less than that: inferred from the liquid's bulk modulus, 2.2GPa). This ratio is far less promising than a gas, or even than a boiling liquid.

 

For a shape memory alloy (see Wiki), this ratio would be bad also, though some people consider using them - to make electricity from the residual heat in car exhaust pipe, last time I saw it. And from a normal solid it's far worse.

 

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This ratio has no theoretical significance. It puts no physical limit on the attainable efficiency. But if you consider how inefficient the technological processes are at a thermal engine, this ratio does then have a direct consequence on the achievable efficiency.

 

No big thermal engine, where efficiency is the prime consideration, will use ice instead of vapour or a gas. But maybe a very special application, where other considerations are more important - just like for electricity from exhaust heat, a dry cycle is more important than efficiency.

 

Could you exploit a huge amount of ice because it's available naturally and you need to add very little man-made apparatus? Similarly, we couldn't produce electricity from rain directly, but if existing mountains make the height drop and existing valleys the reservoirs, we just add a thin dam, turbines and generators, and get cheap electricity.