Re: MIT study touts geothermal energy



On Jan 24, 5:34 pm, "Bret Cahill" <BretCah...@xxxxxxx> wrote:
Instead of screwing around with steam and overpriced 2 phase turbines,
use just one well as the displacer cylinder of a Stirling engine.
Sounds like it might be a good idea.A lot cheaper and at least as efficient. Unlike steam nothing would
need to be insulated.

Stirling requires huge heat transfer areas and therefore is
expensive/watt and has a low power density. It has never really been
suitable or cost effective for most applications.

But these aren't issues when the earth itself is the heat exchanger.

Who cares about size when you are going straight down?

Perhaps you could go into more
detail as to how/why it might work.Very simple:

The displacer is maybe half the length as the well depth with a
diameter somewhat less than that of the casing ID. This is the same
design of ordinary "beta" or "gamma" engines although not nearly as
long.

When you lower the displacer to the bottom of the well the hot air or
gas at the bottom rushes up in the annulus formed between the displacer
and the well casing. The air cools against either the top part of the
well casing and displacer and/or in an above ground cooler.

As it cools a power piston connected to the well casing compresses the
air.

Once the air is compressed the displacer is drawn back up which forces
the cool air to rush back down to the bottom of the well where it
warms.

Then it is expanded isothermally for the power stroke and starts over
again.

Ideally the compression and expansion is supposed to isothermal, but
before now there was never enough time or area for the heat to transfer
so the Stirling never exploited its Carnot efficient thermo cycle.

Carnot is the best any heat engine can do for a given temp. difference.

Carnot efficiency = 1 - Tc/Th.

The Carnot effiiciency is the thermal efficiency
of a physical system, not the efficiceny of a mechanical engine.
Which is really why internal combustion was
invented. Since it's low-end torque to speed
efficiency leaves Carnot cycles in France
where they belong, and ICE's walk on the moon.





But even the best high tech performance Stirlings only reach 50% Carnot
because of the difficulty is transfering heat to a gas.

The well casing, however, provides an enormous area to heat and cool
the gas so it should be much closer to Carnot.

Bret Cahill

.

Relevant Pages

  • Re: MIT study touts geothermal energy
    ... > use just one well as the displacer cylinder of a Stirling engine. ... But these aren't issues when the earth itself is the heat exchanger. ... Carnot is the best any heat engine can do for a given temp. ...
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