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Discussion board help and admin topics => Half Baked Ideas => Topic started by: not_a_mib on March 29, 2006, 07:16:48 AM

Title: Expanding-liquid heat engine
Post by: not_a_mib on March 29, 2006, 07:16:48 AM
Disclaimer:  this idea is so undercooked that it probably isn't even half-baked, more like lightly stir-fried.

Liquids expand when heated, but are nearly incompressible.  This suggests doing a regenerated Brayton
cycle with a liquid, using a pump, hydraulic motor, and countercurrent heat exchanger (regenerator), and
the usual hot-side and cold-side heat exchangers.
This scheme has four steps like most heat engine cycles:

1.  The liquid starts out at room temperature and atmospheric pressure.  It enters the pump, is raised to
some high pressure P, and enters the cold port of side 1 of the regenerator.  For a volume V of liquid,
work PV was required at the pump.

2.  The cold pressurized liquid flows through side 1 of the regenerator, heating and expanding, then
through the hot-side heat exchanger where more heat is added to reach the final hot-side
temperature.  The volume V of liquid has now expanded to V + v, and is still at pressure P.

3.  The hot pressurized liquid now enters the hydraulic motor, doing work P(V+v) on it, drops its
pressure back to atmospheric, then enters the hot port of side 2 of the regenerator.  Net work Pv is available
after subtracting out the work to run the pump.  This could be used for traditional over-unitarian activities
such as illuminating light bulbs or pumping up that big compressed-air tank in the basement below
the demo room.  ;)

4.  The hot liquid flows through side 2 of the regenerator, cooling and contracting, then through the cold-side
heat exchanger, shedding heat to the environment and restoring the initial condition.  Most of the heat flows
through the regenerator into the high-pressure liquid stream, leaving only a small amount to
dump to the environment.

This cycle still has the heat source and sink of a classical heat engine.
If the compression-heating effect in liquids is small, this cycle might not be Carnot-limited.
The Richard Clem engine (
might do something like this.