A group-effort low-temperature-difference Stirling engine
Some like-minded technicians and engineers, having machine
shops at home, decided to design and build a few low-temperature-
difference Stirling engines somewhat along the lines of a Senft
or a Clower engine. The result is shown as "Assembled engine"
above. It is about 175 mm (7 inches) in diameter.
The Stirling is an old engine, patented in 1816 by a Scottish
minister, Robert Stirling. Its charms are that it's very quiet,
can run on any heat source, and does about as well as can be done
given the temperatures of its hot source and cold sink.
The engine shown is not particularly optimized, but it will
do 60 rpm when sitting on two warm hands (~37C), the top plate
exposed to room air temperature of about ~20C (68F). Internal
temperature difference is much less, of course.
Low-temperature-difference heat engines are severely limited
by their Carnot cycle efficiency. The maximum power a heat
engine can extract from the heat flowing through it is the heat
flow times the Carnot efficiency. Carnot efficiency is
(Thigh - Tlow)/Thigh
where the temperatures are in absolute degrees. So for this
engine, the maximum power is
(Thigh - Tlow)/Thigh = (310-293)/310 = 5.8%
of the heat flowing through it. Not much to work with!
Since the engine runs fairly quickly, another problem is to
transfer heat into and out of the working fluid (air) quickly
enough. This problem can be ameliorated by changing the working
fluid to a highly conductive gas. We tried charging the engine
with helium (at 1 atmosphere) and for the conditions given above,
got a new engine speed of 175 rpm. This tells us heat transfer
is a problem in this engine.
And how does a Stirling engine work? It is very clever in its
simplicity. There is a cylinder which is hot at one end and
cold at the other, and inside of it is a "displacer" which is
shuttled up and down in the cylinder to make the gas in it move
from one end of the cylinder to the other and so the gas is
alternately heated and cooled. When gas is heated and cooled,
it expands and contracts. There is a port which connects the
gas in this cylinder to the power cylinder, so when the gas
expands and contracts, it pushes and "pulls" on the piston in
the power cylinder. This particular style of engine is called
the "gamma" configuration.
A refinement found in this and many other Stirlings is the
"regenerator". The regenerator is a sort of thermal capacitor.
When the displacer shifts the gas from the hot side of its
cylinder to the cold side, the gas is made to go through the
regenerator and gives up some of its heat to the regenerator.
When the displacer shifts the gas from the cold side of its
cylinder to the hot side, the gas is made to go through the
regenerator and picks up some of the heat in the regenerator.
The regenerator preheats and precools the gas giving the gas
a head start on the direction it's going. Cooling hot gas
and heating cold gas is like having a heat leak from the hot
side of the engine to the cold side, so it is to be avoided.
Instead of calculating Carnot efficiency, why not use a slide rule?
Measuring engine temperature on the cheap with thermocouples
Model of Heinrici-designed Stirling-cycle engine
Pelton wheel
