StirlingEngine(2)

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ADVANCED PHYSICS
H
EADLINE DISCOVERIIE
S
FISHER SCIENCE EDUCATION
MAKING SCIENCE MATTER™
T
STIRLING
ENGINES
he Stirling engine is an old technology
that has been changing in exciting new
ways, making it a viable alternative
energy source with the potential to pro-
vide cheap and renewable energy for
people all over the globe.
The creator of the Stirling engine, Robert
Stirling (1790-1878), was a Minister of the
Church of Scotland. He attended Glasgow and
Edinburgh universities, where he studied Latin,
Greek, logic, metaphysics, math, and rhetoric.
In 1816 he received a patent for an "air engine"
and although there had been previous attempts
to build similar engines, his was by far the
most successful.
Stirling engines competed with steam engines
until electric and internal combustion engines
replaced both at the turn of the 20th century.
Stirling engines have only been used in very
specialized applications ever since; for exam-
ple, in the 1960s a tiny Stirling engine was
developed to power an artificial heart and
today they are commonly used to provide the
cooling for infrared guidance systems in mis-
siles. However, with the increased focus on
environmental concerns and our quest for
cleaner, more efficient power sources, the
Stirling engine is back in the spotlight as a
viable power source for wide-scale use.
It's Hot Outside!
and exhaust valves, fuel injection, carburetors,
etc.
Green Machines
Laboratory. Their new engine uses intense
acoustic energy instead of pistons for the heat
transfer. Constructed of welded pipes, the
engine is remarkably simple, efficient, and inex-
pensive. "The efficiency of conventional
engines is limited by both the laws of thermo-
dynamics and practical concerns over the cost
of building and operating complex engines.
Typically, the highest efficiencies can only be
obtained from expensive engines like the large
turbines used by electrical utilities. "Our engine
is neither mechanically complex nor expen-
sive," says Scott Backhaus, one of the inven-
tors. An innovative use of this engine is to
reclaim wasted natural gas. When companies
drill for oil, they also run into deposits of natural
gas. They often have no way of transporting
the gas and instead it is just "flared" or burned.
Researchers at Los Alamos are currently work-
ing to design a Stirling engine that would cool
the gas so that it becomes a liquid, which
would make it much easier to transport in con-
ventional pipelines.
What Next?
Unlike internal combustion engines, Stirling
engines can carefully control burning for com-
plete combustion and low emissions so that
they release far fewer pollutants into the atmos-
phere. They can also take advantage of wast-
ed heat to produce electricity, like the heat of
landfills or factory incineration rooms. Stirling
engines can also easily use renewable sources
of energy, such as biomass like wood chips or
cow dung and can even be powered by solar
energy. In fact, solar-powered Stirling engines
are nearly twice as efficient as most photo-
voltaic solar cells.
A very promising use of Stirling engines is
cogeneration, where they produce both heat
and electricity for homes. If the excess heat
produced by Stirling engines was directly used
to replace furnaces and water heaters in
homes, this would yield a dramatic increase in
energy efficiency. This is especially useful for
"off-grid" applications where people are too far
from power plants to get electricity over
cables. Many companies are currently looking
into using Stirling engines to replace the cur-
rent systems in refrigerators. Driven in reverse,
Stirling machine pistons manipulate the con-
tained gas to affect temperatures outside the
machine. Stirling engines would use as much
as 50% less electricity and even more impor-
tantly, they do not require CFCs for cooling.
One company, Global Cooling, is prototyping a
solar-powered Stirling refrigerator that could be
used in the developing world for keeping food
and medicine cool.
The Future
Repeated explosions inside the engine power
all internal combustion engines, like the ones
found in most cars. The Stirling engine is com-
monly referred to as an "external combustion
engine" because its heat source is external to
the engine itself. But this is not an accurate
label because Stirling engines do not even
require combustion to operate. They contain a
fixed amount of gas, often in two cylinders,
one for heating and one for cooling. As the gas
inside the cylinder is heated by the external
source, the internal pressure rises and it push-
es a piston outward. The gas is then trans-
ferred to the cooling cylinder where the pres-
sure drops and the gas becomes easier to
recompress. This results in a net gain in power
going to a driveshaft. Most science students
are familiar with the equation PV=nRT. For a
fixed volume, an increase in temperature will
raise the pressure.
Because Stirling engines rely on a constant
external heat source instead of a series of
explosions, their operation is much more con-
tinuous than internal combustion engines.
They never become clogged with the products
of combustion because this is kept separate
from the moving parts of the engine. They are
much quieter and require almost 50% fewer
parts than internal combustion engines
because they do not need spark plugs, inlet
Perhaps the greatest challenge facing Stirling
engines is the popularity of internal combustion
engines. Designers of Stirling engines will need
to offer incredible advantages to be able to
attract manufacturers away from gasoline
engines. In addition, new materials need to be
developed for the hot parts of the engine; this is
the part that is most likely to wear out. Once
they are mass produced, the cost of Stirling
engines will come down greatly and their popu-
larity should increase. Don't be surprised if you
see one soon!
Article submitted by Adam Papania at A3BS.
The most exciting Stirling technology is current-
ly being developed at Los Alamos National
Set Learning in Motion!
The difference is clear – our best-
selling Stirling and steam engines
utilize plexiglass and heatproof
quartz to enable your students to see
all moveable parts. Incorporate one
into your next discussion of heat and
energy change and set your students’
learning in motion!
The Stirling engine includes a motor-generator
and can be operated as a thermal engine, heat
pump, and/or a refrigerator.
• All parts are precision-machined and wear resistant
• Like all 3B products, they carry a three year warranty
DESCRIPTION
CAT. NO.
PRICE
Stirling Engine
HS90391
1035.00
Steam Engine
HS90389
1195.00
On the steam engine, effortlessly remove the
steam engine drive piston to study its inner
workings.
3B Scientific®
…going one step further
GENERATING
ELECTRICITY
MATERIALS:
–
Electricity CD-ROM
(HS68376A)
a. What is the relationship between the
number of turns of wire in the generator
and the electrical output?
b. What is the difference in output between
a generator with a small magnet and a
generator with a large magnet?
c. How did changing the speed at which
the generator was cranked affect
the outcome?
d. How did you arrange the light bulbs in
order to produce the most light? Why
do you think this was the case?
PROCEDURES:
1. Using the "Electricity" experiment on
Discovery School's Physical Science
Simulations CD-ROM, have students create
generators that supply a circuit with power.
Encourage students to make changes in
experimental conditions one at a time rather
than changing all variables simultaneously.
2. Have students use the "Journal" feature to
record data and results for each configura-
tion they create.
Cat. No: HS68376A Price: $59.95
3. Lead a group discussion asking the follow-
ing questions:
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