StirlingEngine(2).pdf

ADVANCED PHYSICS

H EADLINE DISCOVERIIE S

FISHER SCIENCE EDUCATION

MAKING SCIENCE MATTER™

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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