Solar Stirling Engine
This is a page on Solar-powered
Stirling engines that generate electricity. I also have a
page on model Stirlings.
Once upon a time, I came up
with an idea that really started me thinking. Can Home Shop Machinists
build a credible Solar Stirling Electric Generator? According
to Sandia Laboratories, such a system is the most efficient way to
translate the Sun's energy into electricity. Their system is about 30%
efficient, which is roughly twice what a photovoltaic system can do. Sandia,
in conjunction with Stirling Technologies, has
figured out how to build a 37 foot dish that is supposed to generate
enough power for 8 to 10 homes. Apparently this is about 25 kilowatts.
My question is, how successful
would a system be that was based on a concentrator the size of the "old
school" C-band satellite dishes? These should be readily available
as surplus, and they had technology to track the sun. Each one is about
6 feet in diameter. If we take the 37 foot diameter figure above, that
big dish had a surface area of 1,075 square feet. A 6-foot satellite dish
has a surface area of 28 square feet, or about 3% of that. Accordingly,
if it can operate at similar efficiency, such a dish ought to turn out
5 or 6 hundred watts or so of electricity. Think of it as being the solar
equivalent of a Honda power generator. Put up two of them, which still
doesn't take that much space on your property, and you have 1 kilowatt.
Starting to be a respectable amount of juice. Presumably there is a tax
break on it as well, but I'm no expert on that! If you want more juice,
it's going to take a bigger collector, I'm afraid, so the project may
not be all that practical.
What I am curious about is
what sort of Stirling engine is needed to power such a beast? It may be
that the Sandia version is too exotic to fabricate and one would have
to settle for a lower efficiency. I'm not too concerned about that. Even
half as efficient is still just as good as solar cells (photovoltaic)
and I'll bet cheaper. Apparently they used a Scandinavian designed Stirling
called a "Kockums
4-95 Stirling engine".
Apparently it was originally
created to power submarines. It looks a bit imposing for the average home
shop, but recall we only need on with 1/8 the capacity, so it will be
considerably smaller. Considering what this one is capable of, it seems
relatively compact. The one that was finally perfected for this solar
application is even smaller than the picture above, and is apparently
about the size of a 55-gallon oil drum. 1/8 of that is not small, but
ought to be manageable for many home shop machinists.
This is where I run out of
steam, so to speak. I've no idea what's inside all of those intricate
housings, and I haven't seen plans for any large scale Stirling's in the
Model Engineering community. The best I was able to find is the Sunpower
EG-1000 1 kilowatt Stirling generator, whose design
looks simple, compact, and doable. The generator is an integral linear
design, which is cool. A more detailed scientific paper is also available.
Here is what a 1 KW version looks like:
In practice, imagine
flipping that drawing over so the focus of the parabolic reflector was
the heat end. This particular unit is designed for water cooling. Just
like the Sandia/Kockums unit, I would propose small (motorcycle?) radiators
with a fan to cool the water on the side facing away from the solar collector.
few more factoids on the EG-1000:
- The piston and displacer
are only moving about 9mm at peak efficiency.
- It operates using a planar
- There are no linear bearings.
Rather, gas bearings support the design and require no lubrication.
- It incorporates a linear
generator based on moving permanent magnets and a specially wound
I suspect it will
require considerable experimentation to produce a workable Stirling for
this application. It's my understanding that they can be finicky beasts
to get running and are very sensitive to friction and other factors. Were
I to undertake such research, I suspect it would make sense to build the
dish, measure the heat available at its focus, and then build a testing
rig that delivered that heat via electric or gas fire so I could fiddle
with Stirling engines in the comfort of my own shop.
The Rhombic drive
is similar in design, but converts the motion from linear to rotary:
It seems to me
that a working rhombic drive design could be considerably simplified as
a free-piston linear design. Free-piston Stirlings need a load that is
provided by the linear alternator/generator.
thread on CNCZone about Solar Stirling Power Generation :
It has some interesting thoughts:
- Use discarded CD's for
reflectors. I like it. These could easily be affixed to a surplus
parabolic satellite dish.
- Alternatively, get hold
of a "space blanket" - one of those survival sheets of aluminised/gold
coated plastic sheets - and work out how to make a focusing mirror.
I'm not sure how durable this material would be in the dish, but it
certainly is cheap. Perhaps it would simply require laminating on
a new layer every few years.
- "The advantage with
helium in a Stirling is that it has the highest heat conductivity
of any gas except hydrogen. The Stirling cycle requires the operating
gas to gain and lose heat to its surroundings and this is largely
what limits how fasten a Stirling can run. The higher the heat conductivity
of the gas the faster this process can occur." This is why the
Kockums uses hydrogen as its working fluid--it contributes to efficiency.
I know for a fact that hydrogen and helium are finicky to keep confined,
especially if there are moving parts to consider. I wonder how much
efficiency we're talking about here?
Stirling Wiki: A few useful links, but it never went very far.
nice article on the Sunpower EG-1000 Stirling generator.
"V" Stirling May Be A Good Bet for Power
Primer on How the Stirling Works
spreadsheet walkthrough on how to design a Stirling engine
Stirling Engine Plans: A 4-cylinder design capable of claimed 40
watts. It's about the size of a big model airplane engine.
Schematic Overview of Various Stirling Configurations
Stirling Engine Animations
Update on the 5HP Stirling by Merrick Lockwood : The
original 140 page book is available here.
Robot Fish : Interesting Japanese project. Nice scale of Stirling.
and Simulation of a Free-Piston Solar Stirling
3 kW Stirling Generator Set : 218 page pdf report.
Nasa Animation of a free piston linear design
7 kW Free-Piston Natural Gas-Fired Generator
Nasa Intro to Stirling Power Generation
Guide to Stirling Engine Design:
300+ pages. One interesting comment is that Stirling engines running on
air are only 20-25% as efficient as those that use helium or hydrogen
as their working gas. Lots of rhombic drive models are analyzed in this
of Sunpower Papers on Free-Piston Stirlings
Stirling Page: Amateur-built relatively large capacity Stirlings
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