Simple Stirling 1 performance with and without regenerator
June 9, 2008 – 12:02 pm
I’ve tried a variety of regenerators on the Simple Stirling 1 engine and the one shown in the photo is simple to make and performs as well as or better than all of the other ones I’ve tried. The test results on the engine with the original displacer and with the modified displacer containing the regenerator are shown in the plot. As you can see for the same delta T (temperature difference between the hot and cold plates), the displacer with a regenerator provides much higher RPM.
Ideally, a regenerator makes a Stirling engine more efficient because it performs part of the heating and cooling of the working gas as the displacer cycles it back and forth between the hot and cold chambers. After the gas leaves the regenerator it enters the active heating or cooling regions. A regenerator is a passive component. It cools the hot gas as it flows in one direction through the regenerator and heats the gas when it returns back the other direction. The heat is transferred to the steel wool in the regenerator I’m using and is transferred back on the return trip.
The regenerator isn’t totally free. The steel wool material causes some friction with the air, causing a larger pressure differential on the two sides of the displacer that makes additional work for the engine. The volume taken up by the regenerator also adds dead space to the engine, making the engine slightly less efficient. Despite these disadvantages, the net gain is substantial.
Incidentally, making more clearance around the side of the displacer to make the air flow easier (with no regenerator) actually caused such a large drop in power the engine wouldn’t run at any reasonable temperature. I cut the displacer down from approximately 3.4″ diameter to 3.25″. The reason it causes such a drop in power I believe is because the tight clearance between the displacer and the cylinder wall accelerates the air as it flows past. The air speed is high enough to make the air swirl around in the hot or cold chamber and have good heat transfer with the hot or cold plate. The larger clearance reduces the air speed, possibly causing laminar flow instead of turbulent flow, and reducing the heat transfer.
Modifying the displacer to add the regenerator is probably self-explanatory if you look at the photos. I used a spade bit to drill the four 7/8″ diameter holes. You could probably use a 3/4″ hole instead of 7/8″. To keep the steel wool in place I used 5 minute epoxy to attach a disk of aluminum window screen on the bottom of the displacer and then divided up 0.6 grams of #0000 steel wool among the 4 holes. Try to fluff it up to fill the volume and make sure there are no straight through holes where the air can go without going through the steel wool. Test run the displacer to make sure you’re getting reasonable performance and then epoxy a screen on the top of the displacer to lock the steel wool in. In the photos you can see that I put masking tape on the screen to mark the circle and hold the screen in a circular shape.
If you test your engine without the regenerator and then add it, you’ll probably be as blown away as I was that 0.6 grams of steel wool (this is almost nothing) can make such a huge difference.
It’s quite possible you may be able to come up with a regenerator that works even better. I’m still planning to try some fine copper wool.
9 Responses to “Simple Stirling 1 performance with and without regenerator”
Nicely done! Torque remained constant?
By DU on Jun 9, 2008
I don’t have a way to measure torque on this engine, I’m just measuring unloaded RPM. The engine accelerates until the torque balances the friction. It would be nice to measure the torque–I’ll have to think about that.
By admin on Jun 9, 2008
Hallo Doug,
In one of my LTD Stirlings I used succesfully Scotch Brite cleaning pad as regeneratormaterial.
see:
Http://home.hccnet.nl/hvisser.5
next-Gallery- model 23
Greetings from Zeeland, Holland
Huib Visser
By Huib Visser on Jun 15, 2008
Hi Doug,
As air passes through the regenerator as well as around the displacer, could the speed gain be due to less air resistance?
Would it be more efficient to increased the diameter of the displacer, so more air passes through the regenerator?
nice work btw
Al
By Al on May 17, 2009
Hi Doug, I am not an engineer, but i think the size of the displacer and the size of the power piston are directly related. I also think that the space between the displacer and displacer wall is critical to the amount of pressure created to the power piston. Also i believe the sizes of the two pistons are relative to each other. if u change the size of one u must change the size of the other, directly perportional ???
good job even i can understand it….
By Dave on Jun 5, 2009
Hi All
Cheers, John
I’m recently interested in Stirling engines for generation of charging current for electric assist pedal trikes.
I was keen on a tiny diesel engine, but none seem to exist. Nevertheless, the Stirling engine seems ideal for the use of waste
vegetable oil (WVO). Using this in diesels is very problematic wrt engine life, whereas a Stirling should last as long as it’s
built to. I was wondering where the tiny amount of 0.6g of steel wool was derived. I was keen on using aluminium wool as it has
about the best thermal properties for this use, but on finding it costs about $250 for 5 lb, I thought I would compromise on steel
wool. Laying the strands across the flow rather than with the flow should make the axial conduction of heat as low as possible. I’m toying with a magnetic drive for the displacer and a diaphragm for the power piston. This way leaks can be avoided and friction cut. I’m still looking for sites which will give me volume ratios such as power piston displacement to total enclosed volume. The only mention of weight of regenerator mesh I’ve found is here, and I’m guessing that much more could be used, although ideally, the mass of regenerator material would be the same as the mass of air passed through it in proportion to the specific heats. Any comments eargerly absorbed
By John Riley on Jun 27, 2009
ps I just looked up the specific heat of air and it’s a bit more than twice that of iron on a weight basis, so ideally perhaps 2.5 to 3 times the weight of steel wool? j…
By John Riley on Jun 27, 2009
Hi,
I was looking at your Maker Faire 2008 Engine and it seems quite large however you state it only delivers 0.09W. I do not understand that.
Is that because you are only putting in a small amount of power or are there huge losses. For example, if you put in heat at a rate of 100W, how much power would the engine deliver? Thanks.
By Robert Virkus on Jul 2, 2009
Sorry to intrude – I have some real newby questions about all this if you don’t mind…
I have an off-grid house so energy from renewables is of special interest to me. A year or so ago, I saw something on TV about a generator powered by a Sterling engine at the focal point of a solar array (based on an old satellite dish).
I thought the process seemed fairly “developed” and looked for kits or something to come out. (Note – I have a degree in Music and am not allowed to handle sharp objects ’cause I’m accident prone, so trying to build something myself is not in the cards…) However, the discussion here makes me think that (for me) this would not be a practical addition to my system.
Lets face it – putting up a solar dish and engine in the front yard that only produces 1 watt would only irritate my wife while doing little to charge my batteries.
Is this technology just too “immature” to be of real use for someone like me? I’m looking for some alternatives to PV that I can use to expand my generation capacity. Water is not available, wind is good but windmills aren’t and generators based on the Tesla turbine seem to be “vapourware”.
Sorry if you feel this post is off-topic but I figured that you engineering types who are really working on this stuff would have the answers to my naive questions. Thanks for your patience.
Roger
By Roger Priddle on Aug 2, 2009