Free Piston Stirling Engines – Nice Technology for Tinkerers

Posted: April 19, 2013 by tallbloke in solar system dynamics

I’m looking into new ways of generating power in the backwoods. My previous efforts are a bit fragile and fiddly. What we need is a robust system like the free-piston Stirling engine pictured here. Animation below the break for those interested.


From Wiipedia Commons

From Wikipedia Commons

So, how do these things work efficiently with the right timing when there is no mechanical connection between the displacer at the right, and the piston in the centre?

I’m glad you asked. It’s down to the hysteresis of the diaphragm springs supporting the displacer and the piston. The displacer shuttles to and fro, moving the working gas from the hot end to the cool end of the cylinder. That causes contraction and expansion of the working gas as the heat source continues to push energy into the system at the hot end (red). That sucks and blows the piston to and fro. Get the hysteresis of the diaphragm springs right and you’re in business. Some of the power generated by the linear alternator is used to circulate cooling water round the cool end of the cylinder (blue).

Sunpower Inc have perfected this to make a system which generates a kilowatt at 240V and 50Hz – European standard mains electricity specs. They have produced combined heat and power units for domestic central heating systems which also feed 1Kw onto the grid. Eon trialed them in 1000 customers homes, or so I heard. It’s all gone quiet.

So what has all this to do with lightweight backpacking bloggers backwoods charging systems. Well, watch this 9m youtube video. The first and last couple of minutes are the most interesting.

This has a lot of potential, once the system is calmed down with a linear alternator – that’s the heavy bit, coils of copper required. But for 6V battery charging, they don’t need to be all that heavy.

  1. You can see a bunch of free piston engines in Dessau, Germany at the Technikmuseum “Hugo Junkers”. It’s about an hour’s drive from Berlin. If you’re into engineering for people, engines and aircraft, plan to spend half a day there.

    From memory, there are a couple of free-piston compressors on display.

  2. James B says:

    This is not a free piston, as described in the lead, this is an attached piston, created to do a job. Back when I was a child, grade school, in my late 60’s now, used to go too the Ford musem in Detroit, And the GMI in Flint, and watch the engineering going on, They had some designs in motrs working, with protypes, driving the streets.

  3. tallbloke says:

    James, sorry to disagree, but it is a free piston Stirling engine. Defined by the fact the piston and the displacer are not mechanically coupled.

  4. dp says:

    Too complex. Since you also need light you can have both. see the Russian lamp at about 3/4 to the bottom of this page:

    This are still sold in various forms.

  5. AJB says:

    Sorry, wrong video. This is the one with the linear generator …

  6. tallbloke says:

    Hi dp. I’ve already tinkered with a peltier setup over a small woodburner. They do manage to charge up small batteries, but don’t make enough power to charge a small laptop. Also difficult to control, and I’ve melted the solder in a few. The free-piston stirling engine in a beer can concept is something I’d like to try out. The noise might be a bit tedious, although it might be possible to make it pretty quiet with some thought put into the design.

  7. tallbloke says:

    Very nice. The first one uses minimal amount of ethanol and is pretty quiet. It doesn’t make much power though. I’m wanting more power, and would use a small woodburner as a heat source. I make them out of 5 thou’ titanium foil. They weigh almost nothing.

    To minimise rocking couples a vee configuration or horizontally opposed configuration could be considered. For a given output that would half the individual reciprocating masses, but increase overall weight. It would smooth the current generated too.

    I’ve ordered a $5 shaky flashlight off ebay to study the linear alternator and rectifier circuitry.

  8. hunter says:

    I am vaguely familiar with the basic concept of stirling engines. Where in the device is the heated gas introduced in the interesting example you have provided? Sorry to ask such an obvious question, but I am hoping to make certain I follow this well. Stirling engines do offer some real hopes for environmentally sound sensitive energy production. This looks to be an interesting variation on what I looked at many many years ago..

  9. tallbloke says:

    Hi Hunter. The working gas is sealed into the stirling engine when it is made. Then it stays there and none goes in or comes out. The engine works on the principle of the gas being alternately heated and cooled, thus expanding/contracting. That’s what blows/sucks the piston to and fro.

  10. hunter says:

    Thanks for the answer and for not laughing.
    A sealed system like that will require a very good cooling system and access to lots of btu’s, I think.
    How much will moving heat in and the cooling system ‘cost’?
    Perhaps some sort of very low boiling point gas could help? A phase change could be very useful, possibly.
    The piston seals are not that big a deal. Especially since this is basically going to be low rpm, low-ish temps and pressures. There are plenty of good bearing solutions that would allow for low friction operations. God knows that this sort of system would be orders of magnitude easier to maintain than windmills.

  11. AJB says:

    What are your load expectations? Starters for an order of magnetude [sic] …

    Not much to it

    Unless you’re planning a 4×4 backpack surely still an LED league game, even with NdFeB at scrum half. But how big a magnet can you reasonably lug around? Mr Ed would probably suggest an inflatable Stirling engine – tricky but maybe more than a talking horse. Or perhaps you should just settle for powering the 4×4 with one of these :-).

  12. tallbloke says:

    AJB: Nice links, thanks. I’m looking for about 3W @ 10V so the current is low (0.3A) and the copper wire can be pretty thin. Think about the size and weight of the soap on a rope which powers a laptop from the mains. My Sony Vaio z series uses a tiny one. I have a friend who winds coils for fun (!) so I’ll throw that part of the job at him. How heavy will the magnets need to be? Don’t know yet. Anyhow, I’ll try running the shaky torch with a simple pepsi can engine and see how it goes for a first try.

    Edward de Bono – Lateral thinking. Use a cellphone and bluetooth keyboard instead of a laptop….

  13. tallbloke says:

    Hunter, missed your comment earlier. I think a small backpackers woodburner (about the size of a large bean tin) can supply enough heat for a small charging system. The cooling system could be two waterbags with hoses connected to a manifold surrounding the cylinder. Hang one bag up and let the water trickle through the heat exchanger into the other. Then simply swap the bag positions when necessary. No pump required, let gravity do the job.

    Stirling Engines have a pretty limited niche. For large scale power production steam turbines are much more efficient. Noisy though.

  14. dp says:

    Tallbloke – the trick with Peltier and other thermal generators is to not use one. When you gang them, as on every lamp or lantern, the value/complexity index is quite good. You need several things for comfort besides food and water in an off-the-grid situation: heat, light, distributable energy. Nothing approaches the simplicity and reliability of these devices for providing these comforts simultaneously.

    Personally I would add toilet paper to the list of essentials 🙂

    Solid state thermal generators are not the best for this application.

  15. Graeme No.3 says:

    I was told that the Coastwatchers in WW2 were supplied with a portable Phillips Sterling engine for charging their radio batteries. The hot end was placed on a wood fire. Ran silently, an advantage when you are behind japanese lines. Dating about 1943 or 44.
    Possibly the Philips portable “Bungalow” Stirling engine generator, MP 1002 CA. There was a limited production run of these 200w engines in the early 1950’s (primarily to power ham radio sets)

    Unfortunately have no link or confirmation.

  16. tallbloke says:

    Graeme: You can see a Phillips Stirling Engine in action (after a lengthy preamble in part 1) on youtube. Useful for car camping for the quiet operation for sure. Too heavy for lugging around on foot.

    dp: For offgrid generation at home I have a solar hot water panel, some photo-voltaics (not much – pricey!) and a honda 2.2Kva LPG converted 4 stroke generator. The Stirling engine in a beer can idea is for lightweight travel on foot, where my power needs are pretty modest. A rechargeable lithium ion camcorder battery can act as the ‘reservoir’ to smooth the delivery for recharging a smartphone, mini-laptop, reading light, small radio and mini-speakers.

  17. tchannon says:

    Up until I switched main focus professionally to electronics I worked for quite a few years associated with or one way or another in the motor industry, subsequently also contracting, so I have had a very long interest in many of the technical matters. Whole raft of stuff I rarely talk about.

    Stirling is irrelevant as such given you want a product.

    Without going into why I suggest you stick with a thermoelectric generator, perhaps one of the better technologies now available and designed for high temperatures. (metal junction, not semiconductor, far too fragile)

    One of the nicer ideas fan assists the cold side. This gives me the idea of a bleed might air assist combustion. Getting it running as ever one of the fun parts.

    Can’t see it but I thought you mentioned wanting to charge a laptop battery but seemed to have a typo. This implies you need ~30 Watts, maybe overall best considered as needed Watt/hours, maybe 100 to charge a laptop battery given losses.

    This is I think feasible.

  18. tallbloke says:

    Hi Tim,

    My Sony P11Z lappy will run with no battery in at all on about 0.75A at 10.3V.
    If you charge the battery simultaneously it sucks about 1.6A. So 8-16W is required as rectified alternator output This probably equates to double that in terms of engine output, so I’d be struggling with TEGs or Stirlings for a lightweight rig. Charging the phone battery and using a mini folding keyboard is probably more realistic. the one I have gives about 30 hours of life on a charge. Enough for a weeks blogging on the road.

    After a bit more research I’ve found it’s all about pressure. The more molecules you pack in, the better the performance. That and watercooling the upper end of the displacer cylinder. The water cooling bit is easy, and beer cans are designed to take a surprisingly high pressure. Best gas is Helium. Hot end can take around 300C before plastic deformation sets in with a 4 bar pressure.

    With a decent size can (I have a 1.5 litre Asahi in stock) I reckon I could squeeze 8W of AC out of it. That’ll require a linear alternator weighing a couple of hundred grams. Spare phone batteries are probably a simpler and lighter if less satisfying and somewhat expensive option.

    I still want to build a free piston Stirling though. 🙂

  19. tchannon says:

    Seen the demo of hot water powering a 12V car bulb? Something packs up very small and light is feasible and I think a viable niche product.

    > I still want to build a free piston Stirling though.

    Ah that is a different matter.

    I’ve been toying with trying to produce a micro engine for a very real application with enormous marketing potential since it would slash costs elsewhere. This is feasible using conventional methods but is far too complex to be cost effective. I have in mind something simpler but rather novel. The difference here is it must operate on a low temperature differential, therefore with a high heat flow, needs phase change but no toxic gases. Can’t quite get a click yet.

  20. tchannon says:

    [crawls in history]
    This one will do, about half way down. No verification of course.

    Many years ago I used a peltier based fridge in the car, continental camping. Yes frog and toads do crawl inside during a thunder deluge! Funny to watch.

    Surprising how hard the temperature difference will drive the augmentation fan if connections are changed.

    What I don’t know is how good the more exotic high temperature metal junction devices are relative to low temperature semiconductor junction devices. Not a field where I’ve paid much attention recently.

  21. J Martin says:

    This thing burns twigs, boils water for tea and charges USB devices. all at the same time, so perhaps it can charge a laptop as well.

  22. tallbloke says:

    Thanks JM. 5kw of fire mustering 5W of electricity. There must be a better way.

  23. tchannon says:

    That’s what I had in mind but the output is pathetic.

    Not exactly light either, 1kg.

    Built to a minimal price. As a whole thing, brew, do minor charging for a relatively large market, is fair enough. Not many will want to lug other equipment.

    Bet much better could be done. Particularly use much more of the heat output for generation, meaning more TE modules and a large price hike.
    At least some have an upper limit of 500C

  24. tallbloke says:

    Yeah. My version was 3 times lighter with a bit more leccy output. Melted the peltiers. Proper TEGs are pricey!

  25. J Martin says:

    Someone needs to invent a rotary Stirling engine. There are a bewildering range of different Stirling type designs, but no rotary version. What’s needed is a Stirling Wankel type of design or better still some sort of turbine Stirling.

  26. dp says:

    The ultimate off-grid devices are the deep space probes that use RTG and thermoelectric devices. Not near the fun of Stirling principle mechanisms, but successful in the extreme. The madmodder modeler’s forum has some fascinating shop-built Stirling engines.

    A very interesting wind generator I was was just a long piece of strapping, Nylon or other synthetic, anchored at one end at the top of a pole and anchored at the bottom with a linear generator. The strapping is at a 45º angle or so. The strap vibrates in the wind which causes the tension to vary which drives the linear generator by oscillating a powerful magnetic core in a dense winding. That part is exactly the same as the free piston Stirling engine. Instant power from the wind. Very appropriate where fuel and fire is either not practical or not available. Come to think of it, a free piston wind driven device could be made by connecting one end of the strap to the smaller piston in the reference example engine above and use mechanical vibration rather than heat to oscillate it. Its like writing Perl code – so many ways to do it.

    The piston flexures look like a source of energy waste as the elastic response to flexing is to heat up and it puts that heat on the wrong side of the system, but then so too are the generator coils. Anyway – fascinating idea and definitely thinking outside the box.

  27. dp says:

    Found this interesting NASA page. Not quite flight ready at the time of publishing, but the Stirling is in the running.

  28. M Kelsall says:

    Design of these systems is all about control theory. They can be thought of as spring, mass, damper systems.. These can be modelled mathematically and the engine design tweaked to operate correctly. I’ve done the maths before and understand the principles. Sunpower free piston engines use back emf to alter the damping so keeping the engine in its working zone. Having a free piston engine with an indirectly coupled alternator would save the headache of having to worry about an electrical governing system I suppose.

    Glad to see you’re still into off grid generation.