small wood gassifier generator

[seandepagnier]

I have always been interested in stirling engines due to their quiet nature and ability to run on various sources of energy.

I have 250 watts of solar panels, and have been exclusively solar powered for the last 9 years, but in winter, I am degrading my batteries.

I am seeking advise on how it might be possible to construct a very small stirling generator, but not a toy, something that can actually spin a small brushless motor (using mosfets rather than diodes to rectify) and push a few amps into a 12v battery.

Generally I would like to produce 20-50 watts of electricity to be useful, using wood. If I could run a small batch of wood I would have all the electrical power I could need despite the weather. Most generators mentioned elsewhere are producing much more than this (5000 watt swedish version mentioned), which in my opinion is excessive energy use.

I would like to use a very small quantity of wood, so make this as efficient as possible. I think around 7% thermal efficiency would be useful and possible? If it could be better that would be fantastic. The rest of the energy (heat) would dissipate into my boat which is currently heated from wood anyway.

From what I can tell, it is mostly not efficient to burn a crude fire under the engine because it is impossible to control the fire precisely. The engine would be optimized for a very specific power to maximize efficiency. Is this correct?

Instead it makes more sense to build a wood gassifier, and from this, a gas burner can direct the flame directly to the stirling engine. Some sort of valve would control the gas flow to the optimal level. Has anyone managed this? Is there a way to make use of the heat from the gassifier as well.. maybe pre-heating the air intake for the burner?

What size engine would I use? What could I make it from? What sort of tools would I need? I am thinking a gamma configuration is the simplest, but any suggestions are appreciated. I have a lot more questions, but this is a start.

[Bumpkin]

The trouble, as I see it, with small scale wood gasification is it tends to take a lot more tending than just a basic wood stove. Also if it's done inside a living space there's always some degree of danger from carbon monoxide - stuff happens. There are off-grid folks who use wood-gas to run a more efficient I.C. generator to intermitantly charge their systems. That way they can keep the whole contraption outside and it only needs tending for an hour or so a day. But that's not my Idea of fun, and the batteries still take a beating.

As you point out, if you keep the generation inside and you're using a fire for space heat anyway, any conversion to power is 100% efficient and a steady trickle charge could be very useful. A more focused heat source might make a Stirling engine easier, but I think the goal is possible with a more user-friendly wood heater. Actually, we KNOW it's possible, but to make it practical for a low budget home builder is the problem. I was pursuing it a few years ago until a forest fire burned through our area and my priorities changed for a while. Yeah - excuses, but I'd sure be happy if someone else did the work before (if ever) I get back to it. Here's a link to some thoughts from back then: http://spragueriver.com/index.php/bumpkin-tech

Bumpkin

[Ian S C]

For a small gas generator, it is possible to build an acetylene generator if you can get a supply of calcium carbide.
Ian S C

[cbstirling2]

What about a wind turbine?
An off grid wood stove CHP is technically possible but it never took off commercially.

[cbstirling2]

Btw, that Japanese Ross alpha toy that I posted about produces 1 watt of power enough for usb.
That's approximately 0.5 inch bore, so an one inch bore may generate 4 watts. The airpel graphite cylinders are 1 inch bore.
For a drive train there is a $65 usd toy model off eBay, use the flywheel as the crank wheel. Get two stainless cups and jb weld
The glass cylinders inside to make very good cylinders.
Drill a hole in chimney pipe so flue warms hot cylinder.

http://stirlingengineforum.com/download/file.php?id=747

[Ian S C]

unless this is only a trial build, a motor should be soundly built, with a reasonably solid crankshaft. This is the top end of my stove top fan motor. The bearings on this motor are 8 mm bore "skate board" wheel bearings.
Ian S C
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[cbstirling2]

Crankshaft is parts you don't need. Use a 90 degree split engine on the same pin on a flywheel.

[Ian S C]

True, but it still needs to be well made, not a bit of bent coat hanger wire as used in the tin can motors. I don't know about your country, but here there is a "craze" for a toy called "Fidget", a lot of these contain up to 4 ball bearing races, these are shielded, but not grease filled, they are the same size as skate board bearings, but free running, and can be got "cheap", one of these is just right for a motor, 2 for the main bearings, and 1 each for the con-rods big ends.
Ian S C

[Alfista]

The proposed engine that you describe above, something producing 20-50 W is quite similar to one that I built a couple of years ago. It has a 3" piston. At 3 psi (6 hg) it should be producing about 35 W before losses. The video link below shows it connected to a battery powering an 80W load. It gives a rough idea of the effectiveness in battery charging. The use that you propose seems well suited to this sort of unit. You obviously already know a lot about battery charging and this sort of engine would be a small power source to charge the batteries when the sun is down. Once the engine's hot end is hot and there is a good cooling circuit, it takes surprisingly little heat to turn the engine.

By comparison, at the same pressure and rpm, an equivalent 4" engine should produce 88W and a 5" engine 171W. All of this before subracting mechanical and generator losses.

Link to post : http://stirlingengineforum.com/viewtopi ... 263#p10263

[youtube]https://youtu.be/vmNw857wo3A[/youtube]

[Ian S C]

A handy bit of kit that you can easily build your self is a Prony Brake for measuring torque. here is the brake on one of my flame lickers, and disassembled.
Ian S
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[Bumpkin]

Hey Ian, I always wondered how a prony brake could be consistent enough to get a good reading. Now I see the springs and it's clear. Thanks.

Alfista, I watched the video. I would think it might be easier to test your engine's power by finding the biggest load (light bulbs?) it will run up to voltage without the battery. Which reminds me - I have a small spring piped down from above my house and long ago I mounted a little water turbine to a bicycle alternator - the kind that rides against a tire to power a headlight. It charged a 12 volt battery for a sparsely used light, and a tiny 12 volt 4 inch television/radio combination. The television was only rated at 4 watts, but powered from the turbine alone, the picture would shrink to about half-size as the voltage dropped. Ah - the good old days.

[Ian S C]

With the Prony Brake, you need a tacho to get the revs.
I use the formula: rpm x torque(inch / oz) / 1352 = Watts. The metric version is: gms x cm x rpm x .00001026 = Watts.
I have use the arm on the brake resting on a set of digital scales.
Ian S C