Design suggestions for Stirling generator

[TheVictim]

I'm trying to make a small Stirling generator to directly produce electrical power. Instead of a piston I figure a flexible diaphram would work better since there wouldn't be any pressure loss from an imperfect seal. The temperature on the hot side wouldn't be very high, not above 140f, so even latex would work. The cool side is going to be anywhere from just above freezing to room temp (provided by a small earthenware evaporator) so the differential isn't going to be that much, which is another reason for the diaphram so the generator can operate at very small differentials in temperature.

My question is should I have the actuator that is attached to the diaphram turn a reciprocating wheel to generate electricity, or should I just mount the magnets on the actuator (which also triggers the cold side return valve) and have its motion within the generator coils produce power? Which would be more efficient? I'm siding toward the rotational model.

 

[Oznog]

I'm not sure how you'd use a diaphragm because the Stirling engine piston's action exposes the warmed/expanded air to the cooling surface of the cylinder. With a diaphragm, the air will move but will remain exposed to the rubber diaphragm surface which does not have any substantial cooling effect.

 

[TheVictim]

When the diaphram is fully extended, it will also actuate a ball valve on the cool side that will open a connection between the cool side and the hot side, at which point the diaphram will contract again, allowing the ball valve to close and the cycle to repeat.

 

[TheVictim]

I completed the proof-of-concept experiments today, looks like a diaphram WILL work, but I am going to have to use multiple cylinders and a camshaft. I'll also have to ditch the ball valves and go with a flapper valve.

 

[Pommie]

I have 2 low temperature differential (LTD) stirling engines. Both use a polystyrene displacer to move the air between the hot and cold sides and one of them uses a rubber diaphram as the power piston.

Just found their web site.

Mike.

 

[TheVictim]

My design seems to be different than the classic Stirling engines. There will be 8 cyclinders, 4 hot side and 4 cool side. They will be in a V8 arrangement at 60 degrees to the camshaft. Each hot cylinder will be paired with a cool cylinder and linked with a duct. Airflow through the duct will be controlled by a valve flap on the hot side of the duct. The rotation of the camshaft will provide pressure on the cool side diaphram to move cool air into the hot side cylinders.

I'm not sure how much power I am going to get. I'm going to build a small model from food cans and a wire camshaft before I move on to the full sized model, which will have heat exchangers right inside the cylinders.

The eventual plan is to have the hot side heat exchangers powered by a solar collector and the cool side by an earthenware evaporator. I'm just hoping that the pump for the heat exchangers and the fan to the evaporator won't consume more power than the machine can produce. That is a long way off though. I'm going to get the small model working and then move on to some proof-of-concept experiments for the evaporator.

Hopefully I'll have a tiny generator that runs off of sunlight, air, and water!

 

[Nigel Goodwin]

It will be interesting to see how you get on - I was under the impression that Stirling engines produce only very tiny amounts of power?. If it was a practical solution I suspect it would be commonplace by now?.

 

[Pommie]

Nigel,

It is commercially available. Check out Wispergen.

Mike.

 

[TheVictim]

My machine is going to have the same problem that other LTD engines have, it will be very large for the amount of power it can produce. If I can't get the small model to work well I'm not going to bother building a bigger one.

 

[Super_voip]

Stirling engines are very efficient, the power they produce is dependent on the power input. I'm contemplating installing a small 3KW solar gen in my backyard.

 

[Nigel Goodwin]

Stirling engines are very efficient, the power they produce is dependent on the power input. I'm contemplating installing a small 3KW solar gen in my backyard.

How are you going to focus that amount of solar energy to the correct place?.

Also, is 'small' the correct term? the Wispergen link above is house boiler sized, and only 1KW - admittedly it's also a boiler, but the stirling engine and generator look to be the biggest part of the entire assembly.

 

[Super_voip]

**broken link removed** is a vapourware Stirling but I have a company down the road from where I work that installs 25KW solar stirlings. The internals of the Whispergen are interesting, the Stirling for the 25KW is about the size of a oil drum(44gallon) and produces 60dB noise, heat is sored in silicon and can run for 3 days without Sun. The little 3KW one, I won't charge batteries simply sell to the grid. This should offset whatever I use at night.

 

[TheVictim]

Looks like the flapper isn't going to be as easy as the ball valve. All I have to do to duct in air from the cool side is have the hot side diaphram lift the ball off of a tube connected to the cool side. Very simple. I like simple. Now to find a lightweight heat resistant ball of the right size.

Edit (reply to above): If I remember correctly, solar energy delivers ~ 800W per square meter, and that would be at 100% efficiency. If your Stirling SOMEHOW got 50% efficiency (that is a BEST case scenario) you would need almost 4 square meters of solar collector alone. That isn't exactly small. This isn't taking into account the cool side of the gradient. You would get much more benefit if you had something cooler than the Aussie summer air. If it is going to be in the back yard I suppose you can use an underground heat exchanger.

 

[Super_voip]

These things are in deserts all around the world(more sunny days), if the the hot side is very hot then the cold side doesn't have to be very cold rather like the radio isotope reactors in space where the hot side isn't very as the cold side is very cold. It's the temperature difference that matters. A 3metre dish should work as that would have 7sq Metres of area, so a dish less than 3m, mounted 3m off the ground. A total height of 4.5m.

 

[arrie]

Hey TheVictim.

Have you made any further advances with your idea?

 

[greaserig]

as far as your original question about where to place the magnets... it would seem that making the generator built into the sterling engine, without the extra linkage etc... would be more efficient, and also a very clever design!
i thought it was a cool idea

 

[arrie]

Hi greaserig.
I don't know if TheVictim still frequents this post, but I'm interested in the whole sterling side of things. I'm not sure how he will connect the whole lot to the diaphram, or what sort of rpm TheVictim will get from his sterling, but I do know on your genny side you need quite a bit for useful single phase power, at least 3000 to 3600 rpm, and then you need some decent magnets and coils too.
So he might still need some gearing to get it up there.
Further, running a sterling at that low temp sounds risky, what I've seen is motors running 200deg.C plus to make max use of the air inside the system, and that will give your diaphram idea the march.
So it's back to a solid material design and flywheel setup, or so I believe. The flywheel can have some particular benefits, but I agree with him, you need more that two cylinders.
Is there someone who have actually built a successful sterling on the forum? Maybe he/she can give us better pointers in this regard.
I've been toying this idea for very long, and have done some design along the lines, but buying the materials to build all is a pain, and it's going to be an expensive exercise.