Stirling cycle generator

[Moochingaround]

I'm brainstorming on a way to make power from our woodfired stove. This is a hobby project and some things (like the math) are a bit out of my league.

My idea is to base this on the stirling cycle, but adapted. I've gone through many different versions and at the moment I'm stuck on this. I hope someone with more math skills and insight can help me along here.

My current idea is two "pools" of working fluid. One hot (under the fire) and one cold. Between the two a vane motor with ellipse cylinder. One side of this ellipse would be driven by the hot, compressed air from the hot side and release in the cold side. The other side of the ellipse would carry cold air up into the hot pool.
The temperature difference between the hot and cold side could get up to 300 to 400 degrees C. (the cold side would be water cooled)

Let's say the delta is 400 degrees. Then the hot side would carry double the air volume and pressure of the cold side. My barely educated thoughts are that the ellipse would have to be bigger on the hot working side by that same factor.
So, for example, every cycle would carry down 10cc of hot air and up 5cc of cold air. The cold air will get heated in the hot pool and, ideally, this can keep going for as long as there's fire going.

Are my thoughts and assumptions correct here? I'm thinking myself in circles and hope someone can help me out here.

Thank you for your time.

 

[crshears]

Google hot air engine

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[LittleInch]

Sounds like a perpetual motion machine to me.

You need to work in constant mass and not cc of air. otherwise your system will rapidly run out of air in the hot chamber if you're using cc in standard conditions then 10 out and 5 in won't last for very long?

The work released by the hot air moving form hot to cold is reduced considerably by the movement then of the cold air being compressed into the hot chamber.

If you want electricity from a stove there are many options that provide very small power outputs of 50 or 100W. Basically phone chargers.

Getting hot waste is possible, but electricity is a lot harder. No where close to cost effective.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Moochingaround]

Thank you crshears, I googled a while ago and I'm trying to develop my idea further. This is what we're talking about.

And thanks littleinch. I don't think it's a perpetual motion device as it would need the heat from the fire to keep moving. So I'm putting energy in.

I'll try to elaborate on my idea.
Imagine two spaces with the same volume, closed from the outside.
If you close the passage between the spaces and heat one side up, the pressure increases. Thus there is more volume in that space.
Let's assume one side is 10 bar, the other 5 bar.
Now we put a motor in between the spaces. It uses 10cc of the hot air to move, but while doing that it moves 5cc of cold air up.
The 10cc going down will eventually cool down in the cold space and be 5cc, whole the 5cc moving to the hot space will heat up to get to 10cc.

Now this doesn't take into account any losses or anything yet. So the difference probably isn't so exact.

I'm I right in assuming that moving up 5cc will take about half the power that is produced by moving down 10cc?

 

[LittleInch]

Mooching.

There are several things wrong with your assumption so Ill fill them in.

magine two spaces with the same volume, closed from the outside.
If you close the passage between the spaces and heat one side up, the pressure increases. Thus there is more volume in that space.
Err no - You've just said the volumes are fixed as is the mass in the hot side vessel
Let's assume one side is 10 bar, the other 5 bar.
Now we put a motor in between the spaces. It uses 10cc of the hot air to move, but while doing that it moves 5cc of cold air up.
Did you think about what you just wrote? As you open up the hot chamber then yes, you will get some air movement, but the pressure in the hot chamber will decreas and the pressure in the cold chamber increase. "It move 5cc of the cold air up. Up where? What is forcing the cold air anywhere? Why 5cc? you are confusing actual volume with standard volume or mass. Use one of the other otherwise you will get very confused.
The 10cc going down will eventually cool down in the cold space and be 5cc, whole the 5cc moving to the hot space will heat up to get to 10cc.
Like I said use mass or standard volume. Doing this as a batch process won't work and compressing 5cc of cold air to create 10cc of hot air needs a massive temperature.

Also don't forget you need to factor in the efficiencies of the turbine and the compressor. Something like am open cycle gas turbine which uses the power of the expanding gas to drive the front end compressor to simply get going has an overall efficiency of about 25 to 30% at best.

I can see why you're thinking this, but using air, which has a very low density and hence a very low power output is going to find it very diffiuclt to generate any power worth having.

This is also avoiding the issue of how do you then cool down your hot air after the motor?


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cranky108]

I have given thoughts about generating power from my wood stove, but not being a mechanical, I had thought about a different method.
I was thinking about a Seebeck generator, however I was concerned about being too hot on the hot side for the chips I have seen.

 

[Moochingaround]

It turns out that I was trying to explain a brayton cycle in my layman's terms. This brings other limitations compared to a stirling cycle, so not really an advancement in efficiency.

 

[MintJulep]

A good three minute read.

https://www.stirlingengine.com/generators/