Low Temperature Differential Stirling

[epineh]

I had a thought about a heat source for small generation, way back when I was in school there was mention of large amounts of heat generated within grain silo's and large piles of organic material, I wonder if that would be a possible heat source for a stirling engine, granted most people dont have 100 ft grain silo's in their backyard, but even smaller style hobby farms and such might have something similiar, and they are probably the most likely to benifit from alternative energy production.

There are probably good reasons why it wouldn't work but as I said it was just a thought.

Russell.

[greybeard]

Hi Russell.
I would guess that any heat source could be used for a sterling. It's more a case of what is appropriate in a particular case in terms of economic or physical effort.
If you've only got a garden, then the muck heap rotting down organic matter would be a possible source of waste heat, bearing in mind that you need to keep the temperature up to keep the process going.
However, if you've got a silo, you've probably also got a large piece of land feeding it, with lots of sun to ripen your crops. In this case you might be better considering direct solar heating with the sun(admittedly only 8 - 12 hours a day at most) "wasting" more heat on any spare piece of land than your silo is producing in 24 hours.

John

[pminmo]

I like the diaphram, and have seen the 4 phase/piston approach. It's a good way to handle the 90 degree phase shift. It would seem that diaphrams would be eaiser to DIY manuracture rather than machining pistons and sleeves.

Since helium is a gas of choice of some of the commercial stirlings, I wonder what the temperature vs pressure/volume is for it vs. air?

[Geof]

.....Since helium is a gas of choice of some of the commercial stirlings, I wonder what the temperature vs pressure/volume is for it vs. air?

All gases are the same (to within a fraction of a percent) in terms of the temperature/pressure/volume relationship. The advantage with helium in a Stirling is that it has the highest heat conductivity of any gas except hydrogen. The Stirling cycle requires the operating gas to gain and lose heat to its surroundings and this is largely what limits how fasten a Stirling can run. The higher the heat conductivity of the gas the faster this process can occur.

[greybeard]

With helium readily available through part balloon shops, I would certainly seriously consider that as the best working fluid.
As it's inert and not eco-unfriendly, those are also on the plus side, and as the engine can be designed to be a sealed system, there would be the minimum top-up requirements. There will be some, because helium will diffuse through anything eventually
(Hands up who remembers Graham's Law of diffusion - or was it Dalton's )

Geof - just to put one idea to rest, is it deffinitely best to use a gas as the working fluid, rather than a liquid ? I feel there may be pro's and cons, but I'm out of my depth to be honest.

John

[pminmo]

Anybody got a chart on helium vs temperature expansion/contraction or pressure vs temperature?

[Geof]

...Geof - just to put one idea to rest, is it deffinitely best to use a gas as the working fluid, rather than a liquid ? I feel there may be pro's and cons, but I'm out of my depth to be honest. John

Stirling has to use a gas, the Stirling cycle is based on the change in the product of volume multiplied by pressure with temperature. Furthermore, I am fairly sure it has to indeed be a gas not a vapour. And you ask (maybe) what is the difference between a vapour and a gas? At temperatures below the critical temperature for a particular substance the substance exists either as a liquid or as a vapour depending on the pressure. Below the critical temperature it is possible to convert a vapor back to a liquid simply by increasing the pressure. Above the critical temperature no matter what pressure is applied the substance exists as a gas; it cannot be converted back to a liquid. The critical temperature for water is somewhere in the 700's degrees F (I think). With regards to steam engines and turbines, particularly turbines, you might have heard about them operating 'super critical'; this simply means the exit temperature of the steam is above the critical temperature for water.

Stirlings are very different, thermodynamically, to steam engines in that they do not utilize a phase change from liquid to vapour as part of their cycle.

Maybe all I have done is push you into deeper water, sorry, maybe you could find some books to read, all this stuff is quite interesting. Or take a trip up the Bolton Museum near Birmingham I think or the Museum of Industry (or Science and Technology it may have changed one way or the other) in London. As you might have guessed at one time in my history I taught senior high school and junior college level physics and chemistry; I can still drop into lecture mode on occasion.

[fyffe555]

Anybody got a chart on helium vs temperature expansion/contraction or pressure vs temperature?

I think we're moving towards finding a formulaic method of sizing a stirling engine.

Essentially as Geof said earlier the gas itself is irrelevant other than it's heat conductivity. That's true for pure working gases and gets a bit more complicated for mixes.

You might be able to find a helium temp / pressure / volume chart from a Scuba resource, they ddo this to calculate trimix and have to deal with each gas seperately and together.

Normally to plot pressure to temp of a gas you have to add a few factors and follow one of the gas laws. It gets complicated with mixed gases like air but as a noble gas, helium is covered by the Ideal Gas Law. You could use PV=NRT and plot away. P=pressure, V=Volume, T= Temperature in absoluteK, N= Mols of Gas and R= Molar gas constant based upon units used - for bar/atmospheres, degrees K, litres, grams etc R=0.082. Helium weights about 4grams per mol.

To simplify it, if only Volume and Temperature are changing in the system you can get rid of the constants and end up with V1/T1=V2/T2. In other words the temp and volume are independant of the gas weight, the gas doesn't really matter other than its ability to transfer heat. A gas with a higher heat conductivity will work better so helium would be a gas of choice.

If you want to be really precise you'd need to use Van der Waalls equations of state; (P+(n^2*a/V^2)(V-nb)=NRT. Where a & b are the Van der Waalls constants for Helium and I've no idea what they are.. you'd have to look them up. Once you've got them you could apply them to hot and cold end and work out the expansion/contraction at given temps.

Andrew

[fyffe555]

Geof - just to put one idea to rest, is it deffinitely best to use a gas as the working fluid, rather than a liquid ? I feel there may be pro's and cons, but I'm out of my depth to be honest.

John

Jumping on geof's thread here, As he's said gas is needed for Stirlings but have a look at rankin cycle or phase change engines. More complex that Stirlings but use a liquid changing from liquid to gas and back again in the same cycle.

[greybeard]

Hi Fyffe.
Thanks for the idea, but I'm having trouble keeping focused on Sterlings as it is !!

Geof - thanks for pointing out what should have been obvious to me( No problems with lecture mode - I was a lab tech of one sort or another for most of my working life).
I've no idea what I was thinking of. How could I imagine the compression/expansion parts of the cycle would work.
I was too focussed on the heat transfer I guess.

Need another little drink.

John

[Geof]

.....Need another little drink.....John

The next one should be a nightcap, yes?

By the way I was lab tech for a while; one of my employers back in 1969/70 was DAFS - Department of Agriculture and Fisheries for Scotland. That got me interested in stuff so I took a few years out of the trade to get some degrees, did the boffin stuff for a while and then started my own business in 1981.

[CJL5585]

Question:
Anyone have specs for a 1 meter or 4 foot dish?

What would be the temperature at the focus point of a highly reflective (Chromed or polished aluminum finish) 1 meter dish?

Am thinking about constructing an actual model stirling power generation system to see what I can accomplish.

Jerry