Sterling Engine Temperature Difference

Believe it or not, Jim Senft has built an engine (P-19) which will run with just 0.5k difference between the hot and cold plates!

However, LTD engines are really only scientific curiosities and typical hot end temperature for a 'real' stirling engine would be around 900k with the cold end as close to ambient as possible, ie a temperature difference of some 600k.

nickh=== Posted with Qusnetsoft NewsReader 2.2.0.8

I'm no Stirling engine expert, but I have seen some low differential engines running from the heat from one's hand. Therefore I guess that a suitably designed engine will run from a temperature differential of say 10 - 20 degrees C. Clearly, anything capable of doing any real degree of work requires significantly more.

Mark

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Sorry screwed up the pasting of the links

Two book on eBay that might help.

Stirling and Hot Air Engines - Darlington/Strong *NEW*

Stirling and Hot Air Engines - Darlington/Strong *NEW*

A Nice Hot Cup Of Tea

There's a well-known design (kit?) out there that runs on this sort of temperature difference. Most of it's Perspex, the displacer is polystyrene foam and apart from some weird materials around the power piston (graphite seals and some awkward turning, AFAIR from 20 years ago) it's one of the simplest "first engines" to build.

From this you can extrapolate that of the many things to make life awkward, finding a useful heat difference isn't going to be one. Almost anything that burns will do it, or even a few feet of solar reflector (even for a simple all-metal engine)

It's "Stirling" engine BTW

Here is a page with some info on the P-19 and a few other engines. ______________ Andre' B.

Here is a page with some info on the P-19 and some other engines.

Andre' B.

Thanks for all the replies

I'm looking at the possibility of making one for use in Africa. For the heat two containers of water one black in the sun, and one silver in the shade. Then use it to run a small generator.

Michael

In that case these pages may be of more interest.

Andre' B.

That doesn't sound quite right. Black in the sun, yes to get hot but the silver one in the shade won't loose as much heat as a black one so it will warm up and you'll loose your temperature gradient essential for a stirling engine. At least if I remember my school boy physics correctly...

A very interesting idea but I wonder how much power you can get. Several hundred watts per square metre of collector are available as input so on pure input energy there is a fair bit about but how efficient you can convert that into electrical power is a big question.

I wasn't to sure either Dave, I thought if I let the idea loose then I will get feeddback positive or not.

In that case ignore everythign that has been said and just look at the last 30 years of solar Stirling research. 100°C differences are easily available, with simple collapsible Fresnel mirrors.

Well if the rabbit in No 10 read this he would have us all building these things to generate our own power at home.

But on a serious not the technology seems simple and the cost possibly low so why has there never been a consorted push to use these engines in country's were gas and oil are a expensive luxury to the majority ?

It's late and I'll try to be brief. There are technical obstacles.

Although Stirling engines are really fun to play with, it's not practical. One example is Ford's dabbling with the technology in the early '70's. The car got better mileage and higher power output, but production costs were excessive. The working fluid was helium, and sealing up the engine was problematic. Envision the balloon you got at your last birthday party. The helium escaped after a day or two, didn't it?

Why use helium? Because the molecules are really close together and they transfer heat better than air. The characteristic that makes it favorable also makes it difficult.

Another obstacle is the heat transfer characteristics of gasses. First, remember that heat engine power output equals the heat energy input minus the heat energy output. This applies to internal combustion as well as Stirling.

We can't cool the system cooler than ambient temperature, so to increase power we will want to add more heat.

Let's look at how well air transfers heat energy. Ask your missus to cook you a pie. Look at the thermometer. What is it? 400 degrees Farhenheit?

Open the oven and stick your hand inside. That 400 degree air is uncomfortable, but it doesn't transfer much heat energy to your hand, does it?

Now plunge your hand into the 400 degree pie filling. Can you feel the difference in the way the different mediums transfer heat to your hand?

Now consider how hot you have to make a cylinder before you get the air inside to be the same temperature as the air inside an internal combustion engine.

That's just a few things to ponder. Adios. Rob

=-=-=-=-=-=-=-=-= Rob Skinner La Habra, California

"Rob Skinner" wrote (snip):-

I've never heard it put quite that way before, but it is a really good illustration of the problems of getting heat in and out of an external combustion engine, and one which I will undoubtedly steal should I need to explain it to anyone else!

"Michael" wrote :-

Though the two tanks of water bit does sound like something of a non-starter, there is scope for stirling engines to exploit intermediate temperature differances - eg from low-tech solar collectors, geothermal energy, perhaps even nuclear waste - too low for conventional rankine cycle plant. The thermal efficiency would be low (google on carnot efficiency for the 'science bit') if your basic energy source is practically free then the real figure of merit becomes useable energy per unit cost of the plant.

There are a few Yahoo groups into this sort of thing, try wastewatts, hotairenginesociety and sesusa.

Stirlings have expensive capital costs and low output. They're really only useful where energy is _very_ expensive, meaning the transport costs of the fuel are far more than the fuel itself. They seem to have a small but ideal niche powering navigational buoys.

if the UK was to go for solar, the best way to do it would be to encourage good passive solar design and also low-cost flat-panel solar installations on new build housing. Both of these are far more effective on new than old buildings.

List,

The University Bundeswehr in Munich has been developing solar powered LTD Stirling engines since the early 1990's. I visited them in 1995 and saw a 50W engine which had a solar collection area about 2.5 m2.

Roy Darlington (co-author of book) is a good friend, and he has an engine that uses a 450mm diameter parabolic dish to focus the sun onto the hotcap of one of his beta-type engines.

I have seen this run at the show at Sandown racecourse, on New Year's day at 10am (very weak sun) and produce a couple of watts of shaft power.

will convert solar energy to mechanical power with an efficiency of between 1.25% and 2.5%

In the USA, Stirling Energy Systems is planning a large solar Stirling array in the Mojave desert. These will use parabolic dish systems and high pressurised engines.

You can see their recent press cuttings page here

Stirling engines have had a rocky road of development over the last 200 years and I strongly recommend that any newcomer carefully reads the books on the subject, and talks to engineers who have actually produced working model engines, before they assume they can build an engine of more than 10W output. There have been some very expensive mistakes made in recent years.

The closest mchines to production and availability in the UK are those intended for domestic CHP made by Whispertech (partnered with Powergen) and Microgen (wholly owned subsidiary of British Gas BG)

You could always contact the Stirling Engine Society, PO Box 5909, Chelmsford, Essex, CM1 2FG.

Ken

Andy D> >