Stirling Engines as power converters

I notice that people here occasionally mention Stirling Engines, when talking about the possible exploitation of anomalous heat (sometimes it gets misspelled as 'Sterling').

If you happen to be in the UK, want to have a look at some working examples of these peculiar heat engines, and chat to some people who build them as a hobby, there is an opportunity next month. The Stirling Engine Society will be running a stand at the Midlands Model Engineering Exhibition from 13^th-16^th October.

The majority of the engines on show will be relatively low power units, using air as their working fluid. However, there may well be examples of pressurised engines, and those running with alternative fluids, such as helium. There is also a small active group within the society that has been developing engine systems for powering riverboats – either as direct replacements for the usual steam or IC engine units, or as Stirling-Electric hybrid drives.

Unfortunately, Stirling cycle engines are often misrepresented as some mysterious technology that will revolutionise the world's energy systems. But since it is over two centuries since the original patent by Robert Stirling, that revolution is taking a long time in coming. As usual, of course, ideas turn out to be the easy bit. Making devices work – economically, efficiently, reliably, and durably – is hard. It also doesn't help that accurate thermodynamic analyses of the cycle are far from trivial, and textbook examples can be slightly misleading for the unwary.

Actually, Model Engineering shows are good places to see what can be achieved in small home workshops, using tiny budgets. They are also useful places to pick up unusual tooling and sensible quantities of materials, as well as getting hints, tips, and ideas for possible future projects.

I know this online Forum mainly concentrates on laboratory-oriented topics, but all labs need occasional workshop support. It can be frustrating if you have an idea for a particular experiment, or device, but are stymied by an inability to source or build the necessary equipment. Events like this help to show how some R&D equipment log-jams can be overcome, at a hobby scale and budget.

(p.s. Yes, I am a member of the Society, and will be helping on the stand during the first couple of days )

Quote from Curbina

High powerStirlings are very expensive AFAIK

There is no such thing as a free lunch

To achieve a similar power density to (say) an internal combustion engine, then yes - current designs will cost a lot more. But power/weight (or power/volume) is only one of many criteria when designing a powerplant for any particular task.

The air-independent generators in the Kockums submarines, mentioned by gerold.s , were developed from engine concepts that were initially targetted at the automotive market in the 1970s. They were shown to have multiple disadvantages in that role, only one of which was cost of manufacture. The added complexity needed for rapid variable speed control was another factor, and the need for a good heat-sink was yet another. However, if you use it for constant speed operation (as a generator prime mover), in a marine environment (with plenty of water available for cooling), then those problems disappear. You can then take advantage of the engine's silent operation, as Kockums have done.

In the late 19th and early 20th centuries, many thousands of "hot air engines", of various designs, were sold for pumping water. They were relatively low power, and their efficiency wasn't high, but they were highly reliable, quiet, low maintenance, and could be fueled by burning wood, coal, or Producer gas. Eventually they were displaced by electric motor or noisy diesel engine pumps, of course - but the concept of a quiet-running stand-alone engine, that can run on various heat sources, is still valid.

Hi Alan,

I've sent a query to one of my colleagues at the Society - who keeps a log of these various ventures. I'll see what he comes back with. The company is actually American.  

Of course, they might be another one of these companies that continues to hoover-up grant money for "studies", while making money for its founders for years through "investment opportunities" (before going bust).

It looks like they are using a very high pressure sealed system (Edit: helium nitrogen filled). See this screengrab from one of their videos:

The monster 25kW version is an axial "wobble-yoke" design - judging by their 2019 update document.Cool-Energy-ThermoHeart-25kW-Engine-Overview-20190930.pdf

Quote from JedRothwell

the only real-world application

Surely that would be, by definition, out of this world... 

But I'm afraid that project has been cancelled. Personally, I would have thought that a Brayton cycle machine would be better for a space probe - if only because it would be easier to balance.

If you want to see a good real-world application, have a look at this short video, from this summer.

The engine for this boat is pressurised with helium - and as well as providing propulsion, it can be used to boil a kettle (to make tea).

If you want an example of something more industrial, this company makes Stirling generator units that can handle flare gas.

Hi sam12 - thanks for the document link to: "Innovative new type of Stirling engine".

Unfortunately, the concept is far from innovative. Nearly everyone who first comes across Stirling cycle engines has a similar idea (I certainly did, as a teenager, when first reading about them).

However, anyone who actually tries it finds that it doesn't work. The reasons are not intuitive - but then the entire Stirling cycle still causes confusion, even amongst some academic thermodynamicists (mentioning no names).

But it all helps to increase the level of fascination

I mentioned "Wobble Yoke" drives, further up this thread - in relation to the latest incarnation of the "CoolEnergy" engines. The short video below is a good example of such a drive system - this one from WhisperGEN.

This is a recent pre-print (from ResearchGate) showing a small array of modern engines, mostly used for CHP applications.

AreviewofStirling-engine-basedcombinedheatandpowertechnology.pdf

It is a useful review of the kinds of fully engineered devices currently available - which are aimed at the cost-conscious commercial market, rather than the deeper pockets of the military or space markets.

Of course DIY engines don't have to look so crude

Below is a lovely working scale model of the Ted Cooke-Yarborough designed "Thermomechanical Generator".

It is usually known as the Harwell TMG - since it was developed by Ted when at AERE Harwell in the 1960s & 70s.

It was a linear alternator type, with a diaphragm piston & free displacer, and was designed for long maintenance-free operation (usually burning propane). However, there was also one version powered by a lump of Strontium-90, which sat in a corner of a lab and kept generating for many, many years.

Anyone living within easy reach of Hereford, in the UK, can see some of the old test units on display in the Waterworks Museum. (They don't have the Strontium powered one, though.)

nb. The Hereford Museum will be hosting a Stirling Engine Rally on Sunday 9th October 2022. (They have updated their website at last!)

Quote from Alan Smith

Now here's something to ponder. Ed thinks that phonons cannot exist in bulk metal, being a surface phenomenon. I think he's probably wrong. Here's a test that you might care to try and then explain the result. It is a phenomenon ignored by most physicists, but well known to most engineers - especially those who got their fingers burnt.

Take a piece of steel bar, 30 cms long x 1cm in diameter will do. Heat one end of it with a blowlamp as quickly as you can until it is bright cherry red. The end you are holding will barely become warm. Then plunge the hot end into around 10 cms of cold water. The cold end will almost instantly quickly get hot enough to burn your hand. No physics - and no tables of thermal conductivity - can explain that except for it being heat transport by bulk phonons moving through the lattice.

The above fascinating quote stolen from this thread.

I've long suspected that the designers of regenerative "Caloric" engines, such as John Ericsson (1803-1889) believed that Caloric possessed some form of inertia. I don't think Ericsson (or his contemporaries) could quantify it - but it might explain some of the design features of the early engines, which seem slightly bizarre today (with our currently accepted thermal theories).

Some academics, such as Dr Allan J Organ, have made it their life's work to analyse thermal regenerators - but the damned things have been peculiarly difficult to model, over the years, despite the ever increasing power of digital thermal analysis software. I fear that poor Dr Organ may have even been driven slightly mad by the topic - having now retired without being able to fully answer some of the fundamental questions that most people expected to be solved, once computers became powerful enough.

Hmmm. Thanks Alan Smith - some food for thought, there....