NASA Stirling Engines (Stirling "convertors")

[Tom Booth]

I may be off a bit, not sure, but from the compressor specs, 500ft of 1/2"" pipe

That little compressor should transfer all the air through the system about 10X per hour.

Not fast, but in a heat transfer system fast is not necessarily better.

[Tom Booth]

From the inventor of the "free piston" (NASA type) Stirling engine:

Statement by William (July 2016):
I was happy to see that there are so many keynote speeches scheduled at ISEC 2016 from free piston Stirling companies. I gather that free piston products dominate certain niche markets. My goal in this statement is to point out that mass markets are now within reach.

It seems to me possible to dramatically simplify your free piston designs, stamp out excess cost and attack much larger markets. Some obvious steps present themselves. The first - examine what free piston products performed best in the past in terms of balance, sealing, centering, heat exchanger designs and so forth. Look at what NASA chose for deep space power - and the remarkable performance they achieved.

Consider the free piston alpha, which beats Diesel as a vehicle power plant, according to NASA technical memorandum 82992, and also provides high efficiency in a very wide range of sizes from 100 Watts to megawatts, and quick response to load.

Revisit the 1985 text "Free Piston Stirling Engines" by Walker and Senft, to see how well some of the cheap early machines performed, and how versatile these engines were in many applications. An example from the book is the simple hydrogen-charged free cylinder engine, with its automatic load matching and high specific power. Also look up Biowatt, an early wood-pellet fired system.

By learning from such past efforts cost may be reduced greatly, creating big mass market opportunities. Foremost among these, in the carbon-constrained world of the near future, is the biomass-fired home power plant which complements photovoltaics. Any fuel, with or without pre-processing, could be used to power a free piston Stirling that automatically recharges a home battery bank when charge levels get low. If pyrolysis is used for pre-processing and the resulting biochar is returned to the soil, one has the ideal future power plant: un-interruptable, maintenance-free and carbon-negative in all sizes from domestic on up.

The free piston Stirling community history gives examples of opportunities to simplify designs and slash costs to a fraction of the current values. If you succeed in that, you'll see that the age of commercial Stirling is just beginning.

Best wishes to all
William T. Beale (1928-2016)

https://www.ohio.edu/mechanical/stirlin ... Beale.html


I'd like to think, along with myself, that other members and readers of the forum share that vision and inspiring message.

[dlaliberte]

Heating up a thermal storage unit with direct heat, if it is hot enough to do the job, makes a lot more sense than heating with electrical resistance, unless you are going to otherwise throw away the electricity. This is simply because an electric powered heat pump system can move several times more heat energy from outside, or wherever the heat is, into your thermal storage than the electrical resistance could possibly generate. The exception is when it is too cold out to move more than 1 times the energy required. You also have to factor in the cost of the heat pump system, of course.

The temperature and medium of the heat input required for the thermal storage is a factor as well. Phase change material requires a certain temperature range. A sand battery is simpler by not involving a phase change, and can go to higher temperatures so it can hold more heat, but then a higher input temperature would be required to do that. Yes, it can also operate at lower temperature, but how much capacity would that give you?

[Bumpkin]

Tom, I appreciate how in a later post (Where do you get the time?) you demonstrate having come around to a lower, more competitive cost per watt for PV. So we can compare apples to apples and let the best apple win. I think we both root for Stirlings. I just want to be sure we keep fair score for the folks who stumble in here looking for clues to finding their own way.

But your entry (in answer to me) in an earlier post — “Now again you assert: “if you want to be taken with credibility, you need to acknowledge there are limitations…” On that issue, since your meaning seems clear enough to comment on at this point, - that is YOUR opinion, not FACT, and I do not “need” to acknowledge any such thing.” — That says a lot, particularly since it was one of the few places I made a point of beginning with “I think” and ended with “IMO.” I reckon most all statements are opinion; some are fact. Of course you will and should, do as you want. Like Abraham Lincoln said, “Most of the things you read on the internet are false.”

Total other subject; but on thread: I wouldn’t normally do home heating with PV energy except in “limited, niche areas” no matter the storage or conversion process. But it occurs to me that in the case of long energy transmission due to poor local solar possibilities, it might be practical for someone with very deep pockets. My own PVs are hundreds of feet away from the house because of the long wintertime tree shadows. The sad part of that is I wouldn’t even need wintertime solar if I could just get a practical Stirling wood-stove generator working. Bumpkin

[Tom Booth]

Tom, I appreciate how in a later post (Where do you get the time?) you demonstrate having come around to a lower, more competitive cost per watt for PV.

I haven't "come around" to anything. I'm in very close contact with PV dealers in my area and quite familiar with current prices. I just got an estimate for a PV system to go on the roof above my workshop a few months ago.

How you come to these unwarranted conclusions that I'm "anti- PV" just because I point out it is more efficient to use thermal panels where the end use is thermal I have no idea. It's just common sense to anyone familiar with all the various options.

You make up lies about me, then claim I'm not credible based on your previous lies and misrepresentations.

If you think Stirling engines are only useful in niche areas and their usefulness is so limited, why don't you just outline your reasons or your opinions instead of this attempted character assassination?

Well, maybe because your arguments are rather silly, uni formed and easily refuted. Unable to stand on there own merit. I could care less about "credibility". If you have a point to make, make it.

Just because I do not agree with your view or opinion does not mean I do not respect your opinion. I don't need to resort to put downs and character assassination, attacking someone else's "credibility", I'm just pointing out some facts. It's nothing personal.

I'm interested in making progress with heat engines. You seem to think that's a waste of time or a hopeless cause. Just because I don't share your defeatist attitude does not make me any more or less "credible".

So we can compare apples to apples and let the best apple win. I think we both root for Stirlings. I just want to be sure we keep fair score for the folks who stumble in here looking for clues to finding their own way.

It's not a competition.

Anyway, it is PV vs solar thermal to provide heat to the Stirling not PV vs Stirling. If anything.

I think I'd favor some combination of the three. PV to power the heat pump compressor to collect heat from thermal panels.

But your entry (in answer to me) in an earlier post — “Now again you assert: “if you want to be taken with credibility, you need to acknowledge there are limitations…” On that issue, since your meaning seems clear enough to comment on at this point, - that is YOUR opinion, not FACT, and I do not “need” to acknowledge any such thing.” — That says a lot, ...

Really? Like what exactly?

You think I "need" to get in line or something, with YOUR opinions or I'm not "credible". Who the blankity blank are you? What arrogance.

[Tom Booth]

Heating up a thermal storage unit with direct heat, if it is hot enough to do the job, makes a lot more sense than heating with electrical resistance, unless you are going to otherwise throw away the electricity. This is simply because an electric powered heat pump system can move several times more heat energy from outside, or wherever the heat is, into your thermal storage than the electrical resistance could possibly generate. The exception is when it is too cold out to move more than 1 times the energy required. You also have to factor in the cost of the heat pump system, of course.

The temperature and medium of the heat input required for the thermal storage is a factor as well. Phase change material requires a certain temperature range. A sand battery is simpler by not involving a phase change, and can go to higher temperatures so it can hold more heat, but then a higher input temperature would be required to do that. Yes, it can also operate at lower temperature, but how much capacity would that give you?

The relatively low temperature heat output from a conventional heat pump is a potential problem. The type of compressors used are not designed to handle high temperatures over 300°F.

A low temperature Stirling is a possibility, or a heat pump with a compressor specifically designed to handle higher temperatures seems quite possible.

Have you looked at that PDF about air-cycle refrigeration?

One interesting statement from there:

Work is taken out of the air by an expansion turbine, which removes energy as the blades are driven round by the expanding air.

This work can be usefully employed to run other devices, such as generators or fans. Often, though, it is used to power a directly connected (bootstrap) compressor, which elevates the compressed (hot) side pressure
further without added external energy input, essentially recycling the energy removed from the expanding air to compress the high pressure air further. The increase in pressure on the hot side further elevates the temperature and makes the air cycle system produce more useable heat (at a higher temperature)

So, an air cycle can, apparently, have a secondary high pressure/temperature compressor, I assume, after the regular heat pump compressor. The result being a greater temperature differential than could be obtained from a typical heat pump using normal refrigerant.

An air cycle is impractical in many ways for normal household refrigeration, mostly because it tends to put out cold in the cryogenic range, along with a lot of usually wasted heat. It's TOO COLD for ordinary refrigeration purposes.

But to operate a Stirling engine, that's a different story. For a Stirling an extreme temperature differential is very desirable. With that kind of application where both the heat and cold can be utilized, the efficiency is great.

The only real problem is, I don't know of anything like an off the shelf air cycle heat pump available. I haven't looked recently, there were some in development, but running a Stirling with an air cycle system would be a start from scratch project.

But air-cycle is very simple. It's basically just an air compressor and some pipes.

[Tom Booth]

In fact, I think I may just go ahead and get some pipe/tubing at the hardware store and start experimenting.

It is not unusual for a large industrial type air compressor to get blistering hot, like 500°F even while being vigorously cooled by fans and such, which is why they need special non-combustible lubricating oil.

The compressor heads on my shop compressors have metal cages around them for safety, to prevent getting burned.

Air compressors get HOT!!!

[dlaliberte]

I did read the air-cycle previously, and again. Thanks for bringing it up. Each time I reread these docs I get more out of them. Still learning lots.

Seems a cold storage unit would be as useful as a thermal storage unit, depending on individual needs. Could sand be used for both?

So it sounds like compressed air can be used to generate extreme hot and cold, using different devices. Or the vortex tube can do both with one device.

The generation of compressed air creates lots of (waste) heat as well, but I wonder if there is an efficient way to compress air using heat alone. Heating a volume of air will compress it, just as compressing the same volume will heat it up. So they seem to be closely related. Could there be a compression pump analogous to a heat pump?

[Bumpkin]

Hey look, I posted twice in one day. (getting closer to retirement) Guess I’s somewhat bothered about this: “You make up lies about me, then claim I'm not credible based on your previous lies and misrepresentations.” Geez Tom, shi&, but OK— OK. if you can demonstrate my “lies” then have at it. I will happily retract them in the name of science and debase myself in front of the forum . You contribute usefully to the forum, and I and I’m sure others all appreciate that, but DANG.
x

[Tom Booth]

I did read the air-cycle previously, and again. Thanks for bringing it up. Each time I reread these docs I get more out of them. Still learning lots.

Me too.

Seems a cold storage unit would be as useful as a thermal storage unit, depending on individual needs. Could sand be used for both?

I don't really know. But I would guess just ice, due to the phase change when frozen would work quite well, or going to the extreme, liquid air might be something to consider.

So it sounds like compressed air can be used to generate extreme hot and cold, using different devices. Or the vortex tube can do both with one device.

Vortex tubes are really interesting. I became quite interested in them years ago and even built a couple.

One thing I finally came to realize though is that although they can generate a pretty good temperature difference, the actual output is pretty miniscule in volume. You get a tiny spot of extreme cold and a small spot of heat that if spread out, don't really amount to much.

In the meantime the vortex tube is being fed by a great big air compressor that is running scalding hot and pouring out a ton of heat.

The miniscule amount of heat from the vortex tube is nothing compared with the heat generated by the air compressor itself and is only a residual amount left over after the compressor has already wasted 95% of the heat that was available from compressing the air in the first place.

Likewise with the potential cold from the air compressor when the air, cooled down in the tank is subsequently decompressed or expanded. If expanded through an air engine, motor or turbine of some sort to generate power, the resulting cold exhaust is even more extreme, potentially down in the cryogenic range. That is because the power to drive the compressed air motor is deducted from the latent heat left over in the expanding compressed air resulting in an extreme temperature drop.

So I became much more interested in cryogenic air cycle systems, which works similarly to a vortex tube but in great volume to heat or cool entire warehouses or flash freeze food commercially.

The generation of compressed air creates lots of (waste) heat as well,

Exactly! So may as well go to the source. The vortex tube is like the compressors little baby sister.

but I wonder if there is an efficient way to compress air using heat alone. Heating a volume of air will compress it, just as compressing the same volume will heat it up. So they seem to be closely related. Could there be a compression pump analogous to a heat pump?

Take a look at the patent for the Vuilleumier heat pump. It does just that, and I think the patent explains how that is possible better than anything. There is some additional information online but I haven't found anything as informative as the patent itself for describing how it actually works in very simple terms.

I'm not sure what you might have in mind though, as far as incorporating that into some kind of system.

https://patentimages.storage.googleapis ... 275507.pdf

[Tom Booth]

Hey look, I posted twice in one day. (getting closer to retirement) Guess I’s somewhat bothered about this: “You make up lies about me, then claim I'm not credible based on your previous lies and misrepresentations.” Geez Tom, shi&, but OK— OK. if you can demonstrate my “lies” then have at it. I will happily retract them in the name of science and debase myself in front of the forum . You contribute usefully to the forum, and I and I’m sure others all appreciate that, but DANG.
x

I do not wish to perpetuate this any further. "Lies" in the sense of intentional knowing lies may be too strong a word, I don't know, but you have been smearing me with a broad brush since you're first post on this thread with accusations, backhanded insinuations and miss-characterizations which are not true.

To go back and detail every instance would take volumes and as far as I'm concerned this derail attack on my character has gone far enough.

[Tom Booth]

I was not fully aware of this fact when I posted about the "easy to build" Stirling engine constructed (in part) out of 55 and 30 gallon drums, intended for the third world (Bangladesh etc.)

Resize_20221130_055930_0620.jpg (121.76 KiB) Viewed 23843 times

But this engine prototype designed by Sunpower ( Rice Husk Energy Project - (RHEP)) led to a spin-off engine of inferior (by some accounts) design and compromised construction, which came to be known as the ST-5.

Resize_20221204_140429_9513.jpg (129.92 KiB) Viewed 23843 times

I was aware of the resemblance, but did not know that there was an actual historical connection between the two. The earlier Sunpower version was more generally known as the RHEP engine but has also sometimes been referred to as ST-5 as well, though apparently similar only in appearance.

A commenter on another Stirling engine forum wrote:

I don't want to disparage Merrick on the forum. we're still friends and see each
other from time to time. But his book should be titled;

"How I screwed up a perfectly good Stirling Engine Design Project for the Third
World"

https://www.stirlingengine.com/forums/v ... ?f=3&t=534

The long sordid tale can be found still scattered about here and there on the internet.

Apparently the original "Rice Husk Energy Project" prototype (or prototypes there were apparently several) at Sunpower did at least meet the required target of 5 horsepower or better which was a requirement for receiving funding (from USAID - US Agency for
International Development)

Further:

The ST-5 was NEVER designed by Merrick. He was the Asia Foundation Administrator of the USAID funded project that had my team at Sunpower design an earlier, different version that was supposed to be replicated in Bangladesh and then put into production there. Merrick was supposed to lead the implementation of building four copies of the first engine designed and built by Sunpower. I led the engineering team both in the US and Bangladesh. At least I was supposed to in Bangladesh. I arrived in Bangladesh to find that Dr. Lockwood, (Entomologist) had completely redesigned the engine to his own version. It never put out more
than 1 or 2 HP.

In other words, there was, apparently, a more powerful earlier versions of the SD-5.

Obviously I have no direct personal knowledge or experience in regard to any of this, but taken all together, I do believe the early Sunpower prototype probably did put out at least 5 hp. Which would be about 4kw (3.7 kw)

It might also be noted that although this engine was intended to be simple and easy to build, the funding from USAID was on the order of $ 2 million $ dispersed in stages with certain specific goal requirements.

As a publicly funded project, the plans for the engine were supposed to have been made available in the public domain.

It seems we have been left with a dumbed down version with Merrick's ST-5 book.

[Tom Booth]

Looks like specifications and drawings for the older Rice Husk Energy Project can be downloaded here:

https://pdf.usaid.gov/pdf_docs/PDAAV016.pdf

BTW, This is a legitimate USAID government website download.