NASA Stirling Engines (Stirling "convertors")

[Bumpkin]

Hey guys; Long time forum member and big Stirling fan here, but dang, there’s no such thing as a good no-compromise plan, and a little credibility might beat a lot of fanatical hype in the long run. I would suspect that the 1,000 degree operating temp of the sand battery might have something to do with the vastly greater thermal capacity of phase change. Not even interested enough to look it up, but I would suspect 20% PV to 70 % battery to 90 % inverter would still come out more efficient than 90% thermal battery to 20% Stirling to 75 % generator to 90% inverter (inverter still likely needed to clean up the AC,) no matter how you weigh efficiency — (Somewhat silly to restrict solar efficiency to its footprint when the supply is basically infinite.) OK, I just did the math and the numbers are basically a wash, but PV is now well under a dollar a watt and anyone supposedly into alternate energy who doesn’t know that is not credible. Speaking of credibility, someone needs to prove that ridiculous output claim (5 KW) for that supposed water pump engine. And a PV to heating element can be a perfectly efficient application in the case of, oh I don’t know; perhaps me? Such as if I have enough PV to get through winter then I have too much in summer (not grid connected) so I should just waste it? Or maybe I could just do as I do and heat water with it since my only other water heater is the wood stove which is great in the other three seasons but is sort of a drag in the hot summer. (There’s all kinds of heat elements available, but I sure as H don’t need a 4,000 watt element when a 200 watt engine block heater on my hot water tank works out pretty well with my system and gives warm showers at the end of a sweaty day.) It’s a niche use, great for some, not useful for most, sorta like Stirlings if we’re honest. We’ve gotta pick our spots. If we’re honest. Bumpkin

[Tom Booth]

Depends on your situation.

If your talking an all electric home, electric car or two and plenty of acreage for a solar farm, (and plenty of sunshine) something like the original photovaltaic idea might be more practical just using the Stirling/sand battery for storing overflow excess power.

But that idea also included "municipal backup" already, so a secondary backup for when the grid is down?

A Tesla power wall? That alone is way beyond my budget. What is that $15,000? Great if you have an unlimited budget, you can be as "uncompromising" as you like.

If your a truck farmer living in a cabin in the north (limited sunshine) off-grid and your main supply house is the local scrap yard and what you can put together yourself, your solar footprint is limited and every watt matters - converting photovaltaic to heat is incredibly wasteful and impractical, you're immediately throwing away 90% or so of your valuable solar heat/energy.

[Tom Booth]

Hey guys; Long time forum member and big Stirling fan here, but dang, there’s no such thing as a good no-compromise plan, and a little credibility might beat a lot of fanatical hype in the long run. I would suspect that the 1,000 degree operating temp of the sand battery might have something to do with the vastly greater thermal capacity of phase change. Not even interested enough to look it up, but I would suspect 20% PV to 70 % battery to 90 % inverter would still come out more efficient than 90% thermal battery to 20% Stirling to 75 % generator to 90% inverter (inverter still likely needed to clean up the AC,) no matter how you weigh efficiency — (Somewhat silly to restrict solar efficiency to its footprint when the supply is basically infinite.) OK, I just did the math and the numbers are basically a wash, but PV is now well under a dollar a watt and anyone supposedly into alternate energy who doesn’t know that is not credible. Speaking of credibility, someone needs to prove that ridiculous output claim (5 KW) for that supposed water pump engine. And a PV to heating element can be a perfectly efficient application in the case of, oh I don’t know; perhaps me? Such as if I have enough PV to get through winter then I have too much in summer (not grid connected) so I should just waste it? Or maybe I could just do as I do and heat water with it since my only other water heater is the wood stove which is great in the other three seasons but is sort of a drag in the hot summer. (There’s all kinds of heat elements available, but I sure as H don’t need a 4,000 watt element when a 200 watt engine block heater on my hot water tank works out pretty well with my system and gives warm showers at the end of a sweaty day.) It’s a niche use, great for some, not useful for most, sorta like Stirlings if we’re honest. We’ve gotta pick our spots. If we’re honest. Bumpkin

So,.. we are going to use photovaltaic panels to power a 200 watt engine block heater to heat a refrigerator size box of sand to power a Stirling engine continuously from the heat to generate electricity to power an entire home?

[Tom Booth]

.... Speaking of credibility, someone needs to prove that ridiculous output claim (5 KW) for that supposed water pump engine. ... Bumpkin

The author of the document referenced is William Beale:

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

And has several technical reviewers:

https://pdf.usaid.gov/pdf_docs/Pnaas739.pdf

What evidence do you have for supposing the source of this information is not credible?

[Tom Booth]

FWIW a representative from Sunpower I just contacted confirms the output of that engine (5kw) and says he has personally seen that engine, years ago.

It was self pressurized (and slightly leaky at the crankshaft seal) with air to about 5 atmospheres by its own on board compressor. Not quite as crude as it appears, though the heater section was indeed just a 55 gallon barrel (or "garbage can")

[Bumpkin]

Whatever the heat source, running a Stirling inside your house during heating season would be 100% efficient and one of the very few practical uses that haven’t been replaced by other technologies. OK Tom, this should be fun. you have a habit of arguing apples against oranges with one extremely fresh and the other extremely rotten. As usual you took a practical example I gave in one situation and placed it in another situation to make it look silly. Not very scientific. You are the one who tried to make it sound like you had to have 4000 watts before you could heat anything with PV. I was only pointing out that the orange isn’t always rotten. I think I made it clear that some things are useful in some situations and ridiculous in others. And if entertainment is the purpose, then ridiculous can be useful too. I still have great hopes of developing what I would call a practical Stirling, but as I’ve said for years, I think practical uses are very limited, and the need should lead the technology, not vice-versa. There are reasons Stirlings aren’t used more and it’s not some grand conspiracy. I, and probably most other gear-heads, would certainly want to know a whole lot more when I read: “a crank-drive Stirling engine pumping water is shown in figure 3. While this engine is unusually large for the small amount of power (5 kilowatts) it produces, it is nevertheless very simple to make and operate.” (This was led by your statement that it was apparently built with two barrels, and accompanied by their silly and evidently oversimplified drawing, which if any of it is true, is probably depicting the engine smaller than it actually was.) You can believe that if you want, but then to support your belief you bring forth a witness who says the engine was actually more involved than the previously quoted excerpt insinuates. So one or the other MAY be true, but it would then point out that the other is false, and batting 500 in science is very poor. I would think you would see it that way too. Bumpkin

[Tom Booth]

Whatever the heat source, running a Stirling inside your house during heating season would be 100% efficient...

. Bumpkin

Not if your heat source is photovaltaic panels running an electric heating element. Your already loosing 75 - 80% of the potential solar heat outside at the panels. More is lost in the wires, more is lost in heating element (if any heating element is 100% efficient it's news to me, not all the electricity is converted back into heat) the storage medium (sand battery) looses heat (there is no perfect insulation) the Stirling is perhaps 25% efficient at converting whatever is left BACK into electricity.

Sorry but that does not seem at all practical to me and certainly nowhere near 100% efficient.

[Tom Booth]

Something else worth looking into would be the so-called "thermodynamic" solar panels. Basically a solar heat assisted heat pump. Relatively new and so far I think unavailable in the US but possibly doable as a DIY system using an air cycle.

Get a compressor to compress air into your sand battery through some pipes, use the compressed air to run an air motor to generate some electricity as the pipe exits the sand battery, and send the exhaust from the air motor through the pipes (hot air solar collector) on the roof (or wherever there is sun available).

This should boost the heat put into the sand dramatically over a simple ordinary thermal solar panel by several times over, with a much smaller footprint and works to some extent even at night.

But if your object is to put heat into the sand for the Stirling ANY direct solar heat system would be at least 70% more efficient that photovaltaic running an electric heating element.

[Bumpkin]

Well Tom, like I said this could get fun. Once again as is your wont, you took an example out of context to argue for the sake of getting the last word or defeating the evil “anything that isn’t a hot air engine.” So - I repeat, energy used INSIDE YOUR HOUSE during the heating season for other purposes is 100 % efficient.Your petulant answer could come from any bright five-year-old but unfortunately has nothing to do with the question. If I am wrong please show me where the energy goes? Geez. Bumpkin

[Tom Booth]

If you have cheap PV panels producing loads of excess heat you want to save some of great.

If you want to run a Stirling engine off solar to produce electricity as a primary energy system it makes no sense IMO.

https://youtu.be/omsjv__nedQ


An electrical heating element produces 0 power. The power comes from the PV panels OUTSIDE at maybe 25 percent efficiency at best,being very generous.

Now if you wish to imagine the heating element INSIDE alone is your power source, be my guest. Your entitled to your opinion, though I find it rather bewildering.

It seemed to me you introduced the 200 watt heating element into the context of this discussion as some kind of viable alternative to heat a sand battery to run a Stirling engine. That is the context. I did not change the context. If you intended some other context, then your argument is irrelevant to the discussion.

[Tom Booth]

Some interesting videos on thermodynamic solar:

https://youtube.com/playlist?list=PLpx2 ... eVheOEbAZx


If such a solar assisted heat pump type system can heat hot water 24 hours a day 365 days a year, I see no reason why it could not run a Stirling engine to produce electricity in the same way.

Though I believe an air-cycle based system for that purpose would be better.

Air cycle heat pumps require no chemical refrigerant, work more efficiently at colder outdoor temperatures and produce a more extreme temperature differential so both the hot and the cold output can be utilized, and the system can also be bootstrapped so that the compressor is partially powered by the expander.

A regular refrigerant based heat pump uses an expansion valve rather than an expansion engine so bootstrapping is not a possibility.

[Bumpkin]

OK Tom, here goes.
“If you want to run a Stirling engine off solar to produce electricity as a primary energy system it makes no sense IMO.” — I think you mean “P.V.” solar there and I totally agree, in fact it’s basic enough that I would probably leave off the “IMO.”

“An electrical heating element produces 0 power. The power comes from the PV panels OUTSIDE at maybe 25 percent efficiency at best, being very generous.” — That is indeed very generous. But once again such a basic truth that you left off the “IMO.” As I would too.

“Now if you wish to imagine the heating element INSIDE alone is your power source, be my guest. Your entitled to your opinion, though I find it rather bewildering.” — Such an opinion would indeed be very bewildering, and reading back, I can’t see how you would see it as mine, without your having made some very negative assumptions and leaps of tunnel-vision.

“It seemed to me you introduced the 200 watt heating element into the context of this discussion as some kind of viable alternative to heat a sand battery to run a Stirling engine. That is the context. I did not change the context. If you intended some other context, then your argument is irrelevant to the discussion.” — It’s your thread and if any other context is irrelevant, I will leave if asked, but like the others, that’s just silly. I’ll try to explain better: If you want to be the Stirling guy and possibly even turn it into a living, I can honestly and sincerely say I wish you the best. I think there are still places for Stirlings. But I think that if you want to be taken with credibility, you need to acknowledge there are limitations and treat other technologies or even points of view with a bit more respect. Now I’ll insert the “IMO.” Bumpkin

[Tom Booth]

OK Tom, here goes.
“If you want to run a Stirling engine off solar to produce electricity as a primary energy system it makes no sense IMO.” — I think you mean “P.V.” solar there and I totally agree, in fact it’s basic enough that I would probably leave off the “IMO.”

“An electrical heating element produces 0 power. The power comes from the PV panels OUTSIDE at maybe 25 percent efficiency at best, being very generous.” — That is indeed very generous. But once again such a basic truth that you left off the “IMO.” As I would too.

“Now if you wish to imagine the heating element INSIDE alone is your power source, be my guest. Your entitled to your opinion, though I find it rather bewildering.” — Such an opinion would indeed be very bewildering, and reading back, I can’t see how you would see it as mine, without your having made some very negative assumptions and leaps of tunnel-vision.

“It seemed to me you introduced the 200 watt heating element into the context of this discussion as some kind of viable alternative to heat a sand battery to run a Stirling engine. That is the context. I did not change the context. If you intended some other context, then your argument is irrelevant to the discussion.” — It’s your thread and if any other context is irrelevant, I will leave if asked, but like the others, that’s just silly. I’ll try to explain better: If you want to be the Stirling guy and possibly even turn it into a living, I can honestly and sincerely say I wish you the best. I think there are still places for Stirlings. But I think that if you want to be taken with credibility, you need to acknowledge there are limitations and treat other technologies or even points of view with a bit more respect. Now I’ll insert the “IMO.” Bumpkin

OK Mr. "Bumpkin". You entered into this thread making some rather vague insinuations about "credibility",

Seemed as though it was directed at me, but I wasn't really sure what you were driving at. Thanks for making yourself clear.

Most of your posts, in retrospect appear to be, as I suspected, an attack on my credibility. I make various statements, which I consider factual and reasonable or common sense and you come back with a barrage of counter arguments.

Overall, your motive appears to be backing up your personal belief or opinion that Stirling engines are "limited" to some "niche use" and that to believe or state otherwise, as I do, is dishonest, to quote: "It’s a niche use, great for some, not useful for most, sorta like Stirlings if we’re honest. We’ve gotta pick our spots. If we’re honest. Bumpkin"

So, you step in and basically accuse me of having no credibility and of being dishonest because I believe Stirling engines are actually more useful than what a lot of people give them credit for. Does that accurately sum up your position?

To again quote a previous statement, (out of context, so correct me if I'm somehow misinterpreting your meaning) - "as I’ve said for years, I think practical uses (for Stirling engines) are very limited, "

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 no, I do not "need" to acknowledge any such thing.

I DO believe Stirling engines could potentially replace IC gasoline engines in nearly every department. Solar Stirling engines, in particular, could largely replace coal, oil, gas, wood etc. The nearly unlimited versatility of these engines to run on virtually any fuel or any source of heat has made them of great interest to the military, to NASA, and to the Oil and Gas companies as well. It is, very much a "disruptive technology" no question about it.

As I stated at the very start of this thread, I definitely do not share the opinion that these engines are only historic artifacts or toys without any real practical application.

You seem to be making a concerted effort to criticize and debunk me and my point of view.

Could a Stirling engine be used to power an entire household? Absolutely!

But not by starting out with inefficient PV panels to make electricity to power a heating element to run the Stirling engine to run a generator to turn back into electricity again. Sorry to say but that is not the "uncompromising best option".

I have no problem with Photovoltaics. I'm not against any "other technologies", far from it.

But a PV panel turns sunlight into electricity directly.

Sandwiching a Stirling engine in between and converting the electricity into heat to run a Stirling to convert the heat back into electricity just introduces a lot of needless conversion. Every conversion is a loss. With that arrangement the Stirling engine serves no purpose.

[Tom Booth]

I mean a solar panel connected to a basic heating element. Ive read a standard shipping container sized battery can store 100kw (or more) so something the size of a refrigerator could hold like 10-20kw. Converted at 50% thats still enough to run a house for a week. I like the nasa model because it has few parts to wear out and can run 24/7 maintenance free for decades. Could be just another box behind the garage.

Standard shipping containers come in three different sizes, apparently, so I'm uncertain how you arrive at these figures, but seems like you may be talking about the smallest 10 foot long type?

Anyway, I assume, probably, by 10-20kw for a refrigerator size sand battery you mean kilowatt hours (?)

If that is the case, the average home requires about 29kwh or 100 million joules / day.

A PV panel produces useable power about what? 10 hours a day. Maybe more like 6. Possibly near zero on rainy days.

So, to keep the sand battery charged with PV panels to heat a refrigerator size sand battery, using resistance heating elements, the heating element needs to produce 100 million joules in, say 8 hours on average, maybe?

So that is 28 kwh of heat required to recharge the battery in 8 hours.

I think, if my calculations are right, that would require a 3,500 watt heating element. That's without making any adjustments for cloudy/rainy overcast days or short winter daylight hours

So about $3,000 for eight 450 watt PV panels.

Personally I think I'd double all that, but to build a half dozen DIY solar thermal panels, I can buy this 500 ft. Roll of black pipe for $75

Polish_20221202_212331199_resize_36.jpg (43.89 KiB) Viewed 6000 times

Since thermal hot air panels are about 90% efficient vs. 20% for PV , probably 2 thermal panels might be enough.

[Tom Booth]

I wonder if one of these would work as a heat pump to compress the hot air from the panels into some copper tubing in the sand battery.

It is a 12 volt refrigerator compressor available on eBay for off grid, campers or whatever. For under $200 and draws about 150 watts.

Resize_20221202_215300_0996.jpg (47.47 KiB) Viewed 5996 times

https://www.ebay.com/itm/174259704232?c ... WuMpZm5mu0