NASA Stirling Engines (Stirling "convertors")

[Tom Booth]

NASA has been taking Stirling engines very seriously for decades. So why do so many think of these engines as historic artifacts or only toys with no real practical application.

https://youtube.com/watch?v=jjK-PZX2aZ0&feature=share

Additional videos:

https://m.youtube.com/playlist?list=PLp ... isNWiFsyPx

.

[Tom Booth]

Though these engines were such a failure and had to be decommissioned and sold as scrap, Battelle was proposing putting these things on the moon?

Resize_20220725_011058_8551.jpg (137.2 KiB) Viewed 6212 times

From this PDF:

https://www.nasa.gov/pdf/314629main_bat ... tation.pdf

[Tom Booth]

This is a rather recent video, evidenced by occasional mention of COVID.

The use of Stirling engines in space, to power bases on the moon and Mars, for deep space exploration etc.

AFAIK, the Stirling engines/Convertors under discussion are the same as the well known NASA frictionless free piston engines from around 2010 that were tested and able to run for 14+ years without maintenance or wear.

https://youtu.be/SZNpqW8-T9k

An interesting question from the video was, are these commercially available ? Like are these "off the shelf" Stirling engines being used by NASA.

As far as I know, those test engines that ran for over 14 years are still running.

https://www.spaceflightinsider.com/spac ... asa-glenn/

https://www.nasa.gov/feature/glenn/2020 ... -milestone

About the "record setting TDC#13" (Technology Demonstration Converter #13)

This hardware set includes Technology Demonstration Convertors (TDCs) from Infinia Corporation, of which one pair (TDCs #13 and #14) has accumulated over 60,000 hr (6.8 years) of operation...

That was back in 2012

https://ntrs.nasa.gov/citations/20120013818

[CincyD]

Qnergy sells this but I think they want $$$$. I have a feeling they can be built at scale fairly cheaply. I want one to power my home!

[Tom Booth]

... I want one to power my home!

What would be the heat source?

[CincyD]

A sand battery connected to a solar panel with municipal backup. I imagine a modular system.

[Tom Booth]

By "solar panel", do you mean direct heat transfer to the sand battery or photovoltaic/electric?

Though I suppose it wouldn't matter as far as the Stirling engine is concerned, I think solar heated panels, vacuum tube of parabolic through or dish direct heat to heat transfer to the sand would be many times more economical and efficient.

Azelio has a similar system using aluminum alloy phase change heat storage, but apparently have no intention of supplying residential units. Sand sounds much more feasible. Azelio's Stirling engine does not strike me as being anything particularly efficient or high tech.

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Alpha type Stirling engines are probably the most difficult to construct as well as being the least efficient:

https://www.sciencedirect.com/science/a ... d%201.8%25).


Requiring a very high ∆T to run at all. Though potentially, that could be improved. Also not very practical in terms of economical residential use, but as a system utilizing aluminium phase change heat storage, I guess it makes sense given the high temperature required. Probably the aluminium alloy would be much more compact, lightweight and portable.

I wonder what the equivalent BTU capacity of sand would be by weight and/or volume compared with melted aluminum.

What would be the volume of sand required and where would this sand battery be kept?

[CincyD]

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.

[Tom Booth]

I mean a solar panel connected to a basic heating element....

So, photovaltaic powering an electric heating element?

The reason I asked is, a typical solar panel is only like 20% efficient at converting sunlight to electricity. A vacuum tube solar heater is like 97% efficient, so your loosing like 75% of your potential heat storage.

Then a typical electric heating element, for a water heater for example, draws about 4000 to 5000 watts

At say, $2.50 a watt that's maybe $10,000 in solar panels just for a minimally functional system.

By comparison a DIY direct heat system using black pipe curled up under some window glass or some such could probably do just as well if not better than the $10,000 in photovaltaic panels in terms of actual heat storage in the sand battery.

[CincyD]

The thing is the sand needs to get to 1000f. I was thinking building it around a simple rocket stove would be the cheapest solution -especially if you could grow the fuel. The biggest challenge is getting the engine manufactured. "5-kWe Free-Piston Stirling Engine Convertor" provides some detail but im sure a manufacturer would need everything specced out completely. Ive decided to try to find an engineer that I can afford just to tell me if its possible.

[Tom Booth]

The reason for all the logistics about how the sand is heated could determine the type of Stirling engine.

I'm sure sand CAN get to 1000° or probably better, but I don't know any reason why it "needs to".

I've been thinking for a long time that probably the most versatile, and inexpensive Stirling engine would be something along the lines of one of these little engines:

https://youtu.be/Dp1FT33KEZM


Mainly because the long protruding displacer cylinder could utilize practically any heat source: camp fire, fireplace, rocket stove, wood stove, solar concentrator, hot water, box of hot sand or whatever, as a design starting point anyway. Some modifications could make for some improvement.

The sensitive power generating parts, coils, magnets etc.can be well away from the heat source to avoid heat damage.

[CincyD]

1000 is the temp the nasa converter operates at. Lower temps will work too of course. It would be an easier diy for sure -how large would it have to be to get 3kw?

[Tom Booth]

1000 is the temp the nasa converter operates at. Lower temps will work too of course. It would be an easier diy for sure -how large would it have to be to get 3kw?

Not that it necessarily means anything, as government projects go, but I believe some hundreds of millions of dollars went into NASA Stirling convertor development for something that will continue to run, unattended in outer space on radio isotopes for 30 years until it leaves the solar system. Probably a little bit of overkill there, just to run a Stirling on a crate of hot sand.

I think usually a lowish tech semi - DIY type engine, in the 5 hp range usually looks like a 55 gallon drum. A low temperature engine would probably be a little bigger.

[Tom Booth]

Back in the 1980's, a non-profit VITA (Volunteers In Technical Assistance) developed a very simple Stirling engine using a 55 and a 30 gallon drum.

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Their literature stated:

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

4 to 5 kilowatts seems pretty good IMO for something knocked together out of scrap, essentially for free.

For residential use, running 24/7 far more than what most people would require I think.

Personally I'd go for something much smaller, like, not much more than the minimum continuous load required in the house at any one time.

The reason being, if 5,000 watts are needed, say, for cooking meals at dinner time, there is no point in running a 5,000 watt Stirling generator all day and night just for lighting or whatever the rest of the time.

I'd probably get a few good deep cycle batteries and maybe a 10,000 watt inverter for when there is some heavy load, just using the Stirling/sand heat storage system to keep the inverter batteries charged.

Sure, there would be conversion losses, but probably not as much loss as running a high power Stirling continuously 24/7 with hardly any load.

Many modern inverter-generators are designed that way, since a portable generator is often used only intermittently, it may consist of just a very small 100 watt or something actual generator, a battery and a 5000 watt inverter.

[CincyD]

I was raised by a Marine. I have tendency towards finding the uncompromising best option regardless of the cost/effort -and then realizing it could be easier after the fact. Id imagined a small battery/capacitor set up to buffer engine but using the engine to charge a smaller battery bank sounds more doable. I think most peak demand is at 1kW -the 5kW was future proofing but also based on the nasa model. So maybe a smaller 1kW/hr stirling connected to a 5-10kw tesla knockoff battery pack plus the sand battery -you have 24/hr 360 off grid power for 100% of home needs.