[Semi-OT] Solar-powered Stirling engines

There's a sort of "gee-whiz" article about generating electricity
with solar-powered stirling engines in the August issue of
Discover magazine. The bad news is that the device they're
describing is not a real product (yet), just a research project
funded by a private investor.
The interesting part is that the technology they're describing is
not new at all-- the only question is whether they can get the
cost down to where they can find a market for it. Their target is
250 watts for $250.
Part of the project was designing a stirling engine that could be
manufactured cheaply, and since the "fuel" doesn't cost anything,
they were willing to sacrifice some thermal efficiency to get
more "watts per dollar". The article says its about half as
efficient as the best stirling engines, but *much* cheaper to
build. They don't describe it in any detail, though.
For the solar part they're planning to use a 6'x9' array of small
mirrors, automatically aimed by stepper motors (and somehow
ganged together so they can use fewer steppers). By focusing the
light on a small area, they can produce temperatures up to 1200
degrees farenheit. And because the array is relatively flat, they
can cover it with glass or plastic to protect it from the weather.
My thought was that even if they miss their cost target, as long
as the total price is low some people might buy one just to thumb
their noses at the power company. They say a typical house would
need between 4 and 8 of these, which would be a $1,000-2,000
investment. You might need demand-priced electricity to make it
worthwhile economically (peak sunlight coincides with peak A/C
loads).
Their website at describes an
earlier version of the product and says it will be on the market
next year, but the Discover article sounded like they're now
working on a completely different design (with more reflectors,
and a rectangular array rather than round). Their web designer
really likes the word "revolutionary"...
The basic concept is similar to some government-sponsored
research described at , but on a smaller
scale. The Discover article quotes a guy from Sandia saying
"In principle, this could work.... I'm going to stay skeptical
until I see their data, but if they do pull it off, it could be huge."
BTW the magazine also has a brief article about people who
collect sliderules, with photos of some unusual ones.
For some history on solar-powered steam engines, check out a book
called "A Golden Thread: 2500 years of solar architecture and
technology" by Ken Butti and John Perlin (1980). The first solar
engine was demonstrated in 1866, and the first "practical" one
was built in Egypt in 1913 (it was abandoned during WWI).
Reply to
Ron Bean
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Ron Bean wrote: (clip) You might need demand-priced electricity to make it worthwhile economically (peak sunlight coincides with peak A/C loads). ^^^^^^^^^^^^^ These cost calculations can be very tricky (and deceptive.) In order to use solar electricity in any real sense, you need a lot of storage batteries. They are very expensive, and their life may be too short for their cost. I mean they may have to be replaced before they have returned their investment in energy savings.
Where I live, they tried to distribute the peak load (afternoon) by installing time-of-use meters. The cost of electricity was very low in the morning, and high in the afternoon and evening. So I looked into the possibility of installing a lot of storage batteries, and then buying electricity in the morning and selling it back in the afternoon. It didn't work out, and the main reason was the batteries would not pay for themselves over a 20 year period when they would need replacing.
Reply to
Leo Lichtman
Build your own batteries. It is extremely simple.
Cass
Reply to
Cass
But not easy or free, especially if you'r unhappy with lead fumes and toxic waste generation. You'd ideally want at least 1-200Kwh of batteries. That's around a tonne of batteries. That's one hell of a lot of 'home-made' battery construction.
Reply to
Ian Stirling
As well as topping up electrolyte, cleaning off corrosion, hydrogen fumes and lots of other icky, nasty details that some of the home-power types would just soon forget. And I'd say the 20 year lifespan that somebody estimated is overly optimistic, 5-10 years would probably be closer, less if you're using "free" recycled car batteries like some enthusiasts I've read were promoting.
What I'd like to know is if there's a solar-powered Stirling generator out there that's even close to what you get with a solar panel in terms of watts/square meter. They've always been on the virge of being practical since the '60s, I've still got a Popular Science mag somewhere from that era that had plans for a very small solar-powered Stirling engine. Kind of like fuel cells, the breakthrough was always going to be sometime in the next 5 years.
Stan
Reply to
Stan Schaefer
8 things 6 x 9 feet? That is a lot of "STUFF" sitting in your back yard. A 6 x 9' first-surface reflector should harvest about 5 KW thermal, and a 30% efficiency engine should deliver 1600 W mechanical, or maybe 1 KW electrical output. But, the engine isn't going to be cheap. First, running at 1200 F is going to require some higher-grade materials, or you'll get a very short life. You can't run atmospheric air in the thing, or the engine will be HUGE. Hydrogen at high pressure allows you to use a VERY compact engine, thereby reducing cost. Putting the alternator inside the crankcase allows you to run a hermetic, sealless engine. Using roller bearings and dry piston seals allows you to get rid of the crankcase lube system, and the contamination of the hot section that results. Finally, you need water cooling or some other advanced system to reject heat from the cold side of the engine at the lowest temperature possible. I am planning on building a machine of about this size, and plan to use a car radiator, fan cooled, and will probably need a circulator pump for the water to cool the engine. If 5 Kw is coming in from the sun, and you are delivering 1 Kw electrical to the load, then you have 4 Kw of heat to reject, at the lowest temperature possible. That takes a lot of air, and a lot of surface area.
I've downloaded a computer program that attempts to take into consideration all the internal losses in the engine, such as fluid friction, heat exchanger losses, the sinusoidal piston motion, excess volume above the piston, etc. to produce the most accurate performance predictions. I'm still looking at this, and trying to optimize the design.
Jon
Reply to
Jon Elson
This really won't work, except on the largest scale, due to the losses in the AC -> DC -> AC conversions. Some people at EPRI have been working on superconducting inductor energy storage for this purpose.
But, having a solar, wind or other system, and timing the heavy use of electricity (cooking, clothes drying, water heating) for the cheapest time, and allowing the solar, etc. system to supply all other needs can be a workable system.
Jon
Reply to
Jon Elson
I'm not sure what you mean by "out there". If you mean available off the shelf, no, certainly not. If you want to build your own, however, it would be VERY hard not to get more power/sq. unit than the best photovoltaic systems do. The best PV systems claim 13% efficiency when new, many commercial-grade PV arrays run around 3%! Given all the associated loads having to do with water cooling the cold side of the engine, tracking the collector, etc. a 20% total system efficiency seems quite possible. Getting a 30% conversion to mechanical output is not extremely hard for small Stirling engines. But, you can't make it out of tin cans and string. The hot side will need to run at 600 C or higher, the cold side must have a huge thermal radiator to keep it as close to ambient temp as possible, you have to have a LOT of surface area in the heat exchangers, and in most cases you want to run pressurized working fluid to keep the engine as small as possible. (Smaller engines have less friction and heat losses, of course.)
Search the net, there are a lot of people experimenting with them. A few people are getting very serious, as individuals, in building working Stirling engines.
Jon
Reply to
Jon Elson
IIRC and it has been awhile since I saw the numbers , the stirlings are about 35% efficient compared to 8 % for PV. They have PV's up to around 40% , but they don't say how much they cost and I'm sure they are pricey and you can't buy any. The stirlings run with hydrogen and cost about $100,000 plus the $150,000 tracker. There is a $250,000 concentrated PV tracker about 2 miles from my house that worked for about 3 months , 5 yrs. ago. I just don't see how they can effectivly cool it with static air and heat sinks, they should be about 100 degrees , its a good 100 degrees in the shade. Nevada Power won't talk to me and the company that made it, made it very clear that they don't want any help from me. They wanted to buy my guidance system at a ridiculously low price and I told them to get lost , might have had something to do with it. Let it stare at zenith ! Our federal tax $ at work. The DOE turned me down 4-5 times for a grant for frivolous reasons like my system would burn itself down if it stopped tracking. I tried to reply to them professionally like prosecuting a patent , but couldn't seem to answer such rediculous questions. I remember another one , they said that it couldn't possibly move and track with so few parts. Well, that seems weird , it works just great. Come out and check it out if you don't believe me. "We don't come out to Vegas..." That's a complete lie. They have the test site , EG&G , that $1/4 mil. junk , and gave $6 mil. to the senator to promote solar energy in NV. and he blew it all on paper work and wouldn't really talk to me. Oh, they said they culled it , the only Nevadan to respond to their RFP. They didn't even read it with that $6 mil. !
As soon as I get my tracker into production I'll take a closer look at stirlings. I found out that solar energy is very political and tight lipped. Finding any relevant information for a business plan is hopeless. Using the grid is also hopeless cause they will only pay 1 cent per KWH. Its booze , hookers , and bribes or do it alone.
I've said more than enough , sorry for the rant.
Edison said something about batteries ( they called them something else then) that didn't paint a rosey picture.
Reply to
Sunworshiper
Their claimed breakthrough seemed to be that they had figured out how to make the engine cheaper (and they were going to use several small units, not one big one). But I don't think the article's author understood the problems involved, so they just glossed it over. It's not clear how much testing they've actually done (although they did claim to have a working prototype). They may find out they're not as smart as they thought they were...
Reply to
Ron Bean
"Cass wrote: Build your own batteries. It is extremely simple. And then he wrote: Sorry that I made a post that required someone to think or actually do something. ^^^^^^^^^^^^ I am THINKING about DOING. Would you be kind enough to outline the simple method you know. I am pretty good at doing simple things.
Reply to
Leo Lichtman
Have you ever looked into compressed air or pumped hydro? They are used on a commercial scale to help with peak demand.
Reply to
Stuart
Leo,
A lead-acid battery is very simple to build. Do some research on the Internet. All they consist of is modified lead, a water and sulphuric acid solution and a container.
Batteries were built in tubs long ago and every railroad station had some for its telegraph system.
I was just irritated at all of the nay-sayers as it is easy to try to pick something apart rather than do something. You know, easier to destruct rather than construct.
Cass
Reply to
Cass
Leo,
Here is a useful link about lead-acid cells. You may want to research the patent office, too.
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I think the electrolyte that you mix is a solution of sulphuric acid and distilled water until you get a specific gravity of 1.260. I am not guaranteeing that I recalled that correctly. I know that one can make a L.A. battery hotter by running the s.g. up to 1.300 but the plates don't last as long.
Another thing you need is some separator material to put between the plates to keep them from shorting to each other.
Please let me know if you intend to build some/one as I may join you.
Cass
Reply to
Cass
Cass, it is just as I thought. Your statement that building storage batteries is does not consider the many physical difficulties that come up when you go from the abstract to the concrete. The lead in a lead-acid battery is a special spongy form--not the lead used for joining cast-iron pipe. And the container has to be made of an acid resistant material like hard rubber or glass. The assembly of alternating layers of spongiform lead and porous insulator layers will involve techniques not known to the average shop worker. By the time you get your batteries, large enough to run a houshold, assembled and operating, I believe you will not say "It was easy."
Nor will you say,"They were cheap." I'll bet you can buy batteries of proven design cheaper than you can buy the materials and build them.
I don't think I am being a naysayer. I am being a realist. I don't think building a house is easy, either, even though the separate steps are not hard in themselves.
Reply to
Leo Lichtman
Stuart wrote: (clip) Have you ever looked into compressed air or pumped hydro? They are used on a commercial scale to help with peak demand. ^^^^^^^^^^^ I do know of one example of hydraulic peak-power storage. It is at a dam in northern California, where they use off-peak power to pump water back up into the fore-bay, so they can re-use it to handle peak loads. This is in the mega-watt range. They are able to make use of the turbines and dynamoes which were needed anyway to generate electricity. I don't know of any examples where this is done on a smaller scale, nor where the installation costs are justified by the off-peak load transfer function.
I don't know of any examples where compressed air is used to transfer peak loads. I would be interested in knowing of the ones you are familiar with.
Reply to
Leo Lichtman
Well, the point I was trying to make is that needing EIGHT 6 x 9' reflectors in your backyard to just barely power the house doesn't make a lot of sense. If you can do it instead with 2 reflectors that size, or one slightly larger, that seems a lot better. Also, you need to make all 8 mirrors track, you need to keep them clean, etc. Maybe it starts out cheaper, but it quickly becomes a maintenance nightmare! No thanks!
The stuff about the engines costing $250,000 is just bogus. Yes, if you have a research institute make just one engine, it could cost that much. But, a competent guy with a good home shop could do it for a few thousand, easily.
Jon
Reply to
Jon Elson
Stupid me! I actually thought that you might be interested in building a very simple thing that is very simple to build.
Little did I know that your real intent is just to fight and pick nits.
You can build lead-acid cells much cheaper than you can buy them but you don't think so. So, buy them and quit trying to pick a fight.
By the way, I have built lead-acid cells and as I said, it takes a modified form of lead and I never said it took regular lead.
Now, run along and think that everything that you don't understand is hard.
Cass
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Reply to
Cass
Then please inform us of the details, and also specs such as life, internal resistance, capacity.. Maybe some photos too.
And BTW need I remind you, 90% of this group already has you in their killfile, you might want to keep calm lest you lose what little audience remains.
Tim
-- In the immortal words of Ned Flanders: "No foot longs!" Website @
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Reply to
Tim Williams
That would be interesting: cycle the pressure in a tank between say 0 and 500PSI, just hope you didn't get an aluminum tank! :-o
Tim
-- In the immortal words of Ned Flanders: "No foot longs!" Website @
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Reply to
Tim Williams

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