Okay, I read a fairly brief article sometime back on the web that said that government researchers were having better luck generating power using water heated by the sun to power a stirling engine and using it to run a generator than they were using solar cells. The stirling engine was generating power at about 30% efficiency, whereas with solar cells, by the time you converted the power generated by them into 120 AC, the efficiency was only 16%.
Hmm, says I. I’m broke and bored, and have access to to the gear necessary to build such a set up on a small scale. Then I got to wondering, would it be better to use 1/2 inch dia copper pipe painted black and laid out in a switchback pattern (to save space) or would it be better for me to use 1/4 dia copper pipe, painted black and coiled, then put in a switchback pattern? I could fit more pipe in roughly the same amount of area, but, of course, I’d be losing water volume, but I might be gaining in water temp. Anybody know?
That would be a relatively straightforward calculation if you could provide all the dimensional details, flow rates and other operating conditions.
There was some research done on using “fluted” pipe (pipe with wavy walls instead of straight walls) in heat exchangers and steam generators for improved heat transfer, which is similar to your idea. I think the basic tradeoffs are the added cost of the fluted pipe and the increased flow resistance versus the improved efficiency. In your application, flow resistance would probably be a minor player, so it would be primarily a question of up-front cost.
If this will work, what I’d like to do with it is use it to charge a couple of deep cycle batteries, that I’d use for powering things on the teardrop trailer I want to build (Sample pic can be found here.). So, I’d have a space about 46 inches wide, 36 inches high, and 2 inches thick to work with. Put some thin aluminum behind the coils to reflect the sun on to them, and, if possible, some kind of fresnel lens to magnify the sunlight.
If I can get it to power a stirling strong enough to spin an automotive alternator at the right RPM, I’ll be happy.
I have taken a bit of an interest in Stirling Engines in the past - they are very interesting of themselves and in many ways ideally suited to solar power applications.
However, all the solar Stirling Engines I have ever seen use mirrors to concentrate the Sun’s heat - the greater the temperature difference the better a Stirling Engine will work.
Collecting heat from a network of pipes is great for heating water, but can only give you lots of low-grade heat from which it is difficult to extract mechanical power.
You will find many people sell model Stirling Engines (even on ebay). Small, practical engines are harder to come by, though I have seen many one-off engines at exhibitions and many of these seem to work very well.
Turning a car alternator is quite ambitious and may be more power than you really need.
Of course, googling will generate endless hits. Try “Sterling Engine solar dish” to narrow the search a bit.
I believe I read the same article. My impression was getting this idea to work is the big problem.
Solar cells work, but they are way too expensive. Efficiency isn’t much of an issue until and unless implementation is so wide spread, there’s limited space to place the cells or the collectors. It’s about cost and whether the technology actually works.
I used to be a subscriber to the Stirling Engine Society in the UK, but this lapsed a few years ago. I made a couple of toy engines from tin cans etc; one just about worked. This is all great fun and I was left with the impression that making a Stirling engine that would work would not be too difficult, but making an engine that works really well is another matter.
If you google for “stirling engine society” you will turn up all relevant links.
I have had another look round the web for Stirling engine articles. I am struck by how little has changed since I last took a real interest - I guess that was about five years ago. All a bit disapointing.
I remember feeling that what the world could really use is a simple, cheap, not necessarily particular efficient, solar powered Stirling engine for generating modest amounts of electrical power in poor but hot countries. An approximatly parabolic solar collector is pretty simple to make so most efficiency issues can be answered with a larger reflector. One rule-of-thumb I recall is that the sun delivers about 1kW of power per square metre - there is a lot of power up for grabs!
I am, however, neither a mechanical engineer nor machinist…
My neighbor recently delivered to me 7 solar fence chargers asking"Hey do you know how to check these things out".
In the mix were 4 different brand name fence chargers with 3 different collectors. the “oldest” ,I assume, were made of individual triangle shaped collector chips soldered together in an assembly behind glass.
The others were more modern looking.
The collector surfaces were approximately 6" X 8" except for one big honker which was 7X9.
These power a continuously operating fence charger which provides approximately 6 to 10,000 volt pulses to provide a fence to keep cattle confined. At the same time charge a 9AH 6 volt battery. For use during the nighttime.
These small fence chargers power 10 miles of fence. The new ones will power 200 miles of fence I’m told.
Of the 7 chargers 5 of the collectors worked including all 3 of the old style “handmade” collectors.These put out over 12 volts when put in full sun. About 9 volts with a 40 watt incandecent bulb.That big honker was among the duds only supplying .2 volts.Darn it.
Most of the batteries were duds. I don’t know how long my neighbor has had them.
The only point of this post is that there are probably thousands of these non working solar chargers for sale at farm auctions and they are generally pretty cheap .
You can either fix them up and sell them or salvage parts.
Just a hint for you other guys that are also bored and broke.
How about, rather than making the stirling engine drive a rotary generator, just having it push a sprung piston containing a strong magnet back and forth through the middle of a tubular coil, generating AC that you can then rectify with a few diodes.
I’m not an EE, so I really don’t know how’d that work, nor how to build one. If I had a detailed description of the thing, I could no doubt do it, but puzzling it out on my own is a bit beyond my abilities at this point.
How would you store excess energy when you’re not running something? A battery is the only way I can think of that would be practical, and that’s DC. Power inverters are pretty cheap these days.
Whatever solution you choose for generating the power, you’re going to have to store the energy somehow; the generator itself isn’t likely to produce enough sustained power to charge the battery on its own; I think you’ll need a charge pump circuit (an arrangement that accepts a trickle input and pushes it out as a series of short, high-power pulses).
… the reason I suggest using an oscillating linear generator is that converting the oscillating linear motion of the Stirling piston to a rotary motion, then attaching a rotary dynamo seems to be adding extra steps for loss of efficiency to occur.
Even a rotary dynamo (essentially) generates AC - it isn’t possible to induce a sustained direct current - but many dynamos/alternators internally convert the raw AC by means of a commutator or an inbuilt rectifier.
A linear dynamo is much simpler - it’s just a tube with a coil of wire around it, inside which a magnet moves back and forth - just like this flashlight uses.
Whatever you do, you’re going to have to build a little bit of custom electronics.
