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Old 21st June 2009, 09:13 AM   #1
hayenmill
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Alternative Way to Harness Solar Energy

SOLAR POWERED STIRLING ENGINE:

(again, can't post any links yet, delete spaces between http): h t t p ://digg.com/d1rmzX

Quote:
A Stirling engine is a device that converts heat energy into mechanical power by alternately compressing and expanding a fixed quantity of air or other gas (the working fluid) at different temperatures.
Quote:
Recently, however, stirling engines have been increasingly designed towards using another source of heat: the sun. New stirling engines can work using the heat of the sunand provide a new way for one to harness that energy without only relying on photovoltaic panels.
Furthermore, stirling engines are very easy to make with simple materials
It also links to various websites where they show you how to make one from simple household items

Quote:
If you want it to be more environmentally friendly, simply use mirrors in order to link the sun’s rays into the stirling engine’s heat sinker.
Thoughts?
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Old 21st June 2009, 09:18 AM   #2
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http://digg.com/d1rmzX
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Old 21st June 2009, 09:19 AM   #3
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thanks for linking me
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Old 21st June 2009, 10:58 AM   #4
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Stirling engines are sorta like perpetual motion machines in that they are regularly rediscovered technology that generates a lot of excitement as to the possibilities, then fade away when the tough rules of reality intrude on dreamworld. UNLIKE perpetual motion machines, stirling engines DO work, just not that efficiently. There is no phase change or conversion in the cycle. You're just dumping heat from one place to another.

Stirling engines are attractive because they work on any source of heat -- solar, nuclear, geothermal, etc. They work best when there's a large heat differential between the hot and cold sides. There are low-delta models that will work off the heat from your hand, but they're limited to nothing more than rotating a pretty disc that serves as a flywheel.

Unlike photovoltaics, stirling engines are mechanical devices which wear out, require lubrication, and need regular servicing. Because of their low efficiency, when you couple in the losses inherent with converting mechanical motion to electricity, the end efficiency is even less. They only become attractive if you have a steady source of heat that would otherwise be wasted.

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Old 21st June 2009, 11:49 AM   #5
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Originally Posted by Beanbag View Post
Stirling engines are sorta like perpetual motion machines in that they are regularly rediscovered technology that generates a lot of excitement as to the possibilities, then fade away when the tough rules of reality intrude on dreamworld. UNLIKE perpetual motion machines, stirling engines DO work, just not that efficiently. There is no phase change or conversion in the cycle. You're just dumping heat from one place to another.
Quite right, but there's an important point to note:

Stirling engines use the thermodynamic Stirling cycle, which is theoretically equivalent to the Carnot cycle: it would have the highest possible thermal efficiency. Naturally, real efficiencies are lowered, but a good Stirling engine should still be more efficient than an Otto cycle engine (like used in American cars) or a Diesel engine (like used in European cars).

In practise, however, Stirling engines require very large heat exchangers, and are too heavy for vehicle use. Big heat exchangers are also expensive, so even when weight isn't a concern they aren't very popular. Power plants, where efficiency is incredibly important, generally use modified Rankine or Brayton cycles. The only real advantage they have is that they are quiet. I think some yachts or other luxury boats use Stirling engines simply to reduce noise.

It's also funny that only the Stirling engine gets this attention: no one ever cares about the Ericsson cycle or the tries to build a Carnot engine. The relative sizes of the Wikipedia pages is a nice demonstration of the optimism of Stirling enthusiasts.
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Old 21st June 2009, 11:52 AM   #6
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And to add to Beanbag's post, the heat source has to be lower than 100°c, or steam would work better. So, if a Sterling works in the sun, the use of reflectors to concentrate the heat means Sterling's lose out to steam.

So:
Sun= use steam.
Geothermal= use steam.

Rocks barely too hot to sit on, use Sterling, but only if you can't drill deep enough to hit steam.
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Old 21st June 2009, 11:58 AM   #7
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Many years ago, "New Scientist" (I think) ran a piece on a Stirling cycle engine called the Fluidyne which had no moving parts, except a moving column of water, which caused it to rock from side to side. As I remember, this was going to be used to power an electrical generator for remote applications.

I don't know if it was ever successful, but you could try exploring the net for it.
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Old 21st June 2009, 02:30 PM   #8
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Eh - as a programmer on the Solar One project in Daggett CA (long since decommissioned; http://maps.google.com/?ie=UTF8&ll=3...00957&t=h&z=17) I have a nit to pick with you about "simply use mirrors" to redirect solar energy. Granted the accuracy of such mirrors for pointing is far smaller than the requirements that we had, I would like to challenge you to design a cheap, easy to implement mirror that will statically yield, ohh, let's be easy, about 40% on target radiation for all hours of daylight throughout a year's cycle. Perhaps a system rather than a single mirror, though cost is, of course, of the essence. I don't really care about focus or anything - just get the sunlight on the target (which is, by itself, 100% shaded) 40% of the time. A trough will do it, but it is not exactly cheap to build long parabolic (or even spherical) mirrors, and they still need a control system.

I hear that reflected solar onto a tower boiler is coming back, at least in other countries - why, the foreign countries are trying to sell them here. Too bad that Regan thought they didn't deserve any development funding - we could have been experimenting, developing and selling them to others, rather than buying them. I'm thankful that some people somewhere are carrying on the cando spirit. I wonder how much better it is now than it was last time.

Last edited by shadron; 21st June 2009 at 02:39 PM.
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Old 21st June 2009, 02:37 PM   #9
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Originally Posted by Paul W View Post
Many years ago, "New Scientist" (I think) ran a piece on a Stirling cycle engine called the Fluidyne which had no moving parts, except a moving column of water, which caused it to rock from side to side. As I remember, this was going to be used to power an electrical generator for remote applications.

I don't know if it was ever successful, but you could try exploring the net for it.
yeah i found some info in it in wikipedia. thanks for sharing the name
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Old 21st June 2009, 03:03 PM   #10
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Stirling engines work really well, however, as a "bottoming cycle" for a more conventional power plant, for heat recovery from industrial processes (cogeneration) and for running auxiliaries on heat machines.

For example they make ideal fan motors in some application as their rate of operation will be proportional to the heat that needs to be removed.

However, they are not self-starting, so you need to have a kicker solenoid or similar.
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Old 21st June 2009, 03:34 PM   #11
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BTW, I have a model Stirling engine here in my collection. Fun to play with.

If people really want it I'll post a video of it going, but you can probably find similar on google video search.
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Old 23rd June 2009, 12:37 PM   #12
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Originally Posted by hayenmill View Post
yeah i found some info in it in wikipedia. thanks for sharing the name
Great!
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Old 23rd June 2009, 12:51 PM   #13
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Originally Posted by shadron View Post
Eh - as a programmer on the Solar One project in Daggett CA (long since decommissioned; http://maps.google.com/?ie=UTF8&ll=3...00957&t=h&z=17) I have a nit to pick with you about "simply use mirrors" to redirect solar energy. Granted the accuracy of such mirrors for pointing is far smaller than the requirements that we had, I would like to challenge you to design a cheap, easy to implement mirror that will statically yield, ohh, let's be easy, about 40% on target radiation for all hours of daylight throughout a year's cycle. Perhaps a system rather than a single mirror, though cost is, of course, of the essence. I don't really care about focus or anything - just get the sunlight on the target (which is, by itself, 100% shaded) 40% of the time. A trough will do it, but it is not exactly cheap to build long parabolic (or even spherical) mirrors, and they still need a control system.
What is your opinion on the SkyFuel company with their sun trough out here in Arvada? To be sure, they are using the energy to create steam.

http://www.skyfuel.com/#/TECHNOLOGY/
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Old 23rd June 2009, 02:13 PM   #14
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I can build you a gas-actuated heliostat that tracks the sun by using solar heat. It is not hard to do, but you do need a motor to reset the system each night. It can even be adapted to adjust to inclination using solar heat. Applies to a single-mirror system only, though.
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Old 23rd June 2009, 03:19 PM   #15
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Originally Posted by BenBurch View Post
I can build you a gas-actuated heliostat that tracks the sun by using solar heat. It is not hard to do, but you do need a motor to reset the system each night. It can even be adapted to adjust to inclination using solar heat. Applies to a single-mirror system only, though.
I remember an article in an old Mother Earth News from the 70s, from a guy in New Mexico at a place called Zomeworks, who built a solar tracker by putting freon in a hydraulic system and shading the sides. When the sun hit the freon canister, the gas expanded, causing it to "track" a certain way until it wasn't in sunlight any more.

Just did a google search on Zomeworks, and they're selling "passive solar trackers" with an "integral early morning rapid return system." Maybe that's the motor you're talking about, or maybe they've found another way to do that too. I guess they're probably using something besides freon nowadays.

ETA: According to this how it works link, it looks like the morning return is "expanding gas-powered" too.

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Old 23rd June 2009, 05:27 PM   #16
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JUST like that.
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Old 23rd June 2009, 09:27 PM   #17
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Originally Posted by roger View Post
What is your opinion on the SkyFuel company with their sun trough out here in Arvada? To be sure, they are using the energy to create steam.

http://www.skyfuel.com/#/TECHNOLOGY/
It looks very much like the system Martin developed for a solar plant at Almeria, Spain - it was a multiple trough system with salt working fluid. We were working on that as a subproject to the big plant at Bairstow in the 80s. Might even be some of the guys I worked with in SkyFuel...

It looks good - I particularly like the design of the linear fresnel "Power Tower" that they have. Like Bokonon and Ben says the mirrors can be done gas-operated aiming (John Denver's little solar institute at Snowmass did that with small single mirrors as well), or they could be computer controlled with stepper motors or some such for more flexibility. I like the idea of using salt; that's what we did in Spain, with the tanks planted underground for insulation. You had to have sun sensors and drain the pipes when the sun went behind a cloud; cleaning out the system if the salts cooled too much was a hum-dinger; there's no indication of what they do in the SkyFuel system, but they have to do some such. Using the molten salt as a power reservoir is probably a good thing too, certainly better than batteries. I'd like to know something about their maintenance and upkeep. We had to have a fulltime mirror washing crew to keep the heliostats clean, among other things.

When we did the plant in Bairstow we had to have special FAA airspace declarations; it just wouldn't do to fry a helicopter landing with a VIP. I wonder if they have to do something with these systems? Granted they're smaller, but I imagine they'd blind a pilot right well if approached in the wrong manner.

Last edited by shadron; 23rd June 2009 at 09:33 PM.
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Old 23rd June 2009, 09:32 PM   #18
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Originally Posted by bokonon View Post
Just did a google search on Zomeworks, and they're selling "passive solar trackers" with an "integral early morning rapid return system." Maybe that's the motor you're talking about, or maybe they've found another way to do that too. I guess they're probably using something besides freon nowadays.
That works fine as long as you only want one axis of freedom and don't mind the error of not matching the seasonal n-s tilt angle. Doing that in two dimensions would be difficult, I think. That would be OK for troughs that are light enough to be lifted in a mass, but difficult to scale up, I think.
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Old 23rd June 2009, 10:13 PM   #19
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Shadron. Balance. You can move the 200" on Palomar with one hand.
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