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Forum Index : Other Stuff : Concentrated Solar Thermal
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| 42pole Newbie  Joined: 22/08/2009 Location: AustraliaPosts: 4 |
Hi all, I am new here and can see that you are all well versed in the nature of clean energy. It has been my obsession for a number of years and I have looked into several viable methods to tap this renewable source of energy. Recently I obtained a WW2 searchlight parabolic mirror(1.2m dia) and found that the focal point (120mm dia) exhibits some 600 degrees centigrade of usable energy. I was thinking of using a stirling or steam engine coupled with a modified F&P assembly. I would appreciate any input on this or indeed any other means of converting heat to electricity. Maybe peltier/ seebeck or thermal diodes. Any thoughts? |
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GWatPE Senior Member  Joined: 01/09/2006 Location: AustraliaPosts: 2127 |
Before U start on the engine, U need to perfect the solar tracking system. The focal point is very precise. Gordon. become more energy aware |
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| 42pole Newbie Joined: 22/08/2009 Location: AustraliaPosts: 4 |
Thanks for your response Gordon. I have erected the mirror onto a gimbal assembly and have begun building a sun tracking servo assembly using small 24volt geared motors driven by a photo-transistor tracker aligned with the focal point. My problem now is to find the most productive method of efficiently converting the energy into something storable. Steam is the most obvious as it is easy to produce and has the ability to run an engine as well as being condensed for storage as hot water, but a passive system may be advantageous for noise limitations as I live in suburbia. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5019 |
Sounds like fun. You are capturing about 1 square meter of sunlight, Dependng on where you are, but in Australia during summer on a clear day thats approx 1000 watts. A stirling cycle engine is roughly 20% efficient, so you'll get 200 watts shaft power in full sun. A good alternator at 75% efficient will give you 150 watts electrical power. 150 watts is OK. If you can build it a lot cheaper than 150 watts of solar cells, then it might be worth considering. Remember there will be a lot of moving parts to maintain. What about using peltier with a big heatsink on the cold side? Not as efficient, but no moving parts, apart from the tracker. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| 42pole Newbie Joined: 22/08/2009 Location: AustraliaPosts: 4 |
I knew I could find some help here, thanks for the input Glenn. The maintenance of such a rig is unavoidable, but the trade offs are worthy of exploration. As you noted, the efficiency of peltier is very poor, but I believe high temperature differential devices exist which are suited to extreme temperatures.here I read somewhere there is a new thermal diode array that promises much improved efficiency (>40% ideal Carnot cycle). Unfortunately they are still in the development stage. Lou |
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MacGyver Guru  Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Might I suggest an easier method? Being parabolic in nature, your mirror has a somewhat "exact" focal point. You need to maximize this feature to grab all the power it can concentrate. To do this, the sun's rays must enter parallel to the focal axis, which is at ninety degrees to the mirror plane prescribed by the mirror's circumfrence. An easier method would be to mount the mirror in a STATIONARY position and use a flat-plate heliostat (mirror) to reflect the ever- changing sunlight onto it. This way, the rays of sunlight all enter at ninety degrees to the plane of the mirror. Maneuvering a lightweight flat mirror will be way easier than trying to move a gimbaled heavy curved glass one. A flat-plate mirror will be easier to mount, clean and control than using that heavy paraboloid monster. The curved mirror could be placed into a 'house' or some other structure that would allow you to mount louvers that could be used to control the amount of sunlight entering the mechanism. This would allow you to refine its focus, if the individual mirror pieces are moveable. I've done this on a smaller scale to manufacture steam to run my steam engines. The easiest method to create steam is to pass water through a coil at the focal point. I used oil and stored it in a buried (in beach sand) tank to enable longer steam runs for my toys. I used the hot oil in a heat exchanger (boiler) and pumped in atomized water (fine spray). If you make steam, use distilled or R/O water or you'll plug things up in no time at all (dissolved calcium in the water). The question will arise that you are wasting some of the energy by reflecting it off the first mirror. This much is true, but since it's all FREE stuff (sunlight) who cares? Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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| 42pole Newbie Joined: 22/08/2009 Location: AustraliaPosts: 4 |
Now that's thinking outside the square. You're right as to the monster (40kgs) being difficult to accurately maneuver and my current rig is somewhat 'Heath Robinson' in design. You noted impurities in tap water clogging the works, I didn't think of these implications in my design..thanks for the heads up MacGyver. You were always credited with lateral thinking. The enclosure design is also clever, cage the monster and controlled feeding will keep him regular. Is it possible to use a convex, thereby smaller heliostat or do the convergence losses outweight any control benefits? Also what is the percentage losses in current mirror designs? I know that the accurate mirrors used in laser technology are 99% reflective, but the doping process would be expensive for a larger flat plate reflector. |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
My advise would be to keep it simple. I use a portion of a mirrored glass wardrobe door cut to size. It's still pretty heavy, but not to the extent that it can't be maneuvered easily. An advantage to this is it has a form-fit extruded aluminum border on each side. If you try to go fancy and use a convex, it will throw UN-parallel lines of light at your curved mirror and you'll wind up with zero. As far as % efficient, I don't give that a second thought. If I determine it's not efficient enough, I just make things BIGGER! By the way, as long as we're talking about percentages, about 90% of my building materials come from dumpster diving. When a cop sees a kid in a dumpster, he gives him hell and runs him off. When a cop sees me in a dumpster, he probably thinks I have some business there and leaves me alone. That's an advantage to being 60 and resembling "Gandalf". By the way, that "cage" besides being a way to control the light input, is really important if you have kids around. Think for a minute what would happen if a kid put his hand at the focal length while his buddy fiddled with the heliostat! Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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