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Old 09-03-09, 08:55 PM   #1
Christ
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Lightbulb GeoThermal circulation to run a generator?

Just a quick thought... if the below ground temp is always ~55*, could one not produce electricity with a properly sized thermo-siphon system setup the same way as a geo-heat pump?

Dig a hole 2 feet across, 20 feet down, and 10 feet wide, make a loop of 18 inch dia. pipe with a generator in circuit with it and a burp valve at the highest point, fill it with active propylene glycol, then bury half of it in the hole, and leave half exposed... temperature gets below 45* air temp, the fluid starts to circulate, driving the generator, which (in theory) should then create electricity, right?

Has this been done?

How much electricity could one generate using the specs I've laid out? Would it even be worth it?

Found this:http://www.patentstorm.us/patents/4546608/claims.html


Last edited by Christ; 09-03-09 at 08:59 PM..
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Old 09-04-09, 06:20 AM   #2
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I'm sure it could be done, and its a very interesting idea. However, unless you have a significant thermal differential, there really isn't much power to be harnessed. The medium just won't build up enough momentum/pressure to generate a useful amount of energy.

I wonder if this idea wouldn't be better suited to use in a solar hot water setup? The temperature differential there would be much higher.
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Old 09-19-09, 11:56 PM   #3
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I never noticed a reply to this... sorry

I actually wondered the same thing, because I plan on using the solar-thermal heating system... During the summer, much of that heat generated in the coil and circulated by thermo-siphon would be wasted by the thermo-valves of the house diverting the coolant in the lines that make up the cooling coils... most likely under an outdoor pool or man-pond, perhaps a hot tub as well.

Either way, the temp differential could be up to or beyond 100 degrees if I can cool the upper end of the system to air-temp and the hot end gets up to 170+... 100 degrees over 300-500 feet could develop a significant flow, I'd think... I just don't know how to calculate how much power it could actually generate...
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Old 09-20-09, 10:02 PM   #4
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I suspect you could get an inch or so of head in the dead of winter, I'm sure there are charts out there on thermosyphen temps and the head they produce, you would get about as much power out of it as you could get out of the slope on your floor by spilling a glass of water...
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Old 09-20-09, 10:18 PM   #5
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Quote:
Originally Posted by Ryland View Post
I suspect you could get an inch or so of head in the dead of winter, I'm sure there are charts out there on thermosyphen temps and the head they produce, you would get about as much power out of it as you could get out of the slope on your floor by spilling a glass of water...
I found this, but I'm not quite sure how to gather what it says correctly.

Quote:
Performance of a thermosiphon loop indicates an effectiveness of between 40% and 60% when the overall temperature difference is over 40F and with face velocities of 250 to 600 feet per minute. For example, if entering air at 80F is cooled by the thermosiphon evaporator to 70F and the air off the cooling coil is reheated from 55F to 65F by the condenser section, the effectiveness is 40 % [=(65-55)/(80-55) = 40%].
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Old 09-20-09, 10:21 PM   #6
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The part I'm particularly concerned with is the "Face velocities of 250-600 FPM"... that tells me that it could go anywhere from 4.1 FPS to 10 FPS with a temp difference of over 40 degF... oddly, I can be assured of a temp difference (like I said) of up to 100 degF, and somedays even more, albeit for short periods of time. (Several hours a day, while the sun is between the hills I'll be building between.)
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Old 12-10-09, 09:45 PM   #7
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Whether or not its possible, changing energy forms 3 times just in generation doesn't seem efficient at all (Thermal to hydrolic, hydrolic to mechanical in the generator, mechanical to electrical). This seems like a electrical generation source of last resort. Better to use thermal energy for thermal loads, mechanical energy for mechanical loads, etc, and reduce losses. The energy available to such a device should suit the end use needs.

Though if its waste heat from solar collectors, free is free, technically efficient or not.

Last edited by JimmyClovis; 12-10-09 at 09:52 PM.. Reason: I have solar collectors.
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Old 12-10-09, 09:57 PM   #8
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Quote:
Originally Posted by JimmyClovis View Post
Whether or not its possible, changing energy forms 3 times just in generation doesn't seem efficient at all (Thermal to hydrolic, hydrolic to mechanical in the generator, mechanical to electrical). This seems like a electrical generation source of last resort. Better to use thermal energy for thermal loads, mechanical energy for mechanical loads, etc, and reduce losses. The energy available to such a device should suit the end use needs.
The question precluded another project I'm going to be working on, actually.

I plan on heating my house as much as is possible with a solar thermosiphon system. Before I thought about thermal storage, there was going to be waste heat generated that would need to be cycled off somewhere, and the idea was to use it to create a few extra watts of electricity if it were feasible.

Since I won't have sun every day or for days at a time through the winter, and there is limited sun to begin with on my plot, so thermal storage is a necessity, which means that cycling the fluid back through the thermal storage will be more efficient.

I'm hoping to be able to get about 1,000 gallons worth of tanks for storage. I'm going to see if I can put a tank inside a tank, seal the inner tank and outer tank, and vacuum the space between them, to create a thermos bottle of heated thermal fluid.

I dont' really want to run ethylene glycol, I'd rather use something that's ecologically not destructive, should it leak. I hope I can run a mineral brine, but I'm having a hard time finding info on thermal fluid types.
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Old 12-11-09, 01:25 AM   #9
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A thermosyphon works because of the difference in density in two vertical runs of a loop. If your rising pipe contains 100 lbs of water, and the cool side has 101 in the same volume, you have a one pound force available. Usually friction in the system will take up all the available power, but you could figure flow rates, etc. It is probably the least efficient heat engine ever, but sometimes adequate and convenient as part of another system.
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Old 12-20-09, 05:24 PM   #10
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