Anyone have experience with a solar thermosiphon? I'm thinking about making one to use as a pond pump/heater. Something like a breadbox solaroven with a black coil of tubing inside. the intake side would be at the bottom of a small urban pond. The outlet would have a one way valve to keep the capillary action. The thermo-siphoned water would then go back into the pond via a series of short waterfalls to oxygenate it. Think this is feasible?
The only issue I see is that the heat source needs to be below the cold for the thermosiphon to work. If you put the heat sorce above the cold then the hot will just stay above the cold, it'll have no reason to move down. Whereas, if the cold is above the hot you'll get a constant push from the cold water displacing the warm.
If you're using passive solar to heat the water, you can use a small PV panel and battery to run a pump to circulate it. If the pump is running all the time you may not have an issue with freezing water in your pipes, but if the pump stops and your temps cause the water to freeze in the pipes it could be serious problems. Of course, you could try a hydraulic ram pump, I've never tried one, but I'm sure there are folks here that can give you some good info.
So, do thermosiphons only work on closed systems? I was under the impression that the heated water would continue to force its way up and out, pulling the cooler water into the lower portions of the coil by capillary action.
I am pretty sure as long as you keep part of the system hotter than the other the water will flow along the convection current. In a truly closed system (sealed system) a "heat pump" will cease to function once all the water is the same temperature. From the sounds of your system you wish to utilize gravity and heat to drive the circulation. I would suggest that if you want to incorporate gravity into your system that you use a ram pump or small electric pump because if you cannot create a temperature difference in your system for some reason (due to weather) then your system won't work.
Besides I assume you want constant circulation so a ram pump would work very well for that from what I can tell.
I'm not 100% sure what your asking? I Have set up several thermal siphons for a number of different projects however. But as far as what i think your trying to do a thermal siphon will never work. For several reasons. The first reason is a thermal siphon can not lift water at all. Well maybe a few thousandths of a inch in a 10 foot column if you want to prove me wrong. But for any practical reason there is no lift potential at all. And one of the most common way to brake a thermal siphon in fact is having any kind of air pocket in the system. And no capillary action doesn't play into the effect at all, It actually works to hinder it. That's why people tend to plumb them with as large a diameter pipe as practical. Now because a thermal siphon can not build water pressure it makes it a bad idea to have a check valve in the system. A check valve will actually prevent water flow until the water goes boiling, builds steam pressure and then pushes the check valve open. Or depending on the stem pushes all the cold water out the bottom of the heating coil until the steam pressure escapes. Then cold water rushes back in. In a solar system that effect may only lead to very bad performance but in a system using a wood stove that will make a lot of loud banging and snapping sounds from the pipes. But in general check valves are almost always a bad idea in a thermal siphon system. And you may still experience the above problem if you so much as get a air bubble in the pipes. The other big problem is you can't move heat down, heat only goes up. At least most the time... Funny thing is i read in a old Victorian era trade journal on hydronic heating about a way to make a thermal siphon move heat down. But i will come back to that point latter...
As far as using a ram pump i don't see how that will work. Ram pumps take water from pint A and move it to point B waist 90% and move 10% to point C that can be quite a a bit higher then point A. I have built a few to play around with. There single strong point is they cost nothing to operate. There week point is that they waist most the water that goes into the intake. (the waist valve was given that name for a reason) And you need a good drop for the drive pipe. (there gravity driven critters) If you can make one work in any situation however there awesome. But a back yard pond is not one of them unless your back yard is on a hill side with a good size steam flowing threw it to provide the power and water source for the pump.
Now as far as what will work for you pond heating given the information at hand is what Jon Atkinson suggested. That is to use a PV panel to drive a small water pump to push your pond water up into your solar water heater. Build it as a proper drain back system so it wont freeze up at night and you should be all set to go. I set up a system like that for heating my dads girlfriends above ground swimming pool and its only flaw was that it tended to get the pool too hot till i added a thermostat to the system.
Now the revisit of making a thermal siphon move heat down... This is something i have never done. But its on my to experiment with list. If you like i can move that project up to some time like this Holiday weekend. I'm not even sure how to describe the way this very unusual thermal siphon is set up and i would have to draw up a diagram of what i seen in that old trade journal. So at this point I'm not comfortable trying to tell others how to build one. However Chad if you would like to go the thermal siphon route all i need is a good excuse to go to the shop and play around with old pipe fittings for a evening and see if i can make this silly 100 year old idea work. From there i can work out how to make it into a solar fired, down flowing, pond heating contraption.
Bakerjoe, Thanks so much for the reply! I have never played with one but was under the impression I could lift water with one. The idea was to get away from an electric pump. However pushing heated water to the bottom of the pond is a cool concept. If you can figure that out I could probably keep talapia aluve tn a backyard pond in central Oklahoma. That would be very cool!
I'm not convinced. Sorry to be Mrs. Doubt, but the hotwater out of the system is always higher than the coldwater inlet. So, there is rising.
I've been trying to find the logic of how these things work online, but i just can't figure out the mechanics. I want to play with this...though I am always short on free time .
Here's the plan I was thinking for a pump...
Top of fountain with hose plugged into it under water.
--> water overflows and falls to larger, deeper tank.
-->hose at the bottom of the tank is filled to the deeper tank's water depth based on the water pressure. The rest is filled from syphoning
--> The hose goes to a section set-up for solar heating. The water heats & expands.
---> The hotwater expands and wants to go somewhere. It can't go down because 1) heat rises, 2) there's water pressure pushing up.
---> heated water pops out of the top of the fountain. (go to top)
At the same time, the water falling out of the top of the fountain into the large tank causes the water pressure in the hose to rise, also propeling the movement.
As long as your not trying to lift the water real high, and you make sure that the heating element always has water in it, what's the problem?
As for bubbles in the system ruining it, sometimes bubbles get in people's water pipes, and syphons are used in irrigation where the water probably has a fair amount of oxygen in it. A big bubble can cause problems, but a some isn't so bad.
The one issue would probably be if the heat went away and the system lost pressure, you'll probably end up having to re-start it by re-filling the system with warm water...
I have been known to make things that don't work, but I dunno...I'm trying to mimick natural water circulation with a little push.
Well Chad, I guess we'll just have to keep eachother apprised of our experiments.
I was playing around with pressure calculations this morning and it seems the thing to do to keep the pipe from ever becoming empty of water (the problem I mentioned earlier) is make sure the top "basin" inlet is lower than the bottom pond's water level when the thermosyphon isn't "on".
Oh, and here are my thoughts on the pressures involved.
Water pressure in bottom pond into pipe forcing water up.
Water pressure in top pond forcing water down.
Gravitational pressure of water in pipe forcing water down.
Water pressure caused by temperature change pushing in all directions, but causing the water to be more boyant.
((As you can see, I'm no physacist but I just can't help but try.))
If the pressures add up to an overall pressure going up, then you get your fountain.
If the pressures equalize, then the water stays where it is, but your fountain isn't on.
If there is a general downward pressure, then you end up with an empty top basin and have to re-fill the system.
I was sooo itching to build this this weekend, but sunny California is being blessed by the presence of winter rains. Oh, and I had cleaning, shopping, and planting to do :p
Thermosiphon systems call on Mother Nature to circulate water through the solar collectors to the storage tank using the thermosiphon principle that hot water rises naturally, hence eliminating the need for circulating pumps or other moving parts. Chromagen's thermosiphon systems are the most straightforward, economically viable and energy efficient designs available. A gas or electric backup booster provides energy during cloudy weather, so you can have plenty of hot water when you need it.