I've been reading up on these solar water heating units, and notice a common theme. Everyone likes to brag that "no pump is required". This is because the convection of the heated water draws it up, and cool water flows in below.

But what keeps the heated water from over pressuring and trying to flow backwards into cool stream? Is everyone relying on a positive pressure on the cool side due to city water or a well pump?

What I'm trying to figure out, is if I used a passive water heater in a closed loop (with some way to rapidly dump heat so it circulates readily), could I use that to power something like a water wheel for mechanical gain, or maybe a turbine to lift water in a separate system.

I'm considering an aquaponics system and thought at first I could just use it as a lift mechanism to get water from my sump to the peak and let gravity run it back down, and it would get heated along the way to keep tilapia happy. But these systems can get up 130°F! That would cook my fish and plants. Hence the need for parallel systems where the water heater just circulates water to drive a pump of some sort which then would lift water for the aquaponics.

Did I miss something obvious? Friction and inconsistent flow seem to be the biggest risks. But I'm also wondering if it's a net loss, like putting a wind turbine I front of a fan, or trying to use the refrigerator to cool your house. Also I'd have to figure out how to rapidly dump the heat from the water to circulate, but I figure that be as simple as some shaded riffles to stir it up.

So what am I missing?

Hi Tom, sounds like a fun project. Iam very unfamiliar with aquaponic systems so unsure about what you are trying to do with the system.

Thermosiphoning is the term for solar thermal circulation that avoids pumps. The main requirement is your heater to be below the storage tank. Many variables but some other important ones are 3/4" diameter tubing minimum, avoiding 90 degree bends and keeping the plumbing runs short and insulated.

Usually these systems are closed loop with heat exchangers and not directly connected to the well or city water lines. The flows are slow and weak which makes any mechanical or moving parts very unlikely. The more you block the flow the less it will work.

I think thermosiphoning is great but was convinced by the folks at Radiantech that using a DC circulator with a PV panel results in much better performance for most solar thermal systems. They said 30% better range which I would probably agree with after doing both types of systems. Every situation is different of course.

If you are just wanting to help heat your talipia tank on sunny days, its possible a loop of black PE run downhill could provide some measurable benefit without the extreme temps associated with solar thermal collectors.

Tilapia aside, the question was if I could create a mechanical mechanism from a solar driven source.

I understand the flow would be weak, so a turbine would not work. It would then have to be something like a water wheel instead, where the fluid is only lifted by the heating, and then energy harvested by fluid falling.

But now you're talking an open loop. As long as pressure was high in the system (water lifting) then cool fluid would not necessarily flow into the bottom of the solar collector, unless it was at a higher pressure than than the system.

Now I'm thinking on teeter totter tanks... The heated fluid free flows to a tank where as the tank fills it lowers and raises an opposing tank of equal volume. The raised tank is then allowed to drain into whichever system you are trying to circulate.

As the solar collector cools ( evening to night) the fluid from the hot barrels could be drained back into the solar heater for the next day's haul. Hmm, now I'm thinking of piston drive...

All in all, all of these systems are probably pretty inefficient, but if there is no other option (no city water/electric grid) they may be worth the toils!

Tom Brue : There are novelty entertainment items called rain walls , once you have a working Thermo syphon system you can ether let the warm water run down Rain Wall prior
to it running into your fish tank discharging most of its heat into the air, OR without breaking your syphon, lower the Whole rainfall into the tank and let the water flow directly into
the fishes tank.

You would have two ways to further regulate the temps, lowering the Rain Wall farther into the tank could break your syphon and stop water flow, also covering or blocking your
passive solar coils would work too!

I am generally adverse to sending anyone to U-tube, but you should check out The U-Tube 'web4deb channel' for his Aquaponics set up and watch his fish swimming inches
away from the tempered water flowing in to his system ! Rob T. its creator is a member here and posts to many threads ! For the Good of the Craft ! Big AL