Tub o' Water Solar Heat Exchanger

video Tub o' Water Solar Heat Exchanger
Hey Everyone.

Here's a simple experiment. I hooked a 4'x10' solar hot water panel up to just a small (3 watt) pump and a 60 foot coil of 3/8th" copper pipe set down in a tub of water.

I wanted to see how effective the solar panel would be at transferring heat through that copper coil into the water in the tub.

Since it was below freezing the night before, the tub of water started off with a half inch of ice on it. Three hours later, the water temperature was up to 140 degrees! 

This tub holds between 10 and 15 gallons. When I finish experimenting. I plan on using an insulated 50 gallon tank in my house with TWO coils of copper pipe in it - one for anti-freeze circulating through the solar panel, and another one for fresh water from the well to warm up before going to the traditional water heater.

I think I will also use 3/4 inch diameter copper pipe for the domestic water loop, as it will hold more volume, the main water line is 3/4 inch, and it will give less resistance than smaller diameter pipe.

rimar20004 years ago
Maybe you don't need the water pump. Convection does the work free...
bennelson (author)  rimar20004 years ago
 With the right size hose and an elevated tank, yes convection could work.

On my final house system, both the tank and panel will be at roughly the same height, and about 30 feet apart from each other, so I will need a pump for that anyways. Since I do, I may as well experiment with a pump.
Please be aware, distance and a small diameter pipe will prove to be a problem. Between heat losses along the way (even with insulated pipe) and internal friction in the pipes, flow will be decreased. Indeed, I have been told by plumbers that every 90 degree bend in a pipe was the equivalent of 10 more feet of pipe, friction-wise.
Incidentally, this was found out when they were building the Manhatten Project for the Bomb. Turns out the efforts there to improve efficiency by making pipe runs as straight as possible also reduced amount of pipe significantly.
(Actually, they did it originally to prevent radioactive solids from collecting in corners of pipes, but the efficiency was a bonus.)

I've see three solar heat setups that used a solar powered pump. On cloudy days, the pump does not work as fast (less juice) which is OK as the solar collector does not pick up as much heat as on a sunny day.
Somewhat self-regulating.

AND check valves were added, just to keep the unit from flowing backwards during night and other cooling-off periods.
I take it you do not have space or location to do batch solar water heating?
You could always use a small PV panel to drive the pump...

The stronger the sun; the faster the antifreeze is pumped around and conversely (and more importantly) the weaker the sun the slower the pump leaving the antifreeze in the panel longer to absorb more of the suns heat.

just a thought...
bennelson (author)  Mr_Sea-Breeze3 years ago
Yep, that's the long-range plan. I have an "El-Sid" pump which is designed to run from a 15 watt 12V panel - as you say, the more sunlight on the panel, the higher the voltage, the faster the pump spins.

That way, you really don't need any sort of a pump controller. It's very simple and reliable.
wikkit2 years ago
The heat transfer is related to the velocity of the fluid in the pipe. So while a large diameter pipe will lose less pressure, it will also not get as much heat transferred to it.

In a fuel-cooled rocket engine, for example, the fuel flows through the cooling channels at somewhere around 10 meters per second.
You said in your video that the water that was in the tub with the pump was colder than the other tub. I have a theory for this - it is because the hot water from the solar panel flows into the copper coil thus transferring most of the heat into the bucket of water rather into the other bucket with the pump :)
sparks3 years ago
Thanks for sharing that.