Which transfers more heat - a radiator or long cheap hose?

[idosubaru]

I want to build a solar water heater to put on my garage roof for increasing water temps in a small pool for kids. I already know it’ll work. Black thin walled hose is the easy way. But I’m wondering.....

 

Is there any way to determine which will transfer more heat - a radiator (or two) or a long coil of cheap (thin) black hose? Seems like a radiator painted black could transfer a lot of heat and that’s obviously easy for me to pull off.

[subnz]

Radiators work by transfering heat from  water / air /  refridgerant   that was hot in the first place that needed cooling.   

 

Their surface area with airflow is for dissapating heat away -They don't work well in reverse.

 

Most roof mounted solar heating systems use long lengths of black hose coiled back as forth to create surface area  to induce heating.

Edited May 2, 2018 by subnz

[idosubaru]

black hose is the current plan. Thanks for helping me talk this out. Tne long run of pipe would have no real leak points and probably more durable with no adapters or fittings needed and easier to move/store. I didn’t consider that.

 

Does seem like it should work either direction though? I’d make the radiator black. I realize temp difference and air flow are lacking but hose will see same ambient conditions.

 

Thermodynamics don’t know direction. I think the only practical variables are mostly the effective available surface area and materials properties (“rubber” verses aluminum). aluminum is pretty darn good at heat transfer and thin.

 

I’ll go with hose but if I see an easy way to test it I may try both since I’ve got radiators.

Edited May 2, 2018 by idosubaru

[Olnick]

If you want to try the radiator idea, block the airflow through it with sheet metal or something.  You might even glop a lot of flat black paint or driveway sealant on the fins.

[idosubaru]

Got it I’ll paint it for sure if I do that.

Edited May 3, 2018 by idosubaru

[Subaru Scott]

I've almost done this a couple of times. 

 

Had my eye on the 1/2" black poly pipe that's used for underground service runs.

 

Super cheap, like 25 bucks for 100' roll at Lowes.

 

Then just lay it down in a big spiral, and secure it with a few dabs of construction adhesive.

[idosubaru]

Think there are any styles that are thinner than others, I’m thinking thick pipe won’t conduct as well? I’m wondering if the cheap harbor freight stuff is be thin haha.

[Subaru Scott]

As long as the tubing is black, it will soak up the heat.

 

If you build a thin box around it with a white or reflective bottom and lexan cover, you can substantially increase output, especially in cooler weather.

 

I do recall seeing an example where a guy mounted a truck radiator in his attic with an electric fan which worked quite well. But that’s air to water, not direct solar.

[DaveT]

to gather solar, you want surface area.  The energy density is low.  Long black plastic pipe will be far better than a couple radiators.  The thermal gradient is low, so you need LOTS of surface area.

[1 Lucky Texan]

from what very little I know, subaru scott is on track with putting the unit in a box if possible.

 

there are probably examples/instruction for doing these type of projects on-line. I wouldn't be surprised if some go back to the 70s, but modern materials should be even better.

 

an internet buddy has s imilar device he made for warming a chicken coop/house. His is balanced on some kind of pivot and he uses a wax cylinder (used to vent greenhouses) to 'somewhat' track the sun. It resets the unit to point more easterly overnight to catch the sunrise.

[idosubaru]

57 minutes ago, 1 Lucky Texan said:

there are probably examples/instruction for doing these type of projects on-line. I wouldn't be surprised if some go back to the 70s, but modern materials should be even better.

hmm, sun tracking would be helpful.  the thin hose approach is common and easy.  Was mostly wondering about radiators which are built for heat transfer yet info for alternative uses is scant.  hose is simpler, easier, better looking...not much reason to deviate.

[whynot]

I also think that the small passages in the radiator would increase back pressure and make your pump work harder. the hose would have no restriction in the flow. 

Also, less efficient, but you could use blue pump discharge hose (maybe they sell it in black?) and when you turn the pump off it could self drain, and not have to be removed in the winter... (king of lazy my wife might call me???)

[idosubaru]

Got it guys. hose all the way.  much simpler.

I’ll make an extra coil and replace my radiator with it. Haha!

[1 Lucky Texan]

is this something you would drain in the winter? If not, look for that RV 'safety' antifreeze they put in the drinking water tank/system.

[DaveT]

Put the tubing in a box with a glass front, insulated back will get better gains.  Tracking - you have to analyze the cost complexity & energy use added vs just adding another panel / more area.  Often, it is cheaper and more reliable to just increase the size of the array.  PV or thermal.  This likely varies with where on the earth you are also.

Radiators have very low back pressure - the small tubes are all in parallel, so equal a large pipe.   But for gathering low density heat, that's not what you need.  That's what you need to get rid of LOTS of heat with a large temperature differential in a small area.