Black plastic coiled collector questions

[Sunsneaker]

I have seen a couple of posts warning against enclosing black plastic coil collectors but no one says why. Can anyone elaborate on the reason?

I am starting plans for a set of coiled collectors for a club I belong to. I would think enclosing them with poly-carbonate would increase the amount of heat they collect thus increasing their efficiency.

Thank you in advance for your input.

[russ]

Hi and welcome to Solar Pabel Talk!

Look at Black pipe coil pool heater http://rimstar.org/renewnrg/solar_po...ter_diy_fp.htm

Enclosing the loop in polycarbonate will cost you heat - not needed.

[Sunsneaker]

Hi and welcome to Solar Pabel Talk!

Look at Black pipe coil pool heater http://rimstar.org/renewnrg/solar_po...ter_diy_fp.htm

Enclosing the loop in polycarbonate will cost you heat - not needed.

I have looked over the site at that link it is a very comprehensive write up. Basically when you say that it will cost you heat, you are referring to the PC absorbing some of the energy and not transferring it to the black pipe like an oven would heat a roast?

[russ]

I have looked over the site at that link it is a very comprehensive write up. Basically when you say that it will cost you heat, you are referring to the PC absorbing some of the energy and not transferring it to the black pipe like an oven would heat a roast?

Yep - the poly will absorb/reflect a good portion of what you want - direct is better.

[Sunsneaker]

Yep - the poly will absorb/reflect a good portion of what you want - direct is better.

This makes me wonder why the commercial systems are almost always enclosed.

Do you know of anyone something like this to insulate between the coil and the plywood? Im thinking a think layer of black paint to cut the reflective properties of the foiled insulation.

[Mike90250]

This makes me wonder why the commercial systems are almost always enclosed.

Do you know of anyone something like this to insulate between the coil and the plywood? Im thinking a think layer of black paint to cut the reflective properties of the foiled insulation.

You WANT the reflection, because part of it will warm the underside of the coils.

Reflectix is NOT UV rated - it will rot out in a year on the roof.

Commercial enclosed panels, use metal absorbers, and they will convect heat away, so they need the lid. The metal is much more effective at heat transfer from the hot outside, to the cold water inside. Have you considered SS tubing coiled up ?

[J.P.M.]

This makes me wonder why the commercial systems are almost always enclosed.

Do you know of anyone something like this to insulate between the coil and the plywood? Im thinking a think layer of black paint to cut the reflective properties of the foiled insulation.

Plastic is best used for low temperature applications like pool heating or preheating, probably not much more than 40 deg. C. or less.

If you are referring to "commercial " as in DHW or heating applications, that equipment is usually metallic in nature for a lot of reasons dealing with better durability relative to plastics, etc., particularly at elevated temps. and also for usually more efficient heat transfer characteristics.

Usually, for the applicable low temp. applications, the decrease in performance caused by a cover blocking some solar energy is usually greater than the increased loss by not having a cover.

Plastic for low temp. applications vs. metal for higher temp. applications is an example of appropriate technology for the appropriate application.

Add: +1 on durable reflective material behind the collector surface. And, skip the cover. You'll melt the plastic under no flow conditions/pump failure/etc.

[Sunsneaker]

You WANT the reflection, because part of it will warm the underside of the coils.

Reflectix is NOT UV rated - it will rot out in a year on the roof.

Commercial enclosed panels, use metal absorbers, and they will convect heat away, so they need the lid. The metal is much more effective at heat transfer from the hot outside, to the cold water inside. Have you considered SS tubing coiled up ?

With a light coat of flat black paint on the reflective insulation the black should absorb the light but reflect the infrared heat back towards the coil. I don't think reflecting the light would help any over only reflecting the infrared heat. Metal absorbers as in a metal coil or straight pipe? The club has existing black flex pipe in place, but it is randomly laid out on the dirt; I am trying to make what we have more efficient with as little added as possible.

I would love to upgrade to copper or stainless piping but that would need to be voted on by the board of directors.

[russ]

With a light coat of flat black paint on the reflective insulation the black should absorb the light but reflect the infrared heat back towards the coil. I don't think reflecting the light would help any over only reflecting the infrared heat. Metal absorbers as in a metal coil or straight pipe? The club has existing black flex pipe in place, but it is randomly laid out on the dirt; I am trying to make what we have more efficient with as little added as possible.

I would love to upgrade to copper or stainless piping but that would need to be voted on by the board of directors.

The metal pipe is not really an upgrade - just use more plastic pipe - cheaper

[Sunsneaker]

The metal pipe is not really an upgrade - just use more plastic pipe - cheaper

Agreed

[J.P.M.]

I'm assuming this is a pool heating application? I don't see where anyone specified/confirmed the application. Without more information I'd start out by saying if a pool heating application and plastic tubing coiled up, not painting reflective material will probably result in more heat to the fluid at least partially because:

- The reflective material will absorb less solar energy and heat up less, resulting in less heat transfer to the support material (plywood ?) behind the reflective material. A heat balance will show such heat transfer will be lost out the back and unavailable for fluid heating.
- The short wave reflectivity of the unpainted reflective material will likely result in more of the reflected energy being absorbed by the plastic material than reradiated infrared energy being absorbed by the plastic material.

[Sunsneaker]

not painting reflective material will probably result in more heat to the fluid at least partially because:

- The reflective material will absorb less solar energy and heat up less, resulting in less heat transfer to the support material (plywood ?) behind the reflective material. A heat balance will show such heat transfer will be lost out the back and unavailable for fluid heating.

- The short wave reflectivity of the unpainted reflective material will likely result in more of the reflected energy being absorbed by the plastic material than reradiated infrared energy being absorbed by the plastic material.

J.P.M so you are saying the reflecting of the light from the reflective insulation will better transfer heat to the plastic coils? Wouldn't the light that you can see reflecting back into the atmosphere be wasted energy? I use this same type of reflective insulation in a large window in my house when I don't want the summer sun to heat that room up. in between the glass and the insulation it always really hot; on the room side of the insulation is always very cool. Shouldn't this be attributed to the reflective property of the insulation returning the light to outside of the house instead of allowing it to be absorbed? Wouldn't a coat of black paint allow the light to be absorbed and reflected back by the foil layer as radiant heat instead of simply light? Isn't that the whole technology behind this foil layered insulation, that it reflects radiant heat?

I am new to creating solar pool heaters and my intent is to gain some insight from people that have already accomplished this stuff. My intent is not to sound insolent so forgive me if I do.

[Sunsneaker]

I'm assuming this is a pool heating application?

Sorry I did not include that part in my OP, I assumed that since this is the pool heater board it was understood.

More background:

Pool heater.
Existing gas and rudimentary solar heat.
Unknown pool capacity<-- Next meeting I will be gaining lots of information like this.
Unknown length of existing solar coils.

[J.P.M.]

J.P.M so you are saying the reflecting of the light from the reflective insulation will better transfer heat to the plastic coils? Wouldn't the light that you can see reflecting back into the atmosphere be wasted energy? I use this same type of reflective insulation in a large window in my house when I don't want the summer sun to heat that room up. in between the glass and the insulation it always really hot; on the room side of the insulation is always very cool. Shouldn't this be attributed to the reflective property of the insulation returning the light to outside of the house instead of allowing it to be absorbed? Wouldn't a coat of black paint allow the light to be absorbed and reflected back by the foil layer as radiant heat instead of simply light? Isn't that the whole technology behind this foil layered insulation, that it reflects radiant heat?

I am new to creating solar pool heaters and my intent is to gain some insight from people that have already accomplished this stuff. My intent is not to sound insolent so forgive me if I do.

The idea is to intercept as much solar energy as possible. A black coil 1" coil in , say a parallel pattern on 2" centers will cover half of a 4 X 8 sheet of plywood and have an effective area of 16 ft.^2. A "better" (denser) design will, for example have 1" centers - touching. On the 2" centers, unpainted reflective material will "bounce" solar energy to the back side of the tubing in a more efficient way than a flat black painted surface will reradiate infrared thermal radiation. That, and a lot more reasons make for my best educated opinion. A bit more detail would be necessary to confirm/refute/modify.

Bottom line: Pack it tight and insulate behind the coils with durable reflective material on top of the insulation. Avoid stagnation as much as possible. Do not put a cover or any glazing material between the coils and sunlight.

Better yet: Do it right and get solar pool heating collectors. Better, more efficient, looks nicer, fewer problems.

[Sunsneaker]

[QUOTE=J

Better yet: Do it bright and get solar pool heating collectors. Better, more efficient, looks nicer, fewer problems.[/QUOTE]

Commercial units? We do have 4 commercial collectors, similar to this. We also have an undetermined length of black pipe coiled and laying on the ground.

My desire is to make the black pipe we have coiled and laying on the dirt more efficient so we don't have to use our gas heater as much. This club is not a money making machine so money to buy more of the commercial collectors is tight. With some donated materials I can make the black pipe more efficient without spending much of anything.