Tough clear plastic film recommendations, please ?

[Externet]

Hi.

What tough 'clear-to-heat' plastic film permanently exposed to sun, rain, wind (no snow) would you suggest ?

Something similar to polyethylene. It is to cover a coil of black HDPE as for a solar water heater, shielding it from cooling winds and letting sun heat pass trough ?

A life of 5+ year would be desirable. Greenhouse cover or there is something better ?

 

A coil similar to this attachment, placed on top of a suitable heat insulating polyestyrene foam sheet, and covered by such film instead of glass or polycarbonate sheets.

Edited February 24, 2015 by Externet

[Enthalpy]

Very few plastics are transparent nor even translucent. Very few plastics survive sunlight.

 

Glass would be my choice, much preferred over plastics.

Polycarbonate yes, Pmma also but it's more brittle.

You might check whether PET and PP resist sunlight. Sadly, PP is transparent to thermal infrared.

Some polyvinyl can be translucent but I fear they won't last.

Few more are translucent like FEP or PMP but very expensive.

 

Instead of a different material, I'd suggest to optimize the shape. Similar to a honeycomb sandwich, or more easily an extruded profile, it will give you more strength and stiffness from less mass and insulate much better. Already available on the market, example

http://www.design-composite.com/index.php/en/produkte/architektur-design/clearpep

A multistage custm-made extrusion would insulate even better, like there

http://www.scienceforums.net/topic/85785-drop-tube-elements/#entry828414

antireflective coating wouldn't be bad if feasible. Mma is a perfect glue for Pmma, others exist for PC.

[EdEarl]

Polycarbonate is good and you can get it in single and multi-wall sheets designed for building roofs or greenhouses.

[StringJunky]

Won't the rate of thermal transfer be poor through plastic pipe relative to metal?

Edited February 25, 2015 by StringJunky

[imatfaal]

Won't the rate of thermal transfer be poor through plastic pipe relative to metal?

 

You see them all over rural Greece - most a just a large black hose coiled on a flat roof. At the end of the day they provide enough hot water for the entire family to shower. I don't know if the choice of black hose is economic, practical, or energy-based - but most are simple unadorned hoses.

 

To the OP - any covering would have to be cleaned pretty regularly

[StringJunky]

 

You see them all over rural Greece - most a just a large black hose coiled on a flat roof. At the end of the day they provide enough hot water for the entire family to shower. I don't know if the choice of black hose is economic, practical, or energy-based - but most are simple unadorned hoses.

 

To the OP - any covering would have to be cleaned pretty regularly

OK

[Externet]

Thanks, gentlemen.

If I can find clear PET film, would that be suitable ?

Random pictures :

 

----> http://www.builditsolarblog.com/2010/09/nicely-done-pipe-coil-style-solar-pool.html

 

----> http://media.treehugger.com/assets/images/2012/06/how-make-solar-spa-heater-1.jpg.644x0_q100_crop-smart.jpg

 

----> http://1.bp.blogspot.com/_7x5Njd5SutY/TIuXZFrD3cI/AAAAAAAAANY/T-pzYHgOjqg/s1600/TuscanyPoolHeater1.JPG

[John Cuthber]

PET would almost certainly work (Though it absorbs a lot of UV), so would whatever they make "polytunnels" from; I think it's polythene which is cheap.

[Enthalpy]

PE and PP are the cheapest, but do they resist sunlight for a substantial duration? Shopping bags for instance destroy within one summer, and I have big doubts about PET as well. PC and PMMA at least are known to survive sunlight.

[DrP]

I once wrote a technical document for a solar water heating company who did not have any great English speakers (they were from India). They used squashed or flattened copper pipes painted black. (copper for the heat transfer, flattened to give more surface area of the pipe to face the more perpendicular incident sun rays, painted black for obvious reasons).

 

I 'think' (it was a long time ago) that they used a glass pane in a frame to cover the pipes to stop wind cooling, although I am not sure as it was over 15 years ago.

[John Cuthber]

http://www.polytunnelgardening.co.uk/polytunnel-frequently-asked-questions/2010/how-long-does-the-polythene-cover-last/

[Enthalpy]

Thanks JC!

 

I still have one little doubt: the gardening PE and PP films use to be black or at least dark. Could a UV absorber be added intentionally to protect the polymer's depth and allow to last longer? (They say it's a role of carbon black in tyres) That would explain that they last 3 years while clear (and thinner) shopping bags are quickly ruined. The trick wouldn't be applicable to Solar energy.

 

Slightly off-topic but fun: as an eclipse is expected in Northern Europe, "some people" explain it needs careful preparation by the electricity providers because allegedly 6% of the European electricity is solar. The eclipse is centered on the Faroe and Spitzberg islands - I just doubt someone installed giant photovoltaic plants near Longyearbyen to supply Spain with electricity.

[John Cuthber]

Part of the reason that shopping bags don't last is that they are often designed not to.

http://en.wikipedia.org/wiki/Biodegradable_bag

 

I suspect that the film used for poly tunnels already contains some sort of UV stabiliser.

This sort of thing

http://www.mpi-chemie.com/category/uv-absorbers.html

(Other suppliers are available)

[StringJunky]

Part of the reason that shopping bags don't last is that they are often designed not to.

http://en.wikipedia.org/wiki/Biodegradable_bag

Indeed, I put some darkroom equipment in bin bags all sealed up with tape and a few years later went to get something from my attic, where the items were, and they disintergrated on touching them.