solar collector material

[dirtracer]

is lexan (polycarbonate) desirable to use for a covering on a collector for a solar water heater? the collectors chassis is plywood with insulation. the stuff i found has a light transmission of 92% and a higher r value than glass. i know its considerably stronger too. it seems like it would be easier to mount since its easy to drill. so... what are the drawbacks???

erik

[zap]

Yes, it's desirable for solar collectors.

Some drawbacks are:

Compared to glass, it's more susceptible to scratching and chemical attack.

I don't believe polycarbonate is as prone to crazing as plexiglass but it can craze.

Make sure it has some type of UV resistance.

All in all I think you'll find it excellent for use as solar collector glazing.

Visit http://builditsolar.com/  Gary has tons of info there.

[dirtracer]

is it okay for drilling with regular high-speed steel bits? any recommendations for drill speed?

thanks in advance- erik

[GaryGary]

Hi,

I think polycarbonate is a good all around material for solar collectors.

It has a high temperature capability (270F).

Long life IF it has a coating on the outside surface to protect it from UV -- this is a must.

As mentioned, very good impact resistance.

Its easier to cut and drill that some of the plastics.

Like glass, it has low transmission in the IR, so it loses less heat radiated by the absorber plate.

It comes is several flavors:

Corrugated -- as in the Suntuf corrugated panels that Home Depot sells for $1 per sqft.

As far as I know, only available in 2 ft wide by 8 or 12 ft long.

Twinwall -- this is the stuff that a lot of greenhouses use now.  Very nice to work with.  Prices as low as $1.50 per sqft.  Also comes in 4 ft and 5 ft wide sheets of just about any length -- a convenience over the corrugated.

Sheet -- you can buy thin sheet polycarbonate directly from GE in 300 ft rolls, but (as  far as I know) its hard to get otherwise.

When glazing with polycarbonate, its important to include enough support to keep the panels from buckling as they heat up.  In my experience, this means if you are working with 4 ft wide sheets, you need cross supports about every 2.5 ft.  These don't have to be very heavy -- they are just there to control buckling.  On the last collector, I used EMT conduit for these supports -- its cheap and straight.  Some say that if you are very careful to leave room for thermal expansion of the panel, then it will not buckle -- I've not found this to be the case.

These are links to two collectors that I've built with polycarbonate -- one with corrugated and the other with twinwall:

Corrugated:

http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm

Twinwall:

http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/collectors.htm

I was just finishing up the tank plumbing on the 2nd one today, and found a leak in one of the collectors.  I was able to remove the 4 ft by 12 ft twinwall sheet, fix the leak, and reinstall the glazing inside of an hour -- try that with glass

I like the twinwall.

Are your panels going to be vertical or tilted?  

Gary

[dirtracer]

they are tilted. i'm just building a 4X4 to learn more about this stuff and possibly use to heat a spa. then i'll build another arger system with a large water tank to heat my house with. thanks for all the info!

[GaryGary]

Hi,

I think building a small one to get the design the way you want it is a good way to go.

One thing to be aware of on the tilted collectors is that they will run hotter in the summer when stagnated.  Even with the high temperature capability of polycarbonate, the  stagnated collector can get too hot for good life.

The things that tend to make the stagnation temperature go up (I think) are:

• twinwall glazing rather than single wall (higher R value)
  
• collector absorber with a selective surface (absorber runs hotter)
  
• close spacing of absorber to glazing -- this does not make the collector run hotter, but it does make the glazing run hotter -- I'd leave at least 2 inches, which also allows for a little deflection of the glazing.
  
  

Things that help to keep the stagnation temp down:

• vertical panels because they only get about half the radiation of tilted ones in the summer.  Vertical works well for space heating because they capture a lot of radiation in the winter (low sun), but much less in the summer.  Ground or snow reflection also helps vertical panels in the winter.
  
• For vertical panels, you can also build a small overhang above the collector that shades part of it in the summer when the sun is high.
  
• Covering the panels with something like shade cloth in the summer.
  
• Some form of panel venting that comes into play when the panels reach high temperatures (like the ones at the link above)
  
• not letting the panels go into stagnation by always circulating some fluid during sunny times.
  
  

You may not have any trouble with this overheating issue at all with your panel, but its something to look for with your test panel.

This potential overheating issue is not unique to homemade panels with plastic glazing.  The commercial flat plate and evacuated tube collector manufacturers all want you to protect the panels when stagnated, or not allow them to stagnate.  

Gary

[Drives]

Gary:

In your second link about the commercial absorber panels, you asked the question

"I am still looking for a good way to automatically open a cover on the upper vent when the temperature in the collector exceeds (say) 200F, and close when it goes below this.  Any ideas?"

On my wood stove is a damper vent controlled by a curled metal "spring".  When the wood stove gets hot, it expands and closes down the vent to restrict combustion air intake.  When the stove cools, it contracts, and opens the damper.  I have full adjustment control to set the target operating temp of the stove via a lever arm that fine tunes the spring's motion range.  This allows my stove to keep a smoldering fire say 85 degrees at the damper to a roar of 200 degrees plus (temps are a guess, but I have burn scars to prove it :-)  I can include a picture if your interested.  Of the damper....not the scars!

I recognized the spring when I took the cover off the assembly.....it looks identical to my 1978 Subaru carb "cold Choke" spring.  It is hooked up mechanically opposite to the wood stove, but the same spring.

I am guessing a bunch of the "gearhead" (I mean that title in respect :-) folks on the board could point you to makes and models you could still find in a junk yard that have that spring.  Grab a few, and rig it up on your vents.  I am sure that with some experimentation you would be able to adjust it to open exactly when you need it to.

Hope this helps.

[TomW]

Dean;

First.. How the heck are you?

Second... I made a homebrewed "automatic" damper from a VW cold choke setup and the top of the carb it was attached to bolted to the stove intake. Simple flipped that spring over. It had a deal for adjusting the setting on it and that was my main control of how it reacted to heat. Worked very slick.

Third... If you still have my email address please contact me I need advice on sizing a 1:2 transformer to run my submersible off my 120 volt inverter. I lost yours when my old laptop took a dump.

Cheers.

TomW

[GaryGary]

Hi Dean,

Thanks -- I'll look into that.

I remember the carb cold choke springs well.

I just got the first heat from the Solar Shed yesterday

I thought that getting that long line from the house to the shed purged of air, and getting the pump working on it would be a pain, but it worked the first time.

Just need some sun now.

Gary

[Drives]

Tom:

I'm doing great, been super busy as always.  My sales dept. decided I should become a traveling professor on top of my normal daily duties.  I'm away from home pretty regular.

I hope to get back on IRC next month, and I still owe you some puters.  I'm embarassed how far behind I've gotten in 2006.

Although I've only been flying a 5' and 7' gennie on simple pipe towers, I have been collecting parts for years now, and my goal in 2007 is to finally put the "BIG" gennie and tower up.  (I know your one guy who understands 2-3 year long projects :-)

Very happy to see you active on the board still.  I loved reading about the completion of your projects recently...GREAT JOB!  Sorry to hear of the blade strike/furling incident.

As for transformer sizing...glad to help.  My email is under the subject line above, and in my user info.

DrivesDeanW  @  Yahoo  .   COM

[dirtracer]

little update. the panel worked great, inside temps were hitting 270 F, during this cold (for california) weather. but i left the thing outside and didnt drain it well enough and now there are burst holes all over from freezing

on another note i built a new tank with a heat exchanger for my dad's solar water heater and the first day it heated 55 gallons from 60 F to 140 F in 3 hours. cool.

erik