Glue heat transfer plates to back of panel

davidwillis · October 2016

This may sound crazy, but I would like to try to add some heat transfer plates, and water tubes to the back of my solar panels to heat water in the summer (and possibly even keep panels a little cooler).

I am going to use some of these aluminum plates: https://www.amazon.com/Ft-Aluminum-Transfer-Radiant-Heating/dp/B009KT7PO6

But how do I connect them to the back of the panel? I am thinking maybe just silicone, but am afraid they will fall off. I would like something a little more permanent than silicone, but that will not damage the back of the panel, and can withstand the temperature it may reach.

I was also thinking construction adhesive (liquid nails), but I am not sure if it will damage the panel, or if it will last with the temperatures it may see (liquid nails say up to 140 F, but I would suspect panels may get hotter than that in the summer).

Also if it was thermally conductive, that would help as well.

Thanks for any ideas!

jonr · October 2016

I think silicon would work fine. Consider extruded aluminum heat plates - they should be much flatter and allow removal of the tubing.

BB. · October 2016

Almost any adhesive is going to be a solar insulator. Sundrum Solar makes heat transfer plates for the rear of solar panels and they just clamp them to the rear of the solar panel:
http://www.sundrumsolar.com/products--services.html

https://www.youtube.com/watch?v=aQemNStW9pk
Why do you want to do this? If you do not have enough roof space for solar PV + solar thermal, I could see trying this vs simply two different installations (one solar electric and a second solar thermal).

I am always concerned that mixing electric plus plumbing on your solar PV panels is going to be a plumbing/service nightmare.

If you are trying to reduce the solar PV panel to increase efficiency--I am not sure how much you will gain (water still has to be heated to your needs--pool water does not need to be "hot", just warm and lots of flow. Domestic hot water, you need >120F).

-Bill

jonr · October 2016

You should find that a thin layer of any paste or liquid outperforms the thin layer of air you get when you clamp a large area with no adhesive.

http://www.dansdata.com/goop.htm

BB. · October 2016

The problem is that, for the most part, things that are highly electrically conductive are also thermally conducive. Silicone adhesive (and just about any other standard adhesive) is usually a very good thermal insulator (I have an oven glove made from silicone, and silicone pot handles).

Should the example in the first video have been anodized black (or even painted black)--I would think so... SunDrum has been around for years--But I wonder how many systems they have installed.... Searching the web for Sundrum yields a few results.

-Bill

BB. · October 2016

I looked at your link and you are looking at $10+ or so per 3 grams per tube for ArcticSilver thermal grease (used for electronic/CPU heat sinks).

Plus placing adhesive on the back of a solar panel will probably make warranty service difficult. (and greases can cause problems too).

-Bill

jonr · October 2016

Read further where he discovers that various common substances work even better - it's all about having a thin layer of anything that is a better thermal conductor than air (silicon sealant is ~10x better than air). The same principle applies to where the tubing connects to the plate (ie, silicon would be beneficial there also).

I agree that the whole idea has some practical issues.

davidwillis · October 2016

To answer the question of why.

I already have a grid tie system that is nearly taking my electric bill to zero (it does when I conserve a little). So I would like just a few more solar panels. Also during the sumer I have to heat my hot water with electricity (in the winter I heat it with my wood boiler, but I don't want to run my wood boiler in the summer). I looked into some solar thermal panels, but they are not cheap, and I can actually buy a PV panel for about the same price as I can buy the glass and aluminum to make my own.

So I thought why not just buy a few solar panels and use them for heating my water by adding some pex tubing to the back. I already have pex tubing, and some of those aluminum heat transfer plates left over from when I did my in floor heating, so I would just need to connect them on.

I will drain them out in the winter and only use the panels for PV power, so it is not really worth the cost for dedicated solar thermal panels, but if it just costs me a tube of silicone to get some hot water off my PV panels, then it will be worth it.

Also I love to just try things out, and see how it works. I am sure it will void the PV panel warranty, but if it works for heating my hot water, it is well worth the risk.

So far I am not seeing any suggestions that are any better than just silicone. I may use silicone, but also put a couple braces across to hold them in place... I just don't want to have to repair them if the silicone peals off after a while.

I think something like jb-weld might work, and even transfer heat better, but the cost would just be too high.

I couldn't see a price on the Sundrum Solar makes heat transfer plates, but I would guess they will cost more than I want to spend.

animatt · October 2016

You should not have a problem with silicone provided you have proper cure time and even application.
I made diy solar heating panel out of corrugated galvanized roofing panel
The corrugation are sine wave in shape. I used silicone to connect cpvc to the channels.
I do have supports to hold in the tubes. But span between supports is like 4.5 to 5 feet if i remember correctly. The panel is mounted at about 40 degree angle from horizontal.
I used clear silicone on the back of the panel.
Upper side of panel where I had gravity helping me I used white paintable caulk. The white caulk holds good but needs additional cure times.
And I needed to be able to paint it.

I will see if I can post a picture. Should help out visualizing it.

Depending on how you get paid for excess electricity. I would recommend buying a few solar hot water heating panels.specifically unglazed pool panels. Being you only interested in summer heating should be fine.
My unglazed panel gets about 200 gallons of water up to about 125f daily. I would imagine pool panels mounted flat against roof would do even better. Sometime these panels can be found dirt cheap used.
My panel is a bit large though. About 3 meters wide by 3.8 meters long. So roughly 120sqft.

Did not mean to distract from the conversation. Hope my few cents helped.

animatt · October 2016

These are obviously prepaint. But these are thermoshipon panels. Since they have been painted they have not been empty. So this does help keep temps down. Although unglazed so there is a natural limit to how hot they can get.

davidwillis · October 2016

Thanks animatt Those look good. So are all those lines running paralel from a bottom and top manifold? Can I ask how you made the manifold?
One difference with me gluing to the back of the solar panel is that gravity will be against me. but I think if I put in a brace or two it should hold.

I pool heater is a good idea, and that is probably what I should do, but I want to try this out and see how it works.

animatt · October 2016

Yeah I understand about testing out. This was a way for me to test things out. The same galvanized roofing panel works as a mounting structure and absorber area.

Yes there are cpvc lines in every peak and valley of the corrugated panels. The valleys use silicone and gravity to hold them in place. The peaks are glued with silicone from below the panel. So they face a similiar situation where gravity is fighting against them.
Over about 10 feet wide I believe there is something like 36 peaks and 36 valleys. So roughly 72 risers in total.
The headers are a modified 2 inch Cpvc. kind of expensive. I drilled the header pipe with a drill then used a tap to thread the wholes.
This method of construction is more expensive then pool panels in the usa. But some places have limited access to pool panels.
I had previously built a panel from pvc and had worked well. But this was going to be for my future house and decided to go cpvc and diminish points of failure.
In the future I will have a write up about my collector. But will still be sometime until that happens. I am thinking another 6 months as the system is still not fully implemented. Has alot of uncommon implementations which for most people are just cumbersome or awkward.
Will include pros and cons and setbacks will be provided.

BB. · October 2016

Interesting

http://www.builditsolar.com/Experimental/PEXCollector/SmallPanelAirVsSilicone.htm

-Bill

davidwillis · October 2016

That is a very interesting article. I was never planing on not using any glue, because I needed something to hold it on anyway, and like that article showed, even a non-conductive glue like silicone does better than an air gap.

I also remember reading an article comparing pex to copper (copper is a much better conductor). If I remember correctly it did not do a whole lot better, but I read it years ago. But for me, the cost is too much for copper, since I already have pex. It would be cheaper for me to do two or three panels of pex than one of copper.

Glue heat transfer plates to back of panel

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