Cool your solar panels and heat your water at the same time?

So I'm building a solar panel system. From what I understand, solar panels get less efficient as they warm up - which they do because they're almost black and in the sun, but also because they generate heat as a biproduct of power generation. I also need to generate hot water on site, preferably in as sustainable way as possible.

Has anyone mounted solar water heaters on the *back* of the solar panels as a way of cooling their solar array and generating hot water at the same time?

Heating water is energetically very expensive. Even if I can raise the water temperature 20 degrees with solar, it means I don't have to use so much energy elsewhere to get it up to domestic hot water temperatures. But is it worth the cost of building materials?

I like the idea.  Using the same space for electric and heat saves space. The biggest problem I have in my area is freezing in the winter.  It needs to be set so it completely drains or filled with antifreeze.  An antifreeze loop that only kicked on at a warm temp would work with a water to water heat exchanger.  It sure would be neat if you got a system to work.

Excellent idea! I don't know how practical it would be in real life, but worth exploring. I would try a drainback system, where the water all sits inside the house until the collector is warm enough to give net heating. Then a pump circulates water through the collector and back to storage. No water is exposed to freezing temperatures.

Christopher Shepherd wrote:I like the idea.  Using the same space for electric and heat saves space. The biggest problem I have in my area is freezing in the winter.  It needs to be set so it completely drains or filled with antifreeze.  An antifreeze loop that only kicked on at a warm temp would work with a water to water heat exchanger.  It sure would be neat if you got a system to work.

I'm in Nova Scotia - we do snow and cold weather. It's expected to get down to -13C later on today. Anti-freeze would be the way I would go. Especially considering the system I have in mind, will initially be set up on a site which would only be used during the non-winter months. The eventual plan would be to be living off site, and off the grid. But that's another story. Anti-freeze would be a higher setup cost and more complicated system, but lower maintenance. I'll take the initial pain of the cost over lower maintenance any day!

I'm thinking a 12v water pump hooked up to a small 12v solar panel - no battery or charge controller, so that it would pump the anti-freeze through the system when there was enough light to actually do that. No light would mean no heat and I don't want the heating system to be radiating heat off it during the night. Although it would be an interesting way of de-frosting the panels if they are up on a roof if you could gently warm them with the heating system. Run hot anti-freeze through the heating system and let them melt the ice from the panel surface.

I'm also wondering about how to actually do the solar water heating part. I've seen a few options over the years. Some cheaper than others. The panels I have are pretty big - 7ft by 3 maybe? Currently I have 4 of them, though I may end up buying more.

The backs of them have some kind of plastic backing to the panel, though I'm not sure what that function is. It may be supportive of the glass, or help in the event of an acident and glass shattering. These aren't bi-facial panels so I'd be surprised if they were to reflect light back at the cells to increase productivity, but they could be I suppose. If anyone has information about that, I'd love to know. Either way I don't intend on taking that off.

Glenn Herbert wrote:Excellent idea! I don't know how practical it would be in real life, but worth exploring. I would try a drainback system, where the water all sits inside the house until the collector is warm enough to give net heating. Then a pump circulates water through the collector and back to storage. No water is exposed to freezing temperatures.

See above. Unfortunately the system will be left over winter unattended, and even the tank inside the house would be subject to winter temperatures. I could drain it down when I'm winterising, but ... I guess I'm a bit lazy and would prefer a low maintenance system.

We all know that conductive heat transfers better than radiant heat, so the heat absorbers will need to be in contact with the underside of the panel.

Would the tubes carrying the anti-freeze need to be copper? Would the anti-freeze degrade plastic? I'm not worried about UV degrading plastic pipes since the panel will protect it. Would copper be a better conductor of heat than plastic?

What would be the best way to attach the conducting tubes to the back of the panel? I can hot glue them on I suppose, but that would only create a very minimal conductive surface. I've also seen people using a sheet of metal, with grooves bent into it to carry the tubes. This would increase the surface area on the pipes and therefore the improve the efficiency of the heat transfer.



I'm thinking of using aluminum, since it's readily available, corrosion resistant (not that it's a particular concern since it'll be under the solar panel, but still), cheap and easily worked. Would steel be a better product? If so, why?

I've seen reports from other solar practicioners that indicate that it's a complete waste of time and resources.  Basically, the water never got that hot, and the panels never got cool enough to make a difference.

With panels so cheap these days (shop locally on Craigslist), you'll get far more bang for you buck just adding additional panels.

Michael Qulek wrote:I've seen reports from other solar practicioners that indicate that it's a complete waste of time and resources.  Basically, the water never got that hot, and the panels never got cool enough to make a difference.

With panels so cheap these days (shop locally on Craigslist), you'll get far more bang for you buck just adding additional panels.

OK thanks. I did wonder. The efficiency increase wasn't really the reason I was considering it, but it would have been a nice added bonus. Never mind.

Not yet.  I had been looking at the various commecial PVT(photo voltaic thermal) systems and trying to figure out how to do it with current panels.  Most modern panels are a glass front with some sort of plastic film backing sealing the panel from the elements on the back side.  Also know most of the bulges tend to be aimed back so will be working around them too.

So what would be needed?  1.  An insulated backing.  Will likely need to be polyiso for its greater heat tolerance.(in case of a cooling failure)  2.  Something to gather the heat.(will come back to this)  3.  some sort of heat conductor paste.  4. PV solar panel.


So back to #2 I see 4 possible answers.  

1.  standard copper tube and plate system.  Shot against this is the economics is already questionable on these even before I steal 20% of the energy for PV.  Plus likely localized hot spots that will block over all efficiency gains.

2.  PEX tub and air heat gathering.   Plus fairly cheap and the solar panel should protect the PEX from UV.  Cons.  Will be really difficult to implement as a drain back system.  

3.  Some sort of plastic sack system so the water molds the plastic bag solidly to the back of the solar panel.  Big chance of leaks that could wreck the panel.  Drain back ideal and fairly easily implemented.  Plastic bag failure risk.  Maybe go to TEFZEL? as the plastic then the con is price.

4.  Finally for the answer that looks like it makes the most sense as a possible for me.  How about 2 panes of glass held apart by 2 strings of 1/2" PEX.  Each done as a big "L"  Thus the spacer is the fill and drain system.  Glue this all together with windshield glue.  Use the spacers tubes to carry water in and out allowing for a drain back system design.  Next problem is maintaining a good connection with the solar panels for heat conduction.    Major con is weight.  That will be 3 layers of glass making up the panel plus insulation etc.  This is the one I am leaning towards trying.

Now there are several commercial PVT versions.  Here is my  favorite.  It combines evacuated tube, with pvt and optical concentrator

Still the most efficient  way I've found for heating and hot water is a heat pump promoted by Dennis Lee: The World's Most Efficient Heat Pump
Dennis's Heat Pump Performance

Dennis Lee Heat Pump flv

A very cheap and easy experiment would be to lay some foam on the ground, then lay a black solarshower bag on the foam, then lay a solar panel on the shower bag in full sun. Check the temperature of the water before and after a few hours. It would only take a few minutes to set up. Checking to see if there is a change in power output is also fairly easy.

Products already exist.  I found one about 5 years ago, and can not find it again, but I have just found others.

https://www.convertenergy.co.uk/pv-t-hybrid-solar
https://dualsun.com/en/product/hybrid-panel-spring/

If you wanted to do a DIY type setup, maybe glue (and use thermal paste) the flexible flat pv panels to something like warm board. https://www.warmboard.com/, but I bet that would cost more than just buying a pre-made solution.

I was thinking about putting aluminum pex heat transfer devices on the back of a solar panel.
https://www.pexsuperstore.com/product/25-extruded-4-ft-aluminum-heat-transfer-plates-for-1-2-pex/

On demand water heaters have recirculation pump systems built into them. The other option would be to add your own pump with some sort of controller.

Just some thoughts.

The dual sun website claims 20% greater efficiency. I’ve wondered about using aluminum heat transfer plates with pex tubing on the back of my panels too. I wonder if I could preheat my Diesel engine this way? It seems the is hybrid system might work best in a cold sunny climate like high desert where the delta T of the antifreeze is high. That wa y the panels are being cooled and the heat transfer fluid is being warmed.

Those virtue PVT vacuum tube system looks brilliant..... until it snows.  Have fun trying to clean those off without breaking them.  Keep in mind, that as permiculture would dictate, the same solution will not work in all places.  I am thankful for the vertical angle of my solar panels, most of the snow slides off them on it's own. (they are PV only, it's all I could get while staying up to standards with the "department of making you sad")

Another thing to consider.... you can have too much hot water.  A solar thermal panel will boil water.  In the winter, you may want lots of panels, but come summer, you need to be able to dump heat.  Either a large pool, or better, annual Geo thermal storage well.  Use a heat pump to put it down there, and get it back out in the winter.

Where I am in Canada, a solar thermal system of about 3 panels is enough to do your DHW all summer.

I came across this article today: https://www.psu.edu/news/research/story/form-fit-device-wraps-around-hot-surfaces-turns-wasted-heat-electricity/

It's a flexible kind of heat pipe, that generates electricity based on the heat difference between one side of the device and the other. Their test results sounded super efficient: "In tests being conducted on a gas flue.... A scaled-up version, just over 3-inches squared, .... exhibited a total power output of 56.6 watts when placed on the hot surface...."

I expect cost will be prohibitive right now, there are liquid metals in the device to make it flexible. I wonder just how hot the gas flue is, that 56 watts of electricity is generated from just a 3" by 3" piece. That's just under 900w per square foot! I can see it now, people attach this to the back of a wood stove, and it could supplement their solar electrical when the sun is low but they are already heating the space. Or some fancy heat riser in a RMH that converts some of the heat into electricity.

Great topic.
I teach people how to reduce heat build up in homes by installing an extra roof.
I call it a safari roof and it cuts the roof temp. from 76 to 38 deg. C
I notice solar panels cut the roof temp to 56 deg. C so there is a bit of heat there to be grabbed.
NOTE THIS IS IN AN AREA WHERE THE TEMP> IS 38 -43 deg. C

I've seen reports from other solar practicioners that indicate that it's a complete waste of time and resources.  Basically, the water never got that hot, and the panels never got cool enough to make a difference.

I panic when I see this type of thing said or written dow.
I think its that type of thinking that has contributed to the climate and energy problems we are living in.

John C Daley wrote:I panic when I see this type of thing said or written dow.
I think its that type of thinking that has contributed to the climate and energy problems we are living in.

I think the key of making it worth while is doing it well.  When you are dealing with small differences in temperature, sure it might not be worth it.  But if you design and/or build with high quality (like having maximum surface contact) then it can work really well.

Another thing to do, employ additional systems like a heat pump.  If your liquid going to the panels is as cold as the fridge, it will cool your panels!  If the liquid is already warm, it will do less.

Moving heat around is simple, but not easy.  Nature wants to "balance" all along the way.

If I was doing this, I would 100% use hybrid panels, combined with a heat pump.  I would use the heat for my DHW, and when that's "full", have some place to dump the "waste heat", like seasonal geo storage, or a pool!

Trying and failing can be expensive, so find out what has worked.  Take note of what the nay sayers say, because under that "seemingly stupid" ridicule is some truth.  Why do they say it doesn't work, don't make that mistake, do it better!

I just saw a water cooling system for PV panels that is different.

https://www.pv-magazine.com/2022/01/18/spraying-water-system-for-solar-module-cooling/

The system doesn't kick on until the back of the panel reaches 113 degrees.
It raises the water an average of 86 degrees, cools the panel an average of 50 degrees,and improves power yield a scant 0.5%.
It the water is sprayed/dripped down the front of the panels, so it isn't suitable for use during the winter,but it does seems easy to implement.
I wonder if you could spray the back of the panels as well as the front, for more efficient cooling.

A conductive cooling system like this  should lower roof temperature and consequently structure temperature.
It could be used on roofs without solar pv panels for structure cooling, in humid areas where evaporative cooling spray systems are not effective.