Panel Cooling Idea: Pex Al Pex w/Alum fins

[ScottCrane]

Been reading about cooling panels. Best/cheapest DIY approach appears to be the water spray idea. To me, using collected rainwater would make it feel less wasteful of H20. Other new idea is to use a phase change material on the bottom to absorb and release heat. Lastly, is making some kind of water jacket; like those CPU/GPU coolers. Sounds expensive to me.

But, the water jacket had me thinking about perhaps using Pex Al Pex with aluminium fins to cool the bottoms. Pex Al Pex is a special Pex/Aluminium/Pex construction made for radiant heating and cooling. The "fins" are approx 4" wide and 2' or 4' sections of aluminium flashing type material. The fins are put thru a former to bend a half radius in the middle to fit the Pex-Al-Pex into. The idea is warm (or cold) water is passed thru the Pex and transferred to the fins. Thus warming your tootsies on a cold winter's night Google pex al pex fins and you will see what they look like.

So, as you may have guessed, the idea is to run the Pex-Al-Pex up and down the backside of the panels with fins attached to help with the transfer to heat from the panel to the fins to the cooling water. What would even be a bonus with this idea is if the panels could heat the water up enough to make it worth capturing and saving in a thermal storage for hot water usage.

I'm guessing in your head you are wondering: Dude sounds cool, how much is this gonna cost ya? Well, 1/2" Pex-Al-Pex runs about $0.42/ft (link). The 1/2" PEX Aluminum Heat Transfer Plates, I call them fins, (link) run just under $0.25/ea for a 4' chunk. So, back of the napkin says I want to run four lengths of Pex-Al-Pex up and down my 6' panels. And have four of the 4' fins. So, roughly $10 for the Pex-Al-Pex and another $1 for the fins. Probably add a few more $ for in/out connectors. Then some inexpensive "master controller" Arduino setup, or maybe a Wifi enabled Pi. Of course a warm water pump and cooling, etc.

Without some serious number crunching on the sizing, I picked 4 long runs of the Pex-Al-Pex per panel. To reduce the cost per panel, I might try just a pair of runs on one panel and four on another. Then capture/save/plot the difference between the uncooled panel and its neighbor one with two and four runs. Then compare all three panels apples to apples conditions. If would also be nice if I could find a way to figure out what each panel is putting out by themselves even when wired in series/paralleled in an array. I suppose measuring voltage wouldn't be an issue and current could be done with an external inductance measuring coil type sensor. The assumption is the cooled panels ought to be pushing out more and this should be proportional to the amount of cooling.

This way I get real world data and not just some theoretical guess. And it would help in determining if spending the extra $ for this type of cooling is feasible and what the payback would be. Since I also already have some of the water spray equipment (my drip garden left overs), it wouldn't be difficult to try it on one of the other panels was well. Part of me likes the closed loop Pex-Al-Pex system, but the spray beats it price wise. Let's see what the real world results are going to be.

 

[snoobler]

You need to cool the cells, not the panels. Cooling some cells but not others will have a negligible improvement. Temperature increase reduces current. If you reduce the temp of some cells but not others, the lower current cells (hotter) restrict the higher current cells (cooler) thus bringing all cells to the lower current levels (of the hotter cells).

Compare efficiencies between two temperatures. Is it worth it? My panels indicate a -0.4%/°C change, so a 10°C (18°F) reduction in temperature produces a 4% boost in power

Added weight?

 

[ScottCrane]

Quote: "You need to cool the cells, not the panels". Well if the cells are sitting on a sheet of aluminum and I cool the other side of said sheet, would not the cells also be cooled? Even the ones not directly over where the Pex-Al-Pex is (or the fins). I'm looking at the aluminum sheet the cells are on as a thermal mass in my mind that would more or less maintain even temps across it.

Plus, I'm the one to come up with cooling the backside of the panels. I got some of my info from this You tube Vid. Also this guy Porter that did some spray system directly on top of the glass of the panel (note, not the cells) One of his vids. Check out his other vids for more complete description of his setup.

Man, having to drop 18F to get 4% is a lot. I'll have to run some numbers on that. So, 104% of 330W is like only a 13W gain. A couple more kw across a month. A whopping $2.50 at most per year (using 0.10/kw, 5 hrs day). I'm guessing a five or so year payback.

As for extra weight, good question. I think the alum fins are nothing. The pex-al-pex runs a little less than 6lbs per 100ft. So we are talking less than 7lbs added each panel. My panels already weigh in over 50 lbs, other 7 hopefully won't break my back

I've also been playing with micro mister sprayers which put a fine mist into the air with the idea of using for people cooling while sitting outside. They call it evaporative cooling, aka swamp coolers. But it only works well in a narrow band of temp/humidity. Here is humid S.E. Texas, they only make it mugger than it already is, most of the time. One of my Arduino project idea was to read temp and humidity and give you some indication weather evaporative cooling is effective at current temp/humidity. Evap cooling too inconsistent here in my neck of the woods to use for panel cooling.

 

[fat_old_sun]

You need to cool the cells, not the panels. Cooling some cells but not others will have a negligible improvement. Temperature increase reduces current. If you reduce the temp of some cells but not others, the lower current cells (hotter) restrict the higher current cells (cooler) thus bringing all cells to the lower current levels (of the hotter cells).

Compare efficiencies between two temperatures. Is it worth it? My panels indicate a -0.4%/°C change, so a 10°C (18°F) reduction in temperature produces a 4% boost in power

Added weight?

Temperature increase reduces voltage; current actually increases very slightly.

 

[snoobler]

Temperature increase reduces voltage; current actually increases very slightly.

You are correct. Thanks for the correction.

 

[gnubie]

Quote: "You need to cool the cells, not the panels". Well if the cells are sitting on a sheet of aluminum and I cool the other side of said sheet, would not the cells also be cooled?

Unless you plan on making your own panels they aren't sitting on your aluminium sheet. They are encapsulated in epoxy with a backing sheet on that, then comes your sheet. Add all that up and I'm sure you'll find it has pretty poor thermal conductivity factor (temperature change per watt dissipated) because it's not designed for it.

That said, if you do get to the trial stage please post your results as a matter of curiosity so should anyone else want to try it in the future we can point them at your results. FYI you are not the first person to post on the forum with the intent of doing fins on the back of panels to assist cooling but the waterblock approach is new.

 

[ScottCrane]

Quote: " they aren't sitting on your aluminium sheet. They are encapsulated in epoxy with a backing sheet on that, then comes your sheet. Add all that up and I'm sure you'll find it has pretty poor thermal conductivity factor".
I'm not totally disagreeing with your assessment on the backing sheet. When I next get to my country property with the panels I'm going to take a close look at one of my unfortunate victims of my clumsy handling. Deciding it liked Mr Newton and not me, thus impaled itself, puncturing clean thru the tempered glass, cell, and metal (I assume alum) backing. I'll look at the layers.

Another simple way of testing the thermal conductivity is just to measure the bottom side temp vs the top side from when the sun first starts to warm things up over ambient. You can even use your human built in temp sensor: your hand.

Again, I'm not the one who first came up with the idea of cooling the back of the panels. But, I do appreciate the comments and feedback. Heck, that's why I posted it. Plus it might help someone else and give them a creative spark to something even better.

But, stepping back from the idea into the higher level of making panels more efficient after they leave the factory. Is it worth the effort? Negative Example: Everyone knows that solar tracking is more efficient than fixed array. So, why isn't every install do tracking? Complexity, cost, room for the tracking. Bottom line is years ago, they figured it was better and cheaper just to throw more panels up than to track. And that was when panels were expensive. Now the panels are much cheaper, making it more a logical choice to add more instead of tweaking with existing.

In this water cooling scenario, if I gain 13W per panel over my 24 panels equals roughly another (>300W) panel found locally in the $120-200 range. Granted, saying just throw up another panel (or two) is easier said than done pending upon room and if more rail needs to be put down. Plus wiring, etc. But, once that panel or two is up, there is little to no maintenance required. With any kind of active cooling you have potential leaks and overall future maintenance requirements to account for. Tracker setups also need maintaining too. Factoring this maintenance aspect into the equation tilts the conclusion even further towards more panels.

I've been telling my co workers, friends, and family about my solar project. The point I've emphasized is it isn't the panel cost, its the install stuff, wiring, breakers, grounding, inverter(s), battery, etc that becomes the lion share of the cost plus of course the labor. Since I'm a DIY, the labor is "free" and I enjoy this stuff and have wanted a system for years. Obtaining a 2 yr old used pallet of panels at auction for a under 30 cents/W earlier this year sprung started this project. Couple of the 3K Growatt hybrids in split phase and a slowly building bank of SPIM08HP (3.7/8ah lith polymer) based 48v system with Daly BMS etc. 13s3p and a 13s6p banks for almost 3kwh to start with.

I'm almost 1/2 way to putting up the panels since I've waited to cooler weather here in S.E. Texas. 4 rows of 6 panels divided across the peak on a very shallow metal roof building. There is nothing like being a fried egg on an open roof with the sun beating down on you here in Texas. Dam, why couldn't these panels work in the shade so I can put 'em up without melting me and my equipment. But, fall has arrived here, and this week and weekend look perfect for some panel installing.

 

[boondox]

While it sounds like a fun project in technical terms it seems like a solution in search of a problem. How do you keep the cooling water cool? Does it go from one panel to the next or in strings? Unless each panel gets fresh cool water there will be a temp gradient and not all panels will see the same benefit? And talk about a plumbing nightmare. And then the power to run the pumps. Replacing pumps when they wear out. The list of challenges goes on and the costs climb and as with the trackers throwing another panel up is much easier.

 

[snoobler]

While it sounds like a fun project in technical terms it seems like a solution in search of a problem. How do you keep the cooling water cool? Does it go from one panel to the next or in strings? Unless each panel gets fresh cool water there will be a temp gradient and not all panels will see the same benefit? And talk about a plumbing nightmare. And then the power to run the pumps. Replacing pumps when they wear out. The list of challenges goes on and the costs climb and as with the trackers throwing another panel up is much easier.

In other words... another "juice wasn't worth the squeeze" situation?

 

[boondox]

In other words... another "juice wasn't worth the squeeze" situation?

Exactly. The juice just ain't worth the squeeze. ?

 

[fat_old_sun]

....
Another simple way of testing the thermal conductivity is just to measure the bottom side temp vs the top side from when the sun first starts to warm things up over ambient. You can even use your human built in temp sensor: your hand.

....

The mere act of physically contacting a panel's rear side cladding will affect its temperature. If you're interested in reliable & accurate temperature reading, get a good quality IR non-contact temp probe with emissivity adjustment.

 

[gnubie]

Another simple way of testing the thermal conductivity is just to measure the bottom side temp vs the top side from when the sun first starts to warm things up over ambient. You can even use your human built in temp sensor: your hand.

You don't understand thermal resistance. It will get hot, it's the rate of change that will be restricted. You can put your heatsink on it, and you can water cool the heatsink and you'll find the temperature of the exposed side of the back sheet itself will rapidly collapse if you could actually measure it with your heatsink in place, but the cell temperature will decrease at a much lower rate. Where it settles will depend on the rate of heat in, ie the sun beating down on it at 1kW/sqm, vs the restricted rate at which your cooling system can get it out through the epoxy and backing sheet.

As before though, if you do go ahead with a test build I'd certainly be interested seeing in the results.

 

[fat_old_sun]

Responding to OP & adding to @gnubie response immediately above mine:

The (cell/panel/rear cladding) temperature you measure is the the "open circuit" temperature. As soon as you attempt to draw a heat 'current', the temperature will drop. Then all the the resistances of all the interfaces & materials in the circuit (cell, rear cladding, contact to PEX tubing, heat sink, etc) will come into play and will determine the net heat flux.

If you're determined to go with this idea, try a 'wet' contact between the heat transfer medium (water, oil) and the rear cladding to minimize interface resistances. Build a very thin, leak proof volume bounded by the rear cladding, some sort of pliable potting compound, and another membrane. Pump the fluid at a low rate from one corner, across the back and out diagonally opposite corner. Dissipate this heat elsewhere and recirculate the fluid.

 

[ElBeau]

There was a company that was trying to make a slim tank for the back of the pv panels, low PSI(3-6) I saw the product at ODU, my neigh was taking a solar class there.