Flexible solar panels

[dzl_]

Originally Posted by Danjo

Renogy told me the optimal operating temperature for their panels is 78F. I can imagine being able to fry an egg on directly applied flexible panel mid summer.

Renogy is mistaken, or you may be misinterpreting what they told you. 77/78F is the temperature at which their panel was tested (and the Watt rating is derived from), its not optimal, just the standardized test conditions they use to roughly approximate 'average.'


See: Standard Test Conditions (STC) and/or the image of the renogy specs above.

[john61ct]

On a cold winter morning with good reflection from surrounding snowy surfaces, is when panels can produce way over their standardized ratings, to the point you need to give some cushion when shopping for a controller if your usage may involve such conditions.

In warm weather they very rarely even come close to hitting those numbers.

Originally Posted by Danjo

Renogy told me the optimal operating temperature for their panels is 78F

Not true, at all.

Just that you should install them so that they don't get much **hotter** than that.

The PV chemical reaction is what generates the damaging heat, needs to be dissipated via the underside, where yes it often gets hot enough to fry eggs.

[banman]

Originally Posted by john61ct

On a cold winter morning with good reflection from surrounding snowy surfaces, is when panels can produce way over their standardized ratings, to the point you need to give some cushion when shopping for a controller if your usage may involve such conditions.

In warm weather they very rarely even come close to hitting those numbers.

Not true, at all.

Just that you should install them so that they don't get much **hotter** than that.

The PV chemical reaction is what generates the damaging heat, needs to be dissipated via the underside, where yes it often gets hot enough to fry eggs.

Is this to the point where if I was designing a roof rack for a warm weather bus I would do well to design cooling fins under the panels and or is there a panel you recommend with cooling fins built in?

I can see where home-fabbed cooling fins may inhibit airflow to where they trapped more heat than they shed...

[Danjo]

Originally Posted by dzl_

Renogy is mistaken, or you may be misinterpreting what they told you. 77/78F is the temperature at which their panel was tested (and the Watt rating is derived from), its not optimal, just the standardized test conditions they use to roughly approximate 'average.'


See: Standard Test Conditions (STC) and/or the image of the renogy specs above.

Based on all your recent postings I think you know more than I do about solar, so I am questioning my interpretation of what I’m reading, but that article says the same thing; as the temp goes up, the output goes down, with optimal being somewhere in the mid-70’s.

[john61ct]

Originally Posted by banman

Is this to the point where if I was designing a roof rack for a warm weather bus I would do well to design cooling fins under the panels and or is there a panel you recommend with cooling fins built in?



I can see where home-fabbed cooling fins may inhibit airflow to where they trapped more heat than they shed...

No, ROI on the extra cost not worth it, just ensure air can flow along the underside, 2-3" gap is fine, less if your rack might lift off the roof at highway speeds.

[john61ct]

Please do not imagine inbound telestaff at a cheap mass market outfit like Renogy are going to have a clue.

Sometimes maybe you might get lucky, but that poor soul is getting grossly underpaid and won't stay there for long.

[dzl_]

Originally Posted by Danjo

Based on all your recent postings I think you know more than I do about solar, so I am questioning my interpretation of what I’m reading, but that article says the same thing; as the temp goes up, the output goes down, with optimal being somewhere in the mid-70’s.

I'm not seeing what your seeing in that article. It is true that STC conditions are optimal in some respects (like solar irradiance), but not in terms of temperature, STC conditions to represent real world temperatures (77F panel temperature is unrealistic in warm weather), but its also not optimal from a power output standpoint.


Can you point to the part of the article that you think implies optimal is in the mid-70's? Maybe I'm missing something or maybe I can clarify something you are misinterpreting.


Here is one line from the article that I think should be highlighted:

Because of the physics of solar panels, as they heat up, the voltage they can produce goes down. And since the electrical Power (watts) = Voltage x Current, when the voltage goes down, the power goes down.

[Danjo]

Originally Posted by banman

Is this to the point where if I was designing a roof rack for a warm weather bus I would do well to design cooling fins under the panels and or is there a panel you recommend with cooling fins built in?

I can see where home-fabbed cooling fins may inhibit airflow to where they trapped more heat than they shed...

I just ran across an article on a hybrid solar system. PVT. interesting idea if you have the real estate under floor for a big hot water tank. Hmm...too late for mine and my bus is too small anyway.

[dzl_]

Originally Posted by banman

Is this to the point where if I was designing a roof rack for a warm weather bus I would do well to design cooling fins under the panels and or is there a panel you recommend with cooling fins built in?

I can see where home-fabbed cooling fins may inhibit airflow to where they trapped more heat than they shed...

Short discussion on a similar concept (and a company making a panel like that)



I tend to agree with John though, as cool as the idea sounds, its unlikely that it would make financial sense.


And from a technical standpoint you would need to ensure good heat exchange, probably with some sort of active cooling (fans or maybe some sort of liquid cooling/heat exchanger).

[john61ct]

I just meant for cooling purposes.

If you got passive HWS out of it by pumping down to a heat exchanger, that would be so cool who cares if it's practical or not 8-D

[dzl_]

Here is the NEC (national electrical code) voltage correction table for temperatures under 25C/77F.



You can see the largest correction factor (1.25) is for -32 to -40 Fahrenheit. In lay terms that means voltage and by extension power can be up to 25% higher when its frigid-as-f*** out.

[kazetsukai]

Originally Posted by dzl_

I don't mean to kick a dead horse but I want to be very clear and nip this in the bud, it is very much accepted fact in the solar industry that PV panels perform better in cold conditions and perform worse in hot conditions. We don't need to complicate it.

This keeps bringing me back to the possibility of liquid cooling (rigid) panels somehow.


Coolant loop: Panels -> Heat exchanger (for hot water/etc) -> Ext. Radiator/fan -> Panels


Seems like we're collecting all this thermal energy hoping for it to go away instead of harnessing it.

[Sehnsucht]

Originally Posted by kazetsukai

Seems like we're collecting all this thermal energy hoping for it to go away instead of harnessing it.

Exactly! Where some see a problem to solve I and apparently others see an opportunity. Please refer back to post #4 where this is exactly what I proposed.

[john61ct]

And again, such systems are readily available commercially, dozens of DIY forum build threads, if you want it not rocket science.

Just that IMO it's not worth doing in the first place, unless you enjoy the tinkering for its own sake

far more bang for the buck tapping waste heat off an ICE and buying a couple extra panels.

[banman]

Might not be too difficult/complicated to pump several gallons into a "thermal mass tank" under the solar panels getting heated all day and then using the water at night for showers and dishes... I could see this as potentially viable for a "weekender" build like mine. Otherwise I'm filling water bags to lie in the sun for evening use.

Meanwhile, further proving you guyz are way ahead of me knowledge wise on this topic --
John61ct -- what is this "ICE" you speak of? Google will not give a relevant answer to your jargon...

[Sehnsucht]

ICE = shorthand for Internal Combustion Engine (gas or diesel)

[dzl_]

Originally Posted by kazetsukai

This keeps bringing me back to the possibility of liquid cooling (rigid) panels somehow.


Coolant loop: Panels -> Heat exchanger (for hot water/etc) -> Ext. Radiator/fan -> Panels


Seems like we're collecting all this thermal energy hoping for it to go away instead of harnessing it.

Its an interesting concept, I've been pondering on and off for a few months, that I'm almost positive I'll never pursue.


I've got a bit of an obsession with efficiency and reclaiming wasted or untapped energy, so the concept really appeals to me. But I strongly suspect that the juice might not be worth the squeeze in practice (in terms of efficiency vs other costs).


However, if you get creative it might well make sense, the jury is still out I suppose.


The pros potentially be:
- increased PV panel efficiency in warm weather (probably in the 0-20% range)
- leveraging 'free' heat energy for water heating, radiant heat, or something similar


Cons/hurdles:
- complexity
- weight on the roof
- it would take power to operate this system (this is the hurdle the needs to be epxlored I think)
-



In the system you sketched, you would at a minimum need a water pump and a radiator fan, these things take power, maybe a lot of power if the pump has to pump water from floor level up to roof level continuously. A fan would be needed to cool the water on the return path to the panels. Could the efficiency gains outweigh the added power consumption? Maybe.


Also, unlike a water cooled engine where the cooling system needs to cool something from 'ridiculously hot' down to 'very hot', the cooling system for the pv panels would need to cool something down from 'pretty damn hot' to 'close to ambient.' Could water that is somewhere between ambient and panel temperature make a meaningful difference, and could it both heat water by a meaningful amount, and cool the panels at the same time?


These are open ended questions, that I don't have answers to, but are worth exploring.


On the other hand, if your main consideration is using the excess heat, not improving panel efficiency, and this system would replace or augment traditional water heating, then it might make much more sense.


One way or another its a fun thought experiment.

[Sehnsucht]

Prior to this conversation I was mostly thinking about it from the convert-waste-heat-into-useful-energy perspective because I wasn't well-versed on the significance of ambient temperature on the panels' performance. Now I feel like it's still possible but requires a little more contemplation because the thermal transfer fluid loop does need to achieve nearly ambient temperature before returning to the panels otherwise it becomes self-defeating if it contributes to the panels' heat buildup instead of reducing it. My expectation though was that any necessary power required to pump the fluid would be achieved through the solar energy because basically if the sun's not shining anyways then there's no reason to be pumping fluid around.

[GabbyVerse]

I'm sorry if this was mentioned before but solar panels worked better when they are cooled. Reason for the air gap is to provide airflow to cool the panels so they are more efficient. I know it sounds counter-intuitive but that is how it works.


Gabby

[dwood443]

I’d consider portable panels. This way you don’t have to park directly in the sun. I would think any of the renogy panels with a long enough cable to place the panels within a short distance would be fine. On my VW bus I had quick disconnect mounts but I don’t know how practical that is unless you have easy access to the roof