Solar PV cooling?

[Gotwind Ben]

Has anyone considered cooling the rear of a photovoltaic solar panel?
we know the efficiency is higher at cooler temperatures, cold bright winter days for example.

I wonder, on larger arrays if it would be worthwile running a matrix of tubing on the rear of the panel and pumping coolant around using a low power 12v pump (powered from the panel).

This would only apply to panels over 50 watts at a guess, it might be nonsense also :D
Just thought I would toss the idea in for discussion..

Ben,

I see this as highly problematic from a DIY point-of-view and to a lesser degree (all puns intentional) from an engineering POV, but I certainly have no first hand experience. The problems I see are:

1) How do you sink the roughly .85kW/m^2 of heat effectively w/o adding tremendous cost? Obviously you envisioned using a liquid coolant, but how do you transfer the heat to the liquid? Certainly a hollow aluminum or copper backing would do the trick, but that is a non-trivial additional cost.

2) Assuming a cheap enough heat transfer solution could be found, removing the heat from the solar cells is only the first part of the process, you still need to remove the heat from the cooling media somewhere in the cycle. Adding a radiator and a fan would eat up much of any anticipated gain in a small system.

3) Cost to Benefit ratio seems skewed to me. If we take a 2kW rated array and de-rate 20% (that's just off the top of my head, I have NO IDEA what the actual de-rating factor is) due to heat, this is a loss of 400W. It is unlikely that any cooling system could regain more than 50% of the de-rating, so we are haggling over 200W. @ $10 per installed Watt this has a maximum value of $2000, but because it is only truly a problem in the Summer, we should only count the de-rating 25% of the time. This brings us to $500 to spend on cooling a $20,000 array. If we simply added another 50W to the array, we would cut our "losses" in the summer by ~20%, but in the other three months we should see an average of a 2.5% increase in system output.

The other way to look at this is that in the Summer the longer hours of insolation are built-in compensation for the de-rating.

Finally, on the Cost to Benefit venue, If you size your system based on the actual output rather than the name plate output, you can simply brag about the "free extra power you get in the Winter." :-)

*********************************

If you want to actually give it a go, consider aluminum "roof flashing" bonded directly to the back of the panels. Devise a punch and die to make "cooling fins" prior to attaching. This passive system might help lower temperatures >> than ambient with little actual cost, but I doubt that you are going to see a large improvement.

Keep Thinking :-)

Fish

On the other hand, remembering the news I found a few months ago about a new composite PV material, why not have dual-purpose panels?

Sticking a a PV coating on the sunward surface of an indirect panel-type hot water thingy might perhaps result in something which could be regarded more as a solar hot water installation-with-a-PV-bonus rather than a PV installation-with-a-cooling-penalty.

Might even get enough power off the PV to run the pump. Just seems neat, two birds with one stone, dunno if practical - might not get enough power to justify the cost delta over a normal panel, dunno the new material's conversion efficiency, etc etc ad nauseum.

Obviously wouldn't work with evacuated tube types of course!

[Gotwind Ben]

I wonder.
If a series of alluminium lengths of angle section could be bonded to the rear of the panel to act as a large passive heatsink, dissipating heat (with natural wind airflow) :idea:
For the cost of maybe £15 ($25) for a 60 watt panel, it may have some payback over time.

Ben,

I mean no disrespect, but I too wonder. When I suggested you use thin section aluminum with tabs, it was not because I had failed to considered using 1000lbs of aluminum, or some amount of aluminum between 1000lbs and a few ounces. Honestly it was because you do not want to sink short-term heat or transient heat, but because you wanted to keep the panels as close to ambient as possible. Maximizing heat transfer and minimizing latent heat is a tricky thing, large mass heat sinks do best when there are transients and fans, minimizing heat build-up in a relatively high heat environment with natural aspiration is best achieved with minimal heat-sink mass and maximum transfer, as luck would have it this occurs with cooling area, not mass.

By all means, test it any way you see fit, but my suggestion is small mass, high area, passive cooling to maximize the cooling affect. I believe that the effect will be minimal either way; however, to achieve the maximum benefit you need to minimize mass and maximize area. Thermodynamics is by NO MEANS my specialty, but this seems pretty much a no-brainer. Unless you have access to a large thermal sink (cold running water or Cold Air) then it is unlikely your efforts will prove viable. But by all means, test it and report!! I love being wrong :-)

Fish

Generally natural convection is enough to keep them at a decent operating temp.
That assumes they were properly mounted, at a decent tip angle, with nothing within about 6" (15cm) of the rear or sides.
Mounting them flat against the roof, or using those 'mounting clips' that only hold them 1" (25mm) above the roof, do not allow for decent circulation.

I recall someone efficiently pumping a small amount of water over them. It had a bit of gain in system output, but had issues with residue and water deposits. IIRC, he decided the money would have been better spent on another panel.

I wondered about a small muffin fan to help move the air below the panels along a bit easier. 150ma isn't that much. A direct drive fan from a 3W PV would do it.
But I do not have any reasonable way to tell if it actually helped! A slight change in wind speed or direction, solar intensity, etc, would make it impossible for me to know EXACTLY what caused any PV temp changes.

Most systems are battery based, and the panels would tend to be hot in summer (long days) and good sun (high output), when there is generally a surplus of power anyway.

I believe the peak power voltage decreases with higher temps. I don't believe the amps (actual power) change all that much unless the system is using MPPT. The increased rated power would not really translate directly into much more battery charging amps.
If the peak power voltage drops from 19V in winter, to 17.5V normal, to 15V on a very hot day, that really does not have any significant impact on the type of system I do.
G-

[Gotwind Ben]

Thanks Guys.
Ideas, as daft as they may seem are always worth suggesting I think on a forum, brainstorming is an old 'buzz' word but still very valid today.
You never know what can be imagined/developed collectively.

Fish.
I will give the cooling fin design a test run, how i will compare results will be a challenge :ugeek:

Cheers.

In the UK the mains water supply is fairly cool all year round. There might be some benefit to using incoming cold water to cool the panel and pre-heat the water. Just a thought...

Might find this interesting-

http://www.mantaro.com/downloads/Using_ ... _Cells.pdf