Cleaning panels doesn't really matter - measurement and photo

[scrambler]

The best way to actually measure the difference would be on an array with individual panel monitoring, cleaning half of them only and looking at the panel production of both side at the same time.
From what I have read, people report for 2% to 10% depending how dirty the panels were.

A thin layer of dust probably does not make a huge difference. A thick layer of ashes like we have had recently in CA and or lots of bird dropping probably creates a bigger difference.
It will all depend on the opacity of the layer.

[J.P.M.]

Evaporative cooling is pretty effective. I sprayed my first array in TX once on a hot day (terrible idea I know). The increase in power was enough that I could HEAR the inverter output increasing.

Probably more sensible cooling than evap. cooling, at least if it was noticeable enough to be heard by changes in equipment noise.
Also, given the thermal time constant of most PV panels the cooling effect lasts only about as long as the water is running.
Keeping water running uniformly over an array will cost money for equip., maint. and pumping costs besides the cost and use of the water. If nothing else, careful cost analyses may be a good idea.
Continuous tap water on glazing, hot or otherwise, will increase the buildup of scale on the glazing. That will increase panel fouling and decrease performance. Sounds like the possible makings of a foot shooting party to me.
All in all, spraying water on PV panels seems to have been found about as sensible as spraying water on residential A/C condensers to decrease the working fluid temp. to the coolant before expansion.
Such things don't usually come to a good end for reasons most folks are clueless about.

[nwdiver]

Probably not. I'm pretty sure most panel fouling measurements are of the instantaneous or at least very short period duration, on site and under in situ conditions of semi or quasi steady state irradiance. That means temperatures and other variables are not changing much, if at all, including how wet things are.

Evaporative cooling is pretty effective. I sprayed my first array in TX once on a hot day (terrible idea I know). The increase in power was enough that I could HEAR the inverter output increasing.

[J.P.M.]

The panels are cooler because they're wet and that's why the power output increases not because they're cleaner.

You may have worked in nukes, but you don't seem to know much about testing of solar equipment.
Panels are most probably not wet when having their levels of fouling evaluated.
I'm pretty sure most panel fouling measurements of the before/after cleaning type are of the instantaneous or at least very short period duration, on site and under in situ conditions of semi or quasi steady state irradiance. That means temperatures and other variables are not changing much, if at all, including how wet things are.

Also, under any reasonable insolation where the more reliable measurements can be easier to obtain, panels will usually be well above the dew point temp. meaning any water on a warm panel water will have evaporated long before any measurement.

Also, panels are rarely, if ever cleaned when they are warm or hot as would most likely be the case for testing in situ.

In short, panels being tested or their outputs measured clean and dirty are in all probability not wet as the variability introduced by the moisture would invalidate the testing.

Doing an energy balance on a panel can provide some insights.

Depending on the nature of the fouling and it's characteristics, the clean(er) panel will probably run a bit hotter than a fouled panel.
Reason: For most situations more energy gets into a clean(er) panel.
If heat transfer correlations and rates of heat transfer per degree of temp. difference, panel <-> environment, don't change too more energy into a panel means higher temp. differences will be required.
Yes, more sun turned into electricity by a clean panel means less heat rejection but about 4 times as much excess heat will be generated internally that'll need to be rejected. That's also before decreases in panel efficiency from higher operating temps. are considered. All that will necessitate an increase in the temp. of a clean panel to maintain the energy balance. The net result in most every case will be that a cleaner panel will run hotter.
There are some considerations that may modify the above situation such as a very light colored fouling layer that may also reflect some of the longer IR frequencies while passing some of the UV stuff below the cutoff wavelength for usefulness by silicon solar devices, but that's not too likely unless by design.
I worked a bit with some folks a long time ago on a type of surface coating that passed wavelengths below the useful PV cutoff range and reflected everything above that wavelength. Lots of problems that had no workable/practical solutions at the time. Maybe things have changed.

[nwdiver]

Are you writing that you believe cleaner panels have improved performance because they are running cooler ?

The panels are cooler because they're wet and that's why the power output increases not because they're cleaner.

[J.P.M.]

I read an article at Sandia that said in their testing cleaning panels made about 5% difference. I suspect the folks who say "OMG cleaned my panels and output jumped!!!" are seeing the effects of cooler cells (reduced temperature derating).

Are you writing that you believe cleaner panels have improved performance because they are running cooler ?

[secessus]

I read an article at Sandia that said in their testing cleaning panels made about 5% difference. I suspect the folks who say "OMG cleaned my panels and output jumped!!!" are seeing the effects of cooler cells (reduced temperature derating).

[SunEagle]

Glad the ash is not bad on your panels. But a single days output comparison may not show you the real improvement of having clean panels.

[markofjohnson]

I'm in Oakland California. Did you hear that CA is burning? We've had 4 days of ash fall and blocked sunlight. Today we got some sunlight, so I cleaned the ash of my roof top panels to see what difference it made. Used a hose and a brush and I cleaned 10 out of 14 rooftop panels. See below photos of the panels before cleaning so you can see how dirty, and the energy generated graph. It's not very a scientific test, but I am surprised that dirty panels work almost as good as clean ones.

graph.jpg

dirtyPanels.jpg