In some areas, the water use just from cleaning solar panels becomes a serious issue (let alone using more water to cool them). It's not much water to spray on some panels you are experimenting with, but for a large installation it seems like it would be a lot of water.
The best areas for solar are hot, those are also likely to be the areas where water supply is the most constrained, so this is definitely a no-go solution
Sorry if this is too basic of a question but for places close enough to the sea, would sea water for cooling and cleaning be an option? Too much salt residue?
Now I'm waiting for the seaside solar power plant/salt works proposal to pop up!
Seriously: Maybe residue wouldn't be an issue if it was never allowed to dry, but the corrosion-proof construction, drainage system, and pumping costs would be considerable. Then there's the 100 problems we're not aware of yet...
In the Kochi airport in Kerala, India, the water used to wash the solar panels is used to grow fruits and vegetable plants right under the panels and these are harvested and sold.
The plants under the panels also reduces the heat under the panels and thus increases the efficiency of power generation.
This can be a nice way of getting additional income from the land and also tackles the problem of plants growing under the panels.
For large deployments of Earth-mounted PV you'll need a grid of drainage ponds to buffer runoff water, so you might draw from those. A control system would monitor temperature, PV output, and water turbidity.
The latter is important, since there's no sense dumping dirty water on the panels! Give the suspended solids time to settle out, or even add flocculants via a dispenser.
The wet surface seems better at absorbing sunlight than the 'anti reflective' glass surface, and the evaporating water is a great cooler.
Since it uses so little water, I don't know why industrial installations don't do it.