Cooling Solar PV Panels

The usual recommendation is to leave at least 150mm (6 inches) between the panel and the roof.

Maximum power temperature coefficient (% per °C) The change in module output power at temperatures other than STC (25°C). This specification is used to calculate how much module power is lost or gained due to temperature variations.

This is not hidden but is displayed on panel spec sheets. It is not in the 'small print' but well displayed. Ground mounts have other problems.

Raising the panels and changing the roof color are about all you can do.

Rent a paint sprayer and use Sears Latex Titanium Dioxide White. It will turn a little brown with first coat. Second coat should stay white. Big change in air temp flowing over roof. Also big change in temp inside building, cooler in summer, hotter in winter.

I have a 6.3 kw PV system and I have noticed a marked change in output with changes in temperature. I have done some research on the subject and I have found two possibilities for cooling the PV panels:

1. Install a water misting system to spray the panels every few minutes. Probably the only way to make this a viable and economic solution is to use water captured in rain barrels and using a dedicated water pump. I haven't crunched the numbers on the expense of pumping the water and installing the system. Also, some warn about the possibility of cooling the panel glass too quickly and breaking it.

2. Run water lines underneath the PV panels which would capture some of the heat. The heated water in the lines could be used to preheat water going to the home's water heater.

There are pros and cons in using either system but these are the most practicle solutions I have seen.

Hi Don,

Your answer runs along the lines with my experiment to date. I have installed a soaker hose underneath 20 panels (I have 58). Water to the soaker hose is supplied by a temperature controlled valve that opens when the temperature under the panel rises above 30C and closes when the temperature falls below 30C. Do not know at this time if the cost of water will be off set by increased power production. Not yet sure if the soaker hose is all that effective at cooling, (it is good for watering plants).

I did some measuring of the air temp and panel temp with a min max thermometers. Even though the air temperature here in Toronto has not exceeded 20C this year the max temperature under the panel has exceeded 50C. Hate to think what happens at an air temp of 30C. I do know that my max power production drops by 20% under optimum sun angle on a sunny +20C day.

The cement tile on my roof are never too hot to touch - not even close to it.

1) The tile are light colored

2) The roof is well insulated so it is not holding heat.

I have never measured the backside of my solar thermal panels - tomorrow I will see what it is - nothing to do with PV but would be interesting.

Installing the panels higher, as is always recommended by anyone I know, would help.

Simply painting the roof (at least in the panel area) a light color should help more than the water solutions proposed.

The water should make for a nice green mess on the roof/house.

Hi Russ123

I'm getting some good positive comment but I have to work with what I have for now.

I agree raising the panels to at least six inches above roof level would help - maybe a lot. My installer was supplied with the two inch above roof level brackets by the panel manufacturer - Solgate. Do not know if they considered more airflow. We also get 20-60" of snow in the winter, the higher panel mounting may cause a problem with snow drift build up.

Ashphalt roofs are 99% standard in Canada and are definetly too hot to touch in the summer sun. Tile or cement- slate roofs are prohibitively expensive. Their greater wieght also has structural implications. But intuitively I think that they would be cooler. I could also re roof with white ashphalt or spray the roof white but I'm iffy about that. White reflect the sun rays. The first thing they hit is the panel.

I plan to continue with the water spray experiment and look at raising the panels further off the roof.

The biggest downer on solar PV is that the standard Si panels are only 15% efficient at converting sunlight to electrical power. While tripple junction GaInP or GaAsGe panels convert at 40+% they cost 100X as much. They are used at the Space Station. But when you build a $100 billion home in space a few million for the panels are a tiny expense. Also the heavy government subsidization for the present solar production based in Si wafers is a deterrant to innovation.

Right John - Once installed you live with what you have.

I always make a point of leaving the air gap so others reading can at least be aware of the potential problem.

The tile roof - yes the roof trusses for my house are steel rectangular profiles. Otherwise weight would be a consideration. The tiles do come with a 50 year warranty which is nice.

Your 20'' to 60'' of snow is a LOT! Try this blog - they are also from the frozen north - they maybe they have some useful experience. http://www.greenpowertalk.org/

Russ

Hey John. Having soaker hoses may create a "green mess" as relayed. I don't think a spray would cause that problem. The water would both run off the roof and evaporate. The soaker hose, I think, would tend to hold some moisture in place. The art is in finding the best rate to spray and at what intervals. Please let us know of any further developments. On the Louisiana coast we can't raise the panels over a couple of inches due to the hurricane winds. Good luck