Well that was a mistake! roof heat

[Crowz]

I would think its obvious. You HAD a white roof. You painted it black by adding solar panels. Black roofs get hotter than white roofs.

It used to be a common tactic back in the day to paint car roofs white. Thus red cars with white roofs etc. It actually lowers the temperature pretty dramatically.

 

[McKravitts]

Use spectrophotometer to obtain spectrum falling on roof directly from the sun.
Spectrum falling on roof from backside of a panel.

Spectrum reflected from roof in each case. (ideally a representative white roof ... Oh, you've got one! You're the OP!)

My theory is that the panel does wavelength conversion from visible & UV to IR, and white roof does a poorer job of reflecting that IR.
Spectrum plus intensity for each incident and reflected case should allow calculation of power, and would show if roof does absorb IR.

I played with such an instrument in the short UV range. Had trouble at first getting any signal from the deuterium lamp I was using until I removed the lens on the end of the plastic fiber optic cable. Although they denied it, I thought someone in the lab I borrowed it from must have cooked the lens with a laser. It was basically opaque, according to instrument readings.

Also measure air temperature below panels and over roof without panels as well. That won't say where the heat is coming from, but thermal transfer from hot air could be another theory.

I see shields blocking the gap of some panel installations. Could be for aesthetic reasons, could be to reduce tear-off force of wind. Certainly doesn't help the heat situation.

Aluminum foil reflects both visible and IR with an albedo in the high 90 %
Foil on the bottom of the panel would greatly reduce the hreating of the roof but raise the panel temp and thereby reduce preformance.

If you could see in infrared, you'd see complete blackness at certain wavelengths. These are wavelengths where no atomsheric gases reflect, absorb or emit radiation. A material that radiates in one of these IR bands can actually get cold even in diret sunlight and blocked from convective and conductive heat gain. In essence it is in thermal equlibrium with space.

I've used this method to super cool water when ambient temp was in the low 50's farenheit

Here's an interesting article in ScienceDirect

 

[McKravitts]

I don't understand this part. .

The amount of solar radiation an average solar panel absorbs is more than the average surface of the earth. There are hugh variations in the albedo of the earths surface. Solar panels on an asphalt parking lot would cool the earth, whereas solar panels on a snow covered area would heat the planet.

 

[McKravitts]

Right, so the folks on this thread thinking that these results say solar is a bad idea bc it heats your house need to reevaluate.

I don't think any of us believe that solar is a bad idea.
We are just discussing ways to limit or eliminate the potential heat it could add to your roof.

 

[Starex]

Aluminum foil reflects both visible and IR with an albedo in the high 90 %
Foil on the bottom of the panel would greatly reduce the hreating of the roof but raise the panel temp and thereby reduce preformance.

If you could see in infrared, you'd see complete blackness at certain wavelengths. These are wavelengths where no atomsheric gases reflect, absorb or emit radiation. A material that radiates in one of these IR bands can actually get cold even in diret sunlight and blocked from convective and conductive heat gain. In essence it is in thermal equlibrium with space.

I've used this method to super cool water when ambient temp was in the low 50's farenheit

Here's an interesting article in ScienceDirect

Super cooling water seems like a great school project.
Is it expensive and what exactly is needed?

 

[zanydroid]

I don't think any of us believe that solar is a bad idea.
We are just discussing ways to limit or eliminate the potential heat it could add to your roof.

There was one post back that was inappropriately worried about the issue.

 

[McKravitts]

Super cooling water seems like a great school project.
Is it expensive and what exactly is needed?

Here's a copy and paste I sent to a school teacher a few years ago.
She said she had good results.


What you need:

Styrofoam cooler
Aluminum foil

Crystal clear polyethylene (common food wrap) IR radiation can pass through clear polyethylene with very low loss.

Tape

Remote thermometer. Optional.

Small plastic bowl, preferably black

Spary adhesive.

Procedure:

Using spray adhesive, line the entire inside of the Styrofoam cooler with aluminum foil.

Using a utility knife cut a horizontal line on each side the same distance from the top/bottom.

This cut is to break thermal conduction from the bottom of the cooler to the top.

Place the bowl in the cooler add water and place thermometer probe in the water.

Using tape cover the entire open top with the polyethylene.

Place the cooler with the water in the bowl in an unobstructed location outdoors and lean it towards the north sky on a calm clear night.

Heat in the water and bowl will disapate via IR radiation. The aluminum foils reflects the IR out the top. The Styrofoam blocks conductive heat gain and the clear poly blocks thermal convection.

The thermometer will show the temperature of the water drop and if conditions are favorable it could drop substantially below outdoor ambient temperature and if conditions are good the water temp could drop well below freezing. I have had it get as low as 20 degrees F and still be liquid.

Any slight disturbance to the bowl will cause ice crystals to form instantly.

 

[McKravitts]

There was one post back that was inappropriately worried about the issue.

With the new roofing material available that reflect sun light, people should be aware of the facts.