Thermal heat reduction impact of solar panels on roof vs. shading optimization

[rsilvers]

So maybe this is an argument for not using the lowest-profile mount (like the church photo I posted yesterday).

[SunEagle]

It will all be site specific

higher roofpitch will cause more convection between roof and modules,
most racking has hight adjustments, install a few inches higher will help as well.
effect on the home is also site specific. For example I have energy start shingles which reflect a great deal of the energy, so the PV has less thermal impact than if I didn't have them.
Better insulated homes, better ventilated attics etc, all will decrease the impact of the PV on the home.

The point is that it is a good idea to do these other things before solar as they are usually cheaper than solar and have a larger impact.

Where PV thermal effect on cooling the home would have the greatest impact would be on a home with vaulted ceiling. Insulation is limited, and no attic to ventilate.

I agree.

[ButchDeal]

Cool, please do!

I bet tilted panels on a flat roof would have a larger cooling effect - the hot panels would be well up off the roof, with plenty of ventilation.

It will all be site specific

higher roofpitch will cause more convection between roof and modules,
most racking has hight adjustments, install a few inches higher will help as well.
effect on the home is also site specific. For example I have energy start shingles which reflect a great deal of the energy, so the PV has less thermal impact than if I didn't have them.
Better insulated homes, better ventilated attics etc, all will decrease the impact of the PV on the home.

The point is that it is a good idea to do these other things before solar as they are usually cheaper than solar and have a larger impact.

Where PV thermal effect on cooling the home would have the greatest impact would be on a home with vaulted ceiling. Insulation is limited, and no attic to ventilate.

[DanKegel]

Cool, please do!

I bet tilted panels on a flat roof would have a larger cooling effect - the hot panels would be well up off the roof, with plenty of ventilation.

[SunEagle]

I don't see why the shade from an array would not be a giant benefit. The sun is putting 1000 watts per square meter into your roof. Think of your roof as a solar thermal heater.

I have a FLIR and can do some testing later. I just tested a $30 Duck brand attic stair cover and decided it was not good enough.

I agree that you should see lower temperatures inside a home if you install a pv array on your roof. Although a white roof surface may reflect more heat than what may be absorbed by the black surface of your pv panels.

To make sure the heat the panels absorb does not go into your attic you need to make sure there is enough air circulation between the panels and the roof surface. That will also keep the panels happy.

Also venting the attic and using a thermal barrier near the top inside of the roof frame can lower temperatures inside a home.

[rsilvers]

I don't see why the shade from an array would not be a giant benefit. The sun is putting 1000 watts per square meter into your roof. Think of your roof as a solar thermal heater.

I have a FLIR and can do some testing later. I just tested a $30 Duck brand attic stair cover and decided it was not good enough.

[SunEagle]

[QUOTE=J.P.M.;n315649]

The ceiling is white so the IR sensor isn't really designed for this. QUOTE]

The thermal emissivity of white paint is about the same as most paint colors in the infrared and something of the order of 0.92 -0.95 or so. The color is a perception of the human (and other species) eye. The IR sensor will see it the same way it sees any other color of the same product. If the sensor has an adjustment for emissivity, use it.

Black paint will register and be warmer than some color paint or white paint of the same formulation (except for the pigment) because of its absorptivity in the solar spectrum. If of the same material, and at the same temperature, the color won't make much, if any difference. The IR thermometer will read about the same for either sample. The IR thermometer is "seeing" thermal radiation, not "color" as we know it. The black surface may be warmer than the white one because it's absorptivity to solar radiation is higher than other some color or white.

+1. Good info on IR and emissivity of paint when using an IR camera.

[solarix]

I say - If you live in an area with high roof temperatures, don't put electronics (microinverters or optimizers) up there.

[Willaby]

Well, I just gave JPM a thumps-up-like because he used an emoji, a rare event.

[J.P.M.]

I used my IR temp sensor on the ceiling inside my house the first week of November. My panels are on the vaulted ceiling part of my house. Asphalt shingles and probably R19 insulation then sheetrock. The ceiling is white so the IR sensor isn't really designed for this. Anyway, the ceiling under the array was 2F cooler than the area that wasn't. I measured at the same "height" since my ceiling is hotter the higher one measures. Yes I realize that it is not a true representation since the hot air in the room also heats the ceiling and other heat transfer issues... But, I could outline my panels using this method. I wish I had a FLIR to get better info. If you want I can take readings tomorrow if you are curious. I will probably end up with the same numbers as JPM.

While there is some benefit to the shade provided by an array, it's not the biggest benefit.

My solar magnum opus residence out in Borrego Springs (designed through prelim. plan check but not built) was designed to have two roofs, one on top of the other. The HERS auditor wouldn't sign off on lower roof insulation required from lower roof skin temps, but I left the double roof anyway. I even had a slick way to keep the kangaroo rats and other critters out of the sandwich area. The top section was removable in sections for service.

[J.P.M.]

[QUOTE=FFE;n315644] The ceiling is white so the IR sensor isn't really designed for this. QUOTE]

The thermal emissivity of white paint is about the same as most paint colors in the infrared and something of the order of 0.92 -0.95 or so. The color is a perception of the human (and other species) eye. The IR sensor will see it the same way it sees any other color of the same product. If the sensor has an adjustment for emissivity, use it.

Black paint will register and be warmer than some color paint or white paint of the same formulation (except for the pigment) because of its absorptivity in the solar spectrum. If of the same material, and at the same temperature, the color won't make much, if any difference. The IR thermometer will read about the same for either sample. The IR thermometer is "seeing" thermal radiation, not "color" as we know it. The black surface may be warmer than the white one because it's absorptivity to solar radiation is higher than other some color or white.

[DanKegel]

JPM's helpful when describing how to do something useful, or how solar systems work, as he was here. His data's great.

But there are other data points, e.g.

http://www.sciencedirect.com/science/article/pii/S0038092X11002131

which says
"daytime ceiling temperatures under the PV arrays were up to 2.5 K cooler than under the exposed roof. Heat flux modeling showed a significant reduction in daytime roof heat flux under the PV array. At night the conditions reversed and the ceiling under the PV arrays was warmer than for the exposed roof indicating insulating properties of PV. Simulations showed no benefit (but also no disadvantage) of the PV covered roof for the annual heating load, but a 5.9 kWh m−2 (or 38%) reduction in annual cooling load. "

Another paper (can't find it now) says the effect varies strongly with height above the roof, something about convection.

Anyway, it's complicated. You might want to read up on Cool Roofs in general. Good luck!

[FFE]

I used my IR temp sensor on the ceiling inside my house the first week of November. My panels are on the vaulted ceiling part of my house. Asphalt shingles and probably R19 insulation then sheetrock. The ceiling is white so the IR sensor isn't really designed for this. Anyway, the ceiling under the array was 2F cooler than the area that wasn't. I measured at the same "height" since my ceiling is hotter the higher one measures. Yes I realize that it is not a true representation since the hot air in the room also heats the ceiling and other heat transfer issues... But, I could outline my panels using this method. I wish I had a FLIR to get better info. If you want I can take readings tomorrow if you are curious. I will probably end up with the same numbers as JPM.

[Mike90250]

When designing the system you must allow for voltage droop when panels are hot.

Many schemes have been tried to cool panels, but they consume more power than is recovered.

[J.P.M.]

You guys are so good at this that I kind of wish someone like JPM could do my panels

I'm retired. Ask Dan, Willaby and 55ncs what they think of my opinions.