Is there any hard data out there or any personal experience about reduced cooling load as a result of solar panels installed on a south facing roof? So far I have found 1 study showing a 5 degree roof temperature difference on 1 roof over 3 days...

In my mind you would be able to see a marked difference in summer heat gain from an exposed roof compared with a roof with solar panels.. The home I currently am investigating is in New England, has a dark colored shingles and I'm looking in particular into a finished attic space with a directly south facing roof plane with a low pitch. The room below has a cathedral ceiling and it appears to have R-19 below the roof. We are designing to use about 58% of the area. The panels will be offset 4 inches from the roof, so I'm assuming that they will provide more shade from the sun's radiation than the heat that they will radiate into the roof.

So am I right to say that this room below will stay cooler, would there be a way to put a number to that? And therefore reduce the amount of air conditioning power draw and bring that many kilowatt-hours to using net-zero annual energy.

Tags: j, manual, modeling, net-zero, solar

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fyi, there is moss growing under the bottom edge of the roof under my panels in Maine also.

re: hard data, I recall seeing at least one study that showed some cooling from PV shade on roof.  Suggest you contact David Sailor's groups at AZ State U. re: their  project:

Impacts of Photovoltaic panels on urban climates and building energy use.

https://sustainability.asu.edu/urban-climate/dr-sailors-research-gr...

(their study of PV panels on pedestrian shade found increased radiant temperatures underneath; they are looking at other roof cooling strategies too)

I have not reviewed all the previous Replies, but will throw in a few Comments/Caveats:

1) extremely high roof top temperatures can degrade the pefromance of PV systems.  Manufacturers have correction factors for this effect.  Climate change is expected to exacerbate this problem.  Do you know if building/system designers are including such correction factors ?

2) measures that cool roofs can also increase heating costs.  For example, a "cool roof" was a wash when I used the LBNL Home E Saver online model for my retrofitted ranch style home in Davis, CA.  It is a mixed heating and cooling climate. Our house has a lot of passive cooling features, so heating (with gas furnace) dominates our eneryg costs and carbon emissions.

3) Major fire storms in N. CA over the last 2 years have had big regional effects, including the coating of PV panels with ash.  Even here in Davis, well downwind of the fires but with sustained smoke haze and heavy ash initially, locals reported seeing big drops in their PV output. I think cleaning the panels helped restore the output.

4) re: net, low, or positive energy buildings: As others have commented, passive energy measures are the first choice for various reasons.  One reason to add to the list is that PV systems and the applialnces and systems they run still generate a lot of waste heat, which contributes to urban heat island impacts on energy demand, outdoor air quality, health, etc.

I had noted, and show with graphs (I had posted) that there was some cooling because of the shading of panels.  But it was small compared to the reduction as a result of insulation - more insulation and not just code minimum.

The original posters was from the northeast,  Maine if I remember.  The daytime full sunlight temps are not likely enough to seriously degrade the performance of the panels.  It's not the same kind of climate that you would see in Arizona.

I think it would be hard to prove that PV systems add significant waste heat that adds to the urban heating problem.  The "black" look of PV modules does not mean that they are getting hot and re-radiating.  Even on the hotest days I can put my hand onto a "black" PV module and hold it there without burning... but I would not do the same on a black paved street in California or Arizona.   Heat from houses and appliances will radiate nicely into the night sky - provided insulating heat domes are not created by localized green house gases.  Dense populations tend to localize GHG faster than the can redistribute into a broader area.   Human breath includes CO2 and VOC's  as do the vehicles we drive,  as do the restaurants, hotels (pools/spas), schools and hospitals.

The electrical distribution system - lower voltage wires, transformers on poles and ground mounted ALL result in heat in the nearby environment (at almost ground level).  PV systems can reduce the loads on the local distribution system during the daytime hours resulting in less heat by those systems.   When you look at the impact of the systems of systems... PV is likely to be a very small impact compared to the heat accumulation as a result of the dome created indirectly from the human activities which cause localized GHG.

HI Dennis;

good point re: PV benefit of load shifting re: waste heat.

I was referring to the whole cooling system, which is removing heat from a building.  If the building shell and cooling system are not very efficient, then PV systems are still pumping a lot of  heat into the local area.  When I looked at the literature for urban heat sources, the major sources appeared to be building heating & cooling, combustion engines for vehicles etc., and industrial sources.

More efficient building shells allow smaller PVs, heat pumps, and batteries. Granted, PVs and heat pumps can be more efficient than other systems, but the biggest efficiency gains are in the shell, especially in existing buildings.

Recently, Baniassadi et al. (ASU and UCLA) modeled roof cooling measures in coastal and inland LA homes.  They  found that the direct cooling effects on the building energy and thermal comfort and the indirect cooling effects via neighborhood / ambient environment were of a similar magnitude. They also reviewed studies on effects of cool, green, and PV shaded roofs. Their sensitivity modeling results suggested that the energy and thermal comfort improvements were mainly determined by the level of attic insulation and air exchange rate.

(Direct and indirect effects of high-albedo roofs on energy consumption and thermal comfort of residential buildings.September 2018, Energy and Buildings 178. DOI: 10.1016/j.enbuild.2018.08.048.

re: regional differences, even NE states are predicted to have large increases in cooling demand by mid and late century.  See the recent National Climate Assessment and Stanford estimates in my Slide #2 at https://aceee.org/sites/default/files/pdf/conferences/chee/2018/5c-....  And those are probably underestimates for the NE because they don't factor in increased summer humidity.

We have a radiant barrier that stops roof heat in summer and opens and lets roof heat radiate onto the ceiling and attic ducts on sunny days in winter. Automatically, with no electronics and cheaply. We want to get it made in the USA.

It solves this and other problems, like solar radiation thru windows.... "2) measures that cool roofs can also increase heating costs."

Radiant barriers - especially paints and foils... have been tested many times by various labs and shown to be less than cost effective.  Then simply good old fashion insulation and air sealing.

As for new special radiant materials.... indeed researchers at numerous universities including the folks at University of Colorado (Boulder I believe) have demonstrated a nano material that can effectively radiate heat into the sky during both daytime and nighttime.   Such materials themselves do not require electronics and they could be made cheaply... but to be effective you need systems to move the heat from the building to the exterior radiant panels.

Hopefully your product is based on the new research and not that of the older materials, paints and foils that had already been demonstrated not to be cost effective.

I agree. I want to buy the nano material that radiates heat into space and there is a professor who came up with photocatalytic paint that prevents mold and moss on white roofs. I would like a white product that can be painted onto asphalt shingles to extend their life and reflect heat. I don't think it exists.

Our patented invention can also block hot afternoon sun from coming in windows or coming into glycol solar hot water collectors when people go away on vacation. Automatically. Almost nothing made in USA any more.

Photocatalytic materials often have titanium oxide or other substances in them as part of the trigger to break down materials when hit with the UV in the sunlight.  Adding to paint - is generally an ineffective ensure the proper application of the material.  as needed on the roof.

You're moving the thread further off topic from Tims original query three years ago.

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