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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Yes it can provide shade & you have a nice ventilation chamber under it - with that depending on the panels you can have a lot of heat radiating through... As for how much & will it offset I don't think there is a study out there on that & even if so I would say your results will vary (panels, wind, sun, angle). As for practicality, yes it helps reduce loads, but you still need to address any other issues like air sealing, possibly the insulation, etc.. and maybe even considering a cool roof  to help reflect the heat back away.

As for net-zero - which one? Calcs, you can always try it with Manual J & shaded roof - as for reality, well you will only really know with time & measuring

Sean, 

Thanks for the thoughts! As for net-zero; I am generally considering electricity here because clients I see are all solar customers, and I am working for the solar company; so the clients are much happier if I can give them some solid kilo-watt hour reduction. (side-note: CT Leasing regulations do not allow solar arrays to be designed to accommodate 100% of the annual load)

Up to this point, I've really only worked with heat loss; Hartford CT only has 300 Cooling Degree Days! But seeing the monthly usage spike in the summer months leads me to believe there is some good opportunity here. Just thinking out loud here, but if they have that big of a spike with only this many cooling days, they probably have more of a humidity problem than a hot roof problem..

We have PV panels on our roof,  we tracked the panel temps, microinverter temps,  and attic temps (lots of sensors) for a couple of years.  We did see a definite reduction in attic temps under the panels. Our roof has a low pitch, the north side without PV modules was actually hotter than the southern side!  Five degrees F is reasonable.  But it is also important that the installation has space for air movement under them.   PV integrated as roofing tiles  would not lower temps.   PV mounted 1" above the roof may have less impact.   We have about a six inch space between our panels and the roof. Sufficient for easy cleaning under the panels and good air circulation.

My wife (an engineer also) tracks the data... I'll see if she can provide charts of what we saw a few years ago....

Yes, cooling effect occurs with south facing solar roof top panels.  I am seeing a 5 degree farenheit decrease from late May to early August here in the PNW.

5 degrees less on the roof beneath? Great!

Actually inside the home.

I should add; not using micro invert type panels

I just got back from the NABCEP conference in San Diego.  John Schehl from RISE, indicated there is heat build up that occurs under PV modules.  The heat build-up is not a direct function of UV, rather heat build up from module heat absorption that is transferred by radiation to the roofing materials.  I have a 5 kW PV system on my comp roof.  I plan on using a thermal imaging camera this summer to measure the underside of the OSB roof sheathing both in the array area and the exposed roofing areas.

Agree that shading does work to reduce heat-load nice stats from Dennis, now consider in winter using the panels to heat air by mounting them with 3/4" air-sealed gap ducted to the system to collect it, same system cools in summers storing at night.

While for panels on frames angled to the roof you supply the box & ducting more of a problem, on a good solar section of roof I priced out the roofing costs for metal roofing [for catching rain, some snow load], then how much to box them up from the roof sheathing & it wasn't a factor, it cost about 1/4th more to set up the collector when you don't buy metal roofing for that area.

There are solar panels with thermal built-in that works with a heat-pump, either way to best leverage this as a system requires a thermal-mass to store the therms. [I've posted about a crawl-space thermal-mass that plugs into std rectangular ducting for this.]

Tom, I don'the see any reason why we shouldn't harvest the heat beneath the panels; I would be worried about the roof top temperature becoming too high in the summer and frying the micro inverters (terminals are rated at 75 C) if there was no airflow.
If the heat pump could operate in the summer too and just transfer that heat into a storage tank or a swimming pool that would work out.
Sidenote: has anyone ever put an air source heat pump on a roof?

Sidenote first, the thermal units are a commercial brand sold with the pump, they make custom panels used in a proprietary system, it's pricey yet payback is pretty fast & afaik they can adapt theirs to existing brands.

Yup in summer my sheetmetal & plywood to leverage existing panels or install to a good roof use blowers to vent heat to keep the cells from overheating & uses them for cooling at night to store in a thermal-mass, an active system fans+controls.

It's a giant hunk of AC taken care of done properly, data logging can fine tune to off-grid for many homes balancing collecting & storage seasonally.

Great idea to heat a pool in summer, like it a lot makes perfect sense all levels, reduces evaporation & most people want them warmer in hot weather. For that consider to use air ducting to a heat-exchanger as near to the water heater for the pool as practical, then, air-heat to fluid exchangers work ok in reverse for this good recyclers save them ... data loggers are key, humidity & temp same price most brands now.

I have been told that MI's will not last as long as current warranties guarantee because of heat.

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