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.
I don't think anyone can dispute the fact that the roof would be cooler, although putting a number on it would be difficult.
As far as using it to lower the cooling load and therefore the KWH, in my opinion would be risky and unwarranted. You may just want to say that the roof has a lighter color when entering it into your calculations, although I don't know how accurate it would be, or if it would make a big enough difference to matter.
Any type of shading will help greatly. One of the biggest, if not the biggest, factor for high summertime attic temperature is the type of roofing material used. The most commonly installed roofing is asphalt shingles, which when exposed to the sun, reach temperatures here in California up to 180 degrees in the summer. They transfer their heat to attics and heat them up. When they are shaded they only heat up to ambient temperatures.
Asphalt shingles and composite shingles are probably the worst roofing material because of the asphalt. Asphalt loves absorbing heat from the sun and having burned my hands more than once climbing on roofs without gloves I can say they are also dangerous. I did a study a while back and asphalt and composite shingles consistently heated up attics to 160 - 170 degrees in the summer, which creates a pretty big driving force and the need for adequately insulated ceilings. Shading dropped temperatures significantly even when only a portion of an roof was shaded.
It is interesting that shingle warranties require roofs to have ventilation up to code for the manufacturer to uphold the warrantee; however, it is the shingle and asphalt impregnated felt that is creating all the heat, which our study found was not mitigated by attic ventilation. Attic temps get just as hot with ventilation as they do without and premature shingle failure happens because of the shingle temperature not attic temperature.
Shade is good!
I would assume two things about this idea, first of all you would definitely shade the roof and reduce its solar heat gain by this method of shading, and since the panels have a space of 4" from the roof, this will allow air currents to exist behind the panel that when it heats up will cause hot air convention currents to move air in this space and further provide surface cooling of the roof. I would expect then that the roof would run cooler than it would without the panels. I understand that film type semiconductor panels perform better in heat than other types of PV panels and so would be best for roof use, you might want to look this up and study it.
I had missed your query originally made a couple of years ago. Yes it does make a difference on the temperatures under the roofing sheathing. I know because I had measured that difference for about eighteen months. Both the south side IN THE ATTIC under the solar panels AND on the north side of the roof IN THE ATTIC. I'll see if I can find some of the daily charts that show the impact.
When I collected the data, it was done with multiple points in the attic... some sensors attached to truss right under the plywood. Then in a stack that went down to above the insulation and right against the ceiling sheet rock.
The key point is that the panels make a difference... extra insulation in the attic makes a bigger difference. Also we deliberately set the panels about six inches above the roof line... such that we had air movement below the panels... with the shading on the roof. That becomes important because in the evenings -- during the summer time you need the roof to radiate the heat from the attic back into the night time sky.
With a gap between the roof and the panels it likely means that conduction and convection are the two main pathways to cool the attic.
Also I'm in the Seattle area, I went with nearly a R-40+ cellulose pack in the attic. In my mind an R-19 would not do much to move the heat flow from the hot attic back into the house.
Here come the batches of charts. The data was collected almost nine years ago.. I've included a weather capture (roof top weather station), chart from sensors in the attic, and a chart that shows the temperature and power output of an Enphase inverter mounted on the underside of a PV module on the roof. That inverter temp is when it's operating... and its just behind the PV module so you can pretty much set that as the absolute upper temper limit for the highest temperature under a module. You can also expect the roof temp to be lower.
Where are the solar panels? Are they above the south roof or above the west roof?
Is the south roof cooler than the north roof because of the solar panels? In May there probably is not a lot of sun on the North roof ? Thanks.
Better description of the temps in the attic under solar panels.
The house faces SSW, with the long axis east/west. About 25 degrees off due south. The panels are on the south side of the roof. This roof is a LOW pitch torch down! E.g. roof membrane has good contact with the plywood subsurace! The sensors descriptions can be seen a little better if you click on the image. Otherwise the "key" for the lines is:
#10 South Side roof, sensor attached to roof plywood in attic - four feet from peak
#11 Underside of plywood below panels on west end of the house - about eight feet further down roof from the peak
#13 North side of the roof, sensor attached to roof plywood in attic four feet from peak
#12 North side of house, under attic insulation above ceiling sheet rock.
#7 west side of house under attic insulation against sheet rock (windows open for cooling at night)
#8 west side above attic insulaton
#9 west side below radiant barrier....
Note I used a radiant barrier in the attic - not because I expected it to work... Not much difference really.. not worth the cost. But because the roof has a low pitch - ineeded a means to keep the air channel under the roofing plywood free of the insulation when it was blown in. I refused to allow cardboard impregnated with "fire retardant wax" to be used as barriers. Instead I used a radiant foil to create the air channel from the outer wall up about three feet from the ridge beam. The roof is well ventilated along the ridge beam, but during the hottest time of summer there is a limited spill over of the hot air from each of the "roof channels" into the attic.
I actually have twenty sensors attached in the roof area... more detailed north/south and stacking of the temperature layers in the attic. But the these charts can show... that shading from the solar panels do lower the plywood temps... but as for the temperature in the living space... its insulation. And that radiant foil... well it doesn't do much for keeping the house cool... but it is a far better choice than layering the whole underside of the roof with "waxed cardboard"...that always seemed like a fire waiting to happen.
While the panels do provide some shade and cooling... by time the evening comes... the attic temps even out. The cooling benefit isn't as strong.... and I live in the NW.... I have to deal with moss wanting to grow on the roof under the panel... because its a shaded location!
I would argue that keeping the solar panels cool is more important as you will produce more power. Adding heat adds more resistance on the electrical system decreasing efficiency. I would go for a larger air gap. A study was just done in Des Moines, Iowa that found because of the amount of heating days in the area and it takes more energy to heat than cool, it is arguable that having a dark roof is more beneficial to the overall energy use of a building. Also, helps all the roofing materials stick better w/ less failures. I know, I didn't believe it either at first. But, the delta T's were not as high as you imagine around 15° max. This was a study on multi story flat roof buildings. I know Joe Listenbruck and Allison Bailes have both done research on this in vented attics with I believe similar results. That being said a light colored roof would work better for solar. If you have the opportunity add cont. insulation to the roof. That will trump all. As you are in New England I would look into the pay off between heat gain in the winter compared to heat resistance in the summer.
PV modules are "current" devices. The temperature doesn't alter the resistance, instead it increases the recombination of electrons with the "holes" reducing the electric current that is able to flow out of the module. It is the recombination that reduces efficiency of the photon to electron production.
For roof top PV temperatures - the designers have to consider the WINTER temperatures not the summer temperatures - because the colder temperature - reduces the recombination... which can result in higher current output. If the system designers are not careful to allow for the higher winter time currents - they can produce an array that under the right circumstances can cause the fuses on the PV modules to blow... or the leading that is used to connect cells within the modules to fail. In most cases its not a big problem because winter time PV production is often lower just because the solar insolation also drops. It can be a problem at high altitude (NM, AZ, CO) with a good clear day. But in New England its not likely a problem.
Attached are three files that show the voltage vs temp of an inverter under a PV panel in Seattle and the current vs the temp, temps vs power.
The change in power produced is dependent on the solar insolation. The voltage drops a little because of the MPPT the internal resistance of the module - related - largely to the interconnects. I've tracked modules with dedicated sensors on them... as I mentioned for most of the areas in the northern states... temps are not a problem unless a designer tried to squeeze extra modules into a string to compensate for lower winter insolation. Not a design problem with microinverters.
"Excessive heat can significantly reduce the output of a PV system. This article gives some pointers on avoiding unexpected energy loss in an array.
It may seem counter-intuitive, but solar panel efficiency is affected negatively by temperature increases. Photovoltaic modules are tested at a temperature of 25 degrees C (STC) – about 77 degrees F., and depending on their installed location, heat can reduce output efficiency by 10-25%. As the temperature of the solar panel increases, its output current increases exponentially, while the voltage output is reduced linearly. In fact, the voltage reduction is so predictable, that it can be used to accurately measure temperature.
As a result, heat can severely reduce the solar panel’s production of power. In the built environment, there are a number of ways to deal with this phenomenon.
Different module designs and different semiconductor compounds all react to temperature – here’s a brief intro into what to expect."