Effects on roof lifespan and cooling load.
Solar panels are a highly effective way to produce electricity on your property. Rooftop solar panels have no moving parts and don't take up any space. Besides the electricity they produce, solar panels have two secondary consequences rarely discussed. These consequences are beneficial, especially in the summer and in warm climates.
Roofs Last Longer
One of the most expensive maintenance requirements for a house is replacing the roof. There are many different types of roofing materials, and pretty much all of them fail over time due to three factors: UV degradation, heat, and storm damage.
Roofing is exposed to high ultraviolet radiation from the sun. UV rays can damage roofs physically and chemically. On a physical level, UV rays cause the shingles to dry out, crack, and become brittle over time. Chemical changes occur when UV exposure breaks down asphalt and other molecules, causing them to lose their protective properties.
All the radiation from the sun, UV, visible light, and infrared hits the roof, where much energy is absorbed. This causes the roof to heat up, often to extreme temperatures in the summer. These excessive temperatures can cause thermal stress on the roofing materials by causing them to expand and contract as they heat up and cool down. This can cause cracking, warping, and deterioration. All roofing types, including metal, are susceptible to this thermal degradation. While metal usually handles temperature swings well, the high coefficient of thermal expansion of metal can cause caulking around flashing details to separate and allow water intrusion. The heat can dry out oils in composite shingles, causing them to degrade and lose their waterproofing.
Storms are the other major cause of roof damage. High winds, hail, and debris can all damage roofing materials.
Rooftop solar panels effectively shade the roof from the sun, keeping the roofing material cooler and blocking its exposure to UV radiation. By blocking this UV radiation and keeping the roofing material temperature lower, roofs will last longer. Of course, this is only effective on the parts of the roof covered by solar panels. But solar panels are best located on the parts of the roof that receive the most sun exposure. The solar energy that we want to capture with photovoltaic panels is the same solar energy that degrades roofing. South-facing solar panels will protect the south-facing roof. It's well known amongst roofers that south-facing roofs wear out first.
As for storm damage, solar panels often have very good durability ratings against hail and wind. They effectively shield the roof underneath them from the brunt of wind, hail, and flying debris in a storm.
Houses Stay Cooler
Since solar panels block the sun's energy from hitting the roof and keep the roof cooler, they keep the house cooler in the summer. This reduces the amount of electricity required to cool the house. And if the air handler and ducts are in the attic, solar panels will have even more of a cooling benefit.
Solar panels shade the roof, but the radiation reaching the roof under the panels will still be much more than at night. The panels themselves heat up to hotter than the outdoor air, which causes them to radiate some of that heat away in the form of infrared radiation, some of which is radiated into the roof. The rest of the solar panels' heat is transferred into the outside air through convection.
We can observe this effect by measuring the temperature of the concrete roof tiles underneath the solar panels and confirming that it's hotter than the ambient air. In the early afternoon, when the outdoor air temperature was 105°F, the temperature of the roof tiles in the sun was 162°F and only 130°F underneath the panels.
Now, we must figure out if that roof tile temperature difference translates into different heat fluxes through the roof deck since some thermal energy is radiated away to the outside, and the outdoor air takes some away through convection.
Looking at the temperatures of the bottom of the roof deck from the attic, it's apparent that it's much cooler underneath the solar panels: 126°F vs 141°F. This temperature difference is the net result of many thermal energy interactions between the sun's energy, the roofing materials, and the outside air. Now, we can use this temperature difference on the bottom of the roof sheathing to determine approximately how much thermal energy the solar panels keep out of the attic.
The reason the attic gets hotter than the outside air temperature is because this solar energy heats the roof. When there is a temperature gradient, thermal energy will always flow from higher to lower temperature. The roof's higher temperature causes thermal energy to flow into the attic. After the heat has flowed through the roofing material and roof deck by conduction, convection (heat transfer through fluid movement) and radiation (heat transfer by electromagnetic waves) are the methods by which the heat is transferred into the attic.
The rate of convective heat transfer is given by:
The rate of net radiative heat transfer is given by:
where:
Qnet/t is the net rate of heat transfer by radiation (W)
ϵ is the emissivity of the radiating surface (dimensionless, ranges between 0 and 1)
σ is the Stefan-Boltzmann constant (5.67×10^−8 W/m²·K⁴)
A is the area of the radiating surface (m²)
T2 is the absolute temperature of the radiating surface (K)
T1 is the absolute temperature of the receiving surface (K)
Notice this 4th power dependence on temperature.
I logged the temperatures of various parts of the attic to get two full summer days of data. These days were both partially cloudy, with heavier clouds in the afternoon. I measured the temperatures of the bottom of the roof deck in the sun and under the solar panels, one inch away from the bottom of the roof deck in the sun, the attic air, the outdoor air, the return ducts, and the supply ducts.
To figure out the amount of heat transferred into the attic from the roof deck, we can't just take the average temperatures over a day since this would not yield the correct result. We must calculate the rate of heat transfer by all means for each interval measured; then we can see the amount of heat transferred over a day. I'll leave out the extensive math and present the resulting graphs.
This graph reveals that the bottom of the roof deck under the solar panels is much cooler than the bottom of the deck in full sun. The orange line is the temperature of the air one inch away from the bottom of the roof deck in full sun. This is included to verify that the attic is heating up because of the heat from the bottom of the roof deck.
This graph shows the temperature differences (∆T) between the sunny and solar panel-shaded parts of the roof compared to the attic air and outside air. We can see that the roof deck underneath the panels is about the same temperature as the attic air (light blue line), whereas the roof deck in the sun (orange line) gets much hotter.
The graph below shows the heat transferred into the attic in watts per square meter by both convection and radiation from the roof in full sun and the roof shaded by solar panels. Interestingly, the roof deck under the solar panels stays warmer at night. This is because the panels are blocking the radiative sky-cooling effect.
The average combined heat transfer into the attic from the roof deck in the sun is 1169 watt-hours per square meter per day (which would be 283 kWh/day for the whole roof). It's only half of that for the roof deck under the solar panels, at 583 watt-hours per square meter per day (which would be 141 kWh/day for the whole roof).
Of course, not all of that heat transferred into the attic is transferred into the house. Insulation blocks much of the heat from being transferred into the living space, and attic vents allow the hot attic air to escape. (Critical to these calculations is the mass flow rate of air through the attic, which I've calculated to be an average of 2477 g/s over several days.)
To find the heat that makes it into the house's living space, we need to calculate the conductive heat transfer from the hot attic into the cooler living space across the insulated ceiling and ducts. The ducts contain conditioned air. The graph below shows the amount of heat that would be transferred into the conditioned space if there were no solar panels compared to if the whole roof were covered with solar panels. The spikes are where the air conditioning is running since there will be more heat transfer into the ducts when the air in the ducts is colder relative to the hot attic.
With no solar panels on the roof, 6.026 kWh per day would be transferred into the conditioned space via the attic.
If the whole roof were covered in solar panels, 4.994 kWh per day would be transferred into the conditioned space via the attic.
These are likely underestimates based on assuming perfectly consistent thermal insulation and air sealing.
Putting this all together, if the whole roof were covered in solar panels, 141 kWh per day would be kept out of the attic, and 1.032 kWh per day would be kept out of the conditioned space. That's thermal energy that wouldn't need to be removed from the living space by the air conditioning system. Of course, the whole roof is not covered by solar panels. In this case, only about 23% of the roof is, which means 232 watts per day are being kept out of the house.
That's not a huge difference, but over a summer month, it would reduce the house's cooling load by 7 kWh.
Not only do solar panels produce a significant amount of electricity on your rooftop from the sun, they make roofs last longer and reduce the house or building's cooling load.
Questions for you:
How does this change how you view solar panels?
Are there any other secondary consequences of rooftop solar panels?
How can this knowledge be used to design better roofs?
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