Solar Panels and Temperature

I finally have been getting some good sunny days, unfortunately the solar panels haven’t produced as much as I thought they would because I didn’t factor in temperature.
The panels are rated at 190 watts at 77F and they drop off 0.32% for every degree F over 77F.

The temperature today was in the low 70s, but my roof shingles were 140F and the solar panels were at 100F ( according to my infra red thermometer )
I hosed down every panel in one of the strings until the nearest one was down to about 77F. I did this twice ( I was really thorough the first time, and did a little less the 2nd time. )
Here is the chart:

See those two spikes in the blue line? That is the increased production that came from cooling the panels down. It was about a 10% increase in output. The specifications say that it should be about a 7.5% improvement, but I was getting 10%
Notice that it takes about 40 minutes for the panels to heat back up and the output to return to the curve it was on.
( The “red” string produces slightly better – I don’t know why. It also is shaded for slightly longer in the morning, which is why its production starts later )

I think that if I could cool the solar panels down to ambient air temperature I would get about 10% more total output for the day.


4 responses to “Solar Panels and Temperature

  1. Some tangential thoughts (sorry, I have trouble staying on-topic 🙂 ):

    • The panels getting hot mean solar energy that’s being wasted, turned into heat instead of electricity. I’m looking forward to the future of PV where panels are 90+% efficient (okay, I probably won’t live long enough to see that day…so sue me 🙂 ), and the temp rise is vastly reduced because the absorbed energy is all getting used to shove electrons around instead of cooking your panels. (Useful lifetime of the panels would probably be much improved too).

    • That difference between the two sets of panels is pretty huge! I find myself wondering if solar panels actually have that kind of variation typically, or if there’s something fixable going on. I suspect the latter; since a whole set of panels actually is a large number of smaller components, individual differences in efficiency ought to average out. And if it’s the latter, maybe there’s a 10% improvement to be had if you can just track down what’s holding back that lower-performing set of panels.

    In the meantime, I suppose it’s worth considering your idea of combining a hot water system with PV, where the hot water system acts as a heat sink, carrying away excess energy that would otherwise reduce the efficiency of the PV system, to be used to reduce energy usages elsewhere in the home (hot water systems, home heating).

    • highspeedcharging

      I called Puget Sound Solar and they took a look at the system. There was a problem with the wiring and they fixed it – so now both strings produce about the same amount. I’ll post new data the next time we have a sunny day.

  2. Hi Rich,

    You are observing the phenomena that occasionally causes some of our customers to call us and complain that their system is under-performing. Hot electrons jostling each other with sharp elbows slows the current flow and the voltage drops. Summer temps on the cell surface are around 135-140F but module wattages are rated at 75. I have done the same hose-down on our garage array (because I can walk up to it) just to watch the wattage spike. You are observing one of the reasons that, although we get just 70% of the sun that southern Cal gets, PV systems here produce about 80% of what they do there. Think PV in Florida should produce lots of kWh? Actually, heat+clouds reduces production there to about what you get in Oregon. Our wattage peaks are in April and May; at noonish on a sunny April day, we see just shy of 6kW from our 6.2kW array. Same time and sun in July and it’s down to 4.8kW. There is one or maybe two commercially available solar modules that collect heat from the back of a PV laminate to preheat water. It is, as you guessed, a one-two punch, extracting nice efficiencies overall.

    Because the temps are low, one could use a vacuum-formed polypropylene labyrinth on the back of a PV module’s backsheet to collect heat and cool the module with water and glycol heat transfer fluid, just have to figure out how to bond it to the PV backsheet. It is a PV accessory just waiting to be developed.

  3. My wife and I are recent LEAF owners in North Ballard Seattle and will be adding a PV panel array to our roof in the next few months. Being a tech geek from the late 1970’s assembly coding days I just can’t help over researching, engineering and optimizing our system. One goal is to test a roof mounted single-axis panel tracking system with each panel individually pointed toward the sun rather than grouped into a larger matrix array on traditionally ground mounted trackers. I know single-axis tracking might increase energy harvesting by 25% – 35% if polar mounted (rotation axis aligned north-south, then twist panel around axis), but to reduce wind loads I am trying the less conventional approach of turning (yaw) the upright panel eastward then south then west. Does anyone know how this might change the harvesting efficiency? NREL annual output insolation calculators use the standard polar axis rotation scheme for their single-axis models, not my yaw-the-panel method.

    Thank you for the information regarding PV panel de-rating due to thermal conditions. I knew the 1000w “Standard Test Condition” was at 77-F (25-C) and PV output fell off at about the rate you mentioned for every degree higher, but to see it soaking up 10% to 15% of output potential was eye opening. I have ideas for a hybrid PV panel and solar hot air collector which hopefully may be simpler than heating water by direct thermal contact with the back of the PV panel.

    Talking with two solar PV installers at the recent Shoreline WA SolarFest trade fair I learned different types of inter-wafer wiring on panels can drastically effect shading issues. Partial shading of Silicon Energy PV panels produced in Arlington WA greatly reduces their output, in greater proportion than say a poly crystalline Solar World PV panel. I have no direct knowledge of this, however one fellow I spoke with at SolarFest cited his own experience and seemed pretty convincing. It strikes me as similar to the shading problem described in string inverter systems when shading one panel in the string effects the output of all. Some panels use multiple connection pathway grids wafer to wafer, while Silicon Energy wafers are wired in a less costly (?) series manner. Can anyone verify this supposition?

    Another item, solar PV installers also tell me that in the NW our more frequent defuse light conditions favor poly crystalline PV panels over the slightly more efficient in direct sunlight mono panels. One installer claimed the difference in annual harvesting was as high as 15% – 20% in his experience. On hazy days the differential was 30% but the total for the day was certainly not a lot to begin with, so even if poly panels do very well on diffuse light days, and mono panels have a slight edge on direct sun days the annual harvest differential might be fairly complex to estimate.

    Thanks again for the useful information on your site.

    — mark early

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