Solar panels, basic question - Page 1
Forum section: electronics / beginners

author: honeybadger
author_rank: Contributor
timestamp: January 06, 2020, 03:46:51 pm
content: Hi,I had a thought about solar panels and what happens with the light energy when they are not in use.There is some amount of energy per surface area from the Sun.When it strikes the solar cell - some of it is reflected - some of it is absorbed as a heat energy -the rest is converted to the electricity.So when the solar cell is producing an electric current it gets heated by the absorbed energy and by the ohmic losses (simplified).Now unplug the solar cell.What happens with the "electric" part of the incoming energy?I guess there are only two options:1. The "electric" part is reflected - which means the cell becomes more reflective when unloaded - I don't think so.2. The "electric" part is absorbed as additional heat - which means the cell will be crazy hot. Because of the energy sum it will be hotter than fully loaded cell (short circuit with maximum ohmic loses).I think the second option is the only possibility?

author: David Hess
author_rank: Super Contributor
timestamp: January 06, 2020, 05:17:09 pm
content: With no load attached, the P-N junction becomes forward biased and conducts like any other diode which is what limits the open circuit output voltage. Under these conditions, power dissipation in the cell increases by the cell efficiency so perhaps 20%.

author: honeybadger
author_rank: Contributor
timestamp: January 06, 2020, 11:39:04 pm
content: Thanks, that makes sense.

author: Rick Law
author_rank: Super Contributor
timestamp: January 08, 2020, 04:07:16 am
content: Quote from: honeybadger on January 06, 2020, 03:46:51 pmEven if the electricity continues to be generated, physics "almost" dictates it will be no more heat than "normal" defining normal as the heat that is generated by yourtypical black-tar roof tileswithout solar panels on top.("almost" because black-tar tile is not a perfect "black body", it does reflect some light)The electrical energy the solar panel outputs is converted from the energy of the light falling on it. Black-tar tile is rather "light absorbing." It doesn't reflect much. So black-tar tiles pretty much convert all the light to heat. Even if the solar panel converts that energy into electrical to drive a heater, itwould not be more energy than what fell onto itas light so the electrical heat energy would not be more than a direct black-tile converted heat. Itcould bemore concentrated (say electrical energy driving a high temperature coil heater coming out of a small nozzle), but total energy would be at best the same amount as the energy carried by the light coming in. When the waste-energy is spread and dissipate over theentire areaof the panel, you have at worst (hottest) same as black roof tiles.Now, if you used to have white tile on your roof a lot more energy was reflected away, it would be cooler than black roof tiles. So un-plugged solar panel under the sun would be hotter than the white tiled roof, but not much hotter than the black tiled roof if hotter at all.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: January 08, 2020, 04:22:05 am
content: Semiconductors are photosensitive and in return photoemitting. As @David Hess noted that PN junction turns open when unloaded. It may mean that panel is radiating back at some infrared or visible green.So perhaps the unloaded panel should look different to bystander than when it is loaded.If color change is in visible band, then it is possible to send subconscious pixelated messages to unsuspecting public staring at hills and roofs covered with panels.

author: Rick Law
author_rank: Super Contributor
timestamp: January 08, 2020, 04:27:32 am
content: Quote from: unitedatoms on January 08, 2020, 04:22:05 am"subconscious pixelated messages" !? HMMM... But may be that is why I often wanted French Fries after driving by an unplugged solar panels... Subconscious Google Ads, good for tummy but bad for ticker.

author: CRCLARKE
author_rank: Contributor
timestamp: January 08, 2020, 05:55:48 am
content: Quote from: Rick Law on January 08, 2020, 04:27:32 amsounds like someone's been using too much fluoride toothpaste man.......i once heard somewhere that a solar cell is like the world's most inefficient led............

author: AVGresponding
author_rank: Super Contributor
timestamp: January 08, 2020, 06:07:13 pm
content: Quote from: CRCLARKE on January 08, 2020, 05:55:48 am

author: David Hess
author_rank: Super Contributor
timestamp: January 09, 2020, 03:39:17 am
content: Quote from: Rick Law on January 08, 2020, 04:07:16 amI completely agree. The difference between having an attached load and open circuit is where the light energy converted into electrical energy is deposited. A battery will store it as potential chemical energy. A capacitor will store it as potential electrostatic energy. A heater will release it as heat remotely. And if open circuit, the solar junction becomes forward biased, conducts, and immediately converts it to heat locally so the panels warm up slightly. Forward conduction through the diode represents loss which is balanced out when maximum power point tracking is used.There are a couple of research papers online which discuss the open circuit situation in connection with panel operating life being decreased because of higher operating temperature. I was surprised that the difference was significant but they say they measured it.