# Where does the other 82% go?

Registered Users Posts: 31 ✭✭
So today's garden-variety solar panels are about 18% efficient. I take this to mean that 18% of the incoming solar energy is converted to electrical energy. Where does the remaining 82% of incoming solar energy go?

I think it must be split between absorbed heat and reflected energy. That is, I'm guessing that 18% of the incoming energy is converted to electricity, X% is absorbed (and this is why our panels can get so hot), and (100 - 18 - X)% is reflected back into the atmosphere.

Is this correct, and if so, does anyone know what percentage is absorbed as heat vs what percentage is reflected away? (i.e. what is X?)

• Registered Users Posts: 4,496 ✭✭✭✭✭
I suspect the amount reflected back as light is quite low and most is heat radiated by the warm panels. It's amazing how fast snow melts off partly covered panels even in very cold ambient temp with low sun angle.
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• Solar Expert Posts: 891 ✭✭✭✭
pdh said:
So today's garden-variety solar panels are about 18% efficient. I take this to mean that 18% of the incoming solar energy is converted to electrical energy. Where does the remaining 82% of incoming solar energy go?
A small amount is reflected; the rest is absorbed as heat. Solar panels have albedos of about 5%.  So in your example, 18% is converted to power, 5% is reflected and 77% is dissipated as heat.
• Registered Users Posts: 31 ✭✭
Interesting -- I expected the heat-absorption percentage to be high, but I didn't realize it was quite that high. If only we had an efficient means of capturing and storing that heat energy...

If there was a relatively cheap way to turn the heat into electricity without disrupting the existing electricity-conversion process, even if the heat transformation was only say 25% efficient, the total efficiency would double. (We'd still have 18% of incoming energy turned directly into electricity like we do now, plus we'd be harvesting 25% of the 77% heat, which is roughly another 19%.)

Alas, looking at the Wikipedia article about thermoelectric generators, it says their typical efficiency is only 5 - 8 %, and evidently the technology is expensive. So my cheap 25% idea sounds pretty unreasonable.
Read up on thermodynamics and the fundamental laws.

More or less, the difference between high and low temperatures defines the efficiencies of energy conversion. Using the Kelvin (absolute zero scale).

The best fuel to electrical conversion is about 50% for natural gas fired turbines.

Using waste heat to warm water for processes, home heating, swimming pool, etc. Is usually about the best you can do.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 246 ✭✭✭
Heat. Large arrays actually heat the area they are in significantly. As you drive past a green field you can feel the Temps drop and the air feels coil, as you pass a large array you feel the heat hanging in the air and it's very much hotter around them.
Putting several large arrays in former farm land creates a heat island that will actually change the micro climate and cause up drafts that tend to drive out clouds that used to form there.

As we continue to create more of these we will se large changes that are not always for the better.
• Registered Users Posts: 4,496 ✭✭✭✭✭
Huh? The sun heats areas, not solar arrays. A green area feels cool because of evaporative cooling.
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Here is (apparently) real data about the solar array and local temperatures:

https://www.nature.com/articles/srep35070

## Abstract

While photovoltaic (PV) renewable energy production has surged, concerns remain about whether or not PV power plants induce a “heat island” (PVHI) effect, much like the increase in ambient temperatures relative to wildlands generates an Urban Heat Island effect in cities. Transitions to PV plants alter the way that incoming energy is reflected back to the atmosphere or absorbed, stored, and reradiated because PV plants change the albedo, vegetation, and structure of the terrain. Prior work on the PVHI has been mostly theoretical or based upon simulated models. Furthermore, past empirical work has been limited in scope to a single biome. Because there are still large uncertainties surrounding the potential for a PHVI effect, we examined the PVHI empirically with experiments that spanned three biomes. We found temperatures over a PV plant were regularly 3–4 °C warmer than wildlands at night, which is in direct contrast to other studies based on models that suggested that PV systems should decrease ambient temperatures. Deducing the underlying cause and scale of the PVHI effect and identifying mitigation strategies are key in supporting decision-making regarding PV development, particularly in semiarid landscapes, which are among the most likely for large-scale PV installations.

A solar array over ground appears to prevent cooling at night. Whether that is a reduction in radiative cooling (not my favorite) or convective cooling (my suggestion--the "greenhouse effect"--preventing mixing of air with the ground/vegetation/etc.?), there is an effect at night, not during the day (peak daily temperatures are withing the error bars for most months, except the summer months).

I am not sure that you could tease out a difference with a roof mounted array vs just a house.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 891 ✭✭✭✭
Solray said:
Heat. Large arrays actually heat the area they are in significantly. As you drive past a green field you can feel the Temps drop and the air feels coil, as you pass a large array you feel the heat hanging in the air and it's very much hotter around them.
Putting several large arrays in former farm land creates a heat island that will actually change the micro climate and cause up drafts that tend to drive out clouds that used to form there.
Updrafts actually cause clouds due to adiabatic cooling.  (That's one reason there are often clouds near mountain ranges.)
• Registered Users Posts: 246 ✭✭✭
That's wrong. Clouds form where evaporation occurs and when the clouds are pushed up over mountain ranges, they drop some moisture on the windward side and there is a resulting rain shadow on the leeward side. This is why western Washington and Oregon get so much rainfall and eastern parts of the two states get so much less.

As BB showed, there is a definite heat increase around the arrays, you can feel it as you drive by. As more land is devoted to large arrays, the effect will only increase.

If you don't believe it, go drive by a massive solar array that takes up acres of land and see for yourself.
• Registered Users Posts: 246 ✭✭✭
Estragon said:
Huh? The sun heats areas, not solar arrays. A green area feels cool because of evaporative cooling.
Sunlight heats objects and then in time, the objects heat the air, sunlight does not directly heat air. This is why the coldest temperatures of the day are about 15 to 20 minutes after sunrise when the air is still cooling and before the objects have begun transmitting their heat. Objects like solar panels (and concrete and asphalt and rock landscaping) heat faster and absorb more heat energy than natural growing plants and farmlands with crops and forests of trees. Walk on a sidewalk at noon in July in Phoenix and then walk on a green lawn to see the extreme difference it makes.

• Registered Users Posts: 155 ✭✭
According to that graphic, the earth's surface reflects 30/198 = 15% of the incoming energy. So the immediate area is heated by the remaining 85%. If a panel converts 18% to electricity, and reflects another 5%, then the area is being heated by only 77% of the solar energy. So, there's significantly less heating of paneled areas.

It looks like a bigger difference is in thermals and evapo-transpiration, which remove about 61% of the heat.

I'd think the specific area would determine how that changes when there are panels present. In a dry desert area, paneled areas may be cooler. But replacing grassland with panels would significantly reduce evapo-transpiration, so I'd expect those to be hotter.
• Registered Users Posts: 14 ✭✭
This is certainly interesting thread. I haven't ever thought of the remaining energy actually transforming into the heat. And it amazes me that no one discusses this fact, as urban heat effect is discussed a lot - especially with heat waves getting more intense last few summers.
• Solar Expert Posts: 891 ✭✭✭✭
Solray said:
That's wrong.

Google "adiabatic cooling."  Rising hot air cools. Once the cooling brings the temperature below the dew point, clouds form.

Clouds form where evaporation occurs and when the clouds are pushed up over mountain ranges

That's called orographic lifting - a similar concept, but started by a different process.
• Solar Expert Posts: 1,386 ✭✭✭✭
edited July 2017 #15
> sunlight does not directly heat air.

See the graphic where it says "67 absorbed by the atmosphere".  But I suppose one could argue about the exact definition of "air".

> Objects like solar panels (and concrete and asphalt and rock landscaping) heat faster and absorb more heat energy than natural growing plants and

Light colored concrete will heat slower (mostly due to thermal mass)  and absorb less solar radiation than a dark green plant.  But it has less surface area than plants to lose heat to convection.   Hot concrete hurts your feet because of the high thermal diffusivity (walking on equally hot carpet doesn't hurt).

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Blocking radiative cooling (my suggestion as to what the panels are doing, as well as preventing air mixing at ground level) also keeps ground warmer too (frost on ground, no frost on ground under large oak tree. No frost on windy days--In general).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 246 ✭✭✭
jonr said:
> sunlight does not directly heat air.

See the graphic where it says "67 absorbed by the atmosphere".  But I suppose one could argue about the exact definition of "air".

I see that. It's a very tiny portion indeed and not at the surface where it effects our temperatures greatly. It's absorbed by the gasses in the air specifically.
By contrast, you get 390 coming from the heated ground and another 324 bouncing back off of water vapor in the air (and other greenhouse gasses to a lesser extent)

mike_s said:
According to that graphic, the earth's surface reflects 30/198 = 15% of the incoming energy. So the immediate area is heated by the remaining 85%. If a panel converts 18% to electricity, and reflects another 5%, then the area is being heated by only 77% of the solar energy. So, there's significantly less heating of paneled areas.

It looks like a bigger difference is in thermals and evapo-transpiration, which remove about 61% of the heat.

I'd think the specific area would determine how that changes when there are panels present. In a dry desert area, paneled areas may be cooler. But replacing grassland with panels would significantly reduce evapo-transpiration, so I'd expect those to be hotter.
Try touching the ground in the desert, it is hot but you can hold your hand on the dirt or there, then try touching the glass on a solar panel and holding it there. it will cause a pretty good burn in a short time. Plus there are numerous small weeds, shrubs and even trees like palo verde, mesquite, smoke trees, creosote bushes, cacti and other plantlife that cast shadows so there is no area with a 100% bare ground.
Solar panels are, by design shadowless dark flat non porous surfaces, there is a big difference if you think about it.

It's really just common (or uncommon these days) sense.
• Registered Users Posts: 246 ✭✭✭
BB. said:
Blocking radiative cooling (my suggestion as to what the panels are doing, as well as preventing air mixing at ground level) also keeps ground warmer too (frost on ground, no frost on ground under large oak tree. No frost on windy days--In general).

-Bill
I don't think there is a lot of blockage just because there are gaps around the panels and no seal over the edges to the ground.
My Spa in summer with the lid closed in the shade of it's gazebo covered with  90% shade screen fabric will get too hot to use comfortably in a couple days (110+ degrees), but if I just crack open the lid with a 1" thick piece of wood, it will cool to swimming pool temps in 3 or 4 days (70 - 75 degrees) so you don't need a lot of opening, just a small crack for evaporative cooling to occur which is similar to radiative cooling of a solid slightly moist material like the ground.
• Registered Users Posts: 155 ✭✭
Solray said:
Try touching the ground in the desert, it is hot but you can hold your hand on the dirt or there, then try touching the glass on a solar panel and holding it there. it will cause a pretty good burn in a short time.

Well, sure. The ground is a huge heat sink, the panels are very small in comparison. Instead of heating a large mass of dirt, a small mass of panel is heated. Of course it's going to get hotter.
• Registered Users Posts: 4,496 ✭✭✭✭✭
Blocking radiative cooling and preventing ground level convection is absolutely a factor. Using hay, spruce boughs, loose snow, etc has been used effectively for years to prevent septic and water line freezing in winter. We use old sheets to protect plants from frost in spring/fall, and mound leaves to protect roots. Doesn't need to be anything like airtight.
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• Registered Users Posts: 155 ✭✭
Estragon said:
Blocking radiative cooling and preventing ground level convection is absolutely a factor. Using hay, spruce boughs, loose snow, etc has been used effectively for years to prevent septic and water line freezing in winter. We use old sheets to protect plants from frost in spring/fall, and mound leaves to protect roots. Doesn't need to be anything like airtight.
Radiative cooling is not blocked - the panels are what's heated, and the panels are radiating heat. More so than the ground would in their absence - they get hotter and there's more of a temperature differential which means more heat transfer. The ground is shaded, so there's less heat there to block. There's a reason most panels have white undersides, it doesn't radiate heat well. Black does - the same reason most heatsinks are anodized black. So most of the heat radiated by the panels goes up, not down.

Same for convection, any wind cools the panels, which are absorbing the bulk of the energy and getting hot in the process. Plus, panels have twice the surface area (top and bottom) of the ground.

Insulation is completely different. What you describe works only in areas where there's no permafrost, so the heat stored in the ground only needs to be insulated from the air temperature and the well below freezing temperature of a clear night sky.
• Solar Expert Posts: 1,386 ✭✭✭✭
White surfaces I've seen have emissivities about the same as black ones.  For example:

 Paint: Krylon, flat black        0.95 Paint: Krylon, flat white #1502 0.992
 Paint: plastic, black 0.95 Paint: plastic, white 0.84

Which, in the case of the back of solar panels, is a good thing (you want them to radiate heat off the back side so they can cool down).

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• Registered Users Posts: 14 ✭✭
mike_s said:
Solray said:
Try touching the ground in the desert, it is hot but you can hold your hand on the dirt or there, then try touching the glass on a solar panel and holding it there. it will cause a pretty good burn in a short time.

Well, sure. The ground is a huge heat sink, the panels are very small in comparison. Instead of heating a large mass of dirt, a small mass of panel is heated. Of course it's going to get hotter.
Theoretically, this shade from panels could enable more vegetation grow underneath and vegetation helps to cool down the area, retain moisture in the soil + encourages evapotranspiration, prevents erosion and sequesters carbon. The only question remains how much more heat panels emit compared to the natural state because small plants might not help in this equation significantly.
• Registered Users Posts: 246 ✭✭✭
They don't help at all. You can go to a large array today and compare the Temps to the area across the road without panels. It's obviously hotter in the fields with panels than without them.
• Registered Users Posts: 155 ✭✭
Solray said:
They don't help at all. You can go to a large array today and compare the Temps to the area across the road without panels. It's obviously hotter in the fields with panels than without them.
Is it? Or is that true only during the day when the panels are getting hot and radiating that heat, while the field across the street is absorbing that heat into the soil? Have you compared the night time temperatures on both sides of the road?
• Registered Users Posts: 246 ✭✭✭
Yes, I drove past them at various times of day for a few months. You can really tell the difference with the windows down as you pass the fields.
• Solar Expert Posts: 9,583 ✭✭✭✭✭
jonr said:
White surfaces I've seen have emissivities about the same as black ones. ......
.....
Which, in the case of the back of solar panels, is a good thing (you want them to radiate heat off the back side so they can cool down).
Whoa,  There are more factors in play than just surface emissivity - the materials also have thermal transfer properties. Generally, thicker is less transfer, metals are higher rates than plastics...
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• Registered Users Posts: 395 ✭✭✭
edited July 2017 #28
Urban-Heat islands can actually punch a clear hole completely through low cloud layers / fog.
Look at the "50 km wide Clear Hole" that New Delhi, and other cities, have created in the low clouds ...
https://www.citylab.com/environment/2016/12/india-cities-heat-island-clouds-nasa/510101/
So does rising hot air always cause clouds? No.

• Solar Expert Posts: 1,386 ✭✭✭✭
>  materials also have thermal transfer properties.

Which in the case of a very thin plastic film, is negligible.    So why do solar panels have white (vs other colors) plastic film on the back - my guess is high reflectivity and high emissivity - both good for reducing panel temperature.

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