Is my assumption correct, that

  1. Power generated by photovoltaic solar panels reduces heat energy from the sun (according to the PV cell efficiency of around 20%) on the area they cover

  2. An A/C converts 100% of the input energy to heat eventually

  3. Actually any powered mechanical device that doesn't store energy will eventually transfer all input energy to heat because every movement ends eventually, due to friction

If these assumptions are correct, is it correct to say that PV powered A/Cs are neutral to heat generation? The same goes then for EVs?

(Note I'm strictly about input/output energy conversion during usage - not the production of the devices etc.)

  • $\begingroup$ @SolomonSlow - I didn't say "I'm not thinking about" :). But this one question was strictly about the energy transformation physics in play. Your points would lead to a whole new and big discussion that probably won't fit here. Just so much: I doubt the best option is to wait for EVs, climate-wise. Their effect is not only CO2 neutral usage energy - but also creating large scale privately financed energy storage capacities - which are so much needed with growing renewable energy. Today, still a lot of PV energy is wasted/not used during peak production times - what a waste! $\endgroup$
    – Bachi
    Aug 3 at 16:33
  • $\begingroup$ @whoeverelse_might_read_this: Bachi asked to ignore the cost of manufacturing, and that's a legitimate physics question, but if you're looking at it from an environmental science perspective, then the impact of manufacturing things like air conditioners and cars and the impact of disposing of unwanted things should not be ignored. It can be a substantial part of the total impact of the thing over its entire lifetime. $\endgroup$ Aug 4 at 14:20
  • $\begingroup$ Note: I deleted my prior comment because I failed to make it clear that I really wanted to speak to other readers who might find this question and its answers. $\endgroup$ Aug 4 at 14:21
  • $\begingroup$ @SolomonSlow - From environmental perspective the unwanted CO2 (~5x all other waste) and problem wastes like coal ash aka fly ash are reasons to transition to zero emissions. Starting from industry with high fossil fuel dependency makes using existing industry to build the clean energy tech to replace it the only reasonable option; it isn't hypocritical, just practical. The impacts of waste matter but 100% RE would have to work hard being dirty to come anywhere close to fossil fuels . $\endgroup$
    – Ken Fabian
    Aug 7 at 1:13

Your argumentation is correct.

Nitpicking: a minor non-zero effect might be caused by the darker "albedo" of PV elements vs. the typical earth surface, thus absorbing more of the sun radiation instead of re-radiating it. But, taking into account how little of the earth surface is covered with PV cells, this can surely be neglected.

And even the amount of energy directly consumed/produced by mankind isn't the dominating factor. Even if every single person on earth were constantly producing 10kW heat (and I think, in reality, it's much less), that would be $80\cdot10^{12}W$. Compared with an estimated $140\cdot10^{15}W$ solar radiation hitting the earth, this would be less than 0.1% additional heating, maybe increasing the surface temperature by 0.2K.

The climate problem is more related to changes in the properties of our atmosphere that reduce the amount of heat radiated from earth into space.

So, while your argument is valid, the more important benefit of your PV-powered A/C is that it doesn't use an atmosphere-changing energy source.

  • 1
    $\begingroup$ Re, "...instead of re-radiating it..." IMO, it would be better to say, "...instead of reflecting it..." (But, I'm not a physicist, so I can't say I fully understand the difference between "reflection" and "re-radiation.") $\endgroup$ Aug 3 at 12:47
  • $\begingroup$ That "nit" about reflectance (and black-body reradiation) is in fact an important consideration. What matters here, for solar cells vs. manmade roofing vs. landscape, is how much energy is reflected back into space. But I agree the overriding advantage is that all the energy locked up in subterranean carbon sources remains locked up. $\endgroup$ Aug 3 at 15:44
  • $\begingroup$ @SolomonSlow: "reflectance" is the percent reflected (as a function of wavelength), so you calculate the total energy based on the spectral input. "Re-radiance" depends on the total amount of absorbed energy and on the temperature of the absorber. The black-body curve depends on temperature while the amount absorbed depends on the amount of energy (per wavelength) and absorptance at that wavelength. $\endgroup$ Aug 3 at 15:47
  • $\begingroup$ Regarding the albedo, tought about that, but since most private PV panels are on roofs and most roofs are quite dark, at least here, there shouldn't be a big difference. And I agree the question is more a theoretical one than one of practical impact on climate :-). $\endgroup$
    – Bachi
    Aug 3 at 16:06

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