I'm curious about limits for extracting solar energy, but I never studied this stuff so I'm going to speculate a lot.

My first question deals with the things like wind energy. I assume this energy is largely a result of the atmosphere trying to regain equilibrium from the sun heating things up unevenly. Is this correct? This relates to solar energy since solar panels can get that energy first. However, the electricity they generate is used locally, so eventually it does end up as heat.

This seems like double dipping. We can get the energy first as electricity then again in some other form such as wind. I assume this implies a limit on the conversion efficiency of things like solar panels. Getting the energy from the heat generated by sunlight must be connected heat engines. Therefore, to preserve conservation of energy, there must be a theoretical limit on solar panel efficiency that is equal to the energy in the sunlight minus the energy one can extract from subsequent heat engines. Is this correct? What are these limits?


1 Answer 1


I suspect the OP has looked a site like this one, and imagined that the local waste heat (perhaps via the heat island effect) would mean the generated wind speed would be higher, which then increases the power output of local wind turbines.

Well, consider the following hypothetical thought experiment.

  1. Assume sunlight energy in a day was converted to electricity over a huge area (i.e. no hot air rises/no wind). This energy (minus losses) is used to make ethanol.
  2. The ethanol is then burned/drunk, causing hot air to rise, (and more losses due to radiation into space etc.) but wind forms,
  3. which then drives wind turbines (yet more losses here), which can make electricity (but much less than before),
  4. which can make ethanol again (but much less than the first batch).

You can now see that the efficiency of the solar panels simply is the first ‘loss’, even if they were 100% efficient, there is no double dipping, all the energy in this hypothetical scenario still comes from the original sunlight.

  • $\begingroup$ As you can see in my comment, my main point is that this must constrain the efficiency of the solar panels. I'm not claiming that conservation of energy is being violated. I'm claiming that the energy recovered from the waste heat must limit the solar panels. Therefore your hypothetical is impossible. There is no way the panels could be 100%. $\endgroup$
    – chris
    May 1 at 16:12
  • $\begingroup$ You have it the wrong way round. In this scenario, it is the efficiency of the solar panels that limits the 'downstream' energy able to recovered from the waste heat. Max efficiency is of course 100%. $\endgroup$ May 1 at 23:23
  • $\begingroup$ OK, so I think I see what confused me. If the panel is 100% efficient then I can use that energy to do work. If that work is not 100% efficient in storing the energy in some type of potential then waste heat is created. However all is not lost because, if there is no equilibrum with the waste heat, one can use it to do some more work. Basically, if the sun just hits the Earth, it mostly goes to waste heat, but we can be clever and prevent some of that with a solar panel. I guess the problem is I really don't know much about entropy. $\endgroup$
    – chris
    May 2 at 6:03

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