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> 5 billion years of evolution haven't given us solar powered animals that can run around. It seems like there are some significant hard limits on solar technol

> 5 billion years of evolution haven't given us solar powered animals that can run around. It seems like there are some significant hard limits on solar technology. What do you think animals run on? Gasoline? Plants are like batteries, storing solar energy. Animals, including humans, charge up by consuming these plants. And for those who don't eat plants, they eat charge up by eating the animals who charge up on the plants.

dnautics 11y ago
There are some organisms that get energy off of chemical gradients deep underground.
JoeAltmaier 11y ago
Again, we can call oil 'solar powered' by that means. Of course 'solar powered' should mean 'directly powered by sunlight captured by the creature itself'. No animal can do this, because the light X capture efficiency is far too low.
- mikeash 11y ago
  Solar power for electricity more closely resembles animals getting energy from plants which get energy from the sun, rather than animals which directly get energy from the sun. Aside from calculators and a couple other items, we don't put solar panels on our machines to power them, we connect solar panels to the electrical grid and connect machines to the grid too. Fossil oil qualifies too, but the fact it's all ancient stored solar energy rather than solar energy captured now makes it vastly different. Oil produced by capturing solar power now should qualify as "solar power" just as much as photovoltaics.
- chrischen 11y ago
  Oil is considered a non-renewable resource. Plants however are renewable, so they are fundamentally different. You can consider the solar->plant->animal cycle to effectively be similar to using solar panels. Using solar panels directly would be inefficient without batteries anyways, so the solar->plant->animal cycle is directly analogous to how we would use solar panels.
  - jegutman 11y ago
    Quick google search is about 100 wH per square foot of surface area. I'm just estimating about 8 square feet of surface area (one sided) for a human. Assuming 12 hours a day at that rate you get about 9.6 KwH or ~8000 kcal. With perfect efficiency and a storage system inside the body (which we do have, but not for light energy) that's probably doable. Obviously our tissue is much well less adapted to so many hours of sunlight compared to plants. Also even plants only absorb a relatively small part of the light spectrum (none of the green light). Also even plants required compounds from the earth and the air to actually harvest the energy to store so it's a pretty complicated process to do in an actively moving human.
    - gene-h 11y ago
      It appears you are assuming 100% efficiency for conversion of solar energy. The maximum efficiency possible for solar cells is the carnot efficiency at 86%.
      - jegutman 11y ago
        Yea, this was kind of giving an upper bound. Actually that's not very impressive given that people CAN burn more than 8000 calories in a single day.
- tsotha 11y ago
  Oil is solar powered. If it didn't take so long to form we wouldn't be having the discussion at all. I don't see why you would expect animals to evolve a way to "capture sunlight" (though some do: http://umich.uloop.com/news/view.php/77109/4-incredible-photosynthetic-animals http://umich.uloop.com/news/view.php/77109/4-incredible-phot...) when it's far more efficient from the standpoint of the animal to eat plant matter.

dnautics 11y ago

There are some organisms that get energy off of chemical gradients deep underground.

JoeAltmaier 11y ago

Again, we can call oil 'solar powered' by that means. Of course 'solar powered' should mean 'directly powered by sunlight captured by the creature itself'. No animal can do this, because the light X capture efficiency is far too low.

mikeash 11y ago
Solar power for electricity more closely resembles animals getting energy from plants which get energy from the sun, rather than animals which directly get energy from the sun. Aside from calculators and a couple other items, we don't put solar panels on our machines to power them, we connect solar panels to the electrical grid and connect machines to the grid too. Fossil oil qualifies too, but the fact it's all ancient stored solar energy rather than solar energy captured now makes it vastly different. Oil produced by capturing solar power now should qualify as "solar power" just as much as photovoltaics.
chrischen 11y ago
Oil is considered a non-renewable resource. Plants however are renewable, so they are fundamentally different. You can consider the solar->plant->animal cycle to effectively be similar to using solar panels. Using solar panels directly would be inefficient without batteries anyways, so the solar->plant->animal cycle is directly analogous to how we would use solar panels.
- jegutman 11y ago
  Quick google search is about 100 wH per square foot of surface area. I'm just estimating about 8 square feet of surface area (one sided) for a human. Assuming 12 hours a day at that rate you get about 9.6 KwH or ~8000 kcal. With perfect efficiency and a storage system inside the body (which we do have, but not for light energy) that's probably doable. Obviously our tissue is much well less adapted to so many hours of sunlight compared to plants. Also even plants only absorb a relatively small part of the light spectrum (none of the green light). Also even plants required compounds from the earth and the air to actually harvest the energy to store so it's a pretty complicated process to do in an actively moving human.
  - gene-h 11y ago
    It appears you are assuming 100% efficiency for conversion of solar energy. The maximum efficiency possible for solar cells is the carnot efficiency at 86%.
    - jegutman 11y ago
      Yea, this was kind of giving an upper bound. Actually that's not very impressive given that people CAN burn more than 8000 calories in a single day.
tsotha 11y ago
Oil is solar powered. If it didn't take so long to form we wouldn't be having the discussion at all. I don't see why you would expect animals to evolve a way to "capture sunlight" (though some do: http://umich.uloop.com/news/view.php/77109/4-incredible-photosynthetic-animals http://umich.uloop.com/news/view.php/77109/4-incredible-phot...) when it's far more efficient from the standpoint of the animal to eat plant matter.