3 ms·

The problem with talking about photosynthetic efficiency is that there are different endpoints you can talk about. The more efficient plants (C4 plants like sug

The problem with talking about photosynthetic efficiency is that there are different endpoints you can talk about. The more efficient plants (C4 plants like sugarcane and maize) have something like 4% efficiency converting sunlight to biomass, but they actually absorb a 53% of the incoming light based on spectrum, and lose about 24% of the energy because photons with shorter wavelengths have excess energy which the plants cannot use. We’re not interested in the 4% figure, we’re interested in the 53% and 24% figure because they represent the part of the process that we can change. Doing the math, that’s around 59% loss which you could mitigate by using LEDs that produce the correct spectrum—but solar panels and LEDs have 90% losses, so you’re noticeably worse off. It’s worth remembering that the reason why plants only absorb certain parts of the spectrum is the same reason why photovoltaic panels only absorb certain parts of the spectrum—in both cases, you are using light to move electrons, and these processes only capture energy that corresponds to the underlying band gap. Light with shorter wavelengths has additional energy which is wasted, both for photovoltaics and for plants. You can increase the efficiency by creating multijunction solar panels, which results in multiple band gaps. For most applications, these aren’t cost-effective. If I remember correctly, plants are also “multi-junction”, which explains why they are so efficient.

bsder 6y ago
> It’s worth remembering that the reason why plants only absorb certain parts of the spectrum is the same reason why photovoltaic panels only absorb certain parts of the spectrum Plants are green because they value light consistency instead of total energy. Green light has too many peaks and valleys and can overload the photosynthesis systems so they reflect a lot of it. It's the "renewables without batteries" problem only in biology.
- klodolph 6y ago
  > Green light has too many peaks and valleys and can overload the photosynthesis systems so they reflect a lot of it. This doesn’t make any sense to me. Why would peaks and valleys overload something? Why would green light have more peaks and valleys? I was a bit sloppy with the way I phrased that—what I really meant was “the reason why plants use specific quanta of light is the same reason why photovoltaics absorb specific quanta of light” but I didn’t put much thought into how worded it. Plants absorb light near two different spectral peaks. This is not entirely dissimilar to the idea of a multijunction photovoltaic cell. The color of light between the two peaks is green.
  - Armisael16 6y ago
    I don’t know if this is what the previous poster was getting at, some there is some theoretical evidence that the absorption wavelengths used in photosynthesis are not tuned for maximum power input but for stable power input: https://arxiv.org/abs/1912.12281 https://arxiv.org/abs/1912.12281 If this is the case it would be a motivation that is fundamentally very different than that of a multijunction solar cell.
    - klodolph 6y ago
      I've read that paper, I didn't realize the comment was referring to that. > If this is the case it would be a motivation that is fundamentally very different than that of a multijunction solar cell. Solar cells are designed by humans with human motivations, and plants are "designed" by evolutionary processes which lack motivation entirely. Yet, in spite of this, there are astonishing similarities between the limitations of solar cells and the limitations of plants. Isn't that fascinating, that processes with such disparate origins have such similarities?
      - Armisael16 6y ago
        I don’t see any meaningful similarities beyond ‘they absorb light at more than one frequency’. It doesn’t seem particularly fascinating to me; it’s patently obvious that if you absorb at two frequencies you should absorb at two different frequencies.
        klodolph 6y ago
        > I don’t see any meaningful similarities beyond ‘they absorb light at more than one frequency’. Ah, that's definitely not what I'm talking about. Look at the relationship between efficiency and wavelength for plants and photovoltaics... the similarity is clear. Both photovoltaics and plants are capable of absorbing specific amounts of energy from incoming photons but not other amounts. Any excess energy becomes waste heat. The reason is because in both systems, the incoming light is used to move electrons from one state to another; the energy required to do this must usually come from one photon. Not all solar power systems work this way.

bsder 6y ago

> It’s worth remembering that the reason why plants only absorb certain parts of the spectrum is the same reason why photovoltaic panels only absorb certain parts of the spectrum Plants are green because they value light consistency instead of total energy. Green light has too many peaks and valleys and can overload the photosynthesis systems so they reflect a lot of it. It's the "renewables without batteries" problem only in biology.

klodolph 6y ago
> Green light has too many peaks and valleys and can overload the photosynthesis systems so they reflect a lot of it. This doesn’t make any sense to me. Why would peaks and valleys overload something? Why would green light have more peaks and valleys? I was a bit sloppy with the way I phrased that—what I really meant was “the reason why plants use specific quanta of light is the same reason why photovoltaics absorb specific quanta of light” but I didn’t put much thought into how worded it. Plants absorb light near two different spectral peaks. This is not entirely dissimilar to the idea of a multijunction photovoltaic cell. The color of light between the two peaks is green.
- Armisael16 6y ago
  I don’t know if this is what the previous poster was getting at, some there is some theoretical evidence that the absorption wavelengths used in photosynthesis are not tuned for maximum power input but for stable power input: https://arxiv.org/abs/1912.12281 https://arxiv.org/abs/1912.12281 If this is the case it would be a motivation that is fundamentally very different than that of a multijunction solar cell.
  - klodolph 6y ago
    I've read that paper, I didn't realize the comment was referring to that. > If this is the case it would be a motivation that is fundamentally very different than that of a multijunction solar cell. Solar cells are designed by humans with human motivations, and plants are "designed" by evolutionary processes which lack motivation entirely. Yet, in spite of this, there are astonishing similarities between the limitations of solar cells and the limitations of plants. Isn't that fascinating, that processes with such disparate origins have such similarities?
    - Armisael16 6y ago
      I don’t see any meaningful similarities beyond ‘they absorb light at more than one frequency’. It doesn’t seem particularly fascinating to me; it’s patently obvious that if you absorb at two frequencies you should absorb at two different frequencies.
      - klodolph 6y ago
        > I don’t see any meaningful similarities beyond ‘they absorb light at more than one frequency’. Ah, that's definitely not what I'm talking about. Look at the relationship between efficiency and wavelength for plants and photovoltaics... the similarity is clear. Both photovoltaics and plants are capable of absorbing specific amounts of energy from incoming photons but not other amounts. Any excess energy becomes waste heat. The reason is because in both systems, the incoming light is used to move electrons from one state to another; the energy required to do this must usually come from one photon. Not all solar power systems work this way.