Thanks. As it happened, I had edited my original comment to add a source, shortly before you replied (so you probably missed it). See footnote. Sorry that I didn’t do that when I first posted.
Your first source continues:
In fact, in any case, plants don’t use all incoming sunlight (due to respiration, reflection, light inhibition and light saturation) and do not convert all harvested energy into biomass, which brings about a general photosynthetic proficiency of 3%–6% based on total solar radiation. (See Table 1.)
When we say solar cells are 39% efficient, that’s as a fraction of all incoming sunlight, so the 3-6% is the correct comparison point, not the 11%, right?
Within the 3-6% range, I think (low confidence) the 6% would be lower-intensity light and the 3% would be direct sunlight—I recall that plants start deliberately dumping light when intensity gets too high, because if downstream chemical reactions can’t keep up with upstream ones then you wind up with energetic intermediate products (free radicals) floating around the cell and destroying stuff.
(Update: Confirmed! Actually this paper says it’s even worse than that: “In leaves in full sun, up to 80% of the absorbed energy must be dissipated or risk causing serious damage to the system (41).”)
There are likewise solar cells that also can’t keep up with the flux of direct sunlight (namely dye-sensitized solar cells), but the most commonly-used solar cells are perfectly happy with direct sunlight—indeed, it can make their efficiency slightly higher. The 39%-efficiency figure I mentioned was tested under direct sunlight equivalent. So the best point of comparison would probably be more like 3% than 6%, or maybe even less than 3%? Though it would depend a lot on local climate (e.g. how close to the equator? How often is it cloudy?)
Wikipedia says “C4 plants, peak” is 4.3%. But the citation goes here which says:
A theoretical limit of ~ 12% for the efficiency of photosynthetic glucose production from CO2 and water (based on free energy) can be calculated by considering the chlorophyll band-edge absorption and the two-photosystem structure of oxygenic photosynthesis (6, 13). Taking into account the known losses in light harvesting, overpotentials, and respiration, the maximum limit to photosynthetic efficiency is reduced to 4.6 and 6.0% for C3 and C4 plants, respectively (7). Short-term (rapid-growth phase) conversion efficiencies come within 70 to 75% of meeting these limits.
(Not sure why wikipedia says 4.3% not 4.5%.) Again, we probably need to go down from there because lots of sunlight is the intense direct kind where the plant starts deliberately throwing some of it out.
Anyway, I stand by “≳ 1 OOM below the best human solution” based on what I know right now.
still compatible with pareto-optimality depending on other tradeoffs such as storage density (and the best solar cells + battery tech is far less power dense).
I would say: plants are solving plant problems using plant technology, and humans are solving human problems using human technology. I’m generally kinda negative on how useful it is to frame things like this as a horse-race between the two. ¯\_(ツ)_/¯ (I’m engaging here because it’s fun, not because I think it’s particularly important.)
I agree that your second excerpt is kinda poorly-explained, maybe involving a bit of PR hype. I do think that if people are going to do basic scientific research that is relevant to renewable energy, studying the nuts and bolts of photosynthesis seems like a perfectly reasonable thing to do. But the path-to-applications would probably be pretty indirect, if any.
When we say solar cells are 39% efficient, that’s as a fraction of all incoming sunlight, so the 3-6% is the correct comparison point, not the 11%, right?
No if you look at the Table 1 in that source, the 3-6% is useful biomass conversion from crops, which is many steps removed.
The maximum efficiency is:
28%: (for the conversion into the natural fuel for the plant cells—ATP and NADPH).
9.2%: conversion to sugar after 32% efficient conversion of ATP and NADPH to glucose
3-6%: harvestable energy, as plants are not pure sugar storage systems and have various metabolic needs
So it depends what one is comparing … but it looks individual photosynthetic cells can convert solar energy to ATP (a form of chemical energy) at up to 28% efficiency (53% of spectrum * 70% leaf efficiency (reflection/absorption etc) * 76% chlorophyll efficiency). That alone seems to defeat the > 1 OOM claim, and some algae may achieve solar cell level efficiency.
Overall, this debate would benefit from clarity on the specific metrics of comparison, along with an explanation for why we should care about that specific metric.
Photosynthesis converts light into a form of chemical energy that is easy for plants to use for growth, but impractical for humans to use to power their machines.
Solar cell output is an efficient conversion of light energy into grid-friendly electrical energy, but we can’t exploit that to power plant growth without then re-converting that electrical energy back into light energy.
I don’t understand why we are comparing the efficiency of plants in generating ATP with the efficiency of solar cells generating grid power. It just doesn’t seem that meaningful to me.
I’m simply evaluating and responding to the claim:
I believe that plants are ≳ 1 OOM below the best human solution for turning solar energy into chemical energy, as measured in power conversion efficiency
It’s part of a larger debate on pareto-optimality of evolution in general, probably based on my earlier statement:
But we now know that evolution reliably finds pareto optimal designs:
(then I gave 3 examples: cellular computation, the eye/retina, and the brain)
So the efficiency of photovoltaic cells vs photosynthesis is relevant as a particular counterexample (and based on 30 minutes of googling it looks like biology did find solutions roughly on par—at least for conversion to ATP).
Thanks. As it happened, I had edited my original comment to add a source, shortly before you replied (so you probably missed it). See footnote. Sorry that I didn’t do that when I first posted.
Your first source continues:
When we say solar cells are 39% efficient, that’s as a fraction of all incoming sunlight, so the 3-6% is the correct comparison point, not the 11%, right?
Within the 3-6% range, I think (low confidence) the 6% would be lower-intensity light and the 3% would be direct sunlight—I recall that plants start deliberately dumping light when intensity gets too high, because if downstream chemical reactions can’t keep up with upstream ones then you wind up with energetic intermediate products (free radicals) floating around the cell and destroying stuff.
(Update: Confirmed! Actually this paper says it’s even worse than that: “In leaves in full sun, up to 80% of the absorbed energy must be dissipated or risk causing serious damage to the system (41).”)
There are likewise solar cells that also can’t keep up with the flux of direct sunlight (namely dye-sensitized solar cells), but the most commonly-used solar cells are perfectly happy with direct sunlight—indeed, it can make their efficiency slightly higher. The 39%-efficiency figure I mentioned was tested under direct sunlight equivalent. So the best point of comparison would probably be more like 3% than 6%, or maybe even less than 3%? Though it would depend a lot on local climate (e.g. how close to the equator? How often is it cloudy?)
Wikipedia says “C4 plants, peak” is 4.3%. But the citation goes here which says:
(Not sure why wikipedia says 4.3% not 4.5%.) Again, we probably need to go down from there because lots of sunlight is the intense direct kind where the plant starts deliberately throwing some of it out.
Anyway, I stand by “≳ 1 OOM below the best human solution” based on what I know right now.
I would say: plants are solving plant problems using plant technology, and humans are solving human problems using human technology. I’m generally kinda negative on how useful it is to frame things like this as a horse-race between the two. ¯\_(ツ)_/¯ (I’m engaging here because it’s fun, not because I think it’s particularly important.)
I agree that your second excerpt is kinda poorly-explained, maybe involving a bit of PR hype. I do think that if people are going to do basic scientific research that is relevant to renewable energy, studying the nuts and bolts of photosynthesis seems like a perfectly reasonable thing to do. But the path-to-applications would probably be pretty indirect, if any.
No if you look at the Table 1 in that source, the 3-6% is useful biomass conversion from crops, which is many steps removed.
The maximum efficiency is:
28%: (for the conversion into the natural fuel for the plant cells—ATP and NADPH).
9.2%: conversion to sugar after 32% efficient conversion of ATP and NADPH to glucose
3-6%: harvestable energy, as plants are not pure sugar storage systems and have various metabolic needs
So it depends what one is comparing … but it looks individual photosynthetic cells can convert solar energy to ATP (a form of chemical energy) at up to 28% efficiency (53% of spectrum * 70% leaf efficiency (reflection/absorption etc) * 76% chlorophyll efficiency). That alone seems to defeat the > 1 OOM claim, and some algae may achieve solar cell level efficiency.
Overall, this debate would benefit from clarity on the specific metrics of comparison, along with an explanation for why we should care about that specific metric.
Photosynthesis converts light into a form of chemical energy that is easy for plants to use for growth, but impractical for humans to use to power their machines.
Solar cell output is an efficient conversion of light energy into grid-friendly electrical energy, but we can’t exploit that to power plant growth without then re-converting that electrical energy back into light energy.
I don’t understand why we are comparing the efficiency of plants in generating ATP with the efficiency of solar cells generating grid power. It just doesn’t seem that meaningful to me.
I’m simply evaluating and responding to the claim:
It’s part of a larger debate on pareto-optimality of evolution in general, probably based on my earlier statement:
(then I gave 3 examples: cellular computation, the eye/retina, and the brain)
So the efficiency of photovoltaic cells vs photosynthesis is relevant as a particular counterexample (and based on 30 minutes of googling it looks like biology did find solutions roughly on par—at least for conversion to ATP).