Solar panels can be more efficient, but if your comparison is between “sunlight -> plants” and “sunlight -> solar panels -> electric lights -> plants”, you have to include the actual efficiency of e.g. photovoltaics and LEDs in your calculations.
Photovoltaics these days have something around 15-20% efficiency and LEDs have conversion efficiency around 50-60%. The magenta grow lamps are colored for more efficient use by plants, and you can pack more plants in a smaller space, but at that point you’re trying to offset energy losses on the order of 90%.
Yes, and I updated my comment just as you were posting this to mention losses due to reconversion. However, also keep in mind that even large efficiency losses can still lean in favor of photovoltaics/LEDs, because plants only use about 10% of the sun's energy in the first place.
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.
> 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.
> 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.
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
If this is the case it would be a motivation that is fundamentally very different than that of a multijunction solar cell.
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?
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.
> 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.
I think that's correct, but that difference seems to be within the error margins for this calculation, and some other loss (e.g., transmission, storage, etc) could eat any hypothetical advantage that the vertical farming position might've enjoyed.
Blurple and magenta LEDs seem to be on their way out last time I looked into growing lights. It is white (or white appearing) LEDs now, which are getting 190+ lumens per watt. The light spectrum is still tuned for plant growth efficiency, but it is a broader spectrum per LED rather than more specific spectrums from specific diodes.
It is worth noting too that these newer white LEDs are spectrum efficient enough that people growing with them have had to turn up the overall room temperatures to compensate for the drop in leaf temperatures created by "waste" wavelengths that the sun and old sodium bulbs use to provide.
50% efficiency is pretty massive for a light or most anything really. And you can get higher if you underdrive them.
Just because a light appears white doesn't mean the spectrum is evenly distributed though, generally if you graph the color spectrum of these lights green is in a huge dip. If you go to page 13 of this data sheet they show show spectrum graphs. https://cdn.samsung.com/led/file/resource/2020/09/Data_Sheet... All of which appear white in real life. They may be more cool blue white or warmer reddish white, but white nonetheless. Especially with the intensity that they shine.
Photovoltaics these days have something around 15-20% efficiency and LEDs have conversion efficiency around 50-60%. The magenta grow lamps are colored for more efficient use by plants, and you can pack more plants in a smaller space, but at that point you’re trying to offset energy losses on the order of 90%.