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One question I have had on my mind is about solar powered LED lighting. Solar cells capture a wider spectrum than plants (plants are green ergo not green eater

by Lerc 10y ago

One question I have had on my mind is about solar powered LED lighting. Solar cells capture a wider spectrum than plants (plants are green ergo not green eaters). At what levels of efficiency do solar cells and LEDs have to be before you receive a net gain by having the solar cells in the sun instead of the plants. How far away from that are we now? Underground farming in arid regions seems like the best scope for something like this.

simcop2387 10y ago
Actually if you're doing underground farming and such like that, it could be a net positive pretty quickly because you could be swamped by other concerns such as available water for growing and the energy costs to get it.
- SixSigma 10y ago
  Aquaponics uses much less water than hydroponics plus you don't use energy to make fertiliser. Your inputs are light & fish food and some top up water.
  - deftnerd 10y ago
    I'm a big fan of aquaponics too, but I've never seen numbers on the energy required to source, process, and transport the fish food. It gets even more complicated if you include the energy required to raise the fish that get caught and turned into fish food. In my old system, I used bottle fly larvae to feed the fish so the energy input was less (waste refuse that would have been composted anyway) . It was the least fun part of the system because of the "gross factor" of maggots.
    - SixSigma 10y ago
      There is plenty of efficiency to be gained in shifting dry bulk fish food around but I agree it needs to be more quantified for a real comparison. Micheal Polkas claims 10 calories of energy in for 1 calorie out in our current intensive farming, so any improvement at scale promises returns.
saidajigumi 10y ago
> Underground farming in arid regions seems like the best scope for something like this. This sort of mediated growing environment seems quite interesting as an approach to managing agriculture as environmental changes from global warming progress. In some areas, perhaps conventional greenhouses would suffice alone. But in others, underground agriculture might provide an additional buffer against climate changes. This would allow normalization of growing conditions in regions where the climate has drifted and/or become unpredictable in ways that would severely damage agricultural output.
koverstreet 10y ago
Actually, they do use green - they're more reflective in green than red/blue, but it's still mostly absorbed. Apparently at high illuminations you want to be using green too - the issue is that red and blue are absorbed so well that they don't make it past the surface of the leaf and thus the plant quickly maxes out on them, while green light penetrates further and thus reach more tissue. So when you see pictures of leaf greens being grown under purple light - that's because leaf greens actually need less light than other vegetables, you probably wouldn't want to do that with other crops. IANAbotanist
- zxcdw 10y ago
  Link to a paper talking about this: http://pcp.oxfordjournals.org/content/50/4/684.full http://pcp.oxfordjournals.org/content/50/4/684.full
- washadjeffmad 10y ago
  This is why we still use high yellow/green CFLs per so many red/blue LEDs. It's not all about getting those high peak spectrum binned LEDs for each type and concentration of chlorophyll.
mdorazio 10y ago
I don't have a definitive answer for you, but you can kind of back into a rough approximation (someone please correct my math if I went astray). High-efficiency solar cells are about 20% efficient across the daylight spectrum. For some reason, no one gives hard stats on how much energy LEDs waste as heat, but the claims are that incandescents waste about 90% of energy input as heat, and LEDs are about 80% more efficient than that. So let's say LEDs convert about 80% of input power into light at their operating wavelengths as a guess. So if we put these together, we'll get about 16% of the input daylight back out as light at the wavelengths we want. According to wikipedia [1], about 47% of light that falls on chlorophyll is wasted because it falls outside the ideal range for plants (400-700nm). Another 3% or so is lost due to wavelength conversion. The rest we can't really do anything about because it has to do with biological limitations. Put another way, our total system has to be about 50% efficient to be better than just putting the plants in the sun. As you can see, we're not even close. In fact, it's not even possible since the theoretical maximum efficiency of solar panels is only about 34% [2]. [1] https://en.wikipedia.org/wiki/Photosynthetic_efficiency https://en.wikipedia.org/wiki/Photosynthetic_efficiency [2] https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limi...
- deleted 10y ago
  [deleted]
- TTPrograms 10y ago
  The Shockley-Queisser limit only applies to single-layer cells under unconcentrated light. Different cell designs have surpassed it by significant amounts in practice already.
  - mdorazio 10y ago
    Do you have a link where I can read more? I'm aware of light concentrators being more efficient, but that defeats the whole point since the surface area required is far more than size of the panel itself.
    - 10y ago
kibwen 10y ago
This question was anwered in a prior recent HN thread: https://news.ycombinator.com/item?id=11849172 https://news.ycombinator.com/item?id=11849172 Not an expert so I can't vouch for its veracity, but I'll inline it: "This can theoretically be more efficient than growing plants under sunlight. Chlorophyll mostly absorbs light in the 400nm to 700nm band which is about half of all the photons that hit lights. Even in that band ~24% (https://en.wikipedia.org/wiki/Photosynthetic_efficiency https://en.wikipedia.org/wiki/Photosynthetic_efficiency) of the energy in them in lost due to the higher energy photons (closer to the 400nm side) being converted to lower energy photons. "So right there plants are only 25% efficient at collecting energy from photons. This is why you see indoor plants grown under red LEDs. Modern commercially available solar panels are about 21% efficient so that is actually getting close to plant can do (I'm not going to include inefficiencies in power conversion right now). Solar panels in the high 40% efficiency range exist, but they are crazy expensive and only used on things like satellites. "So using current (very advanced) technology we could actually get about double the plant output per area land by using the most advanced solar panels we have to collect light energy to power LEDs."
- Retric 10y ago
  Assuming zero production costs, 100% efficient LED's, 100% efficient energy transfer, 100% land coverage with Panels that perfectly tile the surface, zero panel degradation, zero dust, etc etc. In the real world wind energy probably works out much better.
- petra 10y ago
  Sure it's great for optimizing land use. Agricultural land is cheap. The real question is cost.
  - yetihehe 10y ago
    Yeah, but imagine this on deserts. You havae too much light and almost no agricultural land. So, use panels to gather light, use hydroponics/aeroponics to grow plants and now you can distribute your sunlight to many levels of racks (which can be underground under panels which means even more efficiency than on normal land.
deleted 10y ago
[deleted]
aaron695 10y ago
> Underground farming Building underground is crazy expensive (albeit very cool sci-fi), why not (cheap) above ground. I think arid regions are kinda cheap for land so why not just a glasshouse? Put solar panels beside it, should they work. Even if the land is very expensive (And localize food production makes sense), floor one is plants, flood 2+ is for businesses. Building multifloors underground is the same structurally as above ground. Exceptions might be places like London where regulations stop you building up.
Joof 10y ago
Plants do still absorb green and are arguably better off if they do (although the latter may be less certain). The bits that absorb green are more sparse, but light will bounce around inside the leaves until it hits one.
- choosername 10y ago
  It depends on the plant, obvious from the different color.
sova 10y ago
I live in Cleveland currently and in the winter time there is no sun. Thus, LEDs are an excellent alternative, and can be used elsewhere during nighttime. The benefits are not "bringing the sun via proxy" ... the benefits are "we have focused on the wavelengths that most rapidly and efficiently excite chlorophylls alpha and beta (harvest sun color and the noon-time color of the summer, if I may approximate the "feeling" of the two active bumps... carotenoids are the main chlorophyll in carrots).

by Lerc 10y ago

simcop2387 10y ago

Actually if you're doing underground farming and such like that, it could be a net positive pretty quickly because you could be swamped by other concerns such as available water for growing and the energy costs to get it.

SixSigma 10y ago
Aquaponics uses much less water than hydroponics plus you don't use energy to make fertiliser. Your inputs are light & fish food and some top up water.
- deftnerd 10y ago
  I'm a big fan of aquaponics too, but I've never seen numbers on the energy required to source, process, and transport the fish food. It gets even more complicated if you include the energy required to raise the fish that get caught and turned into fish food. In my old system, I used bottle fly larvae to feed the fish so the energy input was less (waste refuse that would have been composted anyway) . It was the least fun part of the system because of the "gross factor" of maggots.
  - SixSigma 10y ago
    There is plenty of efficiency to be gained in shifting dry bulk fish food around but I agree it needs to be more quantified for a real comparison. Micheal Polkas claims 10 calories of energy in for 1 calorie out in our current intensive farming, so any improvement at scale promises returns.

saidajigumi 10y ago

> Underground farming in arid regions seems like the best scope for something like this. This sort of mediated growing environment seems quite interesting as an approach to managing agriculture as environmental changes from global warming progress. In some areas, perhaps conventional greenhouses would suffice alone. But in others, underground agriculture might provide an additional buffer against climate changes. This would allow normalization of growing conditions in regions where the climate has drifted and/or become unpredictable in ways that would severely damage agricultural output.

koverstreet 10y ago

Actually, they do use green - they're more reflective in green than red/blue, but it's still mostly absorbed. Apparently at high illuminations you want to be using green too - the issue is that red and blue are absorbed so well that they don't make it past the surface of the leaf and thus the plant quickly maxes out on them, while green light penetrates further and thus reach more tissue. So when you see pictures of leaf greens being grown under purple light - that's because leaf greens actually need less light than other vegetables, you probably wouldn't want to do that with other crops. IANAbotanist

zxcdw 10y ago
Link to a paper talking about this: http://pcp.oxfordjournals.org/content/50/4/684.full http://pcp.oxfordjournals.org/content/50/4/684.full
washadjeffmad 10y ago
This is why we still use high yellow/green CFLs per so many red/blue LEDs. It's not all about getting those high peak spectrum binned LEDs for each type and concentration of chlorophyll.

mdorazio 10y ago

I don't have a definitive answer for you, but you can kind of back into a rough approximation (someone please correct my math if I went astray). High-efficiency solar cells are about 20% efficient across the daylight spectrum. For some reason, no one gives hard stats on how much energy LEDs waste as heat, but the claims are that incandescents waste about 90% of energy input as heat, and LEDs are about 80% more efficient than that. So let's say LEDs convert about 80% of input power into light at their operating wavelengths as a guess. So if we put these together, we'll get about 16% of the input daylight back out as light at the wavelengths we want. According to wikipedia [1], about 47% of light that falls on chlorophyll is wasted because it falls outside the ideal range for plants (400-700nm). Another 3% or so is lost due to wavelength conversion. The rest we can't really do anything about because it has to do with biological limitations. Put another way, our total system has to be about 50% efficient to be better than just putting the plants in the sun. As you can see, we're not even close. In fact, it's not even possible since the theoretical maximum efficiency of solar panels is only about 34% [2]. [1] https://en.wikipedia.org/wiki/Photosynthetic_efficiency https://en.wikipedia.org/wiki/Photosynthetic_efficiency [2] https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit https://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limi...

deleted 10y ago
[deleted]
TTPrograms 10y ago
The Shockley-Queisser limit only applies to single-layer cells under unconcentrated light. Different cell designs have surpassed it by significant amounts in practice already.
- mdorazio 10y ago
  Do you have a link where I can read more? I'm aware of light concentrators being more efficient, but that defeats the whole point since the surface area required is far more than size of the panel itself.
  - 10y ago

kibwen 10y ago

This question was anwered in a prior recent HN thread: https://news.ycombinator.com/item?id=11849172 https://news.ycombinator.com/item?id=11849172 Not an expert so I can't vouch for its veracity, but I'll inline it: "This can theoretically be more efficient than growing plants under sunlight. Chlorophyll mostly absorbs light in the 400nm to 700nm band which is about half of all the photons that hit lights. Even in that band ~24% (https://en.wikipedia.org/wiki/Photosynthetic_efficiency https://en.wikipedia.org/wiki/Photosynthetic_efficiency) of the energy in them in lost due to the higher energy photons (closer to the 400nm side) being converted to lower energy photons. "So right there plants are only 25% efficient at collecting energy from photons. This is why you see indoor plants grown under red LEDs. Modern commercially available solar panels are about 21% efficient so that is actually getting close to plant can do (I'm not going to include inefficiencies in power conversion right now). Solar panels in the high 40% efficiency range exist, but they are crazy expensive and only used on things like satellites. "So using current (very advanced) technology we could actually get about double the plant output per area land by using the most advanced solar panels we have to collect light energy to power LEDs."

Retric 10y ago
Assuming zero production costs, 100% efficient LED's, 100% efficient energy transfer, 100% land coverage with Panels that perfectly tile the surface, zero panel degradation, zero dust, etc etc. In the real world wind energy probably works out much better.
petra 10y ago
Sure it's great for optimizing land use. Agricultural land is cheap. The real question is cost.
- yetihehe 10y ago
  Yeah, but imagine this on deserts. You havae too much light and almost no agricultural land. So, use panels to gather light, use hydroponics/aeroponics to grow plants and now you can distribute your sunlight to many levels of racks (which can be underground under panels which means even more efficiency than on normal land.

deleted 10y ago

[deleted]

aaron695 10y ago

> Underground farming Building underground is crazy expensive (albeit very cool sci-fi), why not (cheap) above ground. I think arid regions are kinda cheap for land so why not just a glasshouse? Put solar panels beside it, should they work. Even if the land is very expensive (And localize food production makes sense), floor one is plants, flood 2+ is for businesses. Building multifloors underground is the same structurally as above ground. Exceptions might be places like London where regulations stop you building up.

Joof 10y ago

Plants do still absorb green and are arguably better off if they do (although the latter may be less certain). The bits that absorb green are more sparse, but light will bounce around inside the leaves until it hits one.

choosername 10y ago
It depends on the plant, obvious from the different color.

sova 10y ago

I live in Cleveland currently and in the winter time there is no sun. Thus, LEDs are an excellent alternative, and can be used elsewhere during nighttime. The benefits are not "bringing the sun via proxy" ... the benefits are "we have focused on the wavelengths that most rapidly and efficiently excite chlorophylls alpha and beta (harvest sun color and the noon-time color of the summer, if I may approximate the "feeling" of the two active bumps... carotenoids are the main chlorophyll in carrots).