Humans make carbon dioxide. Carbon dioxide is (edit: sometimes claimed to be) bad for cognition. But plants turn carbon dioxide back into oxygen. And plants are the one true home decoration strategy. So maybe if you get a lot of plants, you can you can keep carbon dioxide in check and keep your brain working?
It’s theoretically possible. It’s probably just barely possible in practice. But it won’t be easy.
People produce ~1 kilogram of carbon dioxide per day. That’s around 5.7 × 10²³ molecules or 0.948 moles per hour. (You may remember from high school that a mole is a gigantic number made up to avoid having factors of 10²³ everywhere.) Let’s keep it simple and call it one mole per hour.
Meanwhile, plants turn carbon dioxide into oxygen through photosynthesis, i.e. the chemical reaction of (6 water molecules) + (6 carbon dioxide molecules) + (energy) → (1 glucose molecule) + (6 oxygen molecules). The minimum energy physically needed to convert 1 mole of carbon dioxide into glucose and oxygen via this reaction is ~477 kilojoules.
So we’ve already got a lower bound. Say you have magical plants that somehow channel all incoming energy into photosynthesis with perfect efficiency. They’ll need ~477 kilojoules per hour, which converts to a continuous usage of 132.5 watts.1 That’s a bit more than what’s used by two incandescent light bulbs, which isn’t too bad.
But you don’t have magical plants. Real plants do photosynthesis through a physical process with two steps, each of which involves four electrons absorbing a photon. That means you need eight photons per carbon dioxide molecule. If you want to tune your lights for maximum efficiency, you should give each photon exactly the minimum energy necessary to excite an electron, which happens to be ~1.8 eV. That corresponds to pure red light with a wavelength of 680 nm, and a continuous usage of 386 watts.2 No physical system using chloroplasts can neutralize your CO₂ using less than that. Somewhat high, but still manageable.
But your houseplants won’t be able to grab every single photon that hits them and direct it towards photosynthesis. In practice, ~30% of photons will reflect off the plant, or go through it, or hit some part of the plant other than the chloroplasts. That brings us to 551 watts.3
And there’s another issue. After plants make glucose, what happens to it? Some is used to grow more plant, which permanently sequesters carbon from the environment. But lots is also burned by the plant for the general business of staying alive, releasing the carbon back into the air. The exact amount burned in this way varies based on species and conditions, but around 40% loss reasonable,4 bringing us to 918 watts.5
That doesn’t sound that bad. But have you considered what it would be like to live in the same room with 918 watts of pure red light? In terms of radiant power, that’s the same as produced by ~765 incandescent lightbulbs.6 Modern LED grow bulbs are ~50% efficient, meaning you’ll actually need to spend ~1836 watts. If you’re imagining plants that you can actually see, adjust that upwards again for all the light lost to the room. And if you want to use normal light frequencies instead of living Red Life, then your LED bulbs will be less efficient at creating light and your plants will be less efficient at capturing it. Realistically, we’re talking about something like 5,000-10,000 watts, most of which is lost to the room as heat. Imagine five space heaters blasting you on high all the time.
But maybe you’re OK living in a tanning booth. Or maybe you’ll keep your plants in a perfectly reflective chamber. Or maybe your house has a glass ceiling and infinite free sunlight and free climate control. That’s cool. But have you forgotten about your old friend, photosynthetic photon flux density?
Plants can’t absorb infinite amounts of light. Chloroplasts take time to “reset” before they can absorb more photons. Your pet fern can only absorb ~52 watts of energy per square meter of leaf surface area.7 So no matter how much light you can produce, if you want to neutralize the carbon dioxide you make, you will need at least 918 / 52 = 17.6 square meters of fern leaf. Picture a 4.2 meter square wall, packed solid with ferns. If there are any gaps, stems, soil, or wall showing, it needs to be even larger. That’s the absolute minimum.
But maybe that still sounds OK? Fine. But consider one last barrier: Plants obey the laws of physics [citation needed]. If they remove carbon from the air, they must put that carbon somewhere. The only place it can go other than back into the air is into the plant itself.
The 1 kg of carbon dioxide you produce each day corresponds to 273 grams of elemental carbon. The only way for a plant to hide that is by making more plant. But dry plant matter is only ~50% carbon, and for each gram of dry plant matter, plants have 5-10 grams of water (varying a lot by species). So in order to sequester all the carbon you make, each day you will need to grow around
(1 kg carbon dioxide)
× (0.273 kg elemental carbon / kg carbon dioxide)
× (2 kg dry plant / kg elemental carbon)
× (8.5 kg actual plant / kg dry plant)
= 4.6 kg actual plant.
Your garden must grow that much, every day. That’s 140 kg per month. You must prune and discard all that outside, or your garden is not actually sequestering anything.
In conclusion:
- Build an industrial indoor farm.
- Weigh it.
- Wait two weeks.
- Weigh it again.
- Divide the increase in weight by your own body mass.
- That’s the fraction of your CO₂ that you’re removing from the environment.
- Open a window.
-
Behold the power of arithmetic:
(1 mole CO₂ / hour)
× (477 kJ / mole CO₂)
= 132.5 watts. ↩ -
Again using the power of units:
(1 mole CO₂ / hour)
× (8 photons / CO₂ molecule)
× (1.8 eV / photon)
= 385.94 wattsSo chloroplasts are at most ~34% (132.5 / 385.94) efficient at channeling the energy in light into photosynthesis. ↩
-
I find this 30% number amazingly low. (Well done, evolution.) And perhaps it should be somewhat lower. For one thing, the 30% figure comes from sunlight filtered to the 400-700 nm range. If you’ve got pure 680 nm light, absorption should be somewhat higher. Also, if photons are absorbed by some part of the plant other than the chloroplasts, they become heat and the energy is gone. But if they’re reflected or go through the plant, then they might go on to hit some other plant (provided you have a lot of plants around). If you really have pure 680 nm light and you have very densely packed plants, maybe you could drop this to 10-20%. ↩
-
Wikipedia quotes a 35-45% loss just for respiration in the leaf itself. But then this paper shows numbers ranging from 30% to 56% depending on the species and growth rate. ↩
-
I’ve estimated an overall efficiency of 132.5 watts / 918 watts ≈ 14.4%. If you go to Wikipedia, it estimates that ideal leaf efficiency with sunlight is only around 5.4%. That’s because sunlight contains a wide band of wavelengths and my calculation assumed an ideal 680 nm source. Around 47% falls outside the 400-700 nm range, and inside that range, around 24% is lost due to higher-energy photons with energy that gets wasted as heat. If you account for that, my estimate becomes 14.4% × (1-0.47) × (1-0.24) = 5.8%, which is close enough for government work. ↩
-
A traditional “60 watt” incandescent lightbulb is rated based on the power input. But only around 2% of that energy is actually converted to light. So 918 watts of pure red light isn’t what you get from 918 / 60 = 15.3 lightbulbs. It’s what you get from 918 / 60 / .02 = 765 lightbulbs. That said, your eyes aren’t very sensitive to 680 nm light, so the perceived lux wouldn’t be nearly so bad. ↩
-
The saturation point of plants is usually given in units of 300 μmol/m²/s. That the number of photons (in micromoles) that can be absorbed, per square meter of leaf, per second. A typical value for a shade-tolerant houseplant would be ~300 μmol/m²/s. If we assume again that the light is 680 nm so that each photon carries 1.8 eV of energy, then ~300 μmol of photons carries 51.92 joules. That’s 51.92 joules of energy per square meter of leaf surface, i.e. 52 watts. ↩
Does creatine make you smarter? · science health
Life with hazard ratios · science health math
The worthlessness of vitamin D is mildly exaggerated · science health effort
Is "colorectal cancer" rising in "young people"? · science health
Humans make carbon dioxide. Carbon dioxide is (edit: sometimes claimed to be) bad for cognition. But plants turn carbon dioxide back into oxygen. And plants are the one true home decoration strategy. So maybe if you get a lot of plants, you can you can keep carbon dioxide in check and keep your brain working?
It’s theoretically possible. It’s probably just barely possible in practice. But it won’t be easy.
People produce ~1 kilogram of carbon dioxide per day. That’s around 5.7 × 10²³ molecules or 0.948 moles per hour. (You may remember from high school that a mole is a gigantic number made up to avoid having factors of 10²³ everywhere.) Let’s keep it simple and call it one mole per hour.
Meanwhile, plants turn carbon dioxide into oxygen through photosynthesis, i.e. the chemical reaction of (6 water molecules) + (6 carbon dioxide molecules) + (energy) → (1 glucose molecule) + (6 oxygen molecules). The minimum energy physically needed to convert 1 mole of carbon dioxide into glucose and oxygen via this reaction is ~477 kilojoules.
So we’ve already got a lower bound. Say you have magical plants that somehow channel all incoming energy into photosynthesis with perfect efficiency. They’ll need ~477 kilojoules per hour, which converts to a continuous usage of 132.5 watts.1 That’s a bit more than what’s used by two incandescent light bulbs, which isn’t too bad.
But you don’t have magical plants. Real plants do photosynthesis through a physical process with two steps, each of which involves four electrons absorbing a photon. That means you need eight photons per carbon dioxide molecule. If you want to tune your lights for maximum efficiency, you should give each photon exactly the minimum energy necessary to excite an electron, which happens to be ~1.8 eV. That corresponds to pure red light with a wavelength of 680 nm, and a continuous usage of 386 watts.2 No physical system using chloroplasts can neutralize your CO₂ using less than that. Somewhat high, but still manageable.
But your houseplants won’t be able to grab every single photon that hits them and direct it towards photosynthesis. In practice, ~30% of photons will reflect off the plant, or go through it, or hit some part of the plant other than the chloroplasts. That brings us to 551 watts.3
And there’s another issue. After plants make glucose, what happens to it? Some is used to grow more plant, which permanently sequesters carbon from the environment. But lots is also burned by the plant for the general business of staying alive, releasing the carbon back into the air. The exact amount burned in this way varies based on species and conditions, but around 40% loss reasonable,4 bringing us to 918 watts.5
That doesn’t sound that bad. But have you considered what it would be like to live in the same room with 918 watts of pure red light? In terms of radiant power, that’s the same as produced by ~765 incandescent lightbulbs.6 Modern LED grow bulbs are ~50% efficient, meaning you’ll actually need to spend ~1836 watts. If you’re imagining plants that you can actually see, adjust that upwards again for all the light lost to the room. And if you want to use normal light frequencies instead of living Red Life, then your LED bulbs will be less efficient at creating light and your plants will be less efficient at capturing it. Realistically, we’re talking about something like 5,000-10,000 watts, most of which is lost to the room as heat. Imagine five space heaters blasting you on high all the time.
But maybe you’re OK living in a tanning booth. Or maybe you’ll keep your plants in a perfectly reflective chamber. Or maybe your house has a glass ceiling and infinite free sunlight and free climate control. That’s cool. But have you forgotten about your old friend, photosynthetic photon flux density?
Plants can’t absorb infinite amounts of light. Chloroplasts take time to “reset” before they can absorb more photons. Your pet fern can only absorb ~52 watts of energy per square meter of leaf surface area.7 So no matter how much light you can produce, if you want to neutralize the carbon dioxide you make, you will need at least 918 / 52 = 17.6 square meters of fern leaf. Picture a 4.2 meter square wall, packed solid with ferns. If there are any gaps, stems, soil, or wall showing, it needs to be even larger. That’s the absolute minimum.
But maybe that still sounds OK? Fine. But consider one last barrier: Plants obey the laws of physics [citation needed]. If they remove carbon from the air, they must put that carbon somewhere. The only place it can go other than back into the air is into the plant itself.
The 1 kg of carbon dioxide you produce each day corresponds to 273 grams of elemental carbon. The only way for a plant to hide that is by making more plant. But dry plant matter is only ~50% carbon, and for each gram of dry plant matter, plants have 5-10 grams of water (varying a lot by species). So in order to sequester all the carbon you make, each day you will need to grow around
(1 kg carbon dioxide)
× (0.273 kg elemental carbon / kg carbon dioxide)
× (2 kg dry plant / kg elemental carbon)
× (8.5 kg actual plant / kg dry plant)
= 4.6 kg actual plant.
Your garden must grow that much, every day. That’s 140 kg per month. You must prune and discard all that outside, or your garden is not actually sequestering anything.
In conclusion:
- Build an industrial indoor farm.
- Weigh it.
- Wait two weeks.
- Weigh it again.
- Divide the increase in weight by your own body mass.
- That’s the fraction of your CO₂ that you’re removing from the environment.
- Open a window.
-
Behold the power of arithmetic:
(1 mole CO₂ / hour)
× (477 kJ / mole CO₂)
= 132.5 watts. ↩ -
Again using the power of units:
(1 mole CO₂ / hour)
× (8 photons / CO₂ molecule)
× (1.8 eV / photon)
= 385.94 wattsSo chloroplasts are at most ~34% (132.5 / 385.94) efficient at channeling the energy in light into photosynthesis. ↩
-
I find this 30% number amazingly low. (Well done, evolution.) And perhaps it should be somewhat lower. For one thing, the 30% figure comes from sunlight filtered to the 400-700 nm range. If you’ve got pure 680 nm light, absorption should be somewhat higher. Also, if photons are absorbed by some part of the plant other than the chloroplasts, they become heat and the energy is gone. But if they’re reflected or go through the plant, then they might go on to hit some other plant (provided you have a lot of plants around). If you really have pure 680 nm light and you have very densely packed plants, maybe you could drop this to 10-20%. ↩
-
Wikipedia quotes a 35-45% loss just for respiration in the leaf itself. But then this paper shows numbers ranging from 30% to 56% depending on the species and growth rate. ↩
-
I’ve estimated an overall efficiency of 132.5 watts / 918 watts ≈ 14.4%. If you go to Wikipedia, it estimates that ideal leaf efficiency with sunlight is only around 5.4%. That’s because sunlight contains a wide band of wavelengths and my calculation assumed an ideal 680 nm source. Around 47% falls outside the 400-700 nm range, and inside that range, around 24% is lost due to higher-energy photons with energy that gets wasted as heat. If you account for that, my estimate becomes 14.4% × (1-0.47) × (1-0.24) = 5.8%, which is close enough for government work. ↩
-
A traditional “60 watt” incandescent lightbulb is rated based on the power input. But only around 2% of that energy is actually converted to light. So 918 watts of pure red light isn’t what you get from 918 / 60 = 15.3 lightbulbs. It’s what you get from 918 / 60 / .02 = 765 lightbulbs. That said, your eyes aren’t very sensitive to 680 nm light, so the perceived lux wouldn’t be nearly so bad. ↩
-
The saturation point of plants is usually given in units of 300 μmol/m²/s. That the number of photons (in micromoles) that can be absorbed, per square meter of leaf, per second. A typical value for a shade-tolerant houseplant would be ~300 μmol/m²/s. If we assume again that the light is 680 nm so that each photon carries 1.8 eV of energy, then ~300 μmol of photons carries 51.92 joules. That’s 51.92 joules of energy per square meter of leaf surface, i.e. 52 watts. ↩
Does creatine make you smarter? · science health
Life with hazard ratios · science health math
The worthlessness of vitamin D is mildly exaggerated · science health effort
Is "colorectal cancer" rising in "young people"? · science health
Humans make carbon dioxide. Carbon dioxide is (edit: sometimes claimed to be) bad for cognition. But plants turn carbon dioxide back into oxygen. And plants are the one true home decoration strategy. So maybe if you get a lot of plants, you can you can keep carbon dioxide in check and keep your brain working?
It’s theoretically possible. It’s probably just barely possible in practice. But it won’t be easy.
People produce ~1 kilogram of carbon dioxide per day. That’s around 5.7 × 10²³ molecules or 0.948 moles per hour. (You may remember from high school that a mole is a gigantic number made up to avoid having factors of 10²³ everywhere.) Let’s keep it simple and call it one mole per hour.
Meanwhile, plants turn carbon dioxide into oxygen through photosynthesis, i.e. the chemical reaction of (6 water molecules) + (6 carbon dioxide molecules) + (energy) → (1 glucose molecule) + (6 oxygen molecules). The minimum energy physically needed to convert 1 mole of carbon dioxide into glucose and oxygen via this reaction is ~477 kilojoules.
So we’ve already got a lower bound. Say you have magical plants that somehow channel all incoming energy into photosynthesis with perfect efficiency. They’ll need ~477 kilojoules per hour, which converts to a continuous usage of 132.5 watts.1 That’s a bit more than what’s used by two incandescent light bulbs, which isn’t too bad.
But you don’t have magical plants. Real plants do photosynthesis through a physical process with two steps, each of which involves four electrons absorbing a photon. That means you need eight photons per carbon dioxide molecule. If you want to tune your lights for maximum efficiency, you should give each photon exactly the minimum energy necessary to excite an electron, which happens to be ~1.8 eV. That corresponds to pure red light with a wavelength of 680 nm, and a continuous usage of 386 watts.2 No physical system using chloroplasts can neutralize your CO₂ using less than that. Somewhat high, but still manageable.
But your houseplants won’t be able to grab every single photon that hits them and direct it towards photosynthesis. In practice, ~30% of photons will reflect off the plant, or go through it, or hit some part of the plant other than the chloroplasts. That brings us to 551 watts.3
And there’s another issue. After plants make glucose, what happens to it? Some is used to grow more plant, which permanently sequesters carbon from the environment. But lots is also burned by the plant for the general business of staying alive, releasing the carbon back into the air. The exact amount burned in this way varies based on species and conditions, but around 40% loss reasonable,4 bringing us to 918 watts.5
That doesn’t sound that bad. But have you considered what it would be like to live in the same room with 918 watts of pure red light? In terms of radiant power, that’s the same as produced by ~765 incandescent lightbulbs.6 Modern LED grow bulbs are ~50% efficient, meaning you’ll actually need to spend ~1836 watts. If you’re imagining plants that you can actually see, adjust that upwards again for all the light lost to the room. And if you want to use normal light frequencies instead of living Red Life, then your LED bulbs will be less efficient at creating light and your plants will be less efficient at capturing it. Realistically, we’re talking about something like 5,000-10,000 watts, most of which is lost to the room as heat. Imagine five space heaters blasting you on high all the time.
But maybe you’re OK living in a tanning booth. Or maybe you’ll keep your plants in a perfectly reflective chamber. Or maybe your house has a glass ceiling and infinite free sunlight and free climate control. That’s cool. But have you forgotten about your old friend, photosynthetic photon flux density?
Plants can’t absorb infinite amounts of light. Chloroplasts take time to “reset” before they can absorb more photons. Your pet fern can only absorb ~52 watts of energy per square meter of leaf surface area.7 So no matter how much light you can produce, if you want to neutralize the carbon dioxide you make, you will need at least 918 / 52 = 17.6 square meters of fern leaf. Picture a 4.2 meter square wall, packed solid with ferns. If there are any gaps, stems, soil, or wall showing, it needs to be even larger. That’s the absolute minimum.
But maybe that still sounds OK? Fine. But consider one last barrier: Plants obey the laws of physics [citation needed]. If they remove carbon from the air, they must put that carbon somewhere. The only place it can go other than back into the air is into the plant itself.
The 1 kg of carbon dioxide you produce each day corresponds to 273 grams of elemental carbon. The only way for a plant to hide that is by making more plant. But dry plant matter is only ~50% carbon, and for each gram of dry plant matter, plants have 5-10 grams of water (varying a lot by species). So in order to sequester all the carbon you make, each day you will need to grow around
(1 kg carbon dioxide)
× (0.273 kg elemental carbon / kg carbon dioxide)
× (2 kg dry plant / kg elemental carbon)
× (8.5 kg actual plant / kg dry plant)
= 4.6 kg actual plant.
Your garden must grow that much, every day. That’s 140 kg per month. You must prune and discard all that outside, or your garden is not actually sequestering anything.
In conclusion:
- Build an industrial indoor farm.
- Weigh it.
- Wait two weeks.
- Weigh it again.
- Divide the increase in weight by your own body mass.
- That’s the fraction of your CO₂ that you’re removing from the environment.
- Open a window.
-
Behold the power of arithmetic:
(1 mole CO₂ / hour)
× (477 kJ / mole CO₂)
= 132.5 watts. ↩ -
Again using the power of units:
(1 mole CO₂ / hour)
× (8 photons / CO₂ molecule)
× (1.8 eV / photon)
= 385.94 wattsSo chloroplasts are at most ~34% (132.5 / 385.94) efficient at channeling the energy in light into photosynthesis. ↩
-
I find this 30% number amazingly low. (Well done, evolution.) And perhaps it should be somewhat lower. For one thing, the 30% figure comes from sunlight filtered to the 400-700 nm range. If you’ve got pure 680 nm light, absorption should be somewhat higher. Also, if photons are absorbed by some part of the plant other than the chloroplasts, they become heat and the energy is gone. But if they’re reflected or go through the plant, then they might go on to hit some other plant (provided you have a lot of plants around). If you really have pure 680 nm light and you have very densely packed plants, maybe you could drop this to 10-20%. ↩
-
Wikipedia quotes a 35-45% loss just for respiration in the leaf itself. But then this paper shows numbers ranging from 30% to 56% depending on the species and growth rate. ↩
-
I’ve estimated an overall efficiency of 132.5 watts / 918 watts ≈ 14.4%. If you go to Wikipedia, it estimates that ideal leaf efficiency with sunlight is only around 5.4%. That’s because sunlight contains a wide band of wavelengths and my calculation assumed an ideal 680 nm source. Around 47% falls outside the 400-700 nm range, and inside that range, around 24% is lost due to higher-energy photons with energy that gets wasted as heat. If you account for that, my estimate becomes 14.4% × (1-0.47) × (1-0.24) = 5.8%, which is close enough for government work. ↩
-
A traditional “60 watt” incandescent lightbulb is rated based on the power input. But only around 2% of that energy is actually converted to light. So 918 watts of pure red light isn’t what you get from 918 / 60 = 15.3 lightbulbs. It’s what you get from 918 / 60 / .02 = 765 lightbulbs. That said, your eyes aren’t very sensitive to 680 nm light, so the perceived lux wouldn’t be nearly so bad. ↩
-
The saturation point of plants is usually given in units of 300 μmol/m²/s. That the number of photons (in micromoles) that can be absorbed, per square meter of leaf, per second. A typical value for a shade-tolerant houseplant would be ~300 μmol/m²/s. If we assume again that the light is 680 nm so that each photon carries 1.8 eV of energy, then ~300 μmol of photons carries 51.92 joules. That’s 51.92 joules of energy per square meter of leaf surface, i.e. 52 watts. ↩
Does creatine make you smarter? · science health
Life with hazard ratios · science health math
The worthlessness of vitamin D is mildly exaggerated · science health effort
Is "colorectal cancer" rising in "young people"? · science health
人类呼出二氧化碳。二氧化碳（编辑注：有时被声称是）对认知能力有害。但植物能将二氧化碳变回氧气。而植物是唯一正宗的家居装饰策略。所以，如果你养很多植物，或许就能控制住二氧化碳，让你的大脑保持运转？
这在理论上是可能的。在实践中可能也只是勉强可行。但绝非易事。
人每天产生约1公斤二氧化碳。这相当于每小时约5.7 × 10²³个分子，即0.948摩尔。（你可能还记得高中时学过，摩尔是一个巨大的数字，是为了避免到处都是10²³的因子而发明的。）我们简化一下，就算每小时1摩尔。
与此同时，植物通过光合作用将二氧化碳转化为氧气，即化学反应式：（6个水分子）+（6个二氧化碳分子）+（能量）→（1个葡萄糖分子）+（6个氧分子）。通过该反应将1摩尔二氧化碳转化为葡萄糖和氧气所需的最低物理能量约为477千焦。
这样我们就已经得到了一个下限。假设你有魔法植物，能以完美的效率将所有入射能量转化为光合作用。它们每小时需要约477千焦，换算成持续功率就是132.5瓦。1这比两只白炽灯泡的功率略高一点，还不算太糟。
但你没有魔法植物。真正的植物通过一个物理过程进行光合作用，该过程分两步，每一步都涉及四个电子各吸收一个光子。这意味着每个二氧化碳分子需要八个光子。如果你想让灯光效率最大化，就应该给每个光子恰好足以激发电子的最低能量，即约1.8 eV。这对应波长680纳米的纯红光，持续功率为386瓦。2任何使用叶绿体的物理系统都无法用低于这个功率来中和你的二氧化碳。虽然偏高，但还能应付。
但你的室内植物不可能捕捉到照射到它们身上的每一个光子并将其用于光合作用。实际上，大约30%的光子会被植物反射、穿透而过，或者照射到植物上叶绿体以外的部分。这样功率就达到了551瓦。3
还有另一个问题。植物制造出葡萄糖之后，它去了哪里？一部分用于植物生长，从而将碳永久地从环境中固存下来。但还有很大一部分被植物为维持生命活动而燃烧掉，将碳重新释放回空气中。具体燃烧的比例因物种和环境而异，但大约40%的损耗是合理的，4这样功率就达到了918瓦。5
听起来好像没那么糟糕。但你有没有想过，和918瓦的纯红光待在同一个房间里是什么感觉？就辐射功率而言，这相当于约765只白炽灯泡发出的光。6现代LED植物生长灯的能效约为50%，意味着你实际上需要消耗约1836瓦。如果你想要的是肉眼可见的植物，还要把所有散射到房间里的光再重新计算进去。如果你想使用普通的光频而不是过红光人生，那么你的LED灯泡发光效率会更低，植物捕捉光的效率也会更低。实际来说，我们讨论的是大约5000到10000瓦，其中大部分以热量的形式散失到房间里。想象一下五个取暖器同时对着你开到最大档。
但也许你能接受住在美黑舱里。或者你可以把植物放在一个完美反射的房间里。又或者你的房子有玻璃天花板，有无穷无尽的免费阳光和免费空调。那也行。但你有没有忘了你的老朋友——光合光子通量密度？
植物不能无限地吸收光。叶绿体在吸收更多光子之前需要时间"复位"。你的宠物蕨类植物每平方米叶面积只能吸收约52瓦的能量。7所以，无论你能产生多少光，如果你想中和自己产生的二氧化碳，你至少需要918 / 52 = 17.6平方米的蕨类植物叶片。想象一面4.2米见方的墙，密密麻麻地种满蕨类植物。如果有任何缝隙、茎干、土壤或墙面露出来，面积还需要更大。这是绝对下限。
但也许这样听起来也还能接受？好吧。但还有最后一道障碍：植物遵守物理定律\[需要引证]。如果它们从空气中移除碳，就必须把这些碳放在某个地方。除了重新回到空气中，碳唯一能去的地方就是植物本身。
你每天产生的1公斤二氧化碳对应273克元素碳。植物要储存这些碳，唯一的方法就是长出更多的植物。但干植物物质中碳含量只有约50%，而且每克干植物物质对应着5到10克的水分（因物种不同差异很大）。所以，要将你产生的所有碳固存下来，你每天需要种植大约
(1公斤二氧化碳)
× (0.273公斤元素碳 / 公斤二氧化碳)
× (2公斤干植物 / 公斤元素碳)
× (8.5公斤实际植物 / 公斤干植物)
= 4.6公斤实际植物。
你的花园每天都要长出这么多。那就是每个月140公斤。你必须把这些都修剪掉并扔到室外，否则你的花园实际上并没有固存任何碳。
结论：
- 建一个工业化的室内农场。
- 称重。
- 等两周。
- 再次称重。
- 用增加的重量除以你的体重。
- 这就是你从环境中去除的二氧化碳比例。
- 打开一扇窗。
-
(1摩尔二氧化碳 / 小时)
× (477千焦 / 摩尔二氧化碳)
= 132.5瓦。 ↩ -
再次运用单位换算的力量：
(1摩尔二氧化碳 / 小时)
× (8个光子 / 二氧化碳分子)
× (1.8 eV / 光子)
= 385.94瓦所以叶绿体将光能转化为光合作用的效率至多约为34%（132.5 / 385.94）。 ↩
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我觉得30%这个数字低得惊人。（进化干得漂亮。）而且这个数字可能还应该更低一些。首先，30%这个数字来自将阳光过滤到400-700纳米范围后的结果。如果你用的是纯680纳米的光，吸收率应该会高一些。另外，如果光子被植物叶绿体以外的部分吸收，它们就变成了热量，能量就消失了。但如果光子被反射或穿过植物，它们可能会继续照射到另一株植物上（前提是你周围有很多植物）。如果你真的有纯680纳米的光，而且植物非常密集地排列，也许你确实可以把损耗降到10-20%。 ↩
-
维基百科引用35-45%的损耗仅来自叶片自身的呼吸作用。但这篇论文显示的数字在30%到56%之间，取决于物种和生长速率。 ↩
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我估算的总效率为132.5瓦 / 918瓦 ≈ 14.4%。如果你去查维基百科，它估算理想叶片在阳光下的效率只有约5.4%。那是因为太阳光包含很宽的波长范围，而我的计算假设了理想的680纳米光源。大约47%的能量落在400-700纳米范围之外，而在这个范围内，又有约24%因为高能光子的能量以热量形式浪费掉而损失。如果把这些考虑进去，我的估算就变成14.4% × (1-0.47) × (1-0.24) = 5.8%，对政府工作来说已经够精确了。 ↩
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传统"60瓦"白炽灯泡的额定功率是基于输入功率。但其中只有约2%的能量真正转化为光。所以918瓦的纯红光不是918 / 60 = 15.3只灯泡的效果，而是918 / 60 / 0.02 = 765只灯泡的效果。不过，你的眼睛对680纳米的光并不是很敏感，所以感觉到的照度不会那么糟糕。 ↩
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植物的饱和点通常以300 μmol/m²/s为单位给出。这是每平方米叶片每秒可以吸收的光子数（以微摩尔计）。典型的耐阴室内植物大约为300 μmol/m²/s。如果我们再次假设光源为680纳米，每个光子携带1.8 eV的能量，那么约300 μmol的光子携带51.92焦耳的能量。也就是说每平方米叶面积有51.92焦耳的能量，即52瓦。 ↩
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