LIGHT EFFICIENCY: Chemists are working to understand the efficiency with which plants harvest sunlight so that they can capture more of it with artificial systems.
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Plants take advantage of quantum mechanics to harvest sunlight with near-perfect efficiency—though only roughly 2 percent of that capture sunlight ultimately gets stored as chemical energy. Now scientists are studying how this light-harvesting step of photosynthesis is optimized by nature to learn how to mimic it in engineered systems for use in solar cells or artificial leaves that produce fuels directly from the sun.
Plants rely on chromophores—molecules that absorb certain wavelengths of visible light while reflecting others—to harvest energy from the sun. When sunlight hits a plant, electrons in the topmost chromophores absorb energy from incoming photons and then transfer it from the newly energized molecule to another molecule at a lower energy state. That transfer repeats itself via a chain of molecules, a cascade of rapid energy pass-offs that ultimately separates an electron from the last chromophore in the chain, which provides energy that is stored by the plant as a carbohydrate.
In this way chromophores perform three functions: they absorb energy from sunlight (acting as "acceptors"); they donate the energy they absorbed (as "donors"); and they transfer energy to another molecule (as "bridges"). Using measurements from other researchers of the intensity of photons absorbed and emitted by chromophores, chemist Jianshu Cao and his colleagues at the Massachusetts Institute of Technology developed a computer model to arrive at the ratio of acceptors, donors and bridges that optimizes the efficiency of the light-harvesting step of photosynthesis.
The findings: there is an optimal ratio of 10 donors for each acceptor in order to efficiently transfer energy in a natural photosynthetic system with just those two chromophore functions. Adding bridges to an arrangement of donors and acceptors then further increases the efficiency of energy transfer, Cao says.
Chromophores are arranged in bundles in plant cells, and these structures and configurations influence light-harvesting efficiency as well. University of California, Berkeley, chemist Matt Francis created artificial light-harvesting systems by attaching chromophores to tobacco mosaic virus molecules. Modeling these genetically engineered systems, Cao found that one structure—stacks of chromophore disks—could be tuned to improve the overall efficiency by combining multiple disks of similar size but different combinations of bridges, acceptors and donors. One particular configuration of two disks comprising bridges and acceptors stacked between disks made entirely of donors is a good candidate for designing artificial light-harvesting devices, according to the study published October 21 in The Journal of Physical Chemistry B.
Earlier research found that photosynthesis takes advantage of an effect known as quantum coherence. In one study researchers found that the energy absorbed by a chromophore travels through multiple networks at the same time in order to take the quickest path. Other research observed that "noise," or random fluctuations, at the quantum level helps move energy from chromophores to the reaction centers of photosynthesis. Building on this work, Cao and M.I.T. chemist Robert Silbey modeled a light-harvesting system in green sulfur bacteria and found that photosynthesis is most efficient when there is an intermediate amount of noise in the system. "In experimental conditions one always tries to reduce noise," Cao says, "but in a quantum mechanical system, it's actually useful to have some noise."
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21 Comments
Add CommentFrom an engineering perspective, I have to wonder whether following biological models of storing light as any carbon based substance that must be burned to retrieve its energy makes any sense, since, as I understand, our objective is to reduce co2 emissions.
Reply | Report Abuse | Link to thisGood point "jtdwyer". Talk about an oxymoron. Some scientists are working hard to get us away from having to burn anything to produce energy and other scientists are working hard to keep us burning every thing to produce energy.
Reply | Report Abuse | Link to thisBy the way... the first Nissan Leaf came off the production line to its first owner Saturday, and guess what...you can actually use geothermal energy to charge the darn think. Nothing being burnt there, and one of the Leafs have a solar powered spoiler on the back of it.
Look like we are getting closer to inventing an actual tree of life.
Reply | Report Abuse | Link to thisWell I'm no expert, but I think there is a huge difference from burning something you created by pulling carbon from the atmosphere, and burning something that was storing CO2 in the ground...
Reply | Report Abuse | Link to thisIf you create algae fuels, they are carbon neutral, as the carbon burning them adds to the atmosphere is the same carbon they pulled out when created.
If you burn oil, you are taking previously stored carbon and adding to the atmosphere... a much worse scenario.
Two points: firstly, the energy generation from this system could be directly into electrical, invalidating the issue. Secondly, even if the solar energy was used to create hydrocarbons for burning, the raw material for this would be, guess what? CO2.
Reply | Report Abuse | Link to thisThis is, after all, the process that plants themselves go through. The advantages of an artificial system would be the loss of the need for Respiration (the process that burns carbohydrates and produces CO2, that plants and animals must run to survive), therefore it would be "carbon-negative".
Good point - the process would appear to be carbon neutral or, as Vzanchi points out, potentially carbon negative if engineered to directly produce electricity. Thanks!
Reply | Report Abuse | Link to thisOnly in the world of environmental activism can the cognitive dissonance of "harvest sunlight with near perfect efficiency" and "2 percent gets stored as usable energy" be part of the same thought process.
Reply | Report Abuse | Link to thisBefore you try to drop an insult on environmental activists, perhaps you should educate yourself on how photosynthesis works.
Reply | Report Abuse | Link to thisfrgough's jibe was fair game in my opinion: environmental activists are quick to label others "liars" and worse. An article pitched at this level shouldn't have left such an apparent glaring discrepancy unexplained. I'd guess you could read a lot about photosynthesis without twigging how it arises.
Reply | Report Abuse | Link to thisjtdwyer;
Reply | Report Abuse | Link to thisFrom an Engineering Prospective, it makes sense. First, the total energy content of a given volume of fuel is higher for hydrocarbons than it is for pure hydrogen. It is also a higher energy density than any current battery technology can give.
From a biological perspective, using CO2 directly from the atmosphere gives no net gain to atmospheric levels. Forests and grasses will continue to lock up carbon. If it is burned with insufficient carbon, charcoal results, which can be back-filled into old coal mines, thus putting the carbon back into the ground. Giving a net overall reduction in atmospheric carbon.
A bigger engineering problem is the area that this process would take up. To have any meaningful impact, it would need to cover millions of acres (Hectares). That is a lot of land, and might severely impact the ecosystem where it is placed.
The first paragraph is pretty clear is explaining that the near-perfect efficiency refers to the light harvesting step, and that the 2% efficiency refers to the overall process of converting the captured sunlight into usable chemical energy.
Reply | Report Abuse | Link to thisWhen you say: "If it is burned with insufficient carbon, charcoal results", don't you mean that if it's burned incompletely charcoal is produced?
Reply | Report Abuse | Link to thisAs you mention, land use and potable water requirements are bigger issues that could also produce higher food prices.
frgough,
Reply | Report Abuse | Link to thisFirst of all, Plants don't harvest the full spectrum of available energy from the sun. Too much energy and they get damaged. Plants specifically target certain wavelengths to prevent excess energy absorption, this is at least one of the reasons chlorophyl is green. On a hypothetical planet where the characteristic wavelengths of light coming from the star it orbited were different photosynthetic organisms would likely be a different color too.
Having said that you are clearly trolling. Lumping anyone who works on alternative energy in the category of activists is silly and has no logical basis. Using "Activist" as a dirty word is also silly and amounts to intentionally clouding communication.
Maybe you should go troll with the flat earthers, and religious extremists. You'd appreciate their logic, or lack thereof.
reply to kexin:
Reply | Report Abuse | Link to thisIf you understand photosynthesis better than some of us, could you please explain how and why a mere "2 percent capture rate" equates to "near-perfect efficiency?"
I quote the first sentence of the article:
"Plants take advantage of quantum mechanics to harvest sunlight with near-perfect efficiency—though only roughly 2 percent of that capture(d) sunlight ultimately gets stored as chemical energy."
Also, note the missing (d). Maybe Alison Snyder should proof read her articles before publication, both for logic and grammar.
Ultimately we will have to deal with energy demand. There is no such thing as clean energy. There are negative consequences regardless of what scource of energy we use.
Reply | Report Abuse | Link to thisToo many people wanting too many things overloads the global systems that sustain life. It is becoming obvious that we have already exceeded the optimum sustainable population density based on the current consumption of raw materials and the distribution of that consumption is not balanced. The more advanced societies are using more than their fair share. Even if advanced societies reduce their per captia consumption up and coming nations like China are increasing their per captia consumption. There are shortages now and it will only get worse in the immediate future. Shortages will lead to conflict. Sane family planning is one of the tools that gets underfunded on a global basis and we just sent an additional 40+ Luddites to congress in the recent election. If we don't learn to police our destructive habits "Mother Nature" will!
I hate it when I agree with a point you make but I noticed that too.
Reply | Report Abuse | Link to thisAs far as the total efficiency of the system as a fuel source goes the good Fr. is correct. It may absorb 100% of certain spectrums of light but a) it bounces a lot of light away and b) it does not store 100% of the light it absorbs. Most of the light it absorbs is used by the plant and thus not stored as fuel.
Reply | Report Abuse | Link to thisMuch the way hydrogen is a terrible fuel source because it burns more joules than it produces in hydrogen fuel, if the solar energy requires a 98% consumption it is significantly less productive than petroleum which uses very little energy to extract and convert to fuel.
frgough - I understand the confusion between harvesting sunlight with "near perfect efficiency" and then only having 2% efficient energy storage. First of all, harvesting sunlight is a quantum process, as the article correctly states. Therefore the process of turning photons into energy in the chromophores may well be near perfect, in terms of quantum efficiency. Quantum efficiency, it turns out, is also used in assaying whether solar cells are doing a good job of capturing the energy from photons. Energy storage is another story. In a plant, the energy harvested from photons by the chromophores has to be turned into chemical energy - sugars - for storage. There are likely many inefficiencies, some escapable and some built-in, plaguing this process. If, as Vzanchi points out, we use photosynthetic inspiration for direct conversion of photons to electricity, we don't have to deal with this - we will only have to overcome the engineering hurdles of making "stacked" chromophores (I know some researchers working on this now) and figuring out how to build the rest of the framework of the solar cell - to get the electricity out of the chromophore stacks. Anyway this was just a simple explanation from a physics student but I hope it helps.
Reply | Report Abuse | Link to thisMaybe I did not understand the article, but couldn't the energy gathered be used to directly generate electricity or stored in batteries for later use?
Reply | Report Abuse | Link to thisdecoherence desireable??
Reply | Report Abuse | Link to thisnow, moving along the path, What reactions do not require light, sir?
Reply | Report Abuse | Link to this