SUNLIGHT CAPTURE: Which is better at capturing the copious energy in sunlight: photosynthesis or photovoltaics?
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For capturing the sun's copious energy, there are basically two available engineering models: photovoltaic (PV) cells that turn it into flowing electrons or photosynthetic plant cells that turn it into plant food. So which does the job better? After all, such a judgment might help inform policymakers on whether to pursue biofuels or solar electricity.
But the question admits no easy answer, because it begs the deeper question of which one values more: the sheer quantity of electrons produced—so-called efficiency—or the transformation of sunlight into stored chemical energy? After all, storage is a high-value proposition that has made fossil oil, originally derived from plants, so valuable—cheap, energy dense, easy to transport and storable for later use. That is not the case for electricity from the sun—or any other source—which must be captured the instant it is produced and currently has a limited and expensive option for storage: batteries.
"Chemical fuels [hydrocarbons, like those in oil] would be the game-changer if you could directly make them efficiently from sunlight," notes chemist Nathan Lewis, who directs a lab focused on just that prospect: the U.S. Department of Energy's Joint Center for Artificial Photosynthesis. "It pairs the biggest source of energy and the biggest storage."
So, a group of 18 biologists, chemists and physicists set out to answer the question by first creating roughly equivalent systems—comparing apples with apples, as it were rather than apples with oranges. Photosynthesis (conducted by algae) turns roughly 3 percent of incoming sunlight into organic compounds, including yet more plant cells, annually. "Artificial photosynthesis"—comprising a PV cell that provides the electricity to split water into hydrogen and oxygen—turns roughly 10 percent of incoming sunlight into usable hydrogen annually.
That discrepancy suggests there might be room for improvement in photosynthesis, according to the analysis published May 13 in Science. After all, solar cells are capable of absorbing more of the energy in sunlight because they capture it across the electromagnetic spectrum ranging from infrared to ultraviolet, whereas chlorophyll and other photosynthetic pigments absorb only visual light. Introducing pigments to plants that would help them capture ultraviolet or infrared light could change that equation.
Another idea would be to reconfigure photosynthesis itself. Presently plants employ two systems—dubbed photosystem I and photosystem II—to convert sunlight, CO2 and water into carbohydrates. But both of these photosystems rely on capturing visible light photons, which means the two systems compete for each incoming ray of sunlight. If scientists tweaked the system so that photosystem I relied on visible light but II absorbed, say, ultraviolet light—the efficiency of plants would improve considerably.
"It would be the biological equivalent of a tandem photovoltaic cell," or the stacked photovoltaic cells that absorb different wavelengths of light, says biochemist Robert Blankenship of Washington University in St. Louis, lead author of the analysis. Stacked PV has been demonstrated to convert more than 40 percent of incoming sunlight into electricity, albeit at a prohibitively high price. Such synthetic photosynthetic organism could then become the fuel refinery of the future—a prospect being actively pursued by the Advanced Research Projects Agency–Energy (ARPA–e), a recently formed federal agency tasked with taking scientific findings on alternative energy and turning them into deployable technologies.
At the same time, any biological sunlight-capture method faces one significant constraint—the enhanced bugs or plants have to be kept alive. "We don't want them using those resources to make bugs; we want them to use them to make fuel," explains chemist Eric Toone, ARPA–e's deputy director for technology and program manager for so-called electrofuels—an effort to harness extremophiles to make fuels for human use—who was not involved in this analysis. "As you tinker with bugs to turn off pathways that aren't doing what you want them to do, you've got to leave the bug capable of staying alive."
Nor did the scientists consider other factors that could diminish the utility of either or both approaches, such as land or water needs, waste, impacts on food supply or any of a host of other relevant considerations. For example, the fact that hydrogen fuel-cell cars still cost hundreds of thousands of dollars might overwhelm the usefulness of artificial photosynthesis to produce the lightest element. Still, simply on the basis of converting the most sunlight to usable energy, artificial photosynthesis wins.
But don't count out nature, enhanced or otherwise, yet. After all, plants do several things very well that photovoltaic cells—or artificial photosynthesis systems—do not, such as absorb CO2 at low concentrations (382 parts-per-million and rising) directly from the air and use sunlight to turn it into fuel and oxygen.
"Natural photosynthesis turns CO2 into sugars with lots of carbon-carbon bonds," says chemist Andrew Bocarsly of Princeton University, who was not involved with the analysis. "We've been studying CO2 chemistry for a long time, more than 100 years, and there's very little evidence that we could do what a leaf does."
Of course, plants also have another significant advantage—a bad photosynthetic cell can repair itself; in fact, that's part of its normal operation. No artificial system yet devised—super-efficient or otherwise—can heal itself.
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17 Comments
Add Commentyes, there are artificial self-regenerating solar cells...
Reply | Report Abuse | Link to thisfrom Nature Chemistry:
http://www.nature.com/nchem/journal/v2/n11/full/nchem.822.html
or, if I may plug my blog post on this paper here:
http://blog.joerg.heber.name/2010/09/10/in-other-news-self-regenerating-solar-cells/
Joerg
@jheber: Thanks for the links. The author' last sentence does not take this information into account.
Reply | Report Abuse | Link to thisBoth systems are needed. We can cover the exteriors of our buildings with photo voltaic windows or sunshades over our walkways and parking lots. Enough light will filter through to support plant life, that feeds us, gives us out oxygen and sequesters the carbon.
Reply | Report Abuse | Link to thisPlants not only capture solar energy, they store it & enhance the environment in many ways including absorbing Co2
Reply | Report Abuse | Link to thisA catalyst will come along that's going to change everything.And the one who invents it will be the next multimillionaire,bigger than Gates or any before.So keep going after those chemistry decree's.
Reply | Report Abuse | Link to thisCould plants (or bugs ... or euglenized eels) be developed that could use sunlight to either create stored biomass or generate electrical current, depending on circumstances set by controls sensitive to demand, etc.? After all, or living organisms use electrochemical processes and/or transport of electrons.
Reply | Report Abuse | Link to thisIf the web master is smart enough to clobber my old identity because I went after the nincompoop (I hope that is acceptable) that spreads their web ads I would be impressed. As I stated earlier I have been an avid fan of Sciam since 1965 (remember the real science articles not the USA Today we get fed today) but because I had some verbal sounding non doable threats against the web marketeer my ID was removed and so were my comments in every article I participated in.
Reply | Report Abuse | Link to thisFrom now on I am only going to be as repetitive as the Ads are because suddenly I feel like I live in Syria.
I still can't believe every comment was scrubbed that I have made regarding real science because I took a stand against constant abuse. I wonder if Sciam is getting a piece of the action?
I always love their articles on energy research with a Shell Ad on the same page. My oh My what has happened to real science?
Its really amazing that prejudice is so persistent to the scientific mind. WHY must technology be 'superior' to nature's own methods? Technology is a mere analog of the efficiency of nature. WHY not consider that biology can host quite a LOT of the processes and effects we look to technology to produce. Plants are quite compelling because they are worthy of the focus of science. "Worthy"... Why must nature be 'worthy' of human attention and effort? Science may be more a narcissistic activity than most can admit.
Reply | Report Abuse | Link to thisConviction is supported by argument. Argument is part of dialog. Dialog means its a give an take compromise/synthesis of viewpoints in competition for the balance of reason. Your postings dominated the comment forum where readers are INVITED to respond in a way that respects the venue. You failed to remember you are a 'guest' of this website. Courtesies are part of reasoned dialog... Don't impose your compulsive self focus into the face of other readers. Its not polite.
Reply | Report Abuse | Link to thisDid you notice the ad right after your put down. I am looking for intellectual conversations not where to buy sun glasses. Did you read what I wrote or make an assumption. I am having issues with money changers in the temple. Does that make more sense?
Reply | Report Abuse | Link to thisEvery viable site in the world has been occupied by plants. Covering the same surface with solar panels is impossible. We can convert biomass into eco-diesel economically now. (See www.Eprida.com). Or else we can convert biomass into charcoal and hydrogen. Charcoal increases water retention in soils and so is a fetiliser. Hydrogen can be stored in gasometers to power electricity generation when solar, wave or windpower becomes intermittent.
Reply | Report Abuse | Link to thisSince neither technology is currently commercially viable, BOTH should be fully explored. Inorganic and materials chemists like to focus on the efficiency of capturing photons, but this isnt really an issue. Whichever method provides a cheap alternative to gasoline and diesel will reach the market, regardless of how much land it takes up.
Reply | Report Abuse | Link to thisBiofuel work has a long way to go. Current biofuels are not cost competitive with fossil fuels, but they may become so as metabolic pathways are optimized and oil prices rise.
If you're interested in biofuels, check out the work by Craig Venter on photosynthetic algae and Jay Keasling et al at JBEI on engineered E coli that eat biomass and crap biodiesel and isopentanol (an ideal gasoline replacement).
I had the pleasure of attending a conference at which Keasling was the keynote speaker. He claimed that isopentanol could reach the consumer at ~3.50 per gallon IF the metabolic pathway which produces it could be engineered to reach the efficiency of ethanol production in yeast (a lofty and probably unrealistic goal).
Frankly, unlike inorganic catalysis, synthetic biology is a burgeoning field. Personally, I have high hopes for biofuels, although they still have a long way to go.
Reading some of these comments would be much more enlightening if the writers, in the spirit of scientific objectivity, would leave out their rants. I would also urge them to pay greater attention to spelling and punctuation.
Reply | Report Abuse | Link to thisWhat ads?
Reply | Report Abuse | Link to thisIt seems the ads were removed also. So they are listening and I haven't seen them in quite a while. I do enjoy the conversations and if I want to buy something I am sure e-bay or another site is a better choice. Sorry about the rants but they were becoming irritating.
Reply | Report Abuse | Link to thisNow for something entirely different. This article was posted on 12/2010 and has little notice but it fits well with this conversation. It deals with a microbe that poops H2 and O2 prolifically, sustainably and on demand which would cover about half the problems of H2 as a fuel source since storage was always the big issue. To date there may be about 4 or 5 comments and the actual science was discovered in 1993. Please take a minute to read the link and picture a world of just in time H2 versus the Hydro Carbon world we live in.
Reply | Report Abuse | Link to thishttp://www.scientificamerican.com/article.cfm?id=hydrogen-production-comes-natu&posted=1
I hope it gets read and spread.
David
Keep in mind that in order to consume the fuel, it is necessary to oxidize it - this process will, one way or another, release the same amount of carbon dioxide that was originally absorbed by the plant.
Reply | Report Abuse | Link to thisNot to say that this is bad, however. It makes biofuel carbon-neutral (which, environmentally speaking is better than fossil fuels), just like photo-voltaics (or, for that matter other renewables, such as solar-thermal, wind and hydroelectric).
In other words, the comparison here is fair.