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MIT Professor Daniel Nocera claims to have invented the first affordable solar cell capable of mimicking nature and performing photosynthesis -- splitting water.  (Source: Christopher Harting/MIT)

Another study examined how quantum dots may be able to perform "Multiple Exciton Generation" an unusual phenomena that sees an excited quantum dot electron (blue, left) transfer energy to multiple electrons in another quantum dot (green, right)  (Source: Mark T. Lusk, Department of Physics, Colorado School of Mines)
"Her green plastic watering can/For her fake Chinese (silicon) plant/In the fake (silicon) earth."

One of the most promising sources of future energy is solar power.  We're currently only harvesting a minuscule fraction of the estimated 12.2 billion kilowatt-hours of solar energy that hits the Earth every day [source].  While nuclear fission and fusion power also will be critical to the future of man, solar power may be usable on planetoids that lack fissile fuels like uranium and fussile fuels like deuterium.

Before we can get to such ambitious terrestrial or interplanetary objectives, much work needs to be done.  A pair of new studies published by the Massachusetts Institute of Technology and the Colorado School of Mines offer intriguing tools that could one day be applied to making solar power more efficient.

I. Growing a Solar "Tree"

Scientists often find that nature has produced designs that rival any that mankind has cooked up.  A perfect example of that is the tree.  

A dominant plant species across much of the world, trees have solved much of the problems that have puzzled alternative energy scientists.  They not only store collected solar energy in polysaccharides, but also manage an immensely large solar collecting surface via their multipurpose evolutionary invention, the leaf.

Using the chemical model of the leaf, researchers at MIT created a poker card-sized silicon cell, doped with special catalysts and controlling electronics.  The team reports that the device is capable of producing an abundantly positive energy balance by splitting water into hydrogen and oxygen.  The reactions it carries out are similar to those that occur inside chloroplasts in photosynthetic plant cells.

The emitted gases could be harvested and stored for use powering a fuel cell.  Daniel Nocera, Ph.D. [profile], who led the team, describes [press release], "A practical artificial leaf has been one of the Holy Grails of science for decades. We believe we have done it. The artificial leaf shows particular promise as an inexpensive source of electricity for homes of the poor in developing countries. Our goal is to make each home its own power station.  One can envision villages in India and Africa not long from now purchasing an affordable basic power system based on this technology."

Professor Nocera reports that his leaf actually beats nature's design in efficiency by a factor of 10, and may be even more efficient in the future.  Of course it lacks natural leafy plants' ability to heal from damage, self-replicate, and self-generate from ground resources.  Nonetheless, the efficiency mark is an impressive achievement.

The key to that success is special nickel-cobalt catalyst that Professor Nocera cooked up.  Much like photosynthetic pigments that use metal ions as their active center, these catalysts use the harvest solar energy to perform chemical reactions.

John Turner of the U.S. National Renewable Energy Laboratory in Boulder, Colorado created a similar "solar leaf" a decade ago, but it relied on expensive rare metal catalysts.  Since then, many other researchers have created new solar leaf designs, but most of their designs remained quite expensive or lacked efficiency.

By contrast Professor Nocera's design is far cheaper, while maintaining a respectable efficiency.

The key obstacle now to this technology being practically suited for mass production is the lack of availability of cheap, durable fuel cells.  Currently fuel cells capable of producing enough energy to power a modern house remain quite expensive, costing tens, if not hundreds of thousands of dollars.  Still, it is reasonable to hope that similar breakthroughs will one day be able to drop the cost of fuel cells enough that the entire system will become feasible for mass deployment.

MIT's research was funded by The National Science Foundation (NSF) and the Chesonis Family Foundation.  It was presented at the 241st National Meeting of the American Chemical Society(ACS).

II. Connecting the Quantum Dots

Quantum dots are outlandish human-constructed atoms that confine electrons to a three dimensional space in a crystal-like motif.  The electrons are capable of absorbing photons to form excitons and the properties of quantum dots themselves are somewhat like bulk semiconductors, making them an attractive target for photodetectors or solar cells.

Scientists are still struggling to understand the complex structures they've created, though.  The dots operate on quantum physics rules far different from those observed on a macroscopic scale.

New research at the Colorado School of Mines offers evidence in support of a controversial theory called multiple exciton generation (MEG), which suggests that a quantum dot's electron that has absorbed light energy from a single photon can transfer that energy to multiple other electrons.

Previous studies have been remarkably inconsistent on the possible relationship between quantum dot size and MEG events, thus it was an attractive target for simulation, says the research team.  

Using a computer cluster funded by a NSF grant the team revealed that each size of quantum dot is capable of performing MEG for a select slice of the solar spectrum.  Smaller dots have the highest efficiency of electricity generation from their spectrum-dependent MEG events.

The team's leader, Professor Mark Lusk [profile], says that the results indicate that using a mix of quantum dots could produce superior electricity generation capabilities in future solar cells.  He states [press release], "We can now design nanostructured materials that generate more than one exciton from a single photon of light, putting to good use a large portion of the energy that would otherwise just heat up a solar cell."

The results were published in a paper [abstract] in the peer-reviewed journal ACS Nano.

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RE: what's the catch?
By Solandri on 3/29/2011 5:11:28 PM , Rating: 2
The catch is that right now only about 5.5% of the energy in the sunlight is diverted into generating hydrogen. To give you an idea how low that is, photosynthesis in sugar cane is about 7%-8% efficient.

The end product is of course sugar and cellulose (sugar molecules linked together) instead of hydrogen gas. But sugar is a helluva lot more compact and a more transportable energy storage medium than gaseous hydrogen. You can even ferment it to convert it into alcohols, which you can then use as a liquid fuel. That's what biofuels are. The best part is - it costs nothing to build. Plants and algae are self-constructing and self-replicating.

This will be a good invention if they can get the efficiency up above maybe 20%. But as it stands right now, it's inferior to biofuels and burning wood.

RE: what's the catch?
By rs2 on 3/29/2011 11:21:51 PM , Rating: 4
The best part is - it costs nothing to build. Plants and algae are self-constructing and self-replicating.

Self-constructing so long as the required raw materials are available from the environment in sufficient quantities for the plant to replicate and grow. For large scale operations the cost of replenishing these raw materials (typically in the form of fertilizer), while generally manageable, is certainly not "nothing".

Still I like the idea of using nature to produce the things we need. But plants aren't some magic entity that create themselves out of nothing but light and air as you made it sound.

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