Biological Solar

[greencat]

Combining synthetic biology and solar technology could provide a way to trap carbon dioxide and produce fuel.


By harvesting and burning fossil fuels, human beings essentially provide the tail end of a cycle hundreds of millions of years long. Plants and algae that grew by taking in carbon dioxide eventually turned into the deposits of coal and petroleum that we use to power our lives, rereleasing into the atmosphere the same carbon dioxide that nature had previously sequestered. Reducing these emissions will require us to change the way we think about both energy and carbon.

Reforestation is one tactic that has been broadly contemplated to mitigate rising levels of carbon dioxide; carbon capture and sequestration is another. These approaches close the carbon-energy cycle, but they have limited effectiveness in the near term and are difficult to implement on a large scale. Options such as corn ethanol, cellulosic biofuels, and fuels produced by algae offer a way to shorten the cycle: feedstock is grown for several years and then converted to ethanol or diesel. But these processes require input energy, typically from fossil fuels, and are therefore an imperfect solution.

There is a better way. A handful of projects--including an effort by Craig Venter's energy company, Synthetic Genomics--are now under way to use genetically modified photosynthetic organisms to generate fuels with input energy from the sun.

One such effort is Helioculture, an emerging technology pioneered by Joule Biotechnologies of Cambridge, MA, which can uniquely convert sunlight and carbon dioxide directly into a range of fuels and petroleum-derived chemicals that do not require any additional processing steps. The process consumes no fresh water or agricultural land. But while the organism is important, it is not sufficient. Photosynthetic organisms engineered to produce ethanol or other fuels are grown in special chambers shaped much like solar panels, where they absorb sunlight and generate liquid fuels rather than electrons.

Unlike solar energy from photovoltaics, which depends on costly batteries for storage, fuels are efficiently stored in barrels, simplifying distribution and demand management. And because the technology used to grow the organisms is modular, it is easy to scale up. Joule is now gearing up to build a pilot plant in the southwestern United States. I believe that this new fuel source can feasibly replace the 289 billion gallons of gasoline per year that the United States is projected to need in 2050, and it can be produced in an

Technologies of this kind promise a path to true energy independence, enabling us to reduce, or at least stabilize, carbon-dioxide emissions while supporting the power-hungry society we have created.

David Berry is a partner at the venture capital firm Flagship Ventures and a cofounder of Joule Biotechnologies. He was the TR35 Innovator of the Year in 2007.

[greencat]

The efficiency of photovoltaic energy conversion is of critical significance for the practical application of solar installations. Theoretically, every photon absorbed should release one electron. Whereas modern solar cells are far from achieving high efficiency, natural photosynthetic systems achieve nearly 100 per cent quantum yield.

Plants, algae and cyanobacteria are able to almost completely transform captured sunlight into chemical energy. The electrons set free by the photons hitting the photosynthetic elements in these organisms are transported out of the light receptor and are used as the driving force for chemical reactions. The team from the National Institute of Advanced Industrial Science and Technology in Tosu has developed a process to capture light energy with nearly equal efficiency.

Reporting in the journal Angewandte Chemie, they plug a molecular wire directly into a biological photosynthetic system to efficiently conduct the free electrons to a gold electrode.

http://kn.theiet.org/news/feb09/tosu-psi.cfm

To improve the efficiency of synthetic systems, experiments were attempted in which biological light-capturing units were deposited onto electrodes as thin films. However, the transfer of electrons from the light-capturing layer into the circuit in this type of system is so inefficient that most of the electrons don't even make it to the target electrode.

The secret to the success of natural photosystems is the perfect fit of the individual components. The molecules fit precisely together like plugs and sockets and can pass electrons on directly and nearly without loss.

The approach taken by the Tosu team connects a Photosystem I enzyme taken from the cyanobacteria Thermosynechococcus elongatus to a synthetic apparatus. An important component of the electron transmission sequence of Photosystem I is vitamin K1. The researchers removed the vitamin K1 from the Photosystem I protein complex and replaced it with a synthetic analogue. This consists of two molecular plugs and a hydrocarbon chain built to be the same length as that of K1.

One plug connects to Photosystem I, the other is a molecular, viologen group that anchors the ensemble to a coated gold electrode. Electrons released by irradiation of Photosystem I and transmitted along the wire are relayed to the gold electrode by the viologen group.