venerdì 23 settembre 2011

An Efficient Solar Harvest

Solar power could be harvested more efficiently and transported over longer distances using tiny molecular circuits based on quantum mechanics, according to research inspired by new insights into natural photosynthesis. Incorporating the latest research into how plants, algae and some bacteria use quantum mechanics to optimize energy production via photosynthesis, UCL scientists have set out how to design molecular circuitry that is 10 times smaller than the thinnest electrical wire in computer processors. Published in Nature Chemistry, the report discusses how tiny molecular energy grids could capture, direct, regulate and amplify raw solar energy.

Solar fuel production is all about energy from light being absorbed by an assembly of molecules; this electronic excitation is subsequently transferred to a suitable acceptor. For example, in photosynthesis, antenna complexes capture sunlight and direct the energy to reaction centers that then carry out the associated chemistry.
In photosynthesis chlorophyll captures sunlight and directs the energy to special proteins that help make oxygen and sugars. This is no different in principle than a solar cell.
In natural systems energy from sunlight is captured by colored molecules called dyes or pigments, but it is only stored for a billionth of a second. This leaves little time to route the energy from pigments to the molecular machinery that produces fuel or electricity.
The key to transferring and storing energy very quickly is to harness the collective quantum properties of antennae, which are made up of just a few tens of pigments.
Recent studies have identified quantum coherence and entanglement between the excited states of different pigments in the light-harvesting stage of photosynthesis.
Although this stage of photosynthesis is highly efficient, it remains unclear exactly how or if these quantum effects are relevant.
Dr Alexandra Olaya-Castro, co-author of the paper from UCL’s department of Physics and Astronomy said: "On a bright sunny day, more than 100 million billion red and blue colored photons strike a leaf each second."
“Under these conditions plants need to be able to both use the energy that is required for growth but also to get rid of excess energy that can be harmful. Transferring energy quickly and in a regulated manner are the two key features of natural light harvesting systems.
"By assuring that all relevant energy scales involved in the process of energy transfer are more or less similar, natural antennae manage to combine quantum and classical phenomena to guarantee efficient and regulated capture, distribution and storage of the sun’s energy."
Summary of lessons from nature about concentrating and distributing solar power with nanoscopic antennae:
The basic components of the antenna are efficient light absorbing molecules.
Take advantage of the collective properties of light-absorbing molecules by grouping them close together. This will make them exploit quantum mechanical principles so that the antenna can: i) absorb different colors ii) create energy gradients to favour unidirectional transfer and iii) possibly exploit quantum coherence for energy distribution.
Make sure that the relevant energy scales involved in the energy transfer process are more or less resonant. This will guarantee that both classical and quantum transfer mechanisms are combined to create regulated and efficient distribution of energy.


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