Solar energy from nature

One approach to solar energy uses PV -solar photovoltaics to capture sunlight with semiconductor materials that convert it to electric energy. However, this energy is not easily transportable given the electrical grid of many countries. Therefore, we need to rely in an efficient way to collect and stores the sun energy for use whenever or wherever is needed. An answer to this conflict could be found in mimicking the process of photosynthesis.


Process:

The Optical Society of America – Lynn Savage

Photosynthesis begins when the pigments within a plant cell act as antennas that capture photons. These antennas then generate electrons that pass the energy along to other molecules in the multistep process of energy capture, redirection and storage. Plants ranging from daisies to trees chiefly use chlorophyll-containing cells to effect this process (some photoactive bacteria have an analog material, dubbed bacteriochlorophyll). In chlorophyll-containing plants, the process of photosynthesis splits water molecules (H20), releasing oxygen and storing the energy produced by that chemical reaction inside a carbohydrate molecule. The energy resident in each photon is transferred into the final organic compound product, which the plant stores as adenosine triphosphate (ATP) for later use.

One or two types of pigments are necessary for a completely functional system: Photosystem I refers to the absorption of light via the main type of chlorophyll by itself. Photosystem II requires a second pigment. Light-harvesting polymers must be able to absorb sunlight over a significant span of the spectrum as well, in order to not waste photons.

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Using the right synthetic materials, this process can serve to split water molecules, sending the hydrogen to fuel cells such as batteries, for an easier transportation. Unlike PV, which uses readily available water stream sources to hydrogen to be stored and burned as fuel,  mimicking leaves allows for larger fuel production. Another aspect in which this approach focuses is the maximum use of the whole spectrum of sunlight. Ultimately, we look to capture as much energy from every wavelength as possible.

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Daniel Nocera, professor of MIT and researchers are recently working on something called “artificial leaf”: This device, similar to a common leave, is capable of turning the energy of sunlight directly into a chemical fuel that can be stored and used later as an energy source.

The device is a silicon solar cell with different catalytic materials bonded onto its two sides, it does not need external wires or control circuits to operate. When it is exposed to light in a container of water, Simply it quickly begins to generate streams of oxygen bubbles from one side and hydrogen bubbles from the other. If there is a barrier to separate the two sides, the two streams of bubbles can be collected and stored separately, and used later to deliver power.

The artificial leaf is a thin sheet of semiconducting silicon made entirely of inexpensive materials that can be easily found in earth— mostly silicon, cobalt and nickel —. This materials work in ordinary water. Bound onto the silicon is a layer of a cobalt-based catalyst, a material whose potential for generating fuel from sunlight was discovered by Nocera and his co-authors in 2008, which releases oxygen. The other side of the silicon sheet is coated with a layer of a nickel-molybdenum-zinc alloy, which releases hydrogen from the water molecules.

MIT News

David L. Chandler, MIT News Office
September 30, 2011