Solar Energy
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Solar chemical refers to a number of possible processes that harness solar energy by absorbing sunlight in a chemical reaction in a way similar to photosynthesis in plants but without using living organisms. No practical process has yet emerged.
A promising approach is to use focused sunlight to provide the energy needed to split water into its constituent hydrogen and oxygen in the presence of a metallic catalyst such as zinc. This is normally done is a two step process so that hydrogen and oxygen are not produced in the same chamber leading to potentially explosive consequences.

It is also possible to use solar light to drive industrial chemical reactions and applications without a requirement for fossil fuel.

Photodimerization is the photon induced formation of dimers. As early as 1909, the dimerization of anthracene into dianthracene was investigated as a means of storing solar energy. The photodimerization of the napthalene series has also been investigated.

Photoisomerization is the photon induced formation of isomers. Ketone, azepine and norbornadiene among other compounds have been investigated as potential energy storing isomers.

Solar chemical processes use solar energy to drive chemical changes. These processes offset energy that would otherwise be required from an alternate source and can convert solar energy into a storable and transportable fuel. Solar chemical reactions are diverse but can generically be described as either thermochemical or photochemical.

Hydrogen production technologies have been a significant area of solar chemical research since the 1970s. Aside from electrolysis driven by photovoltaic or photochemical cells, several thermochemical processes have also been explored. The seemingly most direct of these routes uses concentrators to split water at high temperatures (2300-2600 °C), but this process has been limited by complexity and low solar-to-hydrogen efficiency (1-2%).^[82] A more conventional approach uses process heat from solar concentrators to drive the steam reformation of natural gas thereby increasing the overall hydrogen yield. Thermochemical cycles characterized by the decomposition and regeneration of reactants present another avenue of hydrogen production. The Solzinc process under development at the Weitzman Institute is one such method. This process uses a 1 MW solar furnace to decompose zinc oxide (ZnO) at temperatures above 1200 °C. This initial reaction produces pure zinc which can subsequently be reacted with water to produce hydrogen.^[83]

Sandia's Sunshine to Petrol (S2P) technology uses the high temperatures generated by concentrating sunlight along with a zirconia/ferrite catalyst to break down atmospheric carbon dioxide into oxygen and carbon monoxide (CO). The CO may then be used to synthesize fuels such as methanol, gasoline and jet fuel.^[84][85]

Photoelectrochemical cells or PECs consist of a semiconductor, typically titanium dioxide or related titanates, immersed in an electrolyte. When the semiconductor is illuminated an electrical potential develops. There are two types of photoelectrochemical cells: photoelectric cells that convert light into electricity and photochemical cells that use light to drive chemical reactions such as electrolysis.^[86]

A photogalvanic device is a type of battery in which the cell solution (or equivalent) forms energy rich chemical intermediates when illuminated. These chemical intermediates then react at the electrodes to produce an electric potential. The ferric-thionine chemical cell is an example of this technology.^[87]

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