Solar chemical

Solar chemical refers to a number of possible processes that harness solar energy by absorbing sunlight in a chemical reaction. The idea is conceptually similar to photosynthesis in plants, which converts solar energy into the chemical bonds of glucose molecules, but without using living organisms, which is why it is also called artificial photosynthesis.^[1]

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 in a two step process so that hydrogen and oxygen are not produced in the same chamber, which creates an explosion hazard. Another approach involves taking the hydrogen created in this process and combining it with carbon dioxide to create methane. The benefit of this approach is that there is an established infrastructure for transporting and burning methane for power generation, which is not true for hydrogen. The problem with this approach is that carbon dioxide must be produced to release the stored energy, so it does not truly give "clean" energy. The other main drawback to both of these approaches is common to most methods of energy storage: adding an extra step between energy collection and electricity production drastically decreases the efficiency of the overall process.

It is also possible to use solar light to directly drive industrial chemical reactions and applications, eliminating the need to burn fossil fuels for energy.

Systems for storing solar energy

Photodimerization is the light 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 naphthalene series has also been investigated.^[2]

Anthracene dimerization

While photodimerization stores the energy from sunlight in new chemical bonds, Photoisomerization, the light induced formation of isomers, stores solar energy by reorienting existing chemical bonds into a higher energy configuration. In azobenzene for instance, the cis-isomer is about 0.6 eV higher in energy than the trans-isomer.^[3]

In order for an isomer to store energy then, it must be metastable as shown above. This results in a trade-off between the stability of the fuel isomer and how much energy must be put in to reverse the reaction when it is time to use the fuel. The isomer stores energy as strain energy in its bonds, the more strained the bonds are the more energy they can store, but the less stable the molecule is. The activation energy, Ea, is used to characterize how easy or hard it is for the reaction to proceed. If the activation energy is too small the fuel will tend to spontaneously move to the more stable state, providing limited usefulness as a storage medium, but if the opposite case is true and the activation energy is very large, the energy expended to extract the energy from the fuel will effectively reduce the amount of energy that the fuel can store. Finding a useful molecule for a solar fuel requires finding the proper balance between the yield, which is how efficient the photoisomerization process is, the light absorption of the molecule, the stability of the molecule in the metastable state, and how many times the molecule can be cycled without degrading.

Various ketones, azepines and norbornadienes among other compounds, such as azobenzene and its derivates, have been investigated as potential energy storing isomers.^[4] The norbornadiene-quadricyclane couple and its derivates are the most extensively investigated systems for solar energy storage processes. Norbornadiene is converted to quadricyclane using energy extracted from sunlight, and the controlled release of the strain energy stored in quadricyclane (about 110 k J/mole) as it relaxes back to norbornadiene allows the energy to be extracted again for use later.

Norbornadiene - Quadricyclane couple is of potential interest for solar energy storage

The attraction of the azobenzene system is that both the cis and trans isomers are photo-switchable with different wavelengths of light, which provides a simple way to trigger the release of stored solar energy. The transition from trans to cis can be done with ultraviolet light, while the transition back from cis to trans can be triggered by blue visible light. However, most research into both the azobenzene and norbonadiene-quadricyclane systems was abandoned in the 1980s as unpractical due to problems with degradation, instability, low energy density, and cost.^[5]

With recent advances in computing power though, there has been renewed interest in finding materials for solar thermal fuels. Researchers at MIT have used time-dependent density functional theory, which models systems at an atomic level, to design a system composed of azobenzene molecules bonded to carbon nanotube templates. This system provides an energy density comparable to lithium-ion batteries, while simultaneously increasing the stability of the activated fuel from several minutes to more than a year and allowing for large numbers of cycles without significant degradation.^[6] Further research is being done in search of even more improvement by examining different possible combinations of substrates and photoactive molecules.

Applications

There are a wide variety of both potential and current applications for solar chemical fuels. These range from portable stoves that can be charged in the sun to providing medical sanitation in off-grid areas, and plans are even in the works to use the system developed at MIT as a window de-icing system in automobiles.

References

External links

• ANU Thermochemical energy storage system - Australian National University, Canberra.
• Laboratory for Solar Technology - Paul Scherrer Institute, Villigen, Switzerland.
• Power & Energy Magazine, March 2004 article on Paul Scherrer Institute work
• Solar Chemistry Project Plataforma Solar de Almería, Spain,
• Isracast - Israel,
• Hydrogen Solar- UK.