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When considering the energy delivered via sunlight to earth in one hour exceeds our total consumption via fossil fuels, nuclear power, and other alternative fuels in a year, the staggering potential of this solar energy is truly perceived.
Enlightened by the realization of the scale of this potential energy source, scientists have striven to devise technology able to harvest and convert this sunlight to a usable form. With the increasing pressure of diminishing fossil fuel reserves (estimated in 2017 to be sufficient until 2067) and the global push to curb climate change, the past 10 years, in particular, has seen huge advances in this field of study and the consequential emergence of solar fuels.
What are Solar Fuels?
Solar fuel, in its most modest description, is the combination of sunlight, water, and carbon dioxide to form a liquid fuel. As opposed to solar cells which convert the absorbed sunlight to electricity, solar fuels store their energy in chemical bonds – the crucial differentiating factor being that these bonds have extremely high energy density, second only to energy stored by an atomic nucleus. As a result of extensive research solar fuels have been developed in two valuable forms: Hydrogen, and carbon-based fuels including methane and carbon monoxide.
How are Solar Fuels Produced?
The production of solar fuels draws on the fundamental natural phenomena of photosynthesis – what better place to take inspiration than a process which has undergone millennia of optimization and is elementary to our continued survival.
Scientists have developed three main approaches to achieve this:
- Natural photosynthesis
- Artificial photosynthesis
- Thermochemical production
Natural photosynthesis and artificial photosynthesis, as the names would suggest, differ by deriving the fuel directly from photosynthesis performed by living organism in the former approach and mimicking this process via a synthetic system in the latter. However, the fundamental principles behind the two methods are the same. Sunlight must first be harvested by absorbing the light photons through an antenna which are then transported to reaction centers. The high energy contained in these protons is used to split charges into their positive and negative ions through a series of redox reactions. Finally, these charges act as oxidizing equivalents to split water molecules into their hydrogen and oxygen constituents which are stored as energy-rich chemical bonds.
In contrast, a thermochemical approach aims to use light photons to split the water molecules directly. By concentrating high energy rays towards a substance in a closed environment, most often a reaction vessel, combined with a source of steam or carbon dioxide, the scorching temperatures can excite direct molecular splitting producing hydrogen or carbon monoxide respectively.
Comparison to Alternative Fuel Sources
One of the greatest obstacles to the widespread employment of renewable energy sources, wind and solar for example, is the significant challenge of storing this harnessed power so that it is readily available independent of whether it is being produced at that instant. As previously mentioned, solar fuel stores the captured sunlight energy in chemical bonds, a form which can be easily stockpiled in a manner akin to oil.
Furthermore, the challenge of transporting this energy to the intended application, in the case of solar fuels, is simply not a challenge. The convenient portable form of these fuels enables existing transportation networks to be used with the hope that solar fuel will be integrated, and eventually replace fossil fuels, in this well-established industry.
With such tantalizing promise, you may be wondering why solar fuel has not already been crowned the great savior in our battle for sustainability and the banishing of fossil fuels. However, solar fuels are by no means a perfect solution and hence are still in their prototype phase. One major flaw which scientists are aiming to combat is the durability; some materials suffer significant degradation from long term exposure to sunlight. The concern is that the energy they harvest in their operational lifetime will not be considerably greater than the energy required to install them. In addition to this is the significant cost associated due to the complex technology and the labor required for installation.
Applications of Solar Fuels
With continued development, one of the main applications of solar fuels could be in our transport industry in the form of either pure hydrogen or the combination of hydrogen and carbon monoxide to create syngas. Perhaps most significant is its suitability as aviation fuel; the task of electrifying this mode of transport is exceptionally wrought with challenges and therefore, unlike cars and buses, has as of yet not been untwined from its unsustainable dependence on petroleum.
A different branch of potential applications is in relation to industrial feedstock – the raw materials supplied to a machine or processing plant. Many products which we so heavily rely on, such as pharmaceuticals, plastics, and fertilizers, are themselves reliant on fossil fuels as their feedstock. Hydrogen, for example, is widely used as a raw material in manufacturing and is conventionally sourced via natural gas steam reforming, not only requiring extensive fossil fuels to power the process but also contributing large quantities of greenhouse gases. Sourcing hydrogen through sunlight harvesting, therefore, will relieve reliance on fossil fuels as well as helping to significantly reduce the energy consumption associated with manufacturing.
Continued Research and Support
The refinement of solar fuels to progress beyond the laboratory and into widespread commercial use continues to be a fixation of extensive investigation. The Engineering and Physical Sciences Research Council (EPSRC) provides funding for numerous university research programmes, most recently to the University of Bath to support their project “Nanostructured metal oxides for solar fuels”. Furthermore, solar fuels are also gaining traction from industry partners with Toyota and Honda amongst some of the names to have launched programmes regarding solar fuel development.
With this ever-increasing support by both subject matter experts and industry leaders, it is evident that the pursuit of novel energy sources is a global issue. The cornerstone powers that be upon which we fuel modern day life are diminishing with every seemingly insignificant drive to the shops and fertilizer sprayed. However, if obstacles such as cost and durability can be overcome, solar fuels offer the potential to be a sustainable candidate to fill this critical role.
Sources and Further Reading