text: The world's dependence on the declining reserves of fossil fuels poses not only environmental problems but also geopolitical ones. Solar fuels, in particular hydrogen, are viewed as an alternative source of energy for replacing fossil fuels especially where storage is essential. Electricity can be produced directly from sunlight through photovoltaics, but this form of energy is rather inefficient to store compared to hydrogen. A solar fuel can be produced when and where sunlight is available, and stored and transported for later usage. This makes it much more convenient, because it can be used in situations where direct sunlight is not available. 
  The most widely researched solar fuels are hydrogen, because the only product of using this fuel is water, and products of photochemical carbon dioxide reduction, which are more conventional fuels like methane and propane. Upcoming research also involves ammonia and related substances (i.e. hydrazine). These can address the challenges that come with hydrogen, by being a more compact and safer way of storing hydrogen. Direct ammonia fuel cells are also being researched. 
  Solar fuels can be produced via direct or indirect processes. Direct processes harness the energy in sunlight to produce a fuel without intermediary energy conversions. Solar thermochemistry uses the heat of the sun directly to heat a receiver adjacent to the solar reactor where the thermochemical process is performed. In contrast, indirect processes have solar energy converted to another form of energy first (such as biomass or electricity) that can then be used to produce a fuel. Indirect processes have been easier to implement but have the disadvantage of being less efficient than the direct method. Therefore, direct methods should be considered more interesting than their less efficient counterparts. New research therefore focusses more on this direct conversion, but also in fuels that can be used immediately to balance the power grid.