Solar Energy Perspectives - IEA

Chapter 9: Solar fuels
“Solar fuels” could have many forms, mainly gaseous or liquid. Gaseous solar fuels could be
pure hydrogen, or a mix of hydrogen and methane (CH 4 ). Liquids are much easier to handle
than gases or solids, especially in transportation. To be liquid at normal (atmospheric)
pressure, however, most fuels need carbon atoms, so they would be hydrocarbons. Producing
liquid fuels from solar requires some carbonaceous feedstock. In practice, it is possible to
make solar gas-to-liquid or solar coal-to-liquid fuels and, of course, solar biomass-to-liquid.
One could also use CO 2 streams, for example captured (but not stored) in the exhaust gases
of a large combustion facility such as a power plant. If one uses gas or coal, or even biomass,
solar energy is not required. Some liquid fuels are already produced, for example, in the
Middle-East and in the Republic of South Africa, from gas or coal. Biofuels represent
a growing global industry, providing about 2% of the global demand for liquid fuels for
transport – 3% of road transport fuels.
Solar pyrolysis or gasification of biomass would greatly reduce the CO 2 emissions involved
in the manufacturing of biofuels. Alternatively, solar processing of the biomass could be seen
as a way to increase the available energy from a given biomass feedstock, avoiding the use
of significant share of this feedstock as energy input into the process.
When solar liquid fuels are burnt, e.g. in the internal combustion engine of a car or other
vehicle, their carbon atoms are oxidised, generating CO 2 emissions. Nevertheless, using solar
as the energy source for the process saves fuel, as a significant share – perhaps one-third on
average – of the initial energy content of the fuel, whether fossil or biomass, is consumed
during the fuel manufacturing process. This also entails significant CO 2 emissions. When the
initial fuel is coal, upstream emissions are even greater than end-of-pipe emissions when the
fuel is burnt. Therefore, if using liquid solar fuels provide only limited climate change
mitigation benefits compared to petroleum products, they accomplish a lot when compared
to ordinary coal-to-liquid fuels. In the “Reference” scenario of ETP 2008 and ETP 2010, there
is a sharp increase in CO 2 emissions after 2030 due to the introduction of large amounts of
coal-to-liquid fuels as substitutes for oil products. Although in scenarios with lower fossil fuel
consumption, keeping fossil fuel prices lower, there would be less coal-to-liquid manufacturing,
it would be very useful to produce such fuels with little or no upstream emissions by
substituting solar heat for the coal energy combusted in the manufacturing process. If natural
gas is reformed with CO 2 which was separated from flue gas of a power plant, the final
product will have half of its carbon atoms from recycled CO 2 .
Producing hydrogen
Solar fuels are usually made from hydrogen. Hydrogen can be produced by electrolysing
water, and if the electricity is of solar origin, it can be considered “solar hydrogen”. Producing
solar hydrogen via electrolysis of water using solar-generated electricity offers an overall
solar-to-hydrogen efficiency of about 10% with current technologies. Heating the water
before it gets electrolysed is an effective means of reducing the required amount of electricity,
and this too can be done with solar energy, thereby increasing the efficiency of the conversion
of electricity into hydrogen. However, it is unclear when and where hydrogen really has to
be preferred to electricity as an energy carrier (see Chapter 5), so the remainder of this
chapter focuses on another way of producing hydrogen from solar energy – concentrating
solar thermal technologies.
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© OECD/IEA, 2011