23
Solar fuels
In this chapter we discuss how to store solar energy in the form of chemical energy in
socalled solar fuels. As we have seen in previous chapters, we are able to convert solar
energy to electricity with efficiencies of up to 44%. Also, we are able to convert solar
energy easily into heat, which we can use for preparing warm water, heating and even
cooling.
The biggest obstacle towards an economy that is driven by renewable energy is not
the generation of sustainable energy sources but the storage of the renewable energy. As
we have seen in Chapter 19, storing electricity with batteries is very difficult. While daynight storage that makes solar electricity generated during daytime available at night is
easily achieved, seasonal storage that allows electricity generated in the summer to be
stored until winter would require very large battery systems that are not usually feasible.
This problem becomes more severe in regions that are far away from the Equator where
the differences in solar irradiance between summer and winter are significant. The same is
also true for solar heat that was discussed in Chapter 22: while a hot water storage tank
with a volume of several hundreds of litres is sufficient as day-night storage, seasonal
storage that would allow solar-heated water to be used throughout the year would require
storage tanks with a volume of tens of thousands of litres. This tank also would need to be
heavily insulated. If we manage to store solar energy as chemical energy, we do not have
this problem as chemicals can be easily stored.
Figure 23.1 shows the Ragone chart that we analyzed in Section 19.3. The chart
shows the amount of energy per mass stored in a certain storage technology versus the
specific power provided by the technology. As we know, by far the most used form of
stored energy is fossil fuels. As we can see from the graph, fossil fuels have high energy
density properties, and are stable and reliable. But, fossil fuels are depleting and their use
is detrimental to the environment. Hydrogen (H 2 ) has an even better gravimetric energy
density than fossil fuels, but it is a very light gas. Hence, the volumetric energy density,
i.e. the energy per unit volume, is much lower. This is one of the reasons it has not been
widely used until now. When we compare batteries and hydrogen as energy storage for
solar electricity, we see that batteries have a lower energy density. Further, they require a
much higher initial investment than hydrogen. Hence, it seems an interesting option to use
Solar fuels
In this chapter we discuss how to store solar energy in the form of chemical energy in
socalled solar fuels. As we have seen in previous chapters, we are able to convert solar
energy to electricity with efficiencies of up to 44%. Also, we are able to convert solar
energy easily into heat, which we can use for preparing warm water, heating and even
cooling.
The biggest obstacle towards an economy that is driven by renewable energy is not
the generation of sustainable energy sources but the storage of the renewable energy. As
we have seen in Chapter 19, storing electricity with batteries is very difficult. While daynight storage that makes solar electricity generated during daytime available at night is
easily achieved, seasonal storage that allows electricity generated in the summer to be
stored until winter would require very large battery systems that are not usually feasible.
This problem becomes more severe in regions that are far away from the Equator where
the differences in solar irradiance between summer and winter are significant. The same is
also true for solar heat that was discussed in Chapter 22: while a hot water storage tank
with a volume of several hundreds of litres is sufficient as day-night storage, seasonal
storage that would allow solar-heated water to be used throughout the year would require
storage tanks with a volume of tens of thousands of litres. This tank also would need to be
heavily insulated. If we manage to store solar energy as chemical energy, we do not have
this problem as chemicals can be easily stored.
Figure 23.1 shows the Ragone chart that we analyzed in Section 19.3. The chart
shows the amount of energy per mass stored in a certain storage technology versus the
specific power provided by the technology. As we know, by far the most used form of
stored energy is fossil fuels. As we can see from the graph, fossil fuels have high energy
density properties, and are stable and reliable. But, fossil fuels are depleting and their use
is detrimental to the environment. Hydrogen (H 2 ) has an even better gravimetric energy
density than fossil fuels, but it is a very light gas. Hence, the volumetric energy density,
i.e. the energy per unit volume, is much lower. This is one of the reasons it has not been
widely used until now. When we compare batteries and hydrogen as energy storage for
solar electricity, we see that batteries have a lower energy density. Further, they require a
much higher initial investment than hydrogen. Hence, it seems an interesting option to use