Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 259

Engineers and scientists have been developing numerous methods for clean hydro-
gen production and CO 2 recycling. This chapter will examine the energy efficiencies,
requirements, and thresholds of different technologies. The major water splitting and
CO 2 recycling technologies will be compared and categorized according to the reac-
tion mechanisms and engineering approaches. The engineering approaches would be
the focus from the perspectives of apparatus, components, materials, equipment and
layout of processes.

9.2 SOLAR-BASED HYDROGEN PRODUCTIONWITH
WATER SPLITTING METHODS

This section examines the technologies of utilizing solar energy for hydrogen pro-
duction with water splitting, which are different from conventional fossil fuel-based
methods. The conventional methods such as modified steam methane reforming and
coal gasification suitable for the usage of solar energy, are still fossil fuel-based although
the green house gas emissions are significantly reduced due to the usage of solar thermal
energy to replace the combustion of an extra amount of methane or coal for the supply
of reaction heat. Therefore, the solar-assisted fossil fuel-based conventional methods
for hydrogen production won't be discussed in this chapter. Since the only feedstock
to a water splitting cycle is water and the products are hydrogen and oxygen with no
greenhouse gas emissions, so the hydrogen production with water splitting is the focus
of this chapter.

9.2.1 Solar-to-hydrogen efficiency of water splitting processes
In water splitting methods, the only feedstock to the hydrogen production cycle is
water, and the only products are hydrogen and oxygen. Since the combustion of hydro-
gen produces water again, the solar-based water splitting methods are fully renewable
compared with the prior modified conventional fossil fuel-based methods for the usage
of solar energy. This section will focus on these unconventional methods. Engineers
and scientists have been developing numerous methods for the clean hydrogen produc-
tion from water splitting with solar energy, such as thermolysis, thermochemical, water
electrolysis, photoeletrolysis, photoeletrochemical, photochemical, photodissociation,
photodecomposition, photolysis, photodegradation, photocatalytic, photobiologi-
cal, and hybrid methods. This section aims to introduce and compare the reaction
mechanisms and the major water splitting technologies and engineering approaches.
Different water splitting mechanisms and engineering approaches may indicate
various hydrogen production efficiencies. Considering the variety of complex efficiency
related factors, the following efficiency definition is adopted for a more consistent
comparison:

H Liq,298
I S

m P ·

=

η (9.2.1)

where η is the solar-to-hydrogen production efficiency, I S is the total incident solar
irradiance of the whole solar spectrum on the basis of the sunlight receiving area of the