Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 282

The solar-to-hydrogen efficiencies of both photoelectrochemical and photochem-
ical technologies are significantly limited by the activity and wavelength range of the
photoelectrodes and photocatalysts. Even if the quantum efficiency reaches as high
as 56%, the solar-to-hydrogen efficiency is still below 16% and 10% for the existing
photoelectrochemical and photochemical cells, respectively.
Both thermochemical cycles and water electrolysis may need additional hydrogen
distribution systems. By comparison, photoelectrochemical and photochemical tech-
nologies are more suitable at hydrogen fueling stations because fewer processes are
needed. Therefore, they are more suitable for serving as hydrogen fueling stations
with no need of extra hydrogen distribution systems. The extension of the working
wavelength of the materials for a solar PV panel, photoelecltrodes, and photo catalysts,
to visible light (400-700 nm) and the infrared (700-2400 nm) range is a useful future
research direction to improve the solar-to-hydrogen efficiency, as these two spectral
ranges occupy more than 90% of the total solar irradiance.
This chapter also examined the energy requirements of synfuel production from
captured CO 2 and hydrogen. It was found that the synfuel production reaction is self-
sustainable, so it was concluded that the energy requirements of synfuel production
are mainly determined by the hydrogen production and CO 2 capture. The hydrogen
production energy consumption is about 5-7 times the level for CO 2 capture. The CO 2
capture methods such as Na 2 CO 3 -based, K 2 CO 3 -based, CaO-based, and MEA-based
processes were examined. A challenge of the solar-based carbonation and calcination
reactors for CO 2 absorption and release is to provide more efficient heat transfer from
the sunlight to solid particles. Improved multiphase flow reactors are needed because
gas and solid are present simultaneously.

Nomenclature
A Area, m 2

C OX Catalyst for oxidization reaction

C R Catalyst for reduction reaction

s − 1

I S Total incident solar irradiance of the solar spectrum, J ·

10 − 34 J

h Planck constant, 6.626 × · s

s − 1 m P Hydrogen production per unit time, mole ·

S Sensitizer

T Temperature, K

Greek
η Efficiency

γ Photon frequency, Hz

REFERENCES

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