Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 281

the major modifications are conducted for the usage of solar thermal energy. A solar-
based spray carbonator and fluidized bed calcinator were tested at a large laboratory
scale (Nikulshina et al., 2006, 2009). Both reactors have a transparent section allow-
ing for the direct heating of the solid reactants. As indicated in Table 9.3.2, solid and
gas must be processed in the same reactor. Sometimes the aqueous solution of the solid
is used for the CO 2 capture, so the heat transfer is a multiphase process and prefer-
able that the solid can be directly heated by the solar irradiation due to the poor heat
transfer performance of solid particles. This explains why the reported reactors are
transparent, which are different from solar hydrogen production reactors, where only
gases are processed.
9.4 SUMMARY
This chapter presented scenarios of using solar-based hydrogen and CO 2 recycling to
provide a sustainable solution to the increasing demand of clean energy and ongo-
ing depletion of conventional fossil fuels. The intermittency issue of solar energy
can also be significantly addressed with the usage of solar-based hydrogen as well as
synfuels produced from recycled CO 2 and H 2 . Then this chapter examined the solar-
to-hydrogen reaction mechanisms and technologies including thermochemical cycles
utilizing solar thermal energy to split water molecules, conventional electrolysis utiliz-
ing solar-generated electricity to split water molecules, and photochemical processes
utilizing photon-activated electrons of auxiliary reagents (sensitizer and catalyst) to
activate and split water molecules. This chapter also examined and suggested some
categorization criteria for technologies from the reaction mechanisms and engineering
approaches, particularly the latter.
The basic components for the hydrogen production apparatus and major advan-
tages and challenges of the technologies were also examined. It was concluded that
conventional water electrolysis powered by solar generated electricity is more mature
than other technologies, but the solar-to-hydrogen efficiency is currently below 16%
due to the energy loss in the conversion of solar irradiation to electricity. Thus, its
efficiency improvement is mainly determined by the increase of solar-to-electricity
conversion efficiency, which is a maximum of about 20% for currently operating
PV panels and solar thermal plants. A hybrid method involving a high temperature
electrolysis and spectrum splitter may utilize more heat than conventional electrolysis.
Consequently, the hydrogen production efficiency is greatly increased to 40-50%, but
it is challenging to find appropriate materials for the electrodes and electrolyte that
must withstand high temperature steam, hydrogen and oxygen.
Thermochemical cycles benefit from large production scales in order to minimize
the energy losses arising from high temperature requirements and multiple auxiliary
processes for an integrated operation of the thermochemical cycle. A high temperature
of 1,500-2,500 ◦ C required by the metal oxide redox pair cycles may keep the cycles
from being utilized in the short term, although the cycles usually have only two chemi-
cal reactions. The solar-to-hydrogen efficiency of thermochemical cycles was estimated
to be in the range of 40-60%, which is higher than conventional electrolysis because
the cycles use thermal energy as the major energy input with no energy loss due to the
conversion of thermal energy to electricity.