Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 261

Currently, some other efficiency definitions are also used to evaluate the solar-to-
hydrogen technologies and equipment. The existence of multiple definitions arises from
the difference of reaction mechanisms, which will be discussed in the following sections.
A direct comparison for different solar-to-hydrogen technologies is challenging because
a wide scope of technologies is involved and many technologies are still at an early
stage of development. This section will provide some general comparisons on the basis
of similar standards reported in literature.
9.2.2 Matching the temperature requirements of solar-based
hydrogen production methods
In many unconventional hydrogen production methods, e.g., thermochemical water
splitting and high temperature electrolysis, high temperature heat is needed (Hinkley
et al., 2011; Monnerie et al., 2011; Corgnale et al., 2011; Summers et al., 2009; Xiao
et al., 2012). Therefore, a large amount of solar irradiance must be concentrated to
reach the temperature requirements and a large additional land area is needed to con-
centrate the solar irradiance. The high temperature requirements bring some significant
engineering challenges. For example, it is still a challenge to find a coating material
for the solar receivers to improve the absorptance and reduce the emittance at 600 ◦ C
(Sergeant et al., 2010; Barshilia et al., 2006; Kennedy et al., 2005). Furthermore, it is
challenging to select an appropriate working fluid and equipment material for a solar
irradiance receiver and reactor. The working fluids include water, thermal oils, molten
salts, steam, air, and other gases. Due to the good heat transfer performance and low
melting points, thermal oils are widely used in the solar concentrating devices such as
solar troughs. However, thermal oils are volatile and toxic, and may decompose at
a high temperature, so the thermal oils are currently operated below 450 ◦ C (Moens
et al., 2003, 2004; Eck et al., 2007; Wu et al., 2001), which is not sufficiently high to
cover the temperature threshold of some thermochemical hydrogen production cycles
(Xiao et al., 2012; Le Gal et al., 2010; Corgnale et al., 2011), which will be discussed
in later sections.
The solar irradiance concentrating devices include solar troughs, lenses, parabolic
dishes, heliostats, and reflection mirrors. Currently, a solar trough can concentrate
more power than a lens and a parabolic dish, but it is challenging to reach up to
500 ◦ C even if it uses a molten salt as the working fluid (Herrmann et al., 2004),
because the receiving area of the tubular irradiance receiver makes the relative number
of suns fewer than that otherwise concentrated to a focal spot by a lens and dish. So
the temperatures provided by solar troughs are not suitable for some thermochemical
cycles with higher temperature requirements (Xiao et al., 2012; Le Gal et al., 2010).
A solar tower capable of concentrating thousands of suns as well as tens of
megawatts of irradiance with heliostats or reflection mirrors can reach a temper-
ature range of 500-1000 ◦ C (Schramek et al., 2009; Spelling 2009; Dersch et al.,
2011). When utilizing a molten salt as the working fluid, the operating temperature
of large lab-scale equipment can reach up to 900 ◦ C (Forsberg et al., 2007; Patel 2011;
Dunn et al., 2012; Moore et al., 2010; Matsunami et al., 2000). Some recent small
industrial scale construction projects utilizing molten salts operate at up to 650 ◦ C
(Khan et al., 2004; Martín, 2007; NREL, 2010). A disadvantage of molten salts is
their higher melting points than thermal oils and gases, which limits the heat transfer