Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 270

(Moens et al., 2003, 2004; Eck et al., 2007; Wu et al., 2001), and molten salts are
planned to be used for gaining a higher temperature than thermal oils (Forsberg et al.,
2007; Patel, 2011; Dunn et al., 2012; Moore et al., 2010), and air and other gases are
used to drive a gas turbine (Schwarzbözl et al., 2006; Ahlbrink et al., 2009; Göttsche
et al., 2010). The currently operational solar thermal plant can generate the work-
ing fluid at more than 500 ◦ C and reach 1,000 ◦ C, so the conversion efficiency from
a working fluid to electricity is in the range of 30-60%. Currently operational solar
thermal plants show that the solar radiation capturing efficiency of solar concentrat-
ing devices with a tracking system is usually higher than 70% (European Commission,
2010; Schmitz, 2009; Taggart, 2008). Therefore, the integration of an electrolyzer
and solar thermal power plant can deliver higher hydrogen production efficiency
(15%-56%) than an electrolyzer and photovoltaic panels.
In addition to the engineering maturity and flexibility of integration with various
solar power generation technologies, another major advantage of water electrolysis is
that the hydrogen production can still operate at nights or days by using power when
sunlight is not available. The power could be either generated from the stored solar
thermal energy for the use at nights and undesirable weather conditions, or directly
from the power grid. In a concentrated solar power plant utilizing solar troughs or
solar towers, a large amount of solar energy can be stored in thermal oils or molten
salts for the times when sunlight is not available (Moens et al., 2003, 2004; Wu et al.,
2001; Herrmann et al., 2004; Patel et al., 2011; Dunn et al., 2012). As for electricity
from the power grid, even if the energy sources on the power grid may not be “clean'',
the impacts of unpredicted weather conditions can be minimized.
Since water electrolysis utilizes an external power source, the design of the power
generation plant does not need to consider the location of the solar power plant. It
can be designed in a compact way that aims at efficient sunlight capturing. A solar
tracking system can be readily utilized for collecting or concentrating the sunlight. In
addition, unpredictable mutual safety impacts are minimized and the distance between
the solar thermal power plant and the facilities of electrolyzer are flexible. The elec-
trolytic facilities do not need to occupy the space where sunlight is more suitable for
the power generation.
9.2.5 Photoelectrolysis and photoelectrochemical
water splitting
As to the water electrolysis presented in the former section, whether the electricity is
fully obtained from an external source does not cause a significant difference from
the perspectives of the chemical reaction mechanisms on the electrode surface. For
example, the electricity can be generated by an electrode on its own if the electrode
is made of materials that can create electric potential due to its exposure to sunlight
(Licht, 2005). The materials could be either n-type or p-type semiconductors. However,
from the aspect of an engineering and equipment setup, the difference is significant.
In this section, it is suggested that “photoelectrolysis'' and “photoelectrochemical''
water splitting are not categorized as “water electrolysis''. Instead, they are adopted
when at least one light absorbing electrode is needed, and only a part or no electricity
for the reduction or oxidization reaction on the electrode is obtained from external
power sources.