Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 262

and storage performance. Therefore, utilizing gas as the working fluid of the solar
tower is another option for obtaining a high operating temperature range of 700-
1000 ◦ C (Schwarzbözl et al., 2006; Ahlbrink et al., 2009; Göttsche et al., 2010). Some
operational small industrial scale solar thermal plants using air and other gases can
operate at a temperature of 1,000 ◦ C. Currently, a high temperature up to 3,500 ◦ C can
be obtained with solar concentrating furnaces of laboratory and pilot scale equipment
(Haueter et al., 1999; Riveros-Rosas et al., 2010). This is a promising scenario for the
high temperature hydrogen production cycles.

9.2.3 Thermolysis, thermal decomposition and
thermochemical methods
The water in Equation (9.2.2) may take part in the reaction in the form of either liquid
water or steam. Since the generation of steam is also from liquid water, then the liquid
form for Equation (9.2.2) is more widely used. The changes of standard enthalpy of
the water splitting at 298K are equal to the negative of the higher and lower heating
values of hydrogen if the same reference temperature is adopted:

H Liq,298 =−

HHV =− 285 . 8kJ / mol = 2 . 97 eV / molecule (9.2.3)

H Gas,298 =−

LHV =− 241 . 8kJ / mol = 2 . 52 eV / molecule (9.2.4)

where the superscript “0'' means the standard state, the subscript 298 means the tem-
perature of 298K (25 ◦ C), and the subscripts Liq and Gas mean the liquid and gaseous
states, respectively. Figure 9.2.1 shows the influence of the temperature on the stan-
dard enthalpy change of water splitting reaction for the gaseous form of water. It can
be found that the reaction enthalpy at 5,000 ◦ C is only about 7% higher than at room
temperature. This means the value of the reaction enthalpy change at room tempera-
ture can well represent the values under current engineering temperature ranges. Also,
the value adopted in Equation (9.2.1) for the efficiency definition is reasonable.
However, the enthalpy change mainly gives the energy balance and requirements.
The balance is not sufficient for examining the spontaneity of direct water splitting,
which is reflected by the changes of the Gibbs free energy. The values of the standard
Gibbs free energy of Reaction (9.2.2) at 298K are (Licht, 2005):

G Liq =

237 . 0kJ / mol = 2 . 47 eV / molecule (9.2.5)

G Gas = 228 . 4kJ / mol = 2 . 38 eV / molecule (9.2.6)

It can be found that the values of the Gibbs energy for liquid and gas forms differ by
only 3.8%. So either value can be used to examine the spontaneity of water splitting.
The large positive values of the Gibbs free energy in Equations (9.2.5) and (9.2.6)
indicate that the direct decomposition of water is far from spontaneous except that the
temperature is increased or a large amount of non-PV work such as electrical work
is injected into the water molecules. To study the spontaneity, Figure 9.2.2 shows the
standard Gibbs energy change of the water splitting reaction at different temperatures.
It can be found that the transition temperature under standard conditions is about