Solar hydrogen production and CO recycling - Solar Energy Sciences and Engineering Applications - page 260

s − 1 . Also, m P in Equation (9.2.1) is

device for the reacting system, and its units are J ·

s − 1 , and H Liq is the standard enthalpy
change of the following water splitting reaction at 298K (25 ◦ C) in units of J

the hydrogen production rate in units of mole ·

mole − 1 ·

with a value of 285,800 J/mol:

H 298 = H 2 O (liquid) = H 2 (gas) + ½O 2 (gas) 285,800 J / mol (9.2.2)

If 25 ◦ C is also used as the reference temperature for the higher heating value (HHV),
then the HHV is equal to the enthalpy change H Liq . The product of m P and H Liq
indicates the theoretical minimum energy needed to split water into hydrogen and
oxygen, or the maximum energy that can be recovered from hydrogen when hydro-
gen is used as a fuel. The reason to use liquid water at 25 ◦ C rather than gaseous water
is because the starting state of the water used in industry for hydrogen production
is mostly in the form of liquid at an ambient temperature, although in the hydrogen
production reactor it could be in other forms and the temperature could be slightly
different. Also, since the efficiency is discussed from the perspectives of production
rather than usage, the actual energy input to the production may attract more engi-
neering interest. If the reported values in past literature are based on H Gas , G Gas
and G Liq , they will be converted to H Liq in this chapter for a consistent comparison.
The reason to use the whole spectrum in Equation (9.2.1) for the efficiency
comparison is to make the devices working at different wavelengths to be more
comparable, and also more convenient for the evaluation of the energy losses
due to the non-use of other wavelengths. The efficiency is also influenced by the
working wavelength of the devices and the sunlight absorbing material may work
only for a specific range of wavelengths. Consequently, other wavelengths of the
solar spectrum will be unused and the efficiency of sunlight usage is reduced.
Different devices may work at different wavelengths of the solar spectrum, so the com-
parison is not made on the same basis when using the working wavelength of the devices
to evaluate their performance. Different wavelengths correspond to different portions
of solar irradiance. Table 9.2.1 shows the energy distribution of different wavelengths
of the solar spectrum from past data (Thuillier et al., 2003). It can be concluded that if
the hydrogen production device works only in the ultraviolet region, then it can only
use a maximum of 10% of the total incident solar energy. By comparison, if it works in
the infrared region, then potentially 50% of the total incident solar energy can be used.
In addition, various devices and technologies have different solar irradiance tracking
and capturing capabilities. As a result, the requirement of land area and dimensions
of auxiliary equipment may be quite different.

Table 9.2.1 Energy distribution of the solar spectrum.

Irradiance Wavelength % of total

Infrared 700-2,400 nm 49.4 91.7

Visible 400-700 nm 42.3

Ultraviolet A 320-400 nm 6.3 8.3

Ultraviolet B 290-320 nm 1.5

Ultraviolet C 200-290 nm 0.5