Environmental Engineering Reference
In-Depth Information
9
Cells for Hydrogen Generation; Aspects of Hydrogen Storage
9.1
Intermittency of Renewable Energy
Hydrogen is available, but tightly bound, in water and many other compounds. To
produce hydrogen from water requires an energy input. Hydrogen is not a source of
energy, but rather a means of storing energy. A hydrogen infrastructure for storing
and transporting energy is a possible alternative to batteries to store energy fromsolar
cells. In a large-scale scenario, liquid hydrogen might be transported by railroad tank
car, analogous to liquid natural gas (LNG), rather than by electric transmission over
power lines, which are costly and require connected plots of land. One, but not the
only, clear route to produce hydrogen from solar energy is via photovoltaic (PV) cells
followed by electrolysis of water. We can take the point of view that the viability of this
route is contingent on availability of large-scale inexpensive solar power, which the
CIGS and CdTe cells seem to offer.
9.2
Electrolysis of Water
Inert electrodes such as stainless steel or platinum, immersed in water (containing a
small addition of ions to promote conductivity), will evolve hydrogen gas at the cathode
and oxygen gas at the anode if
1.9 volts is applied. The chemical potential energy
associatedwith amolecule of hydrogen is 1.23 eV, so the ef
ciency of the electrolyzer is
stated as 65%. According to Turner [109], the ef
ciency of commercial electrolyzers is
in the range 60
-
73%. (Actually, about 4% of commercial hydrogen production is by
electrolysis, and about half that is by electrolysis of brine (NaCl plus water) with
chlorine gas, the primary desired product, hydrogen sometimes being abandoned.)
If the electrolyzer is connected with a photovoltaic array (PV) of efficiency 12%,
connecting enough PV cells in series to achieve a working voltage greater than 1.9 V,
then the overall conversion ef
ciency, sunlight energy into chemical energy in the
form of hydrogen, is 0.65
0.12, which gives 7.8%. This is an off-the-shelf approach
with commercial products available at present. A tandem cell is needed to reach the
