Environmental Engineering Reference
In-Depth Information
minimum 1.9 V, and this, as noted in Chapter 7, section titled, Low-Cost Tandem
Technology: Amorphous Silicon:H-Based Solar Cells, is available in amorphous
silicon tandem cells, providing 2.2 V.
We will see below an example of a monolithic PV-hydrogen electrolytic converter
with ef
ciency 12.4%. For comparison, a high-temperature solar thermal water-
splitting process is predicted to provide 24% ef
ciency [110] in hydrogen production
but requires a large facility.
In general, we regard cells for electrolysis of water, fuel cells, batteries, Stirling
engines, and electrical generators as off-the-shelf items, and will focus here on
research topics that might eventually broaden the set of available devices.
9.3
Efficient Photocatalytic Dissociation of Water into Hydrogen and Oxygen
One element in this category is a cell for splitting water to produce hydrogen with
sunlight, called photocatalytic splitting of water. Direct photocatalytic water splitting
is not easy to accomplish. Ef
ciency is needed in the several steps: absorption of
photons to create electron
hole pairs, in their transport to the water interface, in
dissociating the water, and in separating and collecting hydrogen. The large area
needed for a useful amount of hydrogen production, stemming from the low energy
density in sunlight, is a dif
culty, which could be addressed with focusing mirrors or
Fresnel lenses. We consider a device in Figure 9.1 [111] based on a tandem solar cell.
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Figure 9.1 Schematic diagram [111] of
illuminated monolithic photovoltaic
oxygen bubbles appear at the left (platinum
anode). Mass spectroscopy revealed the evolved
gasestobepurehydrogenandoxygeninratio2to
1. In this device, electrons flow to the illuminated
contact where they act to release hydrogen gas.
This cell is built on the GaAsmultijunction tandem
solar cell technology as described in Chapter 7,
and is clearly an expensive device.
-
photoelectrochemical device for hydrogen
production by water splitting, with 12.4%
efficiency in converting solar light energy into
chemical energy. In this cell, with no external
connection, hydrogen bubbles appear on the
illuminated surface at the right (cathode), and
