Environmental Engineering Reference
In-Depth Information
2.1.3 Photolytic Processes
The photolytic effect represents another technology able to directly exploit the
sunlight, in addition to photovoltaic effect and concentrating solar technology.
This process could be theoretically used to directly dissociate water molecules into
hydrogen and oxygen [
93
]. The recent advances realized in this field [
4
,
94
]
encourage a wide research effort aimed at individuating technological pathways
alternative to thermal, thermochemical and electrolytic approaches, for an useful
contribution to medium-long term hydrogen production.
In particular, two kinds of processes are under investigation:
1. the photoelectrochemical (PEC) process that uses photoactive cells in which
doped semiconductor electrodes are immersed in aqueous solutions or water;
2. the photobiological (PB) water splitting, related to the specific activity of
specialized microorganisms.
In the PEC process, an electronic charge formed at the surface of the anode,
radiated from solar energy, is able to generate an electron-hole pair. In the
presence of an electric field, holes and electrons are forced to move in opposite
directions determining the H
2
O oxidation to oxygen at the anode side and the
hydrogen ion reduction to molecular hydrogen at the cathode side. PEC research is
mainly focused on finding reliable semiconductors able to split water in an
energetically suitable way [
95
-
97
]. On the other hand, PB processes could exploit
the potentialities of algae and bacteria in consuming water and produce hydrogen
as a byproduct of their natural metabolic processes [
98
,
99
]. This research is
focused on the possibility of modifying or engineering them, addressing the solar
energy selectively towards direct hydrogen production.
2.2 Hydrogen Distribution
The transition towards the so-called hydrogen economy requires the development
of infrastructure plans. Currently, few limited networks for hydrogen utilization
exist in the world, mostly concentrated in Europe (UK, Netherlands, Germany) and
USA, and located close to refinery site for petrochemical or other industrial
requirements.
A future massive network can be realized according to two possible scenarios:
1. a centralized management of the worldwide hydrogen production and distri-
bution, corresponding to the existent energy production strategies
2. a distributed territorial production and utilization, for which H
2
is produced on-
site at small-medium-scale filling station.
The analysis of the above strategies needs to include all the stages necessary to
produce and distribute the fuel for a widespread use, and should benefit from the
following two options for hydrogen transport and distribution:
