Environmental Engineering Reference
In-Depth Information
only a step in the overall hydrogen production process. Efficiency, reliability and
costs of overall integrated plants have to be carefully analysed, in particular taking
into account the typical intermittent operation mode of each renewable source.
Several configurations of PV arrays or wind turbines connected to an electrolyser,
based on PEM technology, have been considered evidencing the potentialities of
each solution [ 76 , 77 ]. A possible optimal option is to select the PV panels so that
their voltage-current output matches the polarization curves of the electrolyser.
Solar PV energy has shown good potentialities as an electricity source for water
electrolysis but recent analysis related to environmental and economical issues
evidence that wind energy seems to be, at least for the existent technological level,
a more promising option to produce electrolytic hydrogen [ 85 , 86 ].
The operative temperature could play a crucial role for the development of a
very efficient electrolyser plant. Solid oxide cells (scheme c in Fig. 2.4 ) have
been proposed for high temperature electrolysis (HTE), because of the strong
resistance at high temperatures of the related electrolytes. With respect to tra-
ditional room-temperature electrolysis HTE modules presents two main advan-
tages [ 87 ]:
1. electrical energy requirement is reduced because of better recover of residual
heat, which is cheaper than electricity
2. the power generating cycle, including also electrolysis reaction, is more effi-
cient at higher temperatures.
Currently yttria-stabilized zirconia and doped LaGaO 3 systems seem the most
promising materials for developing high temperature (about or higher than 800C)
and intermediate temperature (between 400 and 800C) electrolysis technologies,
respectively [ 88 ].
This method could be used for nuclear, concentrating solar or geothermal power
plants without carbon dioxide emissions.
The process is based on the following two electrochemical semi-reactions:
H 2 O þ 2e ! H 2 þ O 2
ð 2 : 25a Þ
O 2 ! 1 = 2 O 2 þ 2e
ð 2 : 25b Þ
where the ionic species are oxygen anions. As for alkaline and PEM electrolyser
technologies the overall reaction is Eq. 2.22 .
In Fig. 2.5 , a simplified scheme of a high-temperature electrolysis plant based
on nuclear power is reported [ 89 ].
Water is warmed up by outer heat in the boiler of the nuclear reactor, before
entering as steam into cathode side, where it decomposes according to Eq. 2.25a ,
hydrogen molecule is removed as product, and oxygen anion moves to anode
through a solid oxide electrolyte with high oxygen ion conductivity. Oxygen ion,
loosing electrons at the anode side, is the reactant of the oxidation semi-reaction,
and is recovered as oxygen molecule, according to Eq. 2.25b .
Search WWH ::




Custom Search