H2Tech - Q4 2021 - 30

GREEN HYDROGEN PRODUCTION
lyzer), to meet the required H2
specification. The H2
product
gas exiting the dehydration
unit will be stored and offloaded
either as high-pressure (HP) H2
as cryogenic liquid hydrogen (LH2
Both HP hydrogen gas or LH2
gas or
can be
).
loaded to trucks for transport to offsite
fueling stations and other contracted offtake
markets. H2
is transferred via loading
connections to trailers or road trains
designed to transport either LH2
or HP
H2. The loading station also includes provisions
for metering and pressure regulation,
as required. Presently, the generated
oxygen is vented to the atmosphere. The
roadmap includes plans to condition and
sell the oxygen product in future stages.
DESIGN CHALLENGES
The challenges of electrolyzer technology
selection, the intermittency of
renewable power generation and wastewater
management, as well as the approaches
to overcome them, are discussed
in the following sections.2,3,4,5,6
Electrolyzer technology selection.
The challenge: the choice of the electrolysis
technology to be used depends
on several factors covering technical reThree
key factors
drove the choice of
the technology: cost,
reliability and application.
Cost generally favors
alkaline electrolyzers
... since the cells are
manufactured with
inexpensive and easily
accessible material.
explored during conceptual design, and
two proven types of electrolyzers were
considered: alkaline and PEM (TABLE 1).
The approach: Three key factors
drove the choice of the technology: cost,
reliability and application. Cost generTABLE
1. Comparison of alkaline and PEM electrolyzers
Alkaline
Description: The electrodes are in a liquid alkaline electrolyte solution
-typically a highly concentrated potassium hydroxide (KOH) solution-
and are physically separated by a porous, inorganic and non-conductive
diaphragm, which also separates the H2
and oxygen gases generated
at the electrodes.
Advantages
Simple stack and system design
Relatively easy to manufacture
Established technology with operational track record
Lower cell stack costs compared to PEM
Established supply chain and manufacturing capacity
Limitations
Operates at lower current densities (up to 0.8 A/cm2
compared to PEM
Larger footprints compared to PEM
H2 product contains oxygen, and additional purification required
Lower-pressure H2 product (30 bar max reported)
Slower dynamic response to fluctuations in demand and power
30 Q4 2021 | H2-Tech.com
)
PEM
Description: The electrodes are separated by an electron-insulating
solid electrolyte, which is also an ion-conducting membrane that is
responsible for transporting ions from one electrode to the other
and physically separating the produced H2
and oxygen gases.
Advantages
Compact designs, hence smaller footprint
Faster dynamic response to fluctuations in demand and power
Can operate at up to 70 bar with the oxygen side at atmospheric pressure
Wider electrical load ranges
Operates at higher current densities (up to 2.3 A/cm2
Limitations
Higher cell stack costs compared to alkaline
Not well-established, as reliability and lifetime characteristics of
large-scale stacks are yet to be validated
Use of materials like titanium to withstand harsh oxidative
environment at anode
Sensitive to water impurities like iron, copper and sodium and
can suffer from calcination
)
quirements, economic considerations
and regulatory restrictions. These were
ally favors alkaline electrolyzers, as they
are widely accepted to be less expensive
than PEM since the cells are manufactured
with inexpensive and easily accessible
material. While alkaline electrolyzer
cells may be cheaper, a distinction
must be made about the comparison of
the cell/stack costs and the comparison
of overall system costs. The overall
system costs, which include H2
product
conditioning and plant utility costs, are
yet to be rigorously compared.
Regarding reliability, alkaline electrolyzers
have been in use for decades
with several vendors such as Nel, Thyssenkrupp,
Cummins (Hydrogenics) and
McPhy offering units with proven service
and operating references. Nel, ITM
and Siemens commercially offer PEM
electrolyzers with operating references.
However, for a first-of-a-kind green H2
project of this scale, technology risk is
mitigated by choosing the more established
technology.
A key advantage of PEM is its faster
dynamic response that enables it to robustly
handle the intermittency of renewable
power generation without a
requirement for backup. However, an
essential project requirement is that 25
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