An Optimized Balance of Plant for a Medium-Size PEM Electrolyzer: Design, Control and Physical Implementation

Abstract

The progressive increase in hydrogen technologies' role in transport, mobility, electrical microgrids, and even in residential applications, as well as in other sectors is expected. However, to achieve it, it is necessary to focus efforts on improving features of hydrogen-based systems, such as efficiency, start-up time, lifespan, and operating power range, among others. A key sector in the development of hydrogen technology is its production, renewable if possible, with the objective to obtain increasingly efficient, lightweight, and durable electrolyzers. For this, scientific works are currently being produced on stacks technology improvement (mainly based on two technologies: polymer electrolyte membrane (PEM) and alkaline) and on the balance of plant (BoP) or the industrial plant (its size depends on the power of the electrolyzer) that runs the stack for its best performance. PEM technology offers distinct advantages, apart from the high cost of its components, its durability that is not yet guaranteed and the availability in the MW range. Therefore, there is an open field of research for achievements in this technology. The two elements to improve are the stacks and BoP, also bearing in mind that improving BoP will positively affect the stack operation. This paper develops the design, implementation, and practical experimentation of a BoP for a medium-size PEM electrolyzer. It is based on the realization of the optimal design of the BoP, paying special attention to the subsystems that comprise it: the power supply subsystem, water management subsystem, hydrogen production subsystem, cooling subsystem, and control subsystem. Based on this, a control logic has been developed that guarantees efficient and safe operation. Experimental results validate the designed control logic in various operating cases, including warning and failure cases. Additionally, the experimental results show the correct operation in the different states of the plant, analyzing the evolution of the hydrogen flow pressure and temperature. The capacity of the developed PEM electrolysis plant is probed regarding its production rate, wide operating power range, reduced pressurization time, and high efficiency.

Full text

electronics
Article
An Optimized Balance of Plant for a Medium-Size
PEM Electrolyzer: Design, Control and
Physical Implementation
Julio JoséCaparrós Mancera 1, * , Francisca Segura Manzano 1, JoséManuel Andújar 1,
Francisco JoséVivas 1and Antonio JoséCalderón2
1Department of Electronics Engineering, Computer Systems and Automatic, University of Huelva,
Campus El Carmen, 21071 Huelva, Spain; francisca.segura@diesia.uhu.es (F.S.M.);
andujar@diesia.uhu.es (J.M.A.); francisco.vivas@diesia.uhu.es (F.J.V.)
2Electronics Engineering and Automatic, University of Extremadura, Department of Electrical,
Campus Universitario, 06006 Badajoz, Spain; ajcalde@unex.es
*Correspondence: julio.caparros@diesia.uhu.es; Tel.: +34-669-220-871
Received: 30 April 2020; Accepted: 21 May 2020; Published: 24 May 2020


Abstract:
The progressive increase in hydrogen technologies’ role in transport, mobility,
electrical microgrids, and even in residential applications, as well as in other sectors is expected.
However, to achieve it, it is necessary to focus efforts on improving features of hydrogen-based systems,
such as efficiency, start-up time, lifespan, and operating power range, among others. A key sector in
the development of hydrogen technology is its production, renewable if possible, with the objective
to obtain increasingly efficient, lightweight, and durable electrolyzers. For this, scientific works are
currently being produced on stacks technology improvement (mainly based on two technologies:
polymer electrolyte membrane (PEM) and alkaline) and on the balance of plant (BoP) or the industrial
plant (its size depends on the power of the electrolyzer) that runs the stack for its best performance.
PEM technology offers distinct advantages, apart from the high cost of its components, its durability
that is not yet guaranteed and the availability in the MW range. Therefore, there is an open field of
research for achievements in this technology. The two elements to improve are the stacks and BoP,
also bearing in mind that improving BoP will positively affect the stack operation. This paper
develops the design, implementation, and practical experimentation of a BoP for a medium-size PEM
electrolyzer. It is based on the realization of the optimal design of the BoP, paying special attention
to the subsystems that comprise it: the power supply subsystem, water management subsystem,
hydrogen production subsystem, cooling subsystem, and control subsystem. Based on this, a control
logic has been developed that guarantees efficient and safe operation. Experimental results validate the
designed control logic in various operating cases, including warning and failure cases. Additionally,
the experimental results show the correct operation in the different states of the plant, analyzing
the evolution of the hydrogen flow pressure and temperature. The capacity of the developed PEM
electrolysis plant is probed regarding its production rate, wide operating power range, reduced
pressurization time, and high efficiency.
Keywords: hydrogen production; PEM electrolyzer; balance of plant; design; control; experimental test
1. Introduction
The understanding of energy consumption is changing in a society with demands for more
sustainable energy, where energy policies carried out by governments and companies are creating
a growing social consciousness. In this sense, renewable energies are a fundamental pillar in the
compulsory (i.e., that the Earth cannot wait for) energy transition from fossil fuels to renewable
Electronics 2020,9, 871; doi:10.3390/electronics9050871 www.mdpi.com/journal/electronics

Electronics 2020,9, 871 2 of 25
sources [
1
]. Having said that, the immediacy and security of the fossil fuels to guarantee the energy
needs in any part of the world and under any circumstance need to be beaten with rationality. Except in
very few parts of the world, it is rare that a single renewable source (wind, solar, geothermal, tidal, etc.)
can meet the needs of a community. Therefore, the solution is to carry out an amalgamation of renewable
energies, looking for the synergy between them that can assure production and demand all the time [
2
].
In this scenario, hydrogen can play a pivotal role. Through an electrolyzer (hydrogen production)
and fuel cells (electricity generation from hydrogen) integrated into renewable energy-based systems,
energy demands can be met on a circular greenway (Figure 1) [3].
Electronics 2020, 9, x FOR PEER REVIEW 2 of 27
sources [1]. Having said that, the immediacy and security of the fossil fuels to guarantee the energy
needs in any part of the world and under any circumstance need to be beaten with rationality. Except
in very few parts of the world, it is rare that a single renewable source (wind, solar, geothermal, tidal,
etc.) can meet the needs of a community. Therefore, the solution is to carry out an amalgamation of
renewable energies, looking for the synergy between them that can assure production and demand
all the time [2]. In this scenario, hydrogen can play a pivotal role. Through an electrolyzer (hydrogen
production) and fuel cells (electricity generation from hydrogen) integrated into renewable energy-
based systems, energy demands can be met on a circular greenway (Figure 1) [3].
Figure 1. Use of hydrogen as energy vector.
Regarding hydrogen production, various proposals can be found in the recent scientific
literature on PEM electrolyzers, specifically when it comes to approaching balance of plant (BoP)
design.
In 2011, Balaji et al. proposed an electrolysis plant for portable applications [4]. The result was a
low production electrolyzer with 0.08 Nm
3
/h of hydrogen at 382 W of power consumption and an
efficiency of 77.48%. Since the objective of this design is its mobility, it has a highly reduced BoP. The
water management contains a tank that works as a filling tank as well as an oxygen separator. In the
hydrogen production subsystem, there is a drying stage, made up of two silica gel desiccants that are
alternated by electro valves, before reaching the storage tank. In terms of BoP, the system lacks a
water filtering system, as well as a separation and venting system for the water that can condense
along with the hydrogen produced.
A hydrogen-based system with higher capacity is presented in [5] by means an electrolyzer of 1
kWe (the term kWe is referring to consumed electrical power), and 0.3 Nm
3
/h, which focuses its study
on the relationship between temperature and flow rate to obtain a system efficiency of 65% at 40 °C.
The water subsystem in the BoP has a tank that includes the corresponding cooling system, an
injection pump, and flow regulators through manual valves. In the hydrogen production subsystem
of the BoP, there is an oxygen separator that returns the water to the system and works as a water
inlet tank. The system also includes two gas separator tanks that act as refrigeration and drying
system of the produced hydrogen.
An electrolyzer for direct coupling to photovoltaics is studied in [6]. The study focuses on
finding the best relationship between the number of cells and control technique on photovoltaic
panels, to achieve a hydrogen production rate of 0.48 Nm
3
/h with 11 cells, and power consumption
of 2.25 kWe. In the BoP design, the water comes directly from the oxygen separator tank, which has
external cooling and an injection pump to the stack. The hydrogen goes into an accumulator as the
only component prior to storage, which has its own refrigeration and acts as a pressure separator.
The water obtained in the drying process is not injected into the oxygen separator, but is sent to the
water tank that acts as refrigeration, and has its own filtering system. Then, water must be taken from
Figure 1. Use of hydrogen as energy vector.
Regarding hydrogen production, various proposals can be found in the recent scientific literature
on PEM electrolyzers, specifically when it comes to approaching balance of plant (BoP) design.
In 2011, Balaji et al. proposed an electrolysis plant for portable applications [
4
]. The result was a
low production electrolyzer with 0.08 Nm
3
/h of hydrogen at 382 W of power consumption and an
efficiency of 77.48%. Since the objective of this design is its mobility, it has a highly reduced BoP.
The water management contains a tank that works as a filling tank as well as an oxygen separator.
In the hydrogen production subsystem, there is a drying stage, made up of two silica gel desiccants
that are alternated by electro valves, before reaching the storage tank. In terms of BoP, the system lacks
a water filtering system, as well as a separation and venting system for the water that can condense
along with the hydrogen produced.
A hydrogen-based system with higher capacity is presented in [
5
] by means an electrolyzer of
1 kWe (the term kWe is referring to consumed electrical power), and 0.3 Nm
3
/h, which focuses its study
on the relationship between temperature and flow rate to obtain a system efficiency of 65% at 40
◦
C.
The water subsystem in the BoP has a tank that includes the corresponding cooling system, an injection
pump, and flow regulators through manual valves. In the hydrogen production subsystem of the BoP,
there is an oxygen separator that returns the water to the system and works as a water inlet tank.
The system also includes two gas separator tanks that act as refrigeration and drying system of the
produced hydrogen.
An electrolyzer for direct coupling to photovoltaics is studied in [
6
]. The study focuses on
finding the best relationship between the number of cells and control technique on photovoltaic panels,
to achieve a hydrogen production rate of 0.48 Nm
3
/h with 11 cells, and power consumption of 2.25 kWe.
In the BoP design, the water comes directly from the oxygen separator tank, which has external cooling
and an injection pump to the stack. The hydrogen goes into an accumulator as the only component
prior to storage, which has its own refrigeration and acts as a pressure separator. The water obtained
in the drying process is not injected into the oxygen separator, but is sent to the water tank that acts

Electronics 2020,9, 871 3 of 25
as refrigeration, and has its own filtering system. Then, water must be taken from this refrigeration
tank before starting-up the equipment, and there is no way for direct injection. This implies that the
water that reaches the stack could be contaminated, as it does not have a deionizer system for the stack
inlet water.
Looking for advanced developments with a higher hydrogen production rate, Kosonen et al.
provide in [
7
] a 1 Nm
3
/h electrolyzer that consumes 4.5 kWe, with an efficiency of around 78%.
The system has a large number of cells (66 cells) and a fairly simplified BoP. The water comes directly
from the local water network, and goes through a deionizer, while the hydrogen production goes
through a drying unit that lowers the temperature to
−
70
◦
C, since the system stores hydrogen in a
Nordic location.
The electrolyzer presented in [
8
] is made from two stacks with 48 cells each, producing 5 Nm
3
/h,
at 27 kWe. The efficiency is estimated theoretically at 99%. Although these two stacks-based designs
claim high hydrogen production rates, for the electrolyzer implementation there is required complex
and bulky BoP.
An electrolysis plant that offers a hydrogen production rate of tens Nm
3
is presented in [
9
].
In this case
, the stack technology is based on cells similar to PEM electrolyzer proposed in this paper.
With an active area of 290 cm
2
, stack made up of 60 cells, it provides 10 Nm
3
/h at 46 kWe. The BoP
contains an oxygen separator tank that works as a water filling tank and stack inlet water feed.
The water flow is guaranteed by a pump, along with a filtering system. The hydrogen produced passes
through a gas separator. A single cooling system provides the heat exchange for the water and the
cooling of the hydrogen, with which the humidity is separated, and when it reaches a suitable level it
is injected directly into the stack water feed. In the same operating range, Stansberry et al. developed
in [
10
] another 10 Nm
3
/h electrolyzer, at 60 kWe and overall efficiency of 56% due to a heavy BoP
implementation (drying units and chiller).
Finally, a larger PEM electrolyzer designed for hydrogen refueling stations and big energy
storage systems is discussed in [
11
]. In this case, the electrolyzer is able to produce up to 500 Nm
3
/h,
with a current density of 30 kA/m
2
. According to the analyzed data of the BoPs of the proposals
found in the scientific literature, Table 1shows a qualitatively comparison of the subsystems and
their elements. Here it is verified how the developed BoP in this paper contains elements of large-scale
and moderate-consumption systems. Therefore, it can be seen that the proposal BoP improves previous
solutions by adding elements for better water filtering, such as low pressure separator (LPS), two phase
filtering and recirculation filtering, as well as better hydrogen drying, adding LPS, a pressure swing
adsorption (PSA) dryer (which does not require consumption like temperature swing adsorption (TSA)
dryers), and redesigning the order of the elements by cooling before the high pressure separator (HPS).
In addition to this novel proposal, a differentiating feature of this paper is that all the elements are
described in detail in next sections, both in technical characteristics and in their exact connection,
something that does not occur as precisely in the previous proposals [4–10].
In order to clearly point out the novelty of this article, Table 2compares the developed electrolyzers
in the analyzed works, from their technical specifications, with the one proposed in our research
and presented in this paper (hydrogen production rate of 2.22 Nm
3
/h, at 10 kWe and stack efficiency
between 77% and 91%).
Table 2shows that the authors’ proposal provides a mid-range production, with a fairly
low electrical consumption. This is because the design is based on supplying the stack a high
electrical current, up to 900 A; optimizing the relationship between hydrogen production and
electricity supply. To achieve this design, cells from larger-scale electrolyzers are used, similar to
Reference [
6
], consisting of a stack of 60 cells of 290 cm
2
. The design proposed by authors only
requires 6 cells of 300 cm
2
to provide a 50% higher current density, despite having a similar cell
active area. The current density of 3 A/cm
2
also differentiates the proposed design, since typical
current density reviewed in the literature ranges from 1 A/cm
2
to 2 A/cm
2
. This depends on the
maximum cell current and the active cell area, and it’s limited by these factors. Regarding the

Electronics 2020,9, 871 4 of 25
hydrogen pressure, the developed electrolyzer is capable of supply hydrogen up to 40 bar without
the need of a compressor. As can be seen, this capability from the developed electrolyzer optimizes
the auxiliary consumption, removing any compressor power requirement, which obviously increases
the total efficiency. Additionally, a significant difference between the proposed BoP regarding the
literature review lies in the hydrogen cooling being placed just at the stack hydrogen outlet. Therefore,
in the authors’ proposal, the first gas separator stage, included in all the revised works, receives more
condensed water, so the hydrogen drying is more efficient from its first phase.
Therefore, the novelty of the proposed PEM electrolyzer is characterized by its mid-range
production at optimized consumption, high current density with a low number of cells, high pressure
without the need of compressors, and a BoP that optimizes the hydrogen cooling and drying stages.
The complete design of the proposed BoP, as well as the characteristics of the stack and the electrolyzer
are detailed in the following sections.
Table 1.
Qualitative comparison of proposed polymer electrolyte membrane (PEM) electrolyzer with
previous scientific works.
Water Subsystem Hydrogen
Subsystem
Cooling
Subsystem Control System Advantages Weaknesses
Authors
Proposal
Oxygen separator
tank, water tank,
injection pump,
recirculation pump,
deionizer two phase
filter, recirculation
filter.
HPS (1), LPS
(2), PSA (3)
dryers.
Water and
hydrogen heat
exchangers with
dry cooler.
Water:
conductivity, flow,
level, temperature,
pressure.
Hydrogen: level,
pressure,
temperature.
Laboratory didactic
design with a
mid-size scale.
Optimized BoP with
low consumption.
Hydrogen cooling
before HPS (1)
LPS (2) included
Two water filters
systems
Higher scale.
Reduced size.
[4]Oxygen separator
tank.
Two silica
desiccant
dryers.
-
Water: level,
purity.
Oxygen: sensor.
Hydrogen:
humidity,
pressure.
Portable design.
Water is not
filtered. No
pressure
separator.
[5]
Oxygen separator
tank, water tank,
refilling pump, water
pump.
HPS (1),
buffer.
Water electric
heater.
Water: level,
pressure,
temperature.
Hydrogen: level,
pressure,
temperature.
Compact design.
Water is not
filtered. Only HPS
(1)
as drying stage.
[6]
Oxygen separator
tank, injection pump,
external water tank.
HPS (1).
Water, oxygen and
hydrogen heat
exchangers.
Pre-adjusted. No
controller. PV (5) direct coupling.
Water is not
filtered and has no
direct injection.
Lack of automated
process control.
[7] Water deionizer. Drying unit
to–70 ◦C
Hydrogen cooling.
Water:
conductivity, flow,
pressure,
temperature.
Hydrogen:
pressure,
temperature.
Nordic conditions.
It depends on the
local water
network purity.
[8]
Oxygen separator
tank, water
recirculation filtering,
recirculation pump.
HPS (1), TSA
(4) dryers.
Water
refrigeration.
Hydrogen:
pressure,
temperature.
Semi-industrial scale. TSA (4)
consumption.
[9]
Oxygen separator
tank, water pump,
filtering system.
HPS (1).Water stack
cooling. Not described. High production.
Only one drying
stage for high
production.
[10]
Oxygen separator
tank, water tank,
injection pump,
circulation pump.
HPS (1), PSA
(3) dryer.
Water and
hydrogen heat
exchangers.
-
High production.
Controller integrated
with renewable
sources.
Auxiliaries in BoP
with high power
consumption
involves low
efficiency.
(1)
High pressure separator;
(2)
low pressure separator;
(3)
pressure swing adsorption;
(4)
temperature swing
adsorption; (5) photovoltaic.

Electronics 2020,9, 871 5 of 25
Table 2. Comparison of the findings of the proposed research with previous works.
Production
Rate (Nm3/h)
Power
(kWe)
Efficiency
(%) Cells Cell Area
(cm2)
Cell
Voltage (V)
Current
(A)
Current Density
(A/cm2)
Maximum
Pressure (bar)
Authors
Proposal 2.22 10 77–91 (2)
52–61 (3) 6 300 1.94 900 3 40
[4] 0.08 0.382 77 (2) 2 100 1.91 100 1 5
[5] 0.3 1 86 (2)
65 (3) 10 100 2 50 0.5 6
[6] 0.48 2.25 99 (1) 11 50 1.94 105 2 1
[7] 1 4.5 76–80 (2) 33 69 1.93 70 1 40
[8] 5 27 99 (1) 96 130 2.35 119.6 0.92 13
[9] 10 46 100 (1) 60 290 1.84 414.7 1.43 35
[10] 10 60 72 (2)
56 (3) 65 214 2.15 410 1.92 34.5
(1) Estimated efficiency. It is not an experimental value; (2) stack efficiency; (3) system efficiency.
This paper continues and considerably expands previously developed research [
12
] and contributes
to hydrogen technology implantation into the energy industrial sector, with the design, experimentation
and real implementation of a medium-size proton exchange membrane (PEM) electrolyzer for
hydrogen production. After the design of the BoP, an exhaustive control system is developed to
test the working conditions that will allow the PEM electrolyzer to produce hydrogen in a safe and
efficient way. The aim of this research is to find an equilibrated solution between minimal BoP and
correct performance, always into safety conditions of hydrogen generation. Additionally, although
previous studies have been conducted in the simulation and experimental testing of PEM electrolyzers
as power-hardware-in-loop (PHIL) simulators [
13
], dSPACE Hardware-in-the-Loop simulators [
14
],
multiphysics simulators [
15
], dynamic simulators based on MATLAB [
16
] and mathematical dynamic
Simulink simulators [
17
], this development is oriented to the use of software tools based on totally
integrated automation logic. Therefore, it includes the logic control design, necessary for the safe
and effective performance of the plant, with the experimental tests to evaluate operation parameters,
a monitoring environment, and quality testing.
The paper is organized as follow: Section 2explains material and methods used to develop
the research, including a description of PEM electrolysis technology: main features and highlights.
Next, a detailed description of the design, the developed control logic and implementation is offered in
Section 3. Section 4brings together the experimental results, discussed below in Section 5. Finally,
the overall conclusions are reflected in Section 6.
2. Materials and Methods
2.1. PEM Electrolysis Technology
PEM technology replaces the liquid electrolyte, typical from the alkaline electrolysis, by a solid
polymer electrolyte, which selectively conducts positive ions such as protons. This technology
improves current density, energy efficiency, and dynamic operation [
18
]. The protons participate
in the water-splitting reaction instead of hydroxide, creating a locally acidic environment in the
cell [
19
]. In PEM electrolysis (Figure 2), electrodes are in contact with the solid polymer electrolyte,
usually Nafion. Bipolar plates are also typically added between the solid electrolyte and the electrodes,
made of platinum for the cathode and iridium for the anode, with the aim of adding resistance
to corrosion [
20
], produced during the uncontrolled polarity of the cells and fluctuating charges.
PEM electrolyzers can normally reach a current density up to 2 A/cm
2
, the polymer electrolyte
membrane guarantees a low gas crossover, allowing the PEM electrolyzers to work under a lower
partial load range (0–10%), and it can have a compact design. This allows the obtaining of high enough
operating pressures (30–40 bar), as an effect of the electrochemical compression in PEM technology, [
21
],
to directly fill the pressure hydrogen storage tanks [19].

Electronics 2020,9, 871 6 of 25
Electronics 2020, 9, x FOR PEER REVIEW 6 of 27
The paper is organized as follow: Section 2 explains material and methods used to develop the
research, including a description of PEM electrolysis technology: main features and highlights. Next,
a detailed description of the design, the developed control logic and implementation is offered in
Section 3. Section 4 brings together the experimental results, discussed below in Section 5. Finally,
the overall conclusions are reflected in Section 6.
2. Materials and Methods
2.1. PEM Electrolysis Technology
PEM technology replaces the liquid electrolyte, typical from the alkaline electrolysis, by a solid
polymer electrolyte, which selectively conducts positive ions such as protons. This technology
improves current density, energy efficiency, and dynamic operation [18]. The protons participate in
the water-splitting reaction instead of hydroxide, creating a locally acidic environment in the cell [19].
In PEM electrolysis (Figure 2), electrodes are in contact with the solid polymer electrolyte, usually
Nafion. Bipolar plates are also typically added between the solid electrolyte and the electrodes, made
of platinum for the cathode and iridium for the anode, with the aim of adding resistance to corrosion [20],
produced during the uncontrolled polarity of the cells and fluctuating charges. PEM electrolyzers can
normally reach a current density up to 2 A/cm2, the polymer electrolyte membrane guarantees a low
gas crossover, allowing the PEM electrolyzers to work under a lower partial load range (0%–10%),
and it can have a compact design. This allows the obtaining of high enough operating pressures (30–40 bar),
as an effect of the electrochemical compression in PEM technology, [21], to directly fill the pressure
hydrogen storage tanks [19].
Figure 2. PEM electrolytic cell.
Additionally, in terms of corrosion, although it is not critical in PEM technology, poisoning by
foreign ions appears and thus it has to be highly considered. The water can be easily contaminated
by the impurities it contains, as well as by the corrosion produced in the metallic components of the
system, such as the water pipes or even the stack components themselves. This poisoning will result
in an increase in the cell cathodic overvoltage and a reduction in operating performance [22], in
addition to affecting the membrane in a reduction of its proton conductivity. These are the reasons
why an exhaustive design and control of the BoP (involving water management, conductivity, and
purity) is important to make PEM electrolysis technology become a competitive hydrogen production
option [23].
2.2. Previous Design Considerations
The goal of this research is to develop a PEM electrolyzer capable of producing more than 2
Nm3/h with a maximum operating pressure of 40 bar, and that consumes a maximum power of 10
kWe. For this purpose, the cell used for the electrolyzer stack is a 300 cm2-cell from GINER® (Newton,
MA, USA).
Figure 2. PEM electrolytic cell.
Additionally, in terms of corrosion, although it is not critical in PEM technology, poisoning by
foreign ions appears and thus it has to be highly considered. The water can be easily contaminated
by the impurities it contains, as well as by the corrosion produced in the metallic components of the
system, such as the water pipes or even the stack components themselves. This poisoning will result in
an increase in the cell cathodic overvoltage and a reduction in operating performance [
22
], in addition
to affecting the membrane in a reduction of its proton conductivity. These are the reasons why an
exhaustive design and control of the BoP (involving water management, conductivity, and purity) is
important to make PEM electrolysis technology become a competitive hydrogen production option [
23
].
2.2. Previous Design Considerations
The goal of this research is to develop a PEM electrolyzer capable of producing more than 2 Nm
3
/h
with a maximum operating pressure of 40 bar, and that consumes a maximum power of 10 kWe.
For this purpose
, the cell used for the electrolyzer stack is a 300 cm
2
-cell from GINER
®
(Newton, MA,
USA).
The cells provide a maximum hydrogen production of 0.37 Nm
3
/h, then the design will require
Ncells =6, Equation (1):
Ncells =Stack hydrogen production rate
Cell hydrogen production rate (1)
where:
Ncells is the stack cells number
Cell hydrogen production rate is 0.37 Nm3/h
Stack hydrogen production rate is 2.22 Nm3/h
From the electrolysis reaction, Equation (2), it is possible to calculate the mass balance, Equation (3),
and the volume of hydrogen and oxygen produced with 1 L of water, Equation (4). As the electrolyzer
design has to provide a hydrogen production of 2.22 Nm 3/h, Equation (5) shows that 1.79 L/h of water
will be consumed by the proposed stack in the electrolysis process:
H2O→H2+1
2O2(2)
18 g/mol (H2O)→2g/mol (H2)+0.5·32 g/mol (O2)(3)
1l(H2O)→1.235 Nm3(H2)+0.595 Nm3(O2)(4)
1.79 l/h(H2O)→2.22 Nm3/h(H2)+1.07 Nm3/h(O2)(5)
The stack power consumption can be calculated from Equation (6), resulting in
Pstack
=10.47
kWe
Pstack =Ncells·VBOL ·Istack (6)

Electronics 2020,9, 871 7 of 25
where:
Pstack is the stack power consumption
Ncells is the cells number (6 in this case)
VBOL is the cell voltage at the beginning of life (1.94 V)
Istack is the stack current for maximum hydrogen production (900 A)
2.3. Equipment Selection
From the previous design consideration, Table 3summarizes the selection criteria and the main
technical characteristics of the equipment selected for the implementation of the BoP for the PEM
electrolyzer. All the elements have been selected after a careful market sounding, looking for tested
equipment from warranty companies. The selection criteria and other considered models are also
shown in Table 3.
Table 3. Balance of plant (BoP) implementation. Technical characteristics and selection criteria.
Component Manufacturer Model Main Characteristics Selection Criteria
PEM stack GINER®Merrimack stack
H2production (Max): 2.22 Nm3/h
Current density range: 300–3000
mA/cm2
Current (Max): 900 A
Maximum H2operating pressure: 40
bar
Maximum operating temperature: 70
◦
C
Cell voltage: 1.94 V (BOL (1) )–2.40 V
(EOL (2))
Cell dimensions: Ø 352.44 mm
Number of cells: 6
1. High
operating temperatures.
2. It minimizes the size of
heat exchangers.
3. High operating pressures
can avoid the need for post
electrolysis
compression equipment.
4. Cost.
DC power supply
Green Power®(Beijing, China)
IGBT Power Supply GA-1000 A/15
V-STA
Rated output: 1000 A/15 V
Input line voltage: 380 V
1. It optimizes the current
density up to 3 A/cm2.
2. Air cooling reduces
auxiliary consumption.
Injection pump LAMMERS®(Rheine, Germany)
D-45432
Power: 0.12 kW
Flow rate: 1.36 L/min
Weight: 4 kg
IP56
1. Low power consumption.
2. Adjustable flow.
Recirculation pump
LOWARA
®
(Rye Brook, NY, USA)
1HM07S05T5RVBE
Power: 0.48 kW
Flow rate: 16.67 L/min
Horizontal model
Stainless steel (AISI 304)
Weight: 10 kg
IPX5
1. Compact design.
2. Optimal relation between
flow range and
power consumption.
3. Manufacturing material not
contaminate water.
DI (3) water handling Wasserlab®(Barbatáin, Spain)
SACI001 Type I and Type II filtering
1. Filtering in two stages
reduces the cost
of consumables.
2. Total control of parameters.
3. Preventive maintenance.
H2HPS (4) Custom made Valco®(Nerviano, Italy) level sensors Compact size with ATEX (6)
sealing.
H2LPS (5) Custom made Valco®level sensors Compact size with ATEXsealing.
H2dryer Custom made Pressure drying Compact size. PSA (7) technology.
O2separator Custom made Valco®level sensors High water capacity in vertical
design.
(1)
Beginning of life;
(2)
end of life;
(3)
deionized;
(4)
high pressure separator;
(5)
low pressure separator;
(6)
atmosphere
explosive (devices intended for use in explosive atmospheres); (7) pressure swing adsorption.
Figure 3shows a block diagram describing the PEM electrolysis process of the developed
electrolyzer. The elements in the photos are the ones actually implemented. Deionized (DI) water is fed
to the electrolyzer stack from a DI water handling unit. When power is supplied to the electrolyzer stack,
hydrogen and oxygen gases are generated in it. Oxygen is passed through an O
2
-phase separator where
it is separated and the water is returned to the DI handling unit. From here, using a low-pressure water
pump (or a high-pressure water pump for balanced pressure applications), DI water is injected into
the stack. Regarding the hydrogen line, it is passed through a hydrogen gas-phase separator. The H
2

Electronics 2020,9, 871 8 of 25
gas-phase separator removes water that is electro-osmotically transported through the PEM stack
during the electrolysis process. The hydrogen is then passed through a hydrogen dryer. Throughout
all the hydrogen line, the wastewater is collected and reused. Finally, the produced H
2
usually goes to
a storage tank. Regarding the produced O
2
, it can be vented through the vent unit (as in this case) or it
can be used to any application.
Electronics 2020, 9, x FOR PEER REVIEW 9 of 27
the produced H
2
usually goes to a storage tank. Regarding the produced O
2
, it can be vented through
the vent unit (as in this case) or it can be used to any application.
Figure 3. Block diagram of PEM electrolysis process.
Following Figure 3, the PEM electrolysis process is completed with the two primary elements,
DI water on the one hand and electricity through a DC power supply on the other.
Regarding general operation, when power is applied to the stack, the water supply through it
must be guaranteed at all times; the lack of water will damage the stack. The water flow rate must be
set well above the stoichiometric rate at all times as it also serves to remove excess heat from the stack.
Additionally, during operation, hydrogen (cathode) pressure should be above that of the water
(anode) pressure (see Figure 2). The stack must operate at a hydrogen pressure of at least a 0.068 bar
above the water pressure. This is to ensure that hydrogen can be detected in the water/oxygen outlet
in the event a membrane is breached (membrane failure). This will normally happen in the case of
correct performance, because the pressure of the water hardly requires a value of between 1 bar and
2 bar, while the hydrogen will quickly increase its pressure to reach high pressure in a few minutes,
as shown in the experimental results.
Then, according to operation description, it can be deduced that in PEM electrolysis it is very
important to ensure specific water conditions as well as safe hydrogen production conditions.
Therefore, the correct design of the BoP is crucial to achieve a reliable implementation and an optimal
electrolyzer operation [24].
To ensure a correct performance, a sophisticated control subsystem is also required. This
includes sensors, actuators, and the controller. Table 4 describes the technical characteristics of the
main actuators (electrovalves) and transmitters, such as of transducers level, temperature,
conductivity, flow and pressure, as well as voltage and current sensors used for the implementation
of the PEM electrolysis plant. Table 5 describes the main characteristics of the electrolyzer controller.
The chosen controller has been the well-known Siemens
®
S7-1200 PLC (Munich, Germany) because
it is an industrial tested platform, robust and very suitable for this application.
Table 4. Sensors and actuators technical characteristics.
Component Manufacturer Model Main Characteristics
Electro-Valves Parker
®
(Cleveland, OH, USA) Voltage: 24 V
Level Valco
®
Oxygen separator: 3 levels
HPS: 3 levels
LPS: 2 levels
Temperature RS Pro
®
(Corby, UK) Type: Pt100
Figure 3. Block diagram of PEM electrolysis process.
Following Figure 3, the PEM electrolysis process is completed with the two primary elements,
DI water on the one hand and electricity through a DC power supply on the other.
Regarding general operation, when power is applied to the stack, the water supply through it
must be guaranteed at all times; the lack of water will damage the stack. The water flow rate must be
set well above the stoichiometric rate at all times as it also serves to remove excess heat from the stack.
Additionally, during operation, hydrogen (cathode) pressure should be above that of the water (anode)
pressure (see Figure 2). The stack must operate at a hydrogen pressure of at least a 0.068 bar above
the water pressure. This is to ensure that hydrogen can be detected in the water/oxygen outlet in the
event a membrane is breached (membrane failure). This will normally happen in the case of correct
performance, because the pressure of the water hardly requires a value of between 1 bar and 2 bar,
while the hydrogen will quickly increase its pressure to reach high pressure in a few minutes, as shown
in the experimental results.
Then, according to operation description, it can be deduced that in PEM electrolysis it is very
important to ensure specific water conditions as well as safe hydrogen production conditions. Therefore,
the correct design of the BoP is crucial to achieve a reliable implementation and an optimal electrolyzer
operation [24].
To ensure a correct performance, a sophisticated control subsystem is also required. This includes
sensors, actuators, and the controller. Table 4describes the technical characteristics of the main
actuators (electrovalves) and transmitters, such as of transducers level, temperature, conductivity,
flow and pressure, as well as voltage and current sensors used for the implementation of the PEM
electrolysis plant. Table 5describes the main characteristics of the electrolyzer controller. The chosen
controller has been the well-known Siemens
®
S7-1200 PLC (Munich, Germany) because it is an
industrial tested platform, robust and very suitable for this application.
Finally, with respect to automation software, those presents in the Siemens
®
TIA Portal have
been used both in PLC programming and in SCADA (supervisory control and data acquisition)
software implementation.

Electronics 2020,9, 871 9 of 25
Table 4. Sensors and actuators technical characteristics.
Component Manufacturer Model Main Characteristics
Electro-Valves Parker®(Cleveland, OH, USA) Voltage: 24 V
Level Valco®
Oxygen separator: 3 levels
HPS: 3 levels
LPS: 2 levels
Temperature RS Pro®(Corby, UK) Type: Pt100
Conductivity Metler Toledo®(Columbus, OH,
USA)
Processor: M200 1
4DIN
Sensor: UniCond
Range: 0.01–10 µS/cm
Flow REMAG VISION®(Milwaukee,
WI, USA) 2008 Flow rate: 1–25 L/min
Pressure transmitter Baumer®(Frauenfeld,
Switzerland) Y913
Pressure range: 0–25 bar
Output signal: 4–20 mA
Pressure switches Baumer®RP2Y Pressure range: 1–30 bar
Voltage
Phoenix®(Blomberg, Germany)
Contact MINI MCR-SL-U-UI-NC
2865007
Input signal: 0–24 V/0–30 V
Output signal: 0–10 V/0–5 V/0–20
mA/4–20 mA
Current
LEM
®
(Geneva, Switzerland) HAT
1200-S
Primary nominal current: 1200 A
Output signal: ±4 V
Table 5. Controller technical characteristics. Siemens®S7-1200 PLC.
Module Series Model Signals Notes
CPU CPU 1214C
6ES7214-1AG40-0XB
DI(14), DO(10),
AI(2) DC/DC/DC
Digital inputs SM 1221
6ES7221-1BF32-0XB0
DI(8) DC
Digital outputs SM 1222
6ES7222-1BH32-0XB0
DO(16) DC
Digital outputs SM 1222
6ES7222-1BF32-0XB0
DO(8) DC
Analog inputs SM 1231
6ES7231-4HF32-0XB0
AI(8) 13 bits
Analog inputs/outputs SM 1234
6ES7234-4HE32-0XB0
AI(4), AO(2) 13/14 bits
Resistance temperature
detectors SM 1231
6ES7231-5PF32-0XB0
RTD(8) 16 bits
3. Design and Implementation of the BoP
Going into the blocks of Figure 3, the five subsystems that makes up the BoP and their key parts
are the following:
•
Stack power supply subsystem: AC/DC rectifier, DC voltage transducer and DC current transducer.
•
Water management subsystem: deionized water circulation system (two phase filter and
recirculation filter), inlet water tank, oxygen separator tank, injection pump, recirculation pump,
piping, valves and instrumentation.
•
Hydrogen production subsystem: hydrogen processing: PSA dryers, high pressure
separator (HPS), low pressure separator (LPS) tubing, and valves, and instrumentation.
•
Cooling subsystem: plate heat exchanger, dry cooler, cooling pump, valves and instrumentation.
•
Control subsystem: receives information from sensors and defines operation mode over actuators
according to optimal operation and safety requirements.
Next, a solution for the design, implementation, and control of the BoP of the proposed PEM
electrolyzer in the research is developed.

Electronics 2020,9, 871 10 of 25
3.1. BoP Design
The subsystems and their elements making up the BoP are outlined in Figure 4: the stack power
supply in green, water subsystem in blue, hydrogen subsystem in red, cooling subsystem in orange
and control subsystem in grey.
Electronics 2020, 9, x FOR PEER REVIEW 11 of 27
Next, a solution for the design, implementation, and control of the BoP of the proposed PEM
electrolyzer in the research is developed.
3.1. BoP Design
The subsystems and their elements making up the BoP are outlined in Figure 4: the stack power
supply in green, water subsystem in blue, hydrogen subsystem in red, cooling subsystem in orange
and control subsystem in grey.
Figure 4. Balance of plant (BOP) of the developed PEM electrolyzer.
3.1.1. Stack Power Supply Subsystem
The stack power supply subsystem (in green) is responsible for providing the necessary direct
current for trigger the electrolysis process that produces the hydrogen. Since the electrolyzer is
operated at high power, and with very high-value currents (up to 900 A), current (A) and voltage (V)
sensors are needed to continuously monitor the electrical supply to the stack. In addition, a power
contactor (PC1) is incorporated, to guarantee safe operation, both in production situations and in the
event of an emergency stop.
3.1.2. Water Management Subsystem
The water management subsystem (in blue) starts acquiring water from a DI water tank, which
is convenient to have low conductivity and to ensure a longer stack lifespan. Once the water has been
introduced into the system, an injection pump (P-001) is used to ensure an adequate input flow into
the system. After passing through the injection pump, the water is circulated through a two-phase
filter to give it a low conductivity. Otherwise, the PEM stack could be critically impaired. In the first
phase, it is obtained a Type II conductivity (ASTM Standards for Laboratory Reagent Water (ASTM
D1193-91)) (<1 µScm
−1
) and, in the second phase, the conductivity level drops to the Type I value
(<0.056 µScm
−1
).
After the filtering stage, the water is introduced into an oxygen separator tank that has a triple
function: (1) to be a buffer with the aim to adjust the water flow inside the circuit, (2) to act as a sink
that collects all the wastewaters, and (3) to separate the oxygen from the water. From the oxygen
separator tank, the water continues its flow to the water-control and recirculation phase. The
recirculation pump (P-002) regulates the water flow after the oxygen separator tank, and the sensors’
Figure 4. Balance of plant (BOP) of the developed PEM electrolyzer.
3.1.1. Stack Power Supply Subsystem
The stack power supply subsystem (in green) is responsible for providing the necessary direct
current for trigger the electrolysis process that produces the hydrogen. Since the electrolyzer is operated
at high power, and with very high-value currents (up to 900 A), current (A) and voltage (V) sensors
are needed to continuously monitor the electrical supply to the stack. In addition, a power contactor
(PC1) is incorporated, to guarantee safe operation, both in production situations and in the event of an
emergency stop.
3.1.2. Water Management Subsystem
The water management subsystem (in blue) starts acquiring water from a DI water tank, which is
convenient to have low conductivity and to ensure a longer stack lifespan. Once the water has been
introduced into the system, an injection pump (P-001) is used to ensure an adequate input flow into
the system. After passing through the injection pump, the water is circulated through a two-phase filter
to give it a low conductivity. Otherwise, the PEM stack could be critically impaired. In the first phase,
it is obtained a Type II conductivity (ASTM Standards for Laboratory Reagent Water (ASTM D1193-91))
(<1
µ
Scm
−1
) and, in the second phase, the conductivity level drops to the Type I value (<0.056
µ
Scm
−1
).
After the filtering stage, the water is introduced into an oxygen separator tank that has a triple
function: (1) to be a buffer with the aim to adjust the water flow inside the circuit, (2) to act as
a sink that collects all the wastewaters, and (3) to separate the oxygen from the water. From the
oxygen separator tank, the water continues its flow to the water-control and recirculation phase.
The recirculation pump (P-002) regulates the water flow after the oxygen separator tank, and the
sensors’ line is used by the controller to have information of all critical water parameters, such as
temperature (T), pressure (P), flow (F) and conductivity (C) before being injected into the PEM stack.

Electronics 2020,9, 871 11 of 25
The recirculation line is proposed as a means to correct the conductivity of the water; in case it is not
within the allowed range.
3.1.3. Hydrogen Production Subsystem
The hydrogen production subsystem (in red) must be carefully designed to guarantee all the
safety parameters, as well as the correct hydrogen drying, eliminating the humidity that it may contain,
sending the extracted water to the oxygen separator tank. For this purpose, it can be seen in Figure 4
that the PEM stack output is connected to the HPS. Once a high humidity gradient is reached in
the HPS, this allows the wet hydrogen to flow (dirty hydrogen) into the LPS. Here, the hydrogen that
can be mixed into the atmosphere is released, and the wastewater is sent to the oxygen separator tank.
By contrast, the dry hydrogen (clean hydrogen) from the high-pressure separator, continues to the
drying stage. The drying stage is based on pressure swing adsorption (PSA), a cyclic process that
uses beds of solid adsorbent to remove impurities from the gas. The released water is sent to the LPS,
following the same process previously described. The set of separators takes advantage of the pressure
difference in the water contained in the form of moisture to dry the hydrogen. Throughout the process
of hydrogen production, several sensors are placed; they are used to control the pressure (P) and
temperature (T) parameters of hydrogen flow in the production and drying stages, prior to final storage.
The inertization process makes use of the elements of the hydrogen subsystem; in order to bring it out
in Figure 4, a nitrogen inlet is included in the stack.
3.1.4. Cooling Subsystem
Inside the electrolyzer, the cooling subsystem (in orange) consists of two heat exchangers used in
the water management subsystem and the hydrogen production subsystem. The circulation circuit is
controlled by two electrovalves (TCV106 and TCV113, respectively), one for each subsystem. The water
for the heat exchangers is cooled by an external air cooler, which has its own pump to guarantee water
flow and pressure in the cooling line.
3.1.5. Control Subsystem
The control subsystem processes all the information received from sensors and, based on the
user-defined parameters and the control logic defied, it automatically acts over actuators to put the
system working at the proper operating state. All the above subsystems are controlled through the
control subsystem.
3.2. Design of the Electrolyzer Control Logic
The control system to be implemented into the PEM electrolyzer should be able to have information
and act accordingly into the rest of subsystems that made up the BoP: stack power supply subsystem,
water management subsystem, hydrogen production subsystem and cooling subsystem. Additionally,
it must include the whole sequence of the operating states and the management of the warnings and
alarms generated during the electrolyzer operation.
To follow the development of the control logic in an easy way, all the elements that govern the
electrolyzer operation are named with a number in parentheses that coincide with their numbering
in Figure 4. For the stack power subsystem the control logic receives the information from two
main variables in the controller as shown in Figure 5. Firstly, the operating state of the electrolyzer
is considered. If it is a state where electric current is required to carry out the electrolysis process,
then the power contactor (PC1) will close, allowing the physical connection between stack and the
power supply, and right after that the power supply is activated. Secondly, stack voltage (V) and
current (A) are measured. If their values are not within the adequate range of the stack operation (1.5
V<Cell Voltage <2.4 V and 90 A <Stack Current <900 A), the system is shutdown, which also stops
supplying electricity to the stack.

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Electronics 2020, 9, x FOR PEER REVIEW 13 of 27
Figure 5. PEM electrolyzer control flow diagram: stack power supply subsystem.
On the other hand, the water management subsystem control is shown in Figure 6. It includes
three main parts: water tank level control, water conductivity control and the control of the rest of
the water physicochemical variables. Regarding the first part, the activation of the level sensor (L) in
the oxygen separator tank activates the injection pump (P-001). When the level is high enough, the
pump is deactivated. In case of lowering the level too much, at a low level, the PEM electrolyzer
stops.
Figure 6. PEM electrolyzer control flow diagram: water management subsystem.
Concerning the conductivity control part, the water conductivity is regulated by acting over the
electrovalve (CCV104B) to put in work the recirculation line. When the conductivity is low, the water
is supplied directly to the stack without the need to subject the water to more purification treatment
((CCV104B) closed and (CCV104A) open). If, before production, the conductivity is medium (Type I
< conductivity < Type II), the recirculation circuit (P-002) will be open to recirculate the water back to
the purification filter ((CCV104B) open and (CCV104A) closed). If this occurs during production, a
warning is activated. Finally, if the conductivity rises above Type II, the electrolyzer will be kept
stopped, an alarm will be triggered, and through a process of disconnection and inertization (this
will be explained later in this section), and an alarm will be triggered.
The third branch of Figure 6 concerns the rest of the water physic-chemical parameters like flow,
temperature, and pressure, which are measured with the aim to guarantee that the system parameters
are within its operating specifications; otherwise the system stops.
For the hydrogen subsystem control, Figure 7, it is necessary to take into account the water level
both in the HPS and the LPS. When a mid-level is detected in the HPS, the electrovalve (LCV115) will
be open to letting the accumulated water pass towards the LPS. When the level drops, the valve will
be closed again. In the case of a high level being detected, the electrolyzer stops. Water level control
in the LPS works in a similar way, allowing the water to pass to the oxygen separator tank (by means
of (LCV116) when there is enough water accumulated, as long as the electrovalve (LCV115) is closed.
In an electrolysis process, it is crucial to avoid direct contact between the water and hydrogen lines.
If the level in LPS is low, the valve (LCV115) closes since there is not enough water to transport.
Figure 5. PEM electrolyzer control flow diagram: stack power supply subsystem.
On the other hand, the water management subsystem control is shown in Figure 6. It includes
three main parts: water tank level control, water conductivity control and the control of the rest of the
water physicochemical variables. Regarding the first part, the activation of the level sensor (L) in the
oxygen separator tank activates the injection pump (P-001). When the level is high enough, the pump
is deactivated. In case of lowering the level too much, at a low level, the PEM electrolyzer stops.
Electronics 2020, 9, x FOR PEER REVIEW 13 of 27
Figure 5. PEM electrolyzer control flow diagram: stack power supply subsystem.
On the other hand, the water management subsystem control is shown in Figure 6. It includes
three main parts: water tank level control, water conductivity control and the control of the rest of
the water physicochemical variables. Regarding the first part, the activation of the level sensor (L) in
the oxygen separator tank activates the injection pump (P-001). When the level is high enough, the
pump is deactivated. In case of lowering the level too much, at a low level, the PEM electrolyzer
stops.
Figure 6. PEM electrolyzer control flow diagram: water management subsystem.
Concerning the conductivity control part, the water conductivity is regulated by acting over the
electrovalve (CCV104B) to put in work the recirculation line. When the conductivity is low, the water
is supplied directly to the stack without the need to subject the water to more purification treatment
((CCV104B) closed and (CCV104A) open). If, before production, the conductivity is medium (Type I
< conductivity < Type II), the recirculation circuit (P-002) will be open to recirculate the water back to
the purification filter ((CCV104B) open and (CCV104A) closed). If this occurs during production, a
warning is activated. Finally, if the conductivity rises above Type II, the electrolyzer will be kept
stopped, an alarm will be triggered, and through a process of disconnection and inertization (this
will be explained later in this section), and an alarm will be triggered.
The third branch of Figure 6 concerns the rest of the water physic-chemical parameters like flow,
temperature, and pressure, which are measured with the aim to guarantee that the system parameters
are within its operating specifications; otherwise the system stops.
For the hydrogen subsystem control, Figure 7, it is necessary to take into account the water level
both in the HPS and the LPS. When a mid-level is detected in the HPS, the electrovalve (LCV115) will
be open to letting the accumulated water pass towards the LPS. When the level drops, the valve will
be closed again. In the case of a high level being detected, the electrolyzer stops. Water level control
in the LPS works in a similar way, allowing the water to pass to the oxygen separator tank (by means
of (LCV116) when there is enough water accumulated, as long as the electrovalve (LCV115) is closed.
In an electrolysis process, it is crucial to avoid direct contact between the water and hydrogen lines.
If the level in LPS is low, the valve (LCV115) closes since there is not enough water to transport.
Figure 6. PEM electrolyzer control flow diagram: water management subsystem.
Concerning the conductivity control part, the water conductivity is regulated by acting over the
electrovalve (CCV104B) to put in work the recirculation line. When the conductivity is low, the water
is supplied directly to the stack without the need to subject the water to more purification treatment
((CCV104B) closed and (CCV104A) open). If, before production, the conductivity is medium (Type I
<conductivity <Type II), the recirculation circuit (P-002) will be open to recirculate the water back
to the purification filter ((CCV104B) open and (CCV104A) closed). If this occurs during production,
a warning is activated. Finally, if the conductivity rises above Type II, the electrolyzer will be kept
stopped, an alarm will be triggered, and through a process of disconnection and inertization (this will
be explained later in this section), and an alarm will be triggered.
The third branch of Figure 6concerns the rest of the water physic-chemical parameters like flow,
temperature, and pressure, which are measured with the aim to guarantee that the system parameters
are within its operating specifications; otherwise the system stops.
For the hydrogen subsystem control, Figure 7, it is necessary to take into account the water level
both in the HPS and the LPS. When a mid-level is detected in the HPS, the electrovalve (LCV115) will
be open to letting the accumulated water pass towards the LPS. When the level drops, the valve will be
closed again. In the case of a high level being detected, the electrolyzer stops. Water level control in
the LPS works in a similar way, allowing the water to pass to the oxygen separator tank (by means of
(LCV116) when there is enough water accumulated, as long as the electrovalve (LCV115) is closed.
In an electrolysis process, it is crucial to avoid direct contact between the water and hydrogen lines.
If the level in LPS is low, the valve (LCV115) closes since there is not enough water to transport.

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Electronics 2020, 9, x FOR PEER REVIEW 14 of 27
Figure 7. PEM electrolyzer control flow diagram: hydrogen production subsystem.
After the LPS and HPS stages, the PSA drying stage follows a conventional three-phase cyclic
process during production. This is defined temporarily with the opening and closing of electrovalves
(CV118 and CV119) that allow the hydrogen flow to the final storage, the water accumulation and
further purge through the LPS. During all the process, temperature and pressure are controlled,
entering the system in stop if they are outside the established range.
In a similar way, the cooling subsystem control logic is defined by the temperature of water and
of hydrogen, Figure 8. When they reach a maximum value, (water temperature < 68 °C) and hydrogen
temperature < 72 °C), cooling electrovalves (TCV106 for water temperature control and TCV113 for
hydrogen temperature control) will close and let the cooling water flow through the plate exchange
heaters.
Figure 8. PEM electrolyzer control flow diagram: hydrogen production subsystem.
Once the control logic diagrams of the BoP subsystems have been described, the whole sequence
that gathers these individual control logics into the operating states of the electrolyzer is illustrated
in Figure 9.
Figure 7. PEM electrolyzer control flow diagram: hydrogen production subsystem.
After the LPS and HPS stages, the PSA drying stage follows a conventional three-phase cyclic
process during production. This is defined temporarily with the opening and closing of electrovalves
(CV118 and CV119) that allow the hydrogen flow to the final storage, the water accumulation and
further purge through the LPS. During all the process, temperature and pressure are controlled,
entering the system in stop if they are outside the established range.
In a similar way, the cooling subsystem control logic is defined by the temperature of water
and of hydrogen, Figure 8. When they reach a maximum value, (water temperature <68
◦
C) and
hydrogen temperature <72
◦
C), cooling electrovalves (TCV106 for water temperature control and
TCV113 for hydrogen temperature control) will close and let the cooling water flow through the plate
exchange heaters.
Electronics 2020, 9, x FOR PEER REVIEW 14 of 27
Figure 7. PEM electrolyzer control flow diagram: hydrogen production subsystem.
After the LPS and HPS stages, the PSA drying stage follows a conventional three-phase cyclic
process during production. This is defined temporarily with the opening and closing of electrovalves
(CV118 and CV119) that allow the hydrogen flow to the final storage, the water accumulation and
further purge through the LPS. During all the process, temperature and pressure are controlled,
entering the system in stop if they are outside the established range.
In a similar way, the cooling subsystem control logic is defined by the temperature of water and
of hydrogen, Figure 8. When they reach a maximum value, (water temperature < 68 °C) and hydrogen
temperature < 72 °C), cooling electrovalves (TCV106 for water temperature control and TCV113 for
hydrogen temperature control) will close and let the cooling water flow through the plate exchange
heaters.
Figure 8. PEM electrolyzer control flow diagram: hydrogen production subsystem.
Once the control logic diagrams of the BoP subsystems have been described, the whole sequence
that gathers these individual control logics into the operating states of the electrolyzer is illustrated
in Figure 9.
Figure 8. PEM electrolyzer control flow diagram: hydrogen production subsystem.
Once the control logic diagrams of the BoP subsystems have been described, the whole sequence
that gathers these individual control logics into the operating states of the electrolyzer is illustrated in
Figure 9.
According to the whole sequence, when the plant is turned on and the user is logged correctly, the
electrolyzer starts in Initiate state. In this state, the system is kept waiting for the user to manually
activate the Inertization state. It consists of injecting, for a 2 min duration, nitrogen into the pipelines,
which, as an inert gas, cleans the remaining hydrogen conduits, air or any other gas. If any fault occurs
during inertization, it will return to the Initiate state, otherwise it goes to Standby state.
In this state
,
the system is ready to start the production process under the user’s manual order. In case of remaining
in the Standby state for more than 6 h, the system will return to the Initiate state. On the other hand,
when in the Standby state, if the user activates the inertization button again, the electrolyzer comes
back to it.
When the user activates the production button, the system goes to the Pre-production state. In it,
the water line parameters values are verified: conductivity, flow, temperature, and pressure. If they are
in range, the plant goes to the Purge state; on the contrary the corresponding alarms return the system
to the Standby state. The system can also return to this state if the user presses the standby button.

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Figure 9. PEM electrolyzer control flow diagram: whole operating sequence.
According to the whole sequence, when the plant is turned on and the user is logged correctly,
the electrolyzer starts in Initiate state. In this state, the system is kept waiting for the user to manually
activate the Inertization state. It consists of injecting, for a 2 min duration, nitrogen into the pipelines,
which, as an inert gas, cleans the remaining hydrogen conduits, air or any other gas. If any fault
occurs during inertization, it will return to the Initiate state, otherwise it goes to Standby state. In this
state, the system is ready to start the production process under the user’s manual order. In case of
remaining in the Standby state for more than 6 h, the system will return to the Initiate state. On the
other hand, when in the Standby state, if the user activates the inertization button again, the
electrolyzer comes back to it.
When the user activates the production button, the system goes to the Pre-production state. In it,
the water line parameters values are verified: conductivity, flow, temperature, and pressure. If they
are in range, the plant goes to the Purge state; on the contrary the corresponding alarms return the
system to the Standby state. The system can also return to this state if the user presses the standby
button.
Having arrived at this point where the operation conductions are verified, the Purge state begins.
In this state, the first hydrogen production is carried out, which serves to purge the pipelines of
Figure 9. PEM electrolyzer control flow diagram: whole operating sequence.
Having arrived at this point where the operation conductions are verified, the Purge state begins.
In this state, the first hydrogen production is carried out, which serves to purge the pipelines of
nitrogen previously used during the Inertization stage. Obviously, this hydrogen is not yet used to
be stored, so that it is purged to the atmosphere. This is a temporary process that lasts 2 min, where the
hydrogen line is purged to expel all the nitrogen from the equipment. If the user presses the standby
button, the system returns to the Inertization state.
Once all the previous states have been successfully completed, the Production state is reached,
where the hydrogen produced can be stored at the electrolyzer output. Several cases can occur
from the Production state. Thus, the user can push pause button to goes to Pause state, where the
plant is limited to a minimum production of hydrogen using a minimum DC current. Additionally,
as a security measure, if the hydrogen production flow reaches the maximum allowable pressure,
the systems finishes production and moves to the Pause state. Additionally, the user can stop the
process completely by means of the standby button, with which after performing the Inertization,
the Standby state will be reached. This can also happen automatically if at any time the controller
detects an alarm in the plant.

Electronics 2020,9, 871 15 of 25
In the Pause state, it is possible to recover the Production state just by pushing the production button.
Additionally, after staying at the Pause state more than 30 min, the system returns to the Inertization
state. Finally, in all previous cases, from Pre-production to Pause states, if the standby button is pressed
or an alarm is noticed, the system returns to the Inertization state.
3.3. Implementation of the PEM Electrolysis Plant
Once the BoP of the PEM electrolyzer has been designed and the control logic is defined,
the physical implementation of the electrolysis plant has been carried out. Figure 10a shows the water
management subsystem, as the location of the inlet water tank, oxygen separator tank, injection and
recirculation pumps, the different filtering equipment, as well as all the sensors and actuators that
control the effective and safe operation of this subsystem, including electrovalves, conductivity sensors,
pressure, flow, level, and temperature. Figure 10b shows the hydrogen production subsystem; there can
be found the stack, together with the varied equipment of the hydrogen subsystem. In this area are the
high and low-pressure separators (HPS and LPS), the PSA dryer, as well as the different connection
sockets to the hydrogen storage tank and purging. Finally, the physical implementation includes the
power supply and control subsystems, Figure 10c. In this part, it is located the DC power supply that
provides the DC current to the stack in a controlled manner. It also houses the controller module
(PLC Siemens S7-1200), where all the control logic defined previously has been programmed and
simulated, as well as serving as a platform for the experimental tests shown in the next section. As can
be appreciated, the plate heat exchanger for water and hydrogen, as part of the cooling subsystem are
also shown in Figure 10a,b.
Electronics 2020, 9, x FOR PEER REVIEW 17 of 27
(a) (b) (c)
Figure 10. Detail of the PEM electrolyzer implementation: (a) Water management subsystem; (b)
hydrogen subsystem; (c) stack power supply subsystem, control subsystem and all the power
electronics needed by the electrolyzer. (Renewable Energy Laboratory, Research Group TEP-192,
University of Huelva, Southwest of Spain).
4. Experimental Results
In this section, there will be presented the results that show the proper operation of the plant
following the sequence established by the developed control logic. The plant operation is monitored
by the developed SCADA software. To experimentally verify the achievement of the design
objectives, the I-V characteristic and the hydrogen production vs. power consumption of the stack
will be obtained through measurements. With the aim to show the whole sequence that the system
follows according to the developed control logic, results obtained from a start-stop operation cycle
will be presented.
4.1. Supervisory Control and Data Acquisition (SCADA) Interface
Figure 11Figure 11 shows the interface screen that reflects the stack power supply subsystem
and stack operation. Electrical parameters like cell voltage, stack voltage and current, and power
curve can be shown in Figure 11a, while the hydrogen drying process by PSA is shown in Figure 11b.
(a)
(b)
Figure 11. (a) Stack power supply subsystem monitoring interface; (b) PSA dryers monitoring
interface.
Regarding the water management subsystem, Figure 12 shows its monitoring interface. Figure 12a
shows that the system has started from a low level of water at the oxygen separator tank (WT-O2),
therefore, the controller has activated the injection pump (P-001). In normal operation the injection
Figure 10.
Detail of the PEM electrolyzer implementation: (
a
) Water management subsystem;
(
b
) hydrogen subsystem; (
c
) stack power supply subsystem, control subsystem and all the power
electronics needed by the electrolyzer. (Renewable Energy Laboratory, Research Group TEP-192,
University of Huelva, Southwest of Spain).
4. Experimental Results
In this section, there will be presented the results that show the proper operation of the plant
following the sequence established by the developed control logic. The plant operation is monitored
by the developed SCADA software. To experimentally verify the achievement of the design objectives,
the I-V characteristic and the hydrogen production vs. power consumption of the stack will be obtained
through measurements. With the aim to show the whole sequence that the system follows according to
the developed control logic, results obtained from a start-stop operation cycle will be presented.

Electronics 2020,9, 871 16 of 25
4.1. Supervisory Control and Data Acquisition (SCADA) Interface
Figure 11 shows the interface screen that reflects the stack power supply subsystem and stack
operation. Electrical parameters like cell voltage, stack voltage and current, and power curve can be
shown in Figure 11a, while the hydrogen drying process by PSA is shown in Figure 11b.
Electronics 2020, 9, x FOR PEER REVIEW 17 of 27
(a) (b) (c)
Figure 10. Detail of the PEM electrolyzer implementation: (a) Water management subsystem; (b)
hydrogen subsystem; (c) stack power supply subsystem, control subsystem and all the power
electronics needed by the electrolyzer. (Renewable Energy Laboratory, Research Group TEP-192,
University of Huelva, Southwest of Spain).
4. Experimental Results
In this section, there will be presented the results that show the proper operation of the plant
following the sequence established by the developed control logic. The plant operation is monitored
by the developed SCADA software. To experimentally verify the achievement of the design
objectives, the I-V characteristic and the hydrogen production vs. power consumption of the stack
will be obtained through measurements. With the aim to show the whole sequence that the system
follows according to the developed control logic, results obtained from a start-stop operation cycle
will be presented.
4.1. Supervisory Control and Data Acquisition (SCADA) Interface
Figure 11Figure 11 shows the interface screen that reflects the stack power supply subsystem
and stack operation. Electrical parameters like cell voltage, stack voltage and current, and power
curve can be shown in Figure 11a, while the hydrogen drying process by PSA is shown in Figure 11b.
(a)
(b)
Figure 11. (a) Stack power supply subsystem monitoring interface; (b) PSA dryers monitoring
interface.
Regarding the water management subsystem, Figure 12 shows its monitoring interface. Figure 12a
shows that the system has started from a low level of water at the oxygen separator tank (WT-O2),
therefore, the controller has activated the injection pump (P-001). In normal operation the injection
Figure 11.
(
a
) Stack power supply subsystem monitoring interface; (
b
) PSA dryers monitoring interface.
Regarding the water management subsystem, Figure 12 shows its monitoring interface. Figure 12a
shows that the system has started from a low level of water at the oxygen separator tank (WT-O2),
therefore, the controller has activated the injection pump (P-001). In normal operation the injection
pump is deactivated and the water level begins to decrease. In case the level decreases below the
lowest allowable, the controller stops the electrolyzer and warns of this by an audible and visual alarm
(in red in the screen), Figure 12b.
Electronics 2020, 9, x FOR PEER REVIEW 18 of 27
pump is deactivated and the water level begins to decrease. In case the level decreases below the
lowest allowable, the controller stops the electrolyzer and warns of this by an audible and visual
alarm (in red in the screen), Figure 12b.
(a) (b)
Figure 12. Water management subsystem monitoring interface: (a) normal operation; (b) low level
alarm.
The water management subsystem monitoring interface has more screens; for example, Figure 13a
shows the measurements of the water physic-chemical parameters during production. Figure 13b
shows that the controller has stopped the electrolyzer (Inertization state) because the water
conductivity is Type II, and it activates the recirculation pump (P-002) and an audible and visual
alarm (in red in the screen) warns about this failure.
(a)
(b)
Figure 13. Water management subsystem monitoring interface: (a) physic-chemical parameters; (b)
alarm by high conductivity.
The hydrogen production subsystem is monitored by the interface in Figure 14. Figure 14a
shows that when the level of condensates in HPS, reaches the medium level, the controller opens the
valve (LCV115) to LPS. After this, the condensates start their passage towards LPS, until reaching the
high level of LPS. At this moment, it is necessary to wait for the level to drop in HPS, and with this
the valve of the step to LPS, so that the opening of the LPS outlet valve (LCV116) is allowed, Figure
14b.
Figure 12.
Water management subsystem monitoring interface: (
a
) normal operation; (
b
) low
level alarm.
The water management subsystem monitoring interface has more screens; for example, Figure 13a
shows the measurements of the water physic-chemical parameters during production. Figure 13b
shows that the controller has stopped the electrolyzer (Inertization state) because the water conductivity
is Type II, and it activates the recirculation pump (P-002) and an audible and visual alarm (in red in the
screen) warns about this failure.
The hydrogen production subsystem is monitored by the interface in Figure 14. Figure 14a shows
that when the level of condensates in HPS, reaches the medium level, the controller opens the valve
(LCV115) to LPS. After this, the condensates start their passage towards LPS, until reaching the high
level of LPS. At this moment, it is necessary to wait for the level to drop in HPS, and with this the
valve of the step to LPS, so that the opening of the LPS outlet valve (LCV116) is allowed, Figure 14b.

Electronics 2020,9, 871 17 of 25
Electronics 2020, 9, x FOR PEER REVIEW 18 of 27
pump is deactivated and the water level begins to decrease. In case the level decreases below the
lowest allowable, the controller stops the electrolyzer and warns of this by an audible and visual
alarm (in red in the screen), Figure 12b.
(a) (b)
Figure 12. Water management subsystem monitoring interface: (a) normal operation; (b) low level
alarm.
The water management subsystem monitoring interface has more screens; for example, Figure 13a
shows the measurements of the water physic-chemical parameters during production. Figure 13b
shows that the controller has stopped the electrolyzer (Inertization state) because the water
conductivity is Type II, and it activates the recirculation pump (P-002) and an audible and visual
alarm (in red in the screen) warns about this failure.
(a)
(b)
Figure 13. Water management subsystem monitoring interface: (a) physic-chemical parameters; (b)
alarm by high conductivity.
The hydrogen production subsystem is monitored by the interface in Figure 14. Figure 14a
shows that when the level of condensates in HPS, reaches the medium level, the controller opens the
valve (LCV115) to LPS. After this, the condensates start their passage towards LPS, until reaching the
high level of LPS. At this moment, it is necessary to wait for the level to drop in HPS, and with this
the valve of the step to LPS, so that the opening of the LPS outlet valve (LCV116) is allowed, Figure
14b.
Figure 13.
Water management subsystem monitoring interface: (
a
) physic-chemical parameters;
(b) alarm by high conductivity.
Electronics 2020, 9, x FOR PEER REVIEW 19 of 27
(a)
(b)
Figure 14. Hydrogen production subsystem monitoring interface: (a) mid-level in HPS, valve
(LCV115) open; (b) high level in LPS, valve (LCV116) open.
Figure 15 shows the cooling subsystem monitoring interface. In case the temperature of the water
flow to the stack (Figure 15a) or the temperature of the hydrogen flow from the stack (Figure 15b)
increases above the higher allowed value (68 °C for water and 72 °C for hydrogen), a warning visual
alarm (in yellow) is activated and the respective heat exchanger is put into operation by valves
(TCV106) and (TCV113) respectively.
(a)
(b)
Figure 15. Cooling subsystem monitoring interface: (a) warning by high water temperature; (b)
warning by high hydrogen temperature.
Once the control logic is validated through the SCADA system, the next step is about operating
the developed PEM electrolyzer, monitoring its operation during the different phases of the process.
4.2. Stack Characterization
Figure 16 shows the cell voltage and current during stack operation up to the maximum
allowable current applied by the power supply. The maximum applied current is verified to be safe
below 900 A. The cell electrolysis voltage is experimentally obtained with a value of 1.6 V.
Figure 14.
Hydrogenproductionsubsystem monitoringinterface: (
a
)mid-levelinHPS,
valve (LCV115) open;
(b) high level in LPS, valve (LCV116) open.
Figure 15 shows the cooling subsystem monitoring interface. In case the temperature of the water
flow to the stack (Figure 15a) or the temperature of the hydrogen flow from the stack (Figure 15b)
increases above the higher allowed value (68
◦
C for water and 72
◦
C for hydrogen), a warning visual
alarm (in yellow) is activated and the respective heat exchanger is put into operation by valves (TCV106)
and (TCV113) respectively.
Electronics 2020, 9, x FOR PEER REVIEW 19 of 27
(a)
(b)
Figure 14. Hydrogen production subsystem monitoring interface: (a) mid-level in HPS, valve
(LCV115) open; (b) high level in LPS, valve (LCV116) open.
Figure 15 shows the cooling subsystem monitoring interface. In case the temperature of the water
flow to the stack (Figure 15a) or the temperature of the hydrogen flow from the stack (Figure 15b)
increases above the higher allowed value (68 °C for water and 72 °C for hydrogen), a warning visual
alarm (in yellow) is activated and the respective heat exchanger is put into operation by valves
(TCV106) and (TCV113) respectively.
(a)
(b)
Figure 15. Cooling subsystem monitoring interface: (a) warning by high water temperature; (b)
warning by high hydrogen temperature.
Once the control logic is validated through the SCADA system, the next step is about operating
the developed PEM electrolyzer, monitoring its operation during the different phases of the process.
4.2. Stack Characterization
Figure 16 shows the cell voltage and current during stack operation up to the maximum
allowable current applied by the power supply. The maximum applied current is verified to be safe
below 900 A. The cell electrolysis voltage is experimentally obtained with a value of 1.6 V.
Figure 15.
Cooling subsystem monitoring interface: (
a
) warning by high water temperature; (
b
)
warning by high hydrogen temperature.

Electronics 2020,9, 871 18 of 25
Once the control logic is validated through the SCADA system, the next step is about operating
the developed PEM electrolyzer, monitoring its operation during the different phases of the process.
4.2. Stack Characterization
Figure 16 shows the cell voltage and current during stack operation up to the maximum allowable
current applied by the power supply. The maximum applied current is verified to be safe below 900 A.
The cell electrolysis voltage is experimentally obtained with a value of 1.6 V.
Electronics 2020, 9, x FOR PEER REVIEW 20 of 27
Figure 16. Experimental results. I-V characteristic of the PEM cell.
The relationship between the stack power consumption and hydrogen production rate is shown
in Figure 17. A maximum production rate of 2.2 Nm3/h is verified with a maximum power
consumption of 11.8 kW, still around the 10 kW design consideration.
Figure 17. Experimental results. Relationship between stack power consumption and hydrogen
production rate.
4.3. Start-Stop Operating Cycle. Experimental Test
In this second part, the PEM electrolyzer is subjected to a start-stop operating cycle with the aim
to show the complete sequence of states described in Figure 9. Figure 18 shows the response of
hydrogen pressure at two different point of the hydrogen line. The first is the pressure obtained
directly at the stack output, (please see the pressure transducer (PT112) at stack output in Figure 4)
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00 1000.00
Vcell (V)
Istack (A)
I-V characteristic
0.00
0.40
0.80
1.20
1.60
2.00
2.40
0.00 2.00 4.00 6.00 8.00 10.00 12.00
Hydrogen production rate (Nm3/h)
Stack power (kW)
Power-production characteristic
Figure 16. Experimental results. I-V characteristic of the PEM cell.
The relationship between the stack power consumption and hydrogen production rate is shown in
Figure 17. A maximum production rate of 2.2 Nm
3
/h is verified with a maximum power consumption
of 11.8 kW, still around the 10 kW design consideration.
Electronics 2020, 9, x FOR PEER REVIEW 20 of 27
Figure 16. Experimental results. I-V characteristic of the PEM cell.
The relationship between the stack power consumption and hydrogen production rate is shown
in Figure 17. A maximum production rate of 2.2 Nm3/h is verified with a maximum power
consumption of 11.8 kW, still around the 10 kW design consideration.
Figure 17. Experimental results. Relationship between stack power consumption and hydrogen
production rate.
4.3. Start-Stop Operating Cycle. Experimental Test
In this second part, the PEM electrolyzer is subjected to a start-stop operating cycle with the aim
to show the complete sequence of states described in Figure 9. Figure 18 shows the response of
hydrogen pressure at two different point of the hydrogen line. The first is the pressure obtained
directly at the stack output, (please see the pressure transducer (PT112) at stack output in Figure 4)
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00 1000.00
Vcell (V)
Istack (A)
I-V characteristic
0.00
0.40
0.80
1.20
1.60
2.00
2.40
0.00 2.00 4.00 6.00 8.00 10.00 12.00
Hydrogen production rate (Nm3/h)
Stack power (kW)
Power-production characteristic
Figure 17.
Experimental results. Relationship between stack power consumption and hydrogen
production rate.

Electronics 2020,9, 871 19 of 25
4.3. Start-Stop Operating Cycle. Experimental Test
In this second part, the PEM electrolyzer is subjected to a start-stop operating cycle with the aim to
show the complete sequence of states described in Figure 9. Figure 18 shows the response of hydrogen
pressure at two different point of the hydrogen line. The first is the pressure obtained directly at the
stack output, (please see the pressure transducer (PT112) at stack output in Figure 4) and the second
refers to the pressure obtained at the end of the hydrogen output line (see the pressure transducer
(PT120) at the hydrogen output line in Figure 4).
Electronics 2020, 9, x FOR PEER REVIEW 21 of 27
and the second refers to the pressure obtained at the end of the hydrogen output line (see the pressure
transducer (PT120) at the hydrogen output line in Figure 4).
Figure 18. Experimental results. Hydrogen line pressure evolution during a start-stop cycle.
Additionally, the temperature has been measured in the inflow of water into the stack (see the
temperature transducer (TT105) at stack input in Figure 4), the temperature of hydrogen flow leaving
the stack (temperature transducer (TT121) at stack input in Figure 4) and the temperature of hydrogen
flow after the cooling phase, prior to the HPS (see temperature transducer (TT113) in Figure 4). The
results are showed in Figure 19.
Figure 19. Experimental results. Water and hydrogen temperature evolution during a start-stop cycle.
Figure 18. Experimental results. Hydrogen line pressure evolution during a start-stop cycle.
Additionally, the temperature has been measured in the inflow of water into the stack (see
the temperature transducer (TT105) at stack input in Figure 4), the temperature of hydrogen flow
leaving the stack (temperature transducer (TT121) at stack input in Figure 4) and the temperature
of hydrogen flow after the cooling phase, prior to the HPS (see temperature transducer (TT113) in
Figure 4). The results are showed in Figure 19.
Electronics 2020, 9, x FOR PEER REVIEW 21 of 27
and the second refers to the pressure obtained at the end of the hydrogen output line (see the pressure
transducer (PT120) at the hydrogen output line in Figure 4).
Figure 18. Experimental results. Hydrogen line pressure evolution during a start-stop cycle.
Additionally, the temperature has been measured in the inflow of water into the stack (see the
temperature transducer (TT105) at stack input in Figure 4), the temperature of hydrogen flow leaving
the stack (temperature transducer (TT121) at stack input in Figure 4) and the temperature of hydrogen
flow after the cooling phase, prior to the HPS (see temperature transducer (TT113) in Figure 4). The
results are showed in Figure 19.
Figure 19. Experimental results. Water and hydrogen temperature evolution during a start-stop cycle.
Figure 19.
Experimental results. Water and hydrogen temperature evolution during a start-stop cycle.

Electronics 2020,9, 871 20 of 25
Finally, based on experimental results it is possible to calculate the real stack efficiency by applying
Equation (7),
ηstack =PH2
Pelec
(7)
where:
ηstack is the stack efficiency
PH2is the power produced in form of hydrogen (W)
Pelec is the electrical power consumed by electrolyzer (W)
The power produced in form of hydrogen, PH2, can be expressed as Equation (8):
PH2=.
mH2_stack ·HVH2(8)
where:
HVH2is the hydrogen heating value (J/kg)
.
mH2_stack is the stack hydrogen mass rate (kg/s)
And the mass rate is obtained from Equation (9):
.
mH2_stack =MMH2·.
nH2_stack (9)
where:
MMH2is the hydrogen molar mass (2·10−3kg/mol)
.
nH2_stack is the stack hydrogen molar rate (kg/s)
The stack hydrogen molar, .
nH2_stack, rate can be calculated from Faraday Law, Equation (10):
.
nH2_stack =Ncells
Istack
2F(10)
where:
Fis the Faraday constant (96485 A·s/mol)
Istack is the stack current (A)
Ncells is the stack cells number (6)
On the other hand, the electrical power consumed by the stack,
Pelec
, can be expressed as
Equation (11):
Pelec =Vstack·Istack (11)
where:
Istack is the stack current (A)
Vstack is the stack voltage (V)
And the stack voltage, Vstack, is the addition of the cell voltage, Equation (12),
Vstack =Ncells·Vcell (12)
where:
Ncell is the cells number that make up the stack
Vcell is the cell voltage (V)

Electronics 2020,9, 871 21 of 25
Therefore, taking into account expressions (8)–(12) in expression (7), the stack efficiency results in
Equation (13):
ηstack =MMH2·HVH2
2F·Vcell
(13)
To obtain the numerical value of efficiency, the stack operating point at maximum production
during experimental tests and hydrogen properties are:
HHVH2(hydrogen higher heating value) =141.86·106J/kg
LHVH2(hydrogen lower heating value) =120.86·106J/kg
Vcell =1.6 V (experimental value)
Therefore, the PEM electrolysis stack efficiency rises up to 91% in the best case (
HHVH2
) and it is
not below 77% in the worst case (LHVH2).
To also obtain the real system efficiency, the power generated in the form of hydrogen,
PH2
(theoretically given by (8)) and the electrical power consumed by the stack,
Pelec
(theoretically
given by (11)) are obtained using experimental data. Taking into account that the auxiliary power
consumption is 1.25 kW,
PAux
, the system efficiency can be calculated using Equation (14), as shown in
Figure 20.
ηsystem =PH2
Pelec +PAux
(14)
Electronics 2020, 9, x FOR PEER REVIEW 23 of 27
Therefore, the PEM electrolysis stack efficiency rises up to 91% in the best case (
) and it is
not below 77% in the worst case (
).
To also obtain the real system efficiency, the power generated in the form of hydrogen, 
(theoretically given by (8)) and the electrical power consumed by the stack,  (theoretically given
by (11)) are obtained using experimental data. Taking into account that the auxiliary power
consumption is 1.25 kW, 
, the system efficiency can be calculated using Equation (14), as shown
in Figure 20.
Figure 20. Experimental results. PEM electrolyzer efficiency.
 = 
 +

(14)
Figure 20 shows that the system efficiency rises up to 61% in the best case (
) and it is not
below 52% in the worst case (
).
5. Discussion
Based on the results obtained from the SCADA interface, Figures 11–15 show the proper
operation of the four subsystems that make up the BoP of the developed PEM electrolyzer. Regarding
the water management subsystem, the developed control logic is guaranteed by means of the
injection pump (P-001), the recirculation pump (P-002) and the water level at the oxygen separator
tank is inside the allowed range (Figure 12), as well as the water flow, temperature, pressure and
conductivity (Figure 13) during the production process.
On the other hand, in relation to the hydrogen production subsystem, the controller tracks the
levels in the pressure separators (HPS and LPS) (Figure 14), acting over the electrovalves (LCV115
and LCV116) that communicates with both separators and the oxygen separator. Additionally, the
monitoring interface shows the cooling subsystem operation. Then, when water or hydrogen flows
achieve the highest allowable temperature values, the cooling subsystem is activated (Figure 15).
The I-V curve, Figure 16 verifies that the system works within the range of current indicated for
the cells, and through it, the electrolysis voltage is obtained to perform the stack efficiency
calculations. In the curve that relates to the stack power and the hydrogen production rate (Figure 17),
the maximum production of the design is verified as well as a maximum consumption, which is close
to the expected.
In the second part of the experimental test, Figure 18 shows the processes the system goes
through during a start-stop cycle. At the first phase of the Initiate state, the pressure values are around
0%
10%
20%
30%
40%
50%
60%
70%
0 100 200 300 400 500 600 700 800 900
Efficiency (%)
Time (mm:ss)
System efficiency
System efficiency LHV System efficiency HHV
Figure 20. Experimental results. PEM electrolyzer efficiency.
Figure 20 shows that the system efficiency rises up to 61% in the best case (
HHVH2
) and it is not
below 52% in the worst case (LHVH2).
5. Discussion
Based on the results obtained from the SCADA interface, Figures 11–15 show the proper operation
of the four subsystems that make up the BoP of the developed PEM electrolyzer. Regarding the
water management subsystem, the developed control logic is guaranteed by means of the injection
pump (P-001), the recirculation pump (P-002) and the water level at the oxygen separator tank is
inside the allowed range (Figure 12), as well as the water flow, temperature, pressure and conductivity
(Figure 13) during the production process.

Electronics 2020,9, 871 22 of 25
On the other hand, in relation to the hydrogen production subsystem, the controller tracks the
levels in the pressure separators (HPS and LPS) (Figure 14), acting over the electrovalves (LCV115
and LCV116) that communicates with both separators and the oxygen separator. Additionally,
the monitoring interface shows the cooling subsystem operation. Then, when water or hydrogen flows
achieve the highest allowable temperature values, the cooling subsystem is activated (Figure 15).
The I-V curve, Figure 16 verifies that the system works within the range of current indicated for
the cells, and through it, the electrolysis voltage is obtained to perform the stack efficiency calculations.
In the curve that relates to the stack power and the hydrogen production rate (Figure 17), the maximum
production of the design is verified as well as a maximum consumption, which is close to the expected.
In the second part of the experimental test, Figure 18 shows the processes the system goes
through during a start-stop cycle. At the first phase of the Initiate state, the pressure values are
around 1 bar. When the user presses the inertization button and the Inertization state begins (Figure 18,
coordinate (00:30). When the system completes a successful Inertization (Figure 18, coordinate (02:30, 1)),
it passes to the Standby state.
When the production button is pressed, the equipment goes quickly through the Pre-production
stage (Figure 18, coordinate (03:10, 3), as the adequate conditions in the water subsystem are quickly
reached to start the electrolysis process. This takes a few seconds, and in time 03:30, the Purge state
is reached. At this state, the hydrogen pressure at the stack output starts to rise until 9 bar (Figure 18,
coordinate (08:20, 4)). At this time the system enters into Production state and the hydrogen flows to
the drying stage. Due to the opening of the electrovalves (CV118 and CV119) that communicate HPS
with PSA dryers, a small pressure drop peak occurs at the hydrogen flow leaving the stack. During the
Production stage, both pressures are equalized (Figure 18, time 09:40) and their values coincide during
the entire production process. Once the maximum established pressure of 20 bar is reached (Figure 18,
coordinate (13:30, 4), the hydrogen is ready to be delivered and stored in an external storage tank.
It can be deduced that the pressurization time is 10 min (13:30–03:30), from Purge to Production state.
During the production process it is possible to observe small occasional pressure drops, which are
the ones that occur due to the operation PSA drying stage. The dryers accumulate the humidity
of the hydrogen in its final phase, and periodically purge it to the outside. Because it is a pressure
process, without the use of thermal elements, a small portion (1.6 bar) of the pressure of the production
hydrogen is used to purge the humidity, which is the reason why instantaneous pressure drops occur.
To carry out the controlled shutdown of the plant, the user presses the standby button and it stops
in the Inertization state (Figure 18, coordinate (18:25, 5)), to purge hydrogen from the pipelines with
the use of nitrogen. This process has two phases, firstly the pressure is purged from stack output and
secondly the pressure is purged from the hydrogen output line. After Inertization is complete, the PEM
electrolyzer keeps at Standby state (Figure 18, coordinate (20:20, 1)), with hydrogen depressurized and
ready to re-start the process or on the contrary, to be disconnected.
In a similar way to the hydrogen flow pressure, Figure 19 allows the tracking of the water and
hydrogen flow temperature. The initial temperature in the Initiate state is 22
◦
C, corresponding to the
ambient temperature. In the Inertization state it is observed how there is a small drop in cooled hydrogen
temperature (Figure 19, coordinate (01:50, 1)), this is due to the operation of the cooling subsystem.
Since the hydrogen flow temperature at the stack output is inside the allowed range, the cooling
is deactivated and the temperature value is stabilized again. During the Purge state, water and
hydrogen temperature values go up smoothly until the Production state (Figure 19, coordinate (08:20,
4)), where the temperature curve slopes start to increase. At time 10:50, the controller is warned that
the hydrogen flows temperature needs to be cooled, and it activates the cooling subsystem. Then,
the temperature of the hydrogen flow after the cooling phase decreases until it is stabilized to 15
◦
C. From this moment, the developed control logic guarantees that the cooling subsystem maintains
the hydrogen temperature at 15
◦
C during for the entire duration of the Production state. When the
user presses the standby button, the Production state finishes (Figure 19, coordinate (18:25, 4)), and
the systems enters into the Inertization state. As a consequence to turning offthe system, the water

Electronics 2020,9, 871 23 of 25
and hydrogen flow temperatures are established to ambient temperature and the cooling subsystem
is deactivated.
Finally, Table 6shows a summary of the main characteristics of the developed PEM electrolyzer.
Table 6. Main characteristics of the developed PEM electrolyzer.
Parameter Value
Sustainability 100% renewable
DC power supply
Stack power consumption
Operating range
10 kWe (at max. production)
11.8 kWe (experimental value)
0–100%
Auxiliary consumption 1.25 kW
Stack efficiency 77–91%
Electrolyzer efficiency 52–61%
Hydrogen production rate 0–2.2 Nm3/h
Pressurization time (at 20 bar) 10:00 (mm:ss)
Operating temperature range <68 ◦C (water flow)
<72 ◦C (hydrogen flow)
Water consumption 1.8 l/h
1µScm−1<conductivity <0.056 µScm−1
6. Conclusions
This paper has described the design, implementation, and practical experimentation of a
medium-size PEM electrolyzer for the production of pressurized hydrogen, from water and electric
power (renewable if possible, as in our case). From a commercial stack, the key to achieving its
best performance has been the optimal design of the BoP, paying special attention to the subsystems
that comprise it: the stack power supply subsystem, water management subsystem, hydrogen
production subsystem, cooling subsystem and control subsystem. Based on this, the control logic has
been developed under the criteria of guaranteeing efficient and safe operation.
For this purpose
,
each subsystem has required its own control logic according to plant technical specification.
Additionally, the control logic of the four subsystems has been integrated into the operating states
sequence that governs the electrolyzer performance.
The obtained experimental results validate the control logic in various operating cases, including
warning and failure cases. Additionally, experimental results show correct operation in all the
plant states. To check them, the evolution of the hydrogen flow pressure and temperature as well as
water temperature have been analyzed. Comparing the developed electrolyzer with those found in the
scientific literature, the first is characterized by its high stack efficiency (>77%) and low pressurization
time (10 min) without an external compressor. This feature increases the global efficiency, reducing the
consumption from auxiliaries. On the other hand, the current density of 3 A/cm
2
also differentiates
the proposed design, since the typical current density reviewed in the literature ranges from 1 to
2 A/cm
2
; this allows the achievement of high hydrogen production rates at low cell voltage. The last
improvement of the proposed BoP regarding the literature review is the hydrogen cooling, placed at
the stack hydrogen outlet. Therefore, the first gas separator stage (HPS) receives more condensed
water, so the hydrogen drying is more efficient from its first phase.
In conclusion, the capacity of the developed PEM electrolysis plant regarding its production rate,
wide operating power range, reduced pressurization time and high efficiency has been proved.
Author Contributions:
Conceptualization, F.S.M. and J.M.A.; methodology, J.J.C.M. and F.S.M.; software,
J.J.C.M.; validation, J.J.C.M. and F.J.V.; formal analysis, F.S.M. and J.M.A.; investigation, J.J.C.M. and A.J.C.;
resources, J.J.C.M., F.J.V. and A.J.C.; data curation, J.J.C.M.; writing—original draft preparation, J.J.C.M. and
F.S.M., writing—review and editing, F.S.M. and J.M.A.; visualization, J.J.C.M. and F.S.M.; supervision, J.M.A.;

Electronics 2020,9, 871 24 of 25
project administration, F.S.M.; funding acquisition, J.M.A. All authors have read and agreed to the published
version of the manuscript.
Funding:
This research was funded by “Configuration and management of micro-grid based on renewable
energy and hydrogen technology (H2SMART-
µ
GRID)” Spanish Government, grant Ref: DPI2017-85540-R,”,
and “G2GH2-Going to Green Hydrogen. High efficiency and low degradation system for hydrogen production”
by FEDER 2014/20, grant Ref: UHU-1259316.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
BOL Beginning of life
BoP Balance of plant
DC Direct current
DI Deionized
EMS Energy management system
EOL End of life
ESS Energy storage system
HPS High pressure separator
LPS Low pressure separator
PEM Polymer exchange membrane
PHIL Power-hardware-in-loop
PSA Pressure swing adsorption
SCADA Supervisory Control And Data Acquisition
FFaraday constant (96485 A·s/mol)
Istack Stack current (A)
Ncells Cells number
.
nH2_stack Hydrogen molar rate (mol/h)
ηstack Stack efficiency
HHVH2Hydrogen higher heating value (141.86·106J/kg)
Istack Stack current (A)
LHVH2Hydrogen lower heating value (120·106J/kg)
MMH2Hydrogen molar mass (2·10−3kg/mol)
.
mH2_stack Stack hydrogen mass rate (kg/h)
Vcell Cell voltage (V)
Vstack Stack voltage (V)
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