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On-and Off-Grid Laboratory Test Setup for Hydrogen Production with Solar Energy in Nordic Conditions

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Renewable energy production, such as solar and wind power, require energy storage systems because of their intermittent nature. Hydrogen is potential candidate to store energy, because it has high-energy content capacity also in long term and it is possible to build affordable large-scale energy storages based on hydrogen. This paper studies solar power and water electrolyser cooperation and system designing.
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On- and Off-Grid Laboratory Test Setup for Hydrogen Production with
Solar Energy in Nordic Conditions
Antti Kosonen, Joonas Koponen, Jero Ahola, and Pasi Peltoniemi
LAPPEENRANTA UNIVERSITY OF TECHNOLOGY
P.O. Box 20, FI-53851
Lappeenranta, Finland
Tel.: +358 40 833 7749, Fax: +358 5 621 6799
antti.kosonen@lut.fi
URL: http://www.lut.fi
Keywords
Renewable energy systems, photovoltaic, hydrogen, simulation
Abstract
Renewable energy production, such as solar and wind power, require energy storage systems because
of their intermittent nature. Hydrogen is potential candidate to store energy, because it has high-energy
content capacity also in long term and it is possible to build affordable large-scale energy storages
based on hydrogen. This paper studies solar power and water electrolyser co-operation and system
designing.
Introduction
Solar power has a huge potential to be a remarkable energy source in the world scale. Together with
wind power, they form almost unlimited renewable energy source that is available all around the
whole world. On the other hand, the nature of these productions is intermittent, and hence, these
requires a flexible energy system in parallel. The flexible energy system includes load control,
linkages between different energy sectors, and storages. The storage system is problematic, because it
should be at once energy intensive and suitable for several time scale. Different storage system types
and their features are illustrated in Fig. 1. According to Fig. 1, it is clear that the chemical storages,
such as hydrogen and methane, have the best possibility to be the solution for the intermittent energy
seasonal storage problem.
(a) (b)
Fig. 1: Comparison of different energy storage types. (a) Round-trip efficiency and storage cycle time. (b) Power
vs. energy specific invest cost [1].
Solar power production is rapidly growing all over the world at this moment. Last year, installed solar
PV power capacity increased 38.7 GW, and hence, reached totally the capacity of 177 GW [2].
Germany has installed almost 40 GW solar power during the last 7 years starting in practice from zero
and the annual installation speed has been over 7 GW during the years 2010–2012 [2]. It has been
shown that Finland has almost the same annual solar power production potential as in Germany and
solar power is already profitable also in Finland at this moment, if the energy is mainly self-consumed
[3].
In this paper, laboratory test setup for hydrogen production with water electrolysis from solar energy is
studied. Dimensioning and concept aspects for a solar PV system with hydrogen production are
researched. The system should be able to be used both in off- and on-grid conditions. The work is
based on the data from a real solar PV plant. Both alkaline and PEM water electrolysis methods are
presented and simulated.
This paper is organized as follows. The second section introduces the solar PV production in Finnish
climate. In the third section, water electrolysers are presented and simulation models are formed.
Laboratory test setup is designed in the fourth section. The paper is concluded in the fifth section.
Solar Power
In this paper, both on- and off-grid hydrogen production solutions are presented. The most essential
difference between these two are that an electric grid is a backup in an on-grid system. Hence, there is
no necessarily need for dynamic control of water electrolysers and power electronics in the on-grid
system. On the other hand, an off-grid system should work together with intermittent production, and
hence there should be balance between the load and production all the time. Solar power can be
simulated and/or utilized using real measurement data. The target sampling rate for the solar PV
production is 1 s. In this case, measurements are used, because the data is available for different solar
PV plants. Simulations for LUT solar power plant are already carried out and analyzed in [3].
At this moment, LUT has 208.5 kWp of solar power. Panels are mounted in different places; flat roofs,
carports, walls, and a 2-axis tracking system. The panels of the fixed systems are mainly faced to
south, but half of the wall mounting is faced to west. The slope degree in the all systems is 15° despite
of the wall mounting. The size of the flat roof system is 56.5 kWp, carports 108 kWp, walls 39 kWp,
and tracking 5 kWp. Photos from the solar power plants that are installed are illustrated in Fig. 2.
(a) (b)
(c) (d)
Fig. 2: Solar power plants at LUT. (a) Part of flat roof installation. (b) Carports. (c) Part of wall installations. (d)
2-axis tracking system.
The production of different systems are measured separately, and hence, wide scale of data is
available. The solar power production is measured by five Siemens PAC4200 energy meter systems,
which are read by the LabVIEW software. The software is running by one-second interval. One-
minute average PV productions for different installations during April of 2015 are illustrated in Fig. 3.
Measurements have started on April 2014.
(a) (b)
(c) (d)
(e) (f)
Fig. 3: Solar PV production during the best and worst day on April 2015. (a) Flat roof, 52.5 kWp. (b) Carport,
108 kWp. (c) South wall, 19.1 kWp. (d) West wall, 19.9 kWp. (e) 2-axis tracking system, 5 kWp. (f) Total LUT,
208.5 kWp.
The production of LUT solar power plant can be used for hydrogen production with an on-grid
system. Then the water electrolyser can be driven in real time with the PV production. In the on-grid
system, the electrolyser should be driven directly according to the solar PV power, because the
electrical grid works as a backup system. The profitability of the operation is highest when the own
solar PV production can be used as much as possible.
When the system is working as an off-grid system, the dimensioning and controlling of the system
components is more critical. If the production is not limited then the loads should be dimensioned and
controlled to use all the energy of the production in all cases. In addition, surplus energy that the
electrolyser cannot consume because of practical limitations is supplied into an electrical storage. The
size of the electrical storage should be dimensioned.
Water electrolysis
The principle of water electrolysis is to pass a direct current between two electrodes immersed in an
electrolyte to decompose water. Hydrogen is formed at the cathode and oxygen at the anode. The
production of hydrogen is directly proportional to the current passing through the electrodes [5]. There
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11.04.2015 [513.0 kWh] / 29.04.2015 [52.4 kWh]
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Time (h)
Power (kW)
11.04.2015 [983.8 kWh] / 29.04.2015 [87.8 kWh]
are two main water electrolysis technologies commercially available; 1) alkaline and 2) proton
exchange membrane (PEM) technologies. In water electrolysis, electrical and thermal energy are
converted into chemical energy, which is stored in hydrogen. The energy required for the reaction to
take place is the enthalpy of formation of water H. Only the free energy of this reaction, called Gibbs
free energy change G, has to be supplied to the electrodes in the form of electrical energy. The
remainder is thermal energy, which is the product of process temperature T and entropy change S.
Enthalpy change can be expressed as [6]
∆∆∆ 
 , (1)
where (for hydrogen, = 2) is the number of moles of electrons transferred in the reaction, the
Faraday constant (9.6485·104 C/mol),  the reversible voltage, and the prevailing pressure (Pa).
The reversible cell voltage  is the lowest required voltage for the electrolysis to occur. Without
auxiliary heat the minimum voltage required for water decomposition is higher than the reversible
voltage. Designation “thermoneutral voltage”  is often used in literature for this higher voltage
level. The reversible voltage and the thermoneutral voltage at standard ambient conditions are 1.23 V
and 1.48 V, respectively. However, both voltages are thermodynamic state functions dependent on the
prevailing temperature and pressure [7]. For aqueous water electrolysis, the reversible voltage can be
written as a function of temperature and pressure as
,

ln.
, (2)
where is the universal gas constant, the vapour pressure of the electrolyte solution (atm), and
the vapour pressure of pure water (atm).  in (2) can be expressed as [7]
 1.51841.5421109.52310ln9.84∙10, (3)
where the temperature is in kelvin. Increase in the operating temperature decreases the minimum
voltage required to decompose water, and thus utilization of thermal energy can be beneficial. On the
other hand, increasing the operating pressure results in slightly increased reversible voltage. The cell
voltage is the sum of the reversible voltage and additional overvoltages appearing in the cell [8]
 
   , (4)
where  is the cell voltage,  is the overvoltage due to ohmic losses in the cell elements, 
the activation overvoltage, and  refers to the concentration overvoltage. Ohmic losses are
proportional to the electric current and depend on the electrolyte layer thickness (in PEM membrane
thickness) and its conductivity, which is dependent on the temperature. The activation overvoltage is
nonlinear with respect to the electric current passing through the cell and can be approximated by the
Tafel equation [9]
,/ 2.3026 
/ log
, (5)
where is the current density, the exchange current density, and / the charge-transfer
coefficients at the anode and cathode, respectively. The concentration overvoltage is caused by mass
transport processes. Typically, the concentration overvoltage is much lower than  and  and is
negligible when commercial current density levels are considered. Exemplary overvoltages excluding
the concentration overvoltage in alkaline water electrolysis are illustrated in Fig. 4.
Fig. 4: Exemplary overvoltages as a function of current density in alkaline electrolysis at = 75 °C and
= 30 bar. The resulting polarization curve represents the characteristics of the performance of the water
electrolyser.
In alkaline electrolysers, the electrolyte is usually a highly concentrated aqueous solution of potassium
hydroxide (KOH), of the order of 25–30 wt.%. A gas-tight diaphragm, which is permeable to
hydroxide ions (OH-), separates two electrodes. The minimum partial load of alkaline electrolysers is
limited by the diaphragm, which cannot completely prevent the product gasses from cross-diffusing
through it. The minimum current also protects the electrodes against corrosion in alkaline electrolysis.
Additionally, the inertia in the transport of ions in the electrolyte solution renders the alkaline
electrolysers unable to rapidly react to the changes in the input power [10]. In the alkaline water
electrolysers, the product electrolytic gasses are wet due to the presence of the liquid electrolyte, and
thus gas separators are needed to separate the remaining electrolyte. This additional process then
increases the delay in the hydrogen production.
In PEM electrolysers, thin (50–250 μm) proton conducting membrane is used as a solid polymer
electrolyte rather than the liquid electrolytes typically used alkaline water electrolysers. The water-
assisted proton conduction of PEM electrolysers limits the operation temperature below 80 °C [11].
The gas crossover rate is generally lower than in alkaline water electrolysers enabling the use of
almost the whole range of rated power. Additionally, the solid polymer membrane and compact
system design enables the PEM electrolysers to respond more quickly to fluctuations in the input
power. Thus, the PEM electrolysers can operate in a more dynamic fashion than the alkaline ones.
Commercial PEM electrolysers typically operate at current densities of 0.6–2.0 A/cm2 [12]. The
current density in alkaline ones is typically below 0.5 A/cm2. In the PEM electrolysers, the compact
character of electrolysis modules and the structural properties of the membrane electrode assemblies
allow high operation pressures and give PEM electrolysers the ability to endure significant pressure
differences between electrode compartments. This can enable e.g. production of hydrogen at 35 bar
and oxygen close to atmospheric pressure [8].
Dynamic operation of water electrolysers
Renewable electrolytic hydrogen production can be divided into autonomous and grid-connected
applications based on the presence and employment of an electric grid connection. In autonomous
applications, renewable power production systems and water electrolysers are not connected to the
main electric grid. The water electrolysers may then be required to operate with variable current as
opposed to fixed current operation. Comparison of key dynamic operation characteristics of two main
water electrolysis technologies is presented in Table I.
The lack of liquid electrolyte and the use of thin proton conducting membranes enables PEM water
electrolysers to respond more quickly to the fluctuations in the input power. Additionally, the more
compact structure of PEM electrolysers results in a lower overall heat capacity and heat resistance.
According to (3), the reversible voltage decreases with increasing temperature, and therefore a higher
operating temperature will result in higher hydrogen production efficiency. More compact structure
then allows the water electrolyser to reach its optimal operating conditions faster. The minimum
partial load in water electrolysers is restricted by the hydrogen gas crossover rate to the anode
compartment. The PEM electrolysers are generally regarded to have a lower gas permeability through
the separating membrane than alkaline ones. However, in high-pressure PEM water electrolysis the
operating pressure, especially a large differential pressure, can increase the gas crossover and limit the
safe load range [14].
Table I. Dynamic operation characteristics of alkaline and PEM water electrolysers. [13]
Alkaline PEM
Min. load 2040 %(full load) 510 %(full load)
Ramp-up from minimum load
to full load 0.1310 %(full load)/second 10100 %(full load)/second
Ramp-down from full load to
minimum part load 10 %(full load)/second 10100 %(full load)/second
Simulation of renewable electrolytic hydrogen production
A simple quasi-static MATLAB simulation model is developed to analyse the operation of small-scale
alkaline and PEM water electrolysers, which are connected to the solar PV plant. The loss-estimate
approach used in simulation of alkaline electrolysis is presented in more detail in [15] and modelling
of the PEM electrolysers is described in [16]. The quasi steady-state thermal model, which assumes
constant heat generation and heat transfer rates for a given time interval, is presented in [17]. The
selected technical specifications for the modelled 5 kW water electrolysers are illustrated in Table II.
The maximum change in electrolyser power (max. Δ), is assumed based on values presented in Table
I. Both electrolysers have a nominal hydrogen production rate of 1 Nm3/h (= 0.0899 kg/h). The
simulated alkaline electrolyser is assumed to have a thermal capacity of 174 kJ/°C and thermal
resistance of 0.164 °C/W based on the experimental study conducted in [18]. For the more compact
PEM water electrolyser, a thermal capacity of 42 kJ/°C and thermal resistance 0.08 °C/W are assumed
[19].
Table II. Technical description of the exemplary 5 kW water electrolysers.
Nominal stack
voltage [V]
Nominal stack
current [A]
Effective area per
cell [cm2]
Number of
cells
Min. load
[%]
Max. Δ
[W/s]
Alkaline 43 120 300 22 35 500
PEM 64 80 69 33 10 2000
The nominal power, the minimum partial load, and the maximum ramp-up and ramp-down rate should
be considered especially in the design of the off-grid renewable hydrogen production system. The
minimum partial load gives the minimum electric current for the water electrolyser and limits the off-
grid operation. A battery system should be employed to prevent frequent start/stop cycles by
guaranteeing a protective current for the electrolyser. The capability of the water electrolyser to
respond to the changes in the input power affects also the utilization rate of variable renewable energy
production. In order to maximize the utilization rate of solar PV production and optimize the operation
of a water electrolyser in an off-grid system, the battery system design should be taken into account.
Hence, the nominal capacity of the solar PV system and the water electrolyser technology and its
nominal power define the required battery system capacity. The size of the battery system and solar
PV plant can be optimized for the electrolyser or vice versa.
The operation of water electrolysers described in Table II are simulated based on the measured one-
minute average solar PV production of 5 kWp flat roof installation (south facing, 15° slope). The
simple system control algorithm used to determine the required battery storage capacity is following:
If the battery can provide energy for one hour of water electrolyser operation at minimum load, the
water electrolyser can be started. After the water electrolyser has been started, it can either follow the
PV production or drain energy from the battery system to provide the minimum load in co-operation
with the PV production. Simulation results for the 5 kW alkaline and PEM water electrolysers
connected to 5 kWp solar PV system are presented in Fig. 5.
(a) (b)
Fig. 5: Measured 5 kWp solar PV power production and simulated off-grid hydrogen production rate, stored
energy in batteries, and water electrolyser stack temperature on 3 June 2014 (red) and 11 June 2014 (green).
Both water electrolysers are simulated at the operating pressure of 25 bar. Input of coolant water is included in
simulations to limit the electrolyser stack temperature to 70 °C in order to ensure safe operation. The higher
heating value of hydrogen is 39.4 kWh/kg. (a) Alkaline water electrolyser. (b) PEM water electrolyser.
Table III: Nominal capacity of solar PV, produced energy on 11 June 2014, and the
required storage capacities to utilize the produced energy in the water electrolysis.
Solar PV
[kW
]
Solar energy
[kWh]
Required storage, alkaline
[kWh]
Required storage, PEM
[kWh]
5 34.28 1.70 0.50
10 68.55 16.52 14.75
15 102.83 46.43 44.53
20 137.10 78.96 76.99
The required storage capacity to guarantee continuous operation with a high solar PV production day
(on 11 June 2014) is lower for the PEM water electrolyser than the alkaline one. This is enabled by the
wider stack current control range of the PEM water electrolyser. As the water electrolyser is only
allowed to start after the batteries have enough energy to drive the electrolyser for one hour with
minimum current, the PEM electrolyser operates with more frequent start/stop cycles with low solar
PV production day (on 3 June 2014). As the minimum voltage required to decompose water decreases
with increasing temperature, longer continuous operation, where the stack temperature is allowed to
approach its designated level, may be considered to optimize the stack efficiency. The measured solar
PV production can be scaled up to estimate the required storage capacity to utilize the surplus
production in case of alkaline and PEM water electrolysis. The estimated minimum battery storage
capacities in each simulation case are gathered in Table III. As the nominal PV capacity and surplus
energy increase, the difference between the required storage capacities of the alkaline and PEM
technologies decreases. With 20 kWp nominal capacity, and hence, significant surplus production, the
4 6 8 10 12 14 16 18 20 22
0
1
2
3
4
5
Solar PV production [kW]
Alkaline
3 June 2014, hydrogen produced: 0.034 kg
11 June 2014, hydrogen produced: 0.607 kg
4 6 8 10 12 14 16 18 20 22
0
0.5
1
1.5
2
H
2
production rate [g/min]
4 6 8 10 12 14 16 18 20 22
0
0.5
1
1.5
2
Stored energy [kWh]
4 6 8 10 12 14 16 18 20 22
0
50
100
Time [h]
Stack temperature [°C]
4 6 8 10 12 14 16 18 20 22
0
1
2
3
4
5
Solar PV production [kW]
PEM
3 June 2014, hydrogen produced: 0.051 kg
11 June 2014, hydrogen produced: 0.6 kg
4 6 8 10 12 14 16 18 20 22
0
0.5
1
1.5
2
H2 production rate [g/min]
4 6 8 10 12 14 16 18 20 22
0
0.5
1
1.5
2
Stored energy [kWh]
4 6 8 10 12 14 16 18 20 22
0
50
100
Time [h]
Stack temperature [°C]
battery capacity would enable operating a 5 kW water electrolyser with its nominal current for 15
hours based on the values estimated in Table III.
In the simulations, the batteries are only discharged at a rate corresponding to the minimum load of the
water electrolyser in question. In order to utilize the surplus PV production on a weekly time scale, the
battery discharge rate should be optimized. Weather forecasts could be used to determine whether the
batteries should be discharged for the following day or not. In on-grid operation, the electric grid
connection can serve as a virtual battery for the water electrolyser to provide the minimum stack
current for the water electrolyser at times of low to non-existent PV production. Then, the price of
electricity becomes one determining factor for the use of grid connection. On-grid operation of
alkaline and PEM water electrolysers are simulated, and the time of day was set as a restriction for the
use of grid electricity: Between 06:00 am and 8:00 pm, grid electricity can be used to provide the
minimum load for the water electrolyser. Surplus PV production is injected to the electric grid. The
simulation results are presented in Fig. 6. In Fig. 6(a), the alkaline and PEM water electrolyser uses
20.91 kWh and 3.74 kWh on 3 June, and 4.27 kWh and 0.48 kWh on 11 June of energy from the
electric grid, respectively.
(a) (b)
Fig. 6: Measured 5 kWp solar PV power production and simulated on-grid hydrogen production rate, grid power
balance, and water electrolyser stack temperature on 3 June 2014 (red) and 11 June 2014 (green). Both water
electrolysers are simulated at the operating pressure of 25 bar. Input of coolant water is included in simulations
to limit the electrolyser stack temperature to 70 °C in order to ensure safe operation. (a) Alkaline water
electrolyser. (b) PEM water electrolyser.
In the viewpoint of dimensioning and optimization of the whole system operation, several things
should be taken into account. The solution is depended on the optimization criteria, and hence, there
are several starting points for this. Different parameters that should be included in the optimization
strategy are presented in Fig. 7.
Laboratory Test Setup
Laboratory test setup for hydrogen production will be built to LUT. At the first phase, it will consists
of a 5.5 kW PEM electrolyser. The laboratory system can be used to test the operation of the
4 6 8 10 12 14 16 18 20 22
0
1
2
3
4
5
Solar PV production [kW]
Alkaline
3 June 2014, hydrogen produced: 0.431 kg
11 June 2014, hydrogen produced: 0.689 kg
4 6 8 10 12 14 16 18 20 22
0
0.5
1
1.5
2
H
2
production rate [g/min]
4 6 8 10 12 14 16 18 20 22
2
1
0
1
2
Power balance [kW]
4 6 8 10 12 14 16 18 20 22
0
50
100
Time [h]
Stack temperature [°C]
4 6 8 10 12 14 16 18 20 22
0
1
2
3
4
5
Solar PV production [kW]
PEM
3 June 2014, hydrogen produced: 0.119 kg
11 June 2014, hydrogen produced: 0.609 kg
4 6 8 10 12 14 16 18 20 22
0
0.5
1
1.5
2
H
2
production rate [g/min]
4 6 8 10 12 14 16 18 20 22
2
1
0
1
2
Power balance [kW]
4 6 8 10 12 14 16 18 20 22
0
50
100
Time [h]
Stack temperature [°C]
electrolyser with renewable energy production and frequency control, to verify simulation models, etc.
Power electronics is in essential role when the electrolysers are driven. For solar PV power electronic
based conversion is required to implement maximum power point tracking (MPPT) operation [20].
The system can be operated continuously in MPPT mode when connected to the grid. However,
depending on the battery SOC and the power fed to the water electrolyser, the solar PV power
production may have to be limited when operating in off-grid mode with setups introduced in the
paper. For the water electrolyser AC/DC conversion is required, not only because water electrolyser
consumes DC power but also to execute controlled power consumption. The power converter for
water electrolyser is operated as DC current controlled. In the setups described in the paper also the
battery connection requires AC/DC power conversion unit. How then the battery connection is
operated depends whether the system is in on-grid or off-grid mode. In on-grid mode, the grid is
formed by the existing connection and the conversion unit is needed to convert power to DC for the
batteries as well as to control the SOC of the battery energy storage system. In off-grid mode the
conversion unit needs to be operated as a grid-forming unit and thereby operating as a voltage source
for the islanded grid. It should be noticed that if the system operates mainly in off-grid mode, the
system could also be DC grid based implementation using DC/DC converters. To operate in the off-
grid mode an energy management system (EMS) is required to maintain energy and power balance
[21]. A block diagram of the laboratory test setup for hydrogen production is illustrated in Fig. 8. At
this point, a battery bank is not included because the electric grid can work as a battery. The
electrolyser can be driven virtually as an off-grid system.
Fig. 7: Parameters that should be included in the optimization strategy of hydrogen production.
Fig. 8: Off- and on-grid laboratory test setup for hydrogen production with solar energy. Setup includes the
5.5 kW PEM electrolyser, two hydrogen storage tank of 350 l, and LUT solar power plant. In addition, a small
fuel cell (1.5 kW
el.
+ 1.5 kW
heat
) is included to produce hydrogen back to electricity.
Conclusion
Solar power together with wind power form almost unlimited renewable energy source that cover the
whole world. The nature of these productions is intermittent, and hence, these requires a flexible
energy system that includes load control, linkages between different energy sectors, and storages.
Hydrogen has potential to be a linkage between electricity and other energy sector. This paper studies
hydrogen production with solar power when the system is in on- and off-grids. Measurement of solar
PV combined together with simulations of hydrogen production are carried out to see the co-operation
and dimensioning issues in both cases. Optimization strategy is a multi-disciplinary task.
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... The solution is depended on the optimization criteria, and hence, there are several starting points for this. Different parameters that should be included in the optimization strategy are presented in [9]. In the viewpoint of dynamic, a single solar PV plant production is very challenging to be followed. ...
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