Viable storage of hydrogen in materials with off-board O.M. Løvvik *

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international journal of hydrogen energy 34 (2009) 2679–2683
Available at www.sciencedirect.com
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Viable storage of hydrogen in materials with off-board
recharging using high-temperature electrolysis
O.M. Løvvika,b,*
a
SINTEF Materials and Chemistry, P.O. Box 124 Blindern, NO-0314 Oslo, Norway
University of Oslo, Department of Physics, Gaustadalleen 21, NO-0316 Oslo, Norway
b
article info
abstract
Article history:
Hydrogen storage in materials (hydrides) is challenging for vehicular applications, due to
Received 20 November 2008
the large amounts of heat that must be removed in a short period of time during the filling
Received in revised form
process. It is proposed that rehydrogenation of the hydride instead is performed outside
8 January 2009
the vehicle, and linked to hydrogen production by water electrolysis at the filling station.
Accepted 17 January 2009
The waste heat removed from the hydride during filling can be used to heat the electrolysis
Available online 20 February 2009
cell, increasing its efficiency. This solution can speedup the filling process, increase the
efficiency of the hydrogen production, increase safety during filling, eliminate the need for
Keywords:
massive transportation of hydrogen, and open the arena for many new hydrogen storage
Hydrogen storage
materials.
Electrolysis
ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
Fuel cells
reserved.
Filling stations
1.
Introduction
Hydrogen is commonly believed to become a major energy
carrier, replacing fossil fuels in the energy system [1–5].
Hydrogen should then be produced from renewable energy
sources, to ensure reduced emission of carbon dioxide [6].
Among the most important obstacles for the introduction of
a hydrogen economy is the lack of a proper system for
hydrogen storage, particularly for vehicular applications
[7–10].
Storage of hydrogen in materials, e.g. in the form of metal
hydrides, is often proposed for automotive use of hydrogen –
then hydrogen is absorbed at interstitial sites in intermetallic
compounds (interstitial hydrides) or is stored in complex
metal hydrides via phase transitions [8,11]. Both these options
involve removal of heat when hydrogen is loaded, the amount
of heat given by the hydrogenation enthalpy. However, there
is still much to desire from this solution, and recharging of the
storage tank has been identified as a particularly challenging
task. If hydrogenation is to be done at moderate hydrogen
pressure and temperature, the hydride formation enthalpy
must be around 40 kJ/mol [11], which is heat that must be
rejected during the recharging step. A typical target of the
filling process is to recharge around 5 kg hydrogen during
a few minutes, which implies that heat in the order of MW
must be transferred during hydrogenation. To handle such
large heat rates with on-board heat exchangers would be both
expensive and space demanding [12], if possible at all.
Adding to the complexity, even to obtain a recharging
speed in the order of kg H2/min is a formidable challenge
which is far from solved; this is particularly so for the complex
hydrides, where complicated phase transitions inhibit rapid
(de-)hydrogenation. Thus, the hydrogenation kinetics of most
hydrogen storage materials with acceptable hydrogen density
are far slower than what is required for rapid filling of a large
tank. Distribution of hydrogen is another important technical
* SINTEF Materials and Chemistry, P.O. Box 124 Blindern, NO-0314 Oslo, Norway.
E-mail address: [email protected]
0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2009.01.042
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international journal of hydrogen energy 34 (2009) 2679–2683
challenge [13]. Distribution along city gas pipelines has been
proposed as an option, but this kind of infrastructure is
missing in many places. Also, hydrogen is notoriously difficult
to contain, and many existing pipelines may not be adequate
for hydrogen distribution. Another fundamental issue concerning metal hydrides is the high net energy consumption
associated with their hydrogenation and dehydrogenation
[14], which deteriorates the overall energy efficiency.
An alternative is instead to distribute the energy in the
form of electricity. Hydrogen should then be produced locally
at the filling station, using water electrolysis. One obvious
advantage with such a scheme is the already existing infrastructure in the form of electricity grids [15], even if the
capacity may have to be increased [16]. This is also versatile
with regards to the energy source; the source can easily be
changed without the need to replace any hydrogen production
or distribution units. This has been used in several demonstration projects worldwide (e.g. Refs. [17,18]), linked to
hydrogen stored as liquid or at high pressure.
The efficiency of water electrolysis depends on the operating temperature, and it is a large advantage to operate at
temperatures close to or even above the boiling point of water
[19]. Solid oxide steam electrolysis is a particularly interesting
option with very high potential [20–23]. There can thus be
a need for significant amounts of heat and power at the
hydrogen filling station, in order to increase efficiency of
ordinary electrolysis or to achieve high temperatures needed
for advanced electrolysis.
A previous study compared the efficiency and cost of
production of hydrogen from natural gas centrally or distributed, as well as hydrogen stored in various ways [24]; it was
found that central production and hydrogen storage in
materials were the most energy efficient combination.
Another study focused on exergy as the most important
parameter to assess the different approaches [25]. Distributed
production of hydrogen using small-scale reforming of
methane was recently studied to evaluate the optimal
investment strategy of hydrogen refuelling stations [26].
This paper suggests a new strategy, in which hydrogen is
produced by electrolysis at hydrogen stations, and hydrogen is
stored in materials. A detailed description of the concept is
given in Section 2, along with potential advantages and
requirements. A brief case study is then presented, to
demonstrate the feasibility of the concept.
2.
the filling station. This will reduce the need of tank transportation to a minimum, since the hydrogen tanks will be
stored at the filling station during and after filling. Furthermore, this solution can be coupled with local production of
hydrogen at the filling station, eliminating the need to transport hydrogen completely. The best way of producing
hydrogen would then be by water electrolysis, using grid
electricity (or local electricity sources) as the energy input.
Since heat will be produced during hydrogenation of the
tanks, high-temperature electrolysis should be used in order
for this heat to be exploited; we will come back to the details of
this later. An outline of the concept is shown in Fig. 1.
This concept has many advantages over solutions traditionally proposed for hydrogen fuelled vehicles. One benefit is
that recharging of the storage units may be done during night
and low-peak periods, which opens for utilization of low-cost
electricity from the grid [28]. Using electricity to transport
energy is furthermore relatively simple to introduce, and in
many cases the existing grid would be suitable at least during
initial phases. Electricity also implies flexibility regarding the
energy source, and is particularly suitable for renewable
energy sources producing electricity. The concept is less
suitable for fossil energy sources; production of hydrogen
from fossil fuels should most probably be performed centrally
in a single process, to avoid loss of efficiency and facilitate
carbon sequestration.
A refuelling system involving removal and insertion of
metal hydride tanks between vehicles and the filling station
obviously imposes special requirements on both the vehicles,
the filling station, and their interface. An integrated design
with standardized solutions is crucial to achieve global
implementation of the solution. A more detailed discussion of
the advantages and requirements of the concept is presented
in what follows.
Description of the concept
The crucial point in making solid state storage of hydrogen
viable for vehicular applications is to move the recharging
step out of the vehicle. This has usually been associated with
transportation of the vehicle tanks to a central processing unit
where recharging is to be performed, which is inevitable for
chemical hydrides and non-reversible metal hydrides like
AlH3 [27]. However, such a solution implies large challenges
related to transportation and logistics of hydrogen storage
tanks, and many of the participants in the field are reluctant to
pursue this idea.
The alternative solution which will be advanced in this
paper, however, is to recharge the hydrogen tanks locally at
Fig. 1 – Outline of an automated filling station for hydrogen
fuelled vehicles with solid state hydrogen storage. An
empty storage tank is automatically replaced by a filled
one. Independently, filling of storage tanks is taking place
in a hydrogenation unit. The hydrogenation unit is closely
linked with an electrolysis unit, which produces hydrogen
from water and electricity. The electrolysis unit makes use
of heat from the hydrogenation process, which enhances
efficiency. Any additional heat from the hydrogenation
process can be used for other purposes.
international journal of hydrogen energy 34 (2009) 2679–2683
Starting with the vehicle, the first and most important
benefit is that refuelling may in principle be done during
seconds, since only storage units need to be replaced at the
filling station. This compares very favourably with other
hydrogen storage systems; even refilling a tank with liquid
hydrogen, which ideally is as fast as refuelling gasoline, takes
longer time – in the order of a minute [29]. Filling a highpressure hydrogen tank takes many minutes, especially if
complete filling is to be achieved. The filling time will be
additionally increased if the pressure range is increased to
700 bars. The most dramatic speedup, however, is when
compared to on-board refuelling of solid state hydrogen
materials. Due to limitations of hydrogenation kinetics and
heat management in material based hydrogen storage tanks,
refuelling them can take significantly longer time than filling
a high-pressure tank. Complex hydrides, which include the
materials with highest gravimetric hydrogen density, exhibit
particularly slow kinetics. It has been claimed that this
problem may be the ultimate show-stopper for vehicular
hydrogen storage in materials.
In order to facilitate efficient change of tanks, the tanks
should have a universal design. Vehicles of different size
could then contain a number of tanks corresponding to their
size. This means that a method of switching from one tank to
another as the hydrogen source in large vehicles during
driving operation will be needed. It also means that all tanks
should be easily accessed from the vehicle at the filling
station, for efficient replacement of the tanks. A standardized
placement of tanks (or rather, of their entrance channel)
would further enable adequate design of the vehicle-filling
station interface. Closely linked to this is the launching
system for replacing the tanks. This should be automated,
both since the tanks will be heavy, and to avoid risks associated with manual management. Also, a wireless communication system would most likely be needed for the interface to
be efficient. This could assist in the alignment between
vehicle and delivery system, and aid during the process of
exchanging tanks. Such a communication system could serve
additional roles, including automatic payment.
An automated system will also contribute to reducing the
overall risk related to hydrogen as a fuel, since no manual
handling is required. Adding to this, the concept implies that
no highly pressurized hydrogen is needed during the refuelling process. This is particularly beneficial compared to onboard filling of gas tanks or material based tanks, where highpressure hydrogen must be introduced all the way to the
vehicle.
Management of heat in the order of MW involves large,
heavy, and relatively expensive equipment. To have on-board
rehydrogenation would mean that such heat management
units were needed in every vehicle. When the process is
moved to a central unit at the filling station, however, this
dramatically reduces the need for the size of such an apparatus on-board the vehicle. The remaining requirement of the
on-board heat management system is to provide sufficient
dehydrogenation rates during driving operation.
Moving to the filling station, this will need to have at least
three main parts in addition to the interface with the vehicle:
a water electrolysis unit for hydrogen production, a hydrogenation unit filling tanks with hydrogen, and a storage unit for
2681
empty and filled tanks. All movement of tanks should be
robotized. Furthermore, the electrolysis and hydrogenation
units should be closely linked, allowing for precise management of heat and pressure, which can simultaneously optimize
the conditions for hydrogen production and hydrogenation of
tanks. Thus the overall energy consumption for hydrogen
production and filling of tanks can be minimized.
To perform such an optimization, it is necessary to
compare the amount of heat released during the hydrogenation step to that needed for high-temperature water electrolysis. We assume that the same amount of hydrogen is
produced by electrolysis as is absorbed in the hydrogen
storage material. The target of the hydrogenation enthalpy of
the storage material (hydride) is often stated to be around
40 kJ/mol H2 [11] or 20 MJ/kg H2; this translates to an equilibrium pressure of around 1 bar at room temperature, and
would be ideal for a fuel cell operating at or below 100 C. It is
instructive to compare this to the energy required for a hightemperature solid oxide electrolysis cell (SOEC). The enthalpy
of heat for hydrogen gas at a typical temperature for a SOEC,
800 C, is 123.8 MJ/kg H2. Water electrolysis can at this
temperature run with a cell voltage down to 1.1 eV, which
corresponds to a theoretical electricity consumption of only
105.3 MJ/kg H2. The deficiency of energy must be supplied in
the form of heat; thus approximately 18.5 MJ/kg H2 heat is
needed to keep the reaction running at this temperature
(thermoneutral condition). Taking losses of a few percent into
account, this is exactly the amount of heat which is available
from the hydrogenation reaction! Such a fortuitous coincidence seems to make the proposed concept particularly suitable also from a thermodynamic point of view: all the heat
energy from the hydrogenation reaction which otherwise
would have been lost, could be directly utilized as energy
input to the electrolysis process.
However, the temperature of the hydrogenation waste
heat is considerably lower than 800 C; we have required that
it be around 300 K at atmospheric pressure. The equilibrium
temperature can be increased by increasing the hydrogen
pressure. This is governed by the van’t Hoff equation
ln(P1/P0) ¼ DH/RT þ DS/R, where R is the gas constant, P is the
equilibrium pressure at a given temperature T, and DH and DS
are the enthalpies and entropies of hydrogenation, respectively. From this we see that the equilibrium temperature can
fairly easily be increased to 100 C by using a pressure of
approximately 30 bar. However, to reach an equilibrium
temperature of 800 C, a hydrogen pressure of more than
100 kbar is necessary. This is clearly not feasible, and the
output temperature of the hydrogenation unit will thus be
lower than what would be required for a high-temperature
SOEC.
Alternatively, a hydride with larger hydrogenation
enthalpy could be used. If changing DH to 60 kJ/mol H2, the
equilibrium temperature is 177 C at atmospheric pressure,
which should provide enough heat to move into the regime
of steam. It has been shown that electrolysis operation at
230 C can significantly increase efficiency [30], a temperature
that can be reached with a pressure of only 5 bar if
DH ¼ 60 kJ/mol H2.
It is important that the working temperature and pressure
of the hydride are also matched to the fuel cell on-board the
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international journal of hydrogen energy 34 (2009) 2679–2683
vehicle; that is, the on-board fuel cell and the off-board electrolysis cell should operate at comparable temperatures. Even
if PEM fuel cells are reckoned as most promising today, it is
conceivable that fuel cells with higher temperature can be
operable in the near future [31].
It should be emphasized that this concept is also relevant
for hydrogen fuelled vehicles running on internal combustion
(IC) engines. The amount of waste heat from IC engines is
significantly larger than that available from fuel cell vehicles,
and the temperature is also higher. Thus, releasing hydrogen
from a hydride storage tank by utilizing waste heat should be
straightforward also in the case of IC hydrogen vehicles.
A change of the target for hydrogenation enthalpy to 60 kJ/
mol H2 could mean new possibilities for hydrogen storage in
materials. Many of the lightest hydrides are too stable for use
together with PEM fuel cells [8], but could be much better suited
to this concept. Also, the problem with poor hydrogenation
kinetics is not so imperative when moving the hydrogenation
out of the vehicle, since the time needed for recharging of
a tank can be adapted to the particular hydride being used. In
order to achieve the hydrogenation kinetics required for
a typical filling station, the number of parallel hydrogenation
units can simply be changed – we will come back to this in the
case study of the next section. The number of useable hydrides
can thus with this scheme be dramatically increased; the most
important limitation will most probably be the kinetics of
dehydrogenation in driving mode.
Since the waste heat from the hydrogenation reaction has
too low temperature to be completely utilized by the electrolysis process, there will probably be a net excess heat from
the filling station. This can represent either a problem or an
advantage depending on the circumstances. The excess heat
can improve the economy and efficiency of the filling station if
used for applications in other areas like district heating,
cogeneration, etc.
Another opportunity of improving economy lies in the
produced oxygen. If this is utilized for e.g. medical use or other
industry, the economic feasibility could be further enhanced
[32,33].
Finally, since such filling stations will normally be grid
connected, they can contribute with achieving peak power
control of the electrical grid [34], by reversing electrolysis to
produce electricity when electricity demand and prices are
high.
3.
Case study
In order to illustrate the viability of the concept, a very simple
case study will be presented in this section. It is emphasized
that this is by no means meant to be realistic; its main purpose
is to demonstrate that the magnitudes of the various system
components can be well harmonized to each other.
A small to medium sized filling station is envisioned, with
the ability to serve 120 vehicles per day, each vehicle filling
5 kg H2. With a driving range of around 100 km/kg H2, this
translates to a petrol filling station serving the same number
of vehicles with totally approximately 6000 l of petrol or diesel
per day (assuming an average driving range of 10 km/l petrol
or diesel). Averaged over the whole day this means that
a continuous hydrogen production of 25 kg/h is required. Note
that this is significantly less than the hydrogen flux needed for
on-board filling of hydrogen, which must be in the order of kg/
min if excessive filling time is to be avoided. This means that,
even if only one tank is to be filled at a time, the hydrogenation
kinetics can be considerably decreased compared to a system
in which rehydrogenation must happen on-board the vehicle.
To further decrease kinetics requirements, or to scale up
production, several tanks can be loaded in parallel.
As mentioned above, high-temperature electrolysis requires
a specific power input in the order of 100 MJ/kg H2. For the case
study this means a continuous electricity consumption of
slightly less than 1 MW for the production of hydrogen. It is
relieving to note that this will be accessible at many places using
existing grid infrastructure; as an example, 20 kV electricity
lines have capacities up to 10 MW. For large filling stations
a higher voltage line would be needed, which can have capacities up to hundreds of MW or even the GW range.
When it comes to the hydrogenation unit, the amount and
temperature of heat that must be removed depends on the
hydride material which is chosen. For a material with
DH ¼ 40 kJ/mol H2, the heat removal rate will in average be
140 kW for this test case. If the system is working at around
20 bar, the corresponding temperature will be around 100 C.
This changes to 210 kW at 280 C if DH is changed to 60 kJ/mol
H2, and 245 kW at 310 C if DH ¼ 70 kJ/mol H2. Even the lowest
of these rates is probably too high to be achievable in a single
tank, and hydrogenation should most likely take place in
several tanks in parallel. The rate of heat is too high to be fully
exploited in the electrolysis process, and the excess heat will
have to be either discarded or utilized e.g. as district heating.
4.
Summary
It has been demonstrated how the challenge of hydrogen
storage for transportation can be solved by using material
based storage (hydrides) with rehydrogenation taking place
off-board, at hydrogen filling stations. This should be
combined with local production of hydrogen from water
electrolysis, using waste heat from the hydrogenation process
to increase the efficiency of electrolysis. In this way, energy is
transported to the filling station as electricity, and no transport of hydrogen is necessary.
Challenges remain before this can be implemented in large
scale; the most important are how to exchange the hydride
tanks with the vehicle, and how to optimize the pressure and
temperature needed during electrolysis and rehydrogenation.
However, these are technical challenges that most likely can
be solved. This is different from some of the problems still
facing other proposed solutions to the storage problem, which
are physical restrictions that possibly never can be solved: the
poor volumetric density of gaseous hydrogen storage, large
losses from boil-off associated with liquid hydrogen storage,
as well as long filling times combined with excessive heat
removal rates required by solid state hydrogen storage with
on-board refuelling.
The advantages with the concept are many and profound:
In addition to increasing the overall efficiency of hydrogen
production and storage, it can significantly speedup the filling
international journal of hydrogen energy 34 (2009) 2679–2683
process, since only tanks are exchanged at the filling station.
This also means that safety during filling is improved, and that
transportation of hydrogen is virtually eliminated. Last, but
not least, this could open the arena for many new hydrogen
storage materials with high hydrogen density, possibly
leading to a breakthrough in solid state storage of hydrogen.
Implementation of the proposed concept may thus mean
a major step towards successful introduction of hydrogen as
a fuel for transportation.
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