AckermannTh. (ed) Wind Power in Power Systems
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wind farms, however, can hardly be called simple. Further research, in particular
regarding system solutions that focus on integrating wind turbine design and transmis-
sion solutions, is certainly recommended to create simpler electric design concepts for
offshore wind farms.
Acknowledgement
I would like to thank Per-Anders Lo
¨
f for valuable comments as well as all the presenters
and participants in the Stockholm and Billund workshops for their contribution.
References
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Large-scale Integration of Wind Power and Transmission Networks for Offshore Wind Farms, Eds
J. Matevosyan and T. Ackermann; Royal Institute of Technology, Stockholm, Sweden.
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23
Hydrogen as a Means of
Transporting and Balancing
Wind Power Production
Robert Steinberger-Wilckens
23.1 Introduction
Today’s electricity power generation is based on a complex system of frequency and
voltage control and electricity exchange between subgrids. Power production is ruled by
a time schedule with about 24-hour ahead prediction. The schedule takes the forecast of
load and basic meteorological parameters into account, which are both today fairly well
understood. The intro duction of a fluctuating energy source as constituted by solar or
wind electricity introduces an additional stochastic component to power system sched-
uling. This may lead to power mismatch in the case of conflicts with the flexibility of
conventional (backup) power production. As a result, additional control power (and
energy production) is required from conventional, fast-responding electricity gener-
ation; otherwise, renewable energy pro duction is lost. Such losses also occur when
renewable energy potential is ‘stranded’ as a result of limitations in transmission
capacity, the impossibility of installing transmission lines and so on. Questions relating
to these issues are receiving growing interest in the context of offshore wind projects.
To avoid the spilling of renewable energy production, an energy storage medium
needs to be incorporated into the generation system in order to allow flexible usage of
the power generated. Hydrogen offers several interesting characteristics in this context:
.
It can be reconverted to electricity with a reasonably high efficiency if it is used as fuel cells.
.
It enables peak power production and load following, either from central installations
or from virtual power stations (i.e. it offers decentralised generation capacity).
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.
It can constitute an alternative means of energy transport (e.g. using pipelines where
electricity cables are undesirable) while offering high energy density and low transport
losses.
.
It can be sold as indust rial gas outside the electricity market; thus, on the one hand, it
reduces market pressure and, on the other hand, develops alternative markets for
renewable energies (e.g. in transport fuels).
23.2 A Brief Introduction to Hydrogen
Hydrogen is the elem ent with the lowest atomic weight. Compared with the next lightest
gaseous fuel relevant in power production – natural gas (methane) – it has a heating
value of roughly one third with respect to volume and more than double with respect to
weight. Table 23.1 presents some physical and chemical properties of hydrogen in
comparison with methane (natural gas) and propane. The aspects that are most import-
ant in the context of power production are energy density (defining the storage density
and volume of storage, and the transportation energy effort) as well as ignition limits,
diffusion coefficient and explosion energy (safety concerns).
The important role hydrogen plays in the context of renewable energy arises from the
fact that it can be prod uced easily from water and electricity by the process of electro-
lysis. It can then either be burned as a fuel as a substitute for gaseous fossil fuels or be
converted to electricity in fuel cells in an electrochemical process that exceeds the
efficiency of conventional electricity generation. Within certain limitations, hydrogen
can thus be used for storing renewable electricity and can either be sold off as a product
in its own right or be reconverted to electricity.
Table 23.1 The physical and chemical properties of hydrogen as compared with methane and
propane
Properties Gas
Hydrogen (H
2
) Methane (CH
4
) Propane (C
3
H
8
)
Density (kg/Nm
3
)
a
0.0838 0.6512 1.870
Lower heating value:
kWh/kg 33.31 13.90 12.88
kWh/Nm
3
2.80 9.05 24.08
Upper heating value:
kWh/Nm
3
3.30 10.04 26.19
Diffusivity in air at NTP (cm
3
/s) 0.61 0.16 0.12
Ignition energy (mJ) 0.02 0.29 0.26
Explosion limits in air (vol %) 4–75 5.3–15.0 2.1–9.5
Explosion energy [(kg TNT)/m
3
] 2.02 7.03 20.5
Autoignition temperature (
C)
b
585 540 487
a
Density for a normal cubic metre at 293.15 K, 0.101 MPa.
b
For comparison, the temperature range for gasoline is 228–471
C.
Note: NTP ¼ normal temperature and pressure; Nm
3
¼ norm cubic metres.
Sources: Alcock, 2001; Fischer, 1986.
506 Hydrogen
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An inspection of Table 23.1 shows that hydrogen has a rather low volumetric energy
density (0:0838 kg/Nm
3
), even though the energy density per unit weight (33.31 kWh/kg)
is the highest of all energy vectors. Owing to its low weight, hydrogen disperses very
quickly in the atmosphere (0:61 cm
3
/s). This offsets the low ignition energy (0.02 mJ)
and the unfortunately wide limit of the range of explosive mixtures with air (4–75 vol %) –
much wider than that for the oth er fuel gases listed. The use of hydrogen in closed spaces
(laboratories, garages and so on) therefore calls for appropriate safety measures (e.g. gas
sensors). The explosion energy (2:02 kg TNT/m
3
), though, is considerably lower than
that for methane (7:03 kg TNT/m
3
) for instance. In addition, hydrogen causes problems
with ferric materials, giving rise to embrittlement.
The specific security precautions for hydrogen are not too great. The hazards asso-
ciated with accident s involving hydrogen are not greater than those involving conven-
tional fuel (in some aspects, though, the hazards are different) and are manageable. The
widespread use of hydrogen in industry and the existing transport pipelines and net-
works that have been operated over many decades have proved this (see Section 23.3.3).
23.3 Technology and Efficiency
23.3.1 Hydrogen production
Hydrogen can be generated directly from electricity by using electrolysis. In an electrolytic
solution, water is split into its components – hydrogen and oxygen – at two electrodes.
The gases are produced separately at the electrodes and have a high purity. Electrolysers
can be operated at pressures between ambient pressure and 200 bar. The high-pressure
processes are more efficient but also lead to higher equipment costs and more complicated
systems. However, they can feed a pipeline system directly without any additional com-
pression. Electrolysis requires an input of desalinated and demineralised water.
Electrode lifetime, mechanical stability and process efficiency suffer when input
power varies. For this reason, electrolysers conventionally are operated at constant
rated power. During shutdown, standard electrodes will corrode unless a protective
voltage is applied. This requires an electricity backup. Nowadays, new improved elec-
trode materials (e.g. catal ytic laminated nickel plates) have been developed (Steinberger-
Wilckens and Stolzenburg, 2000). With these electrodes, no protective voltage is
required for stabilisation.
The efficiency of electrolytic hydrogen production is about 65 %, resulting in
4.2–4.8 kWh of energy cost for the electrolysis of 1 Nm
3
of hydrogen at atmospheric
pressure [referring to the lowe r heating value (LHV)]. Using high-temperature and
pressurised electrolysis as well as ad vanced materials, efficiency can be raised consider-
ably, by up to 85–90 % (Pletcher and Walsh, 1990; Wendt and Bauer, 1988).
Electrolysers are available at various sizes and operating pressures. The largest
installations realised to date were rated at around 150 MW of electric power input and
consisted of 2 MW units. Large-scale electrolysis is used predominantly in the synthesis
of ammonia for the manufacture of fertiliser, the two largest installations being in India
and Egypt, both fed by hydro power (Wendt and Bauer, 1988). Depending on process
pressure and system requirements, the hydrogen is either stored directly or is first
pressurised.
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Electrolysers are generally made of high-quality materials that have to be corrosion
proof if applied in a marine environment. Today, they are already used on drilling
platforms and produce hydrogen for welding and cutting.
As well as direct production of hydrogen by electrolysis, other processes of synthesis-
ing hydrogen-rich products are potentially possible. These products would ideally dis-
play a higher volumetric energy content, thus reducing the amount of storage volume
and/or transport energy. Options include the synthesis of products such as ammonia,
methanol, hydrazin and borates (Peschka, 1988). Their production will require the
supply of additional base materials, such as nitrogen or carbon dioxide, which may
both be obtained from the air (with an additional energy cost). In the case of borates,
closed cycles can be established (Mohring, 2002), which implies that the ‘used’ hydrogen
carrier will be returned to the site of the hydrogen source.
23.3.2 Hydrogen storage
Once gaseous hydrogen is produced, the question arises over what type of storage to use.
Hydrogen can be stored in a variety of ways, each with its specific advantages and
disadvantages. The main criteria are safety, efficiency, energy density and volume. In the
following, the different storage methods that are commercially available today are described.
23.3.2.1 Compressed hydrogen
Compressed gaseous hydrogen (CGH
2
) is stored in high-pressure tanks. This requires
compression energy corresponding to 4–15 % of the hydrogen energy content stored,
depending on the intake and outlet pressure of compression (Feck, 2001). Storage volume
per unit of energy is generally high because of the low energy density of compressed
hydrogen compared with that of, for instance, gasoline. Today, high-pressure tanks are
being developed to achieve 700–1000 bar and thus reduce geometric volume. This raises
safety concerns which, in turn, require enhanced security measures. Results from the design,
development and testing of composite-material vessels, though, have shown the technical
feasibility and the manageability of the risk of such an approach (Chaineaux et al., 2000).
23.3.2.2 Liquid hydrogen
Liquid hydrogen typically has to be stored at 20 K (253
C), leading to a considerable
energy cost for compressing and chilling the hydrogen into its liquid state. The cooling
and compressing process results in a net loss of about 30 % of energy (Wagner, Angloher
and Dreier, 2000). The storage tanks are insulated, to maintain the temperature, and are
reinforced to store the liquid hydrogen under (low) pressure. Given the energy required
for liquefaction and the cost of maintaining storage pressure and temperature, liquid
hydrogen storage is expensive in comparison with other aggregate forms.
23.3.2.3 Metal hydrides
Metal hydrides are created from specific metallic alloys that can incorporate hydrogen
into their metal lattice, emitting he at in the process. The hydrogen can be released again
508 Hydrogen
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through heating the metal hydride vessel. The total amount of hydrogen absorbed is
generally 1–2 % of the total weight of the storage medium. This surprisingly low figure is
attributable to the disparity between the very low weight of hydrogen and the high
weight of the metal alloys. In terms of volumetric storage capacity, metal hydride tanks
perform similarly to liquid hydrogen (storing 60 and 70 kg H
2
/m
3
, respectively). Some
metal hydrides are capable of storing 5–7 % of their own weight, but require unloading
temperatures of 250
C or higher (Carpetis, 1988).
The main disadvantage is that the inert mass of metal has to be installed or even
moved around (in the case of vehicle tanks or transport vessels). Still, hydrides offer an
interesting solution for the safe storage of hydrogen as they have the advantage of
delivering hydrogen at co nstant pressure (30 to 60 bar) over a broad range of dischar-
ging levels. The lifetime of a metal hydride storage tank, though, is related directly to the
purity of the hydrogen, since the hydriding process will suffer from impurities that are
deposited in the crystal lattice.
23.3.2.4 Storage systems
Generally, storage containers for gaseous hydrogen are divided into several compart-
ments that are shaped in spheres or, most commonly, cylinders (‘bottles’). These are
then connected into ‘banks’ of storage vessels. A storage container may include several
‘banks’ in order to maintain as constant a pressure as possible during unloading (e.g. in
filling stations). This is the most cost-effecti ve way of pressure storage since the cylinders
are standard industry equipment manufactured in large numbers. More rarely, single-
vessel pressure tanks are found.
Liquid hydrogen, in contrast, is stored in cryotanks, which are built as compact as
possible in order to save costs and minimise losses from evaporation. They do not
usually include a refrigeration unit. The slow warming of the tanks results in an over-
pressure from the formation of a gaseous phase, which regularly has to be relieved by
‘blowing off’ the gas. For large tanks, the rate of evaporative loss amounts to about
0.1 % per day. For smaller, road transport, tanks it still remains below 1 %.
Tanks and all interconnection equipment (such as valves, piping, washers and so on)
are available in materials that are suitable for a marine installation.
23.3.3 Hydrogen transport
Hydrogen is generally transported in the compressed gaseous (CGH
2
) and liquefied
(LH
2
) aggregate state. In the case of small-scale deliveries for industrial purposes,
compressed hydrogen is typically trans ported by vehicle. Bulk delivery generally stems
from hydrogen production at customer-site production plants – so called ‘captive
hydrogen’ – [e.g. via electrolysis or steam reforming of hydrocarbons at refineries,
chemical plants and in the pharmaceutical industry; see Zittel and Niebauer, 1998].
Gaseous hydrogen is also transported via gas pipelines. Today, there are hydrogen
distribution systems in the North of France, in the Benelux region (Belgium, The
Netherlands and Luxemburg) and in Germany, for instance. It might be also an option
to adjust the na tural gas pipeline network in Europe for the transport of hydrogen.
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Modifications to, for example, seals and connections because of the lower viscosity of
hydrogen will be necessary in order to prevent losses and to secure a safe operation.
Hydrogen losses during pipeline transport are very low, at about 0.001 % per 100 km
(Wagner, Angloher and Dreier, 2000). However, the required compression energy to
transfer hydrogen in order to deliver a given energy amount at a given time is ten times
higher than that for natural gas (Feck, 2001).
For the transport of liquid hydrogen by sea, marine vessels will have to be used –
so-called ‘barge carriers’ – which in principle consist of large fibreglass vacuum-insulated
tanks. The storage capacity is about 15 000 m
3
. The energy consumption of such vessels
is about 400 kWh/km (Feck, 2001). The losses can be estimated at 0.1 % per day (Gretz,
2001) or 0.013 % per 100 km (Feck, 2001) of the transported energy. The subsequent
onshore transport of LH
2
by road will consume 4 kWh per km (Fritsche et al., 2001),
with an effective mass per vehicle of 3.5 t. Liquid hydrogen losses from ‘boil-off’
(i.e. evaporation in the steady state) is estimated to average 0.3 % per day (Wagner,
Angloher and Dreier, 2000).
This comparison shows that transport via pipeline would be by far the most energy-
efficient means of delivery. If the hydrogen were transported in a pipeline over a
distance of 100 km offshore followed by 150 km transport on land, pipeline delivery
would ‘cost’ 6.5 % of the energy content of the hydrogen gas flow. Transport by barge in
pressure vessels at 350 bar or in cryogenic tanks for LH
2
followed by rail transport
(preferably using the same containers) would amount to losses of around 17.5 % and
33 %, respectively, of the hydrogen energy content if finally delivered at 20 mbar
pressure (as required for most end-use appliances). If delivery at a pressur e of 350 bar
is necessary (e.g. for hydrogen vehicle-filling stations) the figures change to 16.5 % for
transport by pipeline, 17.5 % for use of pressure vessels and 48 % for use of cryogenic
tanks, because of the final compression for achieving the required end-pressure (all
values here have been calculated with use of data from Feck, 2001; Wagner, Angloher
and Dreier, 2000).
Finally, hydrogen does not need be transported in a ‘pure’ state or in an infrastructure
that is separate from the conventional energy supply system. Similar to town gas that
was used until the middle of the last century in many parts of Europe (and still is used,
for instance, in Stockholm), hydrogen can be mixe d with natural gas and thus reduce the
greenhouse effect of natural gas. We will not discuss the implications of this here, but
will refer anybody inter ested to the NaturalHy project led by GasUnie in the Nether-
lands, a European level integrated project starting in the year 2004.
23.4 Reconversion to Electricity: Fuel Cells
Hydrogen can be reconverted to electricity by means of conventional gas-fired equip-
ment, such as gas motors [e.g. in combined heat and power (CHP) plants] or gas
turbines. These processes have a rather limited electrical efficiency, of 28–35 %. The
total efficiency of converting electricity to hydrogen (electrolyser) and back to electricity
will lie in the range of 18–24 %, neglecting transport losses. Heat recovery in CHP
increases the overall efficiency but the original energy content of the elect ricity gener-
ated from wind power will still be wasted to a large extent.
510 Hydrogen