Low-cost H2 storage options for solar H2 systems for RAPS
Ali
Low-cost hydrogen storage options for solar hydrogen systems for
remote area power supply
Suhaib Muhammed Ali and John Andrews,
School of Aerospace, Mechanical and Manufacturing Engineering
RMIT University,
Bundoora, Melbourne 3083,
AUSTRALIA.
E-mail:s3092612@student.rmit.edu.au
ABSTRACT: Hydrogen obtained from renewable energy sources offers the prospect of a sustainable
energy source free of greenhouse gas emissions. Solar-hydrogen-based power supply systems in remote
areas are a potential early market for such technologies, because of the high cost of conventional fuels in
these areas. The present paper investigates the potential for developing competitive solar-hydrogen
systems for remote area power supply (RAPS) employing photovoltaic (PV) panels, proton exchange
membrane (PEM) electrolysers and fuel cells, and hydrogen gas storage. Since volume constraints for
hydrogen storage systems are generally more relaxed in RAPS applications compared to vehicles, a
number of low-pressure – and hence low cost – storage options are investigated experimentally and using
a system simulation model. These options include low and medium pressure metal cylinders, polyethylene
tanks, and modified gasometers. Two different strategies for determining storage capacity are compared:
unconstrained storage that is, allowing sufficient capacity to store all the hydrogen produced by excess PV
power over load; and constrained storage, that is, limiting storage capacity to an economic minimum. The
unit costs of generated power is evaluated for a range of constrained storage capacities assuming the unit
cost of all other components, namely electrolyser, fuel cell, and balance of system, remain constant within
accepted ranges of values. It is found that the unit cost of power generation with constrained storage is
about 35% lower than that for unconstrained storage for the unit costs assumed. However, this option
involves a very large installed PV area and a relatively small hydrogen storage capacity. If lower-cost
storage systems can be developed, the PV area could be reduced and storage capacity increased with a
consequent lowering of overall unit costs of power supplied.
KEYWORDS: Solar hydrogen, RAPS, PEM Fuel cell, PEM electrolyser, Hydrogen storage, PV panel.
1 INTRODUCTION
The production of hydrogen from renewable energy sources has the advantage of zero or low greenhouse
gas emissions, long term sustainability, security of supply, and widespread availability on a global scale.
Remote area power supply (RAPS) is a potential early market for solar-hydrogen systems because of the
comparatively high cost of conventional energy sources such as diesel generators in remote regions
isolated from main electricity grid. The prospects for solar-hydrogen systems have been further boosted
by the development of proton exchange membrane fuel cells and electrolysers, which both operate with
solid-state electrolytes [Aurora & Duffy 2005; Khole et al. 2003; Lehman et al. 1994].
A number of studies and demonstration projects have focussed on solar hydrogen systems employing
photovoltaic (PV) panels, proton exchange membrane (PEM) electrolysers, storage of hydrogen as
compressed gas and PEM fuel cells [Khan & Iqbal 2005;Kolhe et al. 2003]. It has been found that the cost
of storage forms a significant proportion of total costs of solar hydrogen based raps systems. Indeed the
cost of storage and the associated PEM electrolyser and fuel cell is typically so high that only short-term
storage of hydrogen is economically viable.
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Low-cost H2 storage options for solar H2 systems for RAPS
Ali
In the present paper we describe a simple spreadsheet-based model to size a solar-hydrogen RAPS
system in general and in particular the storage of hydrogen required for supplying a remote homestead.
The implications of unconstrained storage, that is, ensuring there is sufficient capacity to store all the
hydrogen produced by excess power supply over demand, and constrained storage, limiting storage to a
predetermined capacity, for photovoltaic panel sizing and the overall economics of the delivered energy
are discussed. It is found that low-cost storage options are required to make the storage of hydrogen on a
season-to-season basis viable and hence lower unit costs of the electricity supplied. In many remote
applications there is space for relatively large-volume storage systems. Thus some potentially feasible
and low-cost options for storing hydrogen at relatively low pressure such as acrylic cylinders for laboratory
scale work, plastic tanks or containers for larger volumes, and metal cylinders are investigated. The
effects of these low-cost storages on the unit cost of power generation are discussed.
2
MODELLING SOLAR HYDROGEN STORAGE REQUIREMENTS
2.1 A spreadsheet model of a solar hydrogen RAPS system
Figure 1. Schematic diagram of solar hydrogen system.
A schematic diagram of a typical solar-hydrogen system for remote area power supply is shown in Figure
1. The electrolyser is used to produce hydrogen for storage only when there is surplus power from the PV
panel and available storage capacity. The hydrogen is stored in gaseous form. If the storage is full, a
pressure sensor is used to switch off the electricity supply from the PV panel to the electrolyser.
We have developed a spreadsheet model of the operation of such a system with hourly solar radiation
and load to be supplied, and the characteristics of the PV panel, electrolyser, storage system, and fuel cell
as inputs [Andrews et al. 2005]. These characteristics are based on manufacturers’ predicted
performances and independent experimental testing in our laboratory. The model can be used to
determine the area of the PV array, and capacities of the electrolyser, storage, and fuel cell, to meet the
load on a sustainable annual basis [Andrews et al. 2005].
This model has been used to size a solar hydrogen system to meet an idealised electrical demand for a
remote homestead in southern Victoria, Australia. It is assumed conservatively that the total electrical
requirement is constant at 5 kWh/day with a steady load during the day between 6am and 10 pm of 0.253
kW, and a steady but lower load during the night from 10 pm to 6 am of 0.1 kW.
2.2 Unconstrained Storage
In the first case we have examined, it is assumed that there is enough storage capacity to allow all the
hydrogen produced by the excess PV power over the load to be stored for later use. Under this
‘unconstrained storage’ condition, the minimum photovoltaic area required to supply the total load over a
full year, drawing upon hydrogen from storage and the fuel cell to meet the demand when the PV output is
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Low-cost H2 storage options for solar H2 systems for RAPS
Ali
insufficient, is found. In this case, the area of PV panel to meet the annual load tends to be minimized,
while relatively large hydrogen storage capacities are required. The following results were obtained:
•
•
•
A minimum PV panel size of 10.1 m2.
A maximum surplus power of 0.92 kW, so that electrolyser capacity of 1kW would be sufficient.
A required storage capacity of 9.76 kg of hydrogen.
Hence if the hydrogen gas is stored at 25 ºC at and atmospheric pressure, storage with a volume of 117
m3 would be needed. If the hydrogen is compressed to 10 bar a storage volume of only 11.7 m3 is
sufficient.
12
Hydrogen stored(kg)
10
8
`
6
4
2
0
Jan
Feb
Mar Apr
May June
July
Aug
Sep
Oct
Nov
Dec
Figure 2: Annual profile of hydrogen in storage - unconstrained storage condition.
The corresponding variation of hydrogen in storage over a full year is shown in the Figure 2. The
thickness of the curve (in the order of 0.5 kg of hydrogen) is determined by the amount of hydrogen
needed to meet the nighttime demand via the fuel cell, that is, the regular daily variation in amount of
hydrogen stored. The annual variation is approximately sinusoidal with the store being run down to zero at
the end of the winter (end of August in the southern hemisphere) and rising to a maximum after the end
summer in March. Importantly, under the unconstrained storage condition, there is considerable storage of
hydrogen on a relatively long term summer-to-winter basis. Such longer term energy storage would not be
practical with a battery storage system because most batteries will not hold their charge without regular
recharge for longer than a few weeks.
The overall unit cost of power delivered by this system has been estimated on the basis of the following
assumptions:
‰ PV panels with a capital cost of US $5000/kW and a lifetime of 25 years
‰ A PEM electrolyser with a capital cost of US $3000/kW and a 20 year lifetime.
‰ A PEM fuel cell with a capital cost of US $6000/kW and a lifetime of 15 years.
‰ A balance of system (inverter, control system, piping, valves etc.) capital cost of US$6000 and a
lifetime of 25 years.
‰ Annual operating and maintenance costs equal to 2% of capital cost for each of these items.
‰ A real discount rate of 5%.
The unit cost of power delivered was then calculated for a range of capital costs of storage from
$US2000/kg to $US500/kg (Figure 5). At $US2000/kg the unit cost was found to be $US 1.77 /kWh falling
to $US 1.04/kWh at US $500/kg.
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Un it co st o f p o w er($US)
Low-cost H2 storage options for solar H2 systems for RAPS
Ali
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
500
1000
1500
2000
2500
Storage cost($US)
Figure 3: Unit cost of power vs. storage cost – unconstrained storage
2.3
Constrained Storage
In the second case examined, we fixed the storage capacity at the beginning of a model run, and then
determined the PV area to meet the demand throughout the year. Under this condition with a storage
capacity much lower than those used in the ‘unconstrained storage’ runs, clearly there are times when the
storage is full so that not all the available surplus PV power is used to produce hydrogen. With less
storage, a higher proportion of the annual load must be met directly from the PV array. Hence generally
larger PV areas are required compared to unconstrained storage. The results obtained from the model for
a preset storage capacity of 0.7 kg of hydrogen and an electrolyser capacity of 1 kW were:
2
‰ A PV area of 18.40 m .
‰ Not all the surplus power available from PV panel is fully utilized to produce hydrogen, as
expected. The maximum surplus power is 1.88 kW, well above the electrolyser capacity of 1 kW.
At atmospheric pressure and 25oC, 0.7 kg of hydrogen will occupy a volume of 8.4 m3, or 0.84 m3 at 10
bars.
H y d ro g e n s to re d (k g )
0.9
0.8
0.7
0.6
`
0.5
0.4
0.3
0.2
0.1
0
Jan
Feb
Mar Apr
May June July
Aug Sep
Oct
Nov
Dec
Figure 4: Annual profile of hydrogen in storage - constrained storage (0.7 kg maximum) condition.
The annual variation of hydrogen in storage under the constrained storage (0.7 kg maximum) condition is
shown in Figure 4. Storage is still used to meet the nighttime demand, hence the daily variation in the
hydrogen stored (the thickness of the line), but now there is very little season-to-season storage carried
out. The storage is only run down to zero over the two winter months of June and July. The larger PV
panel enables continued hydrogen production to replenish the storage even at relatively lower solar
radiation, except in these two winter months.
The cost of the unit power has been calculated for constrained storage based on the previous
assumptions for the rest of the system components. It is found to be $US 1.08 for a capital storage cost of
$US2000/kg, falling to $US1.03/kWh for a storage cost of $500/kg. On account of the much lower storage
capacity compared to unconstrained storage, the lowering effect on unit power cost of reducing storage
cost is now much less.
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Low-cost H2 storage options for solar H2 systems for RAPS
Ali
The effect of varying the preset storage capacity on the unit power cost is presented in Figure 6 for a
range of storage costs. At $US 2000/kg, the unit power costs falls from $US 1.77/kWh with a storage
capacity of 9.7 kg to $US1.08/kWh with a 0.7 kg storage. At US$500/kg, the economically-optimal storage
capacity rises to 5.0 kg, so that season-to-season storage is employed to a considerable degree, while the
unit cost of power falls to US$1.01/kWh.
Figure 5.Unit cost of power Vs Constrained storage with reduced storage cost
2.4
Comparison between constrained storage and unconstrained storage
The major differences between the two unconstrained and constrained storage conditions on system
sizing and economics can be summarized as follows:
‰ Storage capacity in constrained storage is reduced to only 0.7 kg from the unconstrained value of
9.76 kg.
2
2
‰ PV panel area, on the other hand, is much higher (18.4 m compared to 10.14 m )in constrained
storage (0.7 kg)
‰ Electrolyser capacity is fixed to 1 kW for both conditions.
‰ Fuel cell capacity is solely dependent upon the peak demand load so that it too is same (0.253
kW) in both the cases.
At a unit capital cost of storage of US$2000/kg, the unit cost of power in constrained storage (0.7 kg, the
economically optimal value) is $US1.08, compared to $US 1.77 with unconstrained storage. At
US$500/kg, the economically optimal storage capacity rises to 5.0 kg, and the PV panel area is 13.6 m2,
giving a unit cost of power of US$ 1.01/kWh.
3
LOW-COST HYDROGEN STORAGE OPTIONS
3.1 Options
This analysis shows that low-cost hydrogen storage systems, at a unit capital cost down to $US 500/kg,
can significantly improve the economics of solar –hydrogen RAPS systems A further important
consideration is that in many RAPS applications there is plenty of space available for storage containers,
the exact opposite of the case of hydrogen-powered automobiles. Hence options for hydrogen storage at
relatively low pressure, especially those achievable using a PEM electrolyser itself as the compressor, are
worth investigating for RAPS applications in the drive to achieve lowest-cost storage solutions. Lowpressure storage thus permits hydrogen gas from an electrolyser to be transferred directly to the storage
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Low-cost H2 storage options for solar H2 systems for RAPS
Ali
vessel without the need of an external compressor and its associated considerable parasitic electrical
power consumption. Shapiro has found that the use of PEM electrolysers directly for hydrogen
compression has a significant net energy advantage over use of a separate electrically-driven compressor
[Aurora & Duffy 2005]. Our analysis above indicates that if the cost of storage can be cut to US $500/kg,
season to season storage becomes feasible and unit power costs in the order of US$ 1/kWh can be
achieved.
In RMIT University’s solar hydrogen R&D program we are thus investigating a number of possible low-cost
and relatively low-pressure hydrogen storage options that might be suitable for RAPS applications,
including water displacement based acrylic cylinders for laboratory systems, larger-volume plastic tanks,
and medium-pressure metal cylinders [Pyle 1997].
3.2
Acrylic Cylinders
For laboratory requirements of small volume (10-15 × 103 cm3) of hydrogen, we have constructed a
storage system comprising two acrylic cylinders (dia of 150 mm, height 500 mm) at different levels so that
as hydrogen is collected over water in the lower cylinder the water displaced flows into the second
cylinder which is open to the atmosphere at the top (Figure 6). Similar transparent acrylic cylinders are
used in many of the small scale demonstration solar-hydrogen systems now available commercially, such
as H tec. Appropriate piping and connections for this mechanism is needed in order to facilitate the
smooth passage of incoming hydrogen and subsequent regulation of stored hydrogen during fuel cell
operations.
Connecting
Pipe
Top
cylinder
Bottom
cylinder
H2 Outlet
H2 Inlet
Fig 6: Laboratory scale storage at ambient pressure.
The pressure of hydrogen stored is atmospheric pressure plus the head pressure due to the difference in
water levels in the two cylinders. Since atmospheric pressure is equivalent to about a 10 m head of water
and the height of the cylinders is only 50 cm, this system stores hydrogen at only just over atmospheric
pressure. Importantly atmospheric pressure also acts on the outside surfaces of the lower storage cylinder
so the pressure difference the vessel has to withstand is only the head due to the difference in water
levels.
This type of arrangement provides a self regulating, reliable and safe mechanism for hydrogen storage on
the scale required in many laboratory experiments. The total cost of an 8 litre storage system of this kind
including piping and valves is approximately $US 120.
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Low-cost H2 storage options for solar H2 systems for RAPS
Ali
3.3 Plastic Water Displacement Tanks
For storage of greater volumes of hydrogen at low pressures it may be possible to use modified plastic or
cement water tanks. Plastic water tanks with volumes of 10 000 litres (10 m3) or more are widely available
and used for water storage in rural and remote properties in Australia and many other countries. They are
commonly made from plastics (polyethylene, poly propylene and PVC), and are lightweight. We are
currently investigating the potential of a variety of plastic water tanks, with and without modifications, to be
used for hydrogen storage for RAPS applications in the split level water displacement configuration as
shown in Figure 7.
As with the acrylic cylinder system just described, the storage pressure is only marginally higher than
atmospheric pressure depending upon the amount of water displaced from the bottom tank to the top one.
The difference in level of these two tanks can be positioned in order to generate a greater storage
pressure. Work is under way to test the range of pressures such tanks can safely withstand. In addition
plumbing fittings, valves and pipes suitable for hydrogen usages must be employed.
Fig 7: Split level water tank.
The location of the hydrogen storage tank underground potentially has a strong safety advantage,
particularly since in the Australian context hydrogen storages used for RAPS in many rural and remote
areas must be able to withstand bushfires and lightning strikes. A full safety audit of this method of
storage will be needed before any field trials.
Another factor that must be considered when using plastic tanks is the rate of hydrogen diffusion through
the walls. As mentioned earlier, ideally secure storage of hydrogen from summer to winter is required, so
that the impact of losses through the walls over periods of six months or greater need to be considered.
The effect of diffusion can be evaluated from Fick’s law of diffusion (Askeland 1996):
J = −D ×
Δc
Δx
, For a PVC material, D= 1.3 × 10
Hence J = 1.3 × 10
−12
⎡ 0.0838 ⎤ 2 −1 kg.m
×⎢
⎥.m s × m
⎣ 0.005 ⎦
−3
−12
m2 / s .
= 2.17 × 10
−11
kg
m2s
For a large PVC plastic tank of volume 30 000 litres with a radius of 1.97 m, height of 2.51 m, and wall
thickness of 5 mm, the total hydrogen flux through the walls at NTP is given by:
H2 diffused per sec = J × A = 2.17 × 10
−11
⎡ kg
⎤
× 55.424⎢ 2 × m 2 ⎥ = 1.203 × 10 −9 kg/s
⎣m s
⎦
On the assumption that the pressure inside the tank remains constant (that is, hydrogen lost is
replenished), for one complete year:
Total H2 lost per year = 1.202 × 10
−9
× 8760 × 3600 = 0.038 kg.
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Low-cost H2 storage options for solar H2 systems for RAPS
Ali
Hence approximately 1.5% of the total mass of hydrogen in the tank originally will be lost each year.
Although this value is small, it needs to be accounted for in storage sizing and system design.
Interestingly in the ‘constrained storage’ condition, any hydrogen losses can readily be made up from the
surplus hydrogen production capacity of the PV-electrolyser system.
Current market prices for plastic water tanks of volumes in the order of 10 m3 are US $1200, which
converts to a unit capital cost of $1400/kg of hydrogen stored.
3.4
METAL CYLINDERS
Compressed hydrogen gas at medium pressure stored in suitable metal cylinders is another promising
option for low-cost storage. These medium pressure storage cylinders are smaller and heavier than plastic
low-pressure storage. Metal cylinders are already commonly used for storing LPG at pressures of up to 33
bar. Similar principles applicable to storage of LPG and natural gas in the low to medium pressure range
can be implemented for hydrogen as well. Pyle (1997) has used steel LPG cylinders derated from their
design value of 17 bars (for LPG use) to 8 bar for safe storage of hydrogen refer (Figure 8). Metal
cylinders for storing hydrogen must be made of low carbon steel, so that they do not suffer from hydrogen
embrittlement (Pyle 1997). The material of the tank should be of low-carbon steel and is thus resistant to
hydrogen embrittlement. Pyle (1997) states that tanks made from steel with a high-carbon content, or
which has been cold-rolled or cold-forged, or have weld hard spots in excess of about Vickers Hardness
Number 260, should not be used for storage of hydrogen. Valves and regulators should of course be
hydrogen compatible.
Storage of hydrogen as a compressed gas up to 20 bar in metal or other cylinders would match well the
capabilities of current generation PEM electrolysers to generate hydrogen under pressure for direct
storage without the need of an external. These options could thus be very cost-effective, since the
compression achievable by the PEM electrolysers would greatly reduce the storage volumes required; the
additional cost of an electrically driven compressor would be avoided, as would the electricity consumption
of the latter which would significantly lower the net energy production of the system.
Fig 8: Pressure-derated metal LPG cylinders used for hydrogen storage up to 8 bar (Pyle 1997).
4
CONCLUSIONS
A spreadsheet model has been used to size a solar hydrogen system to meet an idealized electrical
demand for a remote homestead in southern Australia. Two different conditions for determining hydrogen
capacity have been compared: unconstrained storage in which there is enough storage capacity to allow
all the hydrogen produced by the excess PV power over the load to be stored for later use; and
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Low-cost H2 storage options for solar H2 systems for RAPS
Ali
constrained storage in which storage capacity is fixed at the beginning of a model run, and then the PV
area determined to meet the demand throughout the year. In constrained storage, storage capacity is
reduced to only less than one tenth that in unconstrained storage, while PV area is increased by around
80%. At a unit storage cost of US$2000/kg, the unit cost of power in constrained storage is $US1.08,
compared to $US 1.77 with unconstrained storage. However, if the unit storage cost can be cut to
US$500/kg, the economically optimal storage capacity rises considerably (from 0.7 kg to 5.0 kg) as
season-to-season storage becomes economic, and the unit cost of power falls to US$ 1.01/kWh.
Some possible low-cost and relatively low-pressure hydrogen storage options that might be suitable for
RAPS applications are discussed; including water-displacement based acrylic cylinders for laboratory
systems, larger-volume plastic tanks, and medium-pressure metal cylinders.
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