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Stand-alone PEM water electrolysis system for fail safe operation with a renewable energy source
international journal of hydrogen energy 35 (2010) 928–935
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Stand-alone PEM water electrolysis system for fail safe operation with a renewable energy source R.E. Clarke, S. Giddey, S.P.S. Badwal* CSIRO Energy Technology, CSIRO Energy Transformed Flagship, Private Bag 33, Clayton South, VIC 3169, Australia
article info
abstract
Article history:
A complete stand-alone electrolyser system has been constructed as a transportable unit
Received 27 August 2009
for demonstration of a sustainable energy facility based on hydrogen and a renewable
Received in revised form
energy source. The stand-alone unit is designed to support a polymer electrolyte
18 November 2009
membrane (PEM) stack operating at up to w4 kW input power with a stack efficiency of
Accepted 26 November 2009
about 80% based on HHV of hydrogen. It is self-pressurizing and intended for operation
Available online 17 December 2009
initially at a differential pressure of less than 6 bar across the membrane electrode assembly with the hydrogen generation side being at a higher pressure. With a slightly
Keywords:
smaller stack, the system has been operated at an off-site facility where it was directly
Electrolysis balance-of-plant
coupled to a 2.4 kW photovoltaic (PV) solar array. Because of its potential use in remote
PEM electrolyser
areas, the balance-of-plant operates entirely on 12 V DC power for all monitoring, control
Hydrogen generation
and safety requirements. It utilises a separate high-current supply as the main electrolyser
Renewable energy
input, typically 30–40 V at 100 A from a renewable source such as solar PV or wind. The
PV array
system has multiple levels of built-in operator and stack safety redundancy. Control and safety systems monitor all flows, levels and temperatures of significance. All fault conditions are failsafe and are duplicated, triggering latching relays which shut the system down. Process indicators monitor several key variables and allow operating limits to be easily adjusted in response to experience of system performance gained in the field. ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.
1.
Introduction
Currently most hydrogen (w95%) is produced from fossil fuels such as natural gas (steam reforming or partial oxidation) and coal (via gasification). The total global hydrogen generation capacity is over 600 billion m3/Y and is equivalent to about 55 million tons per year [1]. If all this capacity is used as an energy source, it will account for less than 2% of total global energy consumption. However, hydrogen is used for ammonia and fertiliser production (about half), in oil refineries for impurity removal and upgrading of heavy oil fractions into lighter and more valuable products (w35%) with most of the balance being used for methanol production, and in chemical and
metallurgical industries. Only about 1% of the total generation capacity is used as a direct energy source and that too mainly in the space programs. Due to the high cost of hydrogen production, its use for power generation or as an alternative transport fuel has been very limited. This is further exacerbated by the lack of hydrogen transportation, distribution and storage infrastructure. However, fossil fuel resources, especially liquid fuels are declining rapidly and major developed and developing World economies are relying more and more on oil imports. Soon there will be global shortage of oil with oil peak production looming around the corner [2,3]. Along with concern over rising CO2 and other pollutant emissions linked to fossil fuels, there is a requirement to have clean,
* Corresponding author. Tel.: þ61 3 9545 2719; fax: þ61 3 9545 2720. E-mail address: [email protected] (S.P.S. Badwal). 0360-3199/$ – see front matter ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. doi:10.1016/j.ijhydene.2009.11.100
international journal of hydrogen energy 35 (2010) 928–935
Nomenclature A bar BOP DC HAZOP
current in amps atmospheric pressure unit balance-of-plant direct current hazard and operability study (see standards such as IEC 61882)
sustainable and low (or zero) emission fuels and technologies. Hydrogen as an energy carrier and storage media is expected to play a key role in future power generation and with the development of the fuel cell technology as a transport fuel. Hydrogen can be generated by a range of technologies such as reforming of natural gas, liquefied petroleum gas, gasoline, etc.; gasification of coal and biomass; electrolysis of water using nuclear, fossil or renewable energy sources; photo electrochemical/photo catalytic splitting of water; thermolysis and thermo-chemical cycles [4–8]. In the near future, it is likely that fossil fuels (natural gas and coal) would meet a large percentage of the hydrogen demand and for central or large-scale plants, technologies for CO2 capture and sequestration may be incorporated. However, in terms of greenhouse gas reduction potential, renewable energy offers the best possible solution for hydrogen production. As the mandated renewable energy targets increase, there will be much greater emphasis on developing new renewable energy capacity and technologies. Most renewable energy sources (solar, wind, tidal, wave, etc.) are intermittent and variable in nature. In the absence of energy storage, renewable systems must be grossly overdesigned in order to meet instantaneous capacity requirements. Hydrogen offers a flexible energy storage solution for accommodating load variability of long duration (Fig. 1). Water electrolysis is considered to be one of the key technologies for hydrogen generation as it is compatible with existing and future power generation technologies and a large number of renewable technologies (solar, biomass, hydro, wind,
Electricity Water H2 & O2 gases
HHV kW nm3 PEM PV V Y
high heating value kilowatt normal m3 polymer electrolyte membrane photovoltaic PV output voltage, electrolyser voltage, V year
tidal, wave, geothermal, etc.). Other proposed technologies, such as photo-electrolytic, photo-biological, thermolysis and thermo-chemical processes are considered to be well short of meeting the technical and commercial targets at this stage, and in addition, suffer from severe material-related problems [4–8]. Currently most of the commercial water electrolysis technologies use acidic or alkaline electrolyte systems for hydrogen generation. Typical efficiencies quoted are in the 55– 74% range with most commercial systems having efficiencies below 65% [9,10]. The current density is typically around 0.3– 0.4 A/cm2 and there are technical difficulties in maintaining the electrolyte balance and keeping hydrogen and oxygen separated. More recently a solid state water electrolysis technology based on polymer electrolyte membrane has been under development and is being commercialised [11–13]. Its operation can be considered to be reverse to that of a PEM fuel cell. The PEM electrolysis systems can respond rapidly to varying power inputs and therefore can be easily integrated with renewable energy systems. PEM electrolysers operate at relatively low temperatures, typically at 80 C or below and are generally composed of numerous cells stacked in series. Whilst the stack itself may be a compact unit, a laboratory installation will include numerous support components, as well as external connections for electrical supply, water supply and drain, hydrogen and oxygen removal. The assemblage of these components is generally referred to as the balance-of-plant (BOP). Commercialization of electrolyser units requires the development of appropriate packaging for the BOP and all of its
Load Demand (Electric & Hot Water)
Water
Water Storage
H2 Storage Wind Energy Generator
Control System
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Power Management Fuel Cell
Electrolyser
O2 Storage
PV Array
Fig. 1 – The sustainable energy concept based on renewable energy and hydrogen storage solution.
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international journal of hydrogen energy 35 (2010) 928–935
functionality. From a marketing perspective, the appropriate configuration would almost certainly be a single stand-alone unit, a ‘‘black box’’ that simply performs the function of generating hydrogen and oxygen gases when it is supplied with water and electricity. Moving to this form of packaging is also an appropriate step in the overall development and commercialization of capability for hydrogen generation based on water electrolysis. A single package demonstrates successful integration of all aspects of the design, including safety and control systems, into a single functioning entity. Such a system can be transportable, allowing it to be moved to other locations for performance demonstration and it can be sealed for reasons of safety as well as for security. In a previous paper we had described the operation of Oreion Alpha directly coupled to a 2.4 kW solar PV array [14]. This paper describes in detail the construction, sub-system description and commissioning of such a system, the Oreion Alpha.
2.
System overview
The Oreion Alpha is a transportable stand-alone electrolysis unit complete with a balance-of-plant incorporating extensive features to ensure the safety of the electrolysis stack as well as those operating the equipment. It uses a polymer electrolyte membrane (PEM) stack to electrolyse water and is designed to operate over a wide range of input powers, allowing it to utilise renewable power sources which are often quite variable in nature. PEM electrolysers utilise a stack of electrochemical cells in which a pair of electrodes (generally planar in shape) is separated by a proton-conducting polymer membrane. A power source applies a potential to the cell whilst at the same time, water is made available at the positive electrode. Oxidation and reduction reactions occur at the electrodes in the presence of appropriate catalysts and the applied electrical potential. On the positive side of the cell, water is oxidised to protons (hydrogen ions), leaving oxygen which evolves as a gas. Electrical current supplied by the power source provides the potential and the energy to drive the protons across to the other side of the membrane. There they regain the missing electron and evolve as hydrogen gas. The rate of production of hydrogen and oxygen is determined by the amount of DC current that is supplied to the unit. The number of cells and the cell area determine the respective voltage and current capabilities of the electrolyser. The Oreion Alpha is designed for operation from a power supply with a capability of up to 100 A and up to 38 V corresponding to a power input of 3.8 kW at full load. It is selfcontained within a floor-standing equipment cabinet about 1.4 m high, requiring a minimum of external facilities to be operational. It does require a source of deionised water (at a pressure of at least 2 bar). Product gases would generally be stored. In case the gases are to be vented for some reason, an extraction system for dilution and dispersal is required. The system is intended for operation at up to 1 bar on the oxygen side and 6 bar on the hydrogen side. Hydrogen generation at higher pressures is possible but has not been assessed. When running at full load, the PEM electrolysis stack is designed to operate at up to 80 C, which is an optimum
temperature for a PEM-based system. The stack is self-heating due to internal resistive cell losses, although the heat available may be quite variable. At a typical efficiency of 80% at full load, the remaining 20% of the supplied electrical energy is released as heat and contributes to the stack achieving the optimum operating temperature. On part load, the stack operates at lower temperatures. Auxiliary heating is not used because the small efficiency improvement would not justify the energy costs of this heating. The system has an extensive monitoring, control and safety system which encompasses all process values relevant to its safe operation. These include pressures, temperatures, flow rates and water levels at various locations, stack voltage and stack current, and hydrogen gas leakage detection. The system is highly configurable but the usual consequence of detecting any abnormal value or fault is to remove power and shut down the system. Design details of the electrolysis stack itself are not considered in this paper. Instead, the stack is considered as a generic unit, with performance characteristics as described. The stack employed is a CSIRO in-house design, known to have these characteristics [15].
3.
Oreion Alpha design details
3.1.
Design overview
The core support requirements of a PEM electrolyser stack are little more than a supply of pure water at the anode and a supply of electrical current to each cell. If these are present, each PEM cell will consume water and generate oxygen at the anode (which will bubble through the water) and hydrogen at the cathode. However, for commercial (or near commercial) systems, safe operation requires a significant number of additional features to appropriately handle the gases and liquids that are present on both sides of membrane electrode assemblies. One particular complication arises from the fact that conventional PEM membranes allow a significant amount of liquid water to carry across the membrane along with the flow of hydrogen ions, resulting in a steady accumulation of water on the hydrogen generation side. The main operational components of the stand-alone electrolyser system are shown in Fig. 2. This is a simplified process diagram and shows only the key functional elements. A pair of small pressure vessels acts as reservoirs and liquid/ water separators to handle the gas-liquid mixtures that exit on each side of the stack membranes. Water is supplied from an external source to the separator on the oxygen side and maintained at a controlled level. This separator may be pressurized but in the design shown, it is operated at atmospheric pressure. Water is pressurized and circulated at a fairly low rate (several litres/min) by a small pump. It flows through the stack and exits as a 2-phase stream of water and oxygen gas. A back-pressure regulator controls the operational pressure. The water/oxygen stream then returns to the separator, from which the oxygen can be collected or vented away. The hydrogen side operates in ‘‘dead-end’’ mode. Hydrogen gas and a small amount of water evolve from the stack and flow together to the hydrogen/water separator. The
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Electrolysis Stack
Back pressure regulator Flow Pump switch
Pressure switch
Safety Control System
Temperature sensor
Pressure relief valve
Non-return valve
Solenoid valve
Hydrogen Delivery
El ec trolysis Power Supply
Non-return Back pressure valve regulator
Oxygen Delivery
Water Supply
Diff. pressure switch O2 / Water Separator
Pump
H2 / Water Separator
Solenoid valve
Non-return valve
Fig. 2 – A simplified process diagram showing the main operational components of the stand-alone electrolyser system.
water level is controlled so that excess water is recirculated to the oxygen side (or partially/selectively discarded) by a pump and/or a solenoid valve. The hydrogen/water separator is connected to an external hydrogen gas storage vessel. A pressure regulator is used on the output line in order to provide controlled pressure in the separator and for testing and operational purposes. In typical operation with a storage vessel connected, hydrogen gas pressure varies according to the quantity of hydrogen resident in the vessel. The hydrogen and oxygen output streams are saturated with water. In the present design on-board drying capability has not been incorporated but may be included in future designs.
A lower front panel contains the back-pressure regulator controls and analogue gauges to display the pressure in each of the separators. The back of the cabinet contains an electrical panel with terminals for the connection of the main stack power and the 12 V DC auxiliary power. This panel also includes connectors for data logging and an external safety circuit (i.e. ventilation alarm). A higher back panel contains all plumbing connections, which include oxygen and hydrogen gas outlets, water inlet and drains for each separator vessel. A separate hydrogen gas monitoring outlet is also provided. All plumbing is in stainless steel with Swagelok-type fittings.
3.3. 3.2.
Power supply requirements
Oreion Alpha housing
The complete assembly of stack and BOP is housed in a standard commercial 19-inch equipment cabinet (Fig. 3). The cabinet is 1350 mm high and is mounted on castors for manoeuvrability. The upper front panel contains six Eurotherm indicators which display the values of key system variables. These indicators allow for electronic setting of upper and lower limits for the process variables that they monitor, tripping an alarm relay if the values are exceeded. An additional ten status conditions are monitored by sensor switches, for pressure, flow and gas detection, all of which are set to control latching relays. Ten red illuminated switches, also on the front panel, provide display and resetting of these relays. ‘‘Latching’’ is the default mode, so that a manual reset is required to cancel a fault signal and resume operation. For operational flexibility, relays and indicators can be alternatively configured to operate in non-latching mode.
The BOP system has been designed to support an electrolyser stack operating at an output level of up to 3 kW and a current of up to 100 A. PEM electrolysis cells characteristically operate between about 1.5 V at zero current and 1.9 V at full current. The lower voltage defines the energy required for electrochemical hydrogen conversion whilst voltages greater than that may be associated with internal resistive cell losses and consequent heat dissipation. These characteristics suggest a design configuration of 20 cells operating in series at 100 A, with a peak dissipation of 3.8 kW to achieve an efficiency of approximately 80%, with a 38 V total stack voltage. For commissioning of the system and its evaluation with a directly coupled renewable energy source (photovoltaic array), a smaller 14-cell stack was used. At 100 A this represents a potential output in terms of hydrogen generation of approximately 2 kW and a hydrogen generation capacity of approximately 0.585 nm3 per hour.
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international journal of hydrogen energy 35 (2010) 928–935
available stacks [15]. The pressure capability of the stack itself is generally the limiting factor and future developments may see this pressure increased. In many electrolyser applications, if the oxygen that is generated is not required, it is simply vented. However, for optimum management of the stack, the oxygen side may also be operated at a pressure approaching that on the hydrogen generation side. The two gas/water separators have been manufactured as small pressure vessels and may be operated at pressures up to 15 bar. They have an internal volume of approximately 2 l.
3.6.
Water supply sub-system
The Oreion Alpha has been designed to be supplied with highpurity water at a pressure in the range of 0.25 bar–2 bar. At full power (100 A input current), usage is approximately 500 ml per hour for the 2 kW electrolysis stack. This can be supplied either from a reticulated main-pressure source (via a purification system and a pressure regulator) or from a gravity-fed supply tank. Typically a combination of activated charcoal filtering and mixed-bed deionization filters has been found to be adequate. It is generally suggested that PEM electrolysis water supply should have a resistivity of greater than 1 megohm-cm.
3.7. Fig. 3 – A photograph of the completely assembled Oreion Alpha – a stand-alone system for distributed hydrogen generation.
3.4.
Stack efficiency and heat management
For a stack operating at 80% efficiency, the remaining 20% of the supplied electrical energy is converted to heat. This heat may be useful, since the stack operates more efficiently at elevated temperature. Experimental work has shown that a relatively simple but effective control of temperature can be achieved by operation of the electrolyser without cabinet ventilation up to 70 C stack temperature, after which four axial ventilation fans are switched on automatically. These fans are mounted on rear panels of the cabinet, two at a lower level delivering air into the cabinet and two at the top which extract warm air. This regime is intended to maintain the stack temperature in the 70–80 C region (except at lower operational currents), which is close to that required for optimum efficiency.
3.5.
Operating pressures
PEM electrolysis stacks may operate at a wide range of pressures and are self-pressurizing up to quite high values. Although it is possible to post-pressurize (compress) the output streams, allowing the pressurization to be performed by the electrolyser itself may be advantageous since it is thermodynamically efficient and avoids the requirement for a mechanical compressor. The stand-alone electrolyser is intended to generate pressures of up to 6 bar of hydrogen initially, since this matched the capability of currently
Auxiliary power supply
The BOP system operates from an external 12 V, 8 A power supply, to facilitate operation which may be remote from mains power. This could be via a 12 V lead-acid battery at a remote location, or a general-purpose 12 V DC main-operated power supply for situations where main power is available. The average current requirement is approximately 5 A.
3.8.
Monitoring, safety and control sub-systems
Several monitoring, safety and control systems are built into the Oreion Alpha. These are located around the key operational components, which also include the liquid/water separators. Water from the oxygen separator is circulated through the stack by a small stainless-steel pump and returns as a mixed stream of water and oxygen gas. A back-pressure regulator controls the operational pressure on this side of the stack. The separator itself may be operated at atmospheric pressure, from which the oxygen is vented off or collected. On the other side of the stack, operation is basically in ‘‘deadend’’ mode, with hydrogen gas produced within the stack being transferred to the hydrogen gas separator. Liquid water carried across the membrane in association with the transport of hydrogen ions, is also transferred to the separator. The water level is controlled and excess water is recirculated to the oxygen side, via a pump and solenoid valve. Hydrogen exits from this separator via a back-pressure control valve, which may be set to operate the hydrogen generation side (including the hydrogen separator) at up to 6 bar. Water circulation on the oxygen side is monitored by a flow-switch which signals a fault if the flow is below a set value. Water exiting from the stack is monitored for temperature, as is the temperature of the stack surface. Process indicators allow for maximum values of these temperatures
international journal of hydrogen energy 35 (2010) 928–935
30
Stack Voltage, V
28
26
24
22
20 0
20
40
60
80
100
Stack Current, A
Fig. 4 – A representative V–I curve for the complete Oreion Alpha housing a 2 kW stack consisting of 14,100 cm2 cells.
to be set before the process is interrupted. Upstream of the back-pressure regulator, pressure is also monitored by a pressure switch and protected from exceeding a maximum value by a relief valve which will vent oxygen directly. The separator also has a pressure relief valve that would vent oxygen directly if the set pressure was exceeded. The gas outlet line from the hydrogen/water separator is also monitored by pressure switches which shut down the electrolyser if it exceeds a preset value. These switches have been duplicated in order to provide failsafe operation. The hydrogen outlet line also incorporates a flashback arrestor. A process indicator is also used to monitor the differential pressure across the stack. This allows for variable minimum and maximum values to be set. Stack current, stack voltage and inlet water pressure are monitored by individual indicators which allow limits for each to be set. Each gas separator contains a set of float switches which control water level at about half of the vessel volume, but also incorporate high and low safety switches. The cabinet also contains a pair of hydrogen leak detectors. As described above, the monitoring and control system incorporates six process indicators and 10 process switches. An alarm indication from any of these results in interruption of the supply of current to the electrolyser stack via a highcurrent relay. In addition, power is disconnected to the pumps and solenoid valves (which manage water levels in the two separators). The process switches are all adjustable to some degree (mechanically or electrically), and have been configured to provide a process alarm signal at preset levels. The six Eurotherm indicators allow for high and low levels to be optionally programmed and easily modified in software. They are Eurotherm Model 2408i, classified as an ‘‘indicator’’ but they operate effectively as a compact controller when utilising the ‘‘alarm’’ outputs as system control signals. An additional control system has been incorporated to allow the unit to shut down and consume minimal energy when the power supplied by the external source (wind, solar PV, etc.) is too low for the electrolysis stack to operate. Power to pumps and solenoids is removed, leaving just the standby power of the control system. When the correct combination of voltage and current climb back above set
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threshold values (on the following morning, in the case of solar PV), a system restart is initiated. This requires a temporary bypass of the alarm system for circulating water flow, which would otherwise prevent such a restart. If circulating water flow is re-established within a short period of time, the latching alarm is automatically reset and operation continues. The Oreion Alpha was designed to accommodate data logging instrumentation as required. Voltage taps are provided for measuring individual cell voltages. Calibrated shunts have been incorporated to facilitate current measurement. Outputs from the pressure measurement sensors and from thermocouples for temperature measurement are available.
4.
Commissioning and demonstration
A formal HAZOP analysis was undertaken as part of the process of specifying a process flow diagram and the detailed plumbing design. A number of safety-related components were introduced as a result of this analysis. In addition, the plumbing layout and the specification of all significant components were confirmed. The Oreion Alpha was constructed in accordance with the detailed design specification arising from the HAZOP analysis. It incorporated all of the safety and monitoring features that were part of this from the inception. Initial operation was without a stack, using temporary piping connections to complete or bridge the flow circuits. This enabled confirmation that all systems (particularly those related to flow control) operated correctly and also allowed for preliminary settings, such as pump speed and flow-switch set-point, to be made. The 14-cell stack was then installed and operated, starting with lower powers and increasing gradually to full power. In general the system operated extremely well although a number of issues did arise. Pressure losses were initially excessive in some lines. This was remedied by a combination of increased tubing diameter and more-streamlined routing where required. The 12 V pumps proved to have excess capacity and a step-down voltage converter was incorporated to run them more efficiently at 7–8 V. Some fine tuning was also required for mechanical components (such as float switches and flow sensors) and for Eurotherm control setpoints (pressure, temperature voltage and current) and delay timing (as used in the auto-start feature). The 14-cell PEM stack had already been used in a fixed test station but was further tested in the Oreion Alpha in order to verify its satisfactory system operation as installed. Fig. 4 shows a representative V–I curve obtained with Oreion Alpha for the 2 kW electrolysis stack linked to a laboratory power supply. For this trial, readings were taken fairly rapidly, starting at a low current, without allowing time for the system to equilibrate in temperature between current values. It therefore represents the response to a current surge, which produces higher than normal stack voltage and higher power dissipation temporarily, because the stack is initially cool and operating less efficiently. Current was kept constant at 100 A for a short period until the stack temperature approached normal operating value, by which time the stack voltage had fallen from above 29 V (as shown) to below 28 V.
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Current, A
40
30
Current Solar Irradiance
y = 17.435x - 9.451 2 R = 0.947
1000 800
Sunny Days 1200
600 400 200 0 0
10
20
30
40
50
60
800
Current, A
20
Cloudy Days 400
Solar Irradiance, W/m2
50
Solar Irradiance, W/m2
1600 1200
10
0 300
320
340
360
380
0 400
Time, Hours Fig. 5 – A representative diagram showing correlation between the electrolyser stack current and widely varying solar irradiance on different days.
The Oreion Alpha has since been used for a demonstration study at RMIT University which operates a 2.4 kW PV array in association with the Solar Laboratory at its Bundoora East campus. To facilitate comprehensive monitoring, a Doric Digitrend 245 data logging system was employed, using a pair of 20-channel front end modules located inside the cabinet. Each of these modules communicates with the external data logger mainframe via a single data/power cable and may be configured channel by channel for measurement of voltage, temperature and other variables. The system operated for several months and was a successful demonstration of the use of direct coupling between electrolyser and PV array [14,16]. There were some initial difficulties, mostly associated with failures of some commercial components, after which the BOP system performed extremely well. It was also necessary to fine-tune the auto-start system to match the characteristics of the PV array. In the early morning sun, the array produced quite high voltages but negligible current capacity. It was therefore necessary to set a relatively high voltage as the trigger value for starting up the electrolyser stack. Safe operation of the stack required that the pressure across the MEA be maintained at less than 6 bar. The regulator valves performed this function adequately although there was more fluctuation than desirable and a significant dependence on gas and water flow rates. Since both gases were vented for this demonstration program, it proved expeditious to operate the stack with a pressure of approximately 2 bar on the hydrogen side and 1 bar on the oxygen side, well away from the pressure limits. Fig. 5 shows electrolyser current as a function of solar irradiance on four representative days during the fourmonth demonstration period. Also included in this figure (as inset) is a graph of solar irradiance versus current. The correlation between solar irradiance and current and the rapid response of the electrolyser to variation in the solar irradiance and thus the current input is clearly evident. It is
also apparent that the electrolyser operated successfully over a wide range of currents and adapts well to the supply variability. In fact the system accommodated the complete spectrum of weather, ranging from cool and cloudy days to very hot and sunny days when the ambient temperature was above 40 C. There were no failures of the BOP system over this period. The 14-cell stack used for this study proved to be less durable than expected and degraded significantly through the course of the project [14,17]. Even during initial trials, the voltage of a number of individual cells had risen above the target 1.9 V at 100 A. However, the BOP was sufficiently flexible that this did not impede operation, apart from the requirement to set a higher value of stack voltage than would be appropriate for a stack in good condition. The relatively conservative approach taken in the specification of the Oreion Alpha did not place emphasis on compactness or economy of design. Certainly, the relatively spacious cabinet and layout meant that construction was simple and access to components was relatively easy. Subsequent implementations of the same basic design might be considerably more compact, aided additionally by a rationalization of some components that contribute little to the overall functionality of the system. For example, using a pair of high-current solenoids in series may have enhanced safety but a single solenoid would have been adequate. The relatively large water/gas separators were convenient to set up but could be substantially reduced in size.
5.
Conclusions
A stand-alone system complete with 2 kW electrolysis stack and balance-of-plant incorporating multiple levels of operator and stack safety redundancy has been built for integration with a renewable energy source. With the use of an AC/DC
international journal of hydrogen energy 35 (2010) 928–935
converter the system can also be connected to main power for distributing hydrogen generation at end-use sites. The balance-of-plant required to operate a PEM electrolysis stack as an independent stand-alone facility operating at an output power of approximately 2 kW has been successfully demonstrated both in the laboratory during its commissioning phase and also at an external site directly coupled to a 2.4 kW solar PV power source. Housed, along with the stack, in an equipment cabinet of modest size, the balance-of-plant incorporated a relatively sophisticated control and monitoring system providing emphasis on flexible operation and high levels of safety and redundancy. With early careful consideration to control, safety and monitoring sub-systems, the balance-ofplant proved to be relatively straightforward to set up and commission. As a stand-alone system, the only external inputs required were a supply of pure water (tap water through an ion exchange resin column); an auxiliary 12 V DC power supply (possibly a battery) for the control, safety and monitoring system; a DC power source for the electrolysis (up to 100 A, variable voltage in the 20–40 V range to suit the number of cells in the stack). A PV array has proved to be an excellent power source. It is suitable for direct coupling for optimum simplicity and requires only that it is well-matched by having compatible voltage and current characteristics. Product gases (hydrogen and oxygen) were generated over a period of four months in conditions ranging from cool and cloudy days to hot and sunny days when the ambient temperature exceeded 40 C. The system, including all safety systems, operated very effectively and satisfactorily over this period mostly unattended, providing high level of user comfort with relatively new technology for hydrogen generation. In future such a system can be operated with other sources of renewable energy or main power as a distributed hydrogen generator. Coupled to a fuel cell, it has the potential to meet electricity and hot water demands of isolated dwellings. Little attempt was made to achieve a compact package at this stage, resulting in a cabinet of generous size which was straightforward to assemble and is easy to service. Considerable size reduction would be possible in future designs.
Acknowledgement The authors would like to thank Fabio Ciacchi for reviewing this manuscript, and Dr John Andrews, Dr Biddyut Paul and Fabio Ciacchi for assistance with evaluation of the Oreion Alpha at RMIT. The project has been supported by CSIRO’s Energy Transformed Flagship.
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references
[1] Mueller-Langer F, Tzimas E, Kaltschmitt M, Peteves S. Techno-economic assessment of hydrogen production processes for the hydrogen economy for the short and medium term. Int J Hydrogen Energy 2007;32(16):3797–810. [2] Campbell CJ. World: oil and gas industry – peak oil: an outlook on crude oil depletion, http://www.mbendi.com/ indy/oilg/p0070.htm; February 2002. [3] Campbell CJ, Leherre`re JH. The end of cheap oil. Sci Am Mag March 1998:6. [4] Conte M, Lacobazzi A, Ronchetti M, Vellone R. Hydrogen economy for a sustainable development: state-of-the-art and technological perspective. J Power Sources 2001;100:171–87. [5] Badwal SPS, Giddey S, Phair JW. Do all energy crossroads converge to hydrogen. In: Proceedings of Australian Institute of Energy, National Conference – Energy at the Crossroads, 27–29 November 2006, Melbourne, Australia. Paper 8A4, 6 p. [6] Duigou AL, Borgard JM, Larousse B, Doizi D, Allen R, Ewan BC, et al. HYTHEC: an EC funded search for a long term massive hydrogen production route using solar and nuclear technologies. Int J Hydrogen Energy 2007;32(10–11):1516–29. [7] Sexena RC, Seal D, Kumar S, Goyal HB. Thermo-chemical routes for hydrogen rich gas from biomass: a review. Renew Sustain Energy Rev 2008;12:1909–27. [8] McHugh K. Hydrogen production methods. MPR-WP-0001. Alexandria, Virginia, USA: MPR Associates Inc.; February 2005. [9] Friedland RJ, Speranza AJ. Integrated renewable hydrogen utility system. In: Proceedings of the 1999 U.S. DOE Hydrogen Program Review, NREL/CP-570–26938. [10] Prince-Richard S, Whale M, Djilali N. A techno-economic analysis of decentralized electrolytic hydrogen production for fuel cell vehicles. Int J Hydrogen Energy 2005;30(11):1159–79. [11] Grigoriev SA, Porembsky VI, Fateev VN. Pure hydrogen production by PEM electrolysis for hydrogen energy. Int J Hydrogen Energy 2006;31(2):171–5. [12] Barbir F. PEM electrolysis for production of hydrogen from renewable energy sources. Solar Energy 2005;78(5):661–9. [13] Millet P, Andolfatto F, Durand R. Design and performance of a solid polymer electrolyte water electrolyser. Int J Hydrogen Energy 1996;21(2):87–93. [14] Clarke RE, Giddey S, Ciacchi FT, Badwal SPS, Paul B, Andrews J. Direct coupling of an electrolyser to a solar PV system for generating hydrogen. Int J Hydrogen Energy 2009;34(6):2531–42. [15] Badwal SPS, Giddey S, Ciacchi FT. Hydrogen and oxygen generation with polymer electrolyte membrane (PEM) based electrolytic technology. Ionics 2006;12(1):7–14. [16] Clarke RE, Giddey S, Ciacchi FT, Badwal SPS, Paul B, Andrews J. A stand-alone 2 kW PEM electrolyser integrated with a solar PV system for hydrogen generation. In: 17th World Hydrogen Energy Conference, Queensland, Australia, 15–19 June 2008. [17] Giddey S, Badwal SPS, Ciacchi FT, Clarke RE. Investigation of durability issues in polymer electrolyte membrane (PEM) electrolytic cells. In: Fuel Cells Durability & Performance. 3rd ed. The Knowledge Press; 2008 [chapter 13].
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Stand-alone PEM water electrolysis system for fail safe operation with a renewable energy source R.E. Clarke, S. Giddey, S.P.S. Badwal* CSIRO Energy Technology, CSIRO Energy Transformed Flagship, Private Bag 33, Clayton South, VIC 3169, Australia
article info
abstract
Article history:
A complete stand-alone electrolyser system has been constructed as a transportable unit
Received 27 August 2009
for demonstration of a sustainable energy facility based on hydrogen and a renewable
Received in revised form
energy source. The stand-alone unit is designed to support a polymer electrolyte
18 November 2009
membrane (PEM) stack operating at up to w4 kW input power with a stack efficiency of
Accepted 26 November 2009
about 80% based on HHV of hydrogen. It is self-pressurizing and intended for operation
Available online 17 December 2009
initially at a differential pressure of less than 6 bar across the membrane electrode assembly with the hydrogen generation side being at a higher pressure. With a slightly
Keywords:
smaller stack, the system has been operated at an off-site facility where it was directly
Electrolysis balance-of-plant
coupled to a 2.4 kW photovoltaic (PV) solar array. Because of its potential use in remote
PEM electrolyser
areas, the balance-of-plant operates entirely on 12 V DC power for all monitoring, control
Hydrogen generation
and safety requirements. It utilises a separate high-current supply as the main electrolyser
Renewable energy
input, typically 30–40 V at 100 A from a renewable source such as solar PV or wind. The
PV array
system has multiple levels of built-in operator and stack safety redundancy. Control and safety systems monitor all flows, levels and temperatures of significance. All fault conditions are failsafe and are duplicated, triggering latching relays which shut the system down. Process indicators monitor several key variables and allow operating limits to be easily adjusted in response to experience of system performance gained in the field. ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.
1.
Introduction
Currently most hydrogen (w95%) is produced from fossil fuels such as natural gas (steam reforming or partial oxidation) and coal (via gasification). The total global hydrogen generation capacity is over 600 billion m3/Y and is equivalent to about 55 million tons per year [1]. If all this capacity is used as an energy source, it will account for less than 2% of total global energy consumption. However, hydrogen is used for ammonia and fertiliser production (about half), in oil refineries for impurity removal and upgrading of heavy oil fractions into lighter and more valuable products (w35%) with most of the balance being used for methanol production, and in chemical and
metallurgical industries. Only about 1% of the total generation capacity is used as a direct energy source and that too mainly in the space programs. Due to the high cost of hydrogen production, its use for power generation or as an alternative transport fuel has been very limited. This is further exacerbated by the lack of hydrogen transportation, distribution and storage infrastructure. However, fossil fuel resources, especially liquid fuels are declining rapidly and major developed and developing World economies are relying more and more on oil imports. Soon there will be global shortage of oil with oil peak production looming around the corner [2,3]. Along with concern over rising CO2 and other pollutant emissions linked to fossil fuels, there is a requirement to have clean,
* Corresponding author. Tel.: þ61 3 9545 2719; fax: þ61 3 9545 2720. E-mail address: [email protected] (S.P.S. Badwal). 0360-3199/$ – see front matter ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. doi:10.1016/j.ijhydene.2009.11.100
international journal of hydrogen energy 35 (2010) 928–935
Nomenclature A bar BOP DC HAZOP
current in amps atmospheric pressure unit balance-of-plant direct current hazard and operability study (see standards such as IEC 61882)
sustainable and low (or zero) emission fuels and technologies. Hydrogen as an energy carrier and storage media is expected to play a key role in future power generation and with the development of the fuel cell technology as a transport fuel. Hydrogen can be generated by a range of technologies such as reforming of natural gas, liquefied petroleum gas, gasoline, etc.; gasification of coal and biomass; electrolysis of water using nuclear, fossil or renewable energy sources; photo electrochemical/photo catalytic splitting of water; thermolysis and thermo-chemical cycles [4–8]. In the near future, it is likely that fossil fuels (natural gas and coal) would meet a large percentage of the hydrogen demand and for central or large-scale plants, technologies for CO2 capture and sequestration may be incorporated. However, in terms of greenhouse gas reduction potential, renewable energy offers the best possible solution for hydrogen production. As the mandated renewable energy targets increase, there will be much greater emphasis on developing new renewable energy capacity and technologies. Most renewable energy sources (solar, wind, tidal, wave, etc.) are intermittent and variable in nature. In the absence of energy storage, renewable systems must be grossly overdesigned in order to meet instantaneous capacity requirements. Hydrogen offers a flexible energy storage solution for accommodating load variability of long duration (Fig. 1). Water electrolysis is considered to be one of the key technologies for hydrogen generation as it is compatible with existing and future power generation technologies and a large number of renewable technologies (solar, biomass, hydro, wind,
Electricity Water H2 & O2 gases
HHV kW nm3 PEM PV V Y
high heating value kilowatt normal m3 polymer electrolyte membrane photovoltaic PV output voltage, electrolyser voltage, V year
tidal, wave, geothermal, etc.). Other proposed technologies, such as photo-electrolytic, photo-biological, thermolysis and thermo-chemical processes are considered to be well short of meeting the technical and commercial targets at this stage, and in addition, suffer from severe material-related problems [4–8]. Currently most of the commercial water electrolysis technologies use acidic or alkaline electrolyte systems for hydrogen generation. Typical efficiencies quoted are in the 55– 74% range with most commercial systems having efficiencies below 65% [9,10]. The current density is typically around 0.3– 0.4 A/cm2 and there are technical difficulties in maintaining the electrolyte balance and keeping hydrogen and oxygen separated. More recently a solid state water electrolysis technology based on polymer electrolyte membrane has been under development and is being commercialised [11–13]. Its operation can be considered to be reverse to that of a PEM fuel cell. The PEM electrolysis systems can respond rapidly to varying power inputs and therefore can be easily integrated with renewable energy systems. PEM electrolysers operate at relatively low temperatures, typically at 80 C or below and are generally composed of numerous cells stacked in series. Whilst the stack itself may be a compact unit, a laboratory installation will include numerous support components, as well as external connections for electrical supply, water supply and drain, hydrogen and oxygen removal. The assemblage of these components is generally referred to as the balance-of-plant (BOP). Commercialization of electrolyser units requires the development of appropriate packaging for the BOP and all of its
Load Demand (Electric & Hot Water)
Water
Water Storage
H2 Storage Wind Energy Generator
Control System
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Power Management Fuel Cell
Electrolyser
O2 Storage
PV Array
Fig. 1 – The sustainable energy concept based on renewable energy and hydrogen storage solution.
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international journal of hydrogen energy 35 (2010) 928–935
functionality. From a marketing perspective, the appropriate configuration would almost certainly be a single stand-alone unit, a ‘‘black box’’ that simply performs the function of generating hydrogen and oxygen gases when it is supplied with water and electricity. Moving to this form of packaging is also an appropriate step in the overall development and commercialization of capability for hydrogen generation based on water electrolysis. A single package demonstrates successful integration of all aspects of the design, including safety and control systems, into a single functioning entity. Such a system can be transportable, allowing it to be moved to other locations for performance demonstration and it can be sealed for reasons of safety as well as for security. In a previous paper we had described the operation of Oreion Alpha directly coupled to a 2.4 kW solar PV array [14]. This paper describes in detail the construction, sub-system description and commissioning of such a system, the Oreion Alpha.
2.
System overview
The Oreion Alpha is a transportable stand-alone electrolysis unit complete with a balance-of-plant incorporating extensive features to ensure the safety of the electrolysis stack as well as those operating the equipment. It uses a polymer electrolyte membrane (PEM) stack to electrolyse water and is designed to operate over a wide range of input powers, allowing it to utilise renewable power sources which are often quite variable in nature. PEM electrolysers utilise a stack of electrochemical cells in which a pair of electrodes (generally planar in shape) is separated by a proton-conducting polymer membrane. A power source applies a potential to the cell whilst at the same time, water is made available at the positive electrode. Oxidation and reduction reactions occur at the electrodes in the presence of appropriate catalysts and the applied electrical potential. On the positive side of the cell, water is oxidised to protons (hydrogen ions), leaving oxygen which evolves as a gas. Electrical current supplied by the power source provides the potential and the energy to drive the protons across to the other side of the membrane. There they regain the missing electron and evolve as hydrogen gas. The rate of production of hydrogen and oxygen is determined by the amount of DC current that is supplied to the unit. The number of cells and the cell area determine the respective voltage and current capabilities of the electrolyser. The Oreion Alpha is designed for operation from a power supply with a capability of up to 100 A and up to 38 V corresponding to a power input of 3.8 kW at full load. It is selfcontained within a floor-standing equipment cabinet about 1.4 m high, requiring a minimum of external facilities to be operational. It does require a source of deionised water (at a pressure of at least 2 bar). Product gases would generally be stored. In case the gases are to be vented for some reason, an extraction system for dilution and dispersal is required. The system is intended for operation at up to 1 bar on the oxygen side and 6 bar on the hydrogen side. Hydrogen generation at higher pressures is possible but has not been assessed. When running at full load, the PEM electrolysis stack is designed to operate at up to 80 C, which is an optimum
temperature for a PEM-based system. The stack is self-heating due to internal resistive cell losses, although the heat available may be quite variable. At a typical efficiency of 80% at full load, the remaining 20% of the supplied electrical energy is released as heat and contributes to the stack achieving the optimum operating temperature. On part load, the stack operates at lower temperatures. Auxiliary heating is not used because the small efficiency improvement would not justify the energy costs of this heating. The system has an extensive monitoring, control and safety system which encompasses all process values relevant to its safe operation. These include pressures, temperatures, flow rates and water levels at various locations, stack voltage and stack current, and hydrogen gas leakage detection. The system is highly configurable but the usual consequence of detecting any abnormal value or fault is to remove power and shut down the system. Design details of the electrolysis stack itself are not considered in this paper. Instead, the stack is considered as a generic unit, with performance characteristics as described. The stack employed is a CSIRO in-house design, known to have these characteristics [15].
3.
Oreion Alpha design details
3.1.
Design overview
The core support requirements of a PEM electrolyser stack are little more than a supply of pure water at the anode and a supply of electrical current to each cell. If these are present, each PEM cell will consume water and generate oxygen at the anode (which will bubble through the water) and hydrogen at the cathode. However, for commercial (or near commercial) systems, safe operation requires a significant number of additional features to appropriately handle the gases and liquids that are present on both sides of membrane electrode assemblies. One particular complication arises from the fact that conventional PEM membranes allow a significant amount of liquid water to carry across the membrane along with the flow of hydrogen ions, resulting in a steady accumulation of water on the hydrogen generation side. The main operational components of the stand-alone electrolyser system are shown in Fig. 2. This is a simplified process diagram and shows only the key functional elements. A pair of small pressure vessels acts as reservoirs and liquid/ water separators to handle the gas-liquid mixtures that exit on each side of the stack membranes. Water is supplied from an external source to the separator on the oxygen side and maintained at a controlled level. This separator may be pressurized but in the design shown, it is operated at atmospheric pressure. Water is pressurized and circulated at a fairly low rate (several litres/min) by a small pump. It flows through the stack and exits as a 2-phase stream of water and oxygen gas. A back-pressure regulator controls the operational pressure. The water/oxygen stream then returns to the separator, from which the oxygen can be collected or vented away. The hydrogen side operates in ‘‘dead-end’’ mode. Hydrogen gas and a small amount of water evolve from the stack and flow together to the hydrogen/water separator. The
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Electrolysis Stack
Back pressure regulator Flow Pump switch
Pressure switch
Safety Control System
Temperature sensor
Pressure relief valve
Non-return valve
Solenoid valve
Hydrogen Delivery
El ec trolysis Power Supply
Non-return Back pressure valve regulator
Oxygen Delivery
Water Supply
Diff. pressure switch O2 / Water Separator
Pump
H2 / Water Separator
Solenoid valve
Non-return valve
Fig. 2 – A simplified process diagram showing the main operational components of the stand-alone electrolyser system.
water level is controlled so that excess water is recirculated to the oxygen side (or partially/selectively discarded) by a pump and/or a solenoid valve. The hydrogen/water separator is connected to an external hydrogen gas storage vessel. A pressure regulator is used on the output line in order to provide controlled pressure in the separator and for testing and operational purposes. In typical operation with a storage vessel connected, hydrogen gas pressure varies according to the quantity of hydrogen resident in the vessel. The hydrogen and oxygen output streams are saturated with water. In the present design on-board drying capability has not been incorporated but may be included in future designs.
A lower front panel contains the back-pressure regulator controls and analogue gauges to display the pressure in each of the separators. The back of the cabinet contains an electrical panel with terminals for the connection of the main stack power and the 12 V DC auxiliary power. This panel also includes connectors for data logging and an external safety circuit (i.e. ventilation alarm). A higher back panel contains all plumbing connections, which include oxygen and hydrogen gas outlets, water inlet and drains for each separator vessel. A separate hydrogen gas monitoring outlet is also provided. All plumbing is in stainless steel with Swagelok-type fittings.
3.3. 3.2.
Power supply requirements
Oreion Alpha housing
The complete assembly of stack and BOP is housed in a standard commercial 19-inch equipment cabinet (Fig. 3). The cabinet is 1350 mm high and is mounted on castors for manoeuvrability. The upper front panel contains six Eurotherm indicators which display the values of key system variables. These indicators allow for electronic setting of upper and lower limits for the process variables that they monitor, tripping an alarm relay if the values are exceeded. An additional ten status conditions are monitored by sensor switches, for pressure, flow and gas detection, all of which are set to control latching relays. Ten red illuminated switches, also on the front panel, provide display and resetting of these relays. ‘‘Latching’’ is the default mode, so that a manual reset is required to cancel a fault signal and resume operation. For operational flexibility, relays and indicators can be alternatively configured to operate in non-latching mode.
The BOP system has been designed to support an electrolyser stack operating at an output level of up to 3 kW and a current of up to 100 A. PEM electrolysis cells characteristically operate between about 1.5 V at zero current and 1.9 V at full current. The lower voltage defines the energy required for electrochemical hydrogen conversion whilst voltages greater than that may be associated with internal resistive cell losses and consequent heat dissipation. These characteristics suggest a design configuration of 20 cells operating in series at 100 A, with a peak dissipation of 3.8 kW to achieve an efficiency of approximately 80%, with a 38 V total stack voltage. For commissioning of the system and its evaluation with a directly coupled renewable energy source (photovoltaic array), a smaller 14-cell stack was used. At 100 A this represents a potential output in terms of hydrogen generation of approximately 2 kW and a hydrogen generation capacity of approximately 0.585 nm3 per hour.
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available stacks [15]. The pressure capability of the stack itself is generally the limiting factor and future developments may see this pressure increased. In many electrolyser applications, if the oxygen that is generated is not required, it is simply vented. However, for optimum management of the stack, the oxygen side may also be operated at a pressure approaching that on the hydrogen generation side. The two gas/water separators have been manufactured as small pressure vessels and may be operated at pressures up to 15 bar. They have an internal volume of approximately 2 l.
3.6.
Water supply sub-system
The Oreion Alpha has been designed to be supplied with highpurity water at a pressure in the range of 0.25 bar–2 bar. At full power (100 A input current), usage is approximately 500 ml per hour for the 2 kW electrolysis stack. This can be supplied either from a reticulated main-pressure source (via a purification system and a pressure regulator) or from a gravity-fed supply tank. Typically a combination of activated charcoal filtering and mixed-bed deionization filters has been found to be adequate. It is generally suggested that PEM electrolysis water supply should have a resistivity of greater than 1 megohm-cm.
3.7. Fig. 3 – A photograph of the completely assembled Oreion Alpha – a stand-alone system for distributed hydrogen generation.
3.4.
Stack efficiency and heat management
For a stack operating at 80% efficiency, the remaining 20% of the supplied electrical energy is converted to heat. This heat may be useful, since the stack operates more efficiently at elevated temperature. Experimental work has shown that a relatively simple but effective control of temperature can be achieved by operation of the electrolyser without cabinet ventilation up to 70 C stack temperature, after which four axial ventilation fans are switched on automatically. These fans are mounted on rear panels of the cabinet, two at a lower level delivering air into the cabinet and two at the top which extract warm air. This regime is intended to maintain the stack temperature in the 70–80 C region (except at lower operational currents), which is close to that required for optimum efficiency.
3.5.
Operating pressures
PEM electrolysis stacks may operate at a wide range of pressures and are self-pressurizing up to quite high values. Although it is possible to post-pressurize (compress) the output streams, allowing the pressurization to be performed by the electrolyser itself may be advantageous since it is thermodynamically efficient and avoids the requirement for a mechanical compressor. The stand-alone electrolyser is intended to generate pressures of up to 6 bar of hydrogen initially, since this matched the capability of currently
Auxiliary power supply
The BOP system operates from an external 12 V, 8 A power supply, to facilitate operation which may be remote from mains power. This could be via a 12 V lead-acid battery at a remote location, or a general-purpose 12 V DC main-operated power supply for situations where main power is available. The average current requirement is approximately 5 A.
3.8.
Monitoring, safety and control sub-systems
Several monitoring, safety and control systems are built into the Oreion Alpha. These are located around the key operational components, which also include the liquid/water separators. Water from the oxygen separator is circulated through the stack by a small stainless-steel pump and returns as a mixed stream of water and oxygen gas. A back-pressure regulator controls the operational pressure on this side of the stack. The separator itself may be operated at atmospheric pressure, from which the oxygen is vented off or collected. On the other side of the stack, operation is basically in ‘‘deadend’’ mode, with hydrogen gas produced within the stack being transferred to the hydrogen gas separator. Liquid water carried across the membrane in association with the transport of hydrogen ions, is also transferred to the separator. The water level is controlled and excess water is recirculated to the oxygen side, via a pump and solenoid valve. Hydrogen exits from this separator via a back-pressure control valve, which may be set to operate the hydrogen generation side (including the hydrogen separator) at up to 6 bar. Water circulation on the oxygen side is monitored by a flow-switch which signals a fault if the flow is below a set value. Water exiting from the stack is monitored for temperature, as is the temperature of the stack surface. Process indicators allow for maximum values of these temperatures
international journal of hydrogen energy 35 (2010) 928–935
30
Stack Voltage, V
28
26
24
22
20 0
20
40
60
80
100
Stack Current, A
Fig. 4 – A representative V–I curve for the complete Oreion Alpha housing a 2 kW stack consisting of 14,100 cm2 cells.
to be set before the process is interrupted. Upstream of the back-pressure regulator, pressure is also monitored by a pressure switch and protected from exceeding a maximum value by a relief valve which will vent oxygen directly. The separator also has a pressure relief valve that would vent oxygen directly if the set pressure was exceeded. The gas outlet line from the hydrogen/water separator is also monitored by pressure switches which shut down the electrolyser if it exceeds a preset value. These switches have been duplicated in order to provide failsafe operation. The hydrogen outlet line also incorporates a flashback arrestor. A process indicator is also used to monitor the differential pressure across the stack. This allows for variable minimum and maximum values to be set. Stack current, stack voltage and inlet water pressure are monitored by individual indicators which allow limits for each to be set. Each gas separator contains a set of float switches which control water level at about half of the vessel volume, but also incorporate high and low safety switches. The cabinet also contains a pair of hydrogen leak detectors. As described above, the monitoring and control system incorporates six process indicators and 10 process switches. An alarm indication from any of these results in interruption of the supply of current to the electrolyser stack via a highcurrent relay. In addition, power is disconnected to the pumps and solenoid valves (which manage water levels in the two separators). The process switches are all adjustable to some degree (mechanically or electrically), and have been configured to provide a process alarm signal at preset levels. The six Eurotherm indicators allow for high and low levels to be optionally programmed and easily modified in software. They are Eurotherm Model 2408i, classified as an ‘‘indicator’’ but they operate effectively as a compact controller when utilising the ‘‘alarm’’ outputs as system control signals. An additional control system has been incorporated to allow the unit to shut down and consume minimal energy when the power supplied by the external source (wind, solar PV, etc.) is too low for the electrolysis stack to operate. Power to pumps and solenoids is removed, leaving just the standby power of the control system. When the correct combination of voltage and current climb back above set
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threshold values (on the following morning, in the case of solar PV), a system restart is initiated. This requires a temporary bypass of the alarm system for circulating water flow, which would otherwise prevent such a restart. If circulating water flow is re-established within a short period of time, the latching alarm is automatically reset and operation continues. The Oreion Alpha was designed to accommodate data logging instrumentation as required. Voltage taps are provided for measuring individual cell voltages. Calibrated shunts have been incorporated to facilitate current measurement. Outputs from the pressure measurement sensors and from thermocouples for temperature measurement are available.
4.
Commissioning and demonstration
A formal HAZOP analysis was undertaken as part of the process of specifying a process flow diagram and the detailed plumbing design. A number of safety-related components were introduced as a result of this analysis. In addition, the plumbing layout and the specification of all significant components were confirmed. The Oreion Alpha was constructed in accordance with the detailed design specification arising from the HAZOP analysis. It incorporated all of the safety and monitoring features that were part of this from the inception. Initial operation was without a stack, using temporary piping connections to complete or bridge the flow circuits. This enabled confirmation that all systems (particularly those related to flow control) operated correctly and also allowed for preliminary settings, such as pump speed and flow-switch set-point, to be made. The 14-cell stack was then installed and operated, starting with lower powers and increasing gradually to full power. In general the system operated extremely well although a number of issues did arise. Pressure losses were initially excessive in some lines. This was remedied by a combination of increased tubing diameter and more-streamlined routing where required. The 12 V pumps proved to have excess capacity and a step-down voltage converter was incorporated to run them more efficiently at 7–8 V. Some fine tuning was also required for mechanical components (such as float switches and flow sensors) and for Eurotherm control setpoints (pressure, temperature voltage and current) and delay timing (as used in the auto-start feature). The 14-cell PEM stack had already been used in a fixed test station but was further tested in the Oreion Alpha in order to verify its satisfactory system operation as installed. Fig. 4 shows a representative V–I curve obtained with Oreion Alpha for the 2 kW electrolysis stack linked to a laboratory power supply. For this trial, readings were taken fairly rapidly, starting at a low current, without allowing time for the system to equilibrate in temperature between current values. It therefore represents the response to a current surge, which produces higher than normal stack voltage and higher power dissipation temporarily, because the stack is initially cool and operating less efficiently. Current was kept constant at 100 A for a short period until the stack temperature approached normal operating value, by which time the stack voltage had fallen from above 29 V (as shown) to below 28 V.
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Current, A
40
30
Current Solar Irradiance
y = 17.435x - 9.451 2 R = 0.947
1000 800
Sunny Days 1200
600 400 200 0 0
10
20
30
40
50
60
800
Current, A
20
Cloudy Days 400
Solar Irradiance, W/m2
50
Solar Irradiance, W/m2
1600 1200
10
0 300
320
340
360
380
0 400
Time, Hours Fig. 5 – A representative diagram showing correlation between the electrolyser stack current and widely varying solar irradiance on different days.
The Oreion Alpha has since been used for a demonstration study at RMIT University which operates a 2.4 kW PV array in association with the Solar Laboratory at its Bundoora East campus. To facilitate comprehensive monitoring, a Doric Digitrend 245 data logging system was employed, using a pair of 20-channel front end modules located inside the cabinet. Each of these modules communicates with the external data logger mainframe via a single data/power cable and may be configured channel by channel for measurement of voltage, temperature and other variables. The system operated for several months and was a successful demonstration of the use of direct coupling between electrolyser and PV array [14,16]. There were some initial difficulties, mostly associated with failures of some commercial components, after which the BOP system performed extremely well. It was also necessary to fine-tune the auto-start system to match the characteristics of the PV array. In the early morning sun, the array produced quite high voltages but negligible current capacity. It was therefore necessary to set a relatively high voltage as the trigger value for starting up the electrolyser stack. Safe operation of the stack required that the pressure across the MEA be maintained at less than 6 bar. The regulator valves performed this function adequately although there was more fluctuation than desirable and a significant dependence on gas and water flow rates. Since both gases were vented for this demonstration program, it proved expeditious to operate the stack with a pressure of approximately 2 bar on the hydrogen side and 1 bar on the oxygen side, well away from the pressure limits. Fig. 5 shows electrolyser current as a function of solar irradiance on four representative days during the fourmonth demonstration period. Also included in this figure (as inset) is a graph of solar irradiance versus current. The correlation between solar irradiance and current and the rapid response of the electrolyser to variation in the solar irradiance and thus the current input is clearly evident. It is
also apparent that the electrolyser operated successfully over a wide range of currents and adapts well to the supply variability. In fact the system accommodated the complete spectrum of weather, ranging from cool and cloudy days to very hot and sunny days when the ambient temperature was above 40 C. There were no failures of the BOP system over this period. The 14-cell stack used for this study proved to be less durable than expected and degraded significantly through the course of the project [14,17]. Even during initial trials, the voltage of a number of individual cells had risen above the target 1.9 V at 100 A. However, the BOP was sufficiently flexible that this did not impede operation, apart from the requirement to set a higher value of stack voltage than would be appropriate for a stack in good condition. The relatively conservative approach taken in the specification of the Oreion Alpha did not place emphasis on compactness or economy of design. Certainly, the relatively spacious cabinet and layout meant that construction was simple and access to components was relatively easy. Subsequent implementations of the same basic design might be considerably more compact, aided additionally by a rationalization of some components that contribute little to the overall functionality of the system. For example, using a pair of high-current solenoids in series may have enhanced safety but a single solenoid would have been adequate. The relatively large water/gas separators were convenient to set up but could be substantially reduced in size.
5.
Conclusions
A stand-alone system complete with 2 kW electrolysis stack and balance-of-plant incorporating multiple levels of operator and stack safety redundancy has been built for integration with a renewable energy source. With the use of an AC/DC
international journal of hydrogen energy 35 (2010) 928–935
converter the system can also be connected to main power for distributing hydrogen generation at end-use sites. The balance-of-plant required to operate a PEM electrolysis stack as an independent stand-alone facility operating at an output power of approximately 2 kW has been successfully demonstrated both in the laboratory during its commissioning phase and also at an external site directly coupled to a 2.4 kW solar PV power source. Housed, along with the stack, in an equipment cabinet of modest size, the balance-of-plant incorporated a relatively sophisticated control and monitoring system providing emphasis on flexible operation and high levels of safety and redundancy. With early careful consideration to control, safety and monitoring sub-systems, the balance-ofplant proved to be relatively straightforward to set up and commission. As a stand-alone system, the only external inputs required were a supply of pure water (tap water through an ion exchange resin column); an auxiliary 12 V DC power supply (possibly a battery) for the control, safety and monitoring system; a DC power source for the electrolysis (up to 100 A, variable voltage in the 20–40 V range to suit the number of cells in the stack). A PV array has proved to be an excellent power source. It is suitable for direct coupling for optimum simplicity and requires only that it is well-matched by having compatible voltage and current characteristics. Product gases (hydrogen and oxygen) were generated over a period of four months in conditions ranging from cool and cloudy days to hot and sunny days when the ambient temperature exceeded 40 C. The system, including all safety systems, operated very effectively and satisfactorily over this period mostly unattended, providing high level of user comfort with relatively new technology for hydrogen generation. In future such a system can be operated with other sources of renewable energy or main power as a distributed hydrogen generator. Coupled to a fuel cell, it has the potential to meet electricity and hot water demands of isolated dwellings. Little attempt was made to achieve a compact package at this stage, resulting in a cabinet of generous size which was straightforward to assemble and is easy to service. Considerable size reduction would be possible in future designs.
Acknowledgement The authors would like to thank Fabio Ciacchi for reviewing this manuscript, and Dr John Andrews, Dr Biddyut Paul and Fabio Ciacchi for assistance with evaluation of the Oreion Alpha at RMIT. The project has been supported by CSIRO’s Energy Transformed Flagship.
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references
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