The use of PEM unitised regenerative fuel cells in solar- hydrogen systems for remote area power supply

Transcription

1 , 2006 The use of PEM unitised regenerative fuel cells in solar- hydrogen systems for remote area power supply Arun K. Doddathimmaiah and John Andrews* School of Aerospace, Mechanical and Manufacturing Engineering RMIT University Bundoora, Melbourne 3083 AUSTRALIA *Author for correspondence ABSTRACT Remote area power supply (RAPS) is a potential early market for renewable energy hydrogen systems because of the relatively high costs of conventional energy sources in remote regions. Solarhydrogen RAPS systems commonly employ photovoltaic panels, a Proton Exchange Membrane (PEM) electrolyser, a storage for hydrogen gas, and a PEM fuel cell. Currently such systems are more costly than conventional RAPS systems employing diesel generator back up or battery storage. Unitised regenerative fuel cells (URFCs) have the potential to lower the costs of solar hydrogen RAPS systems since a URFC employs the same hardware for both the electrolyser and fuel cell functions. The need to buy a separate electrolyser and a separate fuel cell, both expensive items, is thus avoided. URFCs are in principle particularly suited for use in RAPS applications since the electrolyser function and fuel cell function are never required simultaneously. The present paper reports experimental findings on the performance of a URFC compared to that of a dedicated PEM electrolyser and a dedicated fuel cell. A design for a single-cell PEM URFC for use in experiments is described. The experimental data give a good quantitative description of the performance characteristics of all the devices. It is found that the performance of the URFC in the electrolyser mode is closely similar to that of the stand-alone electrolyser. In the fuel cell mode the URFC performance is, however, lower than that of the stand-alone fuel cell. The wider implications of these findings for the economics of future solar-hydrogen RAPS systems are discussed, and a design target of URFCs for renewable-energy RAPS applications proposed. KEY WORDS: Solar hydrogen, remote area power supply, unitised regenerative fuel cell, proton exchange membrane 1 INTRODUCTION Renewable energy - hydrogen systems for remote area power supply (RAPS) applications are a promising early market for hydrogen production, storage and utilisation technologies, because of the high cost of both conventional RAPS systems relying on diesel or petrol generators, and solar photovoltaic or wind systems employing batteries for storage. Critical limitations of the latter are that batteries can only store electrical energy for a relatively short period, typically only a few days, in the absence of recharging, and have a short overall lifetime. Hydrogen storage systems have the potential on the other hand to provide low-loss storage on a season-to-season basis (that is, for six months or more) and should be able to be designed with a lifetime much longer than batteries. There is a growing body of experience with both solar and wind hydrogen systems for RAPS applications. Some experimental and demonstration solar-hydrogen systems in the power range 1.5 kw to 250 kw have been built and operated [1]. Photovoltaic cells have generally been used as the renewable energy generation technology and Proton Exchange Membrane (PEM) fuel cells to convert stored hydrogen back to electricity. Increasingly PEM electrolysers are also being employed in RAPS applications [2]. However, there is need for considerable further cost reductions for solar/wind hydrogen systems to become economically competitive, and design development and system testing before commercial products can be made and sold [3].

2 The present paper explores a promising opportunity for reducing the cost of the hydrogen subsystem used in a RAPS application, namely the use of recently-developed Unitised Regenerative Fuel Cell (URFC) systems based on PEM technology. A PEM URFC combines the electrolyser and fuel cell functions in a single unit, and the concept has attracted considerable research interest [4]. The URFC concepts is ideally suited for a solar or wind hydrogen RAPS system since in this application the electrolyser function is never needed at the same time as the fuel cell function. If the capital cost of the URFC can be kept much lower than the combined cost of a separate electrolyser and fuel cell, and its efficiencies in both modes are close to those in separate units, then its use in solar-hydrogen RAPS systems could lead to overall cost savings [5, 6]. 2 UNITISED REGENERATIVE FUEL CELLS 2.1 Principles of a URFC In a conventional PEM electrolyser, electrical energy plus some heat drawn from the environment is used to split water into hydrogen and oxygen: H 2 O + electricity + heat H 2 + ½ O 2 In a PEM fuel cell the reverse reaction takes place as hydrogen is recombined with oxygen, the latter usually coming from air to avoid having to store oxygen as well as hydrogen, with the production of electricity, water and heat. H 2 + ½ O 2 H 2 O + electricity + heat. In the usual hydrogen production and storage system for RAPS or other applications, a stack of PEM cells is used for the electrolysis function, and a separate stack of PEM cells for the fuel cell function. The innovative feature of the URFC concept is to use a specially-designed single stack of PEM cells for both the electrolysis and fuel cell functions [4]. Since the basic structure of a dedicated PEM electrolyser and a dedicated PEM fuel cell stack is the same, the use of a single stack to perform both functions offers the prospect of substantial cost reductions. A schematic of a URFC is shown in Figure 1, and the various electrochemical reactions that take place in each mode are presented in Figure 2. In the electrolysis mode (lower half of Figure 2) water is introduced at the anode where it is split by the electric field in combination with the catalyst into oxygen, protons, and electrons. The oxygen evolves as gaseous O 2 at the surface of the electrode, while the protons are driven through the membrane and the electrons move through the external circuit. At the cathode, the protons combine with the electrons to evolve gaseous hydrogen. In the fuel cell mode, the hydrogen and oxygen are supplied to the respective electrodes, and electricity is generated producing water once again. In principle this water can be recycled for use once again in electrolysis. Electrolysis mode Water Fuel cell mode Oxygen from air Water Electricity URFC stack Electricity Oxygen H 2 H 2 Hydrogen storage Heat Figure 1: A schematic of a URFC system for storing electrical energy as hydrogen and then reusing this hydrogen for producing electricity. 2

3 Fuel cell Mode (Discharging) 4e (+) ( ) O 2 + 4H + + 4e Oxygen 2H 2 O Product water Oxygen electrodes Oxygen 4H + 4H + Hydrogen 2H 2 4H + + 4e Proton exchange membrane Hydrogen electrodes Hydrogen 2H 2 O 4H + 4e + O 2 4H + + 4e 2H 2 Process water (+) ( ) 4e Electrolyser cell mode (Charging) Figure 2: Electrochemical reactions taking place in the electrolysis (lower) and fuel cell modes (upper) of a URFC. Note that the entire cell area is used in both modes. The URFC in combination with a storage system for the hydrogen thus serves as a store for electrical energy in RAPS or a range of other applications. In a renewable-energy RAPS system, any surplus solar or wind energy input over the load to be supplied can be fed into the URFC in electrolysis mode (or E-mode ) to produce hydrogen gas for storage. When the solar or wind input is insufficient to meet the load, the deficit can be met by reversing the URFC to fuel cell mode ( FC-mode ) and drawing on stored hydrogen and oxygen from the air as the input fuels. In such an application, simultaneous operation in E-mode and FC mode is never required, so the URFC is ideally suitable. The overall process is silent, relatively efficient, zero greenhouse emissions, and with proper design is very safe. 2.2 Work on URFCs to date Mitlitsky et al. pointed out that regenerative fuel cells have a wide range of potential applications including energy storage devices coupled to RE sources, power plants for automobiles and propulsion systems for satellites [7]. URFCs are also being considered for use in NASA missions [8]. A number of URFCs employing PEM membrane electrode assemblies capable of working reversibly in either the electrolyser or fuel cell modes have been developed [9]. For example, Proton Energy Systems, in association with ATK Thiokol and General Dynamics, have developed a form of URFC system called Unigen primarily for use in backup electricity supplies in spacecraft [10]. However, this system is arguably not a fully-fledged URFC since it still incorporates separate hardware for the electrolyser and fuel cell functions. Lynntech Inc. USA, commercially offers Membrane Electrode Assemblies (MEAs) for URFCs with varying catalytic loading and membrane area [11]. Cisar et al. reported that mixing/alloying platinum black with a mix of iridium oxide and ruthenium oxide is preferred because it reduces the overall cost of MEAs while minimising the over potential for oxygen evolution [11]. Under fuel cell conditions when oxygen reduction is occurring, the platinum component is active while the ruthenium-iridium oxide component is inactive. Under electrolyser conditions and oxygen evolution, both platinum and ruthenium-iridium oxide components are active. 3

4 Dhar [12] reported that PEM URFC energy storage systems can achieve comparable performance to individual electrolyser and fuel cell units. Ongoing research work has focused on bifunctional electrodes and MEA development, for example, the research programs at AIST in Japan and the Dalian Institute of Chemical Physics in China [13]. A project to develop reversible fuel cells to the point of commercial applications in stand-alone PV systems and uninterruptible power supplies, called Revcell, is in progress in The Netherlands [14]. 2.3 The power ratio for a URFC A number of relevant features of URFC operation can be gleaned from a consideration of typical polarisation curves for E-mode and FC-mode functioning of a PEM cell per unit effective membrane area. By normalising the curves in the two modes according to unit area we are in effect representing the URFC situation in which a cell with a given effective membrane area is operating reversibly in the two modes. Figure 3 thus shows indicative polarisation curves (voltage versus current density) for a dedicated PEM electrolyser single cell and a dedicated PEM single fuel cell, as well as the corresponding curves for a PEM URFC. Voltage (V) A A URFC electrolyser URFC fuel cell V oc V rev = 1.23 Dedicated electrolyser Dedicated fuel cell B B i fc,max Current density (A/cm 2 ) Figure 3: Polarisation curves in electrolyser and fuel cell modes in dedicated electrolyser, fuel cells and URFC. Points A and A and B and B, are the maximum power points in the E and FC modes respectively. The corresponding power density versus current density curves are shown in Figure 4. Power density A A B B URFC electrolyser URFC fuel cell Dedicated electrolyser Dedicated fuel cell Current density (A/cm 2 ) Figure 4: Power density versus current density curves corresponding to the URFC in E-mode and FC mode, and a dedicated PEM electrolyser and fuel cell, whose polarization curves are shown in Figure 3. Points A and A, and B and B, are the maximum power points in the E and FC modes respectively. 4

5 The energy efficiency of an electrolyser (η e, defined as energy content of hydrogen produced, based on HHV divided by the electrical energy input) is given by: η e = µ F (1.23/ V cell ) where η F is the Faraday efficiency reflecting any loss of hydrogen produced within the cell system [15]. Hence the performance of an electrolyser improves as the excess of V cutin over the reversible potential of 1.23 V for the water decomposition and recombination reaction decreases, and as the positive slope of the V-I curve decreases. Generally a URFC in E-mode will have energy efficiency a little lower than that of a comparable dedicated electrolyser, and this feature is reflected in Figure 3 by both the higher cut-in voltage and greater slope for the URFC s E-mode curve. The energy efficiency of a fuel cell (η fc, defined as electrical energy output divided by the energy content of hydrogen fuel consumed, again based on HHV for consistency) is given by: η fc = µ fuel (V cell /1.23) where η fuel is the fuel utilisation coefficient reflecting any loss of input hydrogen within the cell system [15]. The performance of a fuel cell therefore increases with the open circuit voltage, and decreases as the modulus of the negative slope of its V-I curve increases (that is, with the rate of decline of cell voltage with current density. The curve for the URFC in FC-mode is thus shown as just below that of the dedicated fuel cell curve in Figure 3, and with a slightly more negative slope. The different signs of the slopes in E-mode and FC-mode can be explained as follows. In FC-mode, the cell is a source of electromotive force that drives current around the external circuit. As the current produced increases, the potential difference across the cell reduces. In effect the current flow acts to reduce the charge build up (net negative on the hydrogen electrode and net positive on the oxygen electrode) produced by the electrochemical reactions at each electrode. In E-mode, the electromotive force driving the effective current in the external circuit and through the cell is provided by the external power source, and the current flows in the reverse direction to that in FC-mode. Now as the voltage is increased above the cut-in voltage, current steadily increases. The positive slope reflects the resistive and other irreversible energy losses in the cell. The greater these losses, the greater the positive slope will be. The power input to the cell in E-mode increases approximately quadratically with increasing current density, and the maximum power input occurs when the maximum allowable current density is reached (point A for the dedicated electrolyser and point A for the URFC in Figures 3 and 4). The power output in FC-mode increases to a maximum and then falls to zero at the maximum current density (short-circuit condition). The maximum power points are shown by the points B and B in Figures 3 and 4 for the dedicated electrolyser and URFC respectively. It can be deduced from this analysis that the maximum power input to a URFC will always be several times the maximum power output. Drawing on typical polarisation curves for a URFC in its two modes as reported in this paper and elsewhere [12, 16] the ratio of power in/power out for a URFC is at best likely to be just over three and in practice four or more. Hence if we want a URFC with a 1 kw peak electrical power output for a RAPS system, its maximum power input is likely to be around 4 kw. This power-ratio feature is clearly important in the design of URFC-based solar-hydrogen systems for RAPS applications. 2.4 Design aims for URFCs Drawing upon this theoretical analysis, and work on URFCs done to date, a number of key design aims for URFCs suitable for RAPS applications can be proposed. Firstly the performance of a URFC in E-mode and FC-mode in terms of energy efficiency needs to be equal to or only slightly lower than that of a comparable dedicated PEM electrolyser and dedicated PEM fuel cell. To achieve this goal, a major technical challenge is to develop a stable and highly active bifunctional oxygen electrode. The choice of electrocatalysts to use is critical in this endeavour [17]. Platinum in its reduced form is the best electrocatalyst for oxygen reduction (reaction with H + ions 5

6 and electrons to form water) but is not is not the best catalyst for water splitting (to form oxygen gas, H + ions and electrons). Swette et al. showed that iridium oxide evolves oxygen at a far lower over potential than platinum and would be a good candidate for oxygen evolution even though a relatively poor candidate for oxygen reduction [18]. Zhigang, Baolin and Ming used Pt black and IrO 2 as oxygen electrode catalysts and have demonstrated that 50 wt% Pt + 50 wt% IrO 2 with a catalyst loading of just 0.4 mg/cm 2 performed well in both electrolyser and fuel cell modes [13]. Liu et al [16] have reported the preparation of composite oxygen electrodes with Pt-black and iridium oxide (50 wt% Pt + 50 wt% IrO 2 ) that functioned in both E- mode and FC-mode with fairly constant performance over 25 cycles between modes. Besides the development of reversible electrodes, proper and reliable reactant management is very important to achieve better URFC performance [7].The management of reactants inside the URFC cell is greatly dependent on the material and construction of the cell/stack. Maintaining sufficient water flow to the catalyst layer in the oxygen electrode during electrolysis mode, while avoiding flooding of gas diffusion channels in the backing layer of this electrode on switching to electrolysis mode, is vital for reliable functioning of URFC. Avoidance of flooding requires keeping the quantity of water in and around the oxygen electrode to the minimum that is needed to allow electrolysis to take place unhindered by water shortage, and effective removal of water (liquid and vapour) that is produced in fuel cell operation. This is usually accomplished by the presence of a diffusion backing/layer which has a mixture of hydrophilic and hydrophobic regions. The hydrophilic regions ensure sufficient delivery of water to the catalyst so that the membrane is hydrated during electrolyser operation. The hydrophobic regions ensure sufficient delivery of oxygen to the electrocatalyst for oxygen reduction reaction to take place during fuel cell reaction. In conventional PEM fuel cells, the gas diffusion backing layer is generally made of a porous carbon material such as carbon cloth or paper. These are not suitable for a URFC because oxidation of material surfaces in the oxygen electrode and backing layers due to the combined presence of water and oxygen must be avoided. Ioroi et al. [19] investigated a variety of titanium gas-diffusion backings (GDBs) coated with different amounts of polytetrafluoroethylene (PTFE) loadings for the oxygen electrode of a URFC. It was found that the URFC performance significantly depended on the amount of PTFE loading in the backing layer on the oxygen electrode, with increasing loadings beyond a certain amount leading to poorer performance. But URFCs with no PTFE-coated backing layer performed very poorly. However, the PTFE coating on the GDB of the H 2 electrode did not affect the cell performance. A second key design aim is to achieve a total system cost for a URFC that is well below the total cost for a system comprising a comparable separate PEM electrolyser and PEM fuel cell. At this stage we are seeking to develop a URFC for a solar/wind hydrogen RAPS system with a delivered power of 1-2 kw that is 40% cheaper than a hydrogen subsystem employing a dedicated electrolyser and fuel cell. This may be accomplished by keeping the costs of materials and construction of a URFC close to those for a PEM electrolyser of the same membrane area and current density. It will also be essential to keep the balance of system costs for the URFC - that is, for water management, thermal regulation, and gas supply and extraction - to approximately the same level as or lower than the balance of system costs for a separate PEM electrolyser and fuel cell with equivalent performance. If it was possible to use a URFC in place of a separate electrolyser and fuel cell and reduce the combine capital cost of these components by 40% without significant adverse effect on their performance, the unit cost of the power from a solar-hydrogen system over its lifetime could be cut by about 10% [20]. There could be an added reduction of cost if there are savings in balance of system costs too. 3 DESIGN OF A SINGLE-CELL URFC A single-cell proton exchange membrane URFC has been designed and is planned to be constructed to compare its performance with a dedicated electrolyser and PEM fuel cell that we have constructed at RMIT University. The design of this URFC seeks to realise optimal performance in both electrolyser and fuel cell modes (Figures 5 and 6). The design is simple and provides for easy assembly and 6

7 disassembly. The Membrane Electrode Assembly (MEA) used was purchased from Lynntech, Inc., USA. and employs a membrane made from Nafion with an active area of cm x cm and a total membrane area of 5.08 cm x 5.08 cm. The catalyst loadings are as follows: Anode catalyst: Pt Black and IrRuOx, 2.0 mg/cm 2 loading Cathode catalyst: Pt Black, 4.0 mg/cm 2 loading. A key feature of the design is the provision of a single water reservoir in the lowest horizontal channel in the end plate in contact with the oxygen electrode. The water level in this reservoir is maintained at a constant level throughout E-mode and FC-mode operation by adding or removing water from an external water supply. The rate of water consumption in E-mode is estimated to be at maximum only 0.48 ml/hour, and the rate of water production in FC mode will be considerably less than this. The flow rate of the water supply and removal is thus very low in both modes. The water wets a strip of the porous backing layer along its lower edge and continually rises through this layer, which is hydrophilic, by capillary action and diffusion through polymer material. Hence a continuous supply of water to the catalyst sites during electrolysis is provided. The membrane itself is hydrated by using humidified hydrogen and oxygen gas inputs from storage. Design features to be investigated experimentally are: The level of the water in contact with oxygen electrode The optimal material and structure for the backing layer on the oxygen electrode to maintain sufficient water supply during electrolysis and water removal during fuel cell operation Catalyst loadings on the membrane electrode assembly, particularly on the oxygen electrode. The configuration of the gas flow channels on both oxygen and hydrogen sides. O 2 from gas storage Hard stop Tension bolts Silicone rubber seal O 2 to gas storage O 2 electrode Catalyst layer Water to wet gas diffusion backing Water level maintenance at level D H 2 to gas storage Membrane electrode assembly H 2 electrode Catalyst layer H 2 from gas storage Metallic end plates and gas flow channels Tension bolts Hard stop Figure 5: A design for a single-cell unitised regenerative fuel cell for use in experimental comparisons with dedicated PEM electrolysers and fuel cells. 7

8 Hole for gas exit/entry Channels cut into metal Hole for water supply Hole for water exit Figure 6: The gas flow channel configuration in the oxygen-side end plate in the URFC design (Figure 5). The hydrogen-side end plate is similar except for the use of the hole at the bottom left for gas supply and absence of the hole at the bottom right. 4 A URFC VERSUS A DEDICATED ELECTROLYSER AND FUEL CELL: EXPERIMENTAL COMPARISONS 4.1 Experimental set-up Experiments have been conducted to compare the performance of a dedicated PEM fuel cell and dedicated PEM electrolyser with that of a purchased single-cell URFC obtained from Fuel Cell Store. The URFC used had an active membrane area of 9 cm 2 and total membrane area of 16 cm 2. The anode and cathode electrodes are catalysed with 3 mg Pt/cm 2. There is no gas diffusion layer in the MEA of this URFC. In fuel cell mode, the polarisation curve of each test cell was measured using the experimental set-up displayed in Figure 7. Gas supply for the fuel cell operation was controlled via valves. Hydrogen was supplied through a storage tank initially filled by electrolyser operation. During cell operation, provisions were made to humidify the incoming hydrogen gas. At each potential, a period of two minutes was allowed for the current and hydrogen usage readings to stabilise. The cell was operated at a room temperature of 18 C and atmospheric pressure. F C + V Range Range 20V DC 10 or 20A DC A Figure 6: Circuit for determining energy efficiency of fuel cell During electrolysis, at the beginning 20 minutes were allowed for the temperature to equilibrate. The cell was polarised to a constant voltage with a D.C power supply. At each potential, two minutes were allowed for the current to stabilise. In most instances, a fairly stable current was obtained, but in a few cases the current continued to drift downward with time. The reason for this drift may have been improper gas removal resulting in a buildup of ohmic resistance at the electrode-membrane interface. The current decay was observed throughout the experiment and was less during measurement of the initial polarisation curve in electrolyser mode. 8

9 Energy efficiency was evaluated as the ratio of energy content of hydrogen produced based on the High Heating Value (HHV) to the energy input to the electrolyser. Faraday efficiency was calculated as the ratio of measured rate of hydrogen production to the theoretical maximum rate calculated from current flow. The volumes of hydrogen produced in both URFC electrolyser mode and dedicated PEM electrolyser were measured using the difference in water level of the storage tank. The variation of current drawn with the input voltage was plotted. Energy efficiency of the fuel cell was calculated as the ratio of energy supplied by the fuel cell to the energy content (based on HHV) of hydrogen consumed. The fuel utilisation coefficient was calculated as the ratio of measured cell voltage to the voltage calculated using the HHV and assuming all input hydrogen is consumed. Hydrogen consumption was determined by the volume of water displaced during fuel cell operation. 4.2 Results The measured polarisation curves in electrolyser and fuel cell modes of the URFC are compared to the corresponding curves for the dedicated PEM electrolyser and fuel cell in Figure 8. Power versus current density curves for the various cells and modes are plotted in Figure 9, and hydrogen production versus current density for the electrolyser modes in Figure 10. The associated variations in energy efficiency with current density are presented in Figure Voltage (V) URFC electrolyser URFC fuel cell Dedicated electrolyser Dedicated fuel cell Current density (ma/cm 2 ) Figure 7: Polarisation curves for the URFC in E and FC modes, and the dedicated PEM electrolyser and fuel cell. 2.0 Power (Output/Input), W Current density (ma/cm 2 ) URFC electrolyser URFC fuel cell Dedicated electrolyser Dedicated fuel cell Figure 8: Power versus current density curves for the URFC in FC mode and the dedicated PEM fuel cell 9

10 200 Current density (ma/cm 2 ) URFC electrolyser Dedicated electrolyser Volume of hydrogen produced (cm 3 /sec) Figure 9: Current density versus volume of hydrogen produced 100 Energy efficiency Current density (ma/cm 2 ) URFC electrolyser URFC fuel cell Dedicated electrolyser Dedicated fuel cell Figure 10: Energy efficiencies of the URFC in E and FC modes compared to those for the dedicated electrolyser and fuel cell. 4.3 Discussion The shapes of the experimentally-determined polarization curves for the dedicated PEM electrolyser and PEM fuel cell (Figure 8) follow closely the forms expected from the general analysis in section 2 (Figure 3), as do the power versus current density curves (Figure 9 compared to Figure 4). From Figure 9 it can be seen that the power ratio (maximum power in divided by maximum power out) is about 4 for this URFC. The hydrogen production rate of the dedicated PEM electrolyser was slightly greater than that of the URFC in E-mode for a given current density. At a current density of 140 ma/cm 2, the hydrogen production rate per unit cell area of the URFC was about 0.05 cm 3 /min, while that of the dedicated PEM electrolyser was 0.06 cm 3 /min. The performance of the URFC in fuel-cell mode was also slightly lower than that of the dedicated PEM fuel cell. For example, at a current density of 60 ma/cm 2, the power output of the URFC was 0.12 W/cm 2 compared 0.16 W/cm 2 for the dedicated fuel cell. The energy efficiency of the URFC in E-mode was only slightly lower than that of the dedicated PEM electrolyser, being no more than 5 % points less than the latter over the full range of current densities. The energy efficiency of the URFC peaks at about 90% and falls to just under 80% at the highest current density. The energy efficiencies of the dedicated PEM fuel cell and that of the URFC in FC- 10

11 mode were much lower, the former peaking at 50% and then falling to below 30% at a current density of 130 ma/cm 2. This could be because of open ended fuel cell operation which might have resulted in low fuel cell utilisation co-efficient. The use of the HHV (rather than the Low Heating Value) in calculating energy efficiencies also tends to accentuate the differences in the efficiencies obtained in the two modes. The energy efficiency of the URFC in FC mode was significantly lower than that of the dedicated PEM fuel cell over the whole range of current densities, indicating that this particular URFC is far from optimal for reversible operation. The low energy efficiency in URFC FC-mode was possibly due to the oxygen electrode retaining too much water after operation in E-mode so that the diffusion of oxygen gas to catalyst sites was inhibited. The observations here that the performance of the URFC in electrolyser mode is very close that of a dedicated PEM electrolyser bode well for the future application of URFCs in solar-hydrogen RAPS systems. However, it clear that that performance of the URFC in FC-mode needs to be improved so that it is much closer to that of a dedicated is only slightly less than that of a comparable dedicated PEM fuel cell. 5 CONCLUSIONS An analysis of typical voltage-current characteristic curves of a PEM cell with a given active membrane electrode assembly active area indicates that the electrical power output of a URFC in FC mode is likely to be less than a third of the maximum electrical power input in E-mode, and may be less than a quarter in practice. For URFCs to be economically preferable to a separate electrolyser and fuel cell in a hydrogen energy storage system for renewable-energy RAPS applications, the following key design goals are proposed: The performance of a URFC in E-mode and FC-mode in terms of energy efficiency that is equal to or only slightly lower than that of a comparable dedicated PEM electrolyser and dedicated PEM fuel cell. Capital costs of the URFC-based storage system that are 40% lower than those for a comparable system employing a dedicated electrolyser and fuel cell. An experimental comparison of a small single-cell URFC with a comparable dedicated PEM fuel cell has been performed. The energy efficiency of the URFC in E-mode was only slightly lower than that of the dedicated PEM electrolyser, being no more than 5 % points less than the latter over the full range of current densities. In FC mode the energy efficiency of the URFC was significantly lower than that of the dedicated PEM fuel cell, indicating that the particular URFC tested was far from optimisation for reversible operation. A design for a single-cell URFC using a commercially-available reversible membrane electrode assembly is described. Experiments to compare the performance of this URFC with comparable dedicated PEM electrolysers and fuel cells are in progress. REFERENCES 1. Agbossou K, Kolbe M, Hamelin J, Bernier, E and Bose TK, 2004, Electrolytic Hydrogen Based Renewable Energy System with Oxygen Recovery and Re-utilization, Renewable Energy, vol. 29, no. 8, pp Schucan, T, 2000, International energy agency hydrogen implementing agreement task 11: Integrated systems Final report of subtask A: Case studies of Integrated Hydrogen energy systems, viewed 08 March Dutton AG, Belies JAM, Diehard H, Falsetto M, Hug W, Rorschach D and Riddell AJ, 2000, Experience in the design, sizing, economics, and implementation of autonomous wind-powered 11

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