- Email: [email protected]
fuel cell system
Solar Energy 79 (2005) 544–550 www.elsevier.com/locate/solener
Solar-powered regenerative PEM electrolyzer/fuel cell system Daniel Shapiro a, John Duffy a
a,*
, Michael Kimble
b,1
, Michael Pien
b
University of Massachusetts Lowell, Solar Engineering Program, 1 University Avenue, Lowell, MA 01854, USA b ElectroChem, Inc., 400 W. Cummings Park, Woburn, MA 01801, USA Received 23 June 2003; received in revised form 2 August 2004; accepted 8 October 2004 Available online 10 February 2005 Communicated by: Associate Editor A.T. Raissi
Abstract An electrolyzer/fuel cell energy storage system is a promising alternative to batteries for storing energy from solar electric power systems. Such a system was designed, including a proton-exchange membrane (PEM) electrolyzer, high-pressure hydrogen and oxygen storage, and a PEM fuel cell. The system operates in a closed water loop. A prototype system was constructed, including an experimental PEM electrolyzer and combined gas/water storage tanks. Testing goals included general system feasibility, characterization of the electrolyzer performance (target was sustainable 1.0 A/cm2 at 2.0 V per cell), performance of the electrolyzer as a compressor, and evaluation of the system for direct-coupled use with a PV array. When integrated with a photovoltaic array, this type of system is expected to provide reliable, environmentally benign power to remote installations. If grid-coupled, this system (without PV array) would provide high-quality backup power to critical systems such as telecommunications and medical facilities. 2005 Published by Elsevier Ltd.
1. Introduction 1.1. Context It is often cited that at least 2 billion people live without access to reliable electricity (Flavin and OÕMeara, 1997). Even modest amounts of electric power can tremendously improve the quality of life of people in underserved regions; two of the authors have seen this * Corresponding author. Tel.: +1 978 934 2968; fax: +1 978 934 3048. E-mail addresses: [email protected] (D. Shapiro), John_duff[email protected] (J. Duffy), [email protected] (M. Pien). 1 Now with MicroCell Technologies, 410 Great Road, Suite C-2, Littleton, MA 01460, USA.
0038-092X/$ - see front matter 2005 Published by Elsevier Ltd. doi:10.1016/j.solener.2004.10.013
firsthand through rural electrification work in Peru. Among the basic applications are: lights, vaccine refrigerators, radio transceivers, and nebulizers. There is a growing consensus (although by no means unanimous) that the global petroleum-based industry is unsustainable from an ecological viewpoint (UN IPCC, 2001), and that alternative sources of energy must be developed. Renewable energy technologies, including solar photovoltaic, solar thermal, geothermal, tidal, wind and others, offer the hope of sustainable development. They are inherently scalable, and lend themselves extremely well to distributed power generation. This development model (many small generation facilities rather than a few large ones) holds the potential for eliminating the need for an all-encompassing grid to transmit power to users—especially helpful in areas where such a grid does not already exist.
D. Shapiro et al. / Solar Energy 79 (2005) 544–550
545
1.2. Regenerative fuel cell system: A definition Regenerative fuel cells, including those using water electrolysis, are not a new concept (McElroy, 1993). Such a system functions like a secondary (or rechargeable) battery. In a regenerative scheme, the energy conversion device (fuel cell) is combined with other components to become a true energy storage system. An external power source supplies initial energy, which is converted within the system for storage. On demand, the stored energy is reconverted into electricity. When the stored energy is exhausted, the system may be replenished by the outside power source.
Fig. 1. Regenerative PV electrolyzer/fuel cell system.
1.3. Scope of project B
A
Fuel Cell
D
C
F
O2
H2
H 2O
H 2O
H 2O
Recent developments in PEM fuel cells are beginning to make possible a promising alternative to batteries for storage of energy from solar electric power systems (Cisar et al., 1999). With this in mind, UML and ElectroChem combined efforts to design an improved energy production/storage system based upon the regenerative fuel cell concept. When integrated with a photovoltaic array, this type of system is expected to provide reliable, environmentally benign power to remote installations. If grid-coupled, this system (without PV array) would provide high-quality backup power to critical systems such as telecommunications and medical facilities. Major project goals include:
E
• Design a system (the ‘‘target system’’) capable of delivering 4 kW for 4 h, and of recharging itself within 40 h with gases stored at up to 2000 psig (13.8 MPa). • Design, build, and test a 200 psig (1379 kPa)-capable prototype system. • Collect operating data from PEM electrolyzer: target was 1.0 A/cm2 at 2.0 V per cell. • Evaluate the systemÕs suitability for direct-coupled use with a PV array. The aim of this study is to contribute to existing knowledge in several ways: • Evaluate performance of experimental PEM device. • Make system design innovations. • Evaluate performance of PEM electrolyzer as compressor.
Fig. 2. Target system components: (A) accumulator, (B) differential-pressure relief, (C) electrolyzer, (D) external power lead, (E) water circulating pump, (F) fuel cell power out.
PEM (Proton-Exchange Membrane) electrolyzer, which breaks water into hydrogen and oxygen and compresses them into high-pressure storage tanks. The gases are used to run a PEM fuel cell, producing on-demand electricity to power a load. The fuel cellÕs only byproducts are heat and pure water, which is recycled for use by the electrolyzer. The only required input is energy to drive the electrolyzer—the water and gases cycle in a closed loop. The round-trip efficiency of the storage system would be roughly 25%, but the marginal cost of added energy storage (i.e., larger gas tanks) would be relatively low. 2.2. Elements of target system design
2. Target system 2.1. Overview The system concept is shown in Fig. 1, with a more detailed view in Fig. 2. A photovoltaic array drives a
2.2.1. Simplification From the outset, a reductionist approach was adopted: simplification was the watchword. Fewer components would mean lower system cost and fewer possible points of failure. To that end, one of the first design
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D. Shapiro et al. / Solar Energy 79 (2005) 544–550
decisions was to integrate gas and water storage in the same pressure vessels. High-pressure, corrosion-resistant containers are expensive, so any reduction in the required number was significant. This decision also meant that the system could dispense with components otherwise needed to produce dry gas: phase separators and dryers to remove moisture from the electrolyzerÕs two product gas streams. 2.2.2. Pressure balancing The electrolyzer is a filter-press style stack of individual electrolytic cells, capped with stiff endplates and held together with compression bolts around the perimeter. This design was intended for use in free atmosphere at moderate internal pressures, up to about 50 psig (345 kPa). Above this pressure, leakage from the seals between cells is expected to be significant. Similarly, if the ambient pressure were much greater than the internal pressure, the stack would leak. To operate successfully at high pressure, therefore, the electrolyzerÕs seals must not experience a large differential pressure between the internal flow fields and the stackÕs environment. It was decided to enclose the electrolyzer in a pressurized environment. Electrolyzers with reinforced plates to operate at high pressure are available commercially, but these would be more costly than those under discussion here. If the electrolyzer were simply enclosed in a pre-pressurized vessel, the systemÕs operating pressure range would still be limited by this pressure—i.e., it could not operate more than about 50 psi (345 kPa) above or below the electrolyzer vessel pressure, whatever that pressure might be. To access the full desired range of system operating pressures, then, the challenge was to match continuously the electrolyzerÕs ambient pressure to that of the fluid within the electrolyzer, entirely eliminating pressure differences across the seals. This requirement led to a radically simplified system design: the electrolyzer is contained within the hydrogen/ water pressure vessel. This concept automatically provides continuous pressure-matching; as the system pressure changes (and thus the pressure of the electrolyzerÕs working fluid), the electrolyzerÕs ambient pressure is of necessity equal to its internal pressure. In addition, this system design reduces the number of fittings and components exposed to the difference between system and room pressure: tubing and connections for hydrogen gas and hydrogen-side water transport are reduced or eliminated. To address the possibility of an oxygen leak within the hydrogen vessel, catalyst-treated material would be placed around the electrolyzer. Any escaping oxygen would immediately recombine on the catalyzed surface with the ambient hydrogen, forming water. This spontaneous reaction is the same as takes place in a PEM fuel cell.
2.2.3. Pressure damping Another key issue in the system design was to minimize any pressure differences within the electrolyzer itself. The core of a PEM device (fuel cell or electrolyzer) is the thin, flexible proton-exchange membrane itself, usually laminated between catalyzed carbon electrodes. It separates the hydrogen side of the system from the oxygen side. This membrane electrode assembly (MEA, see Fig. 3) is relatively fragile; undue stress could compromise the intimate bond between membrane and electrodes or even result in partial or complete rupture, with consequences ranging from impaired system performance to uncontrolled mixing of H2 and O2. It was essential to prevent transient pressure surges, such as might be caused by automated valve actuation, from being transmitted to the MEA. A hydraulic accumulator was chosen to provide a transfer-barrier, pressure–damping interface between the hydrogen and oxygen volumes. The accumulator (at (A) in Fig. 2) is a steel shell containing a flexible bladder. The bladder volume would be connected to the systemÕs hydrogen side, while the shell would be connected to the oxygen side. The accumulator acts as a gas damper, minimizing the magnitude of any pressure surges in the system. 2.2.4. Volume imbalance The accumulator has the additional virtue of compensating for imbalances in water distribution. Under normal operation, water is consumed only on the oxygen side of the system, creating a volume imbalance. Compounding the problem, each proton migrating through the membrane osmotically ‘‘pulls’’ between 1 and 2.5 water molecules along with it to the hydrogen side of the system (US DOE, 2000). Thus for each H2 molecule produced by the electrolyzer, the oxygen side loses 3–6 molecules of H2O, while the hydrogen side gains 2–5 H2O molecules. This cumulative imbalance limits the electrolyzerÕs maximum run time in two ways: first, at some point the oxygen side will run out of water, and second, the mounting pressure imbalance will endanger the electrolyzer. By allowing the relative volume change between the two gases needed to equalize their pressures, the accumulator will extend system run time between cycles, and will permit operation even under conditions where a system problem such as a slow oxygen leak creates improper proportions of oxygen and hydrogen.
Fig. 3. Membrane electrode assembly (MEA).
D. Shapiro et al. / Solar Energy 79 (2005) 544–550
2.2.5. Differential-pressure (D-p) relief Large or sustained pressure imbalances between the oxygen and hydrogen sides are avoided with a differential-pressure relief valve (at (B) in Fig. 2). Excessive pressure from either side causes a piston to move to one side, allowing the higher-pressure gas an avenue to escape. After sufficient gas has escaped, the piston moves back into a central position, again sealing the system.
3. Prototype system With the target system design in hand, a prototype was designed much like the target system, but with the electrolyzer and fuel cell scaled down. However, as it became evident that the project budget would not support the cost of high-pressure hardware, it was decided that the critical mechanisms and concepts could be demonstrated by a prototype designed to operate at up to 200 psig (1379 kPa). The prototype (Fig. 4) produces enough gas at high enough pressures to validate the system design and to serve as a testbed for the experimental electrolyzer. As the analysis of safety issues concerning placing the electrolyzer in a hydrogen atmosphere was incomplete at the time, the electrolyzer was placed in its own pressure vessel and a nitrogen pressurization subsystem was added.
System Schematic Bleed Valve
Relief Valve
Oxygen Tank
Pump
The hydraulic accumulator was omitted due to concerns that it would create difficulty in isolating pressure and volume phenomena—one could not know the quantity of either gas in the system when the gas volumes would be affected by the accumulatorÕs presence. It was desirable, therefore, to collect data without the accumulator. ElectroChem had meanwhile redesigned their electrolyzer stack, greatly improving the support of the proton-exchange membranes at the heart of each cell. This change made the stack much more resistant to damage by pressure transients and imbalances between the cathode and anode sides. Testing could therefore be conducted without the use of an accumulator for pressure balancing. The prototype includes the electrolyzer in its containment vessel, gas/water storage tanks, circulating pump, manual controls and data-acquisition sensors. Also in place are a full complement of relief valves, bleed valves, and a catalyzed hood for the electrolyzer (as described above, for the target system). The electrolyzer is driven by a regulated DC power supply rather than a PV array, for convenience in selecting precise and consistent power levels. This system was tested with very encouraging results.
4. Testing A PC-based data-acquisition system was used to record most system measurements. Thermocouples were placed at the inlet, both outlets, and on the body of the electrolyzer, as well as in each gas tank and in the electrolyzer pressure vessel. Pressure transducers reported the hydrogen and oxygen pressures. Precision current shunts gave the circulating pump and electrolyzer input currents, while the input voltages were measured directly.
Bleed Valve
Hydrogen Tank
Sight Glasses
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4.1. Initial function tests
Relief Valve
The prototypeÕs ‘‘shakedown cruise’’ was a series of gas-generation tests. The system was switched on, H2 and O2 were produced, and the system pressure was allowed to mount. Temperatures were closely monitored, as it was not known how much heat the electrolyzer would release.
Electrolyzer
Fig. 4. Schematic of prototype system.
4.2. Stepwise constant-pressure test Nitrogen
Electrolyzer Vessel
The primary means of investigation was a series of ‘‘stepwise constant-pressure’’ tests. Through the use of back-pressure regulators, the system was tested under constant-pressure operation at a variety of pressure levels. Once each desired pressure level was reached, the pressure was held constant by bleeding additional gas from the system as it was produced. Care was taken to
D. Shapiro et al. / Solar Energy 79 (2005) 544–550
vary the pattern of pressure levels to avoid confusion due to possible hysteresis effects. Throughout the above procedure, the intent was to supply the electrolyzer with power at a constant current level, allowing the stack voltage to fluctuate as the electrolyzerÕs demand for power changed with changing system pressure.
process. It appears that the ‘‘missing energy’’ is drawn from the stack overvoltage, the amount by which the cellsÕ voltages exceed the thermodynamic ideal minimum voltage of 1.23 V, and that the electrolyzer just becomes less inefficient as the pressure rises. The Nernst equation (Eq. (1)) is often used to describe how the ideal cell potential varies with the concentrations of the products and reactants.
5. Results E ¼ E0 5.1. Overall system performance
5.2. Electrolyzer polarization The experimental electrolyzer was tested up to its target current density of 1.0 A/cm2 at a range of pressures, at an average temperature of 65 C (see Fig. 5). At this current level, cell voltage averaged 2.5 V, well above the ideal but approaching the voltage expected from this prototype. Note that the 200 psig data series exhibited an apparent hysteresis effect as current density was changed. 5.3. Electrolyzer as compressor It is known that an electrolyzer can generate gas at high pressure with little more energy than required at low pressure (Cisar et al., 1999). Current work aims at proposing a model for the incremental compression
ð1Þ
where E = ideal cell potential at a given state; E0 = ideal cell potential at STP (‘‘standard’’ potential); n = number of moles of electrons being transferred in the half-reactions; F = FaradayÕs constant, amount of charge carried by 1 mol of electrons, 96,485 C/mol e; R = gas constant, 8.3145 J/K mol; T = temperature in Kelvins; C = concentrations of products or reactants. Here, the products are pure H2 and O2 gas, and their concentration may be taken as their partial pressures (Zumdahl, 1989). If the product gas pressure increases by a given factor X, the ideal cell potential increases by the amount (RT/2F) * ln(X3/2). In one stepwise constant-pressure test, the electrolyzer generated enough gas to raise the system pressure from 32 to 110 psig (221–758 kPa) (see Fig. 6). The measured power demand rose minimally, from 278.1 W to 282.2 W, or an increase of only 1.5%. This type of result has been verified in multiple tests, up to 210 psig (1448 kPa). During this test, the electrolyzer showed a voltage rise significantly greater than the ideal (see Table 1 and Fig. 7). This is being explored in further tests, but is
Electrolyzer Power (watts)
The basic design has proven quite satisfactory. Both the strengths and shortcomings of the prototype have provided crucial lessons for refining the target systemÕs design. For example, integrating the pressure vessels has greatly simplified the system. On the other hand, the original design had to be modified by adding a phase separator at the oxygen sample port, to prevent the escape of water during oxygen gas venting. Pressure balancing across the electrolyzer seals was successfully demonstrated; the electrolyzer stack, designed for 50 psig (345 kPa) use, was operated up to 220 psig (1517 kPa) internal pressure with no evidence of hydrogen or oxygen leakage.
RT j C prod j ; ln nF j C react j
284
120
283
110
282
100
281
90
280
80
.
279
70
278
60
277
50
Electrolyzer Power
276
40
4P O2 (EL out)
275
30 0:45
0:41
0:37
0:33
0:29
0:25
0:20
0:16
0:12
0:08
800
200 psig 150 psig 100 psig 50 psig
0:04
1000
20 0:00
Curr. Density (mA / cm2)
274
1200
System Pressure (psig)
548
Elapsed Time (hours:minutes)
Fig. 6. Power increase of 4-cell, 50 cm2 Temp = 65.4 C, current density = 0.6 A/cm2.
600
electrolyzer.
400 200 0 1.7
1.9
2.1
2.3
2. 5
Cell Potential (volts / cell) 2
Fig. 5. Polarization curves, 50 cm electrolyzer.
Table 1 Pressure and voltage rise, ideal vs. actual Press. factor X
Ideal DV, mV
Actual DV, mV
2.67
20.5
32
9.45
140
9.4
120
9.35
100
9.3
80
9.25
60 Electrolyzer Voltage Oxygen Pressure
9.2 9.15
20 0:45
0:41
0:37
0:33
0:29
0:25
0:20
0:16
0:12
0:08
0:04
0:00
9.1
40
Pressure (psig)
Voltage (volts)
D. Shapiro et al. / Solar Energy 79 (2005) 544–550
0
Elapsed Time (hours:minutes)
Fig. 7. Voltage rise of 4-cell, 50 cm2 electrolyzer (at 65 C).
Fig. 8. PV module I–V curves and load line.
probably related to the electrolyzerÕs stack potential at all pressures being well above the ideal value.
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electrolyzer in a storage tank, accumulators, and differential-pressure relief valve. The experience of building and operating the prototype system with pressures up to 200 psig (1379 kPa) has established proof-of-concept of the placement of the electrolyzer in the hydrogen storage tank. The prototype has also provided valuable lessons for refining the design of a larger scale higherpressure regenerative electrolyzer/fuel cell system. A larger 4 kW fuel cell system has been built and is undergoing initial testing. This system could be grid-coupled to supply ‘‘premium power’’ needs for high-reliability, high-quality backup power, or integrated with a photovoltaic array to serve remote locations. Several relevant issues were not addressed in this work, and remain as subjects for further testing and modeling: Thermal management of this type of system requires investigation. Das et al. (2003) did simulate the heat transfer of this full-scale system with the addition of cooling fins on the gas tubing and oversized fuel cell plates of graphite and showed adequate cooling. Further work is necessary to determine whether the electrolyzer may be a less efficient compressor at high pressures (up to 3000 psig, 20.7 MPa). Aurora (2003) developed a detailed energy flow model of this type of system and predicted the electrolyzer to be more efficient than a mechanical compressor. Especially at high pressures and current densities, side reactions and material breakdown may affect both the purity of the product gases and the longevity and performance of the system. Replaceable ion-exchange resins could be used to mitigate impurity issues.
5.4. Direct-coupling PEM electrolyzer with PV Fig. 8 illustrates conceptually the very favorable match between the PEM electrolyzerÕs load characteristics and the maximum power points of typical PV modules. The ‘‘sample electrolyzer load line’’ is from actual test data, and is superimposed upon a set of current– voltage curves from a typical commercial PV module (here, an ASE Americas ASE-50). Since the electrolyzer stack is modular, at approximately 2 V per cell, an array of PV modules and an electrolyzer can be custom-fit to each other. This indicates that PEM electrolyzer-based systems can probably dispense with the complication, expense, and potential unreliability of interfaces such as maximum power point trackers. Previous work (such as Morimoto et al., 1986; and Arkin and Duffy, 2001) also supports this approach.
6. Discussion Novel pressure regulation concepts have been incorporated into the design of a battery replacement system with a PEM electrolyzer, gas storage, and fuel cell:
Acknowledgment The authors would like to thank the Massachusetts Toxics Use Reduction Institute (TURI) and ElectroChem, Inc. for partial support of this research.
References Arkin, A., Duffy, J.J., 2001. Modeling of PV, electrolyzer and gas storage in a stand-alone solar–fuel cell system. In: Campbell-Howe, R. (Ed.), Proceedings of the National Solar Energy Conference. American Solar Energy Society, Washington, DC, USA. Aurora, P., 2003. Simulation, Modeling and Control of a Solar Hydrogen Fuel Cell System for Remote Applications. MS Thesis. U. Mass. Lowell. Cisar, A., Clarke, E., Salinas, C., Murphy, O.J., 1999. PEM energy storage for solar aircraft. Society of Automotive Engineers. Das, A., Duffy, J., Kimble, M., 2003. Heat transfer improvement for a 4 kW regenerative PEM electrolyzer fuel cell system. In: Campbell-Howe, R. (Ed.), Proceedings of the
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National Solar Energy Conference. American Solar Energy Society, Austin, TX, USA. Flavin, C., OÕMeara, M., 1997. Financing solar electricity. World Watch 10 (3), 1. McElroy, J.F., 1993. Recent advances in SPE water electrolyzer. In: Wilson, R.M. (compiler) (Ed.), Proceedings of Space Electrochemical Research and Technology Conference. National Aeronautics and Space Administration, Cleveland, OH, USA, pp. 193–201. Morimoto, Y., Hayashi, T., Maeda, Y., 1986. Mobile solar energy hydrogen generating system. In: Veziroglu, T.N.,
Getoff, N., Weizierl, P. (Eds.), Proceedings of the 6th World Hydrogen Energy Conference, vol. 1, Vienna, Austria, pp. 326–332. United Nations Intergovernmental Panel for Climate Change Working Group I, 2001. Summary for Policy Makers. Third Assessment Report. US Department of Energy, 2000. Fuel Cell Handbook, fifth ed. US Department of Energy, Morgantown, West Virginia, USA. Zumdahl, S., 1989. Chemistry. D.C. Heath and Company, Lexington, MA, pp. 580, 582, 590, 761, 794.
Solar-powered regenerative PEM electrolyzer/fuel cell system Daniel Shapiro a, John Duffy a
a,*
, Michael Kimble
b,1
, Michael Pien
b
University of Massachusetts Lowell, Solar Engineering Program, 1 University Avenue, Lowell, MA 01854, USA b ElectroChem, Inc., 400 W. Cummings Park, Woburn, MA 01801, USA Received 23 June 2003; received in revised form 2 August 2004; accepted 8 October 2004 Available online 10 February 2005 Communicated by: Associate Editor A.T. Raissi
Abstract An electrolyzer/fuel cell energy storage system is a promising alternative to batteries for storing energy from solar electric power systems. Such a system was designed, including a proton-exchange membrane (PEM) electrolyzer, high-pressure hydrogen and oxygen storage, and a PEM fuel cell. The system operates in a closed water loop. A prototype system was constructed, including an experimental PEM electrolyzer and combined gas/water storage tanks. Testing goals included general system feasibility, characterization of the electrolyzer performance (target was sustainable 1.0 A/cm2 at 2.0 V per cell), performance of the electrolyzer as a compressor, and evaluation of the system for direct-coupled use with a PV array. When integrated with a photovoltaic array, this type of system is expected to provide reliable, environmentally benign power to remote installations. If grid-coupled, this system (without PV array) would provide high-quality backup power to critical systems such as telecommunications and medical facilities. 2005 Published by Elsevier Ltd.
1. Introduction 1.1. Context It is often cited that at least 2 billion people live without access to reliable electricity (Flavin and OÕMeara, 1997). Even modest amounts of electric power can tremendously improve the quality of life of people in underserved regions; two of the authors have seen this * Corresponding author. Tel.: +1 978 934 2968; fax: +1 978 934 3048. E-mail addresses: [email protected] (D. Shapiro), John_duff[email protected] (J. Duffy), [email protected] (M. Pien). 1 Now with MicroCell Technologies, 410 Great Road, Suite C-2, Littleton, MA 01460, USA.
0038-092X/$ - see front matter 2005 Published by Elsevier Ltd. doi:10.1016/j.solener.2004.10.013
firsthand through rural electrification work in Peru. Among the basic applications are: lights, vaccine refrigerators, radio transceivers, and nebulizers. There is a growing consensus (although by no means unanimous) that the global petroleum-based industry is unsustainable from an ecological viewpoint (UN IPCC, 2001), and that alternative sources of energy must be developed. Renewable energy technologies, including solar photovoltaic, solar thermal, geothermal, tidal, wind and others, offer the hope of sustainable development. They are inherently scalable, and lend themselves extremely well to distributed power generation. This development model (many small generation facilities rather than a few large ones) holds the potential for eliminating the need for an all-encompassing grid to transmit power to users—especially helpful in areas where such a grid does not already exist.
D. Shapiro et al. / Solar Energy 79 (2005) 544–550
545
1.2. Regenerative fuel cell system: A definition Regenerative fuel cells, including those using water electrolysis, are not a new concept (McElroy, 1993). Such a system functions like a secondary (or rechargeable) battery. In a regenerative scheme, the energy conversion device (fuel cell) is combined with other components to become a true energy storage system. An external power source supplies initial energy, which is converted within the system for storage. On demand, the stored energy is reconverted into electricity. When the stored energy is exhausted, the system may be replenished by the outside power source.
Fig. 1. Regenerative PV electrolyzer/fuel cell system.
1.3. Scope of project B
A
Fuel Cell
D
C
F
O2
H2
H 2O
H 2O
H 2O
Recent developments in PEM fuel cells are beginning to make possible a promising alternative to batteries for storage of energy from solar electric power systems (Cisar et al., 1999). With this in mind, UML and ElectroChem combined efforts to design an improved energy production/storage system based upon the regenerative fuel cell concept. When integrated with a photovoltaic array, this type of system is expected to provide reliable, environmentally benign power to remote installations. If grid-coupled, this system (without PV array) would provide high-quality backup power to critical systems such as telecommunications and medical facilities. Major project goals include:
E
• Design a system (the ‘‘target system’’) capable of delivering 4 kW for 4 h, and of recharging itself within 40 h with gases stored at up to 2000 psig (13.8 MPa). • Design, build, and test a 200 psig (1379 kPa)-capable prototype system. • Collect operating data from PEM electrolyzer: target was 1.0 A/cm2 at 2.0 V per cell. • Evaluate the systemÕs suitability for direct-coupled use with a PV array. The aim of this study is to contribute to existing knowledge in several ways: • Evaluate performance of experimental PEM device. • Make system design innovations. • Evaluate performance of PEM electrolyzer as compressor.
Fig. 2. Target system components: (A) accumulator, (B) differential-pressure relief, (C) electrolyzer, (D) external power lead, (E) water circulating pump, (F) fuel cell power out.
PEM (Proton-Exchange Membrane) electrolyzer, which breaks water into hydrogen and oxygen and compresses them into high-pressure storage tanks. The gases are used to run a PEM fuel cell, producing on-demand electricity to power a load. The fuel cellÕs only byproducts are heat and pure water, which is recycled for use by the electrolyzer. The only required input is energy to drive the electrolyzer—the water and gases cycle in a closed loop. The round-trip efficiency of the storage system would be roughly 25%, but the marginal cost of added energy storage (i.e., larger gas tanks) would be relatively low. 2.2. Elements of target system design
2. Target system 2.1. Overview The system concept is shown in Fig. 1, with a more detailed view in Fig. 2. A photovoltaic array drives a
2.2.1. Simplification From the outset, a reductionist approach was adopted: simplification was the watchword. Fewer components would mean lower system cost and fewer possible points of failure. To that end, one of the first design
546
D. Shapiro et al. / Solar Energy 79 (2005) 544–550
decisions was to integrate gas and water storage in the same pressure vessels. High-pressure, corrosion-resistant containers are expensive, so any reduction in the required number was significant. This decision also meant that the system could dispense with components otherwise needed to produce dry gas: phase separators and dryers to remove moisture from the electrolyzerÕs two product gas streams. 2.2.2. Pressure balancing The electrolyzer is a filter-press style stack of individual electrolytic cells, capped with stiff endplates and held together with compression bolts around the perimeter. This design was intended for use in free atmosphere at moderate internal pressures, up to about 50 psig (345 kPa). Above this pressure, leakage from the seals between cells is expected to be significant. Similarly, if the ambient pressure were much greater than the internal pressure, the stack would leak. To operate successfully at high pressure, therefore, the electrolyzerÕs seals must not experience a large differential pressure between the internal flow fields and the stackÕs environment. It was decided to enclose the electrolyzer in a pressurized environment. Electrolyzers with reinforced plates to operate at high pressure are available commercially, but these would be more costly than those under discussion here. If the electrolyzer were simply enclosed in a pre-pressurized vessel, the systemÕs operating pressure range would still be limited by this pressure—i.e., it could not operate more than about 50 psi (345 kPa) above or below the electrolyzer vessel pressure, whatever that pressure might be. To access the full desired range of system operating pressures, then, the challenge was to match continuously the electrolyzerÕs ambient pressure to that of the fluid within the electrolyzer, entirely eliminating pressure differences across the seals. This requirement led to a radically simplified system design: the electrolyzer is contained within the hydrogen/ water pressure vessel. This concept automatically provides continuous pressure-matching; as the system pressure changes (and thus the pressure of the electrolyzerÕs working fluid), the electrolyzerÕs ambient pressure is of necessity equal to its internal pressure. In addition, this system design reduces the number of fittings and components exposed to the difference between system and room pressure: tubing and connections for hydrogen gas and hydrogen-side water transport are reduced or eliminated. To address the possibility of an oxygen leak within the hydrogen vessel, catalyst-treated material would be placed around the electrolyzer. Any escaping oxygen would immediately recombine on the catalyzed surface with the ambient hydrogen, forming water. This spontaneous reaction is the same as takes place in a PEM fuel cell.
2.2.3. Pressure damping Another key issue in the system design was to minimize any pressure differences within the electrolyzer itself. The core of a PEM device (fuel cell or electrolyzer) is the thin, flexible proton-exchange membrane itself, usually laminated between catalyzed carbon electrodes. It separates the hydrogen side of the system from the oxygen side. This membrane electrode assembly (MEA, see Fig. 3) is relatively fragile; undue stress could compromise the intimate bond between membrane and electrodes or even result in partial or complete rupture, with consequences ranging from impaired system performance to uncontrolled mixing of H2 and O2. It was essential to prevent transient pressure surges, such as might be caused by automated valve actuation, from being transmitted to the MEA. A hydraulic accumulator was chosen to provide a transfer-barrier, pressure–damping interface between the hydrogen and oxygen volumes. The accumulator (at (A) in Fig. 2) is a steel shell containing a flexible bladder. The bladder volume would be connected to the systemÕs hydrogen side, while the shell would be connected to the oxygen side. The accumulator acts as a gas damper, minimizing the magnitude of any pressure surges in the system. 2.2.4. Volume imbalance The accumulator has the additional virtue of compensating for imbalances in water distribution. Under normal operation, water is consumed only on the oxygen side of the system, creating a volume imbalance. Compounding the problem, each proton migrating through the membrane osmotically ‘‘pulls’’ between 1 and 2.5 water molecules along with it to the hydrogen side of the system (US DOE, 2000). Thus for each H2 molecule produced by the electrolyzer, the oxygen side loses 3–6 molecules of H2O, while the hydrogen side gains 2–5 H2O molecules. This cumulative imbalance limits the electrolyzerÕs maximum run time in two ways: first, at some point the oxygen side will run out of water, and second, the mounting pressure imbalance will endanger the electrolyzer. By allowing the relative volume change between the two gases needed to equalize their pressures, the accumulator will extend system run time between cycles, and will permit operation even under conditions where a system problem such as a slow oxygen leak creates improper proportions of oxygen and hydrogen.
Fig. 3. Membrane electrode assembly (MEA).
D. Shapiro et al. / Solar Energy 79 (2005) 544–550
2.2.5. Differential-pressure (D-p) relief Large or sustained pressure imbalances between the oxygen and hydrogen sides are avoided with a differential-pressure relief valve (at (B) in Fig. 2). Excessive pressure from either side causes a piston to move to one side, allowing the higher-pressure gas an avenue to escape. After sufficient gas has escaped, the piston moves back into a central position, again sealing the system.
3. Prototype system With the target system design in hand, a prototype was designed much like the target system, but with the electrolyzer and fuel cell scaled down. However, as it became evident that the project budget would not support the cost of high-pressure hardware, it was decided that the critical mechanisms and concepts could be demonstrated by a prototype designed to operate at up to 200 psig (1379 kPa). The prototype (Fig. 4) produces enough gas at high enough pressures to validate the system design and to serve as a testbed for the experimental electrolyzer. As the analysis of safety issues concerning placing the electrolyzer in a hydrogen atmosphere was incomplete at the time, the electrolyzer was placed in its own pressure vessel and a nitrogen pressurization subsystem was added.
System Schematic Bleed Valve
Relief Valve
Oxygen Tank
Pump
The hydraulic accumulator was omitted due to concerns that it would create difficulty in isolating pressure and volume phenomena—one could not know the quantity of either gas in the system when the gas volumes would be affected by the accumulatorÕs presence. It was desirable, therefore, to collect data without the accumulator. ElectroChem had meanwhile redesigned their electrolyzer stack, greatly improving the support of the proton-exchange membranes at the heart of each cell. This change made the stack much more resistant to damage by pressure transients and imbalances between the cathode and anode sides. Testing could therefore be conducted without the use of an accumulator for pressure balancing. The prototype includes the electrolyzer in its containment vessel, gas/water storage tanks, circulating pump, manual controls and data-acquisition sensors. Also in place are a full complement of relief valves, bleed valves, and a catalyzed hood for the electrolyzer (as described above, for the target system). The electrolyzer is driven by a regulated DC power supply rather than a PV array, for convenience in selecting precise and consistent power levels. This system was tested with very encouraging results.
4. Testing A PC-based data-acquisition system was used to record most system measurements. Thermocouples were placed at the inlet, both outlets, and on the body of the electrolyzer, as well as in each gas tank and in the electrolyzer pressure vessel. Pressure transducers reported the hydrogen and oxygen pressures. Precision current shunts gave the circulating pump and electrolyzer input currents, while the input voltages were measured directly.
Bleed Valve
Hydrogen Tank
Sight Glasses
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4.1. Initial function tests
Relief Valve
The prototypeÕs ‘‘shakedown cruise’’ was a series of gas-generation tests. The system was switched on, H2 and O2 were produced, and the system pressure was allowed to mount. Temperatures were closely monitored, as it was not known how much heat the electrolyzer would release.
Electrolyzer
Fig. 4. Schematic of prototype system.
4.2. Stepwise constant-pressure test Nitrogen
Electrolyzer Vessel
The primary means of investigation was a series of ‘‘stepwise constant-pressure’’ tests. Through the use of back-pressure regulators, the system was tested under constant-pressure operation at a variety of pressure levels. Once each desired pressure level was reached, the pressure was held constant by bleeding additional gas from the system as it was produced. Care was taken to
D. Shapiro et al. / Solar Energy 79 (2005) 544–550
vary the pattern of pressure levels to avoid confusion due to possible hysteresis effects. Throughout the above procedure, the intent was to supply the electrolyzer with power at a constant current level, allowing the stack voltage to fluctuate as the electrolyzerÕs demand for power changed with changing system pressure.
process. It appears that the ‘‘missing energy’’ is drawn from the stack overvoltage, the amount by which the cellsÕ voltages exceed the thermodynamic ideal minimum voltage of 1.23 V, and that the electrolyzer just becomes less inefficient as the pressure rises. The Nernst equation (Eq. (1)) is often used to describe how the ideal cell potential varies with the concentrations of the products and reactants.
5. Results E ¼ E0 5.1. Overall system performance
5.2. Electrolyzer polarization The experimental electrolyzer was tested up to its target current density of 1.0 A/cm2 at a range of pressures, at an average temperature of 65 C (see Fig. 5). At this current level, cell voltage averaged 2.5 V, well above the ideal but approaching the voltage expected from this prototype. Note that the 200 psig data series exhibited an apparent hysteresis effect as current density was changed. 5.3. Electrolyzer as compressor It is known that an electrolyzer can generate gas at high pressure with little more energy than required at low pressure (Cisar et al., 1999). Current work aims at proposing a model for the incremental compression
ð1Þ
where E = ideal cell potential at a given state; E0 = ideal cell potential at STP (‘‘standard’’ potential); n = number of moles of electrons being transferred in the half-reactions; F = FaradayÕs constant, amount of charge carried by 1 mol of electrons, 96,485 C/mol e; R = gas constant, 8.3145 J/K mol; T = temperature in Kelvins; C = concentrations of products or reactants. Here, the products are pure H2 and O2 gas, and their concentration may be taken as their partial pressures (Zumdahl, 1989). If the product gas pressure increases by a given factor X, the ideal cell potential increases by the amount (RT/2F) * ln(X3/2). In one stepwise constant-pressure test, the electrolyzer generated enough gas to raise the system pressure from 32 to 110 psig (221–758 kPa) (see Fig. 6). The measured power demand rose minimally, from 278.1 W to 282.2 W, or an increase of only 1.5%. This type of result has been verified in multiple tests, up to 210 psig (1448 kPa). During this test, the electrolyzer showed a voltage rise significantly greater than the ideal (see Table 1 and Fig. 7). This is being explored in further tests, but is
Electrolyzer Power (watts)
The basic design has proven quite satisfactory. Both the strengths and shortcomings of the prototype have provided crucial lessons for refining the target systemÕs design. For example, integrating the pressure vessels has greatly simplified the system. On the other hand, the original design had to be modified by adding a phase separator at the oxygen sample port, to prevent the escape of water during oxygen gas venting. Pressure balancing across the electrolyzer seals was successfully demonstrated; the electrolyzer stack, designed for 50 psig (345 kPa) use, was operated up to 220 psig (1517 kPa) internal pressure with no evidence of hydrogen or oxygen leakage.
RT j C prod j ; ln nF j C react j
284
120
283
110
282
100
281
90
280
80
.
279
70
278
60
277
50
Electrolyzer Power
276
40
4P O2 (EL out)
275
30 0:45
0:41
0:37
0:33
0:29
0:25
0:20
0:16
0:12
0:08
800
200 psig 150 psig 100 psig 50 psig
0:04
1000
20 0:00
Curr. Density (mA / cm2)
274
1200
System Pressure (psig)
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Elapsed Time (hours:minutes)
Fig. 6. Power increase of 4-cell, 50 cm2 Temp = 65.4 C, current density = 0.6 A/cm2.
600
electrolyzer.
400 200 0 1.7
1.9
2.1
2.3
2. 5
Cell Potential (volts / cell) 2
Fig. 5. Polarization curves, 50 cm electrolyzer.
Table 1 Pressure and voltage rise, ideal vs. actual Press. factor X
Ideal DV, mV
Actual DV, mV
2.67
20.5
32
9.45
140
9.4
120
9.35
100
9.3
80
9.25
60 Electrolyzer Voltage Oxygen Pressure
9.2 9.15
20 0:45
0:41
0:37
0:33
0:29
0:25
0:20
0:16
0:12
0:08
0:04
0:00
9.1
40
Pressure (psig)
Voltage (volts)
D. Shapiro et al. / Solar Energy 79 (2005) 544–550
0
Elapsed Time (hours:minutes)
Fig. 7. Voltage rise of 4-cell, 50 cm2 electrolyzer (at 65 C).
Fig. 8. PV module I–V curves and load line.
probably related to the electrolyzerÕs stack potential at all pressures being well above the ideal value.
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electrolyzer in a storage tank, accumulators, and differential-pressure relief valve. The experience of building and operating the prototype system with pressures up to 200 psig (1379 kPa) has established proof-of-concept of the placement of the electrolyzer in the hydrogen storage tank. The prototype has also provided valuable lessons for refining the design of a larger scale higherpressure regenerative electrolyzer/fuel cell system. A larger 4 kW fuel cell system has been built and is undergoing initial testing. This system could be grid-coupled to supply ‘‘premium power’’ needs for high-reliability, high-quality backup power, or integrated with a photovoltaic array to serve remote locations. Several relevant issues were not addressed in this work, and remain as subjects for further testing and modeling: Thermal management of this type of system requires investigation. Das et al. (2003) did simulate the heat transfer of this full-scale system with the addition of cooling fins on the gas tubing and oversized fuel cell plates of graphite and showed adequate cooling. Further work is necessary to determine whether the electrolyzer may be a less efficient compressor at high pressures (up to 3000 psig, 20.7 MPa). Aurora (2003) developed a detailed energy flow model of this type of system and predicted the electrolyzer to be more efficient than a mechanical compressor. Especially at high pressures and current densities, side reactions and material breakdown may affect both the purity of the product gases and the longevity and performance of the system. Replaceable ion-exchange resins could be used to mitigate impurity issues.
5.4. Direct-coupling PEM electrolyzer with PV Fig. 8 illustrates conceptually the very favorable match between the PEM electrolyzerÕs load characteristics and the maximum power points of typical PV modules. The ‘‘sample electrolyzer load line’’ is from actual test data, and is superimposed upon a set of current– voltage curves from a typical commercial PV module (here, an ASE Americas ASE-50). Since the electrolyzer stack is modular, at approximately 2 V per cell, an array of PV modules and an electrolyzer can be custom-fit to each other. This indicates that PEM electrolyzer-based systems can probably dispense with the complication, expense, and potential unreliability of interfaces such as maximum power point trackers. Previous work (such as Morimoto et al., 1986; and Arkin and Duffy, 2001) also supports this approach.
6. Discussion Novel pressure regulation concepts have been incorporated into the design of a battery replacement system with a PEM electrolyzer, gas storage, and fuel cell:
Acknowledgment The authors would like to thank the Massachusetts Toxics Use Reduction Institute (TURI) and ElectroChem, Inc. for partial support of this research.
References Arkin, A., Duffy, J.J., 2001. Modeling of PV, electrolyzer and gas storage in a stand-alone solar–fuel cell system. In: Campbell-Howe, R. (Ed.), Proceedings of the National Solar Energy Conference. American Solar Energy Society, Washington, DC, USA. Aurora, P., 2003. Simulation, Modeling and Control of a Solar Hydrogen Fuel Cell System for Remote Applications. MS Thesis. U. Mass. Lowell. Cisar, A., Clarke, E., Salinas, C., Murphy, O.J., 1999. PEM energy storage for solar aircraft. Society of Automotive Engineers. Das, A., Duffy, J., Kimble, M., 2003. Heat transfer improvement for a 4 kW regenerative PEM electrolyzer fuel cell system. In: Campbell-Howe, R. (Ed.), Proceedings of the
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National Solar Energy Conference. American Solar Energy Society, Austin, TX, USA. Flavin, C., OÕMeara, M., 1997. Financing solar electricity. World Watch 10 (3), 1. McElroy, J.F., 1993. Recent advances in SPE water electrolyzer. In: Wilson, R.M. (compiler) (Ed.), Proceedings of Space Electrochemical Research and Technology Conference. National Aeronautics and Space Administration, Cleveland, OH, USA, pp. 193–201. Morimoto, Y., Hayashi, T., Maeda, Y., 1986. Mobile solar energy hydrogen generating system. In: Veziroglu, T.N.,
Getoff, N., Weizierl, P. (Eds.), Proceedings of the 6th World Hydrogen Energy Conference, vol. 1, Vienna, Austria, pp. 326–332. United Nations Intergovernmental Panel for Climate Change Working Group I, 2001. Summary for Policy Makers. Third Assessment Report. US Department of Energy, 2000. Fuel Cell Handbook, fifth ed. US Department of Energy, Morgantown, West Virginia, USA. Zumdahl, S., 1989. Chemistry. D.C. Heath and Company, Lexington, MA, pp. 580, 582, 590, 761, 794.