Michael W. Ellis

Fuel cell systems: efficient, flexible energy conversion for the 21st century

At the beginning of the 21st century, fuel cells appear poised to meet the power needs of a variety of applications. Fuel cells are electrochemical devices that convert chemical energy to electricity and thermal energy. Fuel cell systems are available to meet the needs of applications ranging from portable electronics to utility power plants. In addition to the fuel cell stack itself, a fuel cell system includes a fuel processor and subsystems to manage air, water thermal energy, and power. The overall system is efficient at full and part-load, scaleable to a wide range of sizes, environmentally friendly, and potentially competitive with conventional technology in first cost. Promising applications for fuel cells include portable power, transportation, building cogeneration, and distributed power for utilities. For portable power a fuel cell coupled with a fuel container can offer a higher energy storage density and more convenience than conventional battery systems. In transportation applications, fuel cells offer higher efficiency and better part-load performance than conventional engines. In stationary power applications, low emissions permit fuel cells to be located in high power density areas where they can supplement the existing utility grid. Furthermore, fuel cell systems can be directly connected to a building to provide both power and heat with cogeneration efficiencies as high as 80%.

Single domain PEMFC model based on agglomerate catalyst geometry

Abstract A steady two-dimensional computational model for a proton exchange membrane (PEM) fuel cell is presented. The model accounts for species transport, electrochemical kinetics, energy transport, current distribution, and water uptake and release in the catalyst layer. The governing differential equations are solved over a single computational domain, which consists of a gas channel, gas diffusion layer, and catalyst layer for both the anode and cathode sides of the cell as well as the solid polymer membrane. The model for the catalyst regions is based on an agglomerate geometry, which requires water species to exist in both dissolved and gaseous forms simultaneously. Data related to catalyst morphology, which was required by the model, was obtained via a microscopic analysis of a commercially available membrane electrode assembly (MEA). The coupled set of differential equations is solved with the commercial computational fluid dynamics (CFD) solver, CFDesign™, and is readily adaptable with respect to geometry and material property definitions. The results show that fuel cell performance is highly dependent on catalyst structure, specifically the relative volume fractions of gas pores and polymer membrane contained within the active region as well as the geometry of the individual agglomerates.

The Effect of Mechanical Fatigue on the Lifetimes of Membrane Electrode Assemblies

Long-term durability of the membrane electrode assembly (MEA) in proton exchange membrane (PEM) fuel cells is one of the major technological barriers to the commercialization of fuel cell vehicles. The cracks in the electrode layers of the MEA, referred to as mud-cracks, are potential contributors to the failure in the PEM. To investigate how these mud-cracks affect the mechanical durability of the MEA, pressure-loaded blister tests are performed at 90°C to determine the biaxial fatigue strength of Gore-Primea® series 57 MEA. In these volume-controlled tests, leaking rate is determined as a function of fatigue cycles. The failure is defined to occur when the leaking rate exceeds a specified threshold. Postmortem characterization using bubble point testing and field emission scanning electron microscopy (FESEM) was conducted to provide visual documentation of leaking failure sites. The analysis of the experimental leaking data indicates that the MEA has much shorter lifetimes at the same nominal stress levels than membrane samples without the electrode layers. FESEM photomicrographs of leaking locations identified via the bubble point testing show cracks in the membrane that are concentrated within the mud-cracks of the electrode layer. These two pieces of information indicate that the mud-cracks within the electrode layers contribute to the leaking failures of the MEA assembly. For the fuel cell industry, this study suggests there is an opportunity to reduce the likelihood of membrane pinhole failures by reducing the size and occurrence of the mud-cracks formed during the MEA processing or by increasing the fatigue resistance (including the notch sensitivity) of the membrane material within the MEA.

Lithium oxides precipitation in nonaqueous Li–air batteries

Lithium-air/oxygen battery is a rising star in the field of electrochemical energy storage as a promising alternative to lithium ion batteries. Nevertheless, this alluring system is still at its infant stage, and the breakthrough of lithium-air batteries into the energy market is currently constrained by a combination of scientific and technical challenges. Targeting at the air electrode in nonaqueous lithium-air batteries, this review attempts to summarize the knowledge about the fundamentals related to lithium oxides precipitation, which has been one of the vital and attractive aspects of the research communities of science and technology.

On the Use of Pressure-Loaded Blister Tests to Characterize the Strength and Durability of Proton Exchange Membranes

The use of pressurized blister specimens to characterize the biaxial strength and durability of proton exchange membranes (PEMs) is proposed, simulating the biaxial stress states that are induced within constrained membranes of operating PEM fuel cells. PEM fuel cell stacks consist of layered structures containing the catalyzed PEMs that are surrounded by gas diffusion media and clamped between bipolar plates. The surfaces of the bipolar plates are typically grooved with flow channels to facilitate distribution of the reactant gases and water by-product. The channels are often on the order of a few millimeters across, leaving the sandwiched layers tightly constrained by the remaining lands of the bipolar plates, preventing in-plane strains. The hydrophilic PEMs expand and contract significantly as the internal humidity, and to a lesser extent, temperature varies during fuel cell operation. These dimensional changes induce a significant biaxial stress state within the confined membranes that are believed to contribute to pinhole formation and membrane failure. Pressurized blister tests offer a number of advantages for evaluating the biaxial strength to bursting or to detectable leaking. Results are presented for samples of three commercial membranes that were tested at 80°C and subjected to a pressure that was ramped to burst. The bursting pressures exhibit significant time dependence that is consistent with failure of viscoelastic materials. Rupture stresses, estimated with the classic Hencky’s solution for pressurized membranes in conjunction with a quasielastic estimation, are shown to be quite consistent for a range of blister diameters tested. The technique shows considerable promise not only for measuring biaxial burst strength but also for measuring constitutive properties, creep to rupture, and cyclic fatigue damage. Because the tests are easily amenable to leak detection, pressurized blister tests offer the potential for characterizing localized damage events that would not be detectable in more commonly used uniaxial strength tests. As such, this specimen configuration is expected to become a useful tool in characterizing mechanical integrity of proton exchange membranes.

Development of Thermodynamic, Geometric, and Economic Models for Use in the Optimal Synthesis/Design of a PEM Fuel Cell Cogeneration System for Multi-Unit Residential Applications

Thermodynamic, geometric, and economic models are developed for a proton exchange membrane (PEM) fuel cell system for use in cogeneration applications in multi-unit residential buildings. The models describe the operation and cost of the fuel processing sub-system and the fuel cell stack sub-system. The thermodynamic model reflects the operation of the chemical reactors, heat exchangers, mixers, compressors, expanders, and stack that comprise the PEMFC system. Geometric models describe the performance of a system component based on its size (e.g., heat exchanger surface area), and, thus, relate the performance at off-design conditions to the component sizes chosen at the design condition. Economic models are based on data from the literature and address the cost of system components including the fuel processor, the fuel cell materials, the stack assembly cost, the fuel cost, etc. As demonstrated in a forthcoming paper, these models can be used in conjunction with optimization techniques based on decomposition to determine the optimal synthesis and design of a fuel cell system. Results obtained using the models show that a PEMFC cogeneration system is most economical for a relatively large cluster of residences (i.e. 50) and for manufacturing volumes in excess of 1500 units per year. The analysis also determines the various system performance parameters including an electrical efficiency of 39% and a cogeneration efficiency of 72% at the synthesis/design point.

Optimal Synthesis/Design of a Pem Fuel Cell Cogeneration System for Multi-Unit Residential Applications–Application of a Decomposition Strategy

The application of a decomposition methodology to the synthesis/design optimization of a stationary cogeneration proton exchange membrane (PEM) fuel cell system for residential applications is the focus of this paper. Detailed thermodynamic, economic, and geometric models were developed to describe the operation and cost of the fuel processing subsystem and the fuel cell stack sub-system. Details of these models are given in an accompanying paper by the authors. In the present paper, the case is made for the usefulness and need of decomposition in large-scale optimization. The types of decomposition strategies considered are conceptual, time, and physical decomposition. Specific solution approaches to the latter, namely Local-Global Optimization (LGO) are outlined in the paper Conceptual/time decomposition and physical decomposition using the LGO approach are applied to the fuel cell system. These techniques prove to be useful tools for simplifying the overall synthesis/design optimization problem of the fuel cell system. The results of the decomposed synthesis/design optimization indicate that this system is more economical for a relatively large cluster of residences (i.e. 50). Results also show that a unit cost of power production of less than 10 cents/kWh on an exergy basis requires the manufacture of more than 1500 fuel cell sub-system units per year. Finally, based on the off-design optimization results, the fuel cell system is unable by itself to satisfy the winter heat demands. Thus, the case is made for integrating the fuel cell system with another system, namely, a heat pump, to form what is called a total energy system.

Application of a Decomposition Strategy to the Optimal Synthesis/Design of a Fuel Cell Sub-System

The application of a decomposition methodology to the synthesis/design optimization of a stationary cogeneration fuel cell sub-system for residential/commercial applications is the focus of this paper. To accomplish this, a number of different configurations for the fuel cell sub-system were considered. The most promising candidate configuration, which combines features of different configurations found in the literature, is chosen for detailed thermodynamic, geometric, and economic modeling both at design and off-design. The case is then made for the usefulness and need of decomposition in large-scale optimization. The types of decomposition strategies considered are conceptual/time and physical decomposition. Specific solution approaches to the latter, namely Local-Global Optimization (LGO) are outlined in the paper. Conceptual/time decomposition and physical decomposition using the LGO approach are applied to the fuel cell sub-system. These techniques prove to be useful tools for simplifying the overall synthesis/design optimization problem of the fuel cell sub-system. Finally, the results of the decomposed synthesis/design optimization of the fuel cell sub-system indicate that this sub-system is more economical for a relatively large cluster of residences (i.e. 50). To achieve a unit cost of power production of less than 10 cents/kWh on an exergy basis requires the manufacture of more than 1500 fuel cell sub-system units per year. In addition, based on the off-design optimization results, the fuel cell sub-system is unable by itself to satisfy the winter heat demands. Thus, the case is made for integrating the fuel cell sub-system with another sub-system, namely, a heat pump, to form what is called a total energy system.Copyright

Analysis of Thermal Energy Collection from Precast Concrete Roof Assemblies

The development of precast concrete housing systems provides an opportunity to easily and inexpensively incorporate solar energy collection by casting collector tubes into the roof structure. A design is presented for a precast solar water heating system used to aid in meeting the space and domestic water heating loads of a single-family residence. A three-dimensional transient collector model is developed to characterize the precast solar collector’s performance throughout the day. The model describes the collector as a series of segments in the axial direction connected by a fluid flowing through an embedded tube. Each segment is represented by a two-dimensional solid model with top boundary conditions determined using a traditional flat plate solar collector model. The precast collector is coupled to a series solar assisted heat pump system and used to meet the heating needs of the residence. The performance of the proposed system is compared to the performance of a typical air-to-air heat pump. Using the system model, various designs and operating parameters were analyzed to determine a set of near optimal design values. The annual performance of the near optimal system was evaluated to determine the energy and cost savings for applications in Atlanta, GA and Chicago, IL. In addition, a life cycle cost was completed to determine the economic feasibility of the proposed system. The results of the annual study show that capturing solar energy using the precast collector and applying the energy through a solar assisted heat pump can reduce the electricity required for heating by more than 50 percent in regions with long heating seasons such as Chicago. The life cycle cost analysis shows that the energy savings justifies the increase in initial cost in locations with long heating seasons but that the system is not economically attractive in locations with shorter heating seasons.Copyright

Seals and Sealants in PEM Fuel Cell Environments: Material, Design, and Durability Challenges

Fuel cells have significant potential to improve energy utilization efficiency, but remain quite expensive due to the cost of key components, including the membrane of PEM fuel cells, the catalyst, and the bipolar plates. Due to the cost and significance of these items, extensive research has been devoted to reducing cost and improving the quality and performance of these components. By contrast, seals, sealants, and adhesives play a more mundane role in the overall performance of fuel cells, and yet the failure of these materials can lead to reduced system efficiency, system failure, or even safety concerns. Less attention has been given to the performance and durability of these products, yet as improvements in other fuel cell components are made, these seals are becoming a more critical link in the long term performance of fuel cells. This review paper highlights the importance and background of fuel cell seals; discusses the chemical, thermal, and mechanical environments to which fuel cell seals are subjected; and suggests design and testing protocol improvements that may lead to improved fuel cell system performance.Copyright