By Arnold R. Miller, PhD

Vehicles require both an energy storage medium, such as a fuel, and a power production device, such as an engine or fuelcell. Depending principally on the duty cycle (power versus time), a fuelcell-hybrid powertrain may be advantageous.

Fuelcells

Fuelcells are electrochemical power devices that directly convert the chemical energy of a fuel into electric power. From hydrogen fuel and air (oxygen), they produce electricity and water - the reverse of water electrolysis. While fuelcells share principles of operation with batteries, they differ in that the electrochemically active materials, hydrogen and oxygen, are stored or are available externally and are continuously supplied to the device rather than being stored in the electrodes. They are periodically refueled, like an engine, rather than recharged electrically. Like batteries, individual cells are grouped together into “stacks” to provide any voltage or power required.

Chosen for their safety, reliability, and high power density, fuelcells have provided electric power and drinking water in manned spacecraft for four decades. Since 1992, commercial stationary fuelcell powerplants of 200 kW rating have provided high-reliability, clean, quiet, and safe electric power to hospitals, commercial facilities, universities, airports, and military bases.

By separating the energy storage and power production functions, fuelcells are more convenient, more efficient, and safer than storage batteries. They are more convenient because the refueling process can be completed in a few minutes rather than the hours required for efficient battery recharging. They are more efficient because the electrochemical losses that occur in batteries during recharge, as witnessed by their evolution of heat, do not apply to fuelcells. They are safer because short-circuiting a fuelcell harmlessly dissipates only the energy associated with the small amount of hydrogen present in the cell - in contrast, short-circuiting a battery dissipates all of its stored energy.

Insight into fuelcells follows an understanding of the special place of hydrogen, their natural fuel, among the chemical elements. Most of the elements of nature are metals (see accompanying periodic table), and while most have the familiar metallic properties, not all do. Mercury (Hg), a liquid, lacks hardness. Hydrogen, a gas, would seem to lack all metallic properties. Nonetheless, the most fundamental characteristic of a metal is its tendency to donate electrons in chemical reactions, and on this basis, hydrogen is classified as an alkali metal in the first column of the table. Moreover, solid hydrogen (at low temperature) has decidedly metallic properties. This construct of hydrogen as a gaseous metal allows us to readily see the fuelcell as a special type of battery: Conventional batteries use a metal such as lead, cadmium, or lithium as the anode material (negative plate) - fuelcells use a gaseous metal as anode material, and this is the basis of their advantages of separate energy storage and being refuelable.

This simple fact of using gaseous electroactive materials has far-reaching implications: (1) The energy storage component is separated from the power-producing component, and unlike a conventional battery, in which the metal electrodes or plates serve as both the energy-storage and power-production functions, the fuelcell separates these two functions, and power and energy are not linked. (2) Energy for the vehicle is stored in a fuel tank, analogous to the fuel tank of a conventional engine vehicle, and the vehicle may be rapidly refueled by refilling its fuel tank.

Because fuelcells are electrochemical power devices, essentially “refuelable” batteries, they are not limited in efficiency by the Carnot limit faced by heat engines. Fuelcells do have an analogous limit, namely, “intrinsic maximum efficiency,” which is the Gibbs free energy divided by the enthalpy of the chemical reaction of the fuelcell. Depending on the fuelcell type, the intrinsic maximum efficiency is typically in the range of 80-90%. As a rule of thumb, the overall practical efficiency of a fuelcell powerplant is on the order of 50%.

The type of fuelcell used in our projects and exclusively favored by the auto industry is the proton-exchange membrane (PEM) type, which uses a solid ion-exchange membrane for its electrolyte.

Hydrogen Storage

Storage of hydrogen onboard the vehicle is a greater technical challenge than producing power from a fuelcell. Technically mature methods of onboard storage include (1) direct storage of hydrogen as a compressed gas, (2) direct storage as a liquid, (3) direct storage as a reversible metal hydride [Miller, 2005], (4) onboard chemical transformation to hydrogen of a carbon-based feedstock such as a hydrocarbon or alcohol, and (5) physical dissociation of liquid ammonia to hydrogen.

For industrial vehicles in general, and especially for locomotives, minimum volume of the fuel storage system or powerplant is more important than minimum weight. That is, a high hydrogen volumetric density is more important than a high gravimetric density. Table 1 displays the limits of hydrogen volumetric density for the five fuels abovementioned. These limits are a theoretical construct – they provide a measure of the best possible volumetric density that a given fuel can attain. They omit the volume of the container, associated hardware, and chemical processor. For example, if one had a liter of hydrogen at a pressure of 350 bar, but stored in a tank, with piping, etc, of infinitesimal volume, the liter would store 25 g of hydrogen, corresponding to a volumetric density of 25 g/L. In the case of methanol, which requires reacting the alcohol with water at high temperature over a catalyst to produce hydrogen according to the equation,

CH3OH + H2O → 3H2 + CO2

the limiting volumetric density also omits the volume of the reactant water (in principle, water can be obtained from the fuelcell). The results show that, in the limiting case, the reversible metal hydride is capable of the highest hydrogen volumetric density, namely, 125 g/L, and compressed hydrogen, the lowest.

Real systems require volume for their hardware (e.g., tank, piping, and valves, as well as chemical reactors for methanol and ammonia), and thus the practical hydrogen volumetric densities shown in Table 2 are smaller than the theoretical values of Table 1. The practical densities were computed from the known volumes of actual systems. For example, the hydrogen volumetric density of a real liquid hydrogen storage system is 26 g/L rather than the 70 g/L for the theoretical system. The density of the practical methanol system includes the reactant water, as well as the reformer hardware.

“Storage Efficiency” is here defined as the Practical Density / Theoretical Density x 100%. For example, liquid H2 has a storage efficiency of 26 g/L / 70 g/L x 100% = 37%. Storage Efficiency is a measure of how closely a storage system approaches its volumetric density limit or theoretical density; it is a measure of how well a storage system lives up to its potential, the limits of Table 1.

In conclusion, with today’s technology, liquid ammonia, at 44 g/L, has the highest practical hydrogen volumetric density. Compressed hydrogen, at 10 g/L, has the lowest. Compressed hydrogen and liquid ammonia, at 40% each, have the highest storage efficiency, and reversible metal hydride storage, at 16%, has the lowest.

In choosing a hydrogen storage system for a vehicle, factors other than volume may be important. Three examples are safety, cost, and thermodynamic efficiency.

Because we believe underground vehicles are presently restricted to reversible metal-hydride storage for reasons of safety, we will discuss this method in more detail. Metal hydrides are low-flammability, solid materials that uses metal-hydrogen chemical bonds to store hydrogen safely and compactly. Metals, crystalline solids, consist of a regular array or lattice of spherical atoms. Spheres cannot pack perfectly, and the lattice of atoms also forms a superimposed lattice of holes or interstices (see figure). The interstices interconnect to form a three-dimensional network of channels. Because hydrgen is the smallest atom, it chemically bonds to the metal atoms while occupying the interstices. Transition metals form hydrides that are readily reversible and constitute a safe, solid storage medium for hydrogen. By removing low-temperature heat from the crystal, hydrogen atoms enter the interstices throughout the crystal and charge the metal. Conversely, by providing low-temperature heat (60 - 70 C) to a charged crystal, the process is reversed and the metal is discharged. The gas pressure is approximately constant during the process and can be very low, even below atmospheric.

Unlike liquid or gaseous fuels, metal hydrides are of low flammability. This is because hydrogen is trapped in the metal matrix or lattice, and the rate at which hydrogen atoms can file through the channels, recombine into hydrogen molecules, and be released is limited by the rate of heat transfer into the crystal. Rupture of a hydride system is self-limiting: As hydrogen escapes, the bed naturally cools because chemical bonds are being broken, and the colder bed has a lower rate of atom migration. The metal matrix, moreover, forces the hydrogen atoms close together, as close as in liquid hydrogen, and is responsible for the high volumetric energy density. Although metal-hydride storage is heavy, weight is generally not an issue for locomotives.

Hybrid Powertrain

A fuelcell hybrid powertrain utilizes a fuelcell prime mover, plus an auxiliary power/energy-storage device to carry the vehicle over power peaks in its duty cycle and recover kinetic or potential energy during braking. To allow steady-state operation, the continuous net power of the prime mover must equal or exceed the mean power of the duty cycle. The degree to which the powertrain of a vehicle is hybrid is termed its “hybridity.” The accompanying illustration depicts the general case of such a powertrain. In our mine vehicles, the locomotive is a non-hybrid (h = 0), whereas the loader is a hybrid with hybridity about h = 0.4 (that is, it’s mostly a fuelcell vehicle, with a relatively small auxiliary battery.) As we have shown in published papers, whether a hybrid rail vehicle is worth its extra complexity and generally lower thermodynamic efficiency depends on the application and in particular the duty cycle. For example, freight trains should garner little benefit because they operate at nearly constant power and the kinetic energy is so high that practical auxiliary storage devices can recover only a small fraction of the total available energy during regeneration. On the other hand, we have shown that a hybrid switcher can offer the benefit of reduced capital or recurring costs.

Bibliography

A. R. Miller, Least-cost Hybridity Analysis of Industrial Vehicles. European Fuel Cell News, Vol. 7, January 2001, pp. 15-17

A. R. Miller, J. Peters, B. E. Smith, and O. A. Velev, Analysis of Fuelcell Hybrid Locomotives. Journal of Power Sources, 157, pp. 855-861, 2006

A. R. Miller and J. Peters, Fuelcell Hybrid Locomotives: Applications and Benefits. Proceedings of the Joint Rail Conference, Atlanta, 6 April 2006

A. R. Miller, K. S. Hess, and D. L. Barnes, Comparison of Practical Hydrogen-Storage Volumetric Densities. Proceeding of the National Hydrogen Association Annual Hydrogen Conference, San Antonio, 21 March 2007 (in press)

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