Technology Today

2011 Issue 1

Creating Compact, Reliable and Clean Power With Fuel Cell Technology

The demand for compact, reliable and clean power is an important driver and constraint for large and small systems. Rapid technological advances have been made in fuel cells, which are of interest to Raytheon and our customers as a more effective source of power for key products and as a more sustainable source of power for facilities and large-scale systems.

Raytheon is employing new developments in fuel cell technology that are likely to be part of the next generation of power solutions

How Fuel Cells Work: The Basics

Fuel cells produce direct current (DC) electrical power through an electrochemical reaction in which a fuel (hydrogen or a hydrocarbon) is oxidized, typically with pure oxygen or air. Some designs use chemicals such as chlorine as the oxidizing agent. A fuel cell consists of two electrodes, the anode and cathode, separated by an electrolyte. The electrolyte may be liquid, such as an aqueous alkaline solution, or solid, such as those using polymer membranes or solid oxide fuel cells (SOFC). This distinction defines the two basic classes of fuel cells — liquid and solid — with numerous variants of each.

Fuel cells are distinct from batteries, the other major class of electrochemical power cells. A fuel cell is a thermodynamically open system into which fuel is continuously injected to generate power. In contrast, a battery is a closed system that stores power, though many battery types can have power added through recharging.

Figure 1 illustrates the electrochemistry that powers fuel cells. Because fuel cells create electrical power through an electrochemical reaction rather than through combustion, their efficiency is not limited by the Carnot cycle efficiency of traditional heat engines (e.g., internal combustion engines or turbines), but is potentially higher. Advances in fuel cell technologies during the past decade are helping to realize this theoretical efficiency advantage.

Figure 1. Schematic of a fuel cell. Hydrocarbon fuel or pure hydrogen combines with oxygen in the fuel cell to generate electricity. (Source: Bloom Energy)

Reaction byproducts from fuel cells include water, carbon dioxide (in the case of hydrocarbon fuels), and waste heat as well as useful electrical power. Some fuel cell schemes harness the waste heat to boost efficiency. Fuel cells can be considered a green technology to the extent that they are more efficient, or have a lower carbon footprint, than traditional combustion power technologies, or when renewable (carbon neutral) fuel sources are used.

Fuel cells offer the potential for a number of benefits in providing power to systems large and small versus traditional power sources: higher efficiencies compared to combustion sources, lower carbon profiles depending on the fuel, and possible cost savings depending on relative efficiencies and fuel costs. However, like any emerging technology, fuel cells must overcome a number of challenges prior to widespread adoption. Such challenges include life cycle/durability of SOFC stacks and other components, logistics and supply chain deployment of the alternative fuels consumed by fuel cells, and cost and performance trade-offs versus power sources with similar energy and power densities.

Fuel Cell Applications

At the lower-power end of the spectrum, companies such as MTI MicroFuel Cells Inc., NEAH Power Systems, Inc., Lilliputian Systems Inc., and Angstrom Power, Inc., are focused on developing battery replacement technologies to compactly provide extended power to handheld devices in environments where ready access to the electrical grid for recharging is impossible or impractical.

The key discriminator for such fuel cell systems is the duration of power between refills. They promise to provide as much as two orders of magnitude greater energy density than conventional chemical battery technologies for power densities less than 10 W/kg. Target applications include consumer electronics devices such as mobile phones and laptops. Compact, high energy density fuel cell systems equate to longer effective life between charges. This may be applicable to the power needs for man-portable military devices and may also simplify the logistics of providing such power versus traditional batteries due to fuel cell systems' higher energy density.

Bloom Energy, Fuel Cell Energy, UTC Power and Ballard are examples of companies focused on the higher end of the power spectrum. The power capabilities of these systems can span the range from 100 kW to 50 MW using scalable architecture.

Figure 2. Bloom Energy ES-5000 Energy ServerTM SOFC system. (Source: Bloom Energy)

Figure 2 shows a Bloom Energy system. Each 100-kilowatt EnergyServer™ SOFC power system can be combined with additional units to meet higher, megawatt-scale power requirements, such as those at a large business facility. (As a point of reference, the average electricity usage of a U.S. residence is just over 1 kilowatt, as reported by the U.S. Department of Energy.)

SOFC systems run at high internal temperatures (500–1,000°C), improving electrical efficiency and more easily accommodating the use of alternative fuels. Higher temperature operation, however, increases start-up time and drives material costs. For these reasons SOFCs are currently a less favored solution for certain applications, such as automotive, where lower temperature fuel cell technology is dominant.

Powering automobiles is a much-discussed application of fuel cells. The first fuel-cell vehicle offerings utilize proton exchange membrane (PEM) — also known as polymer electrolyte membrane — solid fuel cells with compressed hydrogen fuel.

PEM fuel cells differ from SOFCs in that they operate at lower temperatures, typically 50 to 100 degrees Celsius. The principal fuel choice is pure hydrogen (although other fuels, including hydrocarbons, have been used). The electrolyte in this type of fuel cell is a polymer membrane that is electrically insulating, but that allows for the flow of protons, which are generated by the interaction of hydrogen fuel with the anode. The anode, typically consisting of a platinum catalyst, ionizes the hydrogen to generate hydrogen ions (i.e., protons) and electrons. Electrons are free to flow in the external load circuit and power the vehicle or other device, and combine with the hydrogen ions and oxygen at the cathode to form water as a waste byproduct of the PEM fuel cell. While the detailed engineering and materials challenges for constructing a PEM versus an SOFC fuel cell differ, the basic concept holds: Hydrogen/hydrocarbon fuel plus oxygen generates electrical power plus water/carbon dioxide and heat as byproducts.

According to the U.S. Department of Energy, the appeal of PEMs for automotive applications is that they hold the promise of clean, reliable power; hydrogen production to power a PEM is typically greener than a gasoline or diesel internal combustion engine. Hydrogen can also be produced domestically, reducing dependence on imported oil. Challenges in producing economically viable PEMs include on-board hydrogen storage; total cost of the fuel cell stack; durability, reliability and life cycle of the fuel cell, including ability to perform in sub-freezing temperatures; and the need for a consumer hydrogen fuel distribution network.

The fuel cell examples cited here are representative of the type of research, development and product creation that is occurring in this rapidly evolving field to provide new types of clean, reliable power solutions. Raytheon's continued pursuit of advances in this area ensures that our customers have access to the best technology in the marketplace, whether developed in-house or through partnerships with industry and academia.

Steve Klepper and Tony Marinilli

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