61 Principles of Operation

Fuel cells are similar to batteries containing electrodes and electrolytic materials to accomplish the electrochemical production of electricity. Batteries store chemical energy in an electrolyte and convert it to electricity on demand until the chemical energy has been depleted. Applying an external power source can recharge depleted secondary batteries, but primary batteries must be replaced. Fuel cells do not store chemical energy but, rather, convert the chemical energy of a fuel to electricity. Thus, fuel cells do not need recharging and can continuously produce electricity as long as fuel and oxidant are supplied (Brown and Jones, 1999).

Figure 6.1 presents the basic components of a fuel cell, which include a positive electrode (anode), a negative electrode (cathode), and an electrolyte. Fuel is supplied to the anode, while oxidant is supplied to the cathode. Fuel is electrochemically oxidized on the anode surface, and oxidant is electro-chemically reduced on the cathode surface. Ions created by the electrochemical reactions flow between the anode and cathode through the electrolyte. Electrons produced at the anode flow through an external load to the cathode, completing an electric circuit.

A typical fuel cell requires both gaseous fuel and oxidants. Hydrogen is the preferred fuel because of its high reactivity, which minimizes the need for expensive catalysts. Hydrocarbon fuels can be supplied, but typically require conversion to hydrogen prior to entering the fuel cell (for lower temperature fuel cells) or within the fuel cell (for higher temperature fuel cells). Oxygen is the preferred oxidant because of its availability in the atmosphere. As indicated in Figure 6.1, the electrolyte serves as an ion conductor. The direction of ion transport depends upon the fuel cell type, which determines the type of ion that is produced and transported across the electrolyte between the electrodes. The various fuel cell types are described in a later section.

FIGURE 6.1

Fuel cell diagram.

6.1.1 Fuel Cell Stack

A single fuel cell is only capable of producing about 1 volt, so typical fuel cell designs link together many individual cells to form a stack that produces a more useful voltage. A fuel cell stack can be configured with many groups of cells in series and parallel connections to further tailor the voltage, current, and power produced. The number of individual cells contained within one stack is typically greater than 50 and varies significantly with stack design.

Figure 6.2 presents the basic components that comprise a fuel cell stack. These components include the electrodes and electrolyte of Figure 6.1 with additional components required for electrical connections and/or insulation and the flow of fuel and oxidant through the stack. These key components include current collectors and separator plates. The current collectors conduct electrons from the anode to the separator plate. The separator plates provide the electrical series connections between cells and physically separate the oxidant flow of one cell from the fuel flow of an adjacent cell. The channels in the current collectors serve as the distribution pathways for the fuel and oxidant. Often, the two current collectors and the separator plate are combined into a single unit called a bipolar plate.

6.1.2 Fuel Cell System

The preferred fuel for most fuel cell types is hydrogen. Hydrogen is not readily available, however, but the infrastructure for the reliable extraction, transport or distribution, refining, and/or purification of hydrocarbon fuels

FIGURE 6.2

Isometric view of the basic components of a fuel cell stack.

FIGURE 6.2

Isometric view of the basic components of a fuel cell stack.

is well established in our society. Thus, fuel cell systems that have been developed for practical applications to date have been designed to operate on hydrocarbon fuels. This typically requires the use of a fuel processing system, or "reformer," as shown in Figure 6.3. The fuel processor typically accomplishes the conversion of hydrocarbon fuels to a mixture of hydrogen-rich gases and, depending upon the requirements of the fuel cell, subsequent removal of contaminants or other species to provide pure hydrogen to the fuel cell.

In addition to the fuel cell system requirement of a fuel processor for operation on hydrocarbon fuels, a power conditioning or inverter system is needed. This is required for the use of current end-use technologies, which are designed for consuming alternating current (AC) electricity, and for grid connectivity in distributed power applications. Since the fuel cell produces direct current (DC) electricity, the power conditioning section is a requirement for fuel cell systems that are designed for AC-based distributed generation. In the

FIGURE 6.3

Schematic representation of a fuel cell system.

FIGURE 6.3

Schematic representation of a fuel cell system.

future, systems and technologies may be amenable to the use of DC electricity, which would allow significant cost savings by avoiding the inverter.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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