13.2: Fuel Cell Technology

Electricity has historically been generated 24/7 by combustion-based power plants. With the deployment of diurnally varying and intermittent renewable solar and wind generation, the 24/7 plants are being operated more dynamically, namely ramping up and down in response to the varying renewable resources. Because combustion emits carbon dioxide and criteria pollutants as unavoidable by-products, an alternative to combustion that can operate (1) more efficiently than combustion (thereby reducing CO₂ per megawatt hour), (2) with a zero-carbon fuel (thereby emitting no CO₂), and (3) without the emission of criteria pollutants would be preferred.

Figure 13.2.1 Power generation options.

An emerging alternative to combustion is fuel cell technology (Figure 13.2.1), which converts fuel and air to electricity in a single step. Intuitively, you can imagine a higher efficiency in the absence of mechanical friction. You can also imagine virtually zero formation and emission of criteria air pollutants, due to relatively low-temperature and relatively benign electrochemistry. In addition, fuel cells are quiet—a welcomed attribute for deployment as a distributed generator in the midst of where the public resides (homes) and works (industry, office buildings, and hospitals, for example).

Figure 13.2.2 Proton exchange membrane fuel cell stack.

The manner by which fuel cells operate is illustrated in Figure 13.2.2. Similar to the electric battery presented in Figure 13.1.7, the fuel cell is composed of an anode and cathode separated by an electrolyte. But rather than storing energy, a fuel cell generates electricity continuously as long as fuel (hydrogen) and oxygen (from the air) are provided.

Hydrogen enters and is dissociated at the anode into protons (H+) and electrons (e−). While the electrolyte is receptive to transporting the protons to the cathode, electrons are rejected and required to find an alternative path. Engineers take advantage of this by providing a path for the electrons to travel through a load, represented in Figure 13.2.2 by a lightbulb. The electrons transfer energy to, and thereby support, the load. While “spent,” the electrons are sufficiently energetic to react with the oxygen entering the cathode channel and the protons exiting the electrolyte, and they close the electrochemical reaction by generating water. The water then mixes with the nitrogen from the air to comprise the fuel cell exhaust.

Types of fuel cells

Figure 13.2.3 Fuel cell types.

The fuel cell stack depicted in Figure 13.2.2 is associated with a particular type of fuel cell, the proton exchange membrane fuel cell (PEMFC). In addition to the PEMFC, the three other major fuel cell types are shown in Figure 13.2.3—the phosphoric acid fuel cell (PAFC), the molten carbonate fuel cell (MCFC), and the solid oxide fuel cell (SOFC). The types vary by the chemistry utilized, the electrolyte used (which provides the name of each fuel cell type), the operating temperature, the time required to turn the fuel cell on and off, and the rate and extent to which the power output can be changed. All operate on hydrogen but can also run off fuels containing hydrogen (for example, natural gas, biogas, and propane) that are re-formed (usually at high temperature with the addition of steam) to release the hydrogen for fueling the stack.*

Because PEMFCs turn on and off like an automobile engine, operate at a relatively low temperature, and rapidly change power output in response to load, they are ideal for powering both ground-based vehicles (from forklifts, to automobiles, to heavy-duty trucks) and space vehicles (for example, space modules, space stations) and for providing backup power in the event of a grid outage (for example, for servers and telephone cell towers). Ballard is an example of a manufacturer of PEMFC systems with applications that include buses, trucks, and urban light-rail trams.

The other fuel cell types require several hours to turn on and off.

As a result, they are dedicated to generating electricity for facilities that have a relatively constant 24/7 load. These loads, while relatively constant, can vary. For example, the load can be different during the day than at night, or during a weekday than on the weekend. The extent to which each fuel cell type can load follow varies. PAFCs are flexible in this regard, whereas MCFCs and SOFCs are less flexible.

PAFCs were the first fuel cell product to be commercialized (in 1992), and today Doosan (their sole manufacturer) offers systems from 400 kilowatts (kW) to 40 megawatts (MW) based on a 400 kW module. (A few kilowatts would be adequate for a home, whereas a megawatt would be appropriate for a hotel.) While the vast majority of the systems deployed worldwide operate on natural gas that is converted to hydrogen through a reformer external to (that is, separated from) the fuel cell stack, Doosan has deployed a 40 MW system that operates directly on hydrogen supplied by a waste stream at a petrochemical plant in South Korea. PAFCs operate at an elevated temperature (200°C), which allows combined heat and power (CHP) and combined cooling, heat, and power (CCHP) applications with efficiencies exceeding 90%.

The basic module of the MCFC commercial unit, 1.4 MW, is replicated to achieve the power ordered by the customer. For example, ten 1.4 MW modules provide 14 MW of power. Typical systems are 2.8 MW, with the largest system, 59 MW, in service in South Korea. MCFCs were first commercialized in 1993 by FuelCell Energy, the sole manufacturer, as the first high-temperature system (650°C). The higher temperature provides both attractive options for CHP and CCHP and the ability to internally reform the fuel (for example, natural gas). The technology has also led to

• The operation of fuel cells on biogas (sourced from water resource recovery facilities), thereby generating carbon-neutral renewable electricity.
• The generation of carbon-neutral hydrogen as well as electricity and heat, referred to as tri-generation.

Bloom Energy has pioneered the introduction of high-temperature (1,000°C) SOFC technology beginning with commercialization in 2009. While the size of the basic module has varied, 250 kW is representative. The technology is purpose-built to be solely an electric generator (that is, not equipped for CHP/CCHP), using the heat instead to generate more electricity with overall fuel-to-electricity efficiencies exceeding 60% and exhaust temperatures as low as 65°C. Similar to MCFCs, SOFCs use internal reformation. A second SOFC manufacturer entering the market is Mitsubishi Hitachi Power Systems with a 250 kW and 1 MW fuel cell (FC) module integrated with a gas turbine (GT) to create a fuel cell/GT hybrid.

Deployment of fuel cells

Figure 13.2.4 Fuel cell deployment.

As shown in Figure 13.2.4, fuel cells are deployed as distributed generators, with sizes ranging from hundreds of kilowatts to tens of megawatts, across a myriad of market segments on the customer side of the electric meter.** These include

• Industry⓵ (Figure 13.2.4).
• Office buildings, commercial developments, universities, and hospitals⓶ .
• Water resource recovery facilities⓷.

Fuel cell technology has been installed throughout the world, with initial market concentrations in Korea, Japan, Europe, and California. In California, over 250 MW of product is installed throughout the state (Figure 13.2.5), with higher concentrations in the two major population centers of northern California and southern California.

Figure 13.2.5 Plot of fuel cell sites in California in 2019. Reproduced from California Stationary Fuel Cell Collaborative.

On the utility side of the meter, large fuel cell systems are being deployed as TIGER (transmission integrated grid energy resource) stations to support local grid constraints (Figure 13.2.4 ⓸). Rather than serving a single customer, these TIGER stations are integrated into the electricity grid. Examples include 10 MW TIGER stations powering “cloud” server farms (for example, eBay, Apple, and Microsoft); a 15 MW TIGER station in Bridgeport, Connecticut; a 30 MW TIGER station in Delaware; and a 59 MW TIGER station in South Korea. Also depicted are fuel cell/GT hybrid systems being developed for 1,000-MW-scale central generation (Figure 13.2.4 ⓹).

Notable in Figure 13.2.4 is the absence of combustion sources of electricity, representing the culmination of the paradigm shift from a combustion-dominant electric grid, with the associated limited efficiencies and emission of criteria pollutants, to an electrochemical-dominant electric grid, with high efficiencies and virtually zero emission of local air pollutants such as nitrogen oxides. While this is notable, it is important to recognize that this paradigm, while having zero emissions of local air pollutants, may not have zero emissions of carbon. If the fuel cells are operating on natural gas, biogas, or syngas, carbon dioxide generated in the reformation process will be liberated in the exhaust. If the fuel cells instead are operating on renewable hydrogen (from otherwise curtailed solar and wind, for example), Figure 13.2.4 represents a 100% renewable grid.

*Reformation (or re-formation) is a process to extract hydrogen from the hydrogen embedded in fossil and bio fuels. The most common fossil fuel reformed is natural gas, which is rich in methane (CH4), using a steam methane reformation (SMR) process. When methane is exposed to heat and steam, the hydrogen can be separated and purified for industrial applications and the refining of gasoline, as two examples.

**Side of the meter refers to the customer side or utility side of the electric utility meter. The customer side of the meter encompasses the circuits owned and managed by the customer. The utility side of the meter encompasses the circuits and electrical resources owned and managed by the utility.