Fuel Cells and Their Applications

Fuel Cells and Their Applications Prof.M.Ghouse Energy Research Institute King Abdulaziz City for Science and Technology (KACST) Riyadh, Saudi Arabia 17th June 2012

Figure 1. Conventional Fuel Cell Plant

-

-Difference between Electrolysis and Fuel Cells

Fig 7. Power System Efficiencies

Electrolysis: Obtaining hydrogen from water: The Basis for a Solar-Hydrogen Economy This project involves a fascinating experiment in electrochemistry that illustrates several important energy related processes, and provides an ideal context for discussion of several issues related to electricity generation

Demonstrated in 1839 by Sir William Grove The principals involved have not changed since, but the materials and cell configurations have

Fuel cell: An electrochemical device, which converts chemical energy to electrical energy without combustion and has its fuel & / or oxidant supplied externally.

This encompasses a wide array of devices Five commonly accepted categories Classified by the electrolyte used

Some Salient features of Fuel Cells •Overall Efficency (η%) =75-80% (40%Elec.+40%Ther.) • Highly efficient in full load and part load operation • Low pollution levels • Minimum water requirements • No major moving parts •Flexible in fuel use •Quick installation • H2/O2 systems produce drinking quality water •Low maintenance •No transmission losses •High co-generation potential.

Fig. 1 Comparison of Energy Transformation Processes in a Diesel Generator and Fuel Cell

Fuel Cells – History How Does a Fuel Cell Work “Cold combustion” (Fuel cell) •Controlled reaction (no flame) •Direct conversion of chemical energy to electrical energy O Fuel

Fuel

Heat

H H Electricity

Movement

Efficiency: 50%

Electricity

Efficiency: 25%

“Warm combustion” (heat engine /motor/ via turbine) • Uncontrolled reaction • Released energy is transferred to a working medium (water, water vapor)

CEV_FCH_K/9

Fuel Cells A Device that Converts Hydrogen to Electricity A Fuel cell is like a battery but with constant fuel and oxidant supply. heat _

hydrogen oxygen

DC electricity

FUEL CELL BATTERY

+

water CEV_FCH_K/10

• Most familiar is the hydrogen oxygen fuel cell: – Cathode:

O2 + 4 e- + 2 H2O

4 OH-

– Anode:

2 H2 + 4 OH– Overall:

2 H2 + O2

4 H2O + 4 e2 H2O

• Current must be routed externally to be used • PEMFC has the greatest immediate potential

Fig 2. Basic principals of a H2-O2 fuel Cell (AFC)

Fig.3a A Single PAFC Cell Assembly

Fig 4. Schematic Diagram of a Basic Fuel Cell

Fuel Cells A Simple PEM Fuel Cell

Combines hydrogen and oxygen in a chemical reaction, producing water and releasing energy.

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Fig.3b Schematic of PAFC Single Cell

Fig 5. Schematic of a Single Fuel Cell

Fig.1 Schematic of Alkaline Fuel Cell (PEMFC)

PEM Fuel Cells PEM Fuel Cell – Electrochemical Reaction Anode:

2H2 → 4H+ + 4e- (oxidation)

Cathode:

O2 + 4e- → 2O22O2- + 4H+ → 2H2O O2 + 4e- + 4H+ → 2H2O (l) (reduction)

Overall reaction:

2H2 + O2

1. Oxidation of hydrogen 2. Reduction of oxygen

→

2H2O (l)

ΔH = - 286 kJ/mol

Theoretical Voltage per single fuel cell: 1.23 V Usable Voltage per fuel cell: (0.6 … 0.9 Volt DC)

Fuel Cells Fuel Cell Types Alkaline (AFC) Polymer Electrolyte Membrane (PEM)

Phosphoric Acid (PAFC)

Types of Fuel Cells

Molten Carbonate (MCFC)

Direct Methanol Direct Methanol (DMFC) (DMFC) Solid Oxide (SOFC)

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Why PAFC ? •To utilize the natural resources of the kingdom of Saudi Arabia. • Ability to use impure H2 Obtained from natural gas or methanol reformers as a fuel and air to use as an oxidant • High tolerance to Fuel impurities like Sulfur • No noble metals • Relatively simple reformer technology and compatibility with Hydrocarbons Water management is not required

Fuel cells are typically classified by the type of electrolyte they use Fuel Cell Type Temperature, ◦C

Applications

Advantages

Proton exchange membrane PEMFC

50 - 100

Appliances, Transportation, Home power generation

Zero emissions potential, Compact Low maintenance, High current density

Solid-oxide SOFC

500 - 1000

Space applications

Low emission, High efficiency, compact

Alkali AFC

50 – 200

Space applications

Reliability, Zero emissions potential

Molten Carbonate MCFC

650

Large vehicles, distributed power, CHP generating systems

~80% efficiency, low emission, quiet, >10 MW

Phosphoric acid PAFC

220

CHP generating systems

~80% efficiency, low emission, quiet, >10 MW

Fig. 6. Schematic of a Single PAFC Cell

But there is no such thing as a free lunch • Hydrogen must be produced, stored, and distributed – This requires electricity for electrolysis of water – Solar power is being perused for this application • Construction of a PEMFC requires: – Platinum catalyst – Exotic membrane materials – Carbon electrodes – Polymers – Copper for wiring

• Low voltages necessitate fuel cell “stacking” • And, lets not forget the biggest potential difficulty with using hydrogen as a fuel ……………………….

Table 1 Major Types of Fuel Cells Type

Operating Temp. oC

Anode

PEMFC

40-90

AFC

65-200

PAFC

200

Pt/C+ PTFE Pt/Pd+ PTFE Pt/C+P TFE

MCFC

650

Ni / Cr

SOFC

1000

NiOYSZ

Electrolyte Cathode Polymer 85-100% KOH 97-100% H3PO4 L/K/Na Carbonate Y2O3/ZrO2 Stabilized

Pt/C+ PTFE Pt/Pd+ PTFE Pt/C+ PTFE NiO Sr-Doped LaMnO3

Fig 8 Various Types of Fuel Cells

Production Hydrogen can be produced from a wide range of feedstocks, and any hydrogen-rich material can serve as a possible fuel source for fuel cells. Hydrocarbon fuels, novel feedstocks such as landfill gas, anaerobic digester gas, and biomass can also produce hydrogen, as can compounds containing no carbon, such as ammonia or borohydride. The vast majority of today’s hydrogen is produced via steam reformation of natural gas (95% in the U.S., roughly 48% globally), but alternative sources such as biogas are growing in popularity. Reformers Hydrocarbon fuels – methanol, ethanol, natural gas, petroleum distillates, liquid propane, and gasified coal – can yield hydrogen in a process called reforming. Natural gas, the feedstock of choice for most of today’s massproduced hydrogen, contains methane (CH4) that can be used to produce hydrogen via a thermal process known as steam-methane reformation. In steam-methane reforming, methane reacts with steam in the presence of a catalyst to produce hydrogen, carbon monoxide, and a relatively small amount of carbon dioxide. Steam reforming is endothermic, meaning heat must be supplied to the process for the reaction to proceed. The process is approximately 72 percent efficient. This type of reforming works similarly for other hydrocarbon fuels, combining the fuel with steam by vaporizing them together at high temperatures. Hydrogen is then separated out using membranes. Another type of reformer is the partial oxidation (POX) reformer. Some CO2 is emitted in the reforming process, but the emissions of NOX, SOX, particulates, and other smog producing agents are cut to zero. Water Electrolysis When an electric current is introduced to water (H2O), hydrogen and oxygen are separated, with hydrogen forming at the cathode and oxygen forming at the anode. Electricity can be provided from any source, but using solar and wind energy to electrolyze water provides the cleanest pathway to produce hydrogen. This model is being used in some hydrogen refueling stations and in renewable energy storage systems that utilize hydrogen. Enzymes Another method to generate hydrogen is with bacteria and algae. Cyanobacteria, an abundant single-celled organism, produce hydrogen through its normal metabolic function. Cyanobacteria can grow in the air or water, and contain enzymes that absorb sunlight for energy and split the molecules of water, thus producing hydrogen. Since cyanobacteria take water and synthesize it to hydrogen, the waste emitted is more water, which becomes food for the next metabolism. Sodium borohydride (NaBH4) is an inorganic compound that can dissolve in water in the absence of a base. Hydrogen can be generated through catalytic decomposition.

Hydrogen Basics Hydrogen: No.1 on the periodic table Fuel cells run on hydrogen, the simplest element and most plentiful gas in the universe. Hydrogen is a diatomic element, meaning that in its liquid and solid states, hydrogen naturally forms into pairs of atoms, which is why hydrogen is often referred to as “H2”. Hydrogen is the lightest element, yet it has the highest energy content per unit weight of all fuels. Hydrogen’s energy density is 52,000 Btu/lb., which is three times greater than that of gasoline. In nature, hydrogen is never found on its own; it is always combined into molecules with other elements, typically oxygen and carbon. Hydrogen can be extracted from virtually any hydrogen-containing compound, including both renewable and non-renewable resources. Regardless of the fuel source, fuel cells utilize hydrogen with little to no polluting emissions, making hydrogen the ultimate clean energy carrier.

The Element Hydrogen 1 H Hydrogen 1.00794 Atomic Number: 1 Atomic Weight: 1.00794 Melting Point: 13.81 K (-259.34°C or -434.81°F) Boiling Point: 20.28 K (-252.87°C or -423.17°F) Density: 0.00008988 grams per cubic centimeter Phase at Room Temperature: Gas Element Classification: Non-metal Period Number: 1 Group Number: 1 Group Name: none What's in a name? From the Greek words hydro and genes, which together mean "water forming."

Fuel cells have the potential to slip into every kind of electronic device. A few applications could include: *Cars- as stated before, fuel cells the size of a printer could provide enough juice to power as well (if not better than) a combustion engine. Slightly larger units are already in place in several bus systems across the United States. The hydrogen for both forms of transportation may be provided through propane, methanol or natural gas. *Personal Devices (Laptops, cell phones, hearing aides) - fuel cells have the tremendous potential to get into every electronic device we come in contact with. Fuel cells offer the possibility of laptops and cell phones with energy life measured in days or weeks, rather than hours. The fuel cell is scalable, which means it can go small enough to power medical devices that normally require battery replacement. *Stationary Power Production and Backup- larger-scale fuel cells could allow every city to have its own power station, rather than a centralized power grid. Power generation could become so decentralized that each housing development or apartment complex could be self-sustained with its own power. This would drastically cut down on pollution and ugly power lines. Hospitals and airports could (some already do) have backup power supplies that kick in, in the event of a power failure.

Fig 8 PEMFC Assembly

Fuel Cells Many applications require more voltage than one single fuel cell delivers

Fuel Cells are stacked in series:

Hydrogen Production Where does the hydrogen come from? H2 Oil or Gas

Hydrogen bottles

Reformer

H2

Solar panel

H2

Hydrogen bottles

Electrolyzer

H2

Algae / Biochemical / Catalytic

H2

H2

Hydrogen bottles HY / 35

Safety and Conclusions - No ignition or heat sources - Don’t inhale the hydrogen or blow - into the balloons - Wear safety glasses at all times - Keep the hydrogen balloons above head level as much as practical

Hydrogen Safety

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