Fuel Cell

THERMODYNAMICS

Dr. Parvez Khalid Butt

Submitted to;Dr. Parveez Khalid Butt

Submitted By;Name Reg. #

Hafiz Ali Haider Sehole FA11-BEC-058Shahid Nawaz FA11-BEC-128Zartab Cheema FA11-BEC-126Hassan Tariq FA11-BEC-106

Labin Ijaz FA11-BEC-064

http://chm100.blogspot.com/By Hafiz Ali Haider Sehole


A fuel cell is a device that converts the chemical energy from a fuel into electricitythrough a chemical reaction with oxygen or another oxidizing agent.

A fuel cell combines hydrogen and oxygen to produce electricity. Fuel cells are greatsubstitutes for other energy conversion techniques such as internal combustion engines (ICEs),turbines, accumulators and batteries. Unlike diesel generators and other ICEs, fuel cells do notuse combustion. Because they convert their energy directly into electricity, they can be morethan twice as efficient as ICEs. A fuel cell operates much like a battery, but it does not store itsenergy internally. A fuel cell does not get depleted. It keeps operating, as long as fuel is supplied.

With hydrogen and oxygen as fuels, and no combustion, fuel cells generate zero-emission energy. Their main by-product is ultrapure water.

One fuel cell provides a tiny amount of direct current (DC) voltage. Fuel cells arenormally combined in stacks to generate a larger voltage. The energy created by fuel cells can beused to power almost every energy end-usage, from small (1W) to large (multi MW)

Principle of Fuel Cell

A fuel cell combines hydrogen and oxygen to produce electricity. The basic principle ofthe fuel cell is illustrated in the figure below. The core of each fuel cell consists of an electrolyteand two electrodes. At the negative anode, a fuel such as hydrogen is being oxidized, while at thepositive cathode, oxygen is reduced. Ions are transported through the electrolyte from one side tothe other. The type of electrolyte determines the temperature window of operation. This windowof operation in its turn determines the catalysts that can be used, and the purity of the fuel to beused. The theoretical open circuit voltage of a hydrogen-oxygen fuel cell is 1.23 V at 298 K, inpractice it is around 1 V at open circuit. Under load conditions, the cell voltage is between 0.5and 0.8 V.



Fuel Cell Types

Six types of fuel cells have evolved in the past decades. They are called after theirelectrolyte, the substance that transports the ions. The electrolyte dictates the operatingtemperature of a fuel cell type. Depending on the operating temperature, a specific catalyst ischosen to oxidize the fuel. Fuel cell types therefore all have different catalysts. A brief summaryof these six fuel cell types is given below.

Fuel Cell Comparison

Various fuel cell types and their operating characteristics.

Each type of fuel cell has different characteristics making it suitable for specificapplications. PEM Fuel Cells have a wide range of applications.


Below is an overview showing various fuel cell types and their typical operating characteristics.

Comparison of low and high Temperature Fuel cells

Different types of fuel cells operate at different temperatures. While PEMFC and AFCare generally called low temperature fuel cells and MCFC and SOFC being called hightemperature fuel cells, the PAFC falls in between and can be called an intermediate temperaturefuel cell. The same holds for PEMFCs using phosphoric acid doped electrolytes. Lowtemperature fuel cells do not require the cooling and thermal shielding necessary for hightemperature fuel cells. For mobile applications, the PEMFC is the obvious choice. AFC, oncethought to be the optimal solution for road transportation, is losing ground. Even NASA isconsidering changing over from AFC to PEMFC.

For stationary applications, both low and high temperature fuel cells seem to qualify.In practice, specific conditions lead to a preference for the one or the other.


Low temperature fuel cells, especially PEM fuel cells, can be characterized by theirrapid start-up, freeze capability, high robustness towards temperature cycles, pressure variations,and redox cycles. Thanks to many automotive oriented R&D programs focusing on aggressivecost reductions, cost levels of PEMFC systems have become competitive in many stationaryapplications, such as backup and remote power, and power generation using hydrogen that isavailable on-site from existing processes.

High temperature fuel cells, such as SOFCs and MCFCs, are able to operate on fuelscontaining large fractions of carbon monoxide, and can thus be operated either with an externalor internal reformer without extensive gas cleaning. In applications where load followingdynamics and start-up times are not critical, and where other fuels than hydrogen are preferred,these high temperature fuel cells are often selected. The critical point for high temperature fuelcells will be whether they can meet the cost and lifetime criteria simultaneously.

Fuel Cell Setup

Single Cell

Besides conducting ions from one electrode to the other, the electrolyte serves as gasseparator and electronic insulator. The electrodes are the sites at which the electrochemicalreactions take place. Besides containing the suitable catalysts, the electrode architecture shouldbe such that transport of reactants to and products from the catalyst/electrolyte interface is takingplace at the maximum possible rate.


A single fuel cell, as displayed in the Figure above, produces the power, whichresults from the area times the current density of the cell times the cell voltage. The typical cellvoltage under load conditions amounts to 0.6 - 0.7 V, which is too low for practical applications.

Stacks

It is therefore common practice to put a number of cells in series, resulting in a so-called fuel cell stack. Flow plates connect two adjacent cells. These flow plates, also calledseparator plates or bipolar plates when a single plate is used for the anode side of one cell and forthe cathode side of the other cell, should have a high electronic conductance, and should act asgas separator between the two adjacent cells. The flow plates contain flow patterns on the cellside to generate an even distribution of reactants across the cell area. On the backside, coolingliquid flow patterns transport the heat to a heat exchanger in the system. The stack power andvoltage is obtained by the number of cells times the individual cell power and voltage. A three-cell stack is schematically drawn in the Figure below. Besides the repeating units displayed inthe Figure above, a stack contains two endplates and two current collector plates from which thecurrent is collected.


System

The fuel cell is the core of each fuel cell system, but it does need a number ofadditional components to make it operate and to let it play its function in its application. TheFigure below gives a schematic, simplified display of a typical fuel cell system. The componentsother than the fuel cell stack and the fuel processor are often called Balance of PlantComponents. These Balance of Plant Components are important drivers of system cost, and ofsystem efficiency and durability.

In low temperature fuel cells, except the DMFC, hydrogen is oxidized at the anode toprotons. The hydrogen can either be fed from a hydrogen storage container, or produced fromanother fuel in a so-called fuel processor. Generally, hydrocarbons or alcohols are used as fuelsto feed fuel processors. The complexity of the fuel processing depends strongly on the fuel cell


type and the primary fuel. In high temperature fuel cells, such as the MCFC and SOFC, fuelprocessing can be done in the fuel cell itself. This process is referred to as internal reforming.

The air pressure needs to be elevated from ambient pressure up to a level whichdepends on the operation pressure and the pressure drop in the complete system. This can rangefrom a gauge pressure of 100 mbar to several bars. The power of the fuel cell stack generallyincreases with increasing pressure; the parasitic loss due to compression however increases aswell.

The voltage of the fuel cell stack is the product of the number of cells times theindividual cell voltage, which is typically 0.6 - 0.7 V DC. For mobile applications, the voltageshould be increased to several hundred Volts and conditioned to the needs of the electric motor.For stationary applications, generally AC voltage is needed, which requires the need for aDC/AC inverter.

System Efficiency

The efficiency of the fuel cell stack (EffFC), the utilization of the hydrogen (UtilH2) and powerconsumed by the balance of plant components (PowerBOC) determine the total systemefficiency:

Effel, sys = EffFC * UtilH2 * (1-(Power BOC / Power Fuel cell system ))

Efficiency gains can be obtained by:

Operating at a high cell voltage; MEAs that offer high power densities at high cellvoltages are clearly to be preferred: The fuel cell efficiency for hydrogen/oxygen fuelcells, can be obtained by dividing the cell voltage at operation by 1.23 V.Hydrogen/oxygen fuel cells operated at 0.7 V thus have an electrical efficiency of 0.57.This energy efficiency number is based on the lower heating value of hydrogen.

Maximizing the utilization of hydroge; Open systems with low hydrogenstoichiometries, or closed systems with low purging frequencies, lead to less waste ofhydrogen and thus to an increased efficiency.

Minimizing the flow of air; As the flow of air is generally a factor of 4 higher than theflow of hydrogen, the energy needed for supplying this air can pose a significant parasiticloss.

Minimizing the pressure drop at anode and cathode; The pressure drop of the flowfield and manifolds requires an increase of the reactant pressure at the inlet that directlyleads to an increase in parasitic energy consumption.

Benefits of Fuel Cell

Benefits of fuel cells are manifold. Characteristics of fuel cells include the following:


Efficient. Fuel cells can be more than twice as fuel efficient as internal combustionengines. Nedstack’s fuel cells offer high efficiency over a wide power range and are notdependent on an optimal production point in order to reach expected economics.

There is neither fuel loss nor aging in standby mode. Powerful. Fuel cells create a powerful current relative to their size. Their current is

highly constant. Versatile. Fuel cells can generate energy on-demand or continuously. They come

modular and scalable to meet changing needs. They operate on a wide range of fuels. Robust. Fuel cells have no moving parts, so their maintenance requirements and costs are

low. They have virtually no downtime. Fuel cells operate reliably in a wide range ofclimates. They can be stationed both indoors and outdoors.

Sustainable. Fuel cells are low emission, PEM Fuel cells even zero emission.Governments stimulate fuel cell technology because of its potential to address issues ofclimate change, urban pollution and dependency on fossil fuels. And you can add noisepollution reduction to the list: as fuel cells have no moving parts, they do their workquietly.

Environmentally friendly. Even at the end of their life span, fuel cells remainenvironmentally friendly: most of their components are recyclable or reusable.

Attractive total cost of ownership. Their total cost of ownership, due to their superiorfuel efficiency and low maintenance profile, are below those of alternatives in a growingnumber of applications.

Fuel Cell

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