Proton Exchange Membrane Fuel Cell - Powering Aviation Understanding PEM Fuel Cell: Thermodynamic Analysis

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PROTON EXCHANGE MEMBRANE
FUEL CELL : POWERING AVIATION
Understanding PEM Fuel Cell: Thermodynamic Analysis
MT-211 (Metallurgical
Thermodynamics & Kinetics)
Prepared by
AMLAN BAISHYA
(12216002)
GOURAV RANJAN SINHA
(12216006)
KUMAR SUBHAM
(12216009)
MOHIT RAJPUT
(12216014)
SHUBHAM AGRAWAL
(12216019)

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ABSTRACT
The present study deals mainly with application of PEM (Proton Exchange
Membrane) fuel cells in airplanes in particular Boeing-787 Dreamliner. It also
provides an insight in to various components of a fuel cell and the thermo
dynamical aspects related to it. It is considered by many the fuel of tomorrow for
automobiles and aircrafts because of its increasing efficiency and reducing our
dependence on foreign oil and lower harmful emissions. It also has the potential for
replacing other fuels in different sectors, however few challenges like high
production cost, fuel storage and limited thermal operations lie ahead of it. The
thermo dynamical aspects of all the components like cathode, anode, heat
exchangers etc. provides a deep insight into the working of the fuel cell and by
optimising the conditions, higher efficiency could be attained. Furthermore
thermodynamics is crucial for the discussion and the thermodynamics analysis
gives us the various parameters for the determination of production of various fuel
cell components like hydrogen storage tanks etc. The module chosen for the basis
of the study is the Hydrogenics HyPM HD 12 Power Module. This is taken as the
representative module for the study of PEMFC. This proves a viable option and
existing resources to meet the energydemands on-board an aircraft. Thesefuel cells
are proved to be feasible for implementation in an airplane taking into account the
safety and designing aspects of the aircraft.

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Acknowledgement
We owe a debt of gratitude to our mentor and professor of our course Dr. R.
Jayaganthan for the vision and foresight which inspired us to conceive this case
study and helped us in the completion of this report.
Amlan Baishya
Gourav Ranjan Sinha
Kumar Subham
Mohit Rajput
Shubham Agrawal

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CONTENTS
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Fuel Cell: An Introduction
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a
chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel,
but hydrocarbons such as natural gas and alcohols like methanol are sometimes used. Fuel cells
are different from batteries in that they require a constant source of fuel and oxygen/air to sustain
the chemical reaction; however, fuel cells can produce electricity continually for as long as these
inputs are supplied.
History
The first fuel cells were invented almost simultaneously in 1838 by two physicists working
independently in two different countries, Welsh physicist William Grove and German
physicist Christian Friedrich Schönbein. The first commercial use of fuel cells came more than a
century later, in NASA space programs to generate power for probes, satellites and space
capsules. Since then, fuel cells have been used in many other applications. Fuel cells are used for
primary and backup power for commercial, industrial and residential buildings and in remote or
inaccessible areas. They are also used to power fuel-cell vehicles, including forklifts, automobiles,
buses, airplanes, boats, motorcycles and submarines.
Description
There are many types of fuel cells, but they all consist of an anode (negative side),
a cathode (positive side) and an electrolyte that allows charges to move between the two sides of
the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit,
producing direct current electricity. As the main difference among fuel cell types is the
electrolyte, fuel cells are classified by the type of electrolyte they use. Fuel cells come in a variety
of sizes. Individual fuel cells produce relatively small electrical potentials, about 0.7 volts, so cells
are "stacked", or placed in series, to increase the voltage and meet an application's requirements.
In addition to electricity, fuel cells produce water, heat and, depending on the fuel source, very
small amounts of nitrogen dioxide and other emissions. The energy efficiency of a fuel cell is
generally between 40–60%, or up to 85% efficient if waste heat is captured for use.
Scope
The fuel cell market is also growing at a healthy pace and according to Pike Research, the
stationary fuel cell market is predicted to reach 50 GW by 2020.
Fuel Cell Vehicles
Fuel cell vehicles (FCVs) have the potential to significantly reduce our dependence on foreign oil
and lower harmful emissions that contribute to climate change. FCVs run on hydrogen gas rather
than gasoline and emit no harmful tailpipe emissions. Several challenges must be overcome
before these vehicles will be competitive with conventional vehicles, but the potential benefits of
this technology are substantial.

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Fuel Cell System Hardware and Concepts
The purpose of the engineering analysis was to determine the feasibility of a PEMFC system on
board a commercial airplane with respect to:
o Major component options and sizes
o Fuel cell
o Hydrogen storage
o Heat exchangers, blowers, and water pumps
o Electrical Loads and Components
o Waste heat recovery methods
o System location possibilities
In this report we attempt to study about all the above components in brief and linking it to the
PEM Fuel Cell system, the analysis is carried out about its feasibility on-board a commercial
aeroplane.
Fuel Cell: Definition
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a
chemicalreaction with oxygen or another oxidizingagent. Hydrogen, however is mostly used and
can produce energy as long as these inputs are provided.
A fuel cell provides, in an electrochemical environment, a way to combine gaseous hydrogen and
oxygen to form water (typically as a liquid), as indicated by Eq. (1):
2H2(g) + O2(g) ➙ 2H2O(l) … (1)
The hydrogen fuel is not literally burned. Rather, the reaction proceeds electrochemically,
producing electrical energy and waste heat. The efficiency of the electrochemical process can be
significantly higher than traditional combustion when compared to conventional H2/O2
combustion with the “electrochemical combustion” provided in a fuel cell. Whereas traditional
combustion has a thermal efficiency of ≅ 35%, limited primarily by the temperatures achievable
in traditional combustion systems, the thermal efficiency of the electrochemical process can be
≅ 50%. The fuel cell’s efficiency can be compared to that with the efficiency of heat engines.
Over the years, there have emerged five general classes of fuel cell systems, which are viable and
commercially available. Table 3 briefly describes these five types
The fuel cell types can be divided into two regimes of operating temperature:

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o low-temperature fuel cells that operate in the range 50°C to 220°C (122 – 392°F) (proton
exchange membrane, alkaline , and phosphoric acid fuel cells),
o High-temperature fuel cells that operate above 650°C (1200°F, molten carbonate, and solid
oxide fuel cells).
There have been comprehensive studies about the feasibility of Solid-Oxide fuel cell and PEM fuel
cell for the aviation applications.
In this study, we examine the low temperature fuel cells. Alkaline fuel cells are damaged by the
small amounts of CO2 in the air and are not practical for this application. Phosphoric acid fuel
cells havetypicallylower efficiencies andlower power densities than proton exchangemembrane
(PEM) fuel cells. This leaves a PEM fuel cell as the only practical fuel cell candidate for low-
temperature fuel cell operation on-board a commercial aircraft. A description of the PEM fuel cell
is found below.
Nafion-based fuel cells operate at low temperatures, around 80°C (180°F). The low-temperature
operation provides for rapidstart-up, which is essential for aircraft power applications. However,
for temperatures at or below 80°C, the reaction product is liquid water, making management of
liquid water an important issue. The MEAs in PEM fuel cells require a Pt catalyst, which is
sensitive to CO poisoning. If properly designed, PEM fuel cells have a low sensitivity to
orientation which is particularly favourable for aircraft applications.
It is envisioned that further advances in PEM stack design, materials, and new PEM materials
may enable fuel cell stack operation temperature in excess of 100°C (212°F) in order to increase its
efficiency, improve heat rejection, further decrease the size of the heat exchanger, and operate
the stack at a state where the water produced at the cathode is water vapour versus liquid. This
would significantly reduce the balance of plant (BOP), since gaseous water vapour is easier to
handle than liquid water.

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PEM Fuel Cell: Specifics for the Study
The commercially available PEM fuel cells that offer high power density (Power/Volume) and
high specific power (Power/Mass), for the order of the loads in this study (> 10 kW), and are
providedas modules requiringonly hydrogen fuel, ambient air,and externalcooling. Thatis, they
include all equipment required for cathode air compression, heat management, and control.
The module chosen for the basis of the study is the Hydrogenics HyPM HD 12 Power Module. The
nominal power output of this PEM fuel cell is 12 kW and the specifics of its operating
characteristics are discussed later. This is taken as the representative technology for the PEM fuel
cell and it shouldn’t be concluded that this is the only type of fuel cell that can be used on-board
an aircraft.
To determine the physical sizes of the fuel cells required to meet the loads in this study, it was
assumed that an appropriately-sized module based on technology identical to the HyPM 12 could
be built, and its size would be proportional to its power in the same ratio as the HyPM 12.
However, the maximum rated capacity was not used to determine this ratio for the HyPM 12. This
is because efficiency, and therefore fuel consumption, changes with power so that a fuel cell
operating at high power will consume more fuel on a per kW basis than one at lower power (see
Figure 9). This affects the hydrogen storage system, changing its weight and volume. So the
optimal operating point from a weight and volume perspective must consider the combined fuel
cell + hydrogen storage system and not just the fuel cell itself.
In light of this, the lightest system (hydrogen and fuel cell) mass and lowest system volume
operating point for the fuel cell was calculated to be that which gives an efficiency of nHHV = 40.9%
(nLHV = 48.4%),which corresponds to a power of approximately 12.8 kW for the Hydrogenics
module. This in turn gives the specific power of the fuel cell module is 149 W/kg (67.6 W/lb) and
the power density is 103 W/L (2,920 W/ft3). These values may be lower than stated for some
manufacturers’ products but it must be emphasized that not only is the modelled module a “high
power” unit and contain the necessary accessories in addition to the stack but also the fuel cell is
not operating at its maximum rated power.

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This study also examines the use of DOE-target technology for the fuel cell system. The 2015
targets for 80 kW integrated transportation fuel cell power systems operating on direct hydrogen
are 650 W/kg (295 W/lb) gravimetric density and 650 W/L (18,400 W/ft3) volumetric density.
The gross fuel cell power required for a given system was determined from the thermodynamic
analysis, the mass and volume were determined and used to calculate the necessary size of the
fuel cell module.
Hydrogen Storage
The hydrogen storage options considered are:
o Metal Hydride
o Liquid
o Compressed gas: 350 bar (5,000 psi) and 700 bar (10,000 psi)
Figure 10 and Figure 11 provide comparisons between these options in terms of storage mass and
volume, respectively, for given amounts of hydrogen stored. Each of these options is described
below in this context.

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Metal Hydride
An alternative to storing hydrogen as either a gas or liquid is to store it in a compound as a metal
hydride. Metal hydrides have been investigated for many years for their interesting technical
properties. Over the years, a number of classic “interstitial” metal hydrides have been studied,
and are commercially available. Examples include LaNi5, Fe-Ti-V alloys, and many others. These
materials are kinetically fast, fully reversible, but typically their gravimetric capacity is low
(typically about 2%), because heavy metals are employed. This poor gravimetric capacity is
particular troublesome for applying classic metal hydrides in weight-sensitive application, for
example in automobiles or on-board aircraft.

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Metal hydrides have been investigated as a way of storing hydrogen for automobile applications.
For example, Sandia has been the lead laboratory for The Metal Hydride Centre of Excellence, a
DOE-funded centre comprised of nine universities, six national laboratories, and four companies,
all collaborating to develop advanced materials for automotive applications that reversibly store
hydrogen with high weight percent and improved volumetric density. A number of interesting
high capacity materials have been developed, for example AlH3 (10 wt. %) and Mg(BH4)2 (14 wt.
%). All of these materials are at this point research materials only, not ready for use in a practical
near term aviation system. One of the best-known of the higher capacity metal hydrides is Ti-
doped sodium alanate, NaAlH4. As a practical matter, only a maximum of approximately 4.5 wt.
% percent is released from the material. This gravimetric capacity is an improvement over the
classic metal hydrides with a weight percent typically of 2 wt. %.
In order torelease hydrogen from NaAlH4, one needstoheat the materialto temperatures around
150°C (300°F). Recently, Sandia has completed a project in which the engineering issues
associated with building a 5 kg (11 lb) NaAlH4 automotive tank, with all the attendant heat
transport and kinetic issues, have been worked out. However, such a tank is at an R&D stage, and
not really ready for a near-term commercial aviation applications.
The only metal hydrides that are readily commercially available are the interstitial metal
hydrides, and so, we report on the weight and volumetric capacities of a commercially available
metal hydride (OV 679) made by Ovonic Hydrogen Systems. It isa proprietary mixture of nominal
formula AB2H3, where A = Ti and Zr, and B = V, Cr, and Mn. The weight and volume estimates we
make also include the tank shell and internal structure including heat exchange tubing. However
the use of metal hydrides to store hydrogen is ruled out given its high weight for storage which is
inappropriate for aviation applications.
Liquid
Hydrogen has been stored for decades in large quantities as a cryogenic liquid (“LH2”) for
industrial and space applications. For much smaller quantities of hydrogen, there are no liquid
hydrogen storage tanks suitable for transportation applications that are available on the open
market. However, some LH2 storage systems have been demonstrated in either prototype form
or in small quantities.
There are two categories of liquid storage. The first is where the hydrogen is stored at
approximately 20 K (-424°F), but the pressure above the liquid is simply the equilibrium vapor
pressure established by the LH2 at that temperature. A second type of liquid storage is one in
which the tank containing the liquid-hydrogen is pressurized and the hydrogen exists as a
supercritical fluid.

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The latter
is referred to as cryo-compressed and offers improved gravimetric and volumetric storage
density over an atmospheric pressure LH2 vessel. A current version of the cryo-compressed tank
is shown in Figure 13.
There are several improvements to the cryo-compressed tanks that are under development, and
they are known as the “Liquid (Proposed)” tanks. These proposed liquid tanks offer a large weight
reduction over other storage methods, being approximately half the weight of the current liquid
technology. The improvements to this new technology , namely optimizing the pressure vessel
and shell designs thereby reducing wall thicknesses and changing to lighter materials, are rather
straightforward, and should lead to robust commercially available cryo-compressed hydrogen
tanks in the future.

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One potential drawback of a liquid hydrogen storage system is the fact that the liquid will absorb
heat from the surroundings and evaporate, leading to pressure rise within the vessel. When the
pressure builds to the maximum level allowed, the hydrogen is vented to the atmosphere so an
un-usedliquid hydrogen tank willexhaust itselfover time. However, thecryo-compressed design
averages less than 0.5% loss per day. Furthermore, this loss becomes negligible in the airplane
application where the hydrogen only needs to be stored for the several hours to complete the
mission. Of course another consideration for LH2 is the fact that it takes a lot of energy to liquefy
hydrogen: up to 30% of the combustion energy of LH2 must be consumed in order to produce
hydrogen at T = 20K. This represents a drawback from LH2 being widely available as part of a
growing H2 infrastructure.
Cryo-compressed hydrogen offers promise for future systems, especially if the issues of liquid
hydrogen generation, storage, and refuelling are satisfactorily solved. However, the gravimetric
storage density per compressed gas was poor and cryo-compressed storage option was not
considered further.
Compressed Gas
The compressed gas storage tanks analysed for this study are composite tanks with polymer
liners, also known as Type IV tanks. They are the highest pressure, lightest weight tanks
commercially available. Compressed gas at 700 bar (10,000 psi) was eliminated during
preliminary screening because although it has a smaller volume than 350 bar (5,000 psi) tanks,
the weight is more and it has additional safety and fuel logistics concerns. The 350 bar tanks are
feasible for use on-board the airplane and were selected for use in the engineering analysis.
For use in this study, 350 bar compressed gas hydrogen tanks were sized according to the linear
trend governed by the equation below. It is assumed that a custom-designed tank would be used
for the airplane application and it would have similar characteristics to the off-the-shelf models.
The equation used for manufacturing purposes was calculated to be:
Tank Mass = 16.19 x H2 Mass + 0.3862 … (14)
For 6 kg (13.2 lb) of hydrogen, this leads to a tank mass of 98 kg (216 lb), or about 6.1% gravimetric
density. Similarly for volume, the linear fit to manufacturers’ data shows that the relationship
between hydrogen mass and volume for the 350 bar (5,000 psi) compressed gas tanks is 17.0
gH2/L (1.06 lb/ft3).
This study also examines the use of DOE-target technology for the hydrogen storage system. The
“ultimate” targets of on-board hydrogen storage systems for light-duty vehicles are 7.5%
gravimetric density and 70 gH2/L (4.37 lb/ft3) volumetric density.
Once the amount of hydrogen required for a given system was determined from the
thermodynamic analysis, the relationships (given above) for mass and volume were used to
calculate the necessary size of the storage tank.

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Heat Exchangers, Blowers, and Water Pumps
A fuel cell system requires a number of components for operation, including pumps, blowers,
fans, and heat exchangers. The primary method to determine the fuel cell system component
sizes (weight and volume) was to use the thermodynamic analysis of the system to find the
performance requirements, and then consult with manufacturers for the appropriate available or
planned product that will satisfy those requirements.
In the sizing process, the type and flow rates of both fluids along with the ratio of required heat
transfer to initial temperature difference (Q/ITD) are calculated using thermodynamics to obtain
an approximate size for the system.
Blowers are used to feed air into the cathode input on the fuel cell, and for supplying cooling air
in the air-cooled system cases. Representative blowers were sized by using the required pressure
rise and flowrate from the thermodynamic analysis to find commercially available blowers that
would meet these needs. For the pumps too, water flowrate and required pressure head were
calculated by the thermodynamic model.
Electrical Load and Components
The fuel cell size depends on the power load that it must serve, and the hydrogen tank depends
on the amount of energy it must store to meet that given load for the specified time. Because all
other components in the system must be sized to go along with the fuel cell’s requirements, they
also depend on the load. Therefore, to find the system size and performance, it is necessary to
specify the load for which it is designed. Table 4 shows the nine load scenarios that considered in
this study.

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Thechoiceofdistribution voltage andtype (AC orDC)can impactthenumber andsize ofthewires
required, so this must be determined before wire mass and volume is estimated. Three possible
distribution methods were considered:
1. Low voltage (50 Volt) DC.
A DC system has the advantage of not requiring a DC-AC inverter and needs two wires
compared to a three-phase AC system. A voltage lower than 50 V provides safety and
maintenance advantages. Wire diameters will be the largest of all options. The aeroplane
used for the study 787, however does not currently have a 50 VDC distribution system, so
this option is mainly considered for stand-alone configuration (a dedicated fuel cell and
load circuit independent of the existing electrical system)
2. High voltage (±270 Volt) DC.
This system has the advantage of being DC, but requires more attention to safety during
maintenance than the 50 VDC system. The 787 already has a ±270 Volt distribution bus, so
this will not add any additional requirements and will allow the fuel cell system to tie
directly into the existing system.
3. 230 Volt AC.
This is the main electrical distribution bus on the 787. While this system requires a DC-AC
inverter, the electricity from the fuel cell sent to this bus can be used in all airplane loads.
Table 5 shows the amperage requirements for the three options and the different possible loads.
The AC current calculations assume a 0.95 power factor. Note that 3-phase AC power inherently
requires less current per wire than equivalent DC power. The 50 VDC option has very large
currents, and high-current DC may not allow application of proper switching and protection
equipment. Therefore, 50 VDC is eliminated from further consideration.

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Piping and Tubing
Gaseous hydrogen is assumed to be distributed at approximately 446 kPa (50 psig) by stainless
steel seamless tubing with a low pressure drop over the length of the run. This resulted in nominal
tubing sizes of ½-inch and ¾-inch OD (outside diameter). Standard tubing wall thicknesses of
0.035 inch and 0.045 inch for the ½-inch and ¾-inch diameter tubing, respectively, were used.
This provides a minimum design pressure of 13,890 kPa (2,000 psig). Although this is well above
the intended distribution pressure, it provides a safety margin in case of regulator failure, and
mitigates the effect of possible tubing wear in the high-vibration environment of an airplane.
Supply and exhaust air streams are assumed distributed by 4-inch (nominal diameter) PVC ducts,
and water by ¾-inch (nominal diameter) nylon-reinforced silicone tubing. Either medium could
be used to transfer heat from the fuel cell to the heat load, but preliminary analysis revealed that
conveying heat via hot water greatly reduced the weight and volume of the piping compared to
using hot air. Furthermore, heat exchangers and fans/pumps are more efficient when handling
water. For thesereasons, conveyingheatviahotairwas rejected andnotconsidered in anyfurther
analysis.
The size of the regulator is taken to be 0.6 L (36.6 in3) and weigh 2.3 kg (5 lb). Each hydrogen tank
is assumed to have one regulator, and to find the number of regulators required, it is assumed that
the maximum capacity per tank is 7 kg (15.4 lb).
Fuel Cell Waste Heat Recovery Options
The PEM fuel cell generates two streams of waste heat. One is the exhaust of oxygen-depleted air
after it passes through the electrochemically active portion of the stack (the cathode), and the

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other is the hot liquid in the cooling loop that is used to maintain stack temperature at an
acceptable level. In this study, the stack was assumed to operate at 70°C (158°F) meaning that both
the air and hot cooling water are expected to come out at this temperature. The cooling water
carries a significantly higher proportion of the heat load: over 90% of the total heat rejected is
through the cooling water.
The fuel cell also releases a small amount of hydrogen. In modern stacks this amount is so small
that it is safely mixed with the oxygen-depleted air within the fuel cell module and released with
that stream. One additional concept that was considered is taking this small amount of hydrogen
and, instead of combining it with the air exhaust, burning it in a combustor to create a high-
temperature waste heat stream. Because the combustor would realistically need to be located in
the fire-rated tail section, the application of this concept is limited to the heat loads in the rear of
the aircraft.
The remainder of this section describes the places on the airplane that may be able to utilize fuel
cell waste heat, and options for fuel cell system configurations that supply varying degrees of
waste heat.
On-board Uses of Waste Heat
There are few places on the airplane that require heat. The largest load is for wing de-icing. On the
787 this is handled by electrical resistance heaters on the leading edges of the wing, with an
electric demand of 30-85 kW per wing3. While heating the leading edges with hot air is common
on commercial aircraft, doing this on the 787 using the hot air derived from fuel cell waste heat
would require a significant re-design of the wing. Therefore, this potential use of waste heat was
not considered.
In the cabin, there are three uses of heat: (1) hot water for washing in lavatories and galleys, (2) hot
water for beverages, and (3) food service ovens. Low-grade waste heat from the fuel cells could
provide wash water at an acceptable temperature (45°C, 115°F), and could pre-heat the beverage
water and/or the food service ovens. The high temperature waste heat concept (burning the
hydrogen) could be constructed using a hydrogen-fueled furnace arrangement, with the resulting
hot air stream used to fully heat the galley ovens, completely replacing their electrical needs.
It should be noted that the air conditioning needs of the cabin are entirely cooling-related, as the
heat released from passengers is in excess of that naturally lost to the surroundings. This effect is
compounded in the 787 due to its all-composite fuselage which is more insulating than the
traditional aluminum structure. The transition to the more electric airplane architecture has also
added heat loads in the form of the inefficiencies of increased electronics and power handling
equipment. In fact, the 787 differs from other commercial airplanes in that it employs a liquid
cooling loop extending throughout the fuselage to help reject the large amount of generated heat.

18. pg. 17
A remaining use of heat is to heat the airplane’s propulsive fuel (Jet-A). Any addition of heat to
the fuel will decrease the amount of fuel burned by the engines, resulting in an efficiency gain.
Military aircraft commonly use this strategy with a variety of on-board heat sources, while on
commercial airplanes the practice is common within the engines themselves, where the fuel is
pre-heated by the engine oil. The only drawbacks are a slightly higher pressure drop in the fuel
system due to the heat exchangers, and the danger of volatilizing the fuel if its temperature
becomes too high. The former problem can be easily solved by appropriately sizing the fuel pump.
For the latter issue, simulation results in this work reveal that for a 20 kW fuel cell system and the
flight-averaged Jet-A flowrate of 1.26 kg/s, the fuel temperature would increase by about 7 °C (13
°F). For a 170 kW fuel cell, the fuel temperature would increase be about 54 °C (97 °F). Jet-A begins
to boil at approximately 200 °C (392 °F) [49], so Jet-A volatilization should not be a concern except
for much larger fuel cell systems.
Location Options for On-board Fuel Cell Systems
There are five main location categories that were considered in this study:
o Fuel cell and hydrogen near load (base case)
o Fuel cell and hydrogen in fairing or “pack bay”
o Fuel cell and hydrogen in tail
o Hydrogen in the fairing, fuel cell at the loads
o Hydrogen in the tail, fuel cell at the loads
A dimensioned outline of the 787-8 is given in Figure 15 showing each of these locations.
The issues influencing the choice of the optimal location of a fuel cell are:
o Available space on the airplane
o Safety of the installed systems
o Tubing, ducting, and wiring mass and volume
o Fuel cell waste heat recovery
o Rejection of waste (warm and moist air from the cathode, condensed water, hot coolant,
and/or excess hydrogen)
These factors are described below, along with the screening procedure
Available Space on the Airplane
There are two locations on the airplane where there is a significant amount of empty space due to
the aerodynamic requirements of the airplane shape: the fairing (where the wing joins the body)
and the tail cone. These two locations have the advantage of being able to host excess equipment
volume without compromising interior space or changing the external shape. If the fuel cell
and/or hydrogen is located outside of the fairing or tail cone, and instead is located near the load,
it must displace an existing or planned piece of equipment such as part of a galley, overhead
storage, under-floor cargo space, or passenger seat space. The inconvenience to an airline

19. pg. 18
customer may make this option less attractive. For example, a typical galley cart occupies
approximately 240 L (8.5 ft3). The 40 kW system sized for the forward galley would occupy
approximately 1150 L (40 ft3), meaning it would displace an equivalent of nearly five galley carts.
By comparison, the available volume in the tail cone area is estimated to be over 2,800 L (100 ft3),
and possibly more in the fairing area.
Safety of the Installed Systems
While fuel cells and hydrogen storage systems have in many respects a successful safety record,
the aviation application exposes the travelling public to this technology in a very proximate
way. The relative locations of people and hydrogen technology hardware strongly influences
where such technology would likely be deployed on the airplane. From a safety perspective, the
fire-rated section of the tail has a significant advantage in that it is behind a firewall and the
space is rated for fire hazards, such as might be a concern for a hydrogen fuel – fuel cell system.
This firewall does not currently exist in the fairing area, although such an upgrade is certainly
possible and reasonable. These safety issues will be important considerations for early adoption
onto an aircraft from both regulatory and customer acceptance points of view.
The separation of hydrogen storage from the fuel cell requires hydrogen to be piped through the
airplane to the point of use. The perceived increase in risk may make this option less attractive.

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Tubing, Ducting, and Wiring
The details of the tubing and ducting for hydrogen delivery and fuel cell thermal management
systems and the electrical wiring were discussed earlier. The location of the fuel cell system
relative to the hydrogen tank, the heat load, and the electrical load will affect the length of these
components and therefore the overall mass and volume.
Waste Heat Recovery
The fuel cells generate a large amount of heat in the form of hot cooling water and gas exhaust
streams. Except for the case with the hydrogen combustor, the streams are hot enough to heat,
but not boil, water and would be suitable for heating tap water for washing, preheating hot water
for beverages, and pre-heating oven systems. The case with the hydrogen combustor can provide
hot enough waste heat for supplying an oven with all its thermal needs.
In all cases, the closer the fuel cell is to the load, the easier it is to utilize the waste heat. The reason
for this is that the uses of waste heat are near the electrical loads in many cases (for example the
galleys require both electricity for refrigeration and heat for hot water). The alternative is to pipe
hot water or air from the location of the fuel cell to the load. This adds a marginal amount of
weight but also reduced the amount of useful heat due to heat loss along the length of the hot pipe.
Rejection of Waste Streams from the Fuel Cell
The fuel cell has between one and four waste streams (depending on the design):
o Warm, moist, oxygen-depleted air from cathode.
o Water condensed from the cathode stream (only if the cathode stream is subsequently
cooled below the dew point).
o Hot cooling water (some fuel cells are cooled in other ways).
o Small amount of excess hydrogen exhaust from anode (most PEM fuel cells do not exhaust
any hydrogen, or mix it with the large quantity of excess air from the cathode to present
no fire hazard).
As mentioned in the Waste Heat Recovery section, the heat and energy may be utilized for other
purposes before discharged into the environment. This would reduce, but not eliminate the waste
generated by the fuel cell. There will always be oxygen-depleted air at above-ambient
temperature that needs to be discharged, and in most cases this will also contain a very small
amount of hydrogen. The amount of oxygen in this stream would typically not be above 15% and
probably 10% or lower for modern stack designs, meaning it is not breathable and could not be
exhausted into the cabin. Furthermore, exhausting it into the cabin would add to the heat load on
the cabin air conditioning system. So it must be sent overboard and should be ducted to the same
line which exhausts cabin air.
If the cathode exhaust is cooled before discharging, some water will condense. This water should
be “pure” although any contaminants in the air stream may also enter the water. However, a
simple in-line filtration system and, if needed, ultraviolet purification (as currently used for the

22. pg. 21
potable water supply on commercial airplanes) could be applied before sending this stream to the
potable water storage tank on board. The advantage of capturing this water instead of sending it
to the waste tank is that it enables the airplane to carry less water at take-off, lowering its take-
off weight. For every 1 L of water (= 1 kg) less on take-off, the airplane could carry 0.16 kg L less of
jet fuel and achieve the same mission. This is indeed very crucial.
Over 90% of the heat generated by the fuel cell is carried away by its internal cooling water
system. For a typical PEM fuel cell operating condition, this amount can be approximated by the
net electrical power of the stack (e.g., for a 10 kW stack, approximately 10 kW of heat will be
exhausted through the cooling water). Because we assume the internal cooling system is closed-
loop, this heat must be rejected to one of the airplane’s cooling systems and/or to a waste heat
recovery option.
The amount of hydrogen exhausted from the stack is small compared to the cathode exhaust
containing oxygen-depleted air and water vapor. Commercial fuel cells will simply combine this
with the cathode exhaust and send to the atmosphere. The resulting mixture will not be
flammable due to the small amount of hydrogen and the reduced oxygen concentration in the
cathode exhaust. Alternatively, the excess hydrogen could be kept separate from the cathode
exhaust stream and burned with an appropriate amount of air in a combustor to eliminate this
exhaust stream entirely. If waste heat recovery is utilized, this has the added benefit of producing
a high temperature waste heat stream.
THERMODYNAMIC ANALYSIS
Fuel Cell Module
All power generation systems require an energy balance to demonstrate the functioning of the
system in detail. In a similar fashion the fuel cell system requires an energy or heat balance
analysis .The energy balance analysis in the fuel cell should be based on energy conversion
processes like power generation, electrochemical reactions, heat loss, etc. Based on the
electrochemical reaction taking place inside a fuel cell, the energy balance for various fuel cell has
to be analysed.
The enthalpy of the reactants entering the system should match the sum of the enthalpies of the
products leaving the cell, the net heat generated within the system, the dc power output from the
cell, and the heat loss from the cell to its surroundings.
ΔHr = ΔHp + Qsys + Poutput power + Qloss
The energy balance analysis is done by determining the fuel cell temperature at the exit by having
information of the reactant composition, the temperatures, H2 and O2 utilization, the power
produced, and the heat loss. The most simple and common reaction encountered in fuel cells is -
H2 + ½ O2 ➙ H2O … (2.1)

23. pg. 22
Analysing from a thermodynamic point of view, the maximum work output obtained from the
above reaction is related to the free-energy change of the reaction. Treating this analysis in terms
of the Gibbs free energy is more useful than that in terms of the change in Helmholtz free energy,
becauseit is more practicaltocarry out chemicalreactions ata constant temperature and pressure
rather than at constant temperature and volume.
The above reaction is spontaneous and thermodynamically favoured because the free energy of
the products is less than that of the reactants. The standard free energy change of the fuel cell
reaction is indicated by the equation
ΔG = –nFE …(2.2)
Where ΔG is the free energy change, n is the number of moles of electrons involved, E is the
reversible cell potential, and F is Faraday’s constant.
If the reactants and the products are in their standard states, the equation can be represented as
ΔG0 = –nFE0 …(2.3)
The value of ΔG corresponding to (2.1) is −229 kJ/mol, n = 2, F = 96500 C/g.mole electron, and
hence the calculated value of E is 1.229 V.
The enthalpy change ΔH for a fuel cell reaction indicates the entire heat released by the reaction
at constant pressure. The fuel cell potential in accordance with ΔH is defined as the thermo-
neutral potential, Et,
ΔH = –nFEt …(2.4)
Where Et has a value of 1.48 V for the reaction represented by Equation 2.1.
The minimum temperature for optimum operating conditions varies from cell to cell. Low to
medium-temperature fuel cells such as polymer electrolyte fuel cells (PEMFC), alkaline fuel cells
(AFC), and phosphoric acid fuel cells (PAFC) are limited by the requirement of noble metal electro
catalysts for optimum reaction rates at the anode and cathode, and H2 is the most recommended
fuel. Carbon monoxide can poison a noble metal electro catalyst such as platinum (Pt) in low-
temperature fuel cells.
The ideal performance of a fuel cell can be represented in different ways.
The most commonly used practice is to define it by the Nernst potential represented as the cell
voltage. The Nernst equation is a representation of the relationship between the ideal standard
potential E0 for the fuel cell reaction and the ideal equilibrium potential E at other temperatures
and pressures of reactants and products.

24. pg. 23
Once the ideal potential at standard conditions is known, the ideal voltage can be determined at
other temperatures and pressures through the use of these equations. According to the Nernst
equation for hydrogen oxidation, the ideal cell potential at a given temperature can be increased
by operating the cell at higher reactant pressures. Improvements in fuel cell performance have
been observed at higher pressures and temperatures. The symbol E represents the equilibrium
potential, E0 the standard potential, P the gas pressure, R the universal gas constant, F Faraday’s
constant and T the absolute temperature.
Acid fuel cell (including PEM)
H2 ➙ 2H+ + 2e- (Anode)
½ O2 + 2H+ + 2e- ➙ H2O (Cathode)
H2 + ½O2 ➙ H2O (Net Reaction)
E = E0 + (RT/2F) ln [PH2/PH2O] + (RT/2F) ln [PO2]½ (Nernst Equation)
The ideal standard potential of an H2/O2 fuel cell (E0) is 1.229 V with liquid water as the
product and 1.18 V for water with gaseous product. This value is normally referred to as the
oxidation potential of H2.
The potential can also be expressed as a change in Gibbs free energy for the reaction of hydrogen
and oxygen. The change in Gibbs free energy increases as cell temperature decreases and the ideal
potential of a cell is proportional to the change in the standard Gibbs free energy.
It is very clear that the influence of temperature on the standard potential is more pronounced
for high-temperature fuel cells. The ideal and actual performance of a fuel cell is quite different,
especially when one analyses the potential current response of a fuel cell.
Figure 2.2 displays the ideal and actual responses of a fuel cell. Electrical energy is obtained from
a fuel cell when a current is drawn, but the actual cell potential is lowered from its equilibrium
potential because of irreversible losses due to various reasons. Several factors contribute to the
irreversible losses in a practical fuel cell. The losses, which are generally called polarization or
over potential, originate primarily from activation polarization, ohmic polarization, and gas
concentration polarization. These losses result in a cell potential for a fuel cell that is less than its
ideal potential.

25. pg. 24
The effect of temperature and pressure on the cell potential may be analysed on the basis of the
Gibbs free energy variation with respect to temperature and pressure in a fuel cell. This may be
written as
(ðE/ðT)P = ∆S/nF
(ðE/ðP)T = -∆V/nF
… (2.9)
where -∆V is the change in volume, ∆S is the entropy change, E is the cell potential, T the
temperature, P the reactant gas pressure, n thenumber of electrons transferred, and F is Faraday’s
constant.
Since the entropy change for the H2/O2 fuel cell reaction is negative the reversible potential of the
H2/O2 fuel cell decreases with an increase in temperature, assuming that the reaction product is
liquid water. For the above reaction the volume change is negative, hence the reversible potential
increases with an increase in pressure. The influence of temperature on the fuel cell voltage is
shown schematically in Figure 2.4, where the fuel cell performance data from typical operating
cells and the dependence of the reversible potential of H2/O2 fuel cells on temperature are given.
An increase in the operating temperature is beneficial to fuel cell performance because of the
increase in reaction rate, higher mass transfer rate, and usually lower cell resistance arising from

26. pg. 25
the higher ionic conductivity of the electrolyte. In addition, the CO tolerance of electro catalysts
in low-temperature fuel cells improves as the operating temperature increases.
An increase in operating pressure has several positive effects on fuel cell performance. The partial
pressures of reactant gases, solubility, and mass transfer rates are higher at higher pressures. The
electrolyte loss by evaporation is reduced at higher operating pressures. The system efficiency is
increased by the increase in pressure. The benefits of increased pressure may be compared with
the problems associated with fuel cell materials and other associated system instrumentation.
Especially higher pressures increase material problems in MCFC and SOFC. Pressure differences
must be minimized to prevent reactant gas leakage through the electrolyte and seals. High
pressure favours carbon deposition and methane formation in the fuel gas.
The maximum electrical obtainable in a fuel cell operating at constant temperature and pressure
is given by the change in the Gibbs free energy of the electrochemical reaction,
W = ΔG = –nFE … (2.10)
Where n is the number of electrons participating in the reaction, F is Faraday’s constant (96,487
coulombs/g-mole electron), and E is the ideal potential of the cell. If we consider the case of
reactants and products being in the standard state, then
ΔG0 = –nFEo … (2.11)
The maximum work available from a fuel source is related to the free energy of reaction in the
case of a fuel cell, whereas the enthalpy of reaction is the pertinent quantity for a heat engine, i.e.,
ΔG = ΔH – TΔS (2.12)
Where the difference between ΔG and ΔH is proportional to the change in entropy ΔS. This
entropy change is manifested in changes in the degrees of freedom for the chemical system being
considered. The maximum amount of electrical energy available is ΔG as mentioned above, and
the total thermal energy available is ΔH.
The amount of heat that is produced by a fuel cell operating reversibly is TΔS. Reactions in fuel
cells that have negative entropy change generate heat, while those with positive entropy change
may extract heat from their surroundings.
Differentiating Equation 2.12 with respect to temperature or pressure, and substituting
it into Equation 2.10 gives
at constant pressure,
ðG/ðT = -ΔS
ðG/ðT = -nF (ðE/ðT)
Comparing the above equations, we get

27. pg. 26
(ðE/ðT)P = ∆S/nF ... (2.13)
Similarly, we can derive
(ðE/ðP)T = -∆V/nF … (2.14)
This was demonstrated earlier in this section.
The purpose of the fuel cell module model is to accurately model and predict the hydrogen, air,
and cooling requirements of a fuel cell that provides the electrical power demand specified by the
load.
PEM Fuel Cell Module
The PEM fuel cell model consists of electrochemical, heat transfer, and mass transfer calculations.
The electrochemical calculations follow the typical engineering equations as found in common
references on fuel cells. As a summary, on the single cell level the system of equations is:
Voperating = E – Vactivation – Vohmic – Vmass transfer
Where the Nernst voltage (E) is:
E = Eo + RT/nF ln (pH2 (pO2)0.5/pH2O)
Where pX is the partial pressure of species X, n is the number of electrons transferred per mole of
reactant (2 in the case of hydrogen oxidation), and F is the Faraday constant.
PEMFC systems: Water and Heat management
Introduction
It is quite common to operate proton exchange membrane fuel cell (PEMFC) systems at pressures
of 2–3 bar (a) and at or below 80 8C. Pressurised operation requires that the system includes a
compressor to reclaim part of the energy that was required for compression. A typical air
management system is given in Fig. 1. The relevant components are
o a compressor or blower;
o the humidifier, which may be integrated with the cooling circuit;
o the stack (cathode);
o a water separator;
o a pressure release device (backpressure controller or expander);
o a cooling circuit: pump, buffer and heat exchanger.

28. pg. 27
In case of pressurisation above about 1.6 bar an air cooler is required.
Small systems require low airflows, for which efficient compressors are not available. Such
systems are operated close to atmospheric pressure. The operating temperature will be to 60–65
oC. This may or may not be acceptable, depending on the application. Some applications however,
would benefit from a higher operating temperature in order to make use of the waste heat or to
reduce heat rejection problems.
Theory
Application of Faraday’s law yields the relation between the current I and the dry air feed (g/s).
Where I is the current produced (A), n the number of electrons involved in the reaction (in this
case, n = 2), e is elementary charge (1.6022E-19 Coulomb), N the Avogadro’s constant
(6:022045E23 mol-1), is the air stoichiometry (the inverse of the oxygen utilisation), x the
fraction of oxygen in air (0.20946), Mair the molar mass of air (28.964 g/mol)
For the remainder, all calculations are based on the general gas law.
PV/T=R
and the associated equations
Fig. 1. Simplified lay-out (intercooler not shown).
to calculate the partial pressure for each component in a gas mixture and

29. pg. 28
for calculations on gas mixtures, in which the weighted harmonic mean M of the individual
molecular weights Mi of the gases present in masses mi in the gas mixture is calculated using
The calculation of pmax;H2O(T), the maximal water vapour pressure as function of temperature, was
done using a polynomial approximation of water vapour pressure data taken from.
Experimental
Although the PEMFC can be operated over a wide range of the displayed curves, it should be noted
that cell efficiency is proportional to cell potential, and is in first approximation given by Zcell =
0.8Vcell (Vcell in volts, Zcell is with respect to LHV, at 100% H2 utilisation). For that reason, operation
at a potential lower than 0.6–0.7 V hardly is attractive. Power density curves are given in Fig. 3.
Opting for the highest power density is tempting in order to reduce the power specific size, weight
and cost of the stack.
However, compression of air takes energy, and these losses should be accounted for. The
polarisation curves given in Fig. 2 can be corrected to account for the losses for compression. A
straightforward way to do this is to
Fig. 2. PEMFC polarisation curves.

30. pg. 29
Fig. 3. Power density curves.
reduce the cell potential at given current I with a voltage ∆V, such that I∆V equals the power
required for compression of the air required at that current. The amount of air depends on the
operating conditions. At an individual cell potential of 0.675 V and an air stoichiometry of 2, the
air consumption of a 50 kW system is 150 Nm3/h. The calculated compressor power as function of
pressure ratio is given in Fig.4 (based on 20 ˚C inlet temperature and 70% compressor efficiency).
Fig. 4. Calculated shaft power and gas exit temperature for compression of 150 Nm3/h air.
Using the calculated energy consumption for air compression, the curves of Fig. 2 were
corrected, see Fig. 5. For the 50 kW (gross power) system referred to in this paper the
compressor uses 8.6 kW, equivalent to 17% of the power produced by the fuel cell. For very small
systems (less than several kW), more severe losses must be accounted for, since small air
compressors are not likely to attain 70% efficiency.

31. pg. 30
Fig. 5. Veffective, based on Fig. 2, corrected for energy consumption for compression.
Ultimately, the two parameters that are of relevance for the system developer and user are the net
power density and the system efficiency. Using the approximate relation between cell potential
and cell efficiency, Zcell = 0.8Vcell, the system efficiency is given by Zsystem = 0.8Veffective. The
effective cell potential can also be used to calculate the net power density. The resulting relation
between net cell power density and system efficiency is given in Fig. 6. From this, it is concluded
that the net effect of pressurisation is negligible at preferred operating conditions, that is, at
current densities of less than about 0.6 A/cm2. So, if for reasons of efficiency, high cell
potential/low cell current density operation is preferred, and if no net system power gain is
achieved under such conditions, then why would one prefer pressurised operation? The reason is
that at pressurised conditions, the water vapour pressure has less effect on water balance, gas
flow rates and gas composition
Fig. 6. Relation between system efficiency and net power density.
Consequences of high relative humidity
The performance of the PEMFC strongly depends on the conductivity of the polymer, which is
strongly related to the relative humidity of the gas the cell is in contact with. The requirement of
working at saturated gas conditions may perhaps not make it impossible to operate the PEMFC at
low P and relatively high T, but such conditions require an adapted design of stack and system.
High water vapour pressures lead to strong dilution of reactants by water vapour and large heat
exchange surfaces.

32. pg. 31
In terms of water management, the following occurs in the cathode line.
Fig. 7. The calculated wet gas volume at various temperatures and pressures. The original dry gas
flow rate used in this calculation is 150 Nm3/h, or 0.042 Nm3/s.
Humidifier
The air is humidified and heats up to stack temperature. First of all, the addition of water vapour
increases the volume of gas flowing through the system (when pressure is kept constant).
Especially at high temperature and low pressure, the increase in gas volume is substantial (Fig. 7).
The hydraulic implications of the increase in gas volume have to be taken into consideration in the
design of the fuel cell stack and the system. The pressure drop in a tube is given by the equation
Where ∆p is the pressure drop over length ∆l, the dimensionless friction factor, d the inside pipe
diameter, the average gas density over length dl, and v the average gas velocity. The pressure
drop is proportional to the product of the gas density and the square of the gas velocity, although
is dependent on the Reynolds number, and therefore gas velocity as well. The effect of
humidification is that due to the addition of water vapour, the gas density decreases whereas the
gas volume increases. The lower gas density results in a lower pressure drop in pipes and bends.
The

33. pg. 32
Fig. 8. The amount of water required to humidify 150 Nm3/h of initially dry air, as function of
temperature and pressure.
increased volume however will have a much stronger effect, resulting in an increased pressure
drop. This can be compensated for by increasing the diameters of flow channels in the fuel cell stack
and system piping. This will however, result in increased system volume and weight.
A trivial consequence of humidification is that water is consumed (Fig. 8). This will particularly
become relevant if the water that is produced in the stack is not sufficient to compensate for the
water used in the evaporator. The evaporation of water takes energy (Fig. 9). This energy can be
taken from the cooling circuit, in which case no extra energy is required. The waste heat of a 50 kW
fuel cell stack operating at 0.675 V individual cell potential (the basis for these calculations) under
condensing conditions is 61 kW. Comparing this number with the values given in Fig. 9, it can be
concluded that there should generally be no problem to fulfil this heat demand.
Fig. 10. Calculated oxygen concentration in fully humidified air, in dependence of pressure and
temperature.

34. pg. 33
4.2. Cathode
The addition of water vapour results in a dilution of the oxygen present in air (Fig. 10). The
concentration shown here is that at the cathode inlet. It can be concluded that working at low P,
high T conditions leads to high oxygen dilution, which decreases cell performance. Since oxygen is
used in the fuel cell reaction, the downstream part of each cell is exposed to substantially lower
concentrations.
Under the conditions assumed here (50% oxygen utilisation), half of the oxygen is used by the
electrochemical reaction taking place in the fuel cell. Since this reduces the volume of gas, some
of the water originally present in the vapour phase will condense out to form liquid water. This
produces heat, which has toberemovedbythestack coolantcircuit. Also, somewater isproduced,
in addition of the water produced by the electrochemical reaction. So, in the cathode, a two-phase
flow is established.
Fig. 9. The power required to fully humidify 150 Nm3/h of initially dry air, as function of
temperature and pressure.
Fig. 11. Water surplus for the system shown in Fig. 1.

35. pg. 34
Water separator
The liquid water is separated from the cathode exit gas by a water separator. The amount of
water that is collected should match the amount needed for humidification. Fig. 11 shows the
water surplus of a system as function of P and T. Negative numbers indicate a net water loss
from the system. One way to prevent a system from turning into a net water consumer is to cool
the water separator, such that it now becomes a condenser. This may seem unattractive since it
apparently increases the amount of heat that has to be discarded to the surroundings, which is
very undesirable for automotive applications. Theoretically, this is not the case however since
under these conditions a large amount of heat is taken from the stack coolant flow, to provide
the heat required for evaporation of the large amount of water for humidification .So, there is
merely a shift in the heat removal.
Fig. 12. The calculated amount of sensible heat that has to be discarded to the surroundings.
Fig. 13. Sum of duties of air cooler, humidifier, condenser and external heat exchanger as
function of stack operating temperature and pressure.

36. pg. 35
Overall heat management
One can try to match heat sources and sinks, to avoid the need for external heating and to reduce
the amount of heat that has to be discarded by a radiator. Even if such matching could be done
perfectly, a net cooling duty is still necessary. This is shown in Fig. 12. At low temperatures, the
water content of the cathode exit gas is low. Little heat is removed in the form of latent heat, but as
sensible heat instead. As temperature increases, cathode exit gas humidity increases, and the net
sensible heat removal from the system is reduced. At even higher temperatures the water separator
has to be cooled in order to become a condenser, e.g. at 60 ˚C for the 1 bar situation. For high
operating pressures, the heat removal by latent heat is relatively low, since most water is in the
liquid form. In addition, a significant cooling duty is required for the inter cooler. 5.
Discussion
From Fig. 12 it seems logical to conclude that working at low P and high T is most attractive. A
compressor is not required, and heat rejection to the surroundings is minimised. However, to make
such asystem work, hugeamounts of water havetobe evaporated in thehumidifier, requiringlarge
amounts of heat. The water condenser has to make up for the water used in the humidifier and
therefore has to withdraw large quantities of heat from the humid cathode exit flow. In the
calculations used to produce Fig. 12, the heat sources and sinks are coupled. This is however, not
realistic at high T, low P combinations, as can be deduced from Fig. 13. Here, the sum of the duties
of the air cooler after the compressor, the humidifier, the condenser and the external heat
exchanger (e.g. car radiator) is presented. From Fig. 13, it can be concluded that working at high T
and low P will lead to huge heat duties (hence sizes) for the heat exchanging components. In
combination with the large wet gas volumes (Fig. 7), there will be a large effect on system size. To
resume the essentials of all the above, an analysis is made of six system cases, each in the range of
40–50 kW (gross) power (Table 1). The systems are operating at 65, 80 8˚C and 1, 1.5 and 2.5 bar
based on single cell performances given in Table 1. It is assumed that the systems all operate at a
current density of 0.6 A/cm2. This assumption implies that the systems use the same amount of
reactants (H2: 30 Nm3/h, air: 145 Nm3/h). In the table, a new term is introduced, the ‘‘relative
pressure drop factor’’ derived from Eq. (2): Here, is the
humid gas density at x˚C/y bar, whereas Vx ˚c;ybar is the wet gas volume at x ˚C/y bar. The reference
is the 80 ˚C/2.5 bar case.

37. pg. 36
Assuming equal stack design and system piping in all 6 sample cases, and neglecting the effect of
the Reynolds number, the ‘‘relative pressure drop factor’’ is presented as an indicator for the effects
of operating conditions on pressure drops in the system.
From Table 1, it can be concluded that the most pronounced differences in the sample systems are
in the amounts of heat that have to be rearranged in the systems, and in the relative pressure drop
factors. It is also concluded that working at atmospheric pressure and temperatures of 65 ˚C and
higher is rather complicated. The 80 ˚C/1 bar case is the most extreme. The high wet gas flow has
significant consequences for the pressure drop in the system. The heat required for humidification
can be equal to the waste heat of the stack, making it difficult to humidify the gases with the stack
coolant water. The oxygen concentration at the cell inlet drops to just 11%. Also, the sum of all heat
exchanger duties is increasing significantly, leading to large system dimensions.
It seems that the use of a compressor is inevitable if heat rejection to the surroundings demands for
operating temperatures of close to 80 ˚C. For many applications a different parameter setting
would be much more attractive though.
Preferably, the temperature should be even higher. That is, close to or above 100 ˚C. The pressure
should be close to atmospheric to avoid compressors. For cars, the higher operating temperature
helps to alleviate the problem of heat rejection to the surroundings1. For stationary applications,
the benefit of a higher operating temperature would be in the more versatile use of the waste heat
for space heating and hot tap water.
The need for operation at such high relative humidity is linked to the characteristics of Nafion and
similar proton conductors. It will be clear that as long as such polymers are used in the PEMFC, the
problems of pressurised operation and heat and water management will not be solved. This will
only be the case if a new type of proton conductor is found, allowing for operation at higher
temperatures and low relative humidity. Preferably, such a polymer should still have the good
properties of Nafion type polymers: cold start capabilities, toughness and good proton
conductivity.

38. pg. 37
Theoretical and Real Fuel Cell Efficiency
The efficiency of any energy conversion device is defined as the ratio between the useful energy
output and the energy input. In a fuel cell, the useful energy output is the generated electrical
energy and the energy input is the energy content in the mass of hydrogen supplied. The energy
content of an energy carrier is called the Higher Heating Value (ΔHHHV). The ΔHHHV of hydrogen is
286.02 kJ mol-1or 141.9MJkg-1. This is the amount of heat that may be generated by a complete
combustion of 1mol or 1 kg of hydrogen, respectively. The ΔHHHV of hydrogen is experimentally
determined by reacting a stoichiometric mixture of hydrogen and oxygen in a steel container at
25◦C. If hydrogen and oxygen are combined, water vapor emerges at high temperatures. Then, the
container and its content are cooled down to the original 25◦C and the ΔHHHV is determined by
measuring the heat released between the identical initial and final temperatures. On the contrary,
if the cooling is stopped at 150◦C, the reaction heat is only partially recovered (241.98 kJ mol-1 or
120.1MJkg-1). This is known as the lower heating value (ΔHLHV) of hydrogen
Assuming that all the Gibbs free energy of hydrogen, ΔG, can be converted into electrical energy,
the maximum possible (theoretical) efficiency of a fuel cell is
However, the ΔHLHV is used very often to express the fuel cell efficiency to compare it with the
internal combustion engine, whose efficiency has traditionally been expressed using the fuel
ΔHLHV. In this case, the maximum theoretical fuel cell efficiency results in a higher number:

39. pg. 38
If both ΔG and ΔHLHV are divided by 2F, where 2 is the number of electrons per molecule of H2 and
F is the Faraday number, the fuel cell efficiency may be expressed as a ratio of two potentials:
where is the theoretical cell potential, and is the potential
corresponding to the ΔHLHV, or thermo neutral potential.
In this section, we analysed the theoretical fuel cell efficiency. However, as explained in the
previous sections, in a real fuel cell the efficiency is quite lower and also depends on the fuel cell
current. The fuel cell efficiency ηFC can also be defined as the ratio between the power produced and
the power of hydrogen consumed
Where Vfc is the generated voltage and Ifc is the fuel cell current. Thus, the FC efficiency is related
to the actual voltage, which is related to the fuel cell current through the polarization curve.
Moreover, in a real system it is necessary to incorporate some auxiliary systems which consume a
fraction of the generated power. As a result, the efficiency of the fuel cell system, ηfcs, is even lower
than that expressed in above Eq.

40. pg. 39
Where Pnet is the net power output, Pfc is the fuel cell power, and Paux is the power consumed by the
auxiliary components, which include, in particular, the air compressor.
The efficiency curve of a fuel cell system of 50 kW modelled in Advanced VehIcle SimulatOR
(ADVISOR) is shown in Fig.1
Exergetic efficiency
Exergetic efficiency, which is defined as the second law efficiency, gives the true value of the
performance of an energy system from the thermodynamic viewpoint. The exergetic efficiency of
a fuel cell system, shown in Fig. 1, is the ratio of the power output Ẇ, over the differences between
the exergy of the reactants (air + hydrogen) and the exergy of the products (air + water), which can
be determined by the following formula:
where Ėair;R, ĖH2;R, Ėair;P and ĖH2O;P are the total exergies of the reactants, air and fuel (hydrogen), and
the products air and water, respectively. Assuming negligible potential and kinetic energy effects
on the fuel cell electrochemical process, the total exergy transfer per unit mass of each reactant and
product consists of the combination of both physical and chemical exergies:
1. Physical exergy
Physical exergy is associated with the temperature and pressure of the reactants and the products
in the fuel cell system. The physical exergy is expressed in terms of the differences of enthalpy from

41. pg. 40
those and entropy from those at standard conditions of temperature and pressure of T0 =298 K and
P0 =1 atm, respectively. The general expression of the physical exergy can be described as:
where h0 and s0 denote the specific enthalpy and entropy evaluated at standard conditions,
respectively. The physical exergy of an ideal gas with constant specific heat Cp and specific heat
ratio k can be written as:
2. Chemical exergy
The chemical exergy is associated with the departure of the chemical composition of a system from
that of the environment. Simply, the chemical exergy considered in the analysis is rather a
standard chemical exergy that is based on the standard values of the environmental temperature
of T0= 298 K and pressure of P0= 1 atm. Generally, these values are in good agreement with the
calculated chemical exergy relative to alternative specifications of the environment. Values of the
chemical exergies for both the reactants and products are given in Table 2. However, the chemical
exergy of air produced from the electrochemical reaction should be calculated in terms of the mole
fraction of each component in the mixture x using the following equation:
The main reason for using the above chemical exergy equation for the product air is that the mole
fractions of mixtures in air would be different than those at the standard condition, especially the
mole fraction of oxygen that will be reduced as a result of the combination with hydrogen to form
water.
3. Mass flow rates of the products and the reactants in the fuel cell
Depending on the power output Ẇ and a fuel cell voltage V, and the stoichiometry of air λ, the mass
flow rates of the reactants and the products in the fuel cell can be easily evaluated from the
equations. The mass flow rates of the inlet air and fuel, hydrogen, can be evaluated through the
following equations:

42. pg. 41
The mass flow rate of the product, air, can be defined as the difference between the amount of
oxygen in the electrochemical reaction and the amount of oxygen consumed by reacting with
hydrogen to produce water:
The amount of water produced by the fuel cell can be calculated by the following equation:
Subsequently, the total exergy of the reactants and the products can be determined through the
following equations:
At standard atmospheric conditions, the air molal analysis (%) would be: 77.48 N2, 20.59 O2, 0.03
CO2 and 1.9 H2O (g). The calculations of the physical and chemical exergies, mass flow rates and
efficiency will be performed at temperature ratios (T/T0) and pressure ratios (P/P0) ranging from 1
to 1.25 and 1 to 3, respectively. These ranges of the operating temperature and pressure are
specifically to be typical for PEM fuel cells. In addition, the analysis will be conducted on 2 cases,
namely at cell voltages of 0.5 and 0.6 V. Moreover, exergetic efficiency calculations will be
performed at air stoichiometries of 2, 3 and 4 and at a cell voltage of 0.5V. The properties of the
PEM fuel cell with the specified operating conditions are presented in Table 1. The analysis will be
performed on an experimentally tested 10 kW PEM fuel stack developed by Energy Partners Inc..
The fuel cell consists of 40 cells with an active cell area of 780 cm2, capable of generating 10 kW DC
power output at 40% efficiency at 300 kPa and 65 ⁰C.

43. pg. 42
Fuel flow controller
The fuel flow controller specifies the mass flow rate of fuel to send to the stack depending on the
power demand and the fuel utilization through the equation:
Where Uf is the fuel utilization, MW is the molecular weight, I is the stack current [A], N is the
number of cells in the stack, n and F are as described in the fuel cell section, and the flow rate is
given in g/s. Uf for commercial PEMFCs is typically close to 1, and in this study is assumed to be
95%.
Air flow controller
The air flow controller is designed similarly to the fuel flow controller in that it calculates the
amount of air to send to the stack depending on the power demand and the oxygen utilization UO2.
The equation it uses is:

44. pg. 43
Note that in this case n = 4 because it is considering the oxygen reduction reaction.
The air control block also incorporates a compressor/blower model to find the power required to
supply the calculated amount of air at 0.17 atm above ambient pressure at a specified efficiency. In
addition, to account for the range limitations of most blowers, a minimum airflow was set so that
even if the stack is operating at low load the blower must supply a minimum amount of airflow.
This is common for commercial stacks and is the reason for the large drop in efficiency as current
is reduced.

45. pg. 44
Waste Heat Recovery methods for LT-PEMFC
Abstract
Fuel cell systems can apply high electricity efficiency. Meanwhile they lose a lot waste heat to the
cooling system or exhaust gas. Recovering the waste heat in fuel cell system might increase the
system efficiency. In this case study, different methods are presented for the heat recovery in fuel
cell systems. The first method is to use waste heat for fuel reforming processes in fuel cell systems.
The second one is to combine with gas turbine, for high temperature fuel cell systems-solid oxide
fuel cells (SOFCs). The third option is to combine heat and power in fuel cell systems.
Introduction
Fuel cell systems become important in energy production research. They are considered to be very
attractive power generation systems, promising highly efficient electricity generation and very
low environmental impact. There are some different kinds of fuel cell systems, according to the
different electrolyte material and working temperature. For high temperature fuel cell, there are
Solid oxide fuel cells (SOFCs) and Molten carbonate fuel cells (MCFCs). The working temperature
of SOFC is about 600° to 1000° c and its electrolyte is made from zirconia doped with yttria. While
the working temperature of MCFCs is around 650°C , its electrolyte consists of liquid (molten)
carbonate, which is a negative ion and an oxidizing agent. For the low temperature (around 40° to
80°C ) fuel cell system, there is Proton Exchange Membrane (PEM) fuel cell, whose electrolyte is a
proton- conducting polymer membrane.
For the different fuel cell systems, one important fuel is hydrogen. But in reality it is difficult to get
pure hydrogen. Reformatted methanol or other hydrocarbons (gasoline or diesel fuel) can be an
alternative to the pure hydrogen based fuel cell systems. But one of the drawbacks of the reformate-
based fuel cell systems is that a large amount of energy is needed for the fuel processing purpose.
The total heat energy requirement for a reformer can be estimated by using the following relation
ΔHtot =ΔH rxn +ΔH vap +ΔHCp +ΔHloss
where ΔHrxn is the enthalpy of reforming reaction; ΔHvap is the enthalpy of vaporization of the
liquid feedstock; ΔHCp is the enthalpy required to reforming temperature; and ΔH loss is the heat
lost to the ambient which could be minimized with adequate insulation.
It was estimated that heating value equivalent to that of about 20-30% of the hydrogen produced
in the reformer is needed to provide a fuel stream with sufficient heating value to meet the heating
requirement of the reformer. On the other hand, according to the fuel cell system energy
distribution, about 50% energy in fuel cell system is converted into electricity, the rest 50% energy
from fuel cell system is lost as heat (fuel cell cooling system and exhaust gas heat). If some part of
the waste heat can be reused, the energy efficiency of fuel cell system would be improved greatly.
For instance, the waste heat may be used for hydrogen reforming process. For the fuel cell research,

46. pg. 45
many people only put their attention to improve the electricity efficiency of fuel cell systems. But
only a little attention is given to the research of waste heat utilization or recovery in fuel cell
system.
In this case study, we will focus on how to reuse the waste heat from fuel cell systems. According to
the survey, we will present some methods for energy recovery system in fuel cell systems. They are
included:
o The high temperature fuel cells: the waste gas is at high temperature. It can be used for fuel
reforming processes in fuel cell systems. Another option is to combine with gas turbine or
micro gas turbine.
o The low temperature fuel cells: the waste gas is at low temperature, but it still can be used
for fuel reforming.
o A combined heat and power system (CHP) for fuel cell system is given for the places, where
fuel cell system not only needs to supply electricity but also needs to provide heat.
High temperature fuel cell systems
High temperature fuel cell systems are included: SOFCs and MCFCs. They have operating
temperatures ranging from as low as 600 ℃ for intermediate temperature operation to above 900
℃ for higher temperature operation. These high temperatures are often viewed as a considerable
disadvantage from a materials point of view, due to the occurrence of unwanted interfacial
reactions, stresses as a result of thermal expansivity mismatches, and so on. But high temperature
is also an advantage of high temperature fuel cell systems.
1) Fuel reforming: The waste heat from high temperature fuel cell systems can be used for fuel
reforming process, which involves the conversion of hydrocarbons into H2 and carbon monoxide
or carbon dioxide. There are three methods of reforming: steam reforming, partial oxidation and
auto thermal reforming.
o Steam reforming of natural gas can be illustrated by the following chemical equation
CH4 + H2O= CO + 3H2 ΔH=206kJ/mol
One of the advantages of steam reforming is that, besides to the two moles of H2 obtained
from methane, an extra mole of H2 is obtained from the added water.
o Partial oxidation of natural gas can be illustrated by the following chemical equation
CH4 + 1/2O2= CO + 2H2 ΔH=-247kJ/mol
Water is not a reactant in the partial oxidation process. Therefore steam generators are not
required, leading to a simpler system.
o Autothermal reforming methane is presented below
CH4 + 3/2O2 = CO + 2H2O ΔH=-519kJ/mol
CH4 + H2O = CO + 3H2 ΔH=206kJ/mol
The autothermal reforming is a combination of partial oxidation and steam reforming.

47. pg. 46
2) Waste heat for fuel reforming: On the other hand, due to the interesting from transport
applications, PEM fuel cells have made rapid technical progress in recent years. But the absence of
a cheap and plentiful supply of hydrogen blocks the development of PEM fuel cells. It is necessary
to process other fuels to produce a suitable gas mixture. So a reforming operation is required for
PEM fuel cells. The benefits of combining high-temperature SOFCs with low-temperature PEM
fuel cells are very huge. Owing to the internal reforming ability of the SOFCs, it is possible to
produce both electrical power, and a stream of reformate gas from an SOFC stack. This eliminates
the need for a reformer upstream of the PEM fuel cells, and should result in improved system
efficiency.
Table 1 :-
Table 2 :-
Now presenting a system combining high and low temperature fuel cell types, in which a high-
temperature SOFC was used to produce electricity and carry out fuel reforming simultaneously.

48. pg. 47
The exhaust stream from an SOFC can be supplied to a low-temperature PEM fuel cell. The results
showed that the overall system efficiency is 61%, which was higher than the overall efficiency of a
Reformer-PEM system (37%-42%), as shown in table 1. Meanwhile the hybrid system had a
significant financial benefit compared to the SOFC-only system, as shown in table 2, and a
substantial benefit compared to the Reformer-PEM system. However, some factors should be
considered for the hybrid system. The first one is the distribution of the power output between the
SOFC and PEM stacks. The second one is the effectiveness of the heat recovery processes within the
system.This one tells that combining a fuel cell cycle with other thermodynamic cycles to provide
higher efficiency. The heat rejected by the fuel cell at higher temperature is used in a bottoming
cycle to generate steam. A conceptual design of a SOFC – PEM fuel cell hybrid power plant , in
which the high temperature SOFC acted both as electricity producer and fuel reformer for the low
temperature PEM fuel cell. The exhaust from the PEM fuel cell went to a waste hydrogen burner
and heat recovery steam generator which can produce steam for further utilizations. The process
should have low pollutant emissions and cost competitive with the existing technology. The
optimal trade-off design solutions for the Pareto set for the hybrid power plant was presented
through a multi-objective optimization framework. The result showed that uncertainty had a
considerable effect on the objectives and the trade-off surfaces were markedly different.
.
SOFC-PEM hybrid fuel cell power plant
Low temperature fuel cells
A large amount of energy is needed for the fuel processing purpose in fuel cell systems. The heat is
usually provided through the combustion of remaining hydrogen/hydrocarbons in the exhaust
gases from the fuel cell anode, burning the hydrogen/hydrocarbons in the byproduct stream of the
reformer, or consumption of additional hydrocarbon fuel other than that being reformed in the
reformer. It is evident that the energy input to the reformer must be reduced if the efficiency of a
fuel cell power plant is to be increased. There is waste energy recovery system for reformate-based
PEM fuel cell systems, as shown in fig. below. There was a throttling valve, a heat exchanger, a
compressor in this system. The feed stock of the fuel reformer, which is a mixture of water and fuel,

49. pg. 48
is vaporized in the heat exchanger and is then compressed to a sufficiently high pressure before it
is ducted into the fuel reformer. The result showed that the power plant efficiency with the energy
recovery system can be increased by more than 20% compared to that of a fuel cell power plant
without the energy recovery system. On the other hand, more than 25% of the waste heat generated
by the fuel cell stack can be brought away because of the energy recovery system. The fuel burned
for the fuel reforming purpose is reduced by more than 70%.
Fig: Schematic of a fuel cell power plant incorporating the energy recovery system.
Combined heat and power (CHP):
In some field as presented in fig. below, fuel cell systems not only need to provide electricity but
also need to provide heat. After getting electricity from fuel cell systems, the electricity is
transferred into heat when heat is needed. In this case, the energy efficiency is very low. Maybe the
heat can come from the waste heat of fuel cell system instead of from electricity. So the CHP is a
very efficient method to use the energy in fuel cell systems, when the electricity and heat both are
needed. We focused on a fuel cell systems not only about electrical efficiency but also about thermal
efficiency. If the electricity is changed as heat to be used, the efficiency would be very low. So it is
better to provide heat directly from fuel cell systems by the exhaust heat. Some necessary and
important properties of CHP fuel cell systems:
o an ability to vary their electrical load rapidly
o an ability to vary their heat to power ratio during operation
o an ability to deliver their waste heat to a useful thermal sink.

50. pg. 49
Fig: Schematic diagram of one of type of combined heat and power (CHP) fuel cell system
Report
Fuel cell systems can provide high efficient electricity and very low environment impact. Instead
of focusing on improving electricity efficiency in fuel cell systems, much attention should be given
to reuse the waste heat. By recovering the waste heat in fuel cell system, the efficiency of fuel cell
system increases a lot.
There are some useful methods to recover the waste heat in fuel cell systems:
o When the waste heat is used for fuel reforming processes, the overall efficiency of fuel cell
system can achieve about 60%. This method can be applied to high temperature fuel cell
systems (SOFC). It can be also suitable for the low temperature fuel cell systems (PEM).
o For the combined solid oxide fuel cell system with gas turbine or micro gas turbine, a net
electrical efficiency can be greater than 60% and the system efficiency is greater than 80%.
o The combined heat and power in fuel cell systems is a good option when heat and electricity
both are need to supply.
The waste heat recovery in fuel cell systems will not cause many problems for the stationary
application. But it may cause some issues for the transportation application. For instance,
compactness, weight, complexity, and so on. Thus, some factors should be considered for the
design of waste heat recovery in fuel cell system.

51. pg. 50
Conclusion
The study discusses to show whether or not a fuel cell system could be feasibly
implemented on board an airplane, and what some of the design aspects and trade-
offs are. This is done by examining different possible system configurations; that is,
both how the components of the system are arranged relative to each other and the
arrangement of the whole system on the airplane. Many conclusions and
assumptions are made at the end of each section and the thermodynamic data for the
same. For e.g. 350 bar compressed gas hydrogen cylinder is used for hydrogen
storage purpose instead of liquid hydrogen and metal hydrides method of storing
hydrogen for fuel cell.
Also, the effect of different loads on the fuel cell system, are compared and
contrasted which in-turn describes its effect on overall airplane performance.
Possible recommendations for the optimization of performance of the airplane and
the fuel cell system are also made by providing various methods for the utilisation of
waste heat generated from it. Thermodynamic analysis provides various conditions
for the study and optimisation conditions, by studying the effect of varying
conditions of operation with parameters like temperature and pressure. Taking into
account the location aspect in the airplane, the pros and cons are discussed keeping
in mind the safety concerns of the passengers. Finally, PEMFC prove to be a viable
and practical option in all respects to meet the energy (electrical) demands of the
Boeing-787 Dreamliner effectively.

52. pg. 51
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53. PROTON EXCHANGE MEMBRANE FUEL CELL: AN ANLYSIS
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