Midterm Flashcards
Reformation vs. Syngas?
Syngas has higher CH_4 and has further reformation possibilities.
*Difference between the two is that reformate has been reformed.
Why would someone propose on-board reformation of a hydrocarbon fuel?
Hydrocarbon fuel can be stored as a liquid, which means it has a higher energy density.
Why is PEMFC performance almost always better than DMFC performance?
Both of these fuel cells have Cathode Losses, but DMFC have Anode Losses too.
What is the role of the anode catalyst in a PEMFC?
Break apart Hydrogen.
What is a Raney Metal?
Heterogenous (doped) metal (not an alloy). This metal is then raised to the lowest melting temperature of the metals it has, and melts that particular metal out. This maximizes surface area of the metal.
*Doesn’t work for PEM as the sharp edges “pop” the membrane, but DMFC works as it is a liquid.
List 3 (or 4) ways of reducing activation losses:
Increase Pressure/Temperature.
Catalyst.
Change Fuel.
Increase “effective area” (i.e. Rough Surface)
What are the cathode exhaust relative humidity limites one uses for the PEMFC?
80-100%
What is Sparging, and why use it?
Sparging is pushing air flow from under water to create bubbles that bring water with them on the surface, and when it busts at the surface, it increases the humidity of the gas above.
Draw a polarization curve and point out the major loss regimes in a fuel cell. (No image, but think about it and list the important parts, and where their located.)
X axis is charge/area (mA/cm^2)
Y axis is Voltage
Ideal/Reversible (all chemical energy is converted to electricity) is the line across the top.
Activation Region is the initial area between the reversible and the rapidly declining line.
Ohmic Region is the minimally declining portion of the line.
Mass Concentration Polarization occurs after the Ohmic Region where the line steeply declines to V=0.
What does a Plate Fin Heat Exchanger look like?
Like corrugated metal sandwiched between two other plates. Gases flow along the vanes.
If there is a large heat difference between the plates, then there may be a problem of different energy in each of the vanes, so perforating or even have a large areas where the gases can be exchanged can improve on this.
What are the 3 main degradation mechanisms in catalytic systems?
Sintering
Fouling
Poisoning
What 3 (or 4) mechanisms limit the conversion in a catalytic steam reformer?
Heat Transfer
Mass Transfer
Chemical Kinetics
Catalyst Degradation
How can a single PEMFC membrane be dehydrated at one point and flooded at another point?
Electro-Osmatic Drag.
What is an MEA?
Membrane Electrode Assembly
What are the seven layers of an MEA? (What is removed if only 5 layers are used?)
1, 7) Gas Diffusion Layer
2, 6) Micropore Level (Hydrophilic)
3, 5) Electrode Catalyst Layer
4) Electrolyte
Micropore Level can be removed for 5 layer.
Water is required for the AFC cathode while it is generated at the anode. Why?
OH- is the mobile ion, and it needs H from the Water.
How can gas foul the surface of a DMFC anode?
CO_2 could block the fluid from hitting the catalyst.
What does 50% excess air mean?
It means that we’re adding 50% more air than stoichiometric equality would require.
What is the first thing to do if the voltage drops?
Purge/Flush the anode.
What’s a good target for Hydrogen Utilization?
90%
What is Turndown Ratio?
Ratio of: (Lowest Operational Power) / (Maximum Operational Power)
This can be important because if there is a long start-up time, it may be more efficient to just keep the cell running at it’s lowest operational power rather than shut it off entirely.
Utilization is INVERSELY related to turndown ratio.
What happens if the hydrogen is not absorbed in a system?
Opens the possibility of overheating.
What is the relationship of power and efficiency as shown by the general fuel cell polarization curve?
As power increases, efficiency drops.
This is the opposite of what occurs in an Internal Combustion engine. Also counter to electric motors, which has the efficiency increase as the VOLTAGE increases.
What is the max temp for a PEMFC, and why?
100 C because at that point water would evaporate.
What does PEM stand for?
Proton Exchange Membrane
What is the stoichiometric equation for a methanol reformer?
CH3OH + H2O -> 3H2 + CO2
What is a “childish” description of PEM?
Spaghetti and Meatballs. The meatballs transfer the electrons (Water Gurps) and as they get fuller, they get constrained by the spaghetti (PTFE, aka Goretex).
What generates Entropy?
Heat Transfer
Friction
Unconstrained Expansion
(Possibly Mass Transfer)
What are the two Boundary Phenomena, and briefly define them:
Heat - Energy that’s exchanged between areas of temperature difference.
Work - Everything else.
What can cause electrical loss?
Resistance (equivalent to Friction) and EMF created fields.
Who is your friend for stoichiometric equations?
“CHON” is your friend!
What is Anthropogenic?
Caused by man.
What does Base-Loaded mean?
A system that is on all the time.
What does Peaky mean?
A system that is turned on/off regularly, and hopefully quickly.
What is the isentropic max efficiency of a system?
(Q_In - Q_Out) / Q_In
or
(T_H - T_C) / T_H
For a combined system with input energy Q_H and outputted work W_Top and W_Bottom, what is the efficiency of the system?
mu = (W_Top + W_Bottom) / Q_H
What are some of the drawbacks of using a combined system?
A combined system can add weight and cost, and may not be very useful if the outputted energy of the first system is not very high.
What is the relationship between current and hydrogen in a fuel cell?
Current is proportional to the amount of Hydrogen used.
Is it possible for electrons to float backwards through the electrolyte?
Absolutely. This is some of what causes non-ideal behavior in a fuel cell.
What is V_c? V_oc?
V_c - Current (as in present) voltage in a cell.
V_oc - Optimal open circuit voltage.
What is the Anode reaction for Methanol? (What can be a problem of one of the products?)
CH3OH + H2O -> CO2 + 6H
CO2 can end up blocking the catalyst, requiring a purge/flush of the anode.
What is the Cathode reaction for Methanol? (And what can the product be used for?)
6H + 1.5O2 -> 3H2O
The resulting H2O is more than enough to be used to support the reaction at the anode.
What can be a problem based on the ideal open circuit voltage?
It’s calculated based on that the system is flooded…
What is impingement?
It is a method to drive the molecule to the surface. This can be used to handle CO2 that may be blocking the catalyst.
What effect does pressure have on Gibb’s Free Energy?
Pressure INCREASES available Gibb’s Free Energy.
What is the best fuel for all types of fuel cells?
Hydrogen!
What exactly is Sintering?
While it can be defined as melting, it is actually when the internal metal melts AROUND the coating metal.
Aside from liquid vs. gas, why do hydrocarbons have a higher energy density than just hydrogen?
Because Hydrogen will actually “sit closer” to each other if there’s something in the middle.
What is the flammability limit?
The concentrations of fuel and air between which combustion is possible.
What is the flammability limit of Hydrogen?
Between 4% and 76%.
What is the stoichiometric equation for Methane?
CH4 + 2 O2 -> CO2 + 2 H2O
What are the “brothers-in-law” to any stoichiometric equation?
This can refer to the nitrogen in the air. Usually when balancing with oxygen as a reactant, this is referring to oxygen from the air, which therefore includes 3.76 moles of N2 for every mole of O2. And they may not react, but they’re along for the ride.
Define (in a very specific way) “to burn”:
To overcome the ignition engergy.
EGR is an acronym for what?
Exhaust Gas Recirculation
What are Zeldovich Knocks?
High Temperature (1500) makes N2 and O2 behave badly and combust.
How can Exhaust Gas Recirculation (EGR) prevent overheating?
By using exhaust, the extra fuel can be absorbed using some of the detritus used in the fuel, “rounding off” any excesses during combustion.
This can also extend the flammability limits.
What is the comparison of the energy density between hydrogen and gasoline?
1 kg Hydrogen is equal to 1 gallon (~3.5 kg) fuel. This means that energy per mass is greater for hydrogen.
HOWEVER
1 kg Hydrogen is ~15 gallons depending on pressure, but much much larger than 1 gallon of fuel. So Energy Density is much higher for gasoline.
Not to mention issues with transporting gas vs. liquid.
What do you know about the surface temperature of a fuel droplet?
That it is equal to the boiling point (i.e. saturation temperature) of the fuel.
How does a candle connect to the rich limit, and the lean limit?
For a candle, very close to the wax and wick, the density of the gaseous fuel is too dense for the flame to light, but at some distance is does. This is the rich limit. The physical outer limits of the flame define the lean limit where there may be fuel still left in the air, but it is not dense enough to actually light on fire.
What is the key advantage of the catalyst?
The catalyst lowers the ignition energy (or activation resistance) enabling “more” reaction.
What are the three types (or 4) of Reformation?
Preferential Oxidation (PrOX)
Pressure Swing Absorbtion (PSA)
ATR (Autothermal reformation. Also known as CPOX)
Steam-reforming
Why is Sulfer super bad?
Because it will chemically bond with the catalyst (poison).
What is a Water Gas Shift?
Chemical reaction to create a higher ratio of hydrogen to carbon.
What is Space Velocity?
The rate of volume entering a reactor (or catalyst bed volume) divided by the volume of the reactor (or catalyst bed volume.
This is \dot{V} / V which gets a unitless rate per second.
What is Conversion Limit?
How much of the fuel converts.
Ratio = (Fuel Changed) / (Fuel Fed).
What is Yield?
Ratio = (Mole of Desired Product Created) / (Mole of Reactant Input)
Could be greater than 1. (An example for a reformer would be H2 / CH4)
CH4 + 2 H2O -> CO2 + 4 H2
Because we expect 4 times the number of H2 over CH4, we therefore divide the total sum of the moles of H2 by 4 before calculating this ratio.
What is HTS?
High Temperature Shift
What is LTS?
Low Temperature Shift
What is NDIR?
Non-Dispersive InfraRed.
Analyzes absorbtivity of gas, so you can determine ratios of gas. (Methanol vs. Water)
What has the largest impact on the viability of an endothermic process?
Correctly handling the heat transfer.
What “dominates” in an ATR Catalyst situation?
Heat Transfer.
What is Light-Off?
When a reaction has become self-sufficient.
True Definition: The temperature at which a catalyst’s efficiency crosses 50%.
Heat for a Steam Reformation comes from where? For ATR?
For SR, heat comes from the outside, so therefore MAXIMIZE the ability for heat to transfer.
For ATR, heat comes from the inside so therefore MINIMIZE the ability for heat to transfer.
Inside of a Reformer, why would you raise the steam/carbon ratio?
To inhibit Coke formation.
Inside of a reformer, what degradation to a catalyst can occur from a low temperature, and what type from a high temperature?
Low Temperature can lead to fouling as the reaction is less likely to occur. (High Temperature would break up the CO formation.)
High temperature can lead to sintering (and possibly fouling from Coke formation)
What is the “Coal Myth”?
Coal is an energy carrier, NOT a molecular carrier. I would guess the myth is the opposite of this…
What can Sodium Boral Hydride do?
It can “steal” the oxygen from H2O enabling the generation of hydrogen.
What can Aluminum do?
Aluminum will react with water and create Hydrogen.
The problem is that most aluminum we interact with has interacted with oxygen, forming a barrier on the aluminum that prevents that reaction described above from happening.
What is the Nusselt Number?
The ratio of Convective Heat Transfer to Conductive heat transfer.
Ratio = h L / k
h - Convective heat transfer coefficient of the flow
L - Characteristic Length
k - Thermal conductivity of the fluid
What is LHSV-M?
Liquid Hourly Space Velocity of Methanol
How do you clean PEM cells?
Clean them as you would a GC column!
1) Non-reacting Carrier Gas
2) Increase the temperature
What is the stoichiometric equation to GENERATE methane?
2 CO + 2 H2 -> CO2 + CH4
Name three flow methods (fields) for PEM?
1) Serpentine
2) Parallel
3) Interdigitated
All of these will drop the percent of hydrogen as it passes through, allowing for the possibility of condensation to block the pathway.
Serpentine is less prone to flooding.
Interdigitated maxes out cross-flow.
What are two of the membranes that can be used in a PEM?
Polyethylene (like Saran Wrap) and Polytetrafluoroethylene (hydrophobic, but “breathes”; corporate name is GoreTex)
Define Electro-Osmatic Drag:
Electron drags water from Anode to Cathode.
This makes the Cathode wet, but the Anode dry. (Super bad.)
What does lambda represent?
Stochiometry Value
What’s the difference between Compressors and Blowers?
Compressors operate at pressure increases greater than 200kPa while blowers operate at pressure increases less than 200 kPa.
Isotropic Efficiency
Reversible Energy / Total Energy
What is a transfer function?
Output / Input
In a steady state.
Define “Boost”:
When the intake pressure is higher than the atmospheric pressure.
What is a way to handle Turbo-lag?
Drop an electric motor in there to juice the system while the turbo warms up.
What’s the relationship between efficiency and the size of the compressor?
Efficiency increases as the size of the compressor increases.
Because of the constraints of Fuel Cells, what are good compressor requirements?
Positive Displacement
Low Flow
Things that can be cooled
“Where” are the problems for PEMFC?
“All problems at the Cathode.”
What changes in the three phase reaction for a DMFC?
Gaseous Hydrogen is replaced with Liquid Methanol.
This is what can cause air bubbles of CO2 to occur within a liquid, requiring pinge-ing or using inherent turbulence in the system.
What happens if the crossover occurs with the water in a DMFC?
The water will act as a carrier bringing the methanol through the polymer without actually allowing it to react with the polymer.
What is the general ideal calculation for the change in enthalpy for a DMFC?
Given:
dh = u + PdV + VdP
but since it’s isentropic (scratch u) and incompressible (scratch dV) then:
dh = VdP
What’s the difference between lean (single) and hybrid?
Hybrid contains some sort of manner to store energy for delayed use in the work output of the system.
Why would we need a DC/DC voltage conversion for a Fuel Cell vehicle?
We may want 500V but often have much less voltage outputted by the system,.
Why can Lightoff be defined as the 50% conversion efficiency of a catalytic converter?
Because the transition from 50% to 90% is very rapid.
What is an RTD and how does it compare to a thermister?
Resistive Temperature Detector, and it has a linear response over a large bandwidth.
Thermisters have an exponential response over a very small range.
What are some (8) primary energy resources?
Nuclear Natural Gas Petrol Coal Hydro Solar Wind Biomass.
What are some (7) methods of producing hydrogen?
Reforming Electrolysis Electrochemical Cycles Chemical Cycles Thermochemical Cycles Thermal Decomposition Photoelectrochemical
What fuels (general) exist (8)?
Hydrogen Gas Liquid Hydrogen Metal Hydrides Methanol Natural Gas Gasoline Diesel Kerosene
What are some advantages (4) of using Hydrogen as a fuel (in general, not just for fuel cells)?
Abundant resource in stable form (i.e. water)
Environmentally benign (though nitrogen oxides can be formed when hydrogen is burned with air)
Compatible with thermal or electrochemical system
Multi-source potential (i.e. renewables, nuclear, fossil)
What are some disadvantages (3) of using Hydrogen as a fuel (in general, not just for fuel cells)?
Few natural resources of available hydrogen exist (energy carrier, not an energy source)
Hydrogen must be produced from another energy source (possible upstream emissions unless renewable primary source is used)
Real and perceived safety issues
What are some (6) advantages of using Reformer/Liquid fuel over gaseous?
Infrastructure exists for liquid fuel.
Fast refueling.
Simple and familiar storage method.
Volumetric energy density of liquid fuel systems are higher than gaseous systems (i.e. range).
Direct methanol fuel cell systems are not as advanced as hydrogen fuel cells.
Safety.
Define the dynamic response, efficiency and type of reaction of Partial Oxidation (PrOx) reformation:
Fast dynamic response.
Poor efficiency.
Gasification reaction.
Define the dynamic response, efficiency and type of reaction of Steam Reforming (SR):
Slow dynamic response.
High efficiency
Externally heated catalytic reaction.
Define the dynamic response, efficiency and type of reaction of Autothermal Reforming (ATR):
Slower response than PrOx, Faster than SR.
Better efficiency than PrOx, Lower than SR.
Oxidation step and (exothermic) catalytic reaction.
Basic 5 step energy flow for an Indirect-Methanol Hybrid Fuel Cell Vehicle:
Methanol: Water Premix Tank Steam Reformer Phosphoric Acid Fuel Cell Stack Power Conditioning (i.e. DC2DC) High Voltage Distribution (to Batteries, or Parastic/Aux. Loads, or Traction Drive/Wheels)
What is the Water Gas Shift Reaction, and why do we use it?
CO + H2O -> H2 + CO2
Allows the capture of hydrogen from carbon energy.
What is the “Coal Myth”, and why is it a myth?
Myth: There is little hydrogen in coal, so it cannot be used.
This is false because coal might not provide hydrogen, but it provides energy which can be used to break up molecules containing energy. (“Not about hydrogen energy content, but about energy content.”)
What are some (6) general problems with current reforming processes?
Large Volume
Very heavy
Long transient response and start-up times
Outgas purity (or conversion efficiency) can be low
Catalyst lifetime can have problems
Not fully integrated into overall fuel cell system
For SOFC, define the operating temperature (C), typical applications, CO tolerance and if it’s CO2 tolerant:
1000 C
Steam cogenerating power plants
Good CO tolerance
It is CO2 tolerant
For MCFC, define the operating temperature (C), typical applications, CO tolerance and if it’s CO2 tolerant:
600 C
Steam cogenerating power plants
Good CO tolerance
It is CO2 tolerant
For PAFC, define the operating temperature (C), typical applications, CO tolerance and if it’s CO2 tolerant:
150 - 205 C
Smaller stationary power supplies with hot water cogenerating, heavy transport (buses, trucks, ships, trains)
Fair CO Tolerance (1%-3%)
It is CO2 tolerant
For PEM, define the operating temperature (C), typical applications, CO tolerance and if it’s CO2 tolerant:
25 - 120 C
Stationary and mobile power supplies, heavy transport, light vehicles
Poor CO Tolerance (
For AFC, define the operating temperature (C), typical applications, CO tolerance and if it’s CO2 tolerant:
65 - 220 C
Space vehicles, submarines, specialty vehicles carrying oxidizer
Poor CO Tolerance (0ppm)
NOT CO2 tolerant
Anode, Electrolyte and Cathode molecule flows for AFC:
Anode: H2 in, H2O out.
Electrolyte: OH- towards Anode
Cathode: O2, H2O in
Anode, Electrolyte and Cathode molecule flows for PEMFC/PAFC:
Anode: H2 in
Electrolyte: H+ towards Cathode
Cathode: O2 in, H2O out
Anode, Electrolyte and Cathode molecule flows for MCFC:
Anode: H2 in, CO2, H2O out
Electrolyte: CO3 - - towards Anode
Cathode: O2, CO2 in
Anode, Electrolyte and Cathode molecule flows for SOFC:
Anode: H2 in, H2O out
Electrolyte: O- - towards Anode
Cathode O2 in
List some (4) stationary applications of fuel cell technology:
Distributed Generation for Subdivisions
Uninterruptible Applications
Small scale residential power
Micro Combined Heat and Power (CHP, Europe and Japan)
What does CHP stand for?
Combined Heat and Power.
List some (4) space and specialty applications:
Vehicles carrying oxidizer and hydrogen
Drinking Water
Stealth (Temperature Signature, Noise)
Unlimited funding (Tech incubator. This is a humorous application.)
What items (besides the fuel) need to be “managed” for proper operation of a fuel cell?
Thermal management
Water management
Air management
Power Conditioning and Energy Storage
What are some (7) of the reasons to be concerned about the safety of Hydrogen?
Flammability Limits Ignition Energy Detonation Characteristics Flame Speed Diffusion Flame Visibility Public Perception
What are some (8) general research and development needs for fuel cells?
Lower Cost
Expand Knowledge base of technology
On-board and off-board hydrogen production/storage
High temperature operation of PEM fuel cells
Codes and standards
Hands-on and theoretical training of engineers and scientists
Public education
What are some (5) general advantages of Fuel Cells?
Efficiency Simplicity Low emissions Silence Fast refueling (with caveats based on technology used)
What are some (5) general challenges of Fuel Cells?
Power density Specific power Reliability Hydrogen storage Cost
What is the OCV and the equation for the Open Circuit Voltage?
E = -[Delta]g_f / zF
(Change in Gibbs over the reaction divided by the number of electrons released times the Faraday constant)
OCV represents the maximum voltage of a Fuel Cell with no losses.
1.48V Liquid, 1.25 V Gaseous.
What is the equation for the thermal efficiency of a fuel cell?
Electrical Energy Produced / (negative) change in enthalpy
What is the equation for fuel utilization?
Mass of fuel reacted / Mass of fuel inputted
What is LHV and HHV?
Lower and Higher Heating Value. Lower corresponds to gaseous products while higher relates to liquid products.
What does the Nernst equations offer for understanding the fuel cell?
Useful for indicating what changes in activity and/or pressures (including gas concentrations) will have on the ideal voltage.
What is the equation for total system power?
Measured Voltage times the current in each cell times the number of cells.
What are some (6) ways to decrease fuel cell losses?
Activation overvoltage Raise cell temperature Use more effective catalysts Increase real surface area of electrodes Increase reactant concentration Increase pressure
What are some ways (3) to decrease Ohmic losses?
Use electrodes with highest possible conductivity
Use high conductivity interconnects and bipolar plates
Make electrolyte as thin as possible
What are some ways (2) to decrease polarization/mass transport losses?
Increase pressure
Increase reactant concentration
What are three things needed for fire to occur?
Fuel
Oxidizer
Heat
What are the 5 simplified combustion steps for hydrocarbon fuels?
Heating of fuel (endothermic) Devolitilzation of Hydrocarbons (endothermic) Breaking of C-C and H-C bonds (endothermic) Mixing with Oxidizer Heat Release (Exothermic)
What are some (10) considerations when designing a reformer?
Power Output (Energy Balance) Conversion limits Catalyst lifetime Space velocity External feed rates (heat source) Outgas purity (yield and selectivity) Temperature gradients Thermal integration (vaporization and superheat) Transient following Overall efficiency
What is conversion?
Reactant consumed / reactant fed
What is selectivity?
Desired product produced / Undesired product produced
What is yield?
Desired product produced / reactant fed
What are physical designs (2), and chemicals used (7) for catalysts?
Physical designs are Monolith and Pelletized.
Chemicals used are Copper, Zinc, Chromium, Gold, Silver, Platinum, Palladium
What are the seven steps of a catalyst converting reactants to products?
Reactants diffuse through bulk stream to catalyst surface
Reactants diffuse through catalyst pores to open reaction site
Reactants absorb onto catalyst
Reaction time governed by chemical kinetics on the catalyst
Product desorb from catalyst
Products diffuse through catalyst pores to surface
Products diffuse from surface to bulk stream
What is the “gist” of LeChatliers Principle?
Decreasing the pressure induces a shift towards products.
What are some limiting mechanisms in reformation?
Note: Same as catalyst limiting mechanisms:
Heat Transfer
Mass Transfer
Chemical Kinetics (function of temperature)
Catalyst degradation
What are 2 effects of increasing the reaction temperature of an autothermal reformation system?
Faster kinetics
Faster diffusion
What are some types of (reversible and non-reversible) catalyst poisoning?
Sulfurs (mercaptan odorants)
Chlorides
Oxygen
What are some types of catalyst fouling?
Condensed species
Coking
What are some types of catalyst sintering?
Substrate
Attrition
After reformation, why may additional clean up of the fuel be necessary (4 items)?
Typical reformate has 1-3% CO (PEM fuel cells require
After reformation, what are some (6) additional methods to clean up the fuel?
Separation Preferential Oxidation Absorbtion Water-gas shift reactors Palladium Membrane Pressure Swing Absorption
What is a general problem with PrOX?
Very low transient capability?
What could you use a Palladium Membrane for, and what are some issues with it?
Can be used to separate H2 after reformation to ensure a fuel with a higher purity.
General Issues: Operating temperature is 250 C - 450 C. Pressure drop (50-300 psi) Durability Need a sink for unpassed H2, CO, etc...
Quick description of Pressure Swing Absorption:
Absorbs CO, CO2, HC
It can be desorbed and reactants dumped (which loses some H2, and requires a sink)
Technical description of PEM membrane:
When hydrated weak acid regions form around hydrophilic ionomers, they are able to pass H+ ions.
Surrounding tough hydrophobic regions.
What are some (6) aspects of Fluorosulphonate Membranes?
Used in PEM Cells Chemical resistant Mechanically strong which allows thin membrane construction Acidic Absorb large amounts of water H+ ions can move if well hydrated.
What is the makeup of PEMFC electrodes?
Platinum used at Anode and Cathode
Carbon and platinum and PTFE for gas diffusion and electrical connection to current collector
What are some aspects of Water Management for the PEMFC at the Anode and Cathode?
Anode:
Back diffusion of water from cathode.
Water supplied via humidification.
Cathode: Water produced at the Cathode Electro-Osmatic drag can occur. Water removed by evaporation. Water supplied by humidification.
Methods (4) to increase humidity:
Lower temperature
Lower air flow rate
Increase pressure
Condense and recirculate water
What are some specific humidification techniques (4 items)?
Sparging
Spray
Counterflow of reactant gases
Interdigitated flow fields
What effect does increasing pressure have on an PEMFC (5 items)?
Increases Nernst potential Decreases activation losses Extends concentration polarization Increases relative humidity Warm air more easily humidified
What are some (6) ways to pressurize the oxidizer stream?
Turbo compressor/expanders Reciprocating compressors Scroll compressors Positive displacement compressors Screw compressors Toroidial Intersecting Vane Machine (TIVM)
What are the full break down of Compressor types?
Dynamic
Centrifugal
Axial
Positive Displacement
Reciprocating
Single-Acting
Double-Acting
Diaphragm
Rotary
Lobe
Liquid Ring
Screw
Scroll
VaneDescribe (in general) Parasitic and Auxiliary loads:
Parasitic Loads are required loads by the system
Auxiliary loads are consumer niceties not required by the fuel cell system.
Vehicle parasitic loads are different from fuel cell system parasitic loads.
What are some parasitic loads for fuel cells (and vehicles)?
Blowers Compressors Pumps Electronics A/C systems heat loads
What is the typical electrolyte for a Alkaline fuel cell?
Aqueous KOH (static or mobile)
What are some (5) advantages of an Alkali fuel cell?
Higher operating voltage due to decreased activation over potential
Potential low cost electrolyte
Potential low cost catalysts
Highly developed (Supplied power for the shuttle and Apollo)
Lots of configurations possible with static of mobile electrolyte (can be used as cooling system)
What are some (7) disadvantages of an Alkali fuel cell?
Not tolerant to CO2 at cathode or anode
Static electrolyte held in Asbestos
Pure oxygen required for static electrolyte
Mobile electrolyte prone to internal short circuits
Mobile electrolyte requires additional equipment
Surface tension of KOH makes it prone to leak
KOH can solidify at cathode if concentration goes too high
What is the chemical equation for AFC (Potassium based)?
2KOH + CO2 -> K2CO3 + H2O
Why would do AFCs use Potassium Carbonate more than Sodium Carbonate?
Potassium Carbonate is much more soluble than Sodium Carbonate.
Why does Potassium Carbonate in the electrolyte have a negative effect on Fuel Cell Performance?
OH- concentration is reduced
Viscosity is increased (mass transport losses)
Carbonate is less soluble (precipitates block channels)
Oxygen solubility reduced (activity at cathode decreases)
Electrolyte conductivity decreases
Electrode performance degrades
What chemicals make up a dissolved fuel system for AFC?
KOH + hydrazine or ammonia
What can be used for electrodes in an AFC?
Sintered nickel powder Raney metals Rolled electrodes (Requires edge connections because there is no bipolar plate as PTFE is an insulator)
What are some potential applications of AFC?
Specialty vehicles using oxidizer and pure hydrogen
Locations using CO2 scrubbing
Regenerative Fuel Cells (possibly)
What are some (11) advantages of DMFC?
Directly oxidize liquid methanol No reformation required Low temperature Based on PEMFC Relatively simple Quick refueling Liquid fuel Infrastructure exists High Energy Density No reformer Water Management
What is the Anode reaction of DMFC?
CH3OH + H2O -> 6H+ + CO2
Anode, Electrolyte and Cathode molecule flows for DMFC:
Anode: H2O, CH3OH in, CO2 out
Electrolyte: H+ (towards cathode)
Cathode: O2 in, H2O out
How does a DMFC polarization curve compare with a PEMFC?
It has a steeper Ohmic slope, but extends for much longer with no indication of mass concentration losses.
What are some additional hardware necessary for DMFC over PEMFC (4 items)?
Methanol concentration sensor
Pumps
Selective CO2 exit
Water recovery
What are some (5) technical challenges of DMFC?
Lower operating voltage than PEM (activation losses at anode and cathode)
Methanol mixes easily with water (fuel crossover)
Evaporated methanol in CO2 exit
CO2 barrier effect at Anode
Difficulties with Methanol as a fuel (Toxicity, readily mixes with water, “invisible” flame, easily ingested)
How can you decrease the Anode activation losses for a DMFC (3 items)?
Increase catalyst loading
Improve catalyst activity
Remove CO2 (better mass transfer)
How can you decrease the fuel crossover for a DMFC (6 items)?
Solve activation losses (stop methanol from traveling in electrolyte)
Control excess methanol at anode
Change PEM composition to reduce electroosmotic drag or only transport water
Thicker electrolyte reduces diffusion (need to reduce ohmic resistance)
Use selective catalysts on cathode (solves half the crossover problem)
Bi-layer PEM
General characteristics of DMFC:
High energy storage capacity Low efficiency (10-30%) Low power (less than 90 mW/cm^2) Good at providing small amounts of power for a long time
List out the 5 Fuel Cell Sub-Systems and their components:
Control Systems
Instrumentation
Water Management
Pumps
Humidifiers
Thermal Management
Radiators
Pumps
Blowers
Air Delivery Systems
Blowers/Compressors
Turbochargers
Hydrogen Delivery Systems
Tanks
Reformers
Heat Exchangers
Ejectors
Power Delivery Systems
DC/DC converters
Inverters
Electric Motors
Energy Storage
Batteries
SupercapsWhat are some (6) general questions for the control system of a Fuel Cell?
What are the response times from the instrumentation?
Is the measurement accurate?
What is the effect of each control measure? (i.e. Is it linear?)
What are the interactions of each control measure? (Stoichiometry effects?)
Steady state oscillations vs. transient behavior?
Minimizing I/O for control systems.
What issues (4) should water management address?
Flooding
Membrane drying
How does one detect and control the water balance? (Individual cell voltages, hygrometers, cathode purges, temperature)
Becoming an integrated part of stack design
What issues (4) should thermal management address?
How does one detect and control thermal balance?
RTDs, thermocouples, thermistors, IR detection
Forced convection through heating/cooling plates
Local temperature gradients exist
Heat exchange efficiencies ([Delta] T)
Increasing enthalpy, area or [Delta] T
Parasitic loads (efficiency tradeoffs)
What is the equation for cooling system effectiveness (CSE) and about what value should be targeted?
Rate of heat removal / electrical power consumed
(This is analogous to COP for refrigeration systems)
Should be in the 20-30 range.
Define some (4) aspects of air management systems:
Oil free
Low flow rates (12-120 SLM)
Back pressure (~10 kPa)
DC power
What are some (6) general benefits to high temperature fuel cell systems?
Lower activation losses due to high temperature
Inexpensive catalysts
Reforming energy available
Tolerance to impurities
CHP applications
Bottoming cycles (Efficiency = Efficiency_top + Efficiency_bottom - Efficiency_top * Efficiency_bottom)
What are some general MCFC components?
Cathode: Porous nickel oxide
Electrolyte 60% (wt) Carbonate in fiber (1 micrometer diameter) matrix of LiOAlO2
Anode: Porous nickel (have to keep from oxidizing)
Bipolar plates are typically Stainless Steel
Large Ohmic resistance due to electrolyte
Long term stability with thicker electrolytes
Challenges with MCFC
Sealing porous anode and cathode (wet sealing is required with electrolyte and bi-polar plate resulting in corrosion, i.e. Aluminum)
Carbonate becomes liquid at 450C (significant shrinkage when carbonate absorbs into matrix)
System durability (thicker electrolytes)
Fuel crossover due to cracks that develop
Need to minimize thermal cycling
Slow heating (14 hours)
Keep anode as reduced nickel (inert purging required at anode)
What are some (5) issues with a Direct Internal Reforming FC System?
Upstream catalyst required
Must be near Anode (similar to three phase requirement)
High fuel conversions (LeChatlier’s principle - hydrogen depleted immediately)
Catalyst lifetime
Impurities (sulphur compounds (ppb), odorants, etc…)
What are some (6) SOFC components?
Lower internal resistance than MCFC
Higher operating temperature than MCFC
Indirect internal reforming works well at high temperatures
Doped zirconia used as electrolyte
Anode is porous metallic nickel on a zirconia skeleton
Cathode is typically a porous p-type semiconductor
What are some SOFC challenges?
Sealing (TCE) Vibration and Brittle Fracture Start up time Temperature dependence Energy required to make device (materials)
What are some methods of large-scale hydrogen production?
Reformation
Production of Aromatics
Steam-Iron Process
What is the chemical equation for Production of Aromatics (hydrogen production technique)?
C8H16 -> C8H10 + 3H2
What is the chemical equation for Steam-Iron Process (hydrogen production technique)?
H2O + 3FeO -> Fe3O4 + H2
When using electrolysis to create hydrogen, how effective is the production in relation to the Thermoneutral voltage and the Reversible Voltage?
Above Thermoneutral: Hydrogen is produced from electricity, but waste heat is also generated.
Below Thermoneutral, Above Reversible: Hydrogen is generated from both Electricity and Heat (Sweet spot)
Below Reversible: Hydrogen is not produced.
What are some methods of Hydrogen production with solar energy conversion?
Photoconversion Photovoltaics with Electrolysis Photoelectrochemical Biological Thermal Conversion Solar heat engines with Electrolysis Thermal decomposition Thermochemical cycles Hybrid cycles
What is the pathway to hydrogen, and some general aspects of Photovoltaics with Electrolysis?
Pathway: Sunlight - Photovoltaics - Electrolysis - H2
Conceptually elegant (low voltage DC)
Demonstrated and commercial
Low conversion efficiency due to photovoltaic conversion
Multijunction photovoltaic can boost efficiency
Cost
What is the pathway to hydrogen, and some general aspects of Solar Heat Engines with Electrolysis?
Pathway: Sunlight - Concentrator - Heat Engine - Generator - Electrolysis - H2
Can use well known cycles such as Rankine, Brayton, Stirling
High conversion efficiency
Requires high concentration ratios for high temperatures (Carnot limited thermal cycles)
Additional conversion step (kinetic to electrical)
Intermittent nature of sunlight can complicate heat engine (numerous start-ups and shutdowns)
List 3 Solar Heat Power Systems for Hydrogen production:
Parabolic concentrator
Heliostats Power Tower
Dish Stirling Engine
What is the pathway to Hydrogen for Photoelectrochemical of solar collection, and how does it relate to another solar method?
Pathway: Sunlight - Photoelectrochemical Cell - H2
This is a miniaturization and “marriage” of the two middle steps of the Photovoltaics with Electrolysis approach.
What is the pathway to Hydrogen for Photochemical of solar collection, and what else is “neat” about it?
Pathway: Sunlight - Chemical Solution - H2/O2 - Separator - H2
Can be used to mineralize pollutants.
What is the pathway for Thermal Decomposition of solar collection?
Pathway: Sunlight - Concentrator - High Temperature Reactor - Separator - H2
What is the pathway to Hydrogen for Biological Methods of solar collection, and name two problems?
Pathway: Sunlight - Bioreactor - Separator - H2
Problems:
Saturation at about 0.3 sunds
Oxygen sensitivity
What are different pathways that Biomass can become energy?
Combustion
Steam Generator
Steam Turbine
Fermentation
Gas Processing
Gas Conditioning
Gas Clean-up
IC Engine
Gas Turbine
Carburation Gasification
Methanol Synthesis
Hydrogen Generation
Guel Cell / Hydrogen refuelingWhat are some methods of storing Hydrogen?
Compressed Gas Liquid H2 Solid (slush) H2 Hydrides Hydrocarbons
What are some important issues to account for in the storage of Hydrogen?
Energy Density
Cycle Lifetime
Ease of uptake and discharge (Energy, time, temperature)
Selectivity (contaminants include N2, O2, CO, CO2, NH3, CH4, and H2S)
What are some desires for effective hydrogen storage?
Useful weight of hydrogen stored per system weight and volume
Reasonable temperature, pressure, and heat required for charge and discharge (ideally at least 30 C less than the fuel cell stack operating temperature)
Adequate H2 charge rate (~5.5 kg H2 charge in under 5 minutes)
Adequate H2 discharge rate (~3 kg/hr for 50 kW stack)
Protection from poisoning by impurities in the Hydrogen supply
Adequate cyclic life (300-500 deep discharge cycles for 100,000 miles range)
Reasonable cost and availability of component metals
Describe some aspects of storing liquid hydrogen:
Stores at -253 C
Typically vents at 690 kPa (phase change)
Lock up time
