From Stack to Fuel Cell System Level

Question 1

Cell to System

Answer 1

• individual cells connected in series to give one or several stacks
• required to reach practicable output voltages
• balance of plant (BoP) systems required

Question 2

Balance of Plant - Component Breakdown - Thermal management

Answer 2

• Coolant & Loops
• Pump
• Heat exchangers

Question 3

Balance of Plant - Component Breakdown - Reactant Supply

Answer 3

• Fuel Management
  - Heater
  - Recirculation system (if required)
  - Sensors & Metering
  - Valves & Flow Control
  - Fuel Tank
• Air Management
  - Heater
  - Compressor
  - Sensor & Metering
  - Valves & Flow Control
• Water Management (if required)
  - Humidifiers
  - Water Storage
  - Water Separator

Question 4

Balance of Plant - Component Breakdown - Power Management

Answer 4

• Power Conditioning & AC/DC Converter
• Controller
• Wiring
• Buffer storage (if required)

Question 5

Thermal Management

Answer 5

• aims to maintain ideal operating T by cooling
• high T offheat more “useable”
• higher T difference to ambient easier to cool
• PEFC:
  - typically liquid cooling
  - high T loop (stack) & low T (FC air)
• SOFC:
  - needs preheating
  - high T is easier to reuse -> heat recuperation (e.g. for reactant preheat)
  - often in two steps (high & low T)
• Types of cooling
  - Liquid Cooling
  - Air Cooling
  - Phase Change Cooling

Question 6

Liquid Cooling

Answer 6

• standard for transport application
• waste heat -> liquid coolant
• cooling channels between cells (usually embedded in bipolar plates)
• high cooling capacity; effective cooling
• heavy -> coolant, pump & line weight
• requirements for coolant:
  - sustain full temperature range of application
  - high heat capacity
  - high thermal conductivity
  - low viscosity but non-volatile (doesn’t evaporate)
  - not corrosive/toxic/harmful

Question 7

Air Cooling

Answer 7

• waste heat -> air
• typically integrated with air supply -> reactant air = cooling air
  - air flow is designed well to avoid T gradients
• lightweight
• less effective than liquid cooling
• requirements:
  - suitable for PEFC app. with small system size/low power requirement
  - either high reactant air flow or separate cooling air
  - suggested for aviation applications of HT-PEFC
  - routinely used for SOFC cooling

Question 8

Phase Change Cooling

Answer 8

• coolant absorbs waste heat during phase change -> evaporation, melting
• simple system
  - not necessarily pumping required
  - potential for small system weight
• performance & suitability strongly dependent on PC material:
  - volume change upon phase change
  - temperature of phase change
  - latent heat absorbed/released during phase change
• typ. only used for small scale applications

Question 9

Reactant Management - General Notes

Answer 9

• lab tests
  
  • use high stoichiometric ratios (>10) to avoid reactant supply being limiting factor
• real application
  - high fuel efficiencies targeted
  - fuel loss needs to be kept at minimum: either low stoichiometric ratio &/ fuel recycling
  - cathodic stoichiometric ratio: trade off considering energy demand & weight of air compression
• choice of operating pressure non-trivial

Question 10

Fuel Management

Answer 10

• key objectives:
  - control of hydrogen flow
  - temperature
  - pressure
  - humidity
  - leak tightness -> sensing required for flow determination & safety
• fuel conditioning path strongly depends on type of hydrogen storage -> pressurisation may be necessary
• use of hydrocarbon fuels may require add. steps (pre-reforming, desulfurisation…)

Question 11

Fuel Utilisation

Answer 11

influenced by
- leakages
- crossover
- reactant mass transport
- reaction rates -> approaches to recycle unused fuel (“anode recycling”)
- describes how much of injected fuel is used

Question 12

Anode Recycling

Answer 12

• significant amount of fuel would be wasted if product is just exhausted -> anode outlet is recycled & mixed with “fresh” fuel
• more sustainable
• more efficient
• SOFCs running on hydrocarbon fuel, looping anode gas is pre-reforming process to produce new hydrogen

Question 13

Air Management in Anode Recycling

Answer 13

• key objectives
  - control of air flow
  - Temperature
  - pressure
  - humidity
• compression required (typ. operation ~2 bar)
  - dep. on post- compression gas T & stack operating T, cooling/heating might needed before reinjection
  - add. heat from compression can be used for water evaporation in humidifiers
• air quality/precleaning -> air may be filtered before inlet

Question 14

Water Management in Anode Recycling

Answer 14

• mainly of concern in PEFC
• key requirements
  - enough water content at every point of the membrane (PEFC) + no humid. gradients -> ideal usually only possible with humidification
  - avoidance of flooding of electrode pores due to excess water
• in practice, reactant paths have humidifiers
• water demand of humidifiers is met by condensing & collecting product water from cathodic outlet

Question 15

Reactant management - PEFC

Answer 15

• product water formed at cathode
• external humidification of relevance due to membrane
• requirement for additional reactant preheating depends on compression & humidification system
• gas crossover through electrolyte: sensing -> key for safety

Question 16

Reactant Management - SOFC

Answer 16

• product water formed at anode
• anode recycling: product water affects processes such as pre-reforming or post-combustion
• preheating of reactants necessary (thermal strain)

Question 17

Power Management in Reuse of Fuel

Answer 17

• Fuel cells provide DC power
• FC rather unique characteristic
  
  • stack is connected in series for practical output voltage
  • output voltage may still not be useable for transmission/to feed loads
    - need for power electronics -> DC/DC conversion& AC/DC conversion
    - esp. for large scale FC systems reliable & lightweight comp. for diff. operating environments are researched

Question 18

BoP component failure

Answer 18

• all BoP components undergo ageing & degradation processes
• regular maintenance & adequate redundancy required
• for road transport humidifier & compressor have been found to be most prone to failure

Question 19

compressor ageing (automotive FCs) reasons

Answer 19

• mechanical
  
  • Praticles
  • Vibration & Shock
  • Rotating Stall
  • Rotary Oscillation/Resonances
  • Pressure Differences
  • Oil contamination (also chemical)
• chemical
  
  • gaseous Contamination
  • Soiling
• thermal
  
  • Temperature

Question 20

Notes on BoP research

Answer 20

• cell component developments -> decrease need for BoP components (not researched as much)
• Better proton exchange membranes -> no ext. humidification needed
• High-Temperature stacks -> less demanding thermal management
• Highly heat conductive heat spreaders within cell
• BoP has similar wight to stack

Question 21

Current major weight contributors

Answer 21

• thermal management system
• compression of ambient air
• (humidification system)
  (also high cost share)

Question 22

Exploitation of Synergy on Full System Level

Answer 22

• application-specific synergies on full system level for most effective use of fuel -> major research focus
• Examples
  - using off-heat fuel cell produces
  - using water fuel cell produces
  - using pressurised air stream from an upstream process for fuel cell
  - hybridise e.g. with batteries &/ turbines

Question 23

Take Away Messages G)

Answer 23

• fuel cell system ≠ stack + balance of plant
• BoP components vary depending on system layout & application requirements
• main subsystems
  - reactant management
  - fuel supply
  - air supply
  - if needed water management
  - thermal management
  - power management
• BoP components contribute significantly to weight & cost
• BoP components consume energy
• for optimised efficiency, synergies with other systems should be exploited