Introduction and Motivation Flashcards
A
Potential measures for aviation emission reduction
- Technological developments (evolutionary + revolutionary)
- Alternative fuels (carbon-based + carbon-free (Hydrogen) )
- Non-combustion based energy conversion (Electrocemical fuel cells)
- Other forms of energy storage
- Operational improvements
→ No olvidar los apuntes mios en rojo
Presentation 1 Slide 12
Define ‘Energy Carrier’
Substance or a phenomenon that can be used to:
1. Produce mechanical work or heat
2. To carry (remember this because of the name) chemical or physical processes
3 ‘types of Hydrogens’, their colors and their process to be obtained
- Types: Fosil Fuels, Electrolysis and Alternative
- Fosil Fuels: Coal (brown), Natural Gas I (grey), Natural Gas II (blue) and Natural Gas III (blue)
Electrolysis: Through nuclear energy (pink), Through mixture of sources (yellow) and Through renewable sources (green).
Alternative: Natural occurrence, Thermochemical and from garbage → Every of them white.
Green Hydrogen Production Processes
- Electrolysis: H2 produced electrochemically from water and renewable electricity (efuel).
- Thermochemical Production: H2 produced thermochemically with the help of a redox material directly employing concentrated sunlight.
Define ‘Volumetric energy density’
Energy stored per volume
Define ‘Specific energy’
Energy stored per mass
Locate in a Specific Energy - Volumetric Energy Density the Kerosene, the Hydrogen (liquid and @ 300 bar) and the Li-ion batteries
Conclusions also
The conclusion basically is that for Hydrogen (liquid + @ 300 bar), the specific energy is high (high energy for low mass), but the energy density is really low (the volume is really high). However, for the kerosene, the specific energy is low (high mass), but the volumetric is high (low volume). That is why it is still used in aviation (Aircraft).
Presentation 1 page 39
Benefit Green Hydrogen vs. other Aviation Fuels
- Cheaper
- Pollution
Why Hydrogen is a good ‘Energy Vector’
Hydrogen is a good energy vector because:
- It does not emit greenhouse gases when used.
- It can be produced from various sources, including renewable energy
- Its only byproduct when used in fuel cells is water.
Scheme of ‘Energy Transformation’ in combustion in heat engines
Chemical energy → Thermal Energy → Mechanical Energy
→ The majority of the losses are in the conversion Thermal Energy → Mechanical Energy
→ Approx. 30% of efficiency
Scheme of ‘Electrochemical Conversion in Fuel-Cells’
Chemical energy → Electrical Energy → Mechanical Energy
→ Limitations arise due to limited mass transport.
→ Approx. 60% of efficiency
Define ‘Fuel Cell’, working principle and what is released
Definition: Type of energy converter that allows directly collecting the chemical energy stored in a fuel by means of an electrochemical process.
Working principle:
1. Separation of the oxidation of a fuel (for example Hydrogen: H2 → 2H+ + 2e-) → You provide the Hydrogen.
2. Reduction of oxygen (for example: 0.5O2 + 2H+ + 2e- → H2O) → The air provides the Oxygen.
3. Electrons travel through the external circuit and power electrical devices.
→ As a consequence, heat is released.
Relevant Fuel Cells Performance Metrics (equations + unities)
- power [W] = voltage * current
- power density [W/L] = power/volume
- specific power[W/kg] = power/mass
- efficiency [-] = output electrical energy/theoretical energy released upon full fuel combustion (heating value of the fuel)
- durability: operating hours performance loss over accelerated stress test
- load response [V/s]
Challenges of Fuel Cells in aviation
- High specific power requirements
- High durability requirements
- Large system size
- Drastic change of ambient conditions (unique in aviation).
Selected Types of Fuel Cells
- Polymer type: PEFC (Polymer Electrolyte Fuel Cell) and HT- PEFC (High Temperature Polymer Electrolyte Fuel Cell)
- Ceramic type: PCFC (Proton Cermaic Fuel Cell) and SOFC (Solid Oxide Fuel Cell)
Comparison of the properties of the 4 differents Fuell Cell types
In the folder Importants
Challenges PEFC
- Requires humidification
- Significant amount of heat to be dissipated at low ΔT
Challenges HT - PEFC
- Durability limited due to high corrosion rates at these temperatures
- Limited power densities
- Limitations of membranes
Challenges PCFC
- Limited performance
Challenges SOFC
- Sealing at high temperatures
- Degradation due to thermo-mechanical strain due to different thermal expansion
behavior of individual components
Trade-Offs of Ceramic Fuel Cells
- High fuel efficiency: less heat to be disipated, heat dissipation at higher ΔT and easy thermal management.
- Low stack specific power
- Less developed for mobile applications, due to: slow load response, thermo-mechanical degradation
How are cells connected in a stack?
How are the voltage and the current?
They are connected in serie
I = I1 = I2 = … = IN
U = U1 + U2 + … UN
Things that affect the performance of a cell and therefore, of the full stack
- Management:
- Water
- Thermal - Transport:
- Electronic
- Ionic - Transfer:
- Mass - Electrochemical reactions
Definition of polarization curve and draw it (theoretical + measured)
Presentation 1 Slide 35
—> Mentione that is the main characterization parameter of the Fuel Cell performance: Cell Voltate vs Current Density
–> As much current density you put, less cell voltage you have; therefore, you are less efficient.
Definition of battery and working principle
Definition: Electrochemical cell that allows converting chemical energy stored in its active materials to electricity
Working principle (Lithium-ion battery): spatial separation of oxidation and reduction reactions. Then, electrons travel through external circuit and power electrical devices.
Fuel Cells vs Batteries table
In the folder importants: CellsvsBatteries
Inconveniences of batteries for aviation
They are:
1. Heavy
2. Large
3. They require thermal management.
