Quiz2 Flashcards
Good and bad about solar energy:
Good:
Clean
High amount of energy
Straight from the source
Bad:
Low energy density (1,4/m2)
Intermittency (Only during daytime
Defined wavelength distribution
Thermal conversion
Paraboler och stirlingmotorer
Quantum conversion
Photon intemittence (nano field)
AM Air Mass:
AM = (1/cos alpha) alpha = solens vinkel/läge
Quantum conversion efficiency:
- absorption
- charge separation
- charge transport
- charge collection
What light do we have from the sun?
- Visual light
- IR
(3. UV too little)
How does light interact with an object?
- Absorption
- Reflection
- Refraction
- Diffraction
Conservation of energy and momentum!
- for large objects each effect is distinct;
- for small objects the difference between the latter three effects is blurred.
Brewster’s angle:
ThetaB = arctan(n1/n2)
Critical angle theta_c:
Where all light is reflected:
Antireflective coatings
Makes more light pass through at larger angles
Nano
Optical antennas:
transmitter to radiation
radiation to receiver
increase absorption!
What determines the size of an antenna?
The size of the wavelength we are trying to capture
Localized particle plasmons:
Since all conduction electrons are involved in the oscillation, plasmons interact strongly with resonant light
Excitation particle plasmon, 2 ways:
radiative decay: photon, equilibrium
Field enhancement
nonradiative decay: e-h pair, Hot e-, phonons, equilibrium
Landau damping
Antennas:
- nano-scale to capture visible light
- 0.5-6.5 eV
- Compared to radio antennas: can capture any wavelength, needs to be efficient in capturing as much as possible.
Routes to higher efficiencies:
- Concentrate sunlight (Micro)
- Multi-junction electrodes (Micro)
- Light trapping (Micro)
- Nano-architectures (Nano)
- Intermediate bandgap materials (Nano)
- Multiple exciton generation (Nano)
- Up- and down-conversion (Molecular)
- Combined “nanomaterials” (Nano)
- Plasmonics (Nano)
What is a Fuel Cell?
Converts chemical energy (fuel) to electricity (and heat).
What is a Battery
A Battery is a “factory” that converts chemical energy to electricity (and heat)
- Primary Battery – Single use, not possible to recharge
- Secondary Battery – Rechargeable battery, can be used many times
Sustainable Energy Dream scenario:
- Renewable energy sources (Solar, Wind etc.)
- Clean fuel production
- Easy distribution and storage
• Clean, high efficiency energy conversion
Combustion engine
\+ Mature technology \+ High energy and power density \+ Low price \+ Lifetime – Low efficiency
Battery
+ Very high efficiency
– Low energy density
– Lifetime?
– Price?
Fuel Cell
\+ High efficiency \+ High energy and power density – High price – Lifetime?
(Typical voltage from an operating Fuel Cell is 0.7 V)
One reson to why gasoline and diesel are more popular than hydrogen etc:
Volumetric energy density is often more important than Gravemetric energy density. Especially for transport applications.
How does a Fuel Cell/Battery work?
Burning of hydrogen
Combustion: 1. Molecules collide (H2 and O2) 2. Bonds are broken/formed • H-H and O-O bonds are broken • H-O bonds are formed • Redistribution of electronic charge 3. Product is formed
Takes place on the order of ps (10^-12)
Energy difference is released as heat
Electrochemistry:
Anode - Oxidation
Cathode - Reduction
Types of Fuel Cells:
- PEMFC – Polymer Electrolyte Fuel Cell
- PAFC – Phosphoric Acid Fuel Cell
- AFC – Alkaline Fuel Cell
- MCFC – Molten Carbonate Fuel Cell
- SOFC – Solid Oxide Fuel Cell
- BFC – Biological Fuel Cell
Fuel Cells vs. Batteries
FC: • Includes only the “reaction zone” • Requires fuel • Simple chemicals • Complex reactions • Safety!
Battery: • Includes reactants and products • Stand alone unit (closed system) • Needs to be replaced/recharged • Complex chemicals • Simple reactions • Safety!
Nanotechnology for Fuel Cells and Batteries:
• Reducing component size
→ Increases capacity
→ Reduces amount of materials needed
• Detailed analysis
→ Accurate descriptions
→ Increases understanding
Cheaper and more efficient devices
N&N for battery development – examples:
- Silicon nanowires: capacity
- CuO nanorods: capacity
- Si/C nanoparticles: stability
Solar cell types:
1 gen: Mono - crystaline Si cells 24%
2 gen: Thin film 20%, Amorphous Si cell 10%
3 gen: Organic 8%, Dye solar cells 11%
Plasmonic:
Goal: Absorbe more with thinner material
(Collidal lithography)
Mechanisms:
• Far field effect
• Near field effect
Photo emission of charge carriers
Dye-sensitized solar cells:
- TiO2 paint, absorbs light
- Charge will stay there long
- Easy to construct
- Colors and looks my be more important than efficiency
Organic solar cells:
- Cheaper
- No lack of resources
- Poor electric selectivity
SUMMARY:
- Challenges for solar cells
- Plasmonics for solar cells
- Thin-film solar cells
- Dye sensitized solar cells
- Organic solar cells
- Exotic cells
• More efficient use of diffuse solar energy
• (Spill over to solar electricity production)
• Potential applications in wind farms, and other
renewable sources
• Cost improvements in traditional methods
• Much applied research needed to bring these new
application to economic reality
Key functional properties of photo-electrodes:
• Band gap • Flat-band potential • Schottky barrier • Electrical resistance - Electrodes - Electrolyte - Electrical leads - Electrical connections • Helmholtz potential barrier • Corrosion and photo-corrosion resistance • Microstructure
The hydrogen problem is:
The materials that are stable in water and can split water into H2 and O2 do not absorb sunlight effectively,
(optical function failure)
and
the materials that absorb sunlight effectively cannot sustain photochemically induced water-splitting. (catalytic function and durability failure)
Band gap
The optimal band gap for high performance photo-electrodes is ~ 2 eV
why it is a wonder?
Because it solves number of problems: - Storage - Efficiency - Emissions - Supply
New materials:
Hematite – great expectations
+favourable valence band edge,
+ reasonably low band gap,
+ high stability
+ low cost.
- very poor conversion efficiencies due to short minority carrier collection length (2-4-6? nm)
- to be overcome with nanostructured electrodes or ???
Grätzel
- GOOD for detailed mechanistic investigations.
GRAPHENE – greater expectations
