Quiz2

Question 1

Good and bad about solar energy:

Answer 1

Good:
Clean
High amount of energy
Straight from the source

Bad:
Low energy density (1,4/m2)
Intermittency (Only during daytime
Defined wavelength distribution

Question 2

Thermal conversion

Answer 2

Paraboler och stirlingmotorer

Question 3

Quantum conversion

Answer 3

Photon intemittence (nano field)

Question 4

AM Air Mass:

Answer 4

AM = (1/cos alpha) 
alpha = solens vinkel/läge

Question 5

Quantum conversion efficiency:

Answer 5

1. absorption
2. charge separation
3. charge transport
4. charge collection

Question 6

What light do we have from the sun?

Answer 6

1. Visual light
2. IR
   (3. UV too little)

Question 7

How does light interact with an object?

Answer 7

• 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.

Question 8

Brewster’s angle:

Answer 8

ThetaB = arctan(n1/n2)

Question 9

Critical angle theta_c:

Answer 9

Where all light is reflected:

Question 10

Antireflective coatings

Answer 10

Makes more light pass through at larger angles

Nano

Question 11

Optical antennas:

Answer 11

transmitter to radiation
radiation to receiver

increase absorption!

Question 12

What determines the size of an antenna?

Answer 12

The size of the wavelength we are trying to capture

Question 13

Localized particle plasmons:

Answer 13

Since all conduction electrons are involved in the oscillation, plasmons interact strongly with resonant light

Question 14

Excitation particle plasmon, 2 ways:

Answer 14

radiative decay: photon, equilibrium
Field enhancement

nonradiative decay: e-h pair, Hot e-, phonons, equilibrium
Landau damping

Question 15

Antennas:

Answer 15

• 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.

Question 16

Routes to higher efficiencies:

Answer 16

• 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)

Question 17

What is a Fuel Cell?

Answer 17

Converts chemical energy (fuel) to electricity (and heat).

Question 18

What is a Battery

Answer 18

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

Question 19

Sustainable Energy Dream scenario:

Answer 19

• Renewable energy sources (Solar, Wind etc.)
• Clean fuel production
• Easy distribution and storage

• Clean, high efficiency energy conversion

Question 20

Combustion engine

Answer 20

\+ Mature technology
\+ High energy and power density
\+ Low price
\+ Lifetime
– Low efficiency

Question 21

Battery

Answer 21

+ Very high efficiency
– Low energy density
– Lifetime?
– Price?

Question 22

Fuel Cell

Answer 22

\+ High efficiency
\+ High energy and
power density
– High price
– Lifetime?

(Typical voltage from an operating Fuel Cell is 0.7 V)

Question 23

One reson to why gasoline and diesel are more popular than hydrogen etc:

Answer 23

Volumetric energy density is often more important than Gravemetric energy density. Especially for transport applications.

Question 24

How does a Fuel Cell/Battery work?

Answer 24

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

Question 25

Electrochemistry:

Answer 25

Anode - Oxidation | Cathode - Reduction

Question 26

Types of Fuel Cells:

Answer 26

* 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

Question 27

Fuel Cells vs. Batteries

Answer 27

``` 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! ```

Question 28

Nanotechnology for Fuel Cells and Batteries:

Answer 28

• Reducing component size → Increases capacity → Reduces amount of materials needed • Detailed analysis → Accurate descriptions → Increases understanding Cheaper and more efficient devices

Question 29

N&N for battery development – examples:

Answer 29

* Silicon nanowires: capacity * CuO nanorods: capacity * Si/C nanoparticles: stability

Question 30

Solar cell types:

Answer 30

1 gen: Mono - crystaline Si cells 24% 2 gen: Thin film 20%, Amorphous Si cell 10% 3 gen: Organic 8%, Dye solar cells 11%

Question 31

Plasmonic:

Answer 31

Goal: Absorbe more with thinner material (Collidal lithography) Mechanisms: • Far field effect • Near field effect Photo emission of charge carriers

Question 32

Dye-sensitized solar cells:

Answer 32

* TiO2 paint, absorbs light * Charge will stay there long * Easy to construct * Colors and looks my be more important than efficiency

Question 33

Organic solar cells:

Answer 33

* Cheaper * No lack of resources * Poor electric selectivity

Question 34

SUMMARY:

Answer 34

* 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

Question 35

Key functional properties of photo-electrodes:

Answer 35

``` • Band gap • Flat-band potential • Schottky barrier • Electrical resistance - Electrodes - Electrolyte - Electrical leads - Electrical connections • Helmholtz potential barrier • Corrosion and photo-corrosion resistance • Microstructure ```

Question 36

The hydrogen problem is:

Answer 36

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)

Question 37

Band gap

Answer 37

The optimal band gap for high performance photo-electrodes is ~ 2 eV

Question 38

why it is a wonder?

Answer 38

``` Because it solves number of problems: - Storage - Efficiency - Emissions - Supply ```

Question 39

New materials:

Answer 39

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