Lectures 1-5

Question 1

Ways of preventing global warming

Answer 1

1. Conserve energy
2. Increase renewable energy resources and reduce coal
3. Remove CO2 from burning fossil fuel
4. Remove CO2 from atmosphere

Question 2

What are the CO2 emission requirements according to IEA 450?

Answer 2

• CO2 emissions need to stay below 450 ppm
• Hold temperature rise below 2 degrees Celsius (Paris Agreement)
• Area under curve of IEA 450 = CO2 accumulation

Question 3

Name 4 human activity sources of CO2.

Answer 3

1. Coal power plants (33%)
2. Transportation (32.9%)
3. Industrial (15%)
4. Electricity Generation (5%) - gas, or other fuels
5. Aviation (3%)

Question 4

What are the ways to decarbonise?

Answer 4

1. Reduce demand for carbon.
2. Use energy more efficiently.
3. Carbon Capture and Storage.
4. Move to low carbon systems and sources.

Question 5

What are the factors involved in the carbon cycle?

Answer 5

The balance between Sinks and Sourcess of CO2.

Sinks of CO2
1. Photosynthesis
2. Diffusion into oceans
3. Ocean carbon stores
4. Marine deposits (Limestones and Dolomites, CaMg(CO3)2.)
5. For the formation of oil, coal, and gas.

Sources of CO2
1. Respiration
2. Decomposition
3. Release from oceans
4. Volcanoes
5 Burning Fossil Fuels

Question 6

What are the sources of Nitrogen?

Answer 6

1. Lightning - strikes N-N bond => N- anions fall as acid rain (from atmosphere)
2. Inorganic fertilisers (NH4NO3, [(NH4)3PO4]
3. Nitrogen Fixation (generation of NH3 from Haber Process)
4. Animal Residues
5. Crop Residues
6. Organic fertilisers

Question 7

What are the forms of Nitrogen?

Answer 7

1. Urea -> CO(NH2)2
2. Ammonia -> NH3 (gaseous)
3. Ammonium -> NH4+
4. Nitrate NO3-
5. Nitrite NO2-
6. Atmospheric N2
7. Organic N

Plants and bacteria: NH4+ and NO3-
Soil and water: N (most limiting nutrient)

Question 8

What are the five processes of the Nitrogen Cycle?

Answer 8

1. Saprobiotic nutrition and microbes
2. Ammonification
3. Nitrification
4. Nitrogen Fixation
5. Denitrification

Question 9

Explain the five processes of the Nitrogen Cycle.

Answer 9

1. Nitrogen gas in the atmosphere, convert N2 into NH4+ or NO3-.
2. Nitrogen-fixing bacteria lives in the root nodules of leguminous plants.
3. Othe nitrogen-fixing bacteria occuring naturally in the soil, convert N2 to NH4+ => Ammonification.
4. NH4+ nitrified to NO2- with nitrifying bacteria nitrified to NO3- => Nitrification.
5. NO3- soluble in water in the soil, absorbed by plants, eaten by animals, producing waste, passed back into soil, converting NO3- to N2 with denitrifying bacteria.
   => NO3- denitrified by denitrifying bacteria to N2 => Denitrification. (Reduces nitrates in the soil)
6. Nitrogen-fixing bacteria converts N2 into NH3 which is then nitrified.

[Animals & plants die and decompose by bacteria in the soil => through ammonification process.]

Question 10

What are the seasonal changes/trends of CO2 concentrations?

Answer 10

1. High concentration in May (spring szn).
2. Lowest concentration in october (autumn szn).
3. CO2 concentrations directly proportional to global temperature.

Question 11

What is the Change observed in CO2 rates snnually, its cause and result in the atmosphere?

Answer 11

Amount of CO2 increases by 5 GtC every year

Cause:
- half of total CO2 added to atmosphere by the oceans
- Carbon Cycle dynamic equilibrium

Result:
- greenhouse gas effect
- CO2 absorbs solar radiation
- increase in temperature

Question 12

What is the effect of sun radiation on Earth?

Answer 12

1. Average radiation from sun is 340 W/m^2 each day.
2. CO2 increases, with absorbed radiation originating from a higher altitude.

Question 13

What are the two main solar technologies?

Answer 13

1. Concentrated Solar Power (CSP)
2. Photovoltaics (PV) - use of solar cells to directly convert sunlight into electricity without any moving parts.

Question 14

What are 2 advantages of concentrated solar power (CSP).

Answer 14

1. Thermal energy storage in competition with gas generating plants.
2. Enables use of solar power even at night.

Question 15

CSP: Explain the Parabolic Trough.

Answer 15

• Light reflected on tube at focal line.
• Tube filled with molten salt which circulates.
• Heat generated used to generate steam driving a turbine-generator.
• Operate at ~400 degrees Celcius.
• 56% at dC.

Question 16

CSP: Explain the Central Receiver.

Answer 16

• Use of central concentrator.
• Molten salt: NaNO3 / KNO3 (60-40 % w).
• Window of operation: 290 - 565 degrees Celcius.
• 65 % efficiency at 565 dC.
  = Better and more efficient due to high temperatures at which they operate.

Question 17

What is the Conversion Efficiency?

Answer 17

A.K.A : Carnot efficiency
η(carnot) = 1 - T(0) / T(H)
T(0): as close to 1 as possible.
T(H): as high as possible.
=> large result, therefore high efficiency.

Question 18

What are Photovoltaics (PV)?

Answer 18

• The conversion of light to power.
• A.K.A the first-generation solar cells.

-Example: silicon solar cells.

• Use crystalline Silicion doped with Phosphorus & Boron / Gallium (n-type / p-type) => semiconductors.
• Efficiency: up to 25%.
• Thickness: 100 - 500 μm.

Question 19

How do photovoltaics work?

Answer 19

1. Charge carriers produced in the bulk of material, charge separation driven by built-in electric field.
   Charge carriers: electrons and holes.
2. Electrons move from n-type to p-type, forming depletion region at the p-n junction.
3. Sunlight strikes -> electron-hole pairs form, electric field drives e&h out the depletion region.
   IF contact between electrodes => e&h extracted from cell, current flows as long as light is absorbed by the cell.
   BAND GAP: 1.1 eV.

Problems:
- expensive (thick layer required for high efficiency)
- Energy consuming manufacturing process.

Question 20

What are Gallium Arsenide solar cells?

Answer 20

• first generation solar cells (1970s).
• a direct band gap semiconductor, absorbing light used for solar cell applications.
• GaAs (III-V) high absorption, absorbing and converting high-energy photons.
• high electron mobility.
• high efficiency -> ideal for high-tech applications.
• thin film - 2μm.

Question 21

What are second generation solar cells?

Answer 21

• thin-film Copper Indium Gallium Selenide (CIGS)
• Cadmium Telluride (CdTe)
• amorphous Silicon (a-Si)
• Thickness: less than first generation (100 - 500μm)
• Thickness of thin-film solar panels: few nm to 10s μm
• Used to make PV
• Efficiency 22.6%
• High manufacturing cost

Question 22

What is a third generation solar cell?

Answer 22

• Dye-synthetized solar cells (DSSC)
• Perovskite solar cells (PSC)
• Organic photovoltaic cells (OPV)

Question 23

What are the advantages of third generation sc?

Answer 23

1. Flexibility (very thin-film).
2. Their use of solution processing for fabrication of cheaper devices.

Question 24

What do dye sensitized cells consist of?

Answer 24

• A.k.a Gratzel cells
• consist of three components:
  1. Solar light harvester (dye, Ru N3-based)
  2. Semiconducting material (TiO2)
  3. Electrolyte solution (I-/I3-)

Question 25

What is the process of dye sensitized solar cells?

Answer 25

- generation of excitons on light excitation - charge separation with TiO2 acceptor - transport of charges to electrodes - Efficiency: up to 11% Problem => solvent leakage due to high T of sun, liquid electrolytes required

Question 26

What are Perovskite Solar Cells (PSC)?

Answer 26

- new solution-based method, preparing high eficient PV devices (PCE > 20%) Advantage: Ease of preparation - Mixture of PbX2 and MeNH3X (in polar solvent) creates crystalline film (on substrate) - Charge transporting layers placed between electrodes - Band gap differs according to nature of halogen (lowest band gap => higher sunlight absorbed) Lowest band gap: Ammonium lead iodide

Question 27

What are the factors affecting the efficiency of photovoltaics using perovskites as active layers?

Answer 27

- energy levels of charge transporting layers - morphology of active layer (important for charge extraction) - stability of devices in operation (reactions at interface of different layers) - protection against degradation (via encapsulation to keep away water and oxygen)

Question 28

What are organic photovoltaics (OPV)?

Answer 28

- Concern the use of organic semiconductors as active layers in solar cells. - Conversion of light to power. - Organic semiconductors: conjugated molecules. - Electronic conjugation: way of controlling the band gap of the materials. - Nature of repeat units: determine energy levels of HOMO and LUMO.

Question 29

What are the main problems in the processibility of conjugated polymers?

Answer 29

1. All conjugated polymers considered as rigid rods. 2. Requirement of precursor route to make them soluble for better interaction with solvents. 3. Not processible. 4. Solubilised polymers required for making films.

Question 30

How to make polymers processible?

Answer 30

- with the use of precursor route to cast films. - substitution of polymers with alkyl substituents enabling their solubility, interacting with solvents.

Question 31

In bulk hetero-junction solar cells, what does the p-n junction consist of?

Answer 31

A combination of two different semiconductor materials: Donor: conjugated polymer, hole conducting Acceptor: Fullerene (C60) or its derivatives (PBCM), electron conducting

Question 32

What are the two requirements for conjugation in polymers?

Answer 32

1. Photon absorption in the visible range (need to absorb sun light) 2. Electrical charge transport Band Gap: >1.9 eV PCE: 3-6%

Question 33

What are the properties of conjugation in polymers?

Answer 33

Good: - High absorption coefficient - easy processability Bad: - Low carrier mobility because of amorphous/ semi-amorphous structures

Question 34

What are the working principles of conjugation of the conjugation in polymers?

Answer 34

1. Glass : enabling light to pass 2. ITO : anode 3. Organic semiconducting layer/film 4. Al-electrode : cathode

Question 35

What are the factors affecting device operation/efficiency?

Answer 35

1. Energy gap of polymer donors - some polymers dont absorb light beyond 670 nm. - Polymers absorbing in near-IR gain as much photons from sunlight as possible. 2. Offset of energy levels of donors and acceptors used - External power conversion efficiency - ηe 3. Morphology control of active layers - bilayer devices - bulk heterojunction devices - molecular heterojunction devices

Question 36

What is the external power conversion efficiency ηe?

Answer 36

1. Shows efficiency of film ηe: external power conversion efficiency η = ( Voc x Isc x FF ) / Pin - ΔΕ >= 0.3 eV (min energy required) IF too high, energetically wasteful. - Voc : open circuit voltage = as high as possible - Isc : Short circuit current (A/cm2) - FF : Fill Factor (area under I/V curve) - Pin : Incident light power - HOMO-LUMO need to be narrow.

Question 37

Explain the morphology control of active layers, mentioning the three devices.

Answer 37

1. Bilayer devices - donor & acceptor stacked together - small interface 2. Bulk heterojunction devices - donor & acceptor mixed in bulk volume - large interface - difficult control of morphology 3. Molecular heterojunction devices - acceptor units covalently bound to donor (mixing of a & d) - Large interface - control of morphology at molecular level

Question 38

What are the use of alternative acceptors to fullerenes?

Answer 38

- Fullerene acceptors have limited molar absorptivities. - Use of organic conjugated molecular acceptors provide more efficient devices.

Question 39

What is Radiate power, and how is it affected by altitude?

Answer 39

At higher altitude, temperature is cooler, emitting less energy to space because power of radiation proportional to T^4. Radiative Power ~ T^4 => Radiative Forcing.

Question 40

How does Concentrated solar power (CSP) operate?

Answer 40

It generates electricity using sunlight to heat molten salt fluid, producing steam, driving a turbine-generator.