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Sustainable > Lectures 1-5 > Flashcards

Lectures 1-5 Flashcards

1
Q

Ways of preventing global warming

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

What are the CO2 emission requirements according to IEA 450?

A
  • 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
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3
Q

Name 4 human activity sources of CO2.

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

What are the ways to decarbonise?

A
  1. Reduce demand for carbon.
  2. Use energy more efficiently.
  3. Carbon Capture and Storage.
  4. Move to low carbon systems and sources.
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5
Q

What are the factors involved in the carbon cycle?

A

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

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6
Q

What are the sources of Nitrogen?

A
  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
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7
Q

What are the forms of Nitrogen?

A
  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)

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8
Q

What are the five processes of the Nitrogen Cycle?

A
  1. Saprobiotic nutrition and microbes
  2. Ammonification
  3. Nitrification
  4. Nitrogen Fixation
  5. Denitrification
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9
Q

Explain the five processes of the Nitrogen Cycle.

A
  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.]

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10
Q

What are the seasonal changes/trends of CO2 concentrations?

A
  1. High concentration in May (spring szn).
  2. Lowest concentration in october (autumn szn).
  3. CO2 concentrations directly proportional to global temperature.
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11
Q

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

A

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

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12
Q

What is the effect of sun radiation on Earth?

A
  1. Average radiation from sun is 340 W/m^2 each day.
  2. CO2 increases, with absorbed radiation originating from a higher altitude.
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13
Q

What are the two main solar technologies?

A
  1. Concentrated Solar Power (CSP)
  2. Photovoltaics (PV) - use of solar cells to directly convert sunlight into electricity without any moving parts.
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14
Q

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

A
  1. Thermal energy storage in competition with gas generating plants.
  2. Enables use of solar power even at night.
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15
Q

CSP: Explain the Parabolic Trough.

A
  • 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.
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16
Q

CSP: Explain the Central Receiver.

A
  • 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.
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17
Q

What is the Conversion Efficiency?

A

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.

18
Q

What are Photovoltaics (PV)?

A
  • 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.
19
Q

How do photovoltaics work?

A
  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.

20
Q

What are Gallium Arsenide solar cells?

A
  • 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.
21
Q

What are second generation solar cells?

A
  • 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
22
Q

What is a third generation solar cell?

A
  • Dye-synthetized solar cells (DSSC)
  • Perovskite solar cells (PSC)
  • Organic photovoltaic cells (OPV)
23
Q

What are the advantages of third generation sc?

A
  1. Flexibility (very thin-film).
  2. Their use of solution processing for fabrication of cheaper devices.
24
Q

What do dye sensitized cells consist of?

A
  • 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-)
25
Q

What is the process of dye sensitized solar cells?

A
  • 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
26
Q

What are Perovskite Solar Cells (PSC)?

A
  • 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
27
Q

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

A
  • 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)
28
Q

What are organic photovoltaics (OPV)?

A
  • 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.
29
Q

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

A
  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.
30
Q

How to make polymers processible?

A
  • with the use of precursor route to cast films.
  • substitution of polymers with alkyl substituents enabling their solubility, interacting with solvents.
31
Q

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

A

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

32
Q

What are the two requirements for conjugation in polymers?

A
  1. Photon absorption in the visible range (need to absorb sun light)
  2. Electrical charge transport
    Band Gap: >1.9 eV
    PCE: 3-6%
33
Q

What are the properties of conjugation in polymers?

A

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

34
Q

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

A
  1. Glass : enabling light to pass
  2. ITO : anode
  3. Organic semiconducting layer/film
  4. Al-electrode : cathode
35
Q

What are the factors affecting device operation/efficiency?

A
  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
36
Q

What is the external power conversion efficiency ηe?

A
  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.
37
Q

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

A
  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
38
Q

What are the use of alternative acceptors to fullerenes?

A
  • Fullerene acceptors have limited molar absorptivities.
  • Use of organic conjugated molecular acceptors provide more efficient devices.
39
Q

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

A

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.

40
Q

How does Concentrated solar power (CSP) operate?

A

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

Sustainable (8 decks)

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