Week 3 - Solar, Biofuels, Geothermal energy

Question 1

What is the photovoltaic effect (what do solar cells do?)

Answer 1

Solar cells convert sunlight to electricity using semiconducting materials (e.g. silicon)

Question 2

What do solar panels do? What is the main component of solar panels?

Answer 2

Solar panels convert sunlight to electricity using solar cells made from semiconducting materials (e.g. silicon)

Question 3

List the hierarchy of a solar system

Answer 3

Cell -> Module -> Panel -> Array

Question 4

What is the working principle of photovoltaic systems?

Answer 4

Generation of electricity via charge separation upon exposure to light

Question 5

What is the formula for energy of a photon?

Answer 5

h*v (h is planck’s constant, v is the wavelength)

Question 6

What is the defining characteristic of n-type semiconductor?

Answer 6

Extra electron(s) as majority charge carriers, high concentration of electrons

Question 7

What defines a p-type semiconductor?

Answer 7

Extra hole(s) as majority charge carriers, high concentration of holes

Question 8

What materials can be used to dope silicon to create p- or n- type sc?

Answer 8

n-type - silicon + phosphorus
p-type - silicon + boron

Question 9

What happens at a p-n junction?

Answer 9

Holes, from the p side, and electrons, from the n side, diffuse. They recombine, and create a depletion region.

Question 10

What is the photovoltaic effect?

Answer 10

Photons excite electrons from valence band in to conduction band. Charge Separation: The built-in electric field in the depletion region separates the electrons (pushed to the n-side) and holes (pushed to the p-side). Current Generation: If an external circuit is connected, electrons flow from the n-side to the p-side, creating electric current.Voltage Generation: The separated charges create a photovoltaic voltage (typically ~0.5–0.7 V for silicon solar cell.

Question 11

What is the 8 stage process that creates a voltage in a solar cell?

Answer 11

1. Attraction of holes and electrons
2. Depletion zone formation
3. Electric field formation
4. Electron-hole pair formation
5. Electron enters conduction band
6. Electron diffuses to n-region; hole remains in p-region
7. Electron moves through external circuit
8. Electron recombines with hole in p-region

Question 12

What is I_L, I_d, I_sh, I_0, k, n, R_s, R_sh in the photodiode equations?

Answer 12

Id : diode current (A)
IL: photo-generated current (A)
ISh: shunt current (A)
I0: reverse saturation current (A)
kB: Boltzmann constant (J/K)
n: Diode ideality factor
Rs: Series resistance (Ohm)
Rsh: Shunt resistance (Ohm)

Question 13

What is the maximum theoretical power output of a solar cell?

Answer 13

P_T = I_sc V_oc

Question 14

What is the maximum power of a solar cell equation?

Answer 14

P_max = I_max V_max

Question 15

Draw the IV curve for solar panels

Answer 15

horizontal, until steep corner into vertical. x axis is voltage, y axis is current. x axis and y axis intercepts are V_oc and I_sc, respectively. The point on the curve where the rectangle with the greatest area is the maximum power output, with V_m and I_m on the x axis. The ratio of that rectangle with the rectangle from I_sc and V_oc is the fill factor.

Question 16

What is n_max,collection for silicon

Answer 16

49%

Question 17

How to increase voltage of PV cells?

Answer 17

Connect in series

Question 18

How to increase current of PV cells?

Answer 18

Connect in parallel

Question 19

How is voltage related with temperature in PV cells?

Answer 19

Voltage drops with increasing temperature

Question 20

How is current related to solar irradiance in PV cells?

Answer 20

Current increases with solar irradiation

Question 21

How can solar cells be made more efficient?

Answer 21

Combining several materials with different band gaps to increase collection efficiency

Question 22

In multi-junction cells, does the material with the biggest band gap go on the bottom or top of the stack?

Answer 22

Biggest band gap on top

Question 23

What are the problems with multi-junction cells?

Answer 23

Challenges:
▪ Matching band gaps and materials
▪ Fabrication

Question 24

What are alternative methods for harvesting solar irradiance, other than PV cells?

Answer 24

Active solar heating using flat plate
collectors to store and circulate thermal energy for air and water heating.
Concentrated solar power (CSP) plants use concentrated sunlight to run a heat engine that spins a turbine and produce electricity

Question 25

What is biomass energy's working principles?

Answer 25

Energy produced via photosynthesis and stored in molecules (e.g. carbohydrates, lignin) is released by breaking chemical bonds.

Question 26

What is the difference between HHV and LHV?

Answer 26

▪ Higher heating value (HHV): includes heat from vaporisation of water contained in the biomass ▪ Lower heating value (LHV): heat from water vaporisation not included

Question 27

What is HHV?

Answer 27

A measure of the energy density of a material. Energy density of biomass is function of fuel’s chemical composition, and expressed as heating value (MJ/kg).

Question 28

What are the four methods of thermochemical conversion?

Answer 28

Combustion, gasification, pyrolysis, hydrothermal liquefaction.

Question 29

What is combustion, and what is its advantage?

Answer 29

Directly burned for energy production e.g. burning wood for heating. Burned in oxygen rich environment. ▪ Waste-to-energy: burning of waste to create electricity ▪ Advantage: Reduction of waste

Question 30

What is gasification, and what is it's advantage?

Answer 30

Heating biomass in presence of oxygen, air, and/or water vapour (lean) ▪ Gas containing various fractions of methane, CO2, CO, H2, water vapour (Syngas)

Question 31

What is syngas useful for?

Answer 31

Syngas can be further used to produce hydrogen, alcohols, hydrocarbon fuels

Question 32

What is pyrolysis, and what does it produce?

Answer 32

Pyrolysis: heating in the absence of oxygen → Gas containing H2 , methane, ethane, liquid hydrocarbons ▪ Gases are combusted for heat and electric power generation ▪ Production of bio-oils for engines

Question 33

What is an issue with using bio-mass energy?

Answer 33

It competes with food supply.

Question 34

What are bio-fuels? List an example

Answer 34

Biofuels: liquid fuels derived from recently living organic material. E.g. ethanol

Question 35

How is ethanol produced from plants?

Answer 35

Ethanol (C2H5OH) ▪ Produced by yeast via fermentation of sugars to alcohols from sugarcane, corn, cellulose etc ▪ Source of ethanol can change net energy balance (ratio between energy output to energy input); Corn: 1.6; Sugarcane: 8 ▪ Energy density: 23.4 MJ/L, 29.7 MJ/kg

Question 36

What are the benefits and drawbacks of using bio-ethanol?

Answer 36

High fuel efficiency Lower production of CO and NOx compared to gasoline Use of agricultural land for the production of fuel→ impact on global food supply Large input of fresh water Produces other toxic chemicals that increase smog

Question 37

List 1 (or 2) bio-fuel, it's source and use.

Answer 37

Biodiesel, produced from vegetable oil, microalgae and animal fat via chemical processes. Used for heavy transport. Biogas ▪ Produced via bacterial anaerobic digestion (landfills or anaerobic digesters) ▪ Main components: methane and CO2 ▪ Can be further purified → high-purity methane for transport and homes

Question 38

List another bio-fuel

Question 39

What are some of the environmental concerns when using bio-fuels, and how can they be managed?

Answer 39

What to watch out for: ▪ Water consumption and pollution ▪ Land usage/transformation ⟶ habitat destruction/deforestation/biodiversity decline ▪ Soil erosion ▪ Use of fertiliser ▪ Competition with food production ▪ High energy input ⟶ should be renewable energy What you should do: ⟶ Use waste biomass ⟶ Reduce use of fertiliser ⟶ Crop rotation and other sustainable measures

Question 40

Describe the process of the heating cycle of a heat pump

Answer 40

Heating cycle 1. Thermal energy from ground absorbed by fluid 2. Heats up refrigerant in evaporator; evaporation of refrigerant 3. Compressor: Gas is compressed, and temperature increased 4. Condenser: Hot gas heats heating fluid; gas temperature decreases, and it condenses 5. Refrigerator passes through expansion valve, expands and cools down

Question 41

What is a geographical limitation to hydrothermal electricity generation?

Answer 41

Hydrothermal resources require a water reservoir close to thermal energy source

Question 42

How does a flash steam power plant work?

Answer 42

Flash steam power plant : 1. Hot water comes out of ground and is depressurised 2. Steam is fed into a turbine 3. Steam is then condensed to low temperatures

Question 43

How does a dry steam power plant work?

Answer 43

Dry-steam: 1. Only steam comes out (no water) and spins a turbine 2. Condensed and pumped back down

Question 44

List three methods of creating new underground reservoirs

Answer 44

Hydraulic stimulation: injecting fluids at high pressure ▪ Thermal stimulation: circulating cold fluid through hot rock mass ▪ Chemical stimulation: circulating chemical compounds that dissolve minerals and create voids

Question 45

What is the advantage and drawbacks of deep well geothermal systems?

Answer 45

Expensive and difficult to dig deep enough (High upfront costs for specialised equipment). Very few site specific limitations, so wide applicability. Do not require creation of new reservoirs and reduce risk of inducing seismicity

Question 46

Is nuclear or wind cheaper?

Answer 46

Wind is cheaper (40 for onshore - 80 offshore, dollars per MWh vs 150-200 for nuclear)

Question 47

What is the Shockley-Queisser Limit on photovoltaic efficiency for silicon based PV?

Answer 47

49%

Question 48

What is the main difference between combustion, gasification, and pyrolysis? What are the main products?

Answer 48

The main difference is the amount of oxygen under which the reaction takes place. Combustion: a lot of oxygen; gasification: lean environment/oxygen-poor; pyrolysis: no oxygen Main products are: Combustion: CO2, water, NOx, SOx Gasification: syngas (CO + H2), CO2, methane Pyrolysis: H2, methane, and bio-oils

Question 49

Explain the cooling cycle in a reverse heat pump/ air-con unit

Answer 49

Cooling cycle: 1) refrigerant liquid reaches expansion valve – temperature being lowered 2) refrigerant passes through evaporator, where it cools down the source media, which cools down the house. 3) The liquid in the evaporator heats up, passes through the compressor, where it gets further heated. 4) Liquid runs through the condenser. Here, it gives away the heat to the sink media and cools down. The sink media runs through the external coils, gives away the heat to the soil and is being cooled down. The refrigerant then passes through the expansion valve again, cooling down the temperature.