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Waste Heat and Power-to-X 5 > Power-to-X > Flashcards

Power-to-X Flashcards

1
Q

In what 4 ways can excess power create useful energy products?

A

Electrolysis to create Hydrogen and Methane
Electro-reduction to create Methanol
Co-electrolysis to create Hydrocarbon Fuels
Heat Pumps to generate Heating

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2
Q

Which carbon-neutral fuels are currently the most promising?

A

Hydrogen and Ammonia

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3
Q

How can Methane be produced without extracting it from natural sources?

A

Catalysing Hydrogen and Carbon Dioxide

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4
Q

Why is it difficult to replace oil and gas with battery-stored power?

A

The energy density of batteries is much lower than oil and gas

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5
Q

What is the main benefit of Power-to-X schemes?

A

Increasing the flexibility of transport, storage and conversion of energy

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6
Q

In 2020, the amount of power generated from wind turbines was double the previous year, why did this cause problems?

A

The network was overloaded and much of the power was wasted

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7
Q

How can surplus energy generation be effectively managed?

A

Sector coupling

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8
Q

Why are hydrogen fuel cells preferred to combustion of hydrogen gas?

A

Combustion of Hydrogen produces NOx emissions

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9
Q

What alternative fuel can be used in marine shipping?

A

Ammonia

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10
Q

Is it possible to fuel airplanes with alternative fuels?

A

Hydrogen can be used in planes, but only for short distance flights

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11
Q

What is a use for waste oxygen produced during electrolysis?

A

It can be supplied to hospitals for respirators

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12
Q

What are the 4 “X” types covered in this course of Power-to-X

A

Power to Hydrogen
Power to Gas (Methane)
Power to Liquids (Methanol)
Power to Chemicals (Ammonia)

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13
Q

What are potential uses of Methanol and Ammonia outside of the energy sector?

A

Methanol can be used to create plastics
Ammonia can be used as fertiliser

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14
Q

What is the enthalpy equation?

A

dH(T,P) = dG(T,P) + T * dS(T,P)

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15
Q

How is Gibbs Free Energy expressed?

A

dG(T,P)

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16
Q

How is Entropy Change expressed?

A

T * dS(T,P)

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17
Q

Where do Hydrogen and Ammonia fall on the storage density graph?

A

Hydrogen has high energy density but low volumetric density
Ammonia has decent volumetric density but low energy density

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18
Q

What are current issues with Power-to-X?

A

High cost
Lacking infrastructure
Poorly standardised

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19
Q

What are the key benefits of Power-to-X?

A

Decentralisation and improved resilience of the energy network

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20
Q

How was the majority of hydrogen fuel generated in 2021? What proportion was this?

A

Natural gas generation, 47%

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21
Q

Why was only 4% of Hydrogen generated by electrolysis in 2021?

A

Electrolysis is expensive and not yet suited to large scale generation

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22
Q

What is Brown Hydrogen? What are its average efficiency and emissions values?

A

Hydrogen from coal gasification
60% efficient
50-642 gCO2/kWh

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23
Q

What is Grey Hydrogen? What are its average efficiency and emissions values?

A

Hydrogen from methane reformation
70-85% efficient
285 gCO2/kWh

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24
Q

What is Blue Hydrogen? What are its average efficiency and emissions values?

A

Hydrogen from methane reformation with carbon capture
70-85% efficient
11-25 gCO2/kWh

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25
Q

What is Green Hydrogen? What are its average efficiency and emissions values?

A

Hydrogen from water electrolysis
60-80% efficient
0-14 gCO2/kWh

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26
Q

What are 3 types of electrolyte for hydrogen production?

A

Alkaline
Acidic
Solid Oxide

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27
Q

What is the equation for charge?

A

Charge (Q) = Current (I) x Time (t)

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28
Q

What is Faraday’s Constant and how is it derived?

A

The charge of one mol of electrons
F = Avogadro’s No. x Charge of 1 electron

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29
Q

Why does an electrolysis system need the capacity for a higher voltage than the calculated operating voltage?

A

A larger current is necessary initially to start the process of electrolysis

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30
Q

What is the Faradaic efficiency?

A

The efficiency of charge flow

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31
Q

How long have alkaline electrolysers been developed for?

A

Around 80 years

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32
Q

What are the pros of alkaline electrolysers?

A

Long track record
Low cost
Reliable and durable
Cheaper catalyst

33
Q

What are the cons of alkaline electrolysers?

A

Not compact
Need moderate current
Slow response time
Not appropriate for variable energy sources

34
Q

How is an alkaline electrolyte constructed?

A

It consists of an aqueous solution of NaOH or KOH, in which is a diaphragm that allows the transfer of OH- ions

35
Q

What is the efficiency of an alkaline cell?

A

70-80%

36
Q

What are the pros of a Proton Exchange Membrane electrolyser?

A

Non-corrosive electrolyte
Higher current density
Compact
Can handle transient power supply

37
Q

What are the cons of a Proton Exchange Membrane electrolyser?

A

Very expensive
Lack of standardisation
New, not yet commercialised
Uses Iridium, a very scarce and expensive element

38
Q

How does a Proton Exchange Membrane work?

A

Protons and O2 are separated from water, the protons pass through the membrane and are evolved into hydrogen atoms by adding electrons

39
Q

What are the porous transportation layers of a PEM electrolyser made of?

A

Titanium

40
Q

Why must the voltage of a PEM cell be carefully controlled?

A

Voltages greater than 2V cause rapid corrosion

41
Q

What are the operating conditions of a PEM cell?

A

Temperature of 60-90 degC
Pressure of 15-20 bar

42
Q

What are 2 benefits of solid oxide electrolysers?

A

They have higher efficiency than other methods
They can reduce input voltage requirements by increasing operating temperature

43
Q

What is the temperature of the steam used in a solid oxide electrolyser?

A

700-1000 degC

44
Q

List 5 Capital Expenditures (CAPEX)

A

Engineering cost
Site preparation
Land cost
Material cost
Cost of systems

45
Q

What are the 2 types of Operational Expenditures (OPEX)? Give some examples of both

A

Fixed: Labour, licensing, taxes, maintenance
Variable: environmental surcharges, waste treatment

46
Q

How is Net Present Value (NPV) calculated?

A

The lifetime sum of CAPEX and OPEX divided by a discount rate

47
Q

How can hydrogen be integrated with existing infrastructure?

A

It can use existing gas pipelines for transport

48
Q

Why can hydrogen gas not be used as a fuel immediately after being generated?

A

It has a low volumetric density, meaning it needs to be compressed or condensed to 350-700 bar in order to efficiently store the energy within

49
Q

Why can’t hydrogen be stored in metal containers?

A

Hydrogen will react with the metal

50
Q

How can hydrogen be stored other than as a pressurised fluid?

A

Adsorption into solid form
Formation of a chemical hydride

51
Q

At what temperature does hydrogen condense at atmospheric pressure?

A

-253 degC

52
Q

How can life-cycle sustainability be improved?

A

Reduce resource use
Improve socio-economic performance
Promote interlinking of key aspects

53
Q

What are the 3 key concerns for choosing a scheme? What has recently been considered an addition to this list?

A

Security, affordability and sustainability
Community benefit is to be added moving forward

54
Q

What is the threshold for blending hydrogen with natural gas at which the risk of explosion increases?

A

5% Hydrogen

55
Q

How are hydrogen fuel cells similar to hydrogen electrolysis cells?

A

They work in revers to turn the hydrogen into a flow of electrons and OH-

56
Q

What is the most efficient type of hydrogen fuel cell and what are its downsides?

A

Solid oxide cell
High operating temperature and long startup time

57
Q

What is the benefit of synthetic carbon based fuels?

A

The process of creating the fuel theoretically removes enough carbon from the atmosphere to balance the emissions of burning it

58
Q

Why is ammonia more commonly used as a fuel than pure hydrogen?

A

It contains a higher density of hydrogen atoms per m3

59
Q

Under what conditions is ammonia produced?

A

Temperatures 300-550 degC
Pressures 15-30MPa
Iron, potassium oxide or aluminium oxide

60
Q

At what temperature will ammonia become liquid?

A

-33.3 degC

61
Q

When is ammonia production most efficient?

A

When the process is running consistently with a constant supply of hydrogen

62
Q

How do you find the charge required to electrolyse a product?

A

Q = e n F
e is the number of moles of electrons involved
n is the number of moles of product involved
F is Faraday’s Constant

63
Q

How do you calculate the Hydrogen production rate in 4 steps?

A
  1. Calculate the charge from the number of electrons involved
  2. Calculate the mol/s rate by using I instead of Q in the calculation of number of moles of Hydrogen
  3. Use the mass of a mole of Hydrogen to find the kg/s production rate
  4. Convert units as needed
64
Q

What is the equation for power generation?

A

W = VI
W is power
V is voltage
I is current

65
Q

What does the value V0th symbolise?

A

The operating voltage at which no heat loss occurs

66
Q

How do calculations change for electrolyser stacks with X number of cells?

A

Molar production rate is multiplied by X
V0th is multiplied by X before being subtracted from operating voltage

67
Q

What is important to check when questions ask about the effects of changing temperature and pressure in an electrolyser cell?

A

If the production rate is independent of these factors or not

68
Q

What is the equation for the power necessary to compress a gas with a molar production rate of n?

A

W = nRT*ln(Pout/Pin)
R is the gas constant
T is the temperature IN KELVIN

69
Q

What are the 3 key components of an alkaline water electrolyser?

A

Anode (+ve)
Cathode (-ve)
Microporous Diaphragm

70
Q

What are the two outputs of water electrolysis and where do they form?

A

O2 at the Anode
H2 at the Cathode

71
Q

In water electrolysis, at which node is water (H2O) absorbed and at which is it produced?

A

Absorbed at the cathode, produced at the anode

72
Q

If you have calculated H2 production rate from electrolysis, how do you find O2 production rate?

A

Divide nH2 (mol/s) by 2, as 2H2 are produced to every 1O2

73
Q

What is the value of Faraday’s Constant,F?

A

96485 C/mol

74
Q

What ions are transferred across PEM and Solid Oxide electrolysers respectively

A

H+ for PEM
O2- for Solid Oxide

75
Q

How do you find the number of compression stages, N, for compression ratio x?

A

log(Pout/Pin)/log(x)

76
Q

What is the equation for the power of a multistage compressor?

A

P = Nz/ηisen * nRT * k/k-1 * [{(Pout/Pin)^k-1/k} -1]
z and k are given factors, z is compressibility factor

77
Q

What is the equation for the Haber-Bosch process, the formation of ammonia?

A

3𝐻2 + 𝑁2⇌ 2𝑁𝐻3

78
Q

If given the thermodynamic parameters of water electrolysis, how do you find the power demand of the electrolyser system?

A

ΔH * production rate of hydrogen (kg/s)

Waste Heat and Power-to-X 5 (2 decks)

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