Power-to-X

Question 1

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

Answer 1

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

Question 2

Which carbon-neutral fuels are currently the most promising?

Answer 2

Hydrogen and Ammonia

Question 3

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

Answer 3

Catalysing Hydrogen and Carbon Dioxide

Question 4

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

Answer 4

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

Question 5

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

Answer 5

Increasing the flexibility of transport, storage and conversion of energy

Question 6

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

Answer 6

The network was overloaded and much of the power was wasted

Question 7

How can surplus energy generation be effectively managed?

Answer 7

Sector coupling

Question 8

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

Answer 8

Combustion of Hydrogen produces NOx emissions

Question 9

What alternative fuel can be used in marine shipping?

Answer 9

Ammonia

Question 10

Is it possible to fuel airplanes with alternative fuels?

Answer 10

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

Question 11

What is a use for waste oxygen produced during electrolysis?

Answer 11

It can be supplied to hospitals for respirators

Question 12

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

Answer 12

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

Question 13

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

Answer 13

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

Question 14

What is the enthalpy equation?

Answer 14

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

Question 15

How is Gibbs Free Energy expressed?

Answer 15

dG(T,P)

Question 16

How is Entropy Change expressed?

Answer 16

T * dS(T,P)

Question 17

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

Answer 17

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

Question 18

What are current issues with Power-to-X?

Answer 18

High cost
Lacking infrastructure
Poorly standardised

Question 19

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

Answer 19

Decentralisation and improved resilience of the energy network

Question 20

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

Answer 20

Natural gas generation, 47%

Question 21

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

Answer 21

Electrolysis is expensive and not yet suited to large scale generation

Question 22

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

Answer 22

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

Question 23

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

Answer 23

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

Question 24

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

Answer 24

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

Question 25

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

Answer 25

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

Question 26

What are 3 types of electrolyte for hydrogen production?

Answer 26

Alkaline Acidic Solid Oxide

Question 27

What is the equation for charge?

Answer 27

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

Question 28

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

Answer 28

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

Question 29

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

Answer 29

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

Question 30

What is the Faradaic efficiency?

Answer 30

The efficiency of charge flow

Question 31

How long have alkaline electrolysers been developed for?

Answer 31

Around 80 years

Question 32

What are the pros of alkaline electrolysers?

Answer 32

Long track record Low cost Reliable and durable Cheaper catalyst

Question 33

What are the cons of alkaline electrolysers?

Answer 33

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

Question 34

How is an alkaline electrolyte constructed?

Answer 34

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

Question 35

What is the efficiency of an alkaline cell?

Answer 35

70-80%

Question 36

What are the pros of a Proton Exchange Membrane electrolyser?

Answer 36

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

Question 37

What are the cons of a Proton Exchange Membrane electrolyser?

Answer 37

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

Question 38

How does a Proton Exchange Membrane work?

Answer 38

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

Question 39

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

Answer 39

Titanium

Question 40

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

Answer 40

Voltages greater than 2V cause rapid corrosion

Question 41

What are the operating conditions of a PEM cell?

Answer 41

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

Question 42

What are 2 benefits of solid oxide electrolysers?

Answer 42

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

Question 43

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

Answer 43

700-1000 degC

Question 44

List 5 Capital Expenditures (CAPEX)

Answer 44

Engineering cost Site preparation Land cost Material cost Cost of systems

Question 45

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

Answer 45

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

Question 46

How is Net Present Value (NPV) calculated?

Answer 46

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

Question 47

How can hydrogen be integrated with existing infrastructure?

Answer 47

It can use existing gas pipelines for transport

Question 48

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

Answer 48

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

Question 49

Why can't hydrogen be stored in metal containers?

Answer 49

Hydrogen will react with the metal

Question 50

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

Answer 50

Adsorption into solid form Formation of a chemical hydride

Question 51

At what temperature does hydrogen condense at atmospheric pressure?

Answer 51

-253 degC

Question 52

How can life-cycle sustainability be improved?

Answer 52

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

Question 53

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

Answer 53

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

Question 54

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

Answer 54

5% Hydrogen

Question 55

How are hydrogen fuel cells similar to hydrogen electrolysis cells?

Answer 55

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

Question 56

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

Answer 56

Solid oxide cell High operating temperature and long startup time

Question 57

What is the benefit of synthetic carbon based fuels?

Answer 57

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

Question 58

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

Answer 58

It contains a higher density of hydrogen atoms per m3

Question 59

Under what conditions is ammonia produced?

Answer 59

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

Question 60

At what temperature will ammonia become liquid?

Answer 60

-33.3 degC

Question 61

When is ammonia production most efficient?

Answer 61

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

Question 62

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

Answer 62

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

Question 63

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

Answer 63

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

Question 64

What is the equation for power generation?

Answer 64

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

Question 65

What does the value V0th symbolise?

Answer 65

The operating voltage at which no heat loss occurs

Question 66

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

Answer 66

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

Question 67

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

Answer 67

If the production rate is independent of these factors or not

Question 68

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

Answer 68

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

Question 69

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

Answer 69

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

Question 70

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

Answer 70

O2 at the Anode H2 at the Cathode

Question 71

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

Answer 71

Absorbed at the cathode, produced at the anode

Question 72

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

Answer 72

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

Question 73

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

Answer 73

96485 C/mol

Question 74

What ions are transferred across PEM and Solid Oxide electrolysers respectively

Answer 74

H+ for PEM O2- for Solid Oxide

Question 75

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

Answer 75

log(Pout/Pin)/log(x)

Question 76

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

Answer 76

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

Question 77

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

Answer 77

3𝐻2 + 𝑁2⇌ 2𝑁𝐻3

Question 78

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

Answer 78

ΔH * production rate of hydrogen (kg/s)