Power-to-X Flashcards
In what 4 ways can excess power create useful energy products?
Electrolysis to create Hydrogen and Methane
Electro-reduction to create Methanol
Co-electrolysis to create Hydrocarbon Fuels
Heat Pumps to generate Heating
Which carbon-neutral fuels are currently the most promising?
Hydrogen and Ammonia
How can Methane be produced without extracting it from natural sources?
Catalysing Hydrogen and Carbon Dioxide
Why is it difficult to replace oil and gas with battery-stored power?
The energy density of batteries is much lower than oil and gas
What is the main benefit of Power-to-X schemes?
Increasing the flexibility of transport, storage and conversion of energy
In 2020, the amount of power generated from wind turbines was double the previous year, why did this cause problems?
The network was overloaded and much of the power was wasted
How can surplus energy generation be effectively managed?
Sector coupling
Why are hydrogen fuel cells preferred to combustion of hydrogen gas?
Combustion of Hydrogen produces NOx emissions
What alternative fuel can be used in marine shipping?
Ammonia
Is it possible to fuel airplanes with alternative fuels?
Hydrogen can be used in planes, but only for short distance flights
What is a use for waste oxygen produced during electrolysis?
It can be supplied to hospitals for respirators
What are the 4 “X” types covered in this course of Power-to-X
Power to Hydrogen
Power to Gas (Methane)
Power to Liquids (Methanol)
Power to Chemicals (Ammonia)
What are potential uses of Methanol and Ammonia outside of the energy sector?
Methanol can be used to create plastics
Ammonia can be used as fertiliser
What is the enthalpy equation?
dH(T,P) = dG(T,P) + T * dS(T,P)
How is Gibbs Free Energy expressed?
dG(T,P)
How is Entropy Change expressed?
T * dS(T,P)
Where do Hydrogen and Ammonia fall on the storage density graph?
Hydrogen has high energy density but low volumetric density
Ammonia has decent volumetric density but low energy density
What are current issues with Power-to-X?
High cost
Lacking infrastructure
Poorly standardised
What are the key benefits of Power-to-X?
Decentralisation and improved resilience of the energy network
How was the majority of hydrogen fuel generated in 2021? What proportion was this?
Natural gas generation, 47%
Why was only 4% of Hydrogen generated by electrolysis in 2021?
Electrolysis is expensive and not yet suited to large scale generation
What is Brown Hydrogen? What are its average efficiency and emissions values?
Hydrogen from coal gasification
60% efficient
50-642 gCO2/kWh
What is Grey Hydrogen? What are its average efficiency and emissions values?
Hydrogen from methane reformation
70-85% efficient
285 gCO2/kWh
What is Blue Hydrogen? What are its average efficiency and emissions values?
Hydrogen from methane reformation with carbon capture
70-85% efficient
11-25 gCO2/kWh
What is Green Hydrogen? What are its average efficiency and emissions values?
Hydrogen from water electrolysis
60-80% efficient
0-14 gCO2/kWh
What are 3 types of electrolyte for hydrogen production?
Alkaline
Acidic
Solid Oxide
What is the equation for charge?
Charge (Q) = Current (I) x Time (t)
What is Faraday’s Constant and how is it derived?
The charge of one mol of electrons
F = Avogadro’s No. x Charge of 1 electron
Why does an electrolysis system need the capacity for a higher voltage than the calculated operating voltage?
A larger current is necessary initially to start the process of electrolysis
What is the Faradaic efficiency?
The efficiency of charge flow
How long have alkaline electrolysers been developed for?
Around 80 years
What are the pros of alkaline electrolysers?
Long track record
Low cost
Reliable and durable
Cheaper catalyst
What are the cons of alkaline electrolysers?
Not compact
Need moderate current
Slow response time
Not appropriate for variable energy sources
How is an alkaline electrolyte constructed?
It consists of an aqueous solution of NaOH or KOH, in which is a diaphragm that allows the transfer of OH- ions
What is the efficiency of an alkaline cell?
70-80%
What are the pros of a Proton Exchange Membrane electrolyser?
Non-corrosive electrolyte
Higher current density
Compact
Can handle transient power supply
What are the cons of a Proton Exchange Membrane electrolyser?
Very expensive
Lack of standardisation
New, not yet commercialised
Uses Iridium, a very scarce and expensive element
How does a Proton Exchange Membrane work?
Protons and O2 are separated from water, the protons pass through the membrane and are evolved into hydrogen atoms by adding electrons
What are the porous transportation layers of a PEM electrolyser made of?
Titanium
Why must the voltage of a PEM cell be carefully controlled?
Voltages greater than 2V cause rapid corrosion
What are the operating conditions of a PEM cell?
Temperature of 60-90 degC
Pressure of 15-20 bar
What are 2 benefits of solid oxide electrolysers?
They have higher efficiency than other methods
They can reduce input voltage requirements by increasing operating temperature
What is the temperature of the steam used in a solid oxide electrolyser?
700-1000 degC
List 5 Capital Expenditures (CAPEX)
Engineering cost
Site preparation
Land cost
Material cost
Cost of systems
What are the 2 types of Operational Expenditures (OPEX)? Give some examples of both
Fixed: Labour, licensing, taxes, maintenance
Variable: environmental surcharges, waste treatment
How is Net Present Value (NPV) calculated?
The lifetime sum of CAPEX and OPEX divided by a discount rate
How can hydrogen be integrated with existing infrastructure?
It can use existing gas pipelines for transport
Why can hydrogen gas not be used as a fuel immediately after being generated?
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
Why can’t hydrogen be stored in metal containers?
Hydrogen will react with the metal
How can hydrogen be stored other than as a pressurised fluid?
Adsorption into solid form
Formation of a chemical hydride
At what temperature does hydrogen condense at atmospheric pressure?
-253 degC
How can life-cycle sustainability be improved?
Reduce resource use
Improve socio-economic performance
Promote interlinking of key aspects
What are the 3 key concerns for choosing a scheme? What has recently been considered an addition to this list?
Security, affordability and sustainability
Community benefit is to be added moving forward
What is the threshold for blending hydrogen with natural gas at which the risk of explosion increases?
5% Hydrogen
How are hydrogen fuel cells similar to hydrogen electrolysis cells?
They work in revers to turn the hydrogen into a flow of electrons and OH-
What is the most efficient type of hydrogen fuel cell and what are its downsides?
Solid oxide cell
High operating temperature and long startup time
What is the benefit of synthetic carbon based fuels?
The process of creating the fuel theoretically removes enough carbon from the atmosphere to balance the emissions of burning it
Why is ammonia more commonly used as a fuel than pure hydrogen?
It contains a higher density of hydrogen atoms per m3
Under what conditions is ammonia produced?
Temperatures 300-550 degC
Pressures 15-30MPa
Iron, potassium oxide or aluminium oxide
At what temperature will ammonia become liquid?
-33.3 degC
When is ammonia production most efficient?
When the process is running consistently with a constant supply of hydrogen
How do you find the charge required to electrolyse a product?
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
How do you calculate the Hydrogen production rate in 4 steps?
- Calculate the charge from the number of electrons involved
- Calculate the mol/s rate by using I instead of Q in the calculation of number of moles of Hydrogen
- Use the mass of a mole of Hydrogen to find the kg/s production rate
- Convert units as needed
What is the equation for power generation?
W = VI
W is power
V is voltage
I is current
What does the value V0th symbolise?
The operating voltage at which no heat loss occurs
How do calculations change for electrolyser stacks with X number of cells?
Molar production rate is multiplied by X
V0th is multiplied by X before being subtracted from operating voltage
What is important to check when questions ask about the effects of changing temperature and pressure in an electrolyser cell?
If the production rate is independent of these factors or not
What is the equation for the power necessary to compress a gas with a molar production rate of n?
W = nRT*ln(Pout/Pin)
R is the gas constant
T is the temperature IN KELVIN
What are the 3 key components of an alkaline water electrolyser?
Anode (+ve)
Cathode (-ve)
Microporous Diaphragm
What are the two outputs of water electrolysis and where do they form?
O2 at the Anode
H2 at the Cathode
In water electrolysis, at which node is water (H2O) absorbed and at which is it produced?
Absorbed at the cathode, produced at the anode
If you have calculated H2 production rate from electrolysis, how do you find O2 production rate?
Divide nH2 (mol/s) by 2, as 2H2 are produced to every 1O2
What is the value of Faraday’s Constant,F?
96485 C/mol
What ions are transferred across PEM and Solid Oxide electrolysers respectively
H+ for PEM
O2- for Solid Oxide
How do you find the number of compression stages, N, for compression ratio x?
log(Pout/Pin)/log(x)
What is the equation for the power of a multistage compressor?
P = Nz/ηisen * nRT * k/k-1 * [{(Pout/Pin)^k-1/k} -1]
z and k are given factors, z is compressibility factor
What is the equation for the Haber-Bosch process, the formation of ammonia?
3𝐻2 + 𝑁2⇌ 2𝑁𝐻3
If given the thermodynamic parameters of water electrolysis, how do you find the power demand of the electrolyser system?
ΔH * production rate of hydrogen (kg/s)
