Off-shore wind power provides the UK with the electricity for hydrogen production and its storage by electrolysis. With the excessive carbon emissions for uranium extraction, during long construction times and with the massive concrete structures to be built, the energy needed for spent fuel disposal and decommissioning, nuclear power is disqualified for hydrogen production as its later carbon-free generation never catches up with its historical emissions.
hydrogen from natural gas would create more CO2 than simply burning CH4.
properties and production of hydrogen
higher and lower calorific values of hydrogen are:-
heating value (HHV) =
142 MJ/kg = 39.4 kWh/kg
heating value (LHV) after latent heat is subtracted = 120 MJ/kg = 33.33 kWh/kg
lower value is obtained by combustion in an engine.
does not occur naturally and has to be extracted and processed before it can be
used as a transport fuel.
are four processes, viz.,
(i) Steam reforming of methane
(ii) Electrolysis of water
can be combined as (i)
and (iii) or (i) and (iv)
or (ii) and (iii) or (ii) and (iv)
Steam reforming of methane
can be extracted from methane by steam reforming in two stages.
2CH4+ 3H2O = CO + CO2+ 7H2and
CO + H2O = CO2+ H2
kg methane with 72 kg steam yields 16 kg H2and releases 88 kg CO2, but the process is only 70%-90%
efficient, so the yield is reduced to 12.8 kg, assuming 80% efficiency.
2.5 kg methane (2.5 x 55 MJ = 137.5 MJ) is needed to yield 1 kg hydrogen (120 MJ)
while releasing 7 kg CO2.
7 kg steam required contains 24 MJ total heat, bringing the input to 161.5 MJ/kg
or 45 kWh/kg.
equivalent power used to obtain the energy of 33.33 kWh/kg in the hydrogen is 45
kWh/kg and 7 kg of CO2is
released. Thus more energy is used to extract hydrogen from the methane
than obtained in it.
Electrolysis of water
can consume between 3.7 and 4.5 kWh/Nm3of hydrogen, which taking the mean is gravimetrically 58.6 kWh/kg
energy used to compress hydrogen to a suitable storage pressure is around 12% of
the HHV or 0.12 x 142 MJ/3600 KJ = 4.7 kWh/kg (Say 5)
large scale plants the energy used to liquefy hydrogen is around 40% of the HHV
or 0.40 x 142 MJ/3600 KJ = 15.8 kWh/kg (Say 16)
(ii) + (iii) is 59 + 5 = 64 kWh/kg
(ii) + (iv) is 59 + 16 = 75 kWh/kg
quantity required for UK transport in 2019 as an example
use compressed hydrogen (Honda) or liquefied hydrogen (BMW and GM) while
aircraft will need to use liquefied hydrogen because of weight and space
requirements. An effective energy content of 120 MJ/kg H2,
means that vehicle energy of 2520 PJ (equivalent to 60 million tonnes /annum of
petrol and diesel) would require 21 x 109kg H2/annum, while aircraft energy of 630 PJ (equivalent
to 15 million tonnes of jet fuel) would require 5.25 x 109kg H2/annum.
works out at 26 x 64 x 109=
1664 TWh for compressed gas
26 x 75 x 109=
1950 TWh for liquefied gas.
aircraft it works out at 5.25 x 75 x 109= 394 TWh
the UK this means that to substitute for the 2019 level of road and air
transport fuel with hydrogen would require from 1664 TWh to 1950 TWh of
electricity generation, compared to the total UK generation of 300 TWh in 2020.
natural gas to hydrogen releases more carbon dioxide than simply burning it
means that for domestic consumers this is an impractical prospect.
air to water heat pumps for hot water yields a coefficient of performance of
just 2, compared with air to air heating of 4. It cannot compete with off-peak
electrical heating at half price.
in the future a favoured minority will use hydrogen and electricity-propelled
road vehicles, this would be such an inefficient use of renewable electricity
that road transport will be substituted by rail (which can use electricity
directly in traction engines).
substitute hydrogen for liquid fuels in the UK would require 5 to 7 times more
electricity generation – an impossible concept.
establishment of a global hydrogen infrastructure for air transport is an
impracticable prospect. A trebling of 2000 global air traffic once envisaged by
2030 would require electricity generation of 7000 TWh/annum to be able to
substitute liquid hydrogen for 260 million tonnes/annum of jet fuel. It would,
in any case, mean that every airport in the world would have to have hydrogen
fuel available for re-fuelling for return to base
John Busby 26 November 2021