A UK hydrogen economy?

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.

Making hydrogen from natural gas would create more CO2 than simply burning CH4.

The thermodynamics.

Hydrogen

Thermal properties and production of hydrogen

The higher and lower calorific values of hydrogen are:-

Higher heating value (HHV)  = 142 MJ/kg = 39.4 kWh/kg

Lower heating value (LHV) after latent heat is subtracted = 120 MJ/kg = 33.33 kWh/kg

The lower value is obtained by combustion in an engine. 

 

Hydrogen does not occur naturally and has to be extracted and processed before it can be used as a transport fuel.

 

There are four processes, viz.,

  (i) Steam reforming of methane

 (ii) Electrolysis of water

 (iii) Compression

 (iv) Cryogenic liquefaction.

 

These can be combined as             (i) and (iii) or (i) and (iv)

       or (ii) and (iii) or (ii) and (iv)

 (i) Steam reforming of methane

 

Hydrogen can be extracted from methane by steam reforming in two stages. 

2CH4 + 3H2O = CO + CO2 + 7H2 and CO + H2O = CO2+ H2

32 kg methane with 72 kg steam yields 16 kg H2 and 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.

The 7 kg steam required contains 24 MJ total heat, bringing the input to 161.5 MJ/kg or 45 kWh/kg.

 

The equivalent power used to obtain the energy of 33.33 kWh/kg in the hydrogen is 45 kWh/kg and 7 kg of CO2 is released. Thus more energy is used to extract hydrogen from the methane than obtained in it.

While natural gas remains available it is more efficient to use methane directly and less carbon for the useful energy obtained would thereby be released. The production of hydrogen from natural gas does not appear to be worthwhile. 

(ii) Electrolysis of water

Electrolysis can consume between 3.7 and 4.5 kWh/Nm3 of hydrogen, which taking the mean is gravimetrically 58.6 kWh/kg (Say 59)

 

(iii) Compression

The 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)

 

(iv) Liquefaction

For 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)

 

So,       (ii) + (iii) is 59 + 5 = 64 kWh/kg

and    (ii) + (iv) is 59 + 16 = 75 kWh/kg

 

Equivalent quantity required for UK transport in 2019 as an example

 

Cars 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 109 kg H2/annum, while aircraft energy of 630 PJ (equivalent to 15 million tonnes of jet fuel) would require 5.25 x 109 kg H2/annum.

 

This works out at 26 x 64 x 109 = 1664 TWh for compressed gas

and 26 x 75 x 109 = 1950 TWh for liquefied gas.

For aircraft it works out at 5.25 x 75 x 109 = 394 TWh

 

For 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.


 

Conclusion 

 

Domestic heating

 

Reforming natural gas to hydrogen releases more carbon dioxide than simply burning it means that for domestic consumers this is an impractical prospect.

 

Using 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.

 

Transport

 

While 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).

 

To substitute hydrogen for liquid fuels in the UK would require 5 to 7 times more electricity generation an impossible concept.

A better option, if sufficient quantities of liquefied natural gas (LNG) will be imported, would be to transfer it directly from an ocean gas tankship, via interim storage at an import terminal, to containers on large road vehicles otherwise fuelled with diesel. This would avoid re-gasification at the terminal and liquefying the gas a second time.   

 

Global air transport

 

The 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