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Hydrogen versus Methane (aka Natural Gas)


Image https://unsplash.com/@andywatkins on https://unsplash.com

Yesterday Whitby residents were given the choice of staying with Natural Gas instead of being forced to take part in a Hydrogen home heating & cooking trial as a result of a tremendous amount of pressure by residents on the local council, government, and Cadent.


It’s worth witnessing the effort in the video recording of the council’s engagement session earlier in February 2023 to get to this point.


Burning Hydrogen mostly just emits water vapour (more below) whereas burning natural gas (which is 90% Methane, so from now on I’ll refer to it as Methane) emits Carbon Dioxide. It’s also created by electrolysis so is as green as the electricity it’s produced from so Hydrogen is claimed to be far better for the environment. But that’s about as good as it gets.


There are plans to mix up to 20% into the Methane network with the long term aim eventually for 100% Hydrogen for heating our homes and cooking our meals. And that’s where a bunch of issues occur.


First, where does the Hydrogen come from and is it efficient?

Hydrogen doesn’t naturally occur so has to be produced by electrolysis; pass electricity through water and you get bubbles of Hydrogen released. That gets captured, compressed, transported (more below) then decompressed and burnt. The result is about 50% efficient; you lose half the energy through that process, so if a wind turbine generates 1kWh of electricity we’ll get 0.5kWh of usable heat when it’s burnt.


Air Source Heat Pumps (ASHP) have an efficiency between 250% and 450% so for example that 1kWh of wind generated electricity results in 2.5kWh to 4.5kWh equivalent of heat in our homes rather than 0.5kWh from a Hydrogen boiler.

If we allow for some losses in electricity transmission and a lowish average heat pump efficiency and take 300% efficiency, the maths is easy - heating with an ASHP is 6 times as efficient as burning Hydrogen, or we can also say ASHP heating requires 1/6th the electricity needed for producing and burning Hydrogen.


Read that again - we require 1/6th of the extra electricity generation to heat our homes with heat pumps versus Hydrogen when we completely transition off Methane.


Transitioning from Methane to electrified heat over the next 10 to 20 years does mean a large electricity generation increase - so it is a big figure. But if all you see is a big electricity generation increase without a reference point then it’s too easy to get blinded by the size of the number but not the comparison. For example at a recent event I spoke at a senior Ofgem official, talking about Hydrogen, exclaimed to me “think of all the electricity needed to power all those heat pumps if we come off [Methane] gas” to which my reply was of course “yes and you’ll need 6 times that if they’re on Hydrogen instead”.



Second, how do you transport it?

Hydrogen molecules are ⅛ the size of Methane so they’re going to squeeze and leak through pipework materials, joints, seals and valves that were only designed for Methane. Hydrogen also has a greenhouse gas warming effect as much as 11 times worse than CO2 so the slightest leak has a much greater impact than the CO2 emitted from burning Methane.


That’s a lot of gas network upgrading and leak risk mitigation to sort out. On Twitter I shared my experience of asking a Cadent gas engineer about upgrading our street to Hydrogen. Looking at his app that showed the local pipelines he could see the gas pipe down our street was metal so would require a new plastic pipe pushed down inside the existing metal pipe, then a hole dug outside each property to then push and connect a plastic pipe to each house. Then a new meter installed. Our house (1960’s) has metal pipework from our gas meter to our living room fire so that would need replacing too which means opening up the ceiling and walls. That’s a lot of work, a lot of disruption and very costly.


It’s interesting to note that the Whitby Hydrogen trial is now going to involve a completely new gas grid being installed in parallel to the existing Methane pipework as too many residents have objected to being forced to take Hydrogen. The trial is now much more likely to fail as a dual rollout of Hydrogen and Methane is an even sillier idea.


Third, energy density is a problem.

The energy density of a Kg of Hydrogen is 2.5x that of a Kg of Methane - that’s because those very small molecules are packed more densely. If you have the kit for receiving and managing highly compressed Hydrogen then the energy density by weight is an advantage. There’s a very small number of Hydrogen cars around and they have high compression tanks on board so there's enough energy to get any distance from a Hydrogen powered car. And in fact by weight Hydrogen has 3 times the energy density of petrol and diesel.


But our homes are supplied by pipes at much lower pressure so the volume-density matters instead. By volume it has only ⅓ the energy density of Methane so that means 3 times the flow rate through the gas system to the boiler to get the same heating energy. That means higher pressure to force through that Hydrogen 3 times as fast through all the gas pipes to your home. Will the pipes take it, what flow issues will there be, will the pipes whistle, a leak will become very obvious and very dangerous very quickly.


Fourth, Hydrogen is much more flammable than Methane and has NOx emissions.

Pure Hydrogen or Methane is hard to burn so has to be mixed with Oxygen from the air around us. Methane is fussy and burns properly with a mixture of 7% to 20% Oxygen - hence opening the window if you’ve got a gas leak will dilute the Methane enough that it’s much less at risk of catching light. Hydrogen isn’t as fussy and will burn with anywhere from 4% to 75% Oxygen mix. A gas boiler is therefore designed to create the best mix of gas and air when it’s burnt and you’d have thought the broader mix for Hydrogen is an advantage but really it means it’s got a greater chance of going boom if there’s a leak and that boom will be bigger than with Methane. That means careful design of the combustion chamber and tightly controlling the flow of Hydrogen and air mix.


That boom is also going to be more impressive because of the ‘flame speed’ - this is how fast the flame travels through air. Hydrogen’s flame speed is 10 times faster than Methane so that bang is going to be bigger and faster.


I mentioned above that Hydrogen mostly emits water vapour but there are also nitrogen oxide emissions too. Hydrogen burns at a higher temperature which means the Nitrogen naturally present in air can result in higher nitrogen oxide emissions than when burning Methane. This can be mitigated by design of the combustion chamber and air mix to keep the temperature lower.



The health and safety report into trialling 100% Hydrogen says “The consequences of the largest domestic hydrogen leak and subsequent explosion scenario are predicted to be more severe than those of the largest domestic natural gas explosion by the consequences model”. This is to be mitigated with two Excess Flow Valves to automatically close the supply and the meter to be installed outside the property - as a homeowner I’d want those to be out on the street and not in the normal external meter cabinet on the front or side of the house (our gas meter, until we had it removed, was in our utility room - so more change to be Hydrogen compatible).

There’s also a requirement for a permanent vent in each room there’s a gas appliance; for us that would’ve been the utility room (gas boiler), kitchen (gas hob) and living room (fireplace). The vent needs to be a minimum of 10,000mm2 in size, permanently open and no less than 500mm from the ceiling (just above head height), so that’s a 10cm x 10cm hole that I could easily put my fist through in each of those rooms. A total hole size of 30,000mm2 (half a sheet of A4) is quite a substantial amount of heat loss I’d not fancy in my house!


In summary if we transition to Hydrogen for heating:

  • We’ll build 6 times as much extra electricity generation,

  • Risk having to install a whole parallel Methane grid,

  • Have bigger explosions,

  • Have to re-pipe our home supply and replace & relocate our gas meters,

  • Have more leaks and those leaks will have an 11 times greater greenhouse gas warming impact than CO2,

  • Have greater NOx emissions than burning Methane,

  • Have several prominent ventilation holes high on the wall totalling A5 size.

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