Since my German is limited to:
'Eine steiner, danke'
I will not attempt to research the studies showing that it is OK to transport hydrogen largely in existing natural gas pipes, albeit at a lower energy flow rate than for the same pipe and pressure as NG.
I did dig out the recent findings of the National grid here in the UK, and document it here:
https://www.greencarcongress.com/2024/07/20240724-pennstate.html

Tech paper:
https://www.nationalgas.com/document/146076/download

No doubt Germany and other countries in Europe have done or are doing equivalent studies.

I also argue that since a lot of energy presently supplied by natural gas will be done with heat pumps, solar etc, and home fuel cells rather than gas boilers use hydrogen with very high electrical plus thermal efficiency, less capacity is needed anyway, so the lower flow rate is acceptable, although of course more expensive per KWh

For the UK transportation and distribution costs make up around 34% of the bill:
https://www.sefe-energy.co.uk/insights-resources/gas-transportation-charges-explained/

Writing off the gas network and going all electric would also be a darn expensive as well as disruptive proposition though.

Converstion of the NG network in Europe does not directly read across to the US, as specification differ.

But those who confidently proclaimed its impossibility were arguing in my view from an agenda.

There are massive battles ahead here in the UK on electricity transmission lines, with the new government planning a huge expansion of onshore wind turbines, after a decade or so of none being built.

https://www.theguardian.com/environment/article/2024/jul/20/pylon-english-councils-fight-ed-miliband-clean-power-revolution-rural-areas

' National Grid said the cost of the Lincolnshire pylon scheme is about £1.1bn, while the cost would be about £6.5bn for underground cabling and about £4.4bn for undersea cabling.'

To nail my colours to the mast, I am completely in favour of the transition to clean energy, and electric transmission of it as well as using hydrogen where that is more appropriate.

I prioritise respect for the land however, and don't regard that as something to be traded off or sacrificed.

So power transmission, yes, but underground or undersea, not with pylons marching across the vistas which make living in the UK a joy.

And solar farms too, with agrivoltaics, where it is blended in with farming, not by creating a desert of factory floor landscapes.

The Canadian Transition Accelerator group has published a very readable technical brief called "The TechnoEconomics of Hydrogen Pipelines" (https://transitionaccelerator.ca/wp-content/uploads/2023/06/The-Techno-Economics-of-Hydrogen-Pipelines-v2.pdf).

My understanding from a quick read through this brief is that the network of smaller diameter low pressure pipes which deliver natural gas to end users is built of low strength steel which resist H2 embrittlement well and can therefore be re-purposed to carry hydrogen without a significant reduction in the volume of delivered energy. These pipes will require new valves and a more careful inspection regime which will add to the cost of operation.

On the other hand the large diameter, high pressure, high flow volume pipelines are built of high strength steel to reduce cost since you need less high strength steel to withstand a given amount of pressure. Unfortunately high strength steel does not resist H2 embrittlement nearly as well as low strength steel and therefore re-purposing these pipes to carry H2 may require significantly lower H2 flow rates which could conceivably be an economic deal breaker. Even for purpose built H2 pipelines the technical brief claims that the energy flow rate will be 12% less than for natural gas pipes.

Building new large diameter pipelines optimized for H2 transport will be an expensive proposition. The levelized costs are reasonable. In the technical brief they go through a sample calculation for a 1500km 36 inch pipe and get cost 0.87C$/Kg H2=0.62US$/Kg H2. However this cost is for a mature pipeline which has been carrying high volumes of H2 for many years. Getting from here to there is not a trivial economic feat.

Here is the link Transition Accelerator mention on hydrogen compression:

https://transitionaccelerator.ca/reports/technical-brief-the-techno-economics-of-hydrogen-compression/

I ain't got the technical chops from that to work out how much moving hydrogen about instead of natural gas is likely to bump up costs, but they do say in 'Summary and Outlook':

' Although compression of natural gas is widely used, the compression of H2 is significantly challenging due to its low molecular weight and density. Currently available compressors which rely on mechanical pistons are expensive and reported efficiencies are low when compressing H2 to high pressures (> 200 bar).
Moreover, they suffer from frequent mechanical failure which increases operating expenses. Further research and development activities are needed to design high efficiency compressors that can deliver H2 at high pressures without compromising on the purity and reliability. The development of new technologies such as those based on ionic liquids or metal hydrides is promising. In particular, ionic liquid
compressors which have been particularly developed by, Linde, could be the key to efficient and low cost H2 compression. These compressors do not require bearings or seals, two of the common sources of failures in piston and diaphragm compressors.
Finally, as we move towards the implementation of net-zero energy systems, the capital costs associated with compressors is forecasted to drop sharply with economy of scale. These are exciting times and present both challenges and opportunities for different stake holders involved in the H2 economy'

This report is from 2021, and things will have moved on since then, in particular I believe in the field of ionic compressors.

@dursan

You have to have power there to transmit.
Most of the power for both Germany and the UK is used in the winter, and both have lousy solar resources.

Fortunately the winter is when the wind blows strongest, but of course not reliably exactly when the power is needed.

Battery storage is realistically only economic for around 4 hours.

Using hydrogen creates an inherent buffer, both in the pipelines and it can of course be stored in salt caverns - we have to get a move on to excavate some out in the UK, as the Germans are doing.

If you are relying exclusively on the electric grid, and not using nuclear, when you turn the switch on, it may well not work, as there is no power available, which is somewhat annoying.

@dursan

Norway:
Yep.
A battery which is already used for as many valleys as the Norwegians are prepared to flood and which especially in dry years can only cover a fraction of European demand even at present levels of electrification