Heating with Hydrogen + Storage

Summary

This blog addresses the energy requirements of heating UK homes with green hydrogen or heat pumps and examines the effects of adding hydrogen storage into the system.

Key Take-aways (TLDR)

In theory, the electricity generating requirement for a green hydrogen heating system could be halved by installing a storage capability for hydrogen.
The amount of hydrogen storage required would be approximately 2000 salt caverns, each containing 500,000 m³ of hydrogen.
No hydrogen or methane storage project of this magnitude has ever been attempted before, and nothing like it is currently underway or planned, anywhere in the world. There is little chance that it could be achieved by 2050.
With or without the hydrogen storage, the amount of renewable electricity required is gigantic, requiring 3.7 to 7.3 times more renewable electricity than the UK’s 50 GW target for 2030.
About 500 TWh of electricity would be wasted every year by the green hydrogen solution. This would cost the economy approximately 1.1% of GDP every year, probably paid-out as subsidies to consumers to make heating affordable.
The previous conclusion remains – that the only practical way to decarbonise the UK’s heating is with heat pumps and heat networks. Heating the UK with green hydrogen, with or without storage, is near to impossible.

Power Requirements with no energy storage

The UK uses about 300 TWh of natural gas per year for ‘domestic use’ – mainly heating buildings. Assuming that the heating occurs over 6 months of the year, that 300TWh corresponds to an average heating power of about 70GW [1].

Some time ago I produced a graphic that shows the various energy flows needed to deliver this 70 GW of heat to households, starting with renewable electricity. The two parts of that graphic are reproduced below: for heating with green hydrogen in Figure 1 and for heating with heat pumps in Figure 2. (Note that two minor changes have been made since the original figure: (i) the ‘Storage’ and ‘Transmission’ blocks have been drawn separately; (ii) the ‘transmission’ efficiency has been increased slightly to correct a small error in the original figure.)

These diagrams are based on the assumption that there would be no significant energy storage in the system, which implies that the hydrogen (and electricity) has to be generated at the same time as it is used for heating.

Fig 1 – Energy flow: Heating UK homes using green hydrogen, with no hydrogen storage

Fig 2 – Energy flow: Heating all UK housing stock using heat pumps

Figure 1 shows that it would take 367 GW of installed offshore wind capacity to generate sufficient electricity to provide the 70GW of heat using green hydrogen (requiring 30,600 offshore wind turbines and covering a sea area of of 49,600 km²). Figure 2 shows that it would take 67GW of installed offshore wind capacity to deliver the same amount of heat using heat pumps (5,800 x 12MW turbines, covering a sea area of 9,000 km²). Consequently, it would take 367/67 = 5.5 times more electricity generating capacity to heat the UK’s housing stock with green hydrogen than with heat pumps. This is the ratio of the blue areas of wind turbines shown in Figures 1 and 2.

Although the renewable electricity used to generate the hydrogen (in Fig 1) would be available all year round, the analysis above assumes it would only be used for heating in the 6 winter months. An alternative use would need to be found for that electricity during the summer: for example it could conceivably be used to make hydrogen for other purposes, such as to manufacture fertilizer or steel. Similarly the electricity used to heat homes with heat pumps (in Fig. 2) would need to be used for something else in the summer when it is not being used for heating… It could be used to run air conditioners or industrial processes or it could be exported via High Voltage DC (HVDC) interconnector to another part of the world.

Power Requirements with maximum energy storage

The assumption of zero hydrogen storage is probably not realistic. In practice there would likely be some storage available in the hydrogen system: in the ‘linepack’ inherent in the gas distribution network and possibly in some additional underground salt caverns [2]. This would mean that some of the green hydrogen generated in the summer could be stored and used to reduce the demand for renewable electricity in the winter heating months.

It is useful to consider the upper limit of this approach, which would be to store all of the hydrogen generated during the summer months. It would then be possible to generate green hydrogen all year round, at half the rate of Figure 1, and use the stored, summer-generated hydrogen to double the winter-generated hydrogen. This ‘upper bound’ of storage would correspond to half of the annual heating demand. So instead of requiring 367 GW of installed offshore wind turbines (as in Figure 1), it would only be necessary to generate half this amount: 183 GW. This concept is shown in Figure 3.

Fig 3 – Heating all UK housing stock using green hydrogen with sufficient hydrogen storage to get 100% utilization for the electricity generation and electrolysis system.

Amount of Hydrogen Storage Needed

In this scheme, the equivalent of half of the 300 TWh of natural gas used to heat houses in winter would have to be stored (ie 150 TWh). Accounting for the energy losses, using the transmission and boiler efficiencies in Figure 3, the amount of hydrogen to be stored would be about: 150 TWh /(0.85×0.9) = 196 TWh.

According to The American Clean Power Association, up to 100 GWh of Hydrogen could be stored in a spherical salt cavern with volume of 500,000 m³ (diameter of 100m), at a pressure of 200 bar [3]. To store 196 TWh would therefore require 1,960 such salt caverns. This is thought to be possible within the available onshore salt deposits in the UK [4], however construction of salt caverns on this scale is completely unchartered territory. Nothing like it has ever been attempted before, in any country and no significant efforts are underway around the world to create such storage.

Discussion

It is interesting to estimate the cost to the economy of the energy waste associated with hydrogen heating. Assuming full hydrogen storage, 71GW of electric power would be needed year round to generate the green hydrogen to heat the UKs homes in winter. This corresponds to a total of 71 x 365 x 24 = 622 TWh of electricity per year. With an electric solution based on heat pumps, 26 GW of electric power would be needed in the 6 months of winter. This would be a total of 114 TWh of electricity per year. The difference between these is 622-114 = 508 TWh. This is the additional electricity needed for hydrogen heating compared to heat pumps. If this electricity was costed at a low wholesale price of £50 /MWh, its value would be about £25b. This would correspond to about 1.1% of the UK’s GDP of £2.2 trillion. So, by using green hydrogen instead of heat pumps for heating, the UK would waste approximately 1.1% of GDP, which would most likely be paid-out as grants to consumers to make their heating affordable.

According to this simplified analysis, heating the UK’s housing stock reduces to a choice between three possible solutions:

67 GW of offshore wind generation, powering heat pumps; or
367 GW of offshore wind generation, with no hydrogen storage. (5.5 times more electricity generation than the heat pump system); or
183 GW of offshore wind generation (2.75 times more electricity than the heat pump system), with 196TWh of hydrogen storage.

It is unlikely that either system (2) or (3) could be built by 2050. The magnitude of the infrastructure task is far too big. The UK currently has about 14 GW of offshore wind [5] and the national target for 2030 is just 50GW. So 183 GW would be a massive stretch …and don’t forget those 1,960 salt caverns! Building 367 GW of renewable electricity generation by 2050 or 2060 or 2070 is also inconceivable.

Of course, it would be possible to have a system that was a mixture of these three scenarios and it is straightforward to determine the energy requirements by a simple average for whatever mix you choose. For example, If you think that 50% of houses will eventually be heated by hydrogen and 50% by heat pumps, then you would need 50% of one of the hydrogen pathways (Fig 1 or Fig 2) and 50% of the heat pump route (Fig 3).

David Cebon, 6 September, 2023