The most frequently proposed ways to heat buildings in a low carbon future are using hydrogen to power hot water boilers or electricity to power heat pumps. There are two low-carbon ways to make hydrogen: These are known as ‘green’ and ‘blue’. This article compares the various options on the basis of energy efficiency, carbon emissions, infrastructure requirements and technology readiness.
Key Take-Aways (TLDR)
- Heat pumps are far more efficient than Green Hydrogen for heating buildings. The ‘wind-to-heat’ energy consumption of heat pumps is 1/6 that of green hydrogen, for delivery of the same amount of heat. Therefore the energy generation costs for heat pumps are 1/6 of those for Green Hydrogen.
- As a consequence of the inefficiency of the Green Hydrogen route, it would require an inordinate amount of renewable electricity to heat the UK’s buildings: approximately 40 times the current installed capacity of offshore wind. The heat pump route would require significantly less additional renewable electricity.
- In the UK in 2020, heating a building with an electric space heater creates about 20% lower carbon emissions than heating the same building with a natural gas boiler. This percentage will improve with time as the electricity supply becomes cleaner (lower carbon).
- Electric space heating will always generate about half the carbon emissions of Green Hydrogen boilers. Electric space heaters are available off the shelf now and require no additional infrastructure.
- The Blue Hydrogen route for heating buildings would require a 25% increase in the amount of natural gas imported into the UK, taking the imports to 60% of national consumption. This would be detrimental to the balance of trade and energy security.
- Neither the Green Hydrogen nor the Blue Hydrogen route is ‘clean’ Both generate substantial carbon emissions. In 2020, the Green Hydrogen route emits 50% more carbon than burning natural gas in a condensing boiler.
- Blue Hydrogen will always generate significant ‘fugitive’ CO2 emissions, that escape into the atmosphere. Consequently, use of Blue Hydrogen for heating would prevent the UK government from meeting its legal commitments for ‘net zero’ emission by 2050.
- Heat pumps generate 1/4 of the emissions of a natural gas boiler in 2020 and this will reduce significantly with time as the electricity grid becomes cleaner. They are the most effective way to reduce carbon emissions from heating. Heat pumps are available off-the-shelf, now.
- It is unlikely that the infrastructure needed for Blue or Green Hydrogen could be built in time for 2040.
- Government policy should promote the of use of heat pumps for heating new and retrofitted buildings and should reject Hydrogen as an option for heating.
‘Green hydrogen’ is created by passing renewable electricity through pure (distilled) water. This splits the water into hydrogen and oxygen gases. The hydrogen can be compressed and pumped through pipes to consumers and then burned in hydrogen-ready condensing boilers to heat buildings.
The efficiencies of the main steps in the Green Hydrogen process are shown in the right path of figure 1. Multiplying these efficiencies together gives an overall end-to-end efficiency of 46%. The largest proportion of losses occurs in the electrolysis step, which is at best 75% efficient. The consequence is that 100kWh of renewable electricity would yield 46 kWh of heat in the building.
The middle branch of figure 1 shows transmission of the electricity via the electricity grid to a consumer, where it powers a 95% efficient electric space heater (of the type you can purchase in a department store). This route would deliver 86kWh to the building: nearly 90% more heat than a Green Hydrogen boiler, for the same amount of renewable electricity. A space heater can be plugged-into the existing electrical infrastructure.
The left hand branch of figure 1 shows use of grid electricity to power a heat pump. The big advantage of this route is that the heat pump delivers 3-4 times more heat into the building than the electricity it uses. (See a nice explanation of how this works in ‘Sustainable energy without hot air‘ by the late David Mackay). Assuming a realistic ‘Coefficient of Performance’ (COP) of 3.0; the output of the heat pump route would be 270kWh of heat.
Conclusions: An electric space heater will deliver nearly 90% more heat per kWh of input electricity than a Green Hydrogen boiler. A heat pump would deliver (270/46) = 6 times (600%) more heat from the same amount of electricity as the Green Hydrogen route.
What are the implications of this factor of 6?
Firstly, the country would have to build six times the number of additional wind turbines or solar panels or nuclear power stations to generate the electricity in the Green Hydrogen case than for heat pumps. Consequently, consumers would have to pay 6 times the price for the energy to heat their homes. Alternatively, the government would have to subsidise the cost of hydrogen, which would have a permanently damaging effect on the economy. There would also be very substantial environmental impact in building and operating the turbines.
Secondly, in2018, the UK used about 300 TWh of natural gas for ‘domestic use’ – mainly heating buildings . (1 TWh is 1 billion kWh). Making the crude assumption that the heating was done over 6 months of the year, the 300TWh would correspond to an average heating power of about 70GW.
If this 70GW of heat was supplied by heat pumps, it would require 70/2.7 = 26 GW of additional renewable electricity. (For comparison the average electric power demand of GB throughout 2019 was 31GW.) This 26GW could be provided by offshore wind turbines, as follows:
- The average ‘power factor’ of wind energy in the North Sea is 38.9% . So the 26GW of electricity would require 26/0.389 = 67 GW of installed wind turbine capacity, corresponding to 5,600 of the largest (12MW) offshore wind turbines.
- The average power density of recent large scale wind turbine projects in the North Sea is 7.4W/m2 , so these turbines would require a sea area of approximately 9,000 km2 (a circle of radius 107km).
If the 70GW of heat was supplied by boilers burning green hydrogen, it would require 70/0.46 = 150GW of additional renewable electricity.
- Using the same calculation as above, this would require 70/(0.46×0.389)=385 GW of installed wind turbine capacity, which could be supplied by 32,000 x 12MW turbines.
- This would correspond to a sea area of 52,000km2 ( a circle of radius 257km).
The scale of these numbers is shown in figure 2. This chart was adapted from a figure in , by overlaying scaled areas of 9,000 km2 and 52,000km2. It can be seen that if the UK was to use green hydrogen for heating, a very large area of the North Sea would need to be covered in wind turbines. Has the scale of this challenge been recognised by those advocating Green Hydrogen?
For comparison, all existing offshore wind installations in the North Sea are plotted in the small red polygons on figure 2. There are currently 2,300 offshore wind turbines in UK waters, with an installed capacity of 10.4GW . So generating sufficient green hydrogen for heating UK buildings would require this to be increased by a factor of nearly 40. Some of the sea areas under consideration for future expansion in ‘Round 3’ of the UK’s offshore wind development are the blue polygons labelled ‘East’ and ‘West’ in figure 2. In total the Round 3 projects amount to another 18GW, of planned capacity which will take the total to about 30GW. This is less than half that required to decarbonise the UK’s heat by the heat pump route.
There is one important caveat here. There is a very significant variation in gas demand for heating from hour to hour. The peak demand can be as much as 170GW, with a ramp rate up to 60 GW/hour. . Natural gas infrastructure can absorb this variation for short periods as it is equipped with various kinds of effective storage. Improved thermal insulation and increased ‘thermal mass’ in buildings can help reduce the peak. However any future system for heat energy must include significant energy storage. This is the subject of another blog post, in progress… It will appear here when it is finished!
Conclusion: The renewable energy generating capacity needed for heating with green hydrogen would be extreme: requiring the UK’s existing offshore wind generating capacity to be increased by a factor of 40. Use of heat pumps would require the existing offshore wind generating capacity to be increased by a factor of around 6.5… still a very significant amount.
It is commonly thought that heating using green or blue hydrogen is ‘clean’, ie zero carbon. Unfortunately this is not correct.
‘Green’ hydrogen is generated using electricity that comes from the national electricity grid. Projections of the future ’emissions intensity’ of the electricity grid are published by the UK governments Department of Business, Energy and Industrial Strategy (BEIS) . In 2020, every kWh of electricity generated results in emission of 136 gCO2. The ’emissions intensity’ of the grid, is therefore 136 gCO2/kWh. This intensity is gradually reducing as coal-fired power stations are phased out and the amount of renewable generation (wind, solar) increases. So electricity is becoming ‘cleaner’ with time. (Note that other estimates are different to those in , for example Drax reported a grid intensity of approximately 200 gCO2/kWh in 2019.)
The projected future carbon emissions from heating the UK’s buildings can be estimated roughly using the ‘EEP 2018’ carbon intensity projections in  and dividing by the ‘wind-to-heat’ efficiencies in figure 1. For example, in 2020 a heat pump using grid electricity will generate carbon emissions of 136/2.7=50 gCO2 per kWh of heat delivered. Similarly, a space heater will generate carbon emissions of 136/0.86 = 158gCO2 per kWh of heat delivered and a Green Hydrogen boiler will generate carbon emissions of 136/0.46=300gCO2 per kWh of heat delivered. (The ratio of the Green Hydrogen to the heat pump, ie 300/50 is the same factor of 6).
The results of this calculation using future projections of emissions intensity are shown in figure 3 for Green Hydrogen boilers, space heaters and heat pumps. (Note that this calculation is approximate only.. If the huge amount of Green Hydrogen implied by this calculation was generated by new renewable electricity plant, the grid intensity would fall significantly below the BEIS projections in .) Also plotted in red on figure 3 is the level of carbon emissions generated by a modern condensing boiler burning natural gas, which is approximately 200 gCO2 per kWh of heat delivered.
From figure 3 it can be seen that Green Hydrogen boilers would emit about 50% more carbon than natural gas boilers in 2020 and will not deliver lower carbon emissions than natural gas boilers until nearly 2030. Conversely, heat pumps will deliver about 1/4 the CO2 emissions of a natural gas boiler in 2020 (ie 75% reduction), reducing to just 15gCO2 per kWh of heat delivered by 2035. Green Hydrogen boilers won’t reach the emissions performance of 2020’s heat pumps until around 2040. In 2020, heating buildings using electric space heaters generates lower carbon emissions than gas boilers and much lower emissions than Green Hydrogen.
Conclusion: Heat pumps would reduce carbon emissions by 75% in 2020 compared to gas boilers: delivering immediate, deep cuts in carbon emissions. Green Hydrogen would emit 50% more carbon than conventional gas boilers in 2020 and will not drop to the same level as existing gas boilers until around 2030.
- Technology for large-scale electrolysis is not well developed. The largest electrolysis plant in the world is currently being built by Hydrogenics in Canada. It has a capacity of 20 MW. Approximately 7,500 of theses plants would be needed to generate the 150GW of Green Hydrogen, just to heat the UK’s buildings. Unless there is a major development in electrolysis technology, it seems unlikely that the installed capacity of electrolysis plants could reach the necessary scale by 2040.
- Heat pumps (and space heaters) are standard, mature, off-the shelf technology that can be purchased now. The International Institute of Refrigeration estimates that there are currently 220m heat pumps in use around the world . They come in two main varieties: ‘air-source’ heat pumps are better for retrofitting existing buildings and ‘ground-source’ heat pumps are more efficient, though more expensive, and easier to install in new buildings .
There are some disadvantages of using heat pumps – for example, they require some modification to the heating system in each building and they are currently more expensive than gas boilers (though the prices are expected to drop as demand increases). However these issues pale into insignificance compared with the magnitude of the task of generating sufficient electricity for green hydrogen.
Conclusion: Heat pumps are available off-the-shelf now. It is unlikely that Green Hydrogen could be scaled to heat the UK’s building stock by 2040.
The alternative method for generating Hydrogen is to make it from methane (natural gas) by a process called steam methane reforming (SMR). SMR strips the Carbon atoms from Methane (CH4), creating CO2 and Hydrogen (H2). If the resulting CO2 is captured and stored ‘sequestered’ permanently underground (see figure 4), the resulting hydrogen is labelled ‘Blue’. The Blue Hydrogen can be compressed and pumped through pipes to consumers and then burned in hydrogen-ready condensing boilers to heat buildings.
It has been argued that ‘Blue’ Hydrogen generated by SMR could replace natural gas for heating and transport in a Hydrogen Economy. For example, the ‘H21’ project in the North of England  proposes to extract natural gas from the North Sea oil fields, convert it to Hydrogen by SMR at facilities on the UK coast, inject the Hydrogen into the National Transmission System (NTS = the ‘gas grid’) and pump the CO2 back into empty oil/gas wells, to be sequestered under the sea (‘Carbon Capture and Storage’ – CCS).
Hydrogen has a significantly lower energy content per unit volume (‘Lower Calorific Value’ LCV=10.8 MJ/m3 ) than Methane (LCV=35.8 MJ/m3 ) . See the top two central boxes in Fig. 4. The factor of 35.8/10.8 = 3.3 means that transferring the same amount of energy to consumers through the NTS using Blue Hydrogen instead of Methane, at the same transmission pressure, would require all gas pipes in the system to carry 3.3 times higher volume flow rate of gas. To do this, as a rough approximation, they would need 3.3 times larger flow area or 1.8 times larger internal diameter. Consequently, it is not simply a matter of using the existing gas grid and pumping Hydrogen instead of Methane. The entire gas grid would have to be replaced by pipes with 3.3 times the capacity, in underground trenches nearly twice the diameter. This problem is recognised by the H21 project, which plans to install an extensive network of new Hydrogen gas mains across the North of England .
Of course, it would also be necessary to replace all natural gas boilers with a hydrogen-ready variety and to build the SMR and CCS plant to generate the hydrogen. There is no commercial-scale CCS facility in the UK. It would have to be developed and built before any blue hydrogen could be generated. (In fact, there are only two commercial scale power plants with CCS plants operating in the world. Of these, the Petra Nova plant in Texas, USA was recently mothballed, 3 years after opening. It failed to meet its carbon capture target of 90% and ran at a financial loss. This questions the viability of CCS as part a Blue Hydrogen solution).
Conclusion: The infrastructure required to generate and transmit blue hydrogen would require rebuilding of the NTS (gas grid) and development of CCS technologies. The ‘technology readiness’ of the Blue Hydrogen route is therefore low.
Carbon emissions due to the Blue Hydrogen process are not zero. Carbon capture from flue gases is not a perfect process. There are a number of available methods and technologies. In general, the higher the effectiveness of carbon capture, the more energy it takes and the less efficient the SMR process becomes . Figure 4 includes one version of the SMR+CCS process, (‘SMR with CO2 capture from flue gas using mono-ethanolamine’), from . This is the most effective available CCS process and results in 90% of Carbon being captured from the SMR. However, this particular SMR+CCS process has an energy efficiency of only 69%. Other SMR+CCS processes have higher energy efficiencies (up to 76%) but CO2 capture rates as low as 53% (ie 47% of carbon escapes into the atmosphere). See  for details.
Using data from Table 3 in  it is possible to calculate the CO2 emissions from several different available SMR+CCS processes. The range of results is plotted as a horizontal shaded area on Figure 3. It can be seen that the most effective CCS process will emit the same amount of CO2 as a heat pump in 2021 but forever after, it will emit more carbon than a heat pump. The level of fugitive carbon emissions would make it impossible to reach the net zero carbon commitments of the UK government. The worst SMR+CCS process generates carbon emissions nearly as high as a natural gas boiler.
Conclusion: If heat was provide by Blue Hydrogen boilers, fugitive emissions of CO2 would always be significant- at least 10% of the carbon in the input methane, which would prevent ‘net zero’ emissions targets being reached. This contrasts with the heat pump route which would have decreasing emissions with time, reaching near zero by 2040.
It seems that Blue Hydrogen is not so green after all!
Natural gas consumption
Substantially more natural gas would need to be imported or fracked to supply the blue hydrogen process.
From figure 4 it can be seen that for each 100 kWh of ‘input’ natural gas, 45kWh of heat are delivered via the blue hydrogen route. An equivalent calculation for natural gas heating (including compression and pumping losses and the efficiency of a condensing boiler) indicates that for each 100 kWh of natural gas, approximately 65 kWh of heat reaches the consumer. Consequently, the blue hydrogen route requires 65/45 = 1.45 times (45%) more input gas. Since the UK uses 300TWh of gas per year for domestic purposes, mainly for heating , this 45% would correspond to importing or fracking an additional 135 TWh of natural gas. This would increase gas imports by 25%, from 520TWh to 655TWh . This would mean that 60% of the nation’s natural gas would be imported (mainly from Qatar and Russia), severely impacting energy security. (In fact, National Grid’s future energy scenario sees the UK importing 100% of gas by 2040, because of depleting North Sea reserves. So the additional gas consumption for Blue Hydrogen would make the energy security situation considerably worse.)
Conclusion: The UK would need to use 45% more natural gas for heating if the energy was delivered via blue hydrogen than by natural gas boilers.
The push towards use of Hydrogen for heating is misguided. Burning hydrogen is very inefficient compared with the alternatives. Consequently hydrogen is wasteful of renewable electricity and/or would substantially increase the amount of natural gas used in the country. The carbon emissions caused by burning Blue or Green Hydrogen are significantly higher than those of heat pumps. It is unlikely that the infrastructure needed for a hydrogen economy could be built by 2040.
Hydrogen is a fundamentally poor choice for heating buildings. It should not be on the agenda. A far better strategy is to convert the country’s heating systems to heat pumps. This should be government policy.
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