Adapted from the ‘Hydrogen for heating?’ article by David Cebon – Director of the Centre for Sustainable Road Freight and the Cambridge Vehicle Dynamics Consortium; Professor of Mechanical Engineering, University of Cambridge.
The two most frequently proposed ways to heat buildings in a low-carbon future in the UK are: hydrogen to power hot water boilers, or electricity to power heat pumps. Hydrogen is being suggested in two low-carbon forms, known as ‘green’ and ‘blue’. See our data page for more information on how hydrogen is produced.
This two-part article compares the effectiveness of the options available using these methods on the basis of energy efficiency and carbon emissions (Part 1), and technology readiness and infrastructure requirements (Part 2).
Green hydrogen boilers vs heat pumps
The overall end-to-end efficiency of heating a building with ‘green’ hydrogen produced using renewable electricity is approximately 46% (see Figure 1). The largest losses occur during the electrolysis process, which is 75% efficient at best. As such, 100 kWh of renewable electricity yields just 46 kWh of space heating using green hydrogen.
By contrast, a heat pump delivers 3-4 times more heat than the electricity it uses. Assuming a realistic ‘Coefficient of Performance’ (COP) of 3.0, a heat pump powered by 100 kWh of renewable electricity delivers about 270 kWh of heat into a building (Figure 1) – six times (600%) more heat than supplied from a green hydrogen boiler using the same amount of electricity.
Figure 1: Heating buildings using electricity or hydrogen.
What are the implications of this heat pump factor of 6?
- Six times more wind turbines, solar panels or nuclear power stations are needed to generate the electricity for green hydrogen heating than for heat pumps.
- Consumers pay six times the price for the energy to heat their homes.
- Or, the government subsidises the cost of hydrogen heating, causing a significant drain on the economy.
- Significant environmental impact from building and operating the very large amount of electricity generation needed to produce the hydrogen.
Has the scale of the challenge been recognised by the green hydrogen advocates?
In 2018, the UK used about 300 TWh of natural gas for ‘domestic use’ – mainly heating. Roughly assuming the use was over six months, this is equivalent to an average heating power of 70 GW.
Supplying this with heat pumps (as per Figure 1) would require 70/2.7 = 26 GW of additional renewable electricity. This 26 GW could be provided by 67 GW of installed offshore wind turbines. However, supplying the 70 GW of heat with green hydrogen boilers would require 70/0.46 = 150 GW of additional renewable electricity. That’s 385 GW of installed offshore wind turbine capacity, which would cover a very large area of the North Sea (Figure 2).
Figure 2: Sea areas of wind turbines needed to supply the UK’s heat.
It is commonly thought that using green or blue hydrogen for heating is zero-carbon and therefore ‘clean’. This is not correct.
Even if sufficient electrolyser capacity was available to generate the hydrogen, it would be necessary to use grid-mix electricity because, as explained above, there will not be sufficient renewable electricity for decades. In the remainder of this article, we refer to hydrogen made from grid-mix electricity as ‘green’ hydrogen, though it is more strictly named ‘yellow’ hydrogen.
According to UK government statistics, in 2020, the carbon ‘emissions intensity’ of the UK grid was 136 gCO2/kWh. (Every kWh of electricity generated resulted in 136 g of CO2 emissions.) This intensity is gradually reducing as coal-fired power stations are phased out and renewable generation increases, making electricity ‘cleaner’ with time.
Projected future carbon emissions from heating the UK’s buildings can be roughly estimated on this basis (see Figure 3). For example, in 2020:
- A heat pump delivering heat using UK grid electricity generated carbon emissions of 50g of CO2 per kWh of heat delivered.
- The figure for a green hydrogen boiler was 300g of CO2 per kWh of heat. (Again, the ratio of carbon emissions of a green hydrogen boiler to a heat pump is 300/50, i.e. the same factor of 6.)
As a further comparison, a modern condensing boiler burning natural gas generates approximately 200g of CO2 per kWh of heat delivered, which is constant with time, and an electric space heater generates about 160 gCO2/kWh.
Figure 3: CO2 emissions from various heat sources.
Figure 3 shows that in 2020, carbon emissions of green hydrogen boilers would be about 50% greater than existing natural gas boilers, with projections showing they would not start to deliver emissions lower than conventional gas boilers until nearly 2030.
Conversely, in 2020 a heat pump would generate just 25% of the CO2 emissions of a natural gas boiler. This is expected to decrease to just 15g of CO2 per kWh by 2035.
The process of manufacturing ‘blue’ hydrogen, which involves CCS (carbon capture and storage), is not zero-emission, as carbon capture is not a perfect process. The most effective CCS process available results in 90% of carbon capture but has an energy efficiency of only 69%. Processes with higher energy efficiencies tend to have lower capture rates (as low as 53%). This range is shown as a shaded band on Figure 3. Manufacture of blue hydrogen also generates fugitive methane emissions which would make it impossible to reach the net-zero carbon commitments of the UK government.
Heat pumps use one-sixth the amount of electricity to deliver the same amount of heat to a building as a green hydrogen boiler. Consequently, the energy costs of heat pumps are one-sixth of those of hydrogen boilers. Heat pumps can deliver immediate, deep cuts in carbon emissions – they reduce carbon emissions by 75% compared to existing natural gas boilers, while green hydrogen boilers won’t reach the emissions performance of a natural gas boiler until 2029 or the performance of a 2020 heat pump until around 2040.