Date Published: May 2022
Why heat pumps can help and hydrogen can’t
The energy used to heat buildings is responsible for around 20% of the UK’s carbon emissions.1 As a nation, we have committed to net-zero carbon by 2050, and providing all heat for buildings from low carbon sources is a key component of the strategy.2 This should be both alarming and empowering to building services engineers. The decisions we make now, and the technologies we implement, will be operational for most, if not all, of the next 30 years.
The world is currently 1°C above pre-industrial levels – Australia is on fire, Cape Town is running out of water and Cambridge hit 38.7°C in summer 2019. Another 0.5°C will exacerbate all of these impacts and many more, and this is the current ‘best case’. We cannot ignore the urgency. This means we need to get on with deploying the technologies that are available to us now and not wait for technological answers that are many decades away. It will be too late.
How far away is hydrogen?
It is impossible to mention low carbon heat without discussing hydrogen. The replacement of methane in our gas network with hydrogen seems instinctively simple. However, we can’t allow ourselves to be lulled into a false sense of security; the technical and economic barriers that stand between us and a functioning hydrogen economy are almost as high now as they have been for the past 20 years.
First, green hydrogen is made by electrolysis of water, using renewable electricity, in hydrolysers and subsequent compression into a network; around 35% of the energy is lost in this process.3
Second, the energy density of hydrogen per unit volume is one third that of methane. If we are to use it as a direct replacement, we will need three times more storage and distribution capacity than we currently have for methane.
Lastly, burning it is inefficient. Hydrogen burns hotter than methane and the thermal losses within the boiler will increase such that current levels of combustion of efficiency are unlikely.4 In total, we lose around 50% of the electricity we put in using hydrogen in this way.
An immediate opportunity
Using hydrogen for heat is not a means to reduce the CO2 emissions of the UK in the 2020s.5 So, what can we do? The answer lies in the proliferation of UK renewables. In the past five years, the carbon intensity of grid electricity has decreased by 60%.
Coal-fired power stations have been decommissioned, and the cost of offshore wind has decreased. Both of these things happened much faster than predicted and the trends are accelerating.6 Meanwhile, the increased efficiency of electrical appliances has reduced peak loads from 61.7GW in 2002 to 50GW in 2018.
Electricity is decarbonising rapidly, the grid has spare capacity, and we know how to turn it into heat with an efficiency of 300-400%.7 Electric heat pumps are completely orthodox in Scandinavia, France and many other countries. Yet, in the UK, we put their widespread deployment in the same bracket as burning hydrogen in boilers – something that is at least 30 years away.
The hydrogen solution is often proposed by those wishing to gain efficiencies by making use of existing ‘infrastructure’ – a completely sensible aim. Heat pumps can provide this when applied to existing heat networks to make excellent use of the physical installations put in place to enable combined heat and power (CHP). We should note that CHP is currently a technology that burns gas to displace low carbon electricity; the net result is that its heat-carbon intensity is 130% that of a gas boiler and 330% more than an electric heat pump.8
Retrofit heat pumps
Retrofit of CHP with heat pumps is possible, and can have an immediate impact. Two stages of vapour compression take heat from air at temperatures as low as -15°C and raise it to 70°C; with water, we can efficiently go to 90°C.9
The refrigerants, either ammonia or a combination of butane and propane, have a near-zero global warming potential. Further, the buffer vessels that have been installed to accommodate the low turndown of CHP engines can be deployed to store heat from heat pumps, shift demand, and reduce pressure on electricity grids.
This is working now in mainland Europe. There are technical hurdles, of course – acoustics, cold plumes, and poor performance of existing heat networks – but these can be overcome. The installations in colder climates show this and will be followed by more in the UK.
Housing served by heat networks is uncommon: approximately 500,000 customers for 20 million buildings. Low-density houses and bungalows are about 90% of the buildings requiring a standalone solution. This sector is significantly easier – efficient, mature heat-pump solutions exist. There are multiple retrofit options from different manufacturers to replace gas boilers.
Consider a 100m2 semi-detached house with solid walls, double glazing and some loft insulation. The peak heat loss will be ~60W/m2, or 6kW. Running a heat pump to supply this heat requires 2kW – less than a kettle and only 14% of the capacity of the 63A supply.
Most heat used by dwellings is supplied to low-density, owner-occupied buildings10 for which the electrical infrastructure was designed for night-storage heaters. The local infrastructure (low-voltage cables, 11kV cables, transformers) has spare capacity. We can transfer the heating load to electricity now and don’t need to wait for hydrogen.
Of course, eventually, this requires an increase in electricity capacity. High-voltage direct current (HVDC) is an established means of transporting large quantities of electricity over long distances with low losses. We have HVDC connections to France, Belgium and the Netherlands; a cable connecting us to Norway will be live in 2021.11 We can transport electricity large distances,12 but this is experimental and inefficient for hydrogen.13
As engineers, we rely on a pragmatic scepticism to filter out real innovation from wishful thinking and spin. We need to stop being distracted by hydrogen and get on with delivering the solutions of this decade.
About the author
Joel Gustafsson is partner, principal engineer and member of MF: Net Zero at Max Fordham
1 2018 UK Greenhouse Gas Emissions, National Statistics
2 Net Zero: The UK’s contribution to stopping global warming Committee on Climate Change, May 2019
3 Proceedings of the IEEE 94(10):1826–1837 Ulf Bossel November 2006
4 Appraisal of domestic hydrogen appliances, work for BEIS February 2018
5 Absolute Zero, UK Fires, November 2019
6 Updated energy and emissions projections: 2018
7 Low carbon heat: heat pumps in London, Greater London Authority, September 2018
8 January 2020 carbon intensity of 209 gCO2/kWh. CHP with 40% thermal efficiency and 40% electrical efficiency
9 Large-scale district heating, Drammen, Norway WWF Scotland 2016
11 North Sea Link, accessed February 2020
12 World’s first 1,100kV HVDC transformer, Siemens, accessed January 2018
13 Japan launches first liquid hydrogen carrier