Decarbonising heating: flying by the heat of our pants?
The 26th UN Climate Change Conference – COP26 – is on the horizon, due to kick off in Glasgow on 1 November. It seems that, despite the socio-economic worries brought on by a global pandemic, heightened societal attention to climate change is here to stay. Now, more than ever, climate change is at the forefront of public discourse and, in November, the UK will be the focal point. In this blog, Dr Laurence Stamford from The University of Manchester outlines the need for radical change in our heating system and the most environmentally sound route to its decarbonisation.
- Compared to other sectors, there is very little progress in decarbonising heat, despite it being one of the biggest contributors to climate change.
- The UK has plans for low-carbon heating in new homes, but nothing substantive for existing buildings.
- Using a life cycle perspective, air source heat pumps appear to be the lowest-impact option but must be combined with efficiency improvements. Hydrogen from various sources can also play a role.
- Policies supporting retrofitting our buildings are needed urgently.
In some areas the UK has made impressive progress on climate change, most notably the electricity sector, where the past 15 years have seen a climate-friendly combination of falling demand (by 18%) and plummeting greenhouse gas intensity (by almost 50%). However, while electrical power is often the centre of attention, we must remember that – as is the case for most countries – electricity accounts for less than a fifth of the UK’s final energy use: 17% to be exact. So, what about the remaining 83%? The biggest contributors are heating and transport, and both are harder to tackle than electricity due their stubborn reliance on combustion of solid, liquid or gaseous fuels.
Change at a glacial pace
This is a global challenge; heating accounted for half of the world’s energy consumption in 2018, and 90% of that was provided by fossil fuels. Altogether, heat is responsible for 40% of global carbon dioxide emissions. Unfortunately – and unlike the transformation of the electricity sector – this picture doesn’t seem to be changing: globally, fossil fuel-based technology and conventional resistive electric heaters accounted for almost 80% of heating equipment sales in 2019. Across Europe, 57% of the heating demand of buildings is met by natural gas with a further 23% provided by oil or coal.
If we have any chance of limiting climate change to 1.5 or 2°C, this has to change fast. In the UK, the government plans to introduce a Future Homes Standard in 2025 which will require energy efficiency measures and prohibit fossil fuel boilers in new homes. This is a great start, but of course there are 27 million existing homes in the UK – plus millions of offices and other commercial premises – all of which will need decarbonising in some way or another over the coming years.
Despite initiatives like the Clean Growth Strategy, currently there is no official plan to tackle this situation: the Green Homes Grant was notoriously ineffective, and the Renewable Heat Incentive expires in March 2022 having received just 87,000 household applications thus far over its 7 year life. Until this is resolved, the average home in the UK will continue to burn nearly 3 cubic metres of natural gas every single day.
Heat in a net zero future
On the plus side, there are various technologies available: hydrogen can be generated in various ways and burned for heat; biomethane can be produced from biomass or biowaste and burned in the same way as natural gas; air-source and ground-source heat pumps can provide around 3 units of heat for every unit of electricity they consume, by operating like a refrigerator in reverse.
This means that we could decarbonise heating via many different routes. For instance, the Committee on Climate Change has modelled future pathways for net zero which rely quite heavily on air-source heat pumps, with the majority of UK homes installing one in the coming decades. In contrast, the National Grid’s Future Energy Scenarios also encompass a net zero future but achieve it with greater emphasis on a hydrogen gas network feeding hydrogen boilers.
One obvious question arising from all of this is: how do our options compare in terms of their environmental sustainability? Are there any we should target via policy, or any we should avoid?
The options
Our recent open-access paper tried to answer these questions by taking into account the whole life cycle, from manufacture and installation to fuel production, use, and end-of-life. It covered various fossil fuelled boilers, hydrogen boilers, electric resistive heaters, and air-source heat pumps. In terms of carbon footprint, a gas boiler has an impact of about 220g CO2e/kWh, almost identical to a resistive electric heater at 226g. An oil boiler comes in at 316g, and an air-source heat pump is considerably lower at 93g.
So, an immediate switch to heat pumps would result in a GHG saving of about 60%. But what about the future? The coming years are likely to see more changes to the gas supply and electricity mix, as well as a potential roll-out of hydrogen piped to people’s homes and offices. To account for this, we used the same scenarios mentioned above from the CCC and National Grid while also varying the gas supply to include increases in either imported liquefied natural gas or domestic shale gas. The results seem to support air source heat pumps as the best option, but first let’s consider hydrogen.
There are two main methods of producing hydrogen: from natural gas, or from electrolysis. We estimate that, by 2050, a boiler burning hydrogen from natural gas (with carbon capture) will have a carbon footprint of 64-73g CO2e/kWh. Counterintuitively, hydrogen from electrolysis can potentially go as low as -59g, ie, net removal of CO2 from the atmosphere. This is due to the extremely high electricity demand that would be created by a national electrolysis programme, some of which could be met, hypothetically, by burning biomass with carbon capture for net-negative emissions.
However, this implies an enormous increase in electricity demand (though a limited percentage of this could be met by off-peak nuclear and renewables), reliance on expensive and uncertain carbon sequestration technology, and increases in non-carbon environmental impacts. In comparison, the 2050 estimate for a heat pump – which requires no long-term storage and monitoring of sequestered CO2 – is 8-21g CO2e/kWh, and could fall to zero by using different refrigerants.
This suggests that heat pumps should receive the bulk of incentivisation effort from policymakers. Existing subsidies for heat pumps, like the Renewable Heat Incentive mentioned earlier, have proved disappointingly ineffective; perhaps this is because they provide payments per unit of heat generated over a 7-year period, leaving homeowners with the arduous task of finding upwards of £10,000 upfront to pay for the installation. Future schemes might consider capital cost subsidy, not dissimilar to the plug-in car grant offered for electric vehicles.
It's not all about carbon
Despite its importance, climate change is only one component of our overarching sustainability goals. In the aforementioned paper, we also assessed 18 other criteria, ranging from particulate matter formation to eco-toxicity and acidification. Frustratingly, ignoring their carbon footprint, modern gas boilers have the lowest impacts in many of these categories. However, looking at a mix of technologies in different future scenarios, it’s likely that decarbonising the whole heating sector by more than 90% can also benefit us in terms of particulate matter formation, photochemical smog creation, ozone layer depletion and terrestrial acidification. Conversely, it’s likely that metal depletion, human toxicity and eco-toxicity will increase in future.
Faced with a trade-off like this, the best solution is often to improve our efficiency: with an aggressive programme to retrofit energy efficiency measures into buildings, we could minimise the demand for heat in the first place, thereby ensuring that we improve a large range of environmental impacts and mitigate the detriment to others.
To be effective, a financial incentive programme like this needs to be easily accessible, low on bureaucracy and, crucially, long-lasting: the biggest failure of the Green Homes Grant was that it only lasted a few months, failing to provide any long-term certainty or ramp-up time for homeowners and industry. It could also be strengthened by a hard commitment to reduce heat demand by a fixed percentage and completely phase out the installation of fossil-fired boilers in existing homes – not just new ones – within the next 10 years.
Overall, it’s clear that we can achieve net zero heating by 2050. On balance it seems that heat pumps are the most environmentally sound route to pursue, but whatever happens we need serious government commitment to incentivising energy efficiency measures and technology retrofitting programmes for buildings. And these commitments can’t arrive soon enough.
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Energy is one of The University of Manchester’s research beacons. Our energy and climate change researchers are at the forefront of the energy transition, collaborating with governments, businesses and institutions to develop innovative, real world solutions to drive a green recovery and help achieve next zero. As the UK prepares to host the COP26 climate summit, read our collection of blogs on climate change for more evidence-based policy solutions.
