Q4 2019: A major milestone for electric heat

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by Dr Iain Staffell – Imperial College London

The UK’s ambition for net zero means that all sectors of the energy system must rapidly decarbonise. 

In the run up to COP26 in Glasgow, Electric Insights will look across these sectors to discuss how clean electricity can help.

The way we heat our buildings presents a major challenge for reaching net-zero emissions. Heat accounts for 80% of the energy consumed in our homes, and is the second largest source of CO2 emissions in the UK behind only transport. Unlike the power sector, emissions from heating show little sign of reducing because measures to reduce heat demand through improving insulation were scrapped, and most of our buildings are still heated by gas boilers as alternatives have yet to gain ground. This is about to change though, as from 2025 it will be illegal to build new homes which use fossil fuels for heating.

Two low-carbon options for the heating sector are hydrogen and electricity. Both are more expensive than natural gas though, so first and foremost it will be critically important to improve building thermal efficiency to bring down energy consumption whilst maintaining comfort levels.

Hydrogen heating

Hydrogen has recently gained attention and could be seen as the ‘continuity candidate’. It has the benefit of using similar appliances to natural gas, so it should feel familiar to consumers and require little change to home life. This is critically important as for everything to change, we need everybody on board.

It should be technically possible to blend 20% hydrogen into our current natural gas pipelines, as is being tested at Keele University. However, this only gives a 7% reduction in methane consumption and carbon emissions, because hydrogen is a much lighter and less energy-dense gas. Alternatively, we could switch entirely to hydrogen, but this would require changing every gas-burning appliance in every home. Manufacturers are already offering ‘hydrogen ready’ boilers, much like the ‘digital ready’ TVs that were marketed in the five years before analogue TV was turned off. Achieving a complete switchover could take decades, but would offer the deep decarbonisation that is needed. 

Pure hydrogen is not naturally occurring, so it must be manufactured from other molecules. Hydrogen can be split out from water (H2O) by electrolysis, or reformed from natural gas (CH4). These will only give environmental benefits if the electricity is clean, or if the reforming is integrated with carbon capture and storage.

Electric heating

Electric heating is already used in around a tenth of British houses, primarily electric radiators or night-storage heaters. Electric heat pumps are a more efficient alternative which use the same technology as a refrigerator, scaled up to suck heat from the outside air or ground (rather than out of your food) and pump it into your home. They offer much higher efficiencies, producing 2–4 units of heat per kWh of electricity consumed, referred to as having a coefficient of performance (COP) of 2–4. However, homes must be much better insulated than is typical in the UK, as heat pumps have lower peak power output than gas boilers. Heat pump performance is sensitive, so high-quality installation and user education are essential (more so than for other technologies).

A large uptake of heat pumps could push up electricity demand during peak winter evenings too far, so hybrid heat pumps (which combine a small heat pump with a boiler for backup) are gaining interest. If these use a natural gas boiler, they will also fall short of being zero-carbon – but this could instead be a hydrogen boiler to combine the best of both technology groups.

The carbon intensity of heat produced from gas and electricity, based on the UK average gas boiler mix, and average generation mix

The rapid changes in Britain’s power system are shifting the goal posts for clean heating. Just as we think about the carbon intensity of electricity, it is useful to compare these technologies on the carbon intensity of the heat they produce.

The move towards condensing boilers means the efficiency of the UK’s gas boilers is improving over time, but too slowly to help with climate change. The average efficiency of gas heating has risen from 82% to 86% over the last decade, meaning that the carbon emissions from producing 1 kWh of central heating or hot water only fell from 225 grams in 2010 to 215 grams in 2019.

In comparison, standard electric heating would have produced 500 grams of CO2 per kWh of heat delivered back at the start of the decade, more than double that from a gas boiler. As electricity production has shifted away from fossil fuels, Britain has reached the point where it is cleaner to use electric heating than a gas boiler for the first time ever.

Averaged over 2019, simple electric heaters produced 207 grams of CO2 per kWh of heat. We would now have to blend 12% hydrogen (by volume) into the mains gas grid for Britain’s boilers to be as clean as simple electric radiators. As the carbon intensity of electricity continues to fall, it will be impossible for hydrogen blending to keep pace with the carbon reductions, and only complete hydrogen switch-over will have the chance to compete on environmental grounds.

Electric heat pumps can boost these savings further, offering heat with as little as 50–100 g/kWh. However, carbon cannot be the only consideration. Retail electricity prices are three times higher than gas (at around 15 p/kWh versus 5 p/kWh), meaning electric heat will be more expensive unless a high-efficiency heat pump is installed. Having time-of-use tariffs (such as Economy 7) can help to reduce these running costs. The cost of producing hydrogen is difficult to quantify as it is not available for the domestic heating sector. 

Ultimately, new hydrogen and electric technologies need to offer low carbon heat without pushing up consumer bills if they are to gain popular support. The next five years are critical for decarbonising Britain’s heat, so pressure is mounting to find the right solutions, to engage wider interest from the public, to build the social licence for a costly programme of transformation, and to decide how to equitably fund such a transformation.

1: Energy efficiency standards meant that for the last decade only ‘condensing boilers’ could be installed. These burn around a tenth less fuel as they capture the heat that is otherwise wasted as steam in the exhaust flue. Their market share has grown from just 2% of gas-heated homes in 2000 to 38% in 2010 to an estimated 74% of homes last year.

2: 466 g/kWh carbon intensity of electricity generation, plus transmission & distribution losses.

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