The new report from the Carbon Trust and commissioned by the Mayor of London concludes that a heat pump retrofit programme for residential and commercial buildings in London will have a critical role to play in meeting the capital’s net zero carbon emissions targets by 2030.

“The Mayor of London has set ambitious aims for London to be a net zero carbon city by 2030 and to have the best air quality of any major city. In addition, 28 London Boroughs have declared climate emergencies with over half of these committed to achieving net zero by 2030. The political will to deliver net zero carbon emissions in London has never been stronger,” says the report

Reducing emissions cause by heating buildings will be one of London’s biggest decarbonisation challenges. Natural gas, says the report, is currently used for most space and water heating and accounts for 37% of London’s greenhouse gas emissions and 22% of Nox emissions, whilst conventional electric heating also contributes significantly to CO2 emissions and increases the peak demand on local energy networks.

The report is intended to provide help and guidance for local authorities, social housing providers, private owners and landlords and also for builders and installers when considering a change of energy source, a heat pump retrofit often being a viable alternative.

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Heat pumps are an efficient form of heating and have the potential to deliver CO2 savings of 60-70% compared to conventional electric heating and 55-65% compared to an A-rated gas

boiler. These savings are expected to increase further, as the grid decarbonises over the coming decades, towards 90-100% CO2 emissions reduction by 2050.

How does it work?

In simple terms, its a bit like domestic refrigerator in reverse: a compressor creates a small amount of higher temperature energy from the large amount of energy collected at a lower temperature. The actual process is a lot more complicated, using the ‘vapour compression cycle’ to harness energy available when vapour returns to liquid.

Heat pumps for central heating systems use the technology to extract energy from the outside air (air source) or from under the ground (ground source). These are low temperatures (below 10°C in winter) but there’s lots of energy when compared to absolute zero (-273°C).

Ground source or air source?

A heat pump has maximum efficiency when the temperature gap between the heat source and the heat demanded is minimised. A ground source heat pump is more efficient than air source because around two metres down the ground stays constant at around 10°C all year round, protected from temperature extremes. This heat there is mainly from the solar energy absorbed by the ground through the summer. Air source heat pumps are not as efficient, but they don’t need available ground and are cheaper to install.

There are a number of factors that can make heat pump retrofits challenging in urban settings and they will only work effectively where the buildings are highly energy efficient, i.e., very well insulated. High rise blocks pose problems as does noise pollution from the pump systems, especially with air source, however, there is a diverse range of solutions that exist to overcome a number of these challenges and new technology is becoming available all the time.

Undertaking energy efficiency upgrades combined with installing heat pump technology will likely require considerable initial investment in most cases but this will be offset by long-term reductions in energy bills.

Most buildings in London will likely require considerable initial investment and perhaps government support to move from oil, gas boilers or electric heating. However, this sort of transition can be phased in as buildings are refurbished over time.

Tom Delay, Chief Executive, the Carbon Trust, says:

“Buildings and heat have been identified by the Committee on Climate Change as key challenge areas for decarbonisation in the coming decades, and so the analysis and recommendations detailed in the report to promote low carbon solutions that are available now is very timely.

“As always, heat pumps are not a silver bullet solution, which is why we have provided a suite of policy recommendations, including investment in energy efficiency in buildings and flexibility in the energy system.”

Shirley Rodrigues, Deputy Mayor for Environment and Energy, Greater London Authority, commented: “Retrofitting heat pumps and improving the energy efficiency of existing buildings are key to achieving the Mayor’s ambitious target for London to reach net zero carbon by 2030.

“Not only will retrofitting heat pumps help support jobs and skills vital to a green, fair and prosperous COVID-recovery, they also reduce energy bills if designed well. However, delivering this at the scale needed will require the Government to step up investment and implement strong supportive policies.”

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