The EXTEND project and longer duration thermal energy storage in homes

Grant-funded field-trial tackling lulls in renewable energy generation and cutting carbon emissions from household heating systems

The EXTEND project aims to replace household boilers fired by fossil fuels with thermal energy storage and heat pumps, with the purpose of moving forward with the electrification of heat.

EXTEND combines a longer-duration thermal storage system and a smart heating controller with household energy systems to tackle periods of low renewable generation, known as “dunkelflaute.”

EXTEND in a nutshell

During the two-year project, we will develop, build and trial the EXTEND solution in 30 homes around the UK, and evaluate the effects a fleet of EXTEND thermal storage systems would have if a large number of them were deployed onto a specific area of the electricity network.

The EXTEND system is built around heat battery storage of sustainable energy, allowing homes to use renewable heat even when renewable generation might not be available. The project is being run in partnership with myenergi, Ripple and Energy Systems Catapult.

Funding is through the UK Government’s Longer Duration Energy Storage Demonstration programme, announced in November 2022.

Space heating and hot water

A heat pump will charge renewable heat into a large-capacity heat battery, delivering space heating and hot water on demand.

Renewable energy

The bulk of input electrical energy will be from on- and off-site wind energy. Householders will have the option of part ownership of a wind farm through Ripple Energy.

Time-shifting control

The proposed system uses smart control logic from myenergi and Sunamp, and a significantly larger thermal storage from Sunamp to overcome lulls in renewable energy supply.

The EXTEND project will trial three different sizes of heat batteries optimised for space heating and compatible with heat pumps

EXTEND partners and granting body


Designs and manufactures space-saving thermal storage


Designs and manufactures smart home energy products

Energy Systems Catapult

Runs real-world, net zero testing

Ripple Energy

Enables consumers to own stakes in large-scale wind projects

The £9.25 million project funding for EXTEND was awarded through the UK Government’s Department for Business, Energy & Industrial Strategy (BEIS) and its Longer Duration Energy Storage Demonstration programme, part of the Net Zero Innovation Portfolio (NZIP), which provides funding for low-carbon technologies and systems.

EXTEND systems in homes will help the UK ride out lulls in renewable energy generation and will allow homeowners to cut their carbon emissions and benefit from lower cost tariffs for flexible demand and participation in grid-supporting measures.”

Andrew Bissell, Sunamp’s chief executive


Project timeline

Research and development

November 2022 to September 2024

Pilot trial

Starting June 2024

Main field trial

June 2024 – September 2024

Trial decommissioning and final reporting

Spring 2025

Frequently asked questions

The field trial will be run using Energy System Catapult’s real-world test environment called the Living Lab. To register your interest in taking part please leave your details below. We will be updating this page to share more relevant information about how to sign up, so please do check back again.

The EXTEND project is developing a new generation of thermal storage optimised for space heating that will allow homeowners to decarbonise their homes and keep relying on cost-effective renewable energy even during “dunkelflaute” periods, where little to no wind or solar energy is being generated. To do this, we will extend our existing energy-dense heat battery technology to a prototype range of thermal batteries that have increased storage duration and capacity. The longer duration energy storage system will be designed to be readily compatible with a large share of the UK housing stock, while still providing the energy efficiency and renewable share of a heat pump system.

The field trial is due to start in December 2023 in 100 homes around the UK. Before the start of the trial, every prospective participant will be given information about the system to be installed in their homes, the scope of the trial and the decommissioning to return their home to its pre-trial state. We’ll share more information on this web page when it is available.

Yes. The EXTEND thermal storage system is an extension of Sunamp’s well-established technology and introduces a doubling of the useful energy content inside the store compared to the normal operating range of Sunamp’s other heat batteries. EXTEND project thermal batteries are for both domestic space heating and hot water, and are specifically designed to be compatible with heat pumps. This means they need to have larger energy storage capacities than our current Thermino battery range, which delivers hot water only. The project is assessing the feasibility of developing three differently sized, demonstration prototype heat batteries - a dishwasher sized model suitable for small or well-insulated new build properties, a refrigerator sized model for typical semi-detached and smaller (or well-insulated) detached properties, and an American fridge-freezer sized model for larger properties. These batteries will benefit from a very high energy density within a relatively small amount of space, ensuring they are highly practical and cost-effective in the average UK home.

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