Power houses: the benefits of heat batteries

The government’s focus on renewable power may have been successful in terms of production, but the lack of infrastructure capacity to deliver it is causing frustration across the plumbing and heating industry.

Negative impact

In 2024, the UK experienced more than 149 hours of negative wholesale electricity prices, and that is predicted to grow to 1,000 hours by 2027.

“As renewable energy surges, these negative prices aren’t just anomalies; they are signals that our energy system needs to adapt, fast,” says Christopher Carver, engineering director at tepeo, who believes the solution lies in technologies such as heat batteries. “Negative prices are becoming a hallmark of large-scale renewable energy deployment.”

Several factors contribute to negative electricity prices, including subsidies for energy producers, which encourage continuous renewable power generation – even when demand and prices are low.

“Big power plants, such as nuclear or coal, can’t always switch off quickly, so electricity suppliers sometimes end up paying others to take the extra energy off their hands,” explains Lukas Bergmann, head of product regulatory affairs at Sunamp. “It sounds strange, but it’s a sign of how our energy system is changing.”

One significant challenge posed by negative pricing is its impact on the profitability of renewable energy investments. Prolonged periods of low or negative prices can undermine future projects.

“Since investors and developers rely on stable returns to justify building new infrastructure, frequent negative prices create uncertainty for businesses dependent on predictable revenue from electricity generation,” explains Carver. “Addressing this issue is crucial for maintaining the momentum of decarbonisation. If renewable energy projects become too financially unstable, the rate of new installations could slow down.”

The UK’s current energy grid was built when power stations provided a steady supply of energy, as opposed to the fluctuating level supplied by renewables. The current grid struggles to handle peaks in energy, which results in waste.

“Upgrading the infrastructure – this is where thermal energy storage makes all the difference, and smarter ways to balance supply and demand will be key to solving this,” adds Bergmann.

Government backing

Government support and policies backing low-carbon technologies make them easier and more affordable for UK consumers to adopt. However, for the UK to meet its zero-carbon goal, companies such as Sunamp are calling for the government to recognise thermal storage as a key part of Britain’s energy future and offer funding.

“The BBC reported how an increase in the Boiler Upgrade Scheme (BUS) grant led to a 52% year-on-year rise in low-carbon heating system heat pump installs,” says Bergmann. “The UK government recently launched the #WarmAndFuzzy campaign with the aim of raising awareness about heat pumps and the BUS. However, to ensure that all homes have access to clean, affordable heating, it’s crucial that the BUS and VAT relief are extended to heat batteries as part of the Warm Homes Plan.”

Meanwhile, to foster consumer confidence in heat storage technology, the Microgeneration Certification Scheme (MCS) is developing standards for thermal storage to ensure heat battery systems are properly designed and installed.

“For heat batteries to achieve widespread adoption, robust industry standards are essential,” says Johan du Plessis, founder and chief executive of tepeo. “Establishing clear guidelines will support market growth and accelerate the integration of heat batteries into homes and businesses across the UK.

“Negative electricity prices highlight the urgent need for better energy storage and flexibility. Heat batteries present a practical, scalable solution to convert surplus renewable energy into valuable heat, reducing waste and supporting net zero goals. With strong policy backing, regulatory reform and continued industry innovation, heat batteries can play a key role in the UK’s transition to a cleaner, more efficient energy system.”

Harnessing excess energy

Industry experts believe that negative energy prices present an opportunity to rethink how energy generation and consumption is managed in the UK. According to tepeo, when it comes to how energy is generated, stored and used, flexibility is the key to integrating renewable energy at scale.

“Energy storage systems play a critical role by decoupling energy demand from generation,” says Carver. “They allow surplus energy produced during periods of low demand to be stored and used when demand rises. While grid-scale storage, enough to power entire towns or cities, has more than doubled in the past year, the UK still lags behind where it needs to be. The National Grid has stated that 50GW of storage is needed by 2050, but as of May 2024, the UK only had 3.5GW of operational battery storage capacity.”

It’s clear that the ability to store and shift energy on a large scale is essential to balance the grid, but for more intermittent renewable sources that operate on a smaller scale, such as domestic uses, heat batteries are becoming more common.

“Instead of letting extra energy go to waste, we need better ways to store it for later,” says Bergmann. “This is where heat batteries, electric batteries and smart grid technology come in. By shifting when we use energy – i.e. storing it when it is cheap and abundant and using it when it is needed – we can balance out demand and supply, reduce strain on the grid and make the most of renewables – all the while cutting energy bills by making the most of off-peak tariffs.”

The UK government has signalled support for heat batteries by including them in the Warm Homes: Social Housing Fund, allowing social housing providers to claim up to £15,000 per property for low-carbon heating solutions.

“Solutions such as grid-scale batteries, pumped hydro storage and flexible demand response play a role, but decentralised storage solutions at the consumer level are arguably even more critical,” says du Plessis. “Heat batteries provide an innovative, scalable solution that can bridge this gap effectively.”

Inner workings

Thermal energy storage systems, such as heat batteries, work by storing excess heat energy – typically from renewable sources including wind, solar panels or heat pumps, as well as grid and microgrid electricity, waste heat, combined heat and power (CHP) and boilers – releasing it later for heating or hot water.

Materials such as phase change materials (PCMs) or solid masses are used to absorb and release thermal energy during a melting and freezing process in a similar way to the gel in a pocket handwarmer.

Heat batteries can store surplus energy for later use, particularly for a household’s heating and hot water needs, making them an energy-dense alternative to hot water cylinders. Instead of converting electricity into heat in real time (which can be inefficient due to heat losses), heat batteries store energy and release it when needed. This helps homes and businesses cut energy costs and reduce reliance on fossil fuels, all while making better use of clean energy when it’s available.

It is worth installers noting that heat pumps require additional thermal storage for hot water, which can make their installation difficult in homes with limited space. However, heat batteries are up to four times smaller than traditional hot water cylinders, making heat pump installations in smaller homes possible.

“For homes where heat pumps aren’t practical, a complete heat battery solution can replace traditional combi boilers,” explains Bergmann. “By combining Sunamp heat batteries with tepeo’s Zero Emission Boiler (ZEB), for example, consumers get a low-carbon, combi-like heating system that replaces gas boilers and shifts consumption to low-demand periods, making heating and hot water efficient and affordable.”

Although challenging, negative energy prices are a sign that the UK is moving in the right direction towards a low-carbon, sustainable energy system. By adopting a variety of flexible low-carbon technologies, including those that store and manage surplus energy, the UK will move closer to decarbonising home heating systems.

User benefits

Heat batteries store excess electricity as thermal energy, which can later be used for heating and hot water. This offers several advantages:

• Demand flexibility: heat batteries enable homes and businesses to shift energy consumption away from peak hours, reducing pressure on the grid.
• Efficiency: converting and storing surplus electricity as heat provides a simple way to make use of excess renewable energy that can deliver the high levels of instant heat output required for heat and hot water in homes.
• Cost savings: consumers benefit from lower energy bills by charging their heat batteries during periods of negative or low electricity prices.

Courtesy of tepeo

Installing thermal storage solutions

• Electric upgrade: Heat batteries run on electricity, so the fuel source is likely to be changed if replacing a gas or oil boiler. This means a new high-current circuit will be required to power the unit directly from the consumer unit via an isolator (50A breaker/RCD, 10mm² min cable size).
• Controls: Most units are compatible with different types of heating controls. For example, the ZEB works with any sort of time or temperature control, including basic timeclocks and thermostats, through to programmable thermostats and smart thermostats.
• Plumbing: The ZEB is a regular boiler (heat only) and integrates into the existing heating system via a 22mm copper flow-and-return connection using solder or compression/ press-fit fittings.
• Additional considerations include: Isolation valves – on both flow and return, so the ZEB can be isolated for maintenance without draining the heating system. Automatic bypass valve – to ensure minimum flow through the ZEB, even when the heating system valves have closed. Pressure relief valve – to ensure the system pressure stays within limits. Magnetic system filter – to help ensure the ZEB heat exchanger stays free from contamination.
• Hot water: If replacing a combi boiler, hot water storage will be needed. This can be an indirect cylinder heated by the thermal storage unit, a direct cylinder heated with immersion or a compact heat battery.

To find out more, visit tepeo.com/faqs