To maximise the use of heat generated by industry and store electricity produced from renewable sources, thermal batteries are emerging as one of the latest solutions in Thermal Energy Storage (TES).
Heat is essential for life and the production of many goods. It accounts for nearly half of the world’s final energy consumption, far surpassing transport (30%) and electricity (20%) [source: Renewables 2021, IEA].
However, heat is often wasted, particularly in industrial processes. It is estimated that between 20% and 50% of the energy used in industry is lost as waste heat and dissipated from equipment surfaces [source: US Department of Energy]. The International Energy Agency, in its Renewables 2023 report, predicts that global demand for industrial heat will increase by 16% (+17.6 EJ) between 2023 and 2028.
Preventing heat waste and utilising it effectively is already possible and has been practised for at least 200 years. However, capturing an ever-greater share of this heat and storing it for conversion into energy is becoming a key factor in industrial decarbonisation and accelerating the energy transition.
The potential for generating electricity from currently unused thermal energy in industrial processes is estimated at a substantial 150 TWh per year [source: TEHAG – Thermal Energy Harvesting Advocacy Group]. This amount of energy is equivalent to the consumption of 20 million people, the annual output of 19 nuclear power plants, or the combined annual energy usage of the Netherlands and Denmark, as highlighted by TEHAG.
Furthermore, thermal storage could significantly boost the potential capacity for long-duration energy storage (LDES) globally. Projections suggest an increase from a range of 1 to 3 TW by 2040 to between 2 and 8 TW [source: LDES Council / McKinsey]. Such growth would lead to investment increases of between $1.6 trillion and $2.5 trillion and could expand the market size to between $1.7 trillion and $3.6 trillion by 2040.
To fully exploit this vast potential, several methods are available, with the most interesting and recent developments taking the form of thermal batteries. In recent years, new industrial ventures, startups, and research projects have emerged, offering solutions that could contribute to a more circular approach to energy production, reducing both consumption and emissions.
TAKEAWAYS
Thermal storage, an old concept renewed
Storing energy as heat is not a new idea. For nearly 200 years, steel producers have captured and utilised waste heat to reduce their energy demand.
Over time, however, methods for thermal storage have become increasingly sophisticated. As part of the broader range of energy storage systems, thermal energy storage (TES) systems have evolved to store heat, allowing it to be used immediately or later, with the potential for long-term storage and utilisation.
There are at least three methods of thermal storage: sensible heat storage, latent heat storage, and thermochemical storage.
Sensible heat refers to the amount of heat exchanged between two bodies, resulting in a change in temperature. It is termed ‘sensible’ because it causes a noticeable change in the temperature of an object. Storage systems of this kind use materials such as rock, sand, or concrete to store thermal energy for later use.
Latent heat storage, on the other hand, involves the amount of heat in a substance undergoing a change of state. Storage systems of this type accumulate energy without changing the material’s temperature but by altering its state; phase change materials (PCMs) are used in this instance.
Thermochemical heat storage involves storing heat through reversible reactions, with specific salts being one of the materials employed.
In addition to storing heat, thermal storage systems can store electrical energy when required. This is where thermal batteries come into play, a relatively recent technology that is gaining increasing interest.
With the anticipated rise in renewable energy sources – particularly solar and wind, which are inherently intermittent – having access to energy storage systems capable of capturing generated electricity, storing it, and releasing it on demand, as well as various forms of storage batteries, is essential.
Thermal batteries: Italian solutions and projects
The materials used for thermal storage in batteries vary, as do the systems and the industrial and research entities working on them. In Italy, some are working with sand as a medium: the company Magaldi has designed, developed, and patented MGTES, a high-temperature system (exceeding 1000 °C) for thermal energy storage based on a fluidised bed of sand, which can acquire properties typical of fluids.
Once charged with electricity from renewable sources, the system can store clean energy for hours, days, or even weeks, and release it on demand. Its operation is structured into three phases: charging, storage, and discharging. During the charging phase, the bed of solid particles can be heated using electric heaters or high-temperature fluid. In this phase, the fluid bed is active to enhance heat transfer. In the storage phase, fluidisation is deactivated, and the sand settles at the bottom of the module, allowing the absorbed energy to be stored. During the discharge phase, the integrated heat exchanger within the fluidised bed of sand particles is reversed to release the stored energy.
Another Italian innovation comes from the deep-tech startup I-Tes, affiliated with the University of Turin, which has developed thermal storage batteries using phase change materials (PCMs) and thermo-chemical materials (TCMs). These materials leverage the physical phenomenon of phase change to store and release large amounts of energy (heat), transitioning from one physical state to another.
In the field of research, the European project BLAZETEC is noteworthy. Coordinated by the CNR-ISM (National Research Council – Institute of Structure of Matter), this initiative began in July 2024 and aims to develop high-temperature thermal batteries (operating between 1200 and 1600 °C) to provide long-term energy storage and conversion solutions.
The project seeks to develop two pilot solutions: an electric thermal battery capable of converting excess electricity into heat and then back into electricity, and a solar thermal battery designed to store concentrated solar radiation and provide electrical energy on demand. Both systems incorporate solid-state energy converters, including thermionic (TIG), thermoelectric (TEG), and thermophotovoltaic (TPV) technologies.
Growing interest and developed solutions worldwide
Internationally, thermal batteries are garnering considerable interest. A notable example is the Californian startup Rondo Energy: in June 2024, it secured €75 million in funding from the European Commission, the European Investment Bank, and Breakthrough Energy Catalyst (a platform founded by Bill Gates to finance highly innovative cleantech projects) for three industrial decarbonisation projects in Europe.
The thermal storage system developed by Rondo Energy is based on a resistance heater that converts electricity (sourced from wind or solar power plants) into heat, much like an electric heater works. This heat is then used to warm stacks of refractory bricks, which serve as the core of the storage system.
When heat is needed, air flows through the stack of bricks and is superheated to over 1000 °C. In addition to hot air, it can deliver other gas streams, thereby replacing fossil fuels in both direct and indirect combustion processes. Alternatively, it can operate in a cogeneration setup with a steam turbine to provide both heat and/or electrical energy.
Another company leveraging refractory bricks, by reengineering their properties, is the MIT spin-off Electrified Thermal Solutions. They have developed a thermal battery capable of storing heat and supplying it for industrial uses, even at very high temperatures, which is particularly useful for energy-intensive sectors such as steelmaking or cement production.
In this case, the thermal battery is charged by passing electricity directly through the bricks to heat them via the Joule effect (a phenomenon where any electrical conductor heats up when an electric current passes through it) up to 1800°C. At this temperature, thermal energy is stored, and the system is then ready to be discharged by passing air or another gas through the channels in the bricks to provide heat to any boiler, turbine, or furnace.
Another notable aspect of this system is its application in the energy sector. The thermal battery can be adapted to an existing gas plant: when there is excess electricity (generated from renewables), it converts the electricity into heat and stores it. Then, when the demand for electricity exceeds supply, it delivers high-pressure hot air to the gas turbine to generate electricity when it is most needed.
Another Californian startup, Antora Energy, uses solid carbon blocks for its thermal batteries, which can be heated up to 2400 °C. The heat is transferred using the light emitted by the glowing blocks, thanks to thermophotovoltaic technology, which is based on the direct conversion process from heat to electricity through photons.
More specifically, this is a power generation technology capable of using thermal radiation to generate electricity in photovoltaic cells. The TPV (thermophotovoltaic) system developed by the startup consists of a thermal emitter capable of reaching temperatures exceeding 1,000 °C and a photovoltaic cell capable of absorbing photons emitted from the heat source.
Buildings as thermal storage solutions and support for the electrical grid
One of the most promising developments in thermal storage involves harnessing the properties of Phase Change Materials (PCMs) to integrate them into construction materials. A study by the National Renewable Energy Laboratory (NREL), titled “Enabling Thermal Energy Storage in Structural Cementitious Composites with a Novel Phase Change Material Microcapsule Featuring an Inorganic Shell and a Bio-Inspired Silica Coating,” has focused on the potential of certain PCM microcapsules as viable solutions for integrating these materials into building construction.
The study particularly highlights the development of a new process to apply a silica coating to specific microcapsules using bio-inspired silica. This process utilises low-cost sodium silicate as a precursor, allowing the production process to occur at room temperature.
The silica coating, composed of unique lightweight, hollow ceramic microspheres, helps to reduce temperature peaks, thereby improving thermal energy storage capacity. As the authors explain:
«This new PCM microcapsule offers an economical solution for incorporating thermal energy storage into cementitious materials, as evidenced by the fact that over 30% of the aggregates (by volume) can be replaced with the microcapsule without a significant loss of strength»
The goal is to store thermal energy and release it as electrical energy when needed, positioning buildings as useful elements to support and balance the grid’s needs.
In the field of construction, water-based thermal storage is being utilised by the Canadian energy company Enwave Energy, which has designed and installed a thermal storage system in Toronto. This system includes a temperature-controlled tank capable of holding over 7.5 million litres of water, based on the Deep Lake Water Cooling (DLWC) system. It uses the deep-water sources of Lake Ontario to provide heating and cooling for connected buildings, creating a low CO2 emissions system.
Conversely, the European Heat-Insyde project has developed a thermochemical storage system based on water and salt (potassium carbonate), aiming to create a thermal battery to store heat and release it for domestic heating, complementing the heat pump.
The operation of this system relies on the properties of the two raw materials. When combined (with water in the form of steam), the water binds to the salt, transforming it into a new crystalline form. This reaction releases heat and is reversible: when heat is applied to separate the water from the new crystal, the original components are restored. The heat generated is stored, and as long as these two components remain separated, the heat is retained without any loss.
This allows for a substantial amount of heat to be stored in a remarkably compact volume. Moreover, the thermal battery is designed to balance the supply of renewable energy, both from the grid and decentralised sources, enabling integration into heating and electrical systems.
Glimpses of Futures
Thermal batteries and, more broadly, thermal storage solutions can be a valuable resource for reducing energy consumption and emissions, particularly in industrial settings. They also offer an alternative form of energy storage to support the growth of renewable energy sources and create the conditions for more efficient building practices.
Let us now try to anticipate possible future scenarios by analysing, through the STEPS matrix, the impacts these thermal storage systems could have on multiple fronts.
S – SOCIAL: thermal batteries can provide a reliable source of energy storage for remote or off-grid communities, which often lack access to the electrical grid. An example of this is a housing project implemented in an area of the Scottish Highlands affected by energy poverty. For the construction of 117 new affordable homes in the Blar Mhor residential complex near Fort William, the local council seized the opportunity to replace the originally planned liquefied petroleum gas (LPG) system with low-carbon heating technologies to reduce carbon emissions and lower energy costs for residents. The houses were equipped with air-source heat pumps, roof-mounted solar panels, and Sunamp thermal batteries. The heat pumps and thermal batteries use electricity generated by the solar panels or from the grid. According to Sunamp, the provider of the thermal storage solutions, the council estimates a 40-60% reduction in domestic energy costs compared to the originally planned LPG system. The Blar Mhor project was supported by the Scottish Government’s Low Carbon Infrastructure Transition Programme.
T – TECHNOLOGICAL: the growing need to harness heat for reuse or convert it into usable energy is driving research towards new thermal storage solutions, either by exploring the properties of new materials or employing existing ones to maximise their capabilities. Two notable examples are Malta Inc. and Brenmiller. Malta Inc., a Google X spin-off established in 2018, has focused on developing long-duration electro-thermal energy storage that uses molten salts (similar to those used in concentrated solar power systems) to store heat and then release it as energy through a heat engine. Malta’s technology holds potential for sustainably repurposing old coal-fired power plants. A study by the U.S. Department of Energy, titled “Repowering Coal Plants as Pumped Thermal Energy Storage,” confirmed that Malta’s electro-thermal energy storage system can be used to cost-effectively convert a retired or decommissioned coal plant (or another fossil fuel-powered generation unit with a steam turbine) into a long-duration energy storage facility. Meanwhile, Brenmiller leverages the heat-storing properties of crushed rocks through modular systems that can be configured and adapted to any industrial site where they are needed. One such system has been implemented in Italy at an Enel thermoelectric plant, where it stores energy during peak hours and releases it when most needed.
E – ECONOMIC: thermal batteries can make the cost of electricity for industrial heating competitive with natural gas equipment, potentially replacing up to 75% of fossil fuel use in the energy demand of the U.S. industrial raw materials sector, which amounts to about 11,600 petajoules per year. This is equivalent to the total energy consumption of all households in the top 12 U.S. states, as highlighted by the report “Thermal Batteries: Decarbonizing U.S. Industry While Supporting A High-Renewables Grid” by the independent U.S. think tank, Energy Innovation. Moreover, thermal batteries do not degrade over time and have a long service life, thereby reducing long-term maintenance and replacement costs.
P – POLITICAL: in Europe, the approval of the Net-Zero Industry Act identifies batteries as one of the technologies that can receive support through strategic projects. Additionally, energy-intensive industries such as steel, chemicals, or cement, which produce components used in these net-zero technologies and invest in decarbonisation, can also be supported as strategic projects. In the United States, the development and incentivisation of thermal batteries could be supported by the Inflation Reduction Act, which, among its regulatory tools, has approved the 45X Advanced Manufacturing Production Credit for manufacturers of clean energy-related technologies, including battery modules, as reported in U.S. Code § 45X – Advanced manufacturing production credit.
S – SUSTAINABILITY: thermal batteries powered by renewable energy could reduce industrial emissions by about half, according to the report “Opportunities to accelerate decarbonization of industrial heat” by the Center for Climate and Energy Solutions. These batteries can achieve efficiency levels in the range of 90-98% between electricity input and final industrial heating demand. Furthermore, thermal batteries could have a significant impact on sustainability by supporting the development of renewable energy sources, such as solar and wind, ensuring energy storage for electricity produced with zero emissions.
