By Jessica Hernandez, Lux Research

Storing thermal energy generated by large-scale solar energy plants, manufacturing plants, and the building sector is crucial if energy-intensive industries and power companies are to decarbonise and improve their operating efficiency.

Electrochemical energy storage like lithium-ion batteries, which convert energy into electricity, can generally only be used for short-term storage.

But thermal energy storage (TES) – which involves heating or cooling a storage medium like molten salt and minerals to use stored energy later – is larger in scale and has a longer service life as well as lower upfront costs. Its efficiency depends on the storage medium.

Gathering global momentum

TES is gathering momentum globally. Thermal storage startup Antora Energy earlier this year raised US$50 million from investment firms including Bill Gates’ Breakthrough Energy Ventures to accelerate the development of heat-based carbon block energy storage system for heavy industry. 

There was also a US$22 million investment in Rondo Energy, which captures low-cost renewable electricity and delivers continuous high-temperature heat to enable customers in industries such as cement, fuels, food, and water desalination to power their operations with zero-carbon energy.

Scientists at the US Department of Energy’s Argonne National Laboratory have meanwhile developed a new type of thermal battery that is more efficient in absorbing and releasing energy, and has a space-saving modular design.

Keeping the heat in

The storage of thermal energy dates back to the beginning of the Industrial Revolution, but interest in larger-scale, high-temperature TES is relatively recent as industrial players focus on the challenge of decarbonising industrial processes.

The technology has historically been tied to the concentrated solar power (CSP) sector. But the simplicity of storing heat compared to storing electrical energy has led to a range of systems and technologies to store thermal energy. There are three main storage methods:

• Sensible heat storage: Thermal energy is stored or released by heating or cooling a storage medium. Relatively inexpensive materials can be used, such as water, mineral oil, rocks, concrete, and steel. At higher stored temperatures, however, material stability can be a challenge. Another obstacle is heat loss, depending on the level of insulation of the container. The technology is the most advanced and simplest form of energy storage. Innovation is ongoing, often around system integration and the improving of thermal conductivity, especially for systems using solid storage materials.

• Latent storage: Thermal energy causes a phase change of storage material without a large change in temperature. Benefits include the ability to store energy in a relatively small temperature range and a generally higher energy density compared with sensible heat storage. However, the stability and lifetime of the phase change materials (PCM) are challenges, along with heat losses depending on the insulation level of the container.

• Thermochemical storage: Thermal energy drives a chemical reaction, and energy is stored in chemical bonds. Benefits include high energy density and the potential for much longer-term energy storage as the energy is stored in stable chemical bonds, meaning the product of the thermally-driven chemical reaction can even be stored at ambient temperature. However, there are many challenges, including material stability, energy loss, and the need for a more complex system design.

The required temperature of the stored thermal energy is usually the determining factor of what TES technologies are appropriate. The following chart shows temperature ranges for various sensible and latent heat storage materials. This is just a sample of materials, and many potential storage materials are not included. Importantly, the chart does not include thermochemical storage materials or rock-based or ceramic-based materials, which generally store medium to high temperatures in the range of 300°C to 1000°C or higher. 

Weighing the options

While there are specific challenges for each type of technology and application, here are some key factors to consider when assessing TES offerings:

• Thermal material complexity: This is one of the more straight-forward factors, as it is crucial for TES systems to keep a low US dollar per kWh design, and use both a form factor and choice of material that does not present lifetime or cost concerns.

• Footprint of storage: How much space will it take up? Water and mineral oil are inexpensive for low or medium heat storage but require insulated tanks with a sizable footprint. Additionally, water is limited to temperatures below 100°C without more expensive system components to handle higher pressures. A benefit of latent and especially thermochemical TES is the potential for a smaller footprint compared with sensible TES.

• Corrosiveness of the storage medium: Molten salts, which are often used in the CSP industry to store higher temperatures while offering the thermal transfer advantages of liquids compared to solids, can be corrosive. This can raise overall costs by requiring more specialised storage tanks.

• Temperature: Hotter isn’t always better. Higher temperatures generally give better efficiency when integrated into power generation cycles but, depending on the form factor of the thermal material, can complicate designs and make them expensive. Moreover, 600°C is an important benchmark, as steel begins to lose structural integrity at higher temperatures.

• Industry application: Application will ultimately determine the trade-offs for TES systems. In the CSP case, developers often accept the added complexity of molten salts because they allow for higher storage temperatures and greater heat-to-electricity efficiencies. 

Heat storage is an area that has been overlooked in the race towards decarbonisation as many companies focus first on areas such as electrification and renewable power production. But solutions to the heat storage issue can unlock greater flexibility and open potential new revenue streams.

As technologies advance rapidly amid rising global interest, TES is likely to become more efficient and economically viable, providing industries with the energy they need while supporting the decarbonisation of the power generation sector.