“Thermal storage systems provide flexibility and sustainability”

To ensure heat is available flexibly, researchers and practitioners are turning to a range of heat storage technologies. Dr Stefanie Tafelmeier, a heat storage expert at the Bavarian Centre for Applied Energy Research (ZAE Bayern), explains what matters most in this context.


Why are thermal storage systems important for our energy supply?

Tafelmeier: The aim is to make our energy supply sustainable in the future, that is, with significantly reduced greenhouse gas emissions. To make this possible, work is being carried out, on the one hand, to make processes, appliances and systems more energy-efficient and, on the other hand, to integrate renewable energies to a greater extent. Thermal energy storage systems offer solutions for both approaches.

What do these solutions look like?

By integrating thermal storage systems to enable the flexible utilisation of, for example, waste heat from plants, efficiency gains can be achieved. The time lag between demand and supply, caused by the fluctuating availability of renewable energy, can be overcome with energy storage systems. People often think of electrochemical energy storage systems such as batteries. These offer excellent solutions for ensuring the availability of electricity at a later time. For some thermal applications, however, it may be more sustainable and cost-effective to store heat directly using a Power2Heat solution. Furthermore, thermal storage systems can smooth out peak loads in applications. This enables systems to be designed in advance to conserve resources. In short: thermal energy storage systems create flexibility, and flexibility enables sustainability.

Could you please give a few examples of the use of thermal storage systems?

The best-known example is the hot-water storage tank for building supply, which usually has a capacity of just a few hundred litres. In addition, there are water storage systems with a capacity of several thousand cubic metres that help supply entire neighbourhoods. So-called ice storage systems can also be integrated into the heat supply for neighbourhoods. As latent heat storage systems, they utilise, amongst other things, the energy stored during the phase transition from water to ice and vice versa. For industrial processes, sorptive energy storage may be of interest – for example, for drying processes that require warm, dry air.

What is the situation in the metal industries?

There are ongoing projects looking into, amongst other things, high-temperature storage systems that could be suitable for metalworking. One of these is the LIMELISA project run by KIT and DLR in collaboration with industry partners, which is developing next-generation liquid metal and liquid salt storage systems for high-temperature applications. At European level, the HEATERNAL project is working towards a thermal energy storage concept for high-temperature processes in, amongst others, the steel, glass, cement and ceramics industries.
Broadly speaking, thermal energy storage in the metals sector is conceivable for at least two potential areas of application. On the one hand, this involves the storage of unavoidable waste heat to enable its flexible use over time. On the other hand, high-temperature storage systems could be charged using surplus electricity (Power2Heat), thereby integrating renewable energy and providing heat that can be used flexibly over time. In the latter case, the very high temperatures required in the metal industry present a particular challenge.

What types of thermal storage are there?

Thermal storage is categorised into three technologies, which describe the physical nature of the storage process: sensible, latent and thermochemical energy storage. Each of these storage technologies has different properties, which make them more or less suitable for specific applications. The term ‘heat storage’ is often used synonymously – although thermal storage systems can, of course, also store cold.
Hot water storage tanks are a good example of a sensible energy storage technology for building services. High-temperature storage systems, which are becoming increasingly important, particularly for industrial processes, also fall into this category. In sensitive energy storage technologies, thermal energy is stored via the heat storage capacity of a storage material (e.g. water or concrete) – without the material’s physical state changing.
Latent heat storageis based on the principle of a material’s phase change, such as from solid to liquid, and the energy stored as a result (latent heat). During the phase transition, the storage system maintains a virtually constant temperature level. Different materials allow for different temperature levels, depending on their melting point. These can be utilised for both heating and cooling requirements. This means that temperature levels well below zero degrees Celsius are also possible.
Thermochemical energy storage systems are based on thermochemical or sorptive energy storage. In this process, not only is energy stored, but a material transformation also takes place in the form of a reaction or a sorption process. In sorptive energy storage, for example, water vapour is absorbed into the storage material and subsequently released again. This is utilised in drying processes where warm, dry air is required.

What technological advances are currently being made in research into thermal storage systems?

Industrial processes and their thermal requirements are increasingly coming to the fore. Consequently, it is becoming increasingly important to be able to cover the different temperature ranges, outputs and capacities of industrial processes. The focus here is on the requirements for storage materials and components, as well as storage design and system integration. Incidentally, this also applies to the storage of waste heat that may be generated during processes. The use of suitable storage systems also allows for greater flexibility in the utilisation of waste heat. In recent years, sophisticated large-scale storage systems have increasingly been tested and implemented for the flexible supply of heat to buildings and neighbourhoods. Latent heat storage systems for domestic use are also becoming increasingly established as commercial products. Research into thermal storage technologies is important not only to maintain flexibility in the energy supply but also to improve the efficiency of industrial processes, plants and equipment. This means that less energy needs to be provided to meet heating and cooling requirements.

What are the barriers to the wider use of such storage systems?

A switch to a new technology or the retrofitting of an existing system often requires a significant investment. With new technologies, this investment is always associated with a certain degree of uncertainty, as there is little empirical data available. In my opinion, this is true of all new technologies.

How can these hurdles be overcome?

It is important to carry out demonstration projects to demonstrate the feasibility of new concepts using innovative thermal storage technologies. In doing so, the storage solutions can also be optimised where necessary. Users from industry, the commercial sector or local authorities must be supported when they carry out demonstration projects in collaboration with research institutes or universities. The knowledge gained from these projects then needs a platform to be shared transparently with other potential users.

An editorially revised and supplemented version of the original publication on the portal www.energieforschung.de. There you will find information on funding opportunities, results and further projects in applied energy research within the framework of the Energy Research Programme of the Federal Ministry for Economic Affairs and Energy (BMWE). The portal is managed on behalf of the BMWE by Projektträger Jülich, Forschungszentrum Jülich GmbH.
About the LIMELISA project: www.kit.edu/kit/pi_2021_034_erneuerbare-energien-auf-dem-weg-zu-thermischen-grossspeichern.php
About the HEATERNAL project: www.cordis.europa.eu/project/id/101103921/de