To ensure flexible provision of heat, different heat storage technologies are at the focus of research and practice. Heat storage expert Dr Stefanie Tafelmeier from the Bavarian Centre for Applied Energy Research (ZAE Bayern) explains what is important here.
Why are thermal storage tanks important for our energy supply?
Tafelmeier: The aim is to realise a sustainable energy supply in the future, that’s to say, a supply with greatly reduced greenhouse gas emissions. To make this possible, on the one hand we’re working on making processes, appliances and systems more energy-efficient, and on the other, we’re looking into how to step up the integration of renewable energies. Thermal energy storage systems offer solutions that work in both approaches.
What do these solutions look like?
Thermal storage tanks can be integrated, which increase efficiency by flexibly utilising waste heat in systems, for example. Energy storage systems can be used to overcome the time lapse between demand and supply that’s caused by fluctuating supplies of renewable energy. Electrochemical energy storage systems, such as batteries, are the first things that often come to mind here. These offer great solutions for the future availability of electricity. However, for some thermal applications it may be more sustainable and economical to store heat directly using a Power2Heat solution. In addition, thermal storage units can cushion peak loads from applications. This makes it possible to design systems in advance, to save resources. In short: thermal energy storage creates flexibility and flexibility enables sustainability.
Can you give us a few examples of the use of thermal storage?
The best-known example is the hot water storage tank for building supply, which usually only holds a few hundred litres. There are also water storage systems with a volume of several thousand cubic metres that supply entire neighbourhoods. So-called ice storage tanks can also be integrated into the heat supply of neighbourhoods. As latent heat accumulators, they utilise, among other things, the energy stored during the transition from water to ice and vice versa. For industrial processes, for example, sorption energy storage can be of interest – for drying processes, for instance, where warm, dry air is required.
What’s the picture in the metal industries?
There are ongoing projects that are looking at high-temperature storage systems that could be suitable for metalworking, among other things. One of these is the LIMELISA project run by KIT and DLR together with industrial partners, in which next-generation liquid metal and liquid salt storage systems are being developed for the high-temperature range. At the European level, the HEATERNAL project is pursuing the goal of a thermal energy storage concept for high-temperature processes in the steel, glass, cement and ceramics industries, among others.
Roughly speaking, it can be said that thermal energy storage in the metal industry is conceivable for at least two possible areas of application. One is the storage of unavoidable waste heat so that it can be utilised flexibly over time. Another involves charging high-temperature storage units with surplus electricity (Power2Heat), thereby integrating renewable energy and providing flexible heat over time. In the latter case, the very high temperatures required in the metal industry are a particular challenge.
What types of thermal storage are out there?
Thermal storage systems are categorised into three technologies that describe the physical nature of the storage process: sensitive, latent and thermochemical energy storage. Each of these storage technologies has different properties that make it more or less suitable for certain applications. The term “thermal storage” is often used synonymously – although thermal storage tanks can of course also store cold.
Hot water storage tanks are a good example of sensitive energy storage technology for building supply. These also encompass high-temperature storage systems, which are becoming increasingly important for industrial processes in particular. In sensitive energy storage technologies, thermal energy is stored via the heat storage capacity of a storage material (e.g. water or concrete) – with no resulting changes to the material’s aggregate state.
Latent heat storage is based on the principle of a material’s phase change, for example from solid to liquid, and the energy stored in this way (latent heat). The tank provides an almost constant temperature level during phase transition. Depending on the melting point, different materials allow different temperature levels. They can be used for both heating and cooling requirements. Temperature levels far below zero degrees Celsius are also possible.
Thermochemical energy storage systems are based on thermochemical or sorption energy storage. Not only is energy stored, but a material transition also takes place in the form of a reaction or a sorption process. In sorption energy storage, for example, water vapour is absorbed in the storage material and released again. This is utilised for drying processes in which warm, dry air is required.
What technological advances are currently being made in thermal storage research?
Industrial processes and their thermal requirements are increasingly coming to the fore. It is therefore becoming increasingly important to be able to cover the different temperature ranges, outputs and capacities of industrial processes. The focus 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 can accumulate during processes. The use of suitable storage units also allows increased flexibility in waste heat utilisation.
In recent years, increasingly sensitive large storage tanks have been tested and implemented for the flexible heat supply of buildings and neighbourhoods. Latent heat storage systems for domestic use are also increasingly established as commercial products.
Research into thermal storage technologies is important in order to keep the energy supply flexible – but also to increase the efficiency of industrial processes, systems and devices. In this way, less energy needs to be provided for heating and cooling requirements.
What obstacles are there to the increased use of such storage systems?
Switching to a new technology or upgrading an existing system often requires a non-negligible investment. With new technologies, this investment is always associated with a certain degree of uncertainty, since empirical knowledge is scarce. In my opinion, this is a common feature of all new technologies.
How can these hurdles be overcome?
Demonstration projects are important, to prove the feasibility of new concepts that use innovative thermal storage technologies. The storage solutions can also be optimised along the way, if required. Users from industry, trade or municipalities must receive support when they realise demonstration projects in partnership with research institutes or universities. After that, the resulting knowledge needs a platform so that it can be shared transparently with other potential users.
Edited and supplemented version of the first publication on the portal . Here you will find information on funding opportunities, findings and other applied energy research projects as part of the energy research programme of the German Federal Ministry for Economic Affairs and Energy (BMWE). The portal is managed on behalf of the BMWE by Project Management 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
