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Thermal Storage as a Means to Support the Energy Transition

Sources: USGS, MII, Public domain

Nowadays, many homes use solar collectors to provide them with warm water. During summer, when heating demand is not high, this works quite well. However, in winter more heating is needed, which is why thermal storage was designed to store a part of the excess heat for later use. Up until now, the excess heat was stored in large water tanks filled with hot water. The disadvantage of this method is that tanks have to be large as well as heat is lost despite good insulation. Thermochemical storage, on the other hand, makes it possible to store thermal energy produced during summer for later use during the winter months with little loss of heat.

A possible method of achieving this is to use zeolites, as they do not store the heat directly but desorb the water which is present within the material and store it instead. The advantage of this is that the energy is not stored in the form of increased heat but in the form of a chemical state. This means that heat is not lost during long-term storage. There is, however, one drawback: zeolites have poor thermal conductivity, which makes transferring the heat from the heat exchanger to the material and back difficult.

Now (2021), scientists at the Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP have solved this problem in the course of their work on the ZeoMet project. They coated the zeolite pellets with aluminium which increased the thermal conductivity twofold after just the first attempt without having a negative impact on water adsorption and desorption. The scientists believe that by adjusting the coating the thermal conductivity can be even further increased by five to ten times. The process of coating the pellets presented considerable challenges because the smaller the grain, the more challenging the task becomes. However, smaller grains also increase the specific power density of thermal storage systems. To achieve sufficient thermal conductivity, the coat must also be tens of micrometers thick which is much thicker than is the norm for vacuum coating processes. Therefore, the scientists employed thermal evaporation. In this process, aluminium wire is continuously applied onto a heated ceramic plate in a vacuum, where the aluminium is evaporated and deposited onto the granules as a layer of aluminium. The pellets had to be circulated continually in a barrel so that they were all covered evenly.

For many years, scientists have tried to find new ways to improve thermal storage. In 2016, scientists developed a zeolite-based open sorption heat storage system to provide thermal energy for e.g. space heating needs. The centepiece of the study was a significant scale prototype using zeolite 13X/H_2O as the reactive pair, and its focus was the development of a 1D mathematical model which could be used to predict both the charging (desorption) and the discharging (adsorption) processes occurring inside the storage unit. The experimental campaigns and the numerical results had good results concerning the thermal performances of such a storage unit. With 40 kg of zeolite, a temperature increases of an average 38°C at the outlet of each zeolite’s vessel during 8 h was achieved during the discharging, with an airflow inlet at 20°C, 10 g/kg of dry air of specific humidity, and a flow rate of 180 m"3/h.

In 2019, scientists undertook the experimental characterisation of working pairs made of various liquid sorbates (distilled water, ethanol, and their mixture) and a 13X zeolite sorbent at ambient pressure. The sorbent hydration by liquid sorbates had a lower heat storage performance than vapour hydration; however, it showed similar heat storage density to that obtainable by latent heat storage at comparable costs, robustness and simplicity of the system, at the same time gaining the long-term storage capabilities of sorption-based technologies. To test their application, the scientists analysed the feasibility of a sorption heat storage system with liquid sorbate, which could be used to improve the cold-start of stand-by generators driven by internal combustion engines. The results showed that liquid hydration may be employed as a simple and low-cost alternative to more efficient, but also more expensive techniques for long-term energy storage.

There are several advantages to using zeolites for thermal storage as they are not only a good storage medium, but can also help provide cooling for domestic use alongside solar collectors as well as for mobile applications. For example, in commercial vehicles, heat lost from the power unit could be used for air conditioning as part of a thermochemical cycle.

However, the hybrid materials used for it present new challenges. Therefore, the scientists are making an effort to intensify their connections with materials developers and systems engineers from research and industry, in order to advance solutions for the flexible supply of heating and cooling.