Parasitic heat transport in thermal energy storage for Carnot batteries

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For the efficiency of Carnot-Batteries, the thermal energy storage (TES) is a key component. It should store large amounts of thermal energy over a long time at low cost. Often liquid materials like water or molten salt are used. The storage can be applied as a two-tank system with cold fluid in one tank and the hot fluid in the other tank, or as a stratified storage, where hot fluid is stored directly above the cold fluid. However, even in single storage tanks with initially uniform temperature, thermal stratification develops due to heat losses to the surrounding. Since the wall material typically has a larger thermal conductivity than the storage medium, heat is transferred from the hot region via the wall to colder regions an vice versa. This drives thermal convection close to the wall, which enforces mixing and thus destroys exergy. The effect can be seen in small systems, but also in larger systems in industrial scale. The understanding of these parasitic convective flows is of fundamental interest for many fluid mechanical and heat transfer problems in nature and engineering. More important, it will help to improve the overall efficiency in terms of energy and exergy of TES for Carnot-Batteries, but also for TES in general for other heating applications. Thus, the project aims also to be a valuable contribution to achieve the 'Wärmewende'.