High temperature thermal storage systems
Advances in energy storage solutions have been, and will continue to be fundamental for a successful transition to larger renewable energy supplies. Solar and wind energy are both subject to continual fluctuations caused by weather changes and daily/annual cycles. Energy storage systems can provide energy on-demand and balance fluctuating energy sources. In concentrating solar power plants, solar energy is converted into thermal energy, which can then be stored in a thermal energy storage system. The energy stored in this system is then converted into a flow of electricity via a connected steam power process at a later time.
Using liquid metals makes it possible to store heat at temperatures above 600 °C, which are typical for energy-intensive high-temperature industrial processes. For heat-to-electricity-conversion processes, the high temperature facilitates an improved efficiency, as most advanced power block technology – designed for fossil fuel combustion – can work at these temperatures. Liquid metals can also be used to store waste heat from high-temperature processes and, depending on demand, the thermal energy can be either converted into electricity, or used directly in other processes. The use of liquid metals as heat transfer and storage medium can thus allow for increased flexibility in energy systems.
As part of the work at KALLA, the technical feasibility of liquid metal stratified thermal energy storage systems will be examined. Stratified thermal energy storage tanks are an economically attractive option, as they use a single storage tank. Single storage tanks containing water are already used today in residential and commercial buildings for room heating and hot water supply.
There are several challenges that must be overcome for liquid metal usage in a similar system. These can only be understood by first examining the stratification in such storage system. The stratification in the fluid can be divided into three zones: an upper hot, a lower cold, and a separation layer called the thermocline. Pure liquid metals conduct heat very well, and so a pure liquid metal reservoir would allow the thermocline to spread very rapidly, which would greatly reduce the efficiency of the storage system. To counteract this, a bed of fixed particles with low thermal conductivity and high heat capacity is included in the entire volume of the storage tank. The bed increases both the storage density and reduces the thermocline spread.
Literature [1,2] has shown that single-tank fixed-bed designs using liquid metals are a promising storage configuration for very high temperatures. In the further planned work, a tank containing lead-bismuth eutectic melt and an integrated bed of particles will be set up on a pilot scale to analyze the energy storage density and the efficiency, as well as to gain insights on the operating behavior. The research will focus on the behavior of the thermocline during loading, unloading, and stand-by times, as the latter strongly influence the efficiency of the storage system.
 Niedermeier, K., Flesch, J., Marocco, L., & Wetzel, T. (2016). Assessment of thermal energy storage options in a sodium-based CSP plant. Applied Thermal Engineering, 107 (2016), 386-397.
 Niedermeier, K., Marocco, L., Flesch, J., Mohan, G., Coventry, J., & Wetzel, T. (2018). Performance of molten sodium vs. molten salts in a packed bed thermal energy storage. Applied Thermal Engineering, 141 (2018), 368-377.
Dr.-Ing. Klarissa Niedermeier
Phone +49 721/608-26902