The rising share of renewable energies leads to increased fluctuations in electrical power supply. One possibility to shift the surplus energy based on demand is a Carnot battery (CB). A CB uses a heat pump or resistance heater to convert and store thermal energy into electrical energy. Later, the stored thermal energy is converted back into electrical energy using a heat engine. This study investigates a CB with a two-zone tank for thermal energy storage. A transcritical process with ${\mathrm{CO}}_2$ is applied for charging, while discharging employs a transcritical process with ${\mathrm{CO}}_2$ and six refrigerants operating in a subcritical process. The transcritical process with ${\mathrm{CO}}_2$ and the four most promising subcritical processes are compared regarding round trip efficiency and levelized cost of electricity (LCOE) depending on the pinch points 5 K and 1 K in the heat exchangers. Additionally, the technology readiness level (TRL) is determined for these configurations. The results show round-trip efficiencies between 11.3% and 33.5% and LCOEs ranging from EUR 0.95 ${\left(\mathrm{kWh}\right)}^{-1}$ to EUR 2.09 ${\left(\mathrm{kWh}\right)}^{-1}$ for the considered concepts with TRLs of up to six.

Latent Thermal Energy Storages (LTES) can store thermal energy in a narrow temperature range. Therefore, they are favorable for integration into Rankine-based Carnot Batteries. For the design of such systems, simulations based on accurate models are desirable. However, physical phenomena such as natural convection in LTES units cannot be modeled directly in transient system models. Simplified models are required. Therefore, the objective of this work is to derive simplified LTES unit models for use in system models. In transient simulations the state of charge of the LTES influences its temperature profile. The temperature profile depends on the geometry of the LTES unit. Therefore, the geometry must be considered to model the transient behavior of an LTES unit. The LTES unit under investigation has a shell and tube heat exchanger structure. The phase change material (PCM) is located between the hexagonal fins and in the space between the finned tubes. Aluminum fins are used. They have a high thermal conductivity and thus compensate for the low thermal conductivity of the sodium nitrate used as PCM. The interaction between fins and PCM is complex. Therefore, a numerical approach can be used to gain insight into the behavior of the LTES unit. To transfer the results of a complex model to a simplified model where fins and PCM are not considered individually, the effective thermal conductivity of a single finned tube can be used to approximate the performance of the LTES unit. In this study, a model of a section with a single finned tube is developed using the COMSOL software. The effective thermal conductivity of the system is determined by varying the effective thermal conductivity in a simplified model and comparing the results with reference cases based on a complex modeling approach. The results can serve as model input for simplified system models of Carnot Batteries, among others.