Sizmann shows that the storage capacity is limited by it ability to take up entropy at a given temperature change [1]. This is also valid, if, in addition to the storage of sensible heat, the latent heat of a phase change can be utilized. The storage capacity can be increased significantly by the additional entropy change of the phase change [1].

In order to avoid the limitation by the achievable entropy uptake for “direct” TES systems (like sensible and latent), the entropy coupled to the heat supplied to the storage has to be separated. Such an “indirect” TES system is only storing work. It is now able to do this without any thermodynamic limitations.

However indirect TES are for discharging not independent as direct TES, where energy and entropy are available at the right relation. At discharging heat from the ambience has to be available to provide entropy for the heat release of the storage. This necessary connection to the ambience can be seen in figure 4 and 5. Advantages of such systems compared to direct TES are a higher flexibility concerning charging and discharging temperature and a higher storage capacity.