4.3 Active indirect concept for TES with single tank

Active indirect design with a single storage tank is also possible which is quite efficient than the two-tank storage system. In this design, TES material in cold and hot

Figure 2. Active direct concept for TES [13].

Figure 3. Active indirect concept for TES using two tanks [13].

state is stored in the same tank as illustrated in Figure 4. The materials get separated by itself due to different material properties at different temperature. Thermal stratification causes the hot material to be stored in the upper part of the tank, while cold material is stored in the bottom. This system is also known as thermocline system, which is approximately 35% cheaper than the two-tank storage system.

In thermocline storage systems, a filler material like rocks, concrete, or sand is used so that less quantity of TES materials is required to enhance the thermocline effect. In this system, handling accuracy during charging and discharging of the TES is required using controlled methods and devices to avoid mixing of materials. Among differently investigated structures during discharging of thermocline, packed-bed configuration is reported to be the best performing [13].

## 4.4 Passive concept for TES

This is another concept of thermal energy storage in which TES materials are fixed and do not move during entire processes. The concept is demonstrated in Figure 5. The TES material is kept in a tank through which HTF is circulated. During charging phase, hot HTF is passed through TES and it transfers its heat to the TES. During discharging, cold HTF is passed from TES which is already at very

#### Figure 4.

Active indirect concept for TES integration, single tank [13].

Figure 5. Passive concept for TES [13].

high temperature. In such systems, thermal conductivity of the storage material and surface contact area defines the rate of heat transfer from and to the HTF during charging and discharging, respectively. In this type of design, rocks and concrete are used as TES materials.
