1.8]. The heat loss is more in such systems as the hot and cold fluids are in direct contact. The hot and cold temperature regions are separated by a temperature gradient resulting in a thermocline. The density difference in the fluid thermally stratify the fluid in the tank. Buoyancy effects create thermal stratification of the fluid within the tank, which helps to stabilize and maintain the thermocline. Van Lew et al. (2011), Bayón and Rojas (2014), and Biencinto et al. (2014) carried out theoretical and experimental work on thermocline energy storage system for CSP plants.
Figure 1.8 Active single tank thermocline thermal energy storage.
Figure 1.9 Active two-tank indirect thermal energy storage.
Figure 1.10 Passive thermal energy storage.
High-temperature HTF flows into the top of the thermocline and leaves the bottom at low temperature. The thermocline moves downward and adds thermal energy to the system for storage. The thermocline moves upward and removes thermal energy from the system to generate steam and electricity if the flow is reversed. Puerto Errado 2 Thermosolar [see Table 1.2] operated by Novatec Solar España uses linear Fresnel reflector system currently operational in Calasparra, Spain, and has implemented a single-tank thermocline thermal energy storage system. This plant is operated at a temperature range of 140ºC-270ºC. Other plants also use this type of thermal systems and are listed in Tables 1.1, 1.2 and 1.3. Active two tank indirect thermal energy storage is also given in Figure 1.9 and passive type thermal energy storage in built with solar tower technology is given in Figure 1.10.
1.3.3 Other TES Systems
1.3.3.1 Packed-Bed Storage System
Other types of thermal energy storage system include packed-bed and passive system. Only power plants use a packed-bed system as the storage method is Airlight Energy Ait-Baha Pilot Plant. This plant uses Parabolic Trough Technology at temperature range of 270ºC-570ºC with 5 hours storage capacity.
1.3.3.2 Passive Thermal Storage System
In the passive type TES systems, thermal storage material is fixed and it does not flow, which is in contrast to the active system. The thermal storage material is used only to store thermal energy which can be transferred to and from the heat transfer fluid via thermal charging and discharging. A passive-type thermal storage system can be a solid material (example: concrete), fluid (example: water) or phase change material (example: PCM). In such systems, the heat transfer fluid transfer energy to the thermal storage material where the material stores energy which can be further transferred to the heat transfer fluid. Such systems have not been integrated in solar thermal power plants till date. The works on passive thermal energy storage system are on the laboratory and fundamental level. A good number of research works can be found in literature on the passive thermal energy storage system.
1.3.4 Types of Thermal Energy Storage (TES)
There are three types of TES mechanisms that can be applied to CSP and other applications: sensible energy storage, latent energy storage and thermochemical energy storage. An overview of these technical concepts and their states of development are presented below.
1.3.4.1 Sensible Energy Storage
The sensible thermal storage system stores thermal energy with increases in the temperature of the TES material. The principle of the sensible thermal storage system is simple and it has been widely applied in CSP as well as other applications. The TES material undergoes temperature change during energy storage and release. The physical and chemical changes of storage material is not observed. Sensible thermal energy storage method is simple and inexpensive. One of the disadvantages of sensible thermal energy storage material is low thermal conductivity. This results in lesser energy storage and release capacity. Also, the sensible heat transfer materials have low energy storage density. This further leads to large sizes of the storage devices.
The amount of energy stored in the material (Q) can be calculated as
Where m is the mass of the material, Cp is the specific heat of the material at constant pressure and ΔT is the temperature difference.
Common sensible storage materials include water, steam, synthetic oil, molten salt, gravel, etc. [see Table 1.4]. As seen from Tables 1.1, 1.2 and 1.3, molten salt is widely used for sensible energy storage. Sensible thermal storage systems are mainly seen for low-temperature applications.
Research on sensible thermal storage is comparatively mature and has been developed to a commercially exploitative level. As the density of sensible thermal storage is low, sensible thermal devices typically have certain limitations due to their large sizes. Lucentini (2014) presented thermal storage of sensible heat using concrete modules in solar power plants. Tiskatine et al. (2017) carried out a detailed study on suitability and characteristics of rocks for sensible heat storage in CSP plants. For high-temperature sensible energy storage for CSP systems, the selection and analysis are done by Khare et al. (2013).
Table 1.4 Typical materials used in sensible heat TES storage (Mehling and Cabeza, 2008; Navarro et al., 2012).
| Material | Density (kg/m3) | Specific heat at constant pressure (J/kg K) |
| Clay | 1,458 | 879 |
| Brick | 1,800 | 837 |
| Sandstone | 2,200 | 712 |
| Wood | 700 | 2,390 |
| Concrete | 2,000 | 880 |