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direct or indirect storage systems.

       1.3.1.1 Active Two-Tank Direct

In an active two-tank direct system, solar radiation is converted to thermal energy and the thermal energy is stored in the same fluid which was used to collect it. The fluid is stored in two tanks, one at high temperature and the other at low temperature. Fluid from the low-temperature tank flows through the receiver and gets heated to a high temperature [Figure 1.7]. Thereafter the hot fluid flows to the high-temperature tank for storage. Fluid from the high-temperature tank flows through a heat exchanger. In the heat exchanger the fluid loses heat to generates steam which is further used for electricity production. The cold fluid exits the heat exchanger and returns to the low-temperature tank to continue the cycle.

Figure 1.7 Active two-tank direct thermal energy storage.

Concentrating solar power (CSP) projects that use parabolic trough systems such as Archimede, ASE Demo Plant, Chabei, Gansu Akesai, Solar Electric Generating Station I (SEGS I) use active two-tank direct thermal energy system [see Table 1.1]. Dacheng Dunhuang Molten Salt Fresnel project used two-tank direct storage with a storage capacity of 13 hours and Molten Salt as the heat transfer fluid. Such storage systems have attracted a great deal of attention from power tower receiver concept. Power tower projects such as Atacama-1, Aurora Solar Energy Project, Copiapó, Crescent Dunes Solar Energy Project (Tonopah) implement two-tank direct storage system. Currently, 20% of the commercial plants use two-tank direct TES system for energy storage, and the summary of all plants is listed in Tables 1.1, 1.2 and 1.3.

      1.3.1.1.1 Active Two-Tank Indirect

      In two-tank indirect systems, different fluids are used as the HTF and storage material. In cases when HTF is expensive or HTF is not well suited to be used as storage materials, such a system can be implemented. The storage material from the low-temperature storage tank flows through an extra heat exchanger. The fluid is heated by the high-temperature HTF in the heat exchanger. The high-temperature storage material then returns back to the high-temperature storage tank. The HTF exits the heat exchanger at a low temperature returning to the receiver system to absorb heat again to continue the cycle. As observed in a two-tank direct system, storage material from the high-temperature tank is further used to generate steam. In contrast to the direct system, an extra heat exchanger is needed in an indirect system, making this system costlier.

Table 1.1 CSP projects with parabolic trough receiver concepts integrated with thermal energy storage systems (data collected from NREL).

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Parabolic trough CSP projects	Thermal energy storage (TES)	Thermal energy storage capacity	Storage description and material
Airlight Energy Ait-Baha Pilot Plant	Packed bed	5 hour(s)	Packed-bed of rocks
Andasol-1 (AS-1)	2-tank indirect	7.5 hour(s)	Molten salt 60% sodium nitrate and 40% potassium nitrate.
Andasol-2 (AS-2)	2-tank indirect	7.5 hour(s)	Molten salt 60% sodium nitrate and 40% potassium nitrate.
Andasol-3 (AS-3)	2-tank indirect	7.5 hours	Molten salts
Archimede	2-tank direct	8 hour(s)	Molten salt 60% sodium nitrate and 40% potassium nitrate.
Arcosol 50 (Valle 1)	2-tank indirect	7.5 hour(s)	Molten salt 60% sodium nitrate and 40% potassium nitrate.
Arenales	2-tank indirect	7 hours	Molten salt 60% sodium nitrate and 40% potassium nitrate.
ASE Demo Plant	2-tank direct	4.27 MWh-t	Molten salt
Ashalim (Negev)	2-tank indirect	4.5 hours	Molten salts
Aste 1A	2-tank indirect	8 Hours	Molten salts 60% sodium nitrate and 40% potassium nitrate
Aste 1B	2-tank indirect	8 Hours	Molten salts 60% sodium nitrate and 40% potassium nitrate
Astexol II	2-tank indirect	8 Hours	Molten salt 60% sodium nitrate and 40% potassium nitrate
Bokpoort	2-tank indirect	9.3 hours	Molten salt
Casablanca	2-tank indirect	7.5 hours	Molten salt 60% sodium nitrate and 40% potassium nitrate.
Chabei 64MW Molten Salt Parabolic Trough project	2-tank direct	16 hours	Molten salt
Delingha 50MW Thermal Oil Parabolic Trough project	2-tank indirect	9 hours	Molten salt
DEWA CSP Trough Project	2-tank indirect	15 hours	Molten salt
Diwakar	2-tank indirect	4 hours	Molten Salt
Extresol-1 (EX-1)	2-tank indirect	7.5 hour(s)	Molten salt 60% sodium nitrate and 40% potassium nitrate.