The design and building of two thermal energy storage (TES) pilot plants to be used for concentrated solar power (CSP) applications is explained and compared. Both pilot plants are based on a two-tank molten salt storage technology (composed by a hot and a cold tank) which works under a TES process. This process has three main steps: charge, storage and discharge. Charging and discharging cannot be performed simultaneously, and the whole process is considered to be cyclic: (1) during the charging process, the heat transfer fluid (HTF) is heated up with the solar collectors from the solar field up to a temperature of 393 °C. Then, the HTF exchanges its energy with the molten salts at the heat exchanger. During this process, the molten salts flow from the cold tank through the heat exchanger, arriving at the hot tank at a temperature up to 388 °C. (2) During the storage, the molten salts are stored in the hot tank at a temperature up to 388 °C. (3) Finally, during the discharging process, which corresponds to the period when no more solar radiation is available, the energy stored in the hot tank is used to produce electricity. Molten salts flow from the hot tank through the heat exchanger to the cold tank, heating up the HTF. This HTF carries the thermal energy to the power block, where it is converted to electricity. The first pilot plant presented in this study was designed and built by Abengoa in 2008 in Seville (Spain). It consisted of an 8.1 MWhth facility which contained all the components (successfully tested and optimized) that were lately used in commercial CSP plants. The second pilot plant consisted of a 0.3 MWhth facility designed, built and tested in 2008 at the University of Lleida (Spain) in conjunction with Abengoa. The TES material used in both pilot plants was a mixture of sodium nitrate and potassium nitrate (60% NaNO3 and 40% KNO3). The main differences between the two facilities fall on the size and on the operating mode: while the facility in Seville is charged with solar energy and discharged against a power cycle, the facility in Lleida is charged with an electrical boiler and discharged against a HTF-air heat exchanger. This paper presents the main components of both facilities and the lessons learnt during operation at pilot plant scale, which have been found to be useful to guarantee the correct start-up and operation of new commercial CSP plants built worldwide. These lessons learnt include key aspects like filling up the storage tanks, heat tracing systems and equipment used.

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