Conceptual design selection and development of a latent-heat thermal energy storage subsystem for a saturated-steam solar receiver and load

ABSTRACT:
The conceptual design of a tube intensive latent heat thermal energy storage (TES) subsystem utilizing a eutectic mixture of sodium hydroxide and sodium nitrate as the phase change material (PCM) has been developed. The charging and discharging of the unit is accomplished by the same

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serpentine tube bundle heat exchanger in which heat transfer is augmented by aluminum channels acting as fins. Every tenth channel is made of steel to provide tube support. A tube bundle is composed of 15 rows of 15.9 mm (0.625 in) 0.0. tubes 15.2 cm (6.0 in) apart. Each row is a single tube approximately 78 m (255 ft) long formed into a vertically oriented serpentine. The horizontal runs of the tube are 15.2 cm (6.0 in) apart. The aluminum channels are inserted between the horizontal runs and connect all the tube rows. This assembly is strapped together and the tubes connect to the steam and condensate headers. Five of these bundles are contained in a single rectangular tank which is nominally 4 m x 4 m x 11.6 m (13'x13'x38'). Eight tanks are required to achieve the 148 MWh capacity and 30 MW peak charge and discharge rate established for this study. The augmented heat transfer design resulted from an analytical effort which examined the freezing of salt over the discharge process in a unit cell surrounding a heat transfer tube. The two-dimensional, finite-difference calculations were performed by computer program FREEZE written for this effort. The study included the determination of the effects of conductivity enhancement and extended heat transfer surfaces of various configurations. Tube diameter and spacing were parametrically varied to assess geometric impact. Both steel and aluminum fins were considered. The final design configuration was simulated in the FREEZE model. The spacing of the heat transfer tubes was determined by finding when the target capacity, the peak discharge heat transfer rate, and virtually complete freezing of the PCM occurred simultaneously.

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