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Materials 2022, 15, 2946 2 of 20 1. Introduction The technology of the particle-to-working fluid heat exchanger (PWFHX) is closely related to central receiver solar technology. The idea of a central receiver system based on gas and other power-generation cycles has gained considerable interest during the past two decades, due to its capability of achieving very high temperatures through intense heat-flux concentration in a relatively small area. Various gas-cycle concepts have been proposed and tested, and most involve the direct heating of compressed air or other gases [1–3]. However, one of the major challenges of these systems is the successful incorporation of thermal energy storage (TES), since the effectiveness of using TES with air or gas is relatively poor. There have been several TES solutions proposed over the past three decades. Ho [4] reviewed and summarized the recent advances in solid-particle-based central receivers. One of the most widely accepted TES solutions is the use of molten salts [5,6]. Currently, the use of molten salts for thermal energy storage is limited to temperatures generally less than 600 ◦C due to technical restrictions. Another solution is the use of solid blocks to store energy during the day. This idea has been demonstrated with concrete blocks [7,8], but the temperatures are generally limited to less than 500 ◦C due to concrete’s properties, making this concept unsuitable for high-temperature applications. Furthermore, since solid blocks store sensible heat, their temperature profile during the discharging process causes a gradual decline in cycle efficiency. Yet another solution is to use sand as a storage medium [9]. This concept was developed to work in conjunction with an air receiver. The sand is heated in an air–sand heat exchanger to a very high temperature. The sand then flows to a hot storage tank, and then to a fluidized bed cooler, where its heat is used to generate steam that feeds a steam power cycle. The colder sand returns either to the air–sand heat exchanger or is stored in a cold storage tank. This technology resolves the temperature limit issues faced in the solid-block concept. However, the main issue of an unfavorable temperature profile during discharging still persists. TES in solid particles has been studied by a number of research groups. The research originated at Sandia National Laboratories (SNL) in the early 1980s examined the use of solid carriers as both a storage medium and working fluid for high temperatures [10–17]. The emphasis was on particle material selection and receiver design, along with the optical characterization of master beads and other particle materials. TES materials were also investigated in the past for varying applications. Zunft et al. [18] quantified the heat losses from a subsystem TES, which consisted of rectangular storage composed of four parallel chambers filled with the ceramic storage material studied in the industrial regenerative thermal oxidizer. The temperature at full load reached 630 ◦C, and the total heat losses in a 24 h period amount to 930 kWh. An experimental packed-bed TES designed by CIEMAT- PSA [19] was composed of an insulated stainless-steel vessel filled with 0.1 m3 alumina spheres with a 9 mm diameter to investigate the specific costs of the storage subsystem under 20 EUR/kWth. An air inlet temperature of 570 ◦C and different air mass flow rates have been investigated. Tescari et al. [20] evaluated the mechanical properties of structured reactors/heat exchangers in high-temperature heat storage via the cobalt oxide cyclic-redox scheme. Two different structures of honeycomb and perforated block and two different compositions were evaluated. During thermal cycling in the range of 800–1000 ◦C, different loads were applied to the samples while monitoring their length variations. Calderón et al. [21] re- viewed solid-particle materials to be used as both a heat-transfer fluid and a medium for thermal energy storage. The parameters and properties of solid particles were described from a materials science point of view by illustrating their function and their connection to the performance of the power plant and its durability. The interactions between solid parti- cles and major system components were further discussed in this review. Furio et al. [22] evaluated two coating methods for silica sand to improve the optical properties. They reported a 140% increase in optical properties. Specific heat capacity and energy density for different particulate materials were studied by Kang et al. [23]. These particulate materials included silica sand, quartz sand, cristobalite, alumina, sintered bauxite, silicon carbidePDF Image | Low-Cost Particulates Used as Energy Storage and Heat-Transfer Medium
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