Exergy recovery from solar heated particles to supercritical CO2

Abstract

In this work, the technical feasibility of a fluidized and a fixed bed heat exchanger in a concentrating solar power (CSP) tower for heat recovery applications is analysed using Two-Fluid Model simulations. The heat recovery process analysed in this work corresponds to the discharge of sensible heat from solid particles.In the cases studied, the fluidizing agent of the bed is carbon dioxide (CO 2 ) in supercritical conditions and the particles, which constitute the bed material, are sensible heat storage material. CO 2 is gaining attention in its application as a working fluid in thermodynamic cycles for power generation, especially in transcritical and supercritical conditions due to its high density and excellent heat transfer characteristics. Currently, research is focused on exploring the CO 2 capabilities when used in combination with CSP technologies, together with systems that allow the storage and recovery of the solar thermal energy.Fixed or fluidized beds work as a direct contact heat exchanger between the particles and the working fluid that percolates through the bed material. Several bed configurations are presented to derive the optimal configuration of the bed that enhances the efficiency from both the energetic and the exergetic points of view. The results indicate that a fixed bed heat exchanger produces a maximum increase of availability in the CO 2 flow during longer times than a fluidized bed heat exchanger. Therefore, to maximise the exergy recovery from solar heated particles to supercritical CO 2 a fixed bed heat exchanger is more suitable than a fluidized bed heat exchanger.

Numerical study of the effect of pressure and temperature on the fluidization of solids with air and (supercritical) CO2

“…The 2D computational domain was meshed with square cells of 2.5 mm size and a second order accurate scheme was selected to discretise the convective derivatives of the governing equations. This mesh size has been proved to reproduce satisfactory the fluidization behaviour under conditions of high pressure and temperature [33]. The gas flow is fed uniformly through the bottom of the bed using a velocity inlet boundary condition.…”

Numerical Comparison of Thermal Energy Performance between Spouted, Fluidized and Fixed Beds Using Supercritical Co2 as Fluidizing Agent

“…For instance, a Brayton cycle CSP plant integrated with sensible heat storage and using a granular bed was designed in Ref. [10], in which supercritical CO 2 (sCO 2 ) was used as the heat transfer fluid (HTF). The density of sCO 2 is similar to that of a common liquid [11,12].…”

“…The feasibility of fluidized and fixed beds as direct contact heat exchangers between the particles and gas was also discussed in Ref. [10] in terms of outlet gas temperature and Carnot efficiency. During a short period of time, the fixed bed can maintain a maximum outlet temperature and Carnot efficiency [10].…”

“…[10] in terms of outlet gas temperature and Carnot efficiency. During a short period of time, the fixed bed can maintain a maximum outlet temperature and Carnot efficiency [10].…”

Exergy analysis of solar central receivers

“…Regarding the SPT technologies, [20] tested different HTF alternatives performing an exergy analysis of the receiver, proving the great performance of molten salts; however, it did not take into account the circumferential variations of the tubes temperature nor the multiple reflections between surfaces. Not only the receiver subsystem has been studied but also the concentrators [21] of all CSP technologies and the power block [22], including supercritical cycles [23,24]. In the literature, there can be found several global analysis of SPT plants, which have shown that the receiver is among the elements suffering from greater exergy losses [20,22,25] given the high heat fluxes exchanged on that device.…”