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Research Papers

Optimization Model for Solar Thermochemical Reactor: Efficiency Increase by a Nonuniform Heat Sink Distribution

[+] Author and Article Information
S. Tescari

N. Mazet

PROMES-CNRS, Rambla de la Thermodynamique, Tecnosud, 66100 Perpignan, Francemazet@univ-perp.fr

P. Neveu

 Université de Perpignan Via Domitia, 52 avenue Paul Alduy, 66860 Perpignan, Franceneveu@univ-perp.fr

S. Abanades

PROMES-CNRS, 7 Rue du Four Solaire, 66120 Font Romeu, Francestephane.abanades@promes.cnrs.fr

J. Sol. Energy Eng 133(3), 031011 (Jul 25, 2011) (6 pages) doi:10.1115/1.4004271 History: Received February 16, 2011; Revised May 06, 2011; Published July 25, 2011; Online July 25, 2011

This study focuses on thermochemical cavity-type reactor, with a reactive material directly irradiated by concentrated solar energy. General tendencies of reactor performance are analyzed as a function of the reactor geometry. The objective is to define a simplified model that can be adapted easily to different reactor designs or different operating conditions. For this reason, the chemical reaction is not precisely fixed but rather characterized by a reaction temperature and a useful power consumed by the endothermic reaction, inside the reactive material. In order to increase the efficiency, two new reactor designs are proposed. These designs allow obtaining a nonuniform distribution of the useful power consumed by the reaction with the depth in a circular cylindrical cavity (z-axis). This is done in two ways: by varying the reactive material thickness along the axis or by varying its density at a constant thickness. The results show that these reactor concepts lead to a more uniform temperature distribution along the z-axis and a diminution of the heat losses. Thus, the reactor efficiency can increase significantly. The results of the simplified model can be used as a system predesign. A more accurate CFD model could be used afterward to refine the optimal shape.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Design of three thermochemical cavity reactors. S0 , irradiated surface; Sbase , bottom surface of the cavity; L, reactor length; R, cavity radius; H, external reactor radius (reactors a and b); Hi , external radius of each subdomain i (reactor c). (a) Reactor with uniform heat sink inside the reactive material (in black), studied in Ref. [16]. Slat is the lateral surface of the cavity. (b) Reactor with nonuniform heat sink. The reactive volume is divided in five subdomains of different heat sinks (σq ,i ) and of internal surface Si . (c) Reactor with nonuniform reactive material thickness. The reactive volume (in black) is divided in five subdomains of different volumes (Vm,i  = π·(Hi 2  − R2 )·L/5) and of internal surface Si .

Grahic Jump Location
Figure 2

Efficiency of a cavity reactor as a function of the shape factor f for three different reactors. Dashed lines, uniform heat sink (Fig. 1a); continuous line with crosses, nonuniform heat sink (Fig. 1b); continuous line with squares, nonuniform thickness (Fig. 1c). Operating parameters: I0  = 1 kW; Vm  = 2·× 10−5 m3 , Tr  = 1400 K, k = 1 W/m·K. ϕ = 0.54 for the uniform heat sink reactor and 0.73 for the other two reactors.

Grahic Jump Location
Figure 3

Temperature distribution along the z-axis, between 0 and L, for two cavity reactors. Lines without symbols, uniform heat sink reactor; lines with square symbols, nonuniform heat sink reactor; continuous lines, temperature on the external lateral reactor surface (TH ); dashed lines, temperature on the internal lateral surface (TR ). z/L = 0: closest point to the irradiated surface. z/L = 1: farthest point to the irradiated surface. Operating parameters: I0  = 1 kW; Vm  = 2× 10−5 m3 , Tr  = 1400 K, k = 1 W/m·K. f = 4; ϕ = 0.54 for the uniform heat sink reactor and ϕ = 0.73 for the nonuniform heat sink reactor.

Grahic Jump Location
Figure 4

Heat losses (eq. 13) and useful power (eq. 14) distribution along the z-axis, between 0 and L, for two cavity reactors: lines without symbols: uniform heat sink reactor; lines with square symbols: nonuniform heat sink reactor. Dashed lines: useful power consumed by the reaction. Continuous lines: heat losses toward environment. Operating parameters: I0  = 1 kW; Vm  = 2·× 10−5 m3 , Tr  = 1400 K, k = 1 W/m·K. f = 4; ϕ = 0.54 for the uniform heat sink reactor and ϕ = 0.73 for the nonuniform heat sink reactor.

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