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Research Papers

# Radiative Transfer Within a Cylindrical Cavity With Diffusely/Specularly Reflecting Inner Walls Containing an Array of Tubular Absorbers

[+] Author and Article Information
Tom Melchior

Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland

Aldo Steinfeld1

Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland; and Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen, Switzerlandaldo.steinfeld@eth.ch

The solar flux concentration ratio $C$ is defined as the incident radiative power flux normalized by $1kW∕m2$ and is often reported in units of “suns.”

1

Corresponding author.

J. Sol. Energy Eng 130(2), 021013 (Mar 20, 2008) (7 pages) doi:10.1115/1.2888755 History: Received March 29, 2006; Revised April 24, 2006; Published March 20, 2008

## Abstract

Monte Carlo radiative transfer analysis is applied to a cylindrical cavity-receiver containing an array of high-temperature tubular absorbers directly exposed to concentrated solar power entering through a spectrally selective window. The cavity walls are assumed either diffusely or specularly reflective. The relative dimensions, the number of tubes, and their position are optimized for maximum energy transfer efficiency or maximum absorber temperature. A single-tube absorber operating at $2000K$ performs best when located at 60% relative distance to the cavity’s aperture. Higher absorber temperatures are attained for a specularly reflective cavity that serves as internal infrared mirror but at the expense of lower energy transfer efficiencies. In contrast, diffuse reflecting cavity walls promote a more uniform temperature distribution around the tubular absorber. Decreasing the window-to-cavity areas ratio further results in an increase of the absorber temperature, which peaks for an optimum absorber-to-cavity radii ratio. This optimum ratio shifts to lower values for multiple-tube absorbers. However, the average absorber temperature is not significantly affected by using multiple-tube absorbers of constant total cross sectional area.

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## Figures

Figure 1

Scheme of solar receiver configuration, featuring the cavity, absorber, window, and CPC. (a) Single-tube absorber and (b) multiple-tube absorber.

Figure 2

Optical properties of a 5-mm-thick quartz window for normal incident solar radiation: (a) spectral reflectivity and transmissivity; (b) spectral absorptance, transmittance, and reflectance

Figure 3

Variation of the energy transfer efficiency as a function of the average absorber temperature for the reference case, and corresponding average window and cavity temperatures

Figure 4

Temperature distribution on absorber for the reference case with η=0, 0.3, and 0.5

Figure 5

Emitted, transmitted, and reflected radiation losses by the window, in percentage of Qinput, as a function of the average absorber temperature for the reference case

Figure 6

Variation of the average absorber temperatures as a function of the energy transfer efficiency for three configurations: (a) a diffuse-reflecting cavity with CPC and selective window (reference case), (b) a diffuse-reflecting cavity with CPC but without window (T=1, A=R=0), (c) a specular-reflecting cavity without CPC and without window

Figure 7

Absorber temperature distribution for the three configurations: (a) a diffuse-reflecting cavity with CPC and selective window (reference case), (b) a diffuse-reflecting cavity with CPC but without window (T=1, A=R=0), and (c) a specular-reflecting cavity without CPC or window

Figure 8

Average, highest, and lowest absorber temperature, as a function of the absorber position, varied between 10% and 90% of the maximum distance to the aperture

Figure 9

Absorber temperature distribution when positioned at (a) 10%, (b) 60%, and (c) 90% of the maximum distance to the aperture

Figure 10

Average absorber array temperature as a function of Aw∕Ac for multiple-tube absorbers

Figure 11

Absorber temperature distribution for the reference case (single-tube configuration) and for (a) Aw∕Ac=0.313, (b) Aw∕Ac=0.062, and (c) Aw∕Ac=0.01

Figure 12

Angle β between tubes for three configurations of the four-tube absorber: (a) β=0deg, (b) β=45deg, and (c) β=90deg.

Figure 13

Average absorber array temperature as a function of radii ratio ra∕rc for multiple-tube absorbers

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