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Research Papers

Heat Transfer Analysis for a Small-Size Direct-Flow Coaxial Concentrating Collector

[+] Author and Article Information
Fabrizio Alberti1

 Renewable Energies and Environmental Technology, Fondazione Bruno Kessler, 38123 Povo, Trento, Italyalberti@fbk.eu

Luigi Crema, Alessandro Bozzoli

 Renewable Energies and Environmental Technology, Fondazione Bruno Kessler, 38123 Povo, Trento, Italy

1

Corresponding author.

J. Sol. Energy Eng 134(4), 041009 (Aug 31, 2012) (7 pages) doi:10.1115/1.4007297 History: Received October 12, 2011; Revised June 26, 2012; Published August 31, 2012; Online August 31, 2012

A coaxial evacuated solar tube has been analyzed. The tube is included in a small-scale concentrated solar power (CSP) system, which runs a cogeneration Stirling engine unit. The engine provides electricity and at the same time generates hot water for heating and sanitary purposes, by cooling down the compression cylinder. The present work is focused on the thermodynamic characterization for a forced-flow in the coaxial evacuated tube, which can heat thermal oil up to 300 °C, when coupled with a parabolic trough collector. The single coaxial tube is 2 m long, it has one glass penetration, it is provided with a glass–metal seal and it has an absorber tube in the focal point with a diameter of 12 mm. A model based on heat transfer analysis coupled with fluid dynamic is presented and discussed. The model is then used to investigate spatial temperature profiles and thermal behaviors for the whole solar collector. It improves previous works in the field of concentrating solar collectors and covers the research in small-size concentrating system using thermal oil as heat transfer fluid.

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Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

An example of concentrating optics with a East–West tracking system

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Figure 2

Cogeneration system layout

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Figure 3

Types of vacuum tube collectors: (a) U-pipe configuration, (b) coaxial configuration, and (c) conventional parabolic trough

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Figure 4

Schematic of the two-dimensional heat transfer model. The list of heat fluxes and variable in the image are not exhaustive.

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Figure 5

Temperatures, thermal resistances, and heat fluxes on a collector cross-section

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Figure 6

Efficiency versus fluid inlet velocity in a logarithmic scale

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Figure 7

Temperature profile for v1inlet  = 0.1 m/s, without concentration. Temperature is defined according to Fig. 5 and Table 1: fluid bulk temperature for the inner and outer tubes (T1ave ; T4ave ); annulus inner surface (T5 ); inner pipe–inner surface (T2 ); inner pipe–outer surface (T3 ); absorber surface (T6 ). Temperature differences across the tube walls (T6 −T5 ; T3 −T2 ) are very small and are not appreciable within this scale; only one temperature for each pair is illustrated.

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Figure 8

Temperature profile for v1inlet  = 0.01 m/s, without concentration. Temperature is defined according to Fig. 5 and Table 1: fluid bulk temperature for the inner and outer tubes (T1ave ; T4ave ); annulus inner surface (T5 ); inner pipe–inner surface (T2 ); inner pipe–outer surface (T3 ); absorber surface (T6 ). Temperature differences across the tube walls (T6 −T5 ; T3 −T2 ) are very small and are not appreciable within this scale; only one temperature for each pair is illustrated.

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Figure 9

Maximum film and bulk temperature for the fluid inside the collector, as a function of inlet velocity, with concentration applied. The fluid is entering the collector with a temperature of 300 °C. The dashed line represents the maximum allowable temperature (345 °C) for thermal oil. When a minimum inlet velocity is imposed (0.1 m/s), no relevant internal coupling is observed, and the maximum bulk and film temperature are always located at the outlet.

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Figure 10

Efficiency versus different pressure in the vacuum annulus. Efficiency starts to drop when pressure is at 0.0001 Torr (0.013 Pa).

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