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Research Papers

Evaporation in Solar Thermal Collectors During Operation—Reasons and Effects of Partial Stagnation

[+] Author and Article Information
J. Glembin, D. Eggert, G. Rockendorf, J. Scheuren

 Institut für Solarenergieforschung Hameln (ISFH), Am Ohrberg 1, Emmerthal 31860, Germanyj.glembin@isfh.de

J. Sol. Energy Eng 133(4), 041003 (Sep 20, 2011) (9 pages) doi:10.1115/1.4004384 History: Received October 25, 2010; Revised May 17, 2011; Published September 20, 2011; Online September 20, 2011

Evaporation in solar thermal collectors normally takes place when the collector pump is not running—the so-called full stagnation. But it is possible that part of the heat transfer fluid evaporates inside a solar thermal collector field although the pump is operating and the collector field outlet temperature is significantly below the evaporation temperature. This operating status is called partial stagnation since only parts of the collector are affected by evaporation. Partial stagnation happens at a pronounced nonuniform temperature distribution in combination with a low mass flow rate and/or a high temperature level. A main reason for an irregular temperature distribution is a nonuniform flow distribution inside the solar thermal system. The paper presents an experimental investigation that analyzes the reasons and effects of partial stagnation occurrences. For this, outdoor measurements were made with a direct-flow vacuum tube collector. Criteria that promote partial stagnation have been identified, such as a coaxial tube design, a low system pressure, and a high gas content of the fluid. Performance measurements show no efficiency reduction during partial stagnation in the system investigated at a horizontal or positive collector slope. A high degree of partial stagnation, however, might pass into a complete evaporation of the collector volume although the collector pump is still running. This could lead to a complete blockage of the flow and a high thermal load of the system components. In all cases, partial stagnation leads to an unstable operation and a high load of the collector fluid and should, therefore, be avoided by design measures. A minimized risk for evaporation during operation is achieved by a more equal flow distribution inside the collector and the whole collector field, air bubbles, and solid particles should be completely removed. In addition, the gas content dissolved in the fluid may be reduced and the system pressure level may be increased in order to raise the boiling temperature.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Scheme of collector group (including tube numbering) with one-sided (OSC, solid lines) and reverse return (RRC, dashed lines) connection (left); detailed view of the connection between the tubes and the header with temperature sensors attached to each tube outlet (right)

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Figure 2

Temperature patterns as well as mass flow and global irradiance during an experiment concerning the emergence of partial stagnation versus time of the day (OSC, see Fig. 1 for numbering of tubes)

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Figure 3

Temperature patterns as well as mass flow and global irradiance during an experiment to generate partial stagnation of high intensity versus time of the day (OSC, see Fig. 1 for numbering of tubes)

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Figure 4

Reasons for partial stagnation divided into required and causing factors

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Figure 5

Left: Gas solubility of nitrogen dependent on temperature and pressure level with measured air content for the investigated variants; right: mass flow rate, below which partial stagnation has been observed (i.e. minimum mass flow rate without evaporation), depending on global irradiance level at different levels of gas content in the fluid

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Figure 6

Measured flow distribution of the investigated collector group (60 parallel vacuum tubes) in one-sided connection and reverse-return connection (left, see Fig. 1 for numbering of tubes); mass flow rate, below which partial stagnation has been observed, depending on global irradiance level at different collector connections (right)

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Figure 7

Temperature profiles of three tubes with different tube flow rates under the operating conditions of Fig. 3 at 11:30; evaporation has not been simulated

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Figure 8

Minimum mass flow rate when boiling point is exceeded for different configurations of direct-flow vacuum tube collectors and connection types

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Figure 9

Efficiency curve of solar simulator test according EN 12975-2 and efficiency curve measured with the outdoor testing device of Fig. 1 with the expanded uncertainty range, both valid for 900 W/m2. Efficiency readings during partial stagnation at 900 ± 50 W/m2

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