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Research Papers

Experiments on Natural Convective Solar Thermal Achieved by Supercritical CO2/Dimethyl Ether Mixture Fluid

[+] Author and Article Information
Lin Chen

Department of Energy and
Resources Engineering,
College of Engineering,
Peking University,
Beijing 100871, China

Xin-Rong Zhang

Department of Energy and
Resources Engineering,
College of Engineering,
Peking University,
Beijing 100871, China
Beijing Key Laboratory for Solid Waste
Utilization and Management,
Peking University,
Beijing 100871, China
e-mail: zhxrduph@yahoo.com

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received October 15, 2012; final manuscript received January 17, 2014; published online March 6, 2014. Editor: Gilles Flamant.

J. Sol. Energy Eng 136(3), 031011 (Mar 06, 2014) (11 pages) Paper No: SOL-12-1283; doi: 10.1115/1.4026920 History: Received October 15, 2012; Revised January 17, 2014

The current study proposed an experimental investigation into the basic characteristics of solar thermal conversion using supercritical CO2–dimethyl ether (DME) natural convection. The main goals are to reduce the operation pressure while maintaining relative high solar thermal conversion efficiency. Experimental systems were established and tested in Shaoxing area (around N 30.0 deg, E 120.6 deg) of Zhejiang Province, China. Due to the preferable properties of supercritical fluids, very high Reynolds number natural convective flow can be achieved. Typical summer day results are presented and analyzed into detail in this paper. It is found that the introduction of DME has successfully reduced the operation pressure and the increase in DME fraction leads to further reduction. Different from pure supercritical CO2 systems, the collector pressure follows the trend of solar radiation with its peak value at noon, instead of continuously increasing mode. The mass flow rate and temperature are typically more stable and also more sensitive than pure supercritical CO2 tests due to the moderation of supercritical fluid properties when DME is introduced. At the same time, the averaged collector efficiency is less affected by the DME mass addition. It is also found that there possibly exist some optimal of DME mass fraction when both the system suitability and stable natural circulation can be achieved.

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Figures

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Fig. 1

Variation of thermophysical properties of the CO2–DME mixture refrigerant with different DME mass fractions at 8.0 MPa. (a) Density; (b) specific heat; (c) thermal conductivity; and (d)viscosity.

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Fig. 2

Schematic diagram of the supercritical CO2–DME natural convection based solar water heater

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Fig. 3

Variations of the measured solar radiation and fluid pressures with test time. The mass fraction of DME is 10%. (a) July 8, 2010 and (b) Aug. 9, 2010.

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Fig. 4

Variations of the measured solar radiation and mass flow rate with the test time. The mass fraction of DME is 10%. (a) July 8, 2010 and (b) Aug. 9, 2010.

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Fig. 5

Variations of the measured mass flow rate and fluid temperatures with the test time. The mass fraction of DME is 10%. (a) July 8, 2010 and (b) Aug. 9, 2010.

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Fig. 6

Variations of the collector efficiency with the comprehensive coefficient. The mass fraction of DME is 10%. (a) July 8, 2010 and (b) Aug. 9, 2010.

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Fig. 7

Variations of the measured solar radiation and fluid pressures with the test time. The mass fraction of DME is 20%. (a) Sept. 8, 2010 and (b) Sept. 12, 2010.

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Fig. 8

Variations of the measured solar radiation and mass flow rate with the test time. The mass fraction of DME is 20%. (a) Sept. 8, 2010 and (b) Sept. 12, 2010.

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Fig. 9

Variations of the measured mass flow rate and fluid temperatures with the test time. The mass fraction of DME is 20%. (a) Sept. 8, 2010 and (b) Sept. 12, 2010.

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Fig. 10

Variations of the collector efficiency with the comprehensive coefficient. The mass fraction of DME is 20%. (a) Sept. 8, 2010 and (b) Sept. 12, 2010.

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Fig. 11

Variations of the time-averaged pressures with the mass fraction of DME

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Fig. 12

Variations of the measured collector efficiency with the comprehensive coefficient under three different mass fractions of DME, 0%, 10%, and 20%. Every point shown in this figure represents a time-averaged value during one day.

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