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Research Papers

Real-Size Experiments on Reverse Natural Air Convection Between Inclined Parallel Plates for New Insulation Methods in Solar Flat-Plate Collectors

[+] Author and Article Information
Th. Beikircher

Bavarian Center for Applied Energy
Research (ZAE Bayern),
Division Energy Storage,
Walther-Meißner-Straße 6,
Garching 85748, Germany
e-mail: thomas.beikircher@zae-bayern.de

V. Berger, M. Möckl

Bavarian Center for Applied Energy
Research (ZAE Bayern),
Division Energy Storage,
Walther-Meißner-Straße 6,
Garching 85748, Germany

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received September 19, 2015; final manuscript received September 24, 2016; published online November 10, 2016. Assoc. Editor: Werner Platzer.

J. Sol. Energy Eng 139(2), 021003 (Nov 10, 2016) (7 pages) Paper No: SOL-15-1314; doi: 10.1115/1.4035024 History: Received September 19, 2015; Revised September 24, 2016

Reverse natural air convection (hot plate top) was experimentally investigated between two inclined parallel aluminum plates (1 m × 2 m × 3 mm) with a separation distance of 20 mm to 100 mm. The inclination ϑ to the horizontal was varied from 0 deg to 90 deg. The mean temperatures of the plates have been adjusted to 90 °C and 30 °C resulting in Rayleigh numbers Ra between 2.7 × 104 and 3.3 × 106. The experimental conditions correspond to the back side of an absorber in a typical solar flat-plate collector, where the conventional insulation has been removed. The upper hot plate simulates the absorber and was electrically heated by an area heater, while the temperature distribution over the plate was measured. The lower cold plate was held isothermally by integrated water tubes and a thermostat. The side walls of the rectangular cavity were thermally connected to the colder plate and had a distance of 10 mm to the hot plate, comparable to a typical collector casing. The experimentally obtained results for Nu (Ra,ϑ) were mathematically described and compared to rare reverse convection data of other authors, gained at smaller aspect ratios/flow lengths and for adiabatic side walls: The formula of Elsherbiny approximately (within 10%) describes solar flat-plate collectors between 0 deg and 60 deg inclination, while the relations of Arnold, Ozoe, and Inaba show large errors up to 50%. Additionally, we experimentally showed that pure air gap insulation (30–50 mm) has surprisingly acceptable loss coefficients between 1.3 and 2.5 W/m2K depending on collector slope. It can be used as a cheap insulation method for low temperature collector applications. Additionally, inserting an 25–50 μm thick aluminum film symmetrically between the plates, a new and efficient insulation method for the absorber of a solar flat-plate collector was experimentally investigated: At plate distances of 30–50 mm, temperatures below 100 °C and slopes below 45 deg, this compact and cheap film insulation was proven to be equivalent to dry mineral wool and avoids its disadvantage of worsening insulation properties due to humidity.

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References

Beikircher, T. , Berger, V. , Osgyan, P. , Reuß, M. , and Streib, G. , 2014, “ Low-e Confined Air Chambers in Solar Flat-Plate Collectors as an Economic New Type of Rear Side Insulation Avoiding Moisture Problems,” Sol. Energy, 105, pp. 280–289. [CrossRef]
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Dropkin, D. , and Sommerscales, E. , 1965, “ Heat Transfer by Natural Convection in Liquids Confined by Two Parallel Plates Which are Inclined at Various Angles With Respect to the Horizontal,” ASME J. Heat Transfer, 87(1), pp. 77–82. [CrossRef]
Hollands, K. G. T. , Unny, T. E. , Raithby, G. D. , and Konicek, L. , 1976, “ Free Convective Heat Transfer Across Inclined Air Layers,” ASME J. Heat Transfer, 98(2), pp. 189–193. [CrossRef]
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Arnold, J. , Bonaparte, P. , Catton, I. , and Edwards, D. , 1974, “ Experimental Investigation of Natural Convection in a Finite Rectangular Region Inclined at Various Angles Between 0 and 180 deg,” 1974 Heat Transfer and Fluid Mechanics Institute, Stanford University Press, Stanford, CA.
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Ozoe, H. , Yamamoto, K. , Sayama, H. , and Churchill, S. , 1974, “ Natural Circulation in an Inclined Rectangular Channel Heated on One Side and Cooled on the Opposing Side,” Int. J. Heat Mass Transfer, 17(10), pp. 1209–1217. [CrossRef]
Churchill, S. , Ozoe, H. , and Sayama, H. , 1975, “ Natural Convection in an Inclined Rectangular Channel at Various Aspect Ratios and Angles: Experimental Measurements,” Int. J. Heat Mass Transfer, 18(12), pp. 1425–1431. [CrossRef]
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Figures

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Fig. 1

Sketch of the geometry under investigation

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Fig. 2

Sketch of the experimental setup

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Fig. 3

(a) Side face of the real-size laboratory setup in inclined position. On the left, the thermostat for the cold plate and the DC power supply for the hot plate as well as the measuring DMM and the side insulation can be seen; (b) experimental setup from the front side with area heater (2000 × 940 × 2 mm3) after removing the front PIR insulation.

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Fig. 4

Area heater with Pt100 temperature sensors for heating the upper plate

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Fig. 5

Comparison between experimental values Nu(Ra,ϑ) according to Table 5 and theory according to Eq. (10). The accordance between theory and experiment is excellent within the error bars.

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Fig. 7

Slope-dependent total U-value [W/m2K] of the upper hot (90 °C) plate to the lower cold (30 °C) plate with different insulation concepts between the plates for 30 mm (circles) and 50 mm (squares) plate distance: No insulation, i.e., air gap (hollow), two- (hollow and crossed) and four-sided (filled) film insulation. For comparison, U-values for different mineral wool insulations plotted (bar ranges, upper bar corresponding to 30 mm, lower to 50 mm insulation strength).

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Fig. 6

Measuring data for Ra = 90,000, a = 67, l = 2 m and comparison to predictions of other authors with smaller a and l. Additionally, Eq. (10) (Beikircher) is plotted. The table characterizes the determination of Nu(90 deg), fitting the experimental data to Eq. (10) with Nu(90 deg) as free parameter.

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