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Research Papers

Applicability of Heat Mirrors in Reducing Thermal Losses in Concentrating Solar Collectors

[+] Author and Article Information
Vikrant Khullar

Mechanical Engineering Department,
Thapar Institute of Engineering and Technology,
Patiala 147004, Punjab, India
e-mail: vikrant.khullar@thapar.edu

Prashant Mahendra, Madhup Mittal

Mechanical Engineering Department,
Thapar Institute of Engineering and Technology,
Patiala 147004, Punjab, India

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF THERMAL SCIENCE AND ENGINEERING APPLICATIONS. Manuscript received October 31, 2017; final manuscript received June 2, 2018; published online August 6, 2018. Assoc. Editor: Nesrin Ozalp.

J. Thermal Sci. Eng. Appl 10(6), 061004 (Aug 06, 2018) (14 pages) Paper No: TSEA-17-1422; doi: 10.1115/1.4040653 History: Received October 31, 2017; Revised June 02, 2018

In the present work, a novel parabolic trough receiver design has been proposed. The proposed design is similar to the conventional receiver design except for the envelope and the annulus part. Here, a certain portion of the conventional glass envelope is coated with Sn-In2O3 and also Sn-In2O3 coated glass baffles are provided in the annulus part to reduce the radiative losses. The optical properties of the coated glass are such that it allows most of the solar irradiance to pass through, but reflects the emitted long wavelength radiations back to the absorber tube. Sn-In2O3 coated glass is referred to as “transparent heat mirror.” Thus, effectively reducing the heat loss area and improving the thermal efficiency of the solar collector. A detailed one-dimensional steady-state heat transfer model has been developed to predict the performance of the proposed receiver design. It was observed that while maintaining the same external conditions (such as ambient/initial temperatures, wind speed, solar insolation, flow rate, and concentration ratio), the heat mirror-based parabolic trough receiver design has about 3–5% higher thermal efficiency as compared to the conventional receiver design. Furthermore, the heat transfer analysis reveals that depending on the spatial incident solar flux distribution, there is an optimum circumferential angle (θ = θoptimum, where θ is the heat mirror circumferential angle) up to which the glass envelope should be coated with Sn-In2O3. For angles higher than the optimum angle, the collector efficiency tends to decrease owing to increase in optical losses.

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References

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Figures

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Fig. 2

Schematic showing (a) parabolic trough collector, (b) conventional receiver design, and (c) the proposed receiver design

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Fig. 1

Details of selected surface absorption based parabolic trough receiver designs in the literature

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Fig. 3

(a) Spectral normal reflectance and (b) Effective emittance as a function of absorber tube temperature of Sn-In2O3 coated glass envelope

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Fig. 6

(a) Schematic showing the thermal resistance model and (b) the heat transfer mechanisms involved in proposed receiver design

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Fig. 4

(a) Local concentration ratio around the receiver of a parabolic trough having rim angle 70 deg (data points taken from Ref. [28]), and (b) schematic showing the regions of the hybrid glass envelope having different optical properties; θ denotes the angle up to which the glass envelope has been coated

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Fig. 5

Optical efficiency of parabolic trough as a function of heat mirror circumferential angle

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Fig. 12

Effect of incidence angle on the effective emittance of the heat mirror for various absorber tube temperatures. Wind speed 1 ms−1 and DNI = 807 Wm−2.

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Fig. 7

For different values of circumferential heat mirror angles (a) envelope temperature as function absorber tube temperature, (b) convection losses as function absorber tube temperature, (c) radiation losses as function absorber tube temperature, and (d) total thermal losses as function absorber tube temperature. Wind speed 1 ms−1 and DNI = 807 Wm−2.

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Fig. 8

Thermal efficiency as a function of absorber tube temperature for different circumferential heat mirror angle. Wind speed 1 ms−1 and DNI = 807 Wm−2.

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Fig. 9

For a given absorber tube temperature and for different values of θ (a) radiation losses as a function of wind speed, (b) convection losses as a function of wind speed, and (c) thermal losses as a function of wind speed

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Fig. 10

For various absorber tube temperatures (a) radiation losses as a function of θ, (b) convection losses as a function of θ, and (c) thermal efficiency as a function of θ. Wind speed 1 ms−1 and DNI = 807 Wm−2.

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Fig. 11

Thermal efficiency as a function of absorber tube temperature for the proposed and conventional receiver designs employing SCHOTT PTR®70 coated absorber tube

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Fig. 13

Thermal efficiency as a function of absorber tube temperature for nonuniform and uniform absorber tube temperature

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Fig. 14

(a) For a given heat mirror emittance, the effect of absorber tube emittance on the thermal efficiency of the receiver and (b) for a given absorber tube emittance, the effect of heat mirror emittance on the thermal efficiency of the receiver. Wind speed 1 ms−1, DNI = 807 Wm−2, and Tr = 600 °C.

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Fig. 15

Schematic showing the application of Hottel's string method. Circular arc “abe” (portion of receiver circumference) represents surface 1, circular arc “dc” (coated or un-coated portion of the glass envelope as the case may be) represents the surface 2, and combined “ce” and “da” (baffles) represent the surface 3, respectively.

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Fig. 16

Effect of baffles on the thermal efficiency of the parabolic trough collector. Wind speed 1 ms−1 and DNI = 807 Wm−2.

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