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Research Papers: Radiative Heat Transfer

Performance of Near-Field Thermophotovoltaic Cells Enhanced With a Backside Reflector

[+] Author and Article Information
T. J. Bright

George W. Woodruff School
of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332

L. P. Wang

School for Engineering of Matter,
Transport, and Energy,
Arizona State University,
Tempe, AZ 85287

Z. M. Zhang

Fellow ASME
George W. Woodruff School
of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332
e-mail: zhuomin.zhang@me.gatech.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received June 5, 2013; final manuscript received January 6, 2014; published online March 7, 2014. Assoc. Editor: Wilson K. S. Chiu.

J. Heat Transfer 136(6), 062701 (Mar 07, 2014) (9 pages) Paper No: HT-13-1281; doi: 10.1115/1.4026455 History: Received June 05, 2013; Revised January 06, 2014

Thermophotovoltaic (TPV) systems are very promising for waste heat recovery. This work analyzes the performance of a near-field TPV device with a gold reflecting layer on the backside of the cell. The radiative transfer from a tungsten radiator, at a temperature ranging from 1250 K to 2000 K, to an In0.18Ga0.82Sb TPV cell at 300 K is calculated using fluctuational electrodynamics. The current generation by the absorbed photon energy is modeled by the minority carrier diffusion equations considering recombination. The energy conversion efficiency of the cell is determined from the generated electrical power and the net absorbed radiant power per unit area. A parametric study of the cell efficiency considering the gap spacing and other parameters is conducted. For an emitter at temperature 1250 K, the efficiency enhancement by adding a mirror, which reduces the sub-bandgap radiation, is shown to be as much as 35% relative to a semi-infinite TPV cell. In addition, the potential for further improvement by reducing surface recombination velocity from that of a perfect ohmic contact is examined. The cell performance is shown to increase with decreasing gap spacing below a critical surface recombination velocity.

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Figures

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Fig. 1

Schematic of the tungsten emitter and the TPV cell with different regions delineated. The In0.18Ga0.82Sb TPV cell is assumed to be at 300 K. The bottom of the active cell is either a gold mirror or a substrate with the same optical properties as the cell.

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Fig. 9

Efficiency improvement for varying thicknesses of the p-region with a 2000 K emitter: (a) with mirror and (b) without mirror. The improvement is due to reduction in bulk recombination. Note that the line styles for (a) and (b) are the same.

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Fig. 2

The efficiency of a TPV cell with a tungsten emitter at 2000 K with and without mirror on the backside. The mirror increases the efficiencies both for the ideal case with 100% quantum efficiency and the more realistic model considering recombination.

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Fig. 3

The effect of mirror on the efficiency for different tungsten emitter temperatures: (a) the absolute efficiency with and without the mirror, and (b) the relative improvement in efficiency using the mirror

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Fig. 4

Contour plot of the Poynting vector as a function of depth and wavelength with a 1500 K emitter for d = 10 nm: (a) with a back side reflector and (b) without a back side reflector. Note that the unit of Sλ is (MW/m2-μm).

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Fig. 5

Spectral Poynting vector at the TPV cell surface (z = 0) as a function of wavelength, showing the reduction of energy flux with mirror at wavelengths beyond the bandgap indicated by the dashed vertical line

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Fig. 6

Efficiency improvement by reducing recombination velocity at the top surface of the TPV cell for an emitter at 2000 K: (a) with a mirror and (b) without a mirror. If a lower enough recombination velocity can be achieved, the trend of efficiency toward smaller gap spacing reverses.

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Fig. 7

Minority carrier concentration at the cell surface (z = 0) and bulk recombination rate Rbulk (in the p-layer) as functions of gap spacing for different surface recombination velocities for an emitter at 2000 K with back side mirror

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Fig. 8

Ratio of the bulk recombination rate to surface recombination rate, showing the transition point between the dominant mechanism as the critical surface recombination velocity, Se,c

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