This work aims to investigate the various factors which may affect a thermophotovoltaic (TPV) system's performance, with a special focus on the importance of incorporating a back surface reflector (BSR), which enables below-bandgap photons' recycling. The possible extent to which common PV materials can be used in TPV applications is investigated by comparing them on a Planck distribution curve. The effects of varying BSR reflectivity, TPV cell's external quantum efficiency, and emitter temperature are investigated on the TPV module's efficiency using open-circuit voltage, empirical relations for fill factor, maximum voltage, and photogenerated current. It is shown that TPV applications require materials with smaller (e.g. 0.6 eV < Eg ≤ 0.74 eV) bandgap energy, e.g. In0.53Ga0.47As (0.74 eV), due to their high percentage of energy (> 26%) abovebandgap without a spectral control and a small difference between peak and bandgap wavelength. It is shown that the inclusion of a BSR (reflectivity = 1) results in an increase of 15% in TPV efficiency. The results show that by the collective changes of an added BSR, high emitter temperature (> 2000 K), and improved external quantum efficiency (EQE ≈ 1), the present TPV systems can attain efficiency values more than 30% which makes them a favorable prospective choice for Concentrated Solar Power.
Satellite communication systems require dual-band circularly polarized (CP) waves to integrate receiving and transmit antennas into one dual-band terminal. Polarization converters play an important role in such a scenario to convert the incoming linearly polarized (LP) signal into the transmitted CP signal. Wide-band and wide-angle performance for the transmission based dual-band polarization converters has been a challenge. In this paper, a transmission-based linear polarization to circular polarization (LP-to-CP) converter is presented using a very simple and thin metasurface based structure. The proposed metasheet is uni-layered, patterned with a circular diagonal split ring enclosed in the square ring. The idea for dual-band polarization conversion is verified with an example in which x(y) polarization is converted into LHCP(RHCP) in Ku-band (15.25~18.9 GHz), and RHCP(LHCP) in Ka-band (29.7~36.7GHz). The converter performs polarization conversion for both bands over 21% of bandwidth which is very wide. Moreover, it maintains performance over wide-angled oblique incidences. Dual-band performance is analyzed theoretically, numerically, and experimentally. The equivalent circuit model is also presented for the unit cell. This metasheet has simple and low-cost design, wide polarization conversion bandwidths, polarization diversity, and wide-angle performance which are very promising for future communication systems.
Polarization converters play an important role in modern communication systems, but their wide and multiple band operation to facilitate volume and size reduction is quite challenging. In this paper, a triple-band Linear Polarization to Circular Polarization (LP-to-CP) converter is presented using a novel design procedure based on geometric parameters optimization of a metasheet. The proposed converter is ultrathin, wideband, stable over a wide range of incident angles, and polarization diverse. The conductor layer of metasheet is patterned with a square ring and five square-patches diagonally intersecting each other. To validate the proposed method, an LP-to-CP convertor in X-band (7.3~9.6 GHz) and dual Ka-bands (25.4~31.4 GHz, 35.4~42.2 GHz) is presented. The performance is quite stable in wide range of frequencies and against the variation of incident angles from −25° to 25°. After performing model-based theoretical paradigm analysis, and full-wave simulation and optimization, the converter is fabricated and the measurements are performed inside the anechoic chamber. The measured results, close to simulation results, depict the validity and reliability of the proposed design.
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