Performance of low bandgap thermophotovoltaic cells in a small cogeneration system

In recent years there has been significant progress in fabrication of low bandgap thermophotovoltaic (TPV) devices, such as InGaAsSb, InGaAs and GaSb cells. However, only limited data are available in the literature with respect to the performance of these TPV cells in combustion-driven radiant sources. In this study, power generation using InGaAsSb TPV cells has been investigated in a gas-fired home heating furnace. The radiant power density and radiant efficiency of a gas-heated radiator were determined at different degrees of exhaust heat recuperation. Heat recuperation is shown to have a certain effect on combustion operation and radiant power output. The electric output characteristics of the InGaAsSb TPV devices were investigated under various operating conditions. An electric power density of 5.4×10³ W m⁻² was produced at a radiator temperature of 1463 K for the small cogeneration system. The cell short circuit density was observed to be greater than 1×10⁴ A m⁻² at a radiator temperature of 1203 K. Furthermore, the design aspects of combustion-driven TPV systems have been discussed. It is shown that development of a special combustion device with high conversion level of fuel chemical energy to useful radiant energy is required, to improve further the system efficiency.     

The total infrared transmittance of polymerised vinyl fluoride films for a wide range of radiant source temperature

The current growing interest on the use of polymer films is mainly attributed to their suitability as glazing and construction materials in various fields of contemporary technology. Polymerised vinyl fluoride (Tedlar) films are nowdays considered as very suitable for construction and energy applications. Most polymer films are partly transparent, showing a strongly selective absorbing behaviour at the infrared spectrum. Their total hemispherical infrared transmittance which depends on radiant source temperature and film thickness, is a very useful fundamental property for energy balance and heat transfer calculations. However it is not always possible to locate information about this property in the literature. In the present investigation the calculation of the total infrared transmittance of Tedlar films is presented, based on recent spectral transmittance measurements of commercial tedlar films for the wavelength band between 2.5–42.5 μm and for a broad range of radiant source temperatures between –20 to 500 C. Results from the present analysis were found to be in reasonable agreement with earlier data, derived for a specific film thickness and for a narrow range of radiant source temperatures by Willier.     

Thermophotovoltaic power generation systems using natural gas-fired radiant burners

Thermophotovoltaic (TPV) power generation in gas-fired furnaces is attracting technical attention. Considerable work has been done in the area of low bandgap GaSb cell-based TPV systems as well as silicon solar cell-based TPV systems. Previous investigations have shown that a radiant burner with a high conversion level of fuel to radiation energy must be developed to realize an efficient TPV system. In our work, we investigated different natural gas-fired radiant burners in order to raise the conversion of fuel energy to thermal radiation. These burners were used as radiation sources to establish and test two TPV prototype systems. It was found that for a non-surface combustion radiant burner, the radiation output can be enhanced using a thermal radiator with a porous structure. Also, we developed a cascaded radiant burner that generates two streams of radiation output. One stream illuminates silicon concentrator solar cells while the other drives low bandgap GaSb cells. In this way, useful radiation output and thus TPV system efficiency are significantly increased due to the cascaded utilization of combustion heat and optimized thermal management.     

Spectral selective radiating materials for direct radiative heating

Conventional infrared radiants for heating emit IR radiation spectrally broadly and make energy loss because of the absorption by the atmosphere. This loss is not negligible in a large space. Using a spectral selective radiant which emits the IR radiation only between 8 and 13 μm in wavelength, we can heat the objectives directly decreasing the energy loss due to the atmospheric absorption. As an example, an SiO film on an Al film was deposited on a glass substrate. Spectral emittance in IR region was measured using an FT-IR. To confirm the direct heating properties, we calculated radiance reaching arbitrary distance from the radiant. As a result, the IR radiation emitted from the radiant at a distance of 10 m was about 10% greater than that from the blackbody, depending on the atmospheric condition.     

Effect of radiant spectral characteristics on performance of a near-infra-red heating module

We simulated the heat transfer phenomena of the heating module that is primarily based on the radiant energy in the near-infra-red(NIR) domain. In the module, the power emitted by the lamp filament is distributed to the lamp glass, reflector, and the target medium, which are cooled by an air flow. The radiant heat transfer is simulated by using the ray-tracing scheme, in which the spectral characteristics of the emission and the materials are incorporated. The heat transport from the lamp glass to the cooling air is analyzed by using the finite volume method. As the lamp-filament temperature rises in the range of 3000–3400K, the NIR radiant power on the target medium increases. However, the lamp-glass temperature also rises, and the proportion of the NIR power to the entire radiation has a peak in the temperature range. The spectral distributions of the absorbed energies in all the components in the module are highly non-uniform, and a monochromatic model of the radiant heat transfer may result in a significant discrepancy.     

Normal spectral emittance of a nickel metal surface with rectangular microcavities

Spectral control of thermal radiation emitted from rectangular microcavities (0.5×0.5×0.5µm³) was investigated through emission experiments at high temperatures. The microcavities were fabricated on a mirror‐finished Ni metal surface. Through measurement of the normal spectral emittance, the maximum emittance was obtained around a wavelength of 0.87µm, which was very close to that (0.894µm) estimated from the cavity resonance theory. The emittance reached a maximum value of 0.95, and then decreased drastically with increasing wavelength, from the cut‐off wavelength. For a longer wavelength range from 1.7µm, it was equal to the emittance of the mirror‐finished surface. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20248     

Spectral emissivity of ytterbium oxide-based materials for application as selective emitters in thermophotovoltaic devices

The spectral emissivity of Yb³⁺ in a series of compounds has been investigated to study the effects of crystal structure type, chemical environment, ytterbium concentration, impurity concentration and temperature on the spectral radiant intensity of the selective emitter peak and emitted power from the material. A figure of merit has been defined which enables the compounds to be ranked for application in a practical thermophotovoltaic energy conversion device. It is shown that significant selective emission can be achieved from compounds in which the Yb³⁺ concentration is as low as 10 mol%. Apart from pure Yb₂O₃, the compounds Yb₃Al₅O₁₂, YbNbO₄ and Y_0.9Yb_0.1O_1.5 are found to have emission spectra suitable for efficient matching to silicon photovoltaic cells.     

Study of the photon down‐conversion effect produced by thin silicon‐rich oxide films on silicon solar cells

In the present work, we studied the photon down‐conversion effect produced by thin films of silicon oxide with embedded silicon nanocrystals also called silicon‐rich oxide (SRO). These films have been used to absorb high energy light and the re‐emission of two or more low energy photons (~1.1 eV) with the goal of improving the external quantum efficiency and consequently the conversion efficiency of silicon solar cells. According to our results, the incorporation of a thin SRO film on the solar cell surface increases the short circuit current and the FF of the silicon solar cells; the enhancement of spectral response is due to the high photoluminescence intensity of the SRO in the visible region when irradiated with UV light. An improvement of 38% in the solar cell efficiency has been observed in our particular solar cell fabrication process by the use of an SRO film with high photoluminescence intensity, which replaces the conventional silicon dioxide film. Copyright © 2016 John Wiley & Sons, Ltd.     

Structural and optical characterizations of CdTe on CdS grown by hot-wall vacuum evaporation

We report on characterizations of polycrystalline CdTe on CdS grown by hot-wall vacuum evaporation. The CdTe film grown on CdS/SnO₂/glass was compared with the other two CdTe films, which were grown directly on #7059 glass and on SnO₂-coated glass. The grown CdTe/CdS film is composed of a grain size 3–15 μm and have a close-packed structure compared to other CdTe films. A weak excitonic peak at 1.59 eV as well as two kinds of donor–acceptor pair emission bands has been observed in PL spectrum of CdTe/CdS film. On the other hand, the excitonic peak cannot be detected in other CdTe films.     

Characterisation of rare earth selective emitters for thermophotovoltaic applications

We investigate selective radiation emitters made from rare earth oxides suitable for thermophotovoltaic (TPV) systems. Yb₂O₃ and Er₂O₃ emitters were fabricated and their radiation power, temperature and emissivity were measured in the entire relevant spectral range. We found temperatures of 1735 K for the Yb₂O₃ emitter and of 1680 K for the Er₂O₃ emitter both heated with a 1.35 kW butane burner. The maximum emissivities of the selective peaks were 0.85 at 1.27 eV, and 0.82 at 0.80 eV for the Yb₂O₃ and the Er₂O₃ emitter, respectively. The emission spectra show gas emission lines originating from the combustion process in addition to the selective emission bands. An estimation based on a simplified combustion model show that a TPV system with a system efficiency of about 10% can be realised using an Yb₂O₃ emitter, silicon photocells and a perfect selective filter.     

Experimental verification of thermal remote sensing method for recovering temperature distribution in glass

The thermal remote sensing method for recovering the temperature distribution in glass from spectral emission data is examined experimentally. An analytical model is formulated and the desired temperature distribution is obtained using an optimization scheme which determines the temperature profile in the form of a polynomial or a set of discrete points. In order to evaluate the accuracy and validity of the thermal remote sensing method, the recovered temperatures are compared with independent measurements using surface thermocouples and a Mach-Zehnder interferometer. Experimental results are reported for fused silica (Corning Code 7940) glass samples using a Perkin-Elmer spectrometer to measure the spectral radiant energy emerging from the layer of glass. Opaque (high and low emittance) boundary conditions at the heated surface of the glass were considered. Temperatures in the range from 500 to about 900 K were examined. Spectral emission data between 3.3 and 4.8 μm were used in recovering the temperature distribution in the glass samples. The results showed that the recovered and interferometrically measured temperature profiles agreed well, with the maximum deviation never exceeding approximately 2 per cent.     

Microdefects and point defects optically detected in Cu(In,Ga)Se2 thin film solar cells exposed to the damp and heating

Optically detected changes have been studied in Cu(In,Ga)Se₂ (CIGS) thin film solar cells, which were exposed to the damp and heat test by IEC 1215 international standard recommendations. High-resolution optical microscopic images at T=300 K and emission properties at T=20 K of ZnO/CdS/CIGS devices were characterized and compared to the tested non-incapsulated device. The near-gap photoluminescence peak at 1.191 eV for the baseline device drastically decreases after the test. Long wavelength emission bands at 1.13 and 1.07 eV, associated with optical transitions through defect levels in absorber, retain their intensity and spectral position. Microscopic surface morphology deteriorates after the test: appearance of micro-scale defects and reduction of optical reflectivity have been observed in blue-violet light and polarization with good contrast. A decrease of conversion efficiency of the exposed solar cell is caused by the degradation of upper wide-gap films and heteroboundary between CdS and CIGS.     

Investigations on copper zinc tin sulfide thin films grown through nebulizer assisted spray pyrolysis technique

An attempt is made in this work to synthesize the nontoxic Cu₂ZnSnS₄ (CZTS) thin film on FTO coated glass substrates through aerosol assisted nebulizer spray pyrolysis technique at different annealing temperatures for photovoltaic energy conversion. The deposited thin film is further used to form a heterojunction interface with a cadmium sulfide layer to fabricate a solar cell with Glass/FTO/CdS/CZTS/Ag superstrate structure to reveal its photovoltaic application. Various characterization techniques are utilized to study its inherent properties. X-ray diffraction (XRD) is employed to investigate the structural parameters such as crystallite size, microstrain and dislocation density which shows a preferential peak for (112) plane around 28.5° confirming the formation of kesterite CZTS. Raman measurements establish the peak for CZTS at 336 cm⁻¹ and confirm the absence of parasitic secondary phases for an excitation wavelength of 488 nm. Scanning electron microscope and atomic force microscope used to examine the surface morphology and roughness of the films reveal a good surface morphology with a grain size of 555.9 nm and roughness of 123.7 nm for an annealing temperature of 350°C. The bandgap of the deposited CZTS films is found to be around 1.5 eV. The I-V characteristics of CZTS seem to be better for the 350°C annealed film. The performance of the optimized CZTS absorber layer is tested by forming a solar cell structure. The devised solar cell exhibited an open circuit voltage of 213 mV and a short circuit current density of 490 μA cm⁻² with a conversion efficiency of 0.68% substantiating the usage of the prepared film as an absorber for photovoltaic conversion.     

Generation of electricity using InGaAsSb and GaSb TPV cells in combustion-driven radiant sources

Electric power generation using InGaAsSb and GaSb thermophotovoltaic (TPV) cells was investigated in a gas-fired heating furnace. Electric output characteristics of the TPV cells in the combustion-driven radiant sources are presented. For the InGaAsSb cells, open circuit voltage was higher than 300 mV at a short circuit current density above 1 A/cm². The short circuit current of the InGaAsSb cells increased more rapidly than that of the GaSb cells in radiator temperature range 930–1215 °C. A mathematical model for the TPV devices was developed to describe the effect of cell bandgap and radiator temperature on electric power output and cell efficiency. Also, the design aspects of the combustion-driven TPV systems are discussed.     

Experimental performance of a thermoelectric ceiling cooling panel

An experiment has been performed to investigate the cooling performance of a thermoelectric ceiling cooling panel (TE‐CCP). The TE‐CCP was composed of 36 TE modules. The cold side of the TE modules was fixed to an aluminum ceiling panel to cool a test chamber of 4.5 m³ volume, while a copper heat exchanger with circulating cooling water at the hot side of the TE modules was used for heat release. Tests were conducted using various system parameters. It was found that the cooling performance of the system depended on the electrical supply, cooling water temperature and flow rate through the heat exchanger. A suitable condition occurred at 1.5 A of current flow with a corresponding cooling capacity of 289.4 W which gives the coefficient of performance (COP) of 0.75 with an average indoor temperature of 27°C. Using thermal comfort test data in literature for small air movements under radiant cooling ceilings, results from the experiments show that thermal comfort could be obtained with the TE‐CCP system. Copyright © 2007 John Wiley & Sons, Ltd.     

The creation of a comprehensive metocean data set for offshore wind turbine simulations

A database of meteorological and ocean conditions is presented for use in offshore wind energy research and design. The original data are from 23 ocean sites around the USA and were obtained from the National Data Buoy Center run by the National Oceanic and Atmospheric Administration. The data are presented in a processed form that includes the variables of interest for offshore wind energy design: wind speed, significant wave height, wave peak‐spectral period, wind direction and wave direction. For each site, a binning process is conducted to create conditional probability functions for each of these variables. The sites are then grouped according to geographic location and combined to create three representative sites, including a West Coast site, an East Coast site and a Gulf of Mexico site. Both the processed data and the probability distribution parameters for the individual and representative sites are being hosted on a publicly available domain by the National Renewable Energy Laboratory, with the intent of providing a standard basis of comparison for meteorological and ocean conditions for offshore wind energy research worldwide. Copyright © 2015 John Wiley & Sons, Ltd.     

Measurement of infrared radiation emitted by the flame of a vegetation fire

An experimental study has been carried out on the radiation emitted in the infrared by a flame produced by the combustion of vegetation (vine branches) in a square tray of size 50 × 50 cm. Measurements have been performed with an FTIR spectrometer and an IR camera on the corresponding flame with characteristic height between 50 cm and 1 m. A specific device has been built in order to allow the simultaneous acquisition of spectra in the range between [1000–6000 cm^?1] and pictures in selected bands of the infrared. Pictures with a camera in the visible range have been taken at the same time. A preliminary calibration using a reference emitter has been done in order to evaluate the emitted radiation quantitatively. By comparing the emission by flames with the one of this reference surface, the radiation emitted by a vegetation flame has been observed and analysed as a function of the wavenumber. The emission is observed to be predominant in a range around 2300 cm^?1, due to CO₂ production, and strong emission bands related to H₂O are also observed. The experimental tools, FTIR spectrometer and IR camera, both in good agreement, show a weak emission due to soots, at least at this scale of relatively small flames.     

一维Si/SiO_2光子晶体应用于热光伏的新进展

针对一维(1D)Si/SiO_2光子晶体滤波器在GaSb热光伏电池特征波长处存在较大反射峰的问题,根据等效折射率理论,将其基本结构由(L/2HL/2)~5改进为[1.10(L/2HL/2)](L/2HI/2)~3[1.10(L//2HI/2)].结果表明:改进的滤波器在特征波长处的透过率有较大提高,且抑制了长波光子的透过.辐射器为1250℃的黑体时,采用改进滤波器的TPV系统的光谱效率为33.3%,输出电功率密度为1.06W·cm~(-2),相对于采用原滤波器的TPV系统增长幅度均超过了10%,电池效率也有所提高.     

Power generation performance of hydrogen-fueled micro thermophotovoltaic reactor

A tubular platinum reactor with a perforated annular array enables fuel/air mixtures to exchange sides, thus sustaining flames and preventing heat loss. Consequently, the combustion efficiency and operational range can be enhanced. A hydrogen/air mixture was introduced into inner and outer chambers at different equivalence ratios and flow velocities to chemically and physically investigate the interplay between the chambers. The benefits of hydrogen include a high gravimetric heating value, flame speed, and diffusion capacity and short chemical reaction time. The coexistence of heterogeneous (surface) and homogeneous (gas) reactions in the micro TPV reactor was examined and elucidated in terms of aerodynamics, mass and heat transfer, and chemical reactivity. Furthermore, a TPV reactor with TPV cell arrays was assembled, and the corresponding radiant efficiency of the emitter and the overall efficiency of the proposed micro TPV system were determined in this study.     

New development of one-dimensional Si/SiO2 photonic crystals filter for thermophotovoltaic applications

One-dimensional (1D) Si/SiO₂ photonic crystals (PhCs) are regarded as the most promising candidates for thermophotovoltaic (TPV) optical filters. The performance of TPV devices can be significant improved with the designed 1D ten-layer Si/SiO₂ PhC of excellent stop-band characteristic. However, large oscillations around 1.45–1.75 μm in the pass band of this PhC filter would reduce the above band-gap power transmitted to the cells, leading to discounts of the system efficiency and power density. This work focused on the pass-band characteristics of the 1D Si/SiO₂ PhCs. The mechanism of the large oscillations mentioned above was discussed and a modified 1D five-unit Si/SiO₂ PhC in which the first and fifth units serve as refractive index match units to smooth the large oscillations in the pass band was presented. The simulation indicated that the modified 1D PhC exhibited much flatter and lower pass band around 1.45–1.75 μm than that of the original 1D PhC, even for large incident angle of 45° for both TM and TE polarizations. Both PhCs in the modified and original structures were prepared through a magnetron sputtering process and the measured optical characteristics showed good coherence with the simulation results. An ideal thermodynamic model was then applied to predict the improvement of the TPV system performance by utilizing the modified 1D PhC filter. The results indicated that the modified 1D PhC would lead to 21.0–5.9% increase of the spectral efficiency and 14.8–5.3% increase of the power density at 1200–1800 K radiator temperature.