Modelling the Thin-Layer Drying Kinetics of Marinated Beef during Infrared-Assisted Hot Air Processing of Biltong

Biltong is a dried meat product that is widely consumed in South Africa. The marinated meat is traditionally dried under ambient winter conditions while commercial biltong producers use hot air driers. Hot air drying is time-consuming and energy-intensive. A combined infrared and hot air drying (IRHAD) is an alternative method of drying meat during biltong processing. The aim of this study was to establish the effect of the infrared (IR) power, the temperature, and velocity of the drying air on the drying kinetics of marinated beef and subsequently select the best thin-layer drying model for IRHAD during biltong processing. Marinated beef samples were dried at IR power levels of 500, 750, and 1000 W; drying air temperatures of 30, 35, and 40°C; and air velocity of 1.5 and 2.5 m∙s-1. Results indicate that increasing the IR power and the drying air temperature increased the IR emitter temperature and the core temperature of the marinated beef sample. Consequently, increasing the drying rate thus reduced drying time. The air velocity had an inverse relationship with the IR emitter temperature, the core temperature of the marinated beef sample, and the drying rate. The drying process was characterised by a rising rate period in the first half an hour, followed by a falling rate period which implies that moisture transport occurred partly by surface evaporation and predominantly by diffusion. The effective moisture diffusivity ranged from 4.560 × 10 − 10 to 13.7 × 10 − 10 m 2 ∙ s − 1 , while, the activation energy ranged between 40.97 and 59.16 kJ∙mol-1. The IRHAD of marinated beef during its processing to biltong was best described by the two-term model since it had the highest R 2 (0.9982-0.9993) and the lowest RMSE (0.0062-0.0099). The power level of the IR emitter of 1000 W combined with a drying air temperature and velocity of 40°C and 1.5 m∙s-1, respectively, showed the highest improvement in the drying kinetics and the lowest drying time of 5.61 ± 0.35 hours; hence, it is recommended as a possible drying alternative for the processing of biltong.

Ultraefficient thermophotovoltaic power conversion by band-edge spectral filtering

Thermophotovoltaic power conversion utilizes thermal radiation from a local heat source to generate electricity in a photovoltaic cell. It was shown in recent years that the addition of a highly reflective rear mirror to a solar cell maximizes the extraction of luminescence. This, in turn, boosts the voltage, enabling the creation of record-breaking solar efficiency. Now we report that the rear mirror can be used to create thermophotovoltaic systems with unprecedented high thermophotovoltaic efficiency. This mirror reflects low-energy infrared photons back into the heat source, recovering their energy. Therefore, the rear mirror serves a dual function; boosting the voltage and reusing infrared thermal photons. This allows the possibility of a practical >50% efficient thermophotovoltaic system. Based on this reflective rear mirror concept, we report a thermophotovoltaic efficiency of 29.1 ± 0.4% at an emitter temperature of 1,207 °C.

Research of an oxide cathode as a cathode-neutralizer for EPSPS

In this research, a laboratory model of a thermionic oxide cathode was tested as part of a diode circuit. The ultimate goal of this work was to obtain the thermionic characteristics of the emitter of the laboratory model and to study the processes of emitter activation. The relevance of the study is due to the increased interest in the possibility of using thermionic cathodes as cathode-neutralizers for electrically-powered spacecraft propulsion system (EPSPS). During the experiment, the following parameters were recorded: the pressure in the vacuum chamber and the emission current to the anode-collector. The current of the emitter and the voltage applied between the anode-collector and the emitter were regulated. The gap between the emitter and the anode-collector was set before the beginning of the experiment and was 2 mm. The emission current was measured in the emitter temperature range from 600 °C to 1260 °C. The temperature of the emitter was controlled by infrared and optical pyrometers. In the course of the work, three emitter activation processes were identified: temperature, time and voltage. The processes of activation by temperature and time are widely known, in contrast to the activation process by voltage, for which there is currently no unambiguous theoretical explanation.

Overview on recent progress in magnetron injection gun theory and design for high power gyrotrons

The magnetron injection gun (MIG) is one of the most critical subcomponents in gyrotrons. The electron beam, which has the primary role on the gyrotron operation, is generated and configured at this part of the tube. The electron beam properties determine the excitation mode in the cavity, the power of the generated microwaves and the gyrotron efficiency. The operation of MIGs could be influenced by several factors such as trapped electrons, manufacturing tolerances, roughness of the emitter ring, emitter temperature inhomogeneity, electron beam neutralization effect, etc. The influence of many of these factors on the electron beam quality has been systematically investigated during the last years. Several novelties have been proposed in order to limit the influence of these factors on the gyrotron operation. In particular, new design criteria have been proposed for the suppression of electron trapping mechanisms, a new type of the emitter ring has been proposed to minimize the influence of the manufacturing tolerances and edge effects on the beam quality, alternative MIG design approaches have been proposed, etc. An overview of all these works will be presented here.

Effects of heat transfer on characteristics of thermionic energy converter

Photon enhanced thermionic emission (PETE) is a new concept in solar energy conversion, combining thermal and photovoltaic carrier excitations with thermionic emission. A solar-power-driven thermionic energy converter operates by illuminating the solar light condensed by a large-scale Fresnel lens to convert heat energy into electrical energy. By enhancing the efficiency of converting solar radiation into the emitter internal energy, the output power and efficiency of the thermionic energy converter can be greatly improved. In this study, using numerical simulations, the effects of emitter temperature and output characteristics on a thermionic energy converter were investigated. The results showed that the higher rate of the heating power represented the higher temperature of an emitter, as well as output current density, and efficiency. In addition, by reducing the diameter of a collector and thermal conductivity of insulation materials, or increasing the diameter of emitter, the temperature of emitter, output current density, and efficiency could be notably improved. It is also worth mentioning that the main factor that affected the emitter temperature in the process of heat transfer was heat conduction between solids. In conclusion, adequate illumination, reasonable size of collector and emitter, as well as appropriate insulation measurements could efficiently improve the output characteristics of thermionic energy converter.

Performance Assessment and Optimization of a Thermophotovoltaic Converter–Thermoelectric Generator Combined System

To evaluate the feasibility of the performance enhancement of a thermophotovoltaic (TPV) converter by using a thermoelectric generator (TEG), a new model of a combined system is established, where the TEG is attached on the backside of the TPV converter to harvest the heat produced in the TPV converter. The effects of the voltage output of the TPV converter, band gap energy of the TPV converter, dimensionless current of the TEG, and emitter temperature on the performance of the combined system are examined numerically. It is found that the performance of the TPV converter can be enhanced by using the TEG. The percentage increment of the maximum power output density is larger than that of the maximum efficiency. There are optimally working regions of the converter voltage, dimensionless current, and band gap energy. The elevated emitter temperature results in the increase of the power output density of the combined system. However, there is an optimal emitter temperature that yields the maximum efficiency of the combined system. Moreover, the TEG is not suitable to harvest the heat produced in the TPV converter when the emitter temperature is sufficiently high.

Optimal Design of Wavelength Selective Thermal Emitter for Thermophotovoltaic Applications

We theoretically and numerically demonstrate optimal design of wavelength selective thermal emitter using one-dimensional (1D) and two-dimensional (2D) metal-dielectric gratings for thermophotovoltaic (TPV) applications. Proposed design consists of tungsten (W) and silicon dioxide (SiO2) gratings which can withstand high temperatures. Radiative properties of 1D grating were calculated using a numerical method, while effective medium approximation was used for 2D gratings. Optimal designs were obtained such that output power is maximum for GaSb photovoltaic (PV) cell at emitter temperature of 1500 K and radiated energy for longer wavelengths is limited to a low value. A constrained optimization was performed using genetic algorithm (GA) to arrive at optimal design.

Modeling of Near-Field Concentrated Solar Thermophotovoltaic Microsystem

Modeling of a near-field concentrated solar thermophotovoltaic (STPV) microsystem is carried out to investigate the use of STPV-based solid-state energy conversion as a high power density MEMS power generator. Near-field radiation can be realized between two closely separated surfaces (i.e. order of radiation wavelength), resulting in the enhancement of the heat radiation flux orders of magnitudes higher than the blackbody limit, consequently increasing cell output power density. The Near-field STPV model consists of an absorber/emitter model used to estimate the net power absorbed from solar irradiance, a near-field radiation transfer model to evaluate the power tunneled from the emitter to the PV cell at different separation distances, and a PV cell model to determine the photocurrent generated due to thermal radiation absorbed. Results reveal that decreasing separation distance between the emitter and the PV cell increases the absorber/emitter thermal efficiency, increases conversion efficiency, and the power density (×100 far-field). The results also predict increase in cooling power requirement as the separation distance is decreased, which may be a limiting design parameter for near-field STPV microsystems. Based on the model, an overall conversion efficiency of 17% at a separation distance of 10 nm and emitter temperature of 2000 K with solar concentration 6000 sun can be reached; this corresponds to an output power density of 9×105 W/m2.