“…As the power conversion efficiency of these devices are bounded by the Carnot limit, their efficiencies would generally increase at high temperature. Thermionic converters also need to operate at high emitter temperature (generally above 1000 °C) to yield a higher current density as described by the Richardson–Dushman equation. The radiation spectrum of thermophotovoltaic emitter has to be matched with the bandgap of the photovoltaic cell, which generally requires the emitter temperature to be at least 730 °C (1000 K) for practical power density and conversion efficiency.…”
High temperature processes are widely used in a variety of existing and emerging industrial and aerospace applications. The thermal properties of high‐temperature materials thus play an important role in controlling the thermal energy, as highlighted by successful applications of thermal barrier coating and aerogels. While thermal transport processes at room and low temperature have witnessed tremendous progress in the past two decades, particularly on the fronts of understanding basic heat transfer properties at the micro‐ and nanoscale, the understanding at high temperature is still at the nascent stage, owing to several unique features at high temperature, such as the dominant Umklapp scattering effect that can render a crystalline material amorphous‐like thermal properties, and the important radiation contribution at high temperature. This lack of systematic understanding, coupled with the challenges in maintaining high‐temperature stability in a large number of materials, has limited the development of materials to meet the thermal transport properties pertaining to several current and emerging high‐temperature applications. This Review is aimed at providing an overview of the basic mechanisms governing thermal transport processes at high temperature, to identify their unique features and challenges, and to explore opportunities in material research for high‐temperature thermal materials.
“…As the power conversion efficiency of these devices are bounded by the Carnot limit, their efficiencies would generally increase at high temperature. Thermionic converters also need to operate at high emitter temperature (generally above 1000 °C) to yield a higher current density as described by the Richardson–Dushman equation. The radiation spectrum of thermophotovoltaic emitter has to be matched with the bandgap of the photovoltaic cell, which generally requires the emitter temperature to be at least 730 °C (1000 K) for practical power density and conversion efficiency.…”
High temperature processes are widely used in a variety of existing and emerging industrial and aerospace applications. The thermal properties of high‐temperature materials thus play an important role in controlling the thermal energy, as highlighted by successful applications of thermal barrier coating and aerogels. While thermal transport processes at room and low temperature have witnessed tremendous progress in the past two decades, particularly on the fronts of understanding basic heat transfer properties at the micro‐ and nanoscale, the understanding at high temperature is still at the nascent stage, owing to several unique features at high temperature, such as the dominant Umklapp scattering effect that can render a crystalline material amorphous‐like thermal properties, and the important radiation contribution at high temperature. This lack of systematic understanding, coupled with the challenges in maintaining high‐temperature stability in a large number of materials, has limited the development of materials to meet the thermal transport properties pertaining to several current and emerging high‐temperature applications. This Review is aimed at providing an overview of the basic mechanisms governing thermal transport processes at high temperature, to identify their unique features and challenges, and to explore opportunities in material research for high‐temperature thermal materials.
“…1 d, the electrons at the cathode would have sufficient energy to overcome a surface barrier, migrate to the anode and drive electricity through the TEC. Figure 1 ( a ) Various mechanisms of producing electron emission 6 , 17 , 30 . ( b ) Unconnected cathode and anode 7 .…”
Section: Introductionmentioning
confidence: 99%
“… ( a ) Various mechanisms of producing electron emission 6 , 17 , 30 . ( b ) Unconnected cathode and anode 7 .…”
Section: Introductionmentioning
confidence: 99%
“…Despite enormous research in TEC, the TEC technology's potential in generating electricity is still being hindered due to high material work function and space charge related problems 7 , 28 – 30 . This has led to the unnatural death of technology in the 90 s. Nevertheless, a recent revival of interests in thermionic energy converters is attributed to the emerging nanomaterials and ever-growing technology in the twenty-first century 5 , 6 , 8 .…”
In this study, five mathematical models were fitted in the absence of space charge with experimental data to find a more appropriate model and predict the emission current density of the graphene-based thermionic energy converter accurately. Modified Richardson Dushman model (MRDE) shows that TEC's electron emission depends on temperature, Fermi energy, work function, and coefficient of thermal expansion. Lowest Least square value of $$S=\sum {\left({J}_{th}-{J}_{exp}\right)}^{2}=0.0002 \,\text{A}^{2}/\text{m}^{4}$$
S
=
∑
J
th
-
J
exp
2
=
0.0002
A
2
/
m
4
makes MRDE most suitable in modelling the emission current density of the graphene-based TEC over the other four tested models. The developed MRDE can be adopted in predicting the current emission density of two-dimensional materials and also future graphene-based TEC response.
“…Generally, solar power can be divided into non-concentrating (e.g. flat-plate photovoltaics) and concentrating solar power systems [3] . Photovoltaic (PV) can provide direct conversion of solar energy into electricity via the Photovoltaic effect [4] however, the conversion efficiencies of the commercially available photovoltaic systems are still low such as 14-20% for silicon solar cells and 25-30% for III-V multi-junction solar cells [5] .…”
A consequence of the integration of photovoltaic module and thermoelectric generator is the presence of thermal contact, which affects the heat transfer in the hybrid system. Thermal and electrical contact are inevitable in a thermoelectric generator and their effects on the performance of the thermoelectric generator can be significant if not properly managed. Therefore, this study presents a comprehensive three-dimensional numerical investigation on the effect of contact resistances on the performance of concentrated photovoltaic-thermoelectric using COMSOL 5.4 Multiphysics software. Four contact resistances are studied including thermoelectric thermal contact resistance, thermoelectric electrical contact resistance, photovoltaic-thermoelectric interface thermal contact resistance and thermoelectric generator-heat sink interface thermal contact resistance. Twelve contact resistance cases are considered, and a comparison study is presented to investigate the most important contact resistance. In addition, a parametric optimization study is performed to investigate the optimum values for thermoelectric leg height, load resistance, concentration ratio and convective heat transfer coefficient. Results show that ignoring all contact resistances in the hybrid system causes an overestimation of overall power output and efficiency by 7.6% and 7.4% respectively using the base values considered in this study. In addition, the thermal contact resistance between the thermoelectric generator and heat sink, and that between the photovoltaic-thermoelectric interface are found to be the most important contact resistances, which should be reduced. Furthermore, results show that the optimum thermoelectric external load resistance in a hybrid system is lower than that of the thermoelectric generator only system. This study will provide valuable guidance on photovoltaic-thermoelectric accurate modelling.
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