Shanhe Su

Parametric design criteria of an irreversible vacuum thermionic generator

A new model of the vacuum thermionic generator (TIG) including internal and external irreversible heat losses is proposed. The energy balance equations of the cathode and anode of the TIG are used to determine the operating temperatures of two electrode plates. Analytic expressions for the power output and efficiency of the system are derived. The power output and efficiency under different conditions are optimized. The effects of the work functions of electrode materials and the output voltage on the performance of the system are discussed. The optimal regions of several important parameters are determined. The optimum design criteria, which may provide some guidance for the choice of electrode materials and the operation of the TIG, are obtained.

Hot-Carrier Solar Cells With Quantum Well and Dot Energy Selective Contacts

In this paper, we simulate hot-carrier solar cells (HCSCs) with quantum well and dot energy selective contacts (ESCs) and derive the power outputs and efficiencies of HCSCs, including three energy loss mechanisms. The effects of the shape of the electron energy spectrum on the electric current densities and heat flux densities of HCSCs are discussed based on the ballistic transport theory. Performance characteristics pertaining to the extraction energy level and the transmission energy width are revealed. The key parameters of HCSCs are optimally designed. The results obtained show that thermalization losses due to non-ideal ESCs are one of the main energy dissipation mechanisms decreasing the efficiency of HCSCs and the performance of an HCSC can be more significantly improved by using quantum dot ESCs than quantum well ESCs.

Thermal electron-tunneling devices as coolers and amplifiers

Nanoscale thermal systems that are associated with a pair of electron reservoirs have been previously studied. In particular, devices that adjust electron tunnels relatively to reservoirs’ chemical potentials enjoy the novelty and the potential. Since only two reservoirs and one tunnel exist, however, designers need external aids to complete a cycle, rendering their models non-spontaneous. Here we design thermal conversion devices that are operated among three electron reservoirs connected by energy-filtering tunnels and also referred to as thermal electron-tunneling devices. They are driven by one of electron reservoirs rather than the external power input, and are equivalent to those coupling systems consisting of forward and reverse Carnot cycles with energy selective electron functions. These previously-unreported electronic devices can be used as coolers and thermal amplifiers and may be called as thermal transistors. The electron and energy fluxes of devices are capable of being manipulated in the same or oppsite directions at our disposal. The proposed model can open a new field in the application of nano-devices.

Effects of nanoscale vacuum gap on photon-enhanced thermionic emission devices

A new model of the photon-enhanced thermionic emission (PETE) device with a nanoscale vacuum gap is established by introducing the quantum tunneling effect and the image force correction. Analytic expressions for both the thermionic emission and tunneling currents are derived. The electron concentration and the temperature of the cathode are determined by the particle conservation and energy balance equations. The effects of the operating voltage on the maximum potential barrier, cathode temperature, electron concentration and equilibrium electron concentration of the conduction band, and efficiency of the PETE device are discussed in detail for different given values of the vacuum gap length. The influence of the band gap of the cathode and flux concentration on the efficiency is further analyzed. The maximum efficiency of the PETE and the corresponding optimum values of the band gap and the operating voltage are determined. The results obtained here show that the efficiency of the PETE device can be sig...

Using a multi-layer graphene-based emitter to improve the performance of a concentrated solar thermionic converter

A new model of the concentrated solar thermionic converter (CSTIC) with a multi-layer graphene (MLG)-based emitter is established. Based on the formulas of the thermionic emission from the MLG with ABA and ABC stacking orders, the power output and the efficiency of the CSTIC are derived. The performance characteristics of the CSTIC are discussed. It is revealed that the maximum efficiency of the CSTIC with the MLG-based emitter is higher than that with the single layer graphene (SLG)-based emitter, and the operating temperature of the MLG-based cathode is lower than that of the SLG-based cathode. It is important to find that the performances of the CSTIC with the ABA stacked graphene are better than those with the ABC stacked graphene. The optimum surface work function of the MLG-based cathode at the maximum efficiency is larger than the work function of the SLG-based cathode so that the CSTIC with the MLG-based emitter can be experimentally implemented more easily than the CSTIC with the SLG-based emitter.A new model of the concentrated solar thermionic converter (CSTIC) with a multi-layer graphene (MLG)-based emitter is established. Based on the formulas of the thermionic emission from the MLG with ABA and ABC stacking orders, the power output and the efficiency of the CSTIC are derived. The performance characteristics of the CSTIC are discussed. It is revealed that the maximum efficiency of the CSTIC with the MLG-based emitter is higher than that with the single layer graphene (SLG)-based emitter, and the operating temperature of the MLG-based cathode is lower than that of the SLG-based cathode. It is important to find that the performances of the CSTIC with the ABA stacked graphene are better than those with the ABC stacked graphene. The optimum surface work function of the MLG-based cathode at the maximum efficiency is larger than the work function of the SLG-based cathode so that the CSTIC with the MLG-based emitter can be experimentally implemented more easily than the CSTIC with the SLG-based emitter.

Transient models integrating photovoltaic, electron-tunneling, and thermoelectric mechanisms

ABSTRACT Photovoltaic cells that harness renewable solar energy have been combined with either electron tunnels or thermoelectric modules to work as efficient hybrid devices. In the present numerical analysis, we integrated a photovoltaic cell, an electron tunnel, and a thermoelectric module into a three-component conjugate assembly, and further studied the characteristics of this assembly in transient states. Mechanisms including (a) thermalization obeying Fermi–Dirac distribution in the cathode; (b) electron transport confined to tunnels; (c) irreversible exchange of electron fluxes between two electrodes; (d) Seebeck effects; (e) Joule heating; (f) transient heat conduction in the thermoelectric module; and (g) convective cooling over the bottom of the assembly are all intricately coupled. The set of highly nonlinear algebraic equations resulting from energy balances over five nodes are iteratively solved using the modified Newton–Raphson method jointly with under-relaxation. Thermal efficiencies of the proposed three-module combined assembly surpass those of two-unit hybrid models previously reported in the literature.