Near-Field Enhanced Thermionic Energy Conversion for Concentrated Photovoltaic Power Generation

This abstract presents the direct conversion of excessive heat from a concentrated photovoltaic power generator into the electrical power by developing near-field enhanced thermionic energy conversion (NETEC). The excess heating of the concentrated photovoltaics (CPV) is the main source of waste energy that should be effectively removed to maintain the performance of photovoltaic cells. To make use of waste heat generated in the (CPV) system, we propose two innovative approaches integrated with thermionic emission: (1) combination of photovoltaic and thermionic effects to enhance electron emission from a cathode [1-4], and (2) use of near-field thermal radiation, typically several orders of magnitude greater than blackbody thermal radiation, to boost photovoltaic emission of electrons. This hybrid model takes advantage of the combination of near-field photo-excitation enhancement with thermionic emission which will significantly improve the energy conversion efficiency and output power for the cases where cathode temperature is much lower than conventional thermionic energy converters. A thermal emitter and a low-bandgap semiconductor cathode are separated by a sub-wavelength gap distance to allow the transfer of a significant amount of thermal radiation between the two due to near-field effects. Near-field thermal radiation enhances the concentration of electrons in the conduction band of the cathode due to the combination of photoexcitation and thermal excitation, leading to the enhancement of electrical current from the cathode to the anode. This hybrid system will be remarkably more efficient with higher power throughput compared to other current direct energy conversion systems, such as thermoelectric, thermophotovoltaic, and thermionic systems [5-7].

Figure 1(a) shows the schematic configuration of the NETEC system, where a nanoscale gap is maintained between cathode and emitter to allow strong enhancement in thermal radiative transfer. The thermal energy required to maintain the emitter at a high temperature can be supplied by the excessive heat coming from solar concentrated energy. The energy diagram in Fig.1(b) illustrates the work functions and energy barriers of this system, where the output voltage is basically given by the difference between the cathode and anode work functions plus any external voltage, Vex .

Figure  SEQ Figure \* ARABIC 1. (a) Schematic of a NETEC structure showing the heat transfer and carrier transport mechanisms in a plain system. (b) Energy diagram showing the work functions and energy barriers.

In order to calculate the current density of the cathode, by following a similar procedure to calculation of simple thermionic current [1] and by taking into account the effect of photoexcited electrons in the conduction band, following equation will be obtained [1].

JC=4πem*k2h3TC2 exp-EC-EF,n+χkTC=ATC2 exp-ϕC-EF,n-EFkTC          (1)

Hence, the output power of a NETEC system can be calculated analogous to thermionic converters, by multiplying the net current density, JC-Ja , and the output voltage,

PNETEC=JnetVnet=JC-JaϕC-ϕa+Vex                                                                            (2)

                                                       

Further, the efficiency of the NETEC system, ηsys  can be defined as the ratio of the output electrical power, Pel  over the total input energy of the system, which in this case, is the summation of the total power emitted by the thermal emitter (calculated from near-field radiative transfer equations), Pemitter  and the net thermal energy needed to maintain the cathode temperature, QCathode ,

ηsys=PelPemitter+QCathode                                                                                                                (3)

The operating temperature of the cathode is a decisive parameter in designing an efficient NETEC system, since the thermal energy of the cathode which is directly related to its temperature, determines the average velocity of the carriers within the material. Thus, by decreasing the electron affinity of the material, χ , cathode should be able to operate in lower temperatures. The effects of electron affinity on the cathode output current, the electrical power and the system efficiency are shown in Fig.2(a) , (b) and (c). As it can be seen in Fig.2(c), for any electron affinity, there exist an optimum temperature which results in the maximum system efficiency, where by increasing the electron affinity, intuitively this optimum temperature will raise as accordingly. So, by wisely manipulating the electron affinity of the cathode through some surface operations to manage it due to its operating temperature, we can obtain a remarkable output power.

Figure  SEQ Figure \* ARABIC 2. (a) J-V characteristics of a NETEC system for different electron affinities. (b) Electrical power as a function of cathode temperature. (c) System efficiency for different electron affinities and cathode temperature.

To summarize, in this study a novel hybrid energy conversion system is introduced by combination of the near-field thermal radiation and conventional thermionic energy conversion systems. Near-field enhanced thermionic energy conversion (NETEC) system benefits from photoexcitation due to incident near-field radiation and also thermal energy of the semiconductor cathode in order to directly generate electricity. We have illustrated the remarkable enhancement in output power and system efficiency achievable using this new concept for the first time. Most importantly, many challenges faced in direct energy conversion schemes such as near-field thermophotovoltaic systems or thermionic converters, being an essentially high temperature or low power throughput, can be addressed and resolved using this method.

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