(128e) 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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