Development of Thermophotovoltaic Generation of Electricity using Spectral-Controlled
Near-Field Thermal Radiation
Katsunori Hanamura**Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan,
hanamura@mech.titech.ac.jp

Abstract Near-field radiation transfer between pillar-array surfaces made of tungsten wasenhanced through an interference of SPP (Surface Plasmon Polariton) compared with that between
flat surfaces. With decreasing pillar-height, the frequency at the maximum radiation flux wasshifted toward a high-frequency side. On the other hand, through an experiment using a flat-surface
emitter made of tungsten and a GaSb cell with a bandgap of 0.67eV, with decreasing gap between
those surfaces, the output power became 3.6 times higher than that obtained by far-field radiation.
Keywords: Near-field radiation, Thermophotovoltaics, Surface Plasmon Polariton

1. Introduction
Nano- and/or micro-technologies will be useful for high density power generation or low
temperature heat recovery when the nanometer-scaled phenomena could be expanded into apractical length scale. There are many publications related to enhancement of radiation flux byevanescent wave effect. However, results of most of them came from experimental alliances far
from a practical scale or configuration, such as a small temperature difference or heat transfer
between sphere and plate. In the current study, nano-gap thermophotovoltaic generation ofelectricity was investigated using a millimeter-scale experimental setup and numerical simulation.
2. Experimental setup of nano-gap thermophotovoltaic generation of electricity
Figure 1 shows a schematic diagram of an experimental setup to make a parallel nanoscaled-gap. Atungsten emitter and a GaSb cell were mounted independently on each stage and were face-to-facein a vacuum chamber. The cell was pasted on a surface of a water-cooled copper block mounted on
piezo-actuator system for making the nano-gap, while the emitter was mounted on double axes
gonio-stage for making the parallel gap. The space-resolution of the piezo-actuator in the directionof the z-axis was 40nm. Figure 2 shows the diagrams of electric current and voltage (I-V curves)
for the GaSb cell under the condition of the emitter temperature of 1000K. The
diode-characteristics of the cell is also shown (symbols described by dark). When the cellapproaches to the emitter surface from an arbitrary origin (0 µm) to a position of 1.1µm, the I-V
curve didn’t change because the view factor was almost unity. On further increase in moving
distance up to 1.2 or 1.5µm from the origin, the short-circuit-current increased drastically. As aresult, the output power by the evanescent wave effect became higher than that by the conventional
propagating wave.

The Center ofRotation

θGonio Stage• x,y -rotation

Emitter
TPV Cellor PV Cell

Cooling water
Piezo Actuator• z-stagez

Micro Stage• z-rotation• x, y, z-stageyx

Fig.1 Schematic diagram ofexperimental setup

Fig.2 Electric current and voltage diagram(I-V curve) with emitter temperature

3. Radiation transfer enhancement by interference of SPP using a pillar-array structure
Figure 3 shows a pillar-array structured surface for numerical simulation model. Using theMaxwell’s equation with emission sources in all nodes with random phase-angles, the radiationtransfer is solved through a Finite Difference Time Domain method. Both emitter-1 (1000K) andemitter-2 (300K) were face-to-face and made of tungsten. The cross-sectional area (a x a = 360nmx 360nm) and the pitch (p = 400nm) of the pillar were fixed through simulation. As shown in Fig.4,
the normalized radiation flux between plane plates increases with decreasing gap as d = 200nm,100nm and 50nm, as described by dashed lines. In the case of pillar-array surfaces with d = 200nm,
the radiation flux for the pillar-height of h = 200nm could be enhanced in a range of angularfrequency from 10x1014 to 20x1014 rad/s, as described by the solid line. The peak frequency was
shifted toward the high-side with decreasing pillar-height. As shown in Fig.5, the radiation flux
increases and decreases periodically with increase in the pillar-height; as a result, a surface wave
like a surface-Plasmon-Plariton (SPP) propagates in narrow channels between pillars and the wave
is reflected at the bottom. Consequently, a kind of resonance wave in the channel might betransferred from the emitter-1 to the emitter-2.
4. Concluding remarks
A high density generation of electricity will be achieved through spectral-controlled near-field

Vacuum gap

dy zx

規格化ふく射流束

radiation transfer using a pillar-array structured surface.
0.8

ω x 1014 rad/s

0.7

12.57

0.6

15.71

0.5

21.99

0.4
0.3
0.2
0.1
0.0
0500

1000

1500

pillarピラー高さ
height h,h,nm
nm

Fig.3 Schematic diagramof pillar-array structuresurfaces

Fig.4 Comparison between
Normalized radiation fluxesof pillar-array and plane platesurfaces

Fig.5 Schematic diagramof pillar-array structuresurfaces