High Density Thermophotovoltaic Generation of Electricity using Near-Field Radiation

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Effect of near-field radiation (evanescent wave) to enhancement of thermophotovoltaic(TPV) generation of electricity has been investigated in the current study. A tungsten emitter and the GaSb TPV cell were put together in face to face in a vacuum chamber. The cell was pasted on a surface of a water-cooled copper block mounted on x-y-z-θ micro-stages, while the emitter is held by two zirconia rings in a water-cooled jacket mounted on double axes gonio-stage controlled by a computer. Using the gonio-stage, a parallel gap between these surfaces was made over the surface area of emitter, i.e., 2 x 8 mm^2. The space resolution, which depends on the minimum incremental motion of the z-axis stage, is 100nm. A CO_2 laser is used for heating of the emitter. The input energy is kept at constant, i.e., 20W. Temperatures of the emitter are measured by thermocouples. As the gap becomes narrower up to 15μm, the electric power obtained, first, increases, and then, is held at almost constant. As a re … More sult, the view factor is almost unity under the condition of the gap narrower than 50μm. On further decrease in gap, the emitter temperature decreases drastically. The electric power, first, decreases accordingly, and then, increases oppositely though the temperature still continues to decrease. Finally, the power went down sharply to almost zero, which means that the emitter has contacted with the cell. On the basis of the scale from the origin at the contact point, variation of the power and the temperature with gap is understood as follows. As the emitter approaches the cell, the near-field effect for long wavelength radiation becomes remarkable, first. In this case, only the heat is transferred quickly, resulting in the drastic temperature decrease. As the gap is narrower than 1μm, the near-field effect for short wavelength radiation becomes remarkable gradually since the radiation with wavelength in a range from visible to 1.85μm is active for conversion into electricity by the TPV cell. Consequently, it is disclosed that the near-field radiation enhances heat transfer as well as energy conversion from the thermal energy into electricity. Less