APPLICATION OF NANOSCALE THERMAL RADIATION TO THERMOPHOTOVOLTAIC SYSTEM; A NOVEL CONCEPT

The exploitation of the conventional sources of energy has reached the extremum already, so switching to renewable sources of energy is the motto of the future. The conventional sources of energy are depleting at an alarming rate. A large amount of heat is released as waste heat from the existing energy conversion systems, so technologies that can help in recycling or recovering the waste heat along with increasing the overall efficiency of the existing energy conversion systems and devices is the need of the hour. The thermophotovoltaic (TPV) devices do the same thing.

Figure 1: Thermophotovoltaic cell 

 TPV helps recycle the waste heat very efficiently. This device makes use of the photovoltaic effect by which the photons whose energy is larger than that of the bandgap of the TPV cell will be able to generate electron-hole pair. In this device, a junction is formed between the semiconductor layers, which are doped unevenly. This junction creates a potential barrier inside the cell. This barrier potential separates charge carriers generated due to the photovoltaic effect. 

Due to this difference in potential between the charge carriers, an emf is generated which allows current to flow through the cell. The photons are emitted by a high-temperature source called the emitter. The temperature of the emitter will affect the performance of the TPV device. The desired thermal radiation can be filtered out by using a wavelength selective filter before radiating the cell, in this way the temperature can be optimized. There are no moving parts inside a TPV system, so there are no effects of friction and noise while operating in this device as compared to other energy conversion devices. As compared to conventional photovoltaic (PV) systems, TPV systems have higher efficiency in energy conversion. In TPV systems, there is provision for matching the exact wavelength of emitted radiation from the emitter to that required for the bandgap excitation, thereby reducing lots of unused radiation. Nowadays the research is focusing more on the micro thermophotovoltaic devices (MTPV) which is a variant of the TPV systems. These MTPV systems are the best choice for MEMS devices for efficient energy conversion applications. MTPV devices will act as the primary power source for the MEMS devices. 

Due to the miniaturization of all the electronic components inside the computers and mobile phones, there is always a stable demand for MEMS devices and it's research opportunities. MTPV can function as the micro-power generators for these micro-electromechanical (MEMS) devices. The energy conversion efficiency of the TPV systems is greater than that of the existing PV systems, but this efficiency is still not up to the mark when these TPV devices are coupled with MEMS devices. In order to increase this conversion efficiency, principles of nanoscale heat transfer need to be applied in the design and fabrication of these TPV devices. The normal emitters and filters need to be replaced by nanostructured emitters and filters. There are lots of advantages while using these nanostructures in the components of TPV systems. Only after understanding the principles of nanoscale heat transfer, can we think about the next level of TPV technology in the future with excellent efficacy and performance. 

Another interesting change in the design of the TPV device is that by reducing the distance between the emitter and TPV cell to around sub-wavelength dimensions, the throughput or energy transfer can be enhanced nearly 10-fold. This enhancement in energy transfer via TPV device is due to the size effect since the length scale is compared to the wavelength of the emitted radiation. This size effect is adopted from the principles of heat transfer in the nano regime. In this nano regime, whenever the size of the particle becomes smaller than the mean free path of the medium, then size effects comes in to picture, as a result of which there is anomalous enhancement in the heat transfer and other parameters contrary to bulk regime. 

 2. Nanoscale Radiation in TPV systems.

 2.1 Application of thin films 

The thin films are used as antireflection coatings on top of cells, emitters, and filters. The reflectance from these materials can be significantly reduced by using these thin-film antireflection coatings. If the reflectance is reduced, the unused radiation getting reflected away while irradiating the cell reduces and also low reflectance implies high absorbance of radiation which increases the overall energy conversion efficiency of the TPV systems. Figure 2 shows the comparison between the reflectance value with and without antireflection coating on the cell. The wavelength range is from 1 to 2.5 µm. From this figure, it is clear that the reflectance reduced from around 0.35 for the bare cell to below 0.1 in the coated cell for the entire spectrum. Double layered thin film coatings can be made to reduce the reflectance completely to zero. 

Figure 2: Normal reflectance from TPV cells with and without antireflection coatings. 

 2.2 Periodic gratings 

By making use of nanostructured gratings as the wavelength selective emitters, the overall conversion efficiency of the TPV systems can be increased. The fabrication of such nanostructured gratings is difficult. Nowadays the most extensively researched area for these types of gratings is in the fabrication of 2-D metallic nanostructured gratings. 1D tungsten grating can also be used as an alternative to the 2D metal gratings for the TPV emitter. As compared to normal metal emitters, these nanostructured gratings contained emitter shows high emittance peaks within the entire spectrum of irradiation wavelengths. The emittance value is enhanced with nanostructured gratings inside a bare metallic emitter. This is clearly evident from fig 3.

 

Figure 3: Comparison of spectral emittance for TM wave from complex grating and plain tungsten at θ=0° and 60°. 

2.3 Near-field TPVs 

Near field thermal radiation is one of the techniques by which the conversion efficiency and energy throughout the TPV systems can be enhanced very efficiently. The main idea is to place the emitter very close to the TPV cell so that the gap between the two is of the order of the emitted wavelength. This brings in the enhanced energy transfer via the cell due to the size effect. The energy of the photons released from the thermal emitter increases while moving through the cell due to photon tunneling. There should be a very small vacuum gap between the thermal emitter and the TPV cell in order to facilitate this photon tunneling. The TPV systems using this principle are called the near field TPVs. Near-field TPVs are a promising technology in the future and this domain attracts lots of research interests. Fig 4 shows the near-field TPV system. 

Figure 4: a) Near-field TPV system and b) illustration of minority carrier diffusion lengths and depletion region in the p-n junction.  

In this figure, the thermal source or thermal emitter is made of tungsten and it is maintained at a high temperature of 2000 k. The characteristic wavelength of this tungsten emitter is 1.5 µm. The TPV cell used is In0.18Ga0.82Sb which is an alloy of InSb and GaSb. The energy gap of this cell is 0.56 eV. In these near-field TPVs, a wavelength selective filter cannot be used between the emitter and cell, since this filter thickness might diminish the near-field effects. In the far-field TPV systems, the wavelength selective filter is one of the ways through which the conversion efficiency of the TPV systems is increased. In the near field TPV devices, one way to increase the conversion efficiency without compromising the near field effects is by depositing a thin metallic layer at the bottom of the TPV cell. This thin layer will act as an electrode as well as a reflective coating for long-wavelength photons which have a large penetration depth. By using this thin film metallic coating, the unused radiation will be reflected back to the emitter. This technique will reduce the overall radiative energy transfer without reducing the photocurrent generation.

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