Date of Award

Fall 12-15-2023

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Advisor

Sheila Edalatpour

Second Committee Member

Bashir Khoda

Third Committee Member

Olivier Putzeys

Abstract

Highly efficient energy harvesting devices that can recover a large amount of waste energy are of significant interest. Thermophotovoltaics (TPVs) are solid-state devices that can convert thermal energy radiated by a heated emitter into electrical power through the process of photovoltaic effect. In a TPV system, heat is converted to electricity by using a high-temperature emitter to radiate photons, which are then absorbed by a photovoltaic (PV) cell, generating electron-hole pairs and producing electrical power.

TPV devices have the advantages of producing High efficiency and being lightweight with low maintenance cost and zero pollution. However, these devices have not been commercialized yet as their power output is low. A promising mechanism for increasing the power output and also the efficiency of the TPV cells is by reducing the gap distance between the emitter and the PV cell of the device to a value that is smaller than the dominant wavelength of thermal radiation (~ 10 mm at room temperature). In this case, thermally radiated evanescent electromagnetic waves, which are only substantial at sub-wavelength distances from the emitter, can also be received by PV cells and contribute to power generation. These devices with a sub-wavelength separation gap between the emitting layer and the received are referred to as near-field thermophotovoltaic (NF-TPV) devices.

It has been proposed that using a reflector at the back side of the PV cell, introducing a thin metal cover on the PV cell, and creating an air gap between the PV cell and the back reflector can increase the power output and/or the efficiency of the NF-TPV devices. However, it is not known how the performance of the NF-TPV devices changes with using air gap and thin metal layer simultaneously.

This thesis examines the effect of the air gap and the presence of the metal cover layer of the NF-TPV devices for a vacuum gap size of 50 nm between the emitter and the PV cell. Also, a new configuration where both the air gap and the metal cover have been used simultaneously, is proposed. Finally, by using genetic algorithms, novel NF-TPV devices with high efficiency and power output are designed. Based on the results, it is seen that utilizing the air gap increases the efficiency of a NF-TV device having a vacuum gap of 50 nm by 10%. However, the power output of the device decreases in the presence of the air gap due to a reduction in the in-band radiation absorption by the PV cell. The air gap is more beneficial at small vacuum gap sizes, such as 10 nm, where both the power output and the efficiency increase with introducing an air gap. It also is shown that, for a NF-TPV device with a vacuum gap size of 50 nm, depositing a very thin metal layer, around 3 nm, on top of the PV cell increases the power output of the NF-TPV nearly by a factor of 4. In this case, efficiency decreases from 30% to 20%. Therefore, placing a thin metal layer on top of the PV cell is practical for waste heat recovery applications where power output is more important than efficiency.

The results of this thesis can provide a guide for designing high-power output and large-efficiency NF-TPV devices that can be used for converting heat from any source, including waste heat, to useful electrical power.

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