| Popis: |
Power generation in a nanoscale-gap thermophotovoltaic (nano-TPV) device can be enhanced, compared to conventional thermophotovoltaic (TPV) systems, due to radiative heat transfer exceeding the blackbody limit. TPV power generation refers to direct thermal-to-electrical energy conversion of near infrared and infrared radiation emitted by a terrestrial source. By separating the radiator and the cell by a gap smaller than the peak emitted wavelength, radiative heat transfer can exceed the blackbody predictions by a few orders of magnitude due to energy transport by waves evanescently confined to the surface of the radiator. This enhanced energy transfer can lead to a significant increase in TPV power generation. This dissertation is divided into two main parts. First, a numerical model is presented which demonstrates increased power generation in nano-TPV devices when compared to conventional TPV systems. The model incorporates near-field radiation, heat and charge transport while accounting for radiative, electrical and thermal losses in the cell. The devices analyzed consist of GaSb cells illuminated by a broadband tungsten and a quasi-monochromatic Drude emitter at 2000 K. Results show an increase in power generation by a factor of 4.7 with a tungsten emitter and a 100-nm-thick gap. Furthermore, it is shown that nano-TPV power generators may perform better with broadband emitters where radiative heat transfer is dominated by frustrated modes rather than surface modes. The second part of this dissertation is devoted to the experimental demonstration of radiative heat transfer exceeding the blackbody limit, which is the fundamental phenomenon underlying enhanced power generation in nano-TPV systems. A MEMS-based experimental device has been fabricated for radiative heat flux measurements between 5 5 mm2 planar intrinsic silicon surfaces separated by a variable gap as small as 150 nm. The separation gap is maintained via rigid spacers and a compliant membrane allows for variation of the gap size via mechanical forces. Results agree well with predictions based on fluctuational electrodynamics. At a gap size of 150 nm, the blackbody limit is exceeded by a factor of 8.4. This is the largest value ever recorded between macroscale planar surfaces at non-cryogenic temperatures. |
