Nano-scale vacuum spaced thermo-tunnel devices for energy harvesting applications
- PDF / 231,080 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 73 Downloads / 140 Views
Nano-scale vacuum spaced thermo-tunnel devices for energy harvesting applications
Amit K. Tiwari, Jonathan P. Goss, Nick G. Wright, and Alton B. Horsfall Electrical and Electronic Engineering, Newcastle University, NE1 7RU, UK
ABSTRACT The output power-density and the efficiency of thermo-tunnel devices are examined as a function of inter-electrode separation, electrode work-function, and temperature. We find that these physical parameters dramatically influence the device characteristics, and under optimal conditions a thermo-tunnel device is capable of delivering a very high output power-density of ∼ 103 Wcm−2 . In addition, at higher temperatures, the heat-conversion efficiency of the thermo-tunnel device approaches ∼ 10%, comparable to that of a thermoelectric generator. We therefore propose that thermo-tunnel devices are promising for solid-state thermal energy conversion.
INTRODUCTION Solid-state thermal energy conversion technologies are becoming increasingly important for both terrestrial and extra-terrestrial applications. Thermionic energy-conversion is of particular interest because of the potential of providing relatively efficient thermalenergy conversion in comparison to commonly used techniques, such as thermoelectric and thermophotovoltaic conversion [1]. Recently, combined thermionic emission and tunneling of hot electrons (thermo-tunneling) has come to the fore, capable of providing very high output power-densities via a nano-scale, vacuum-spaced thermo-tunnel device [2– 10]. The height and the width of a potential barrier between the electrodes which an electron must overcome are controlled by the physical parameters, particularly interelectrode separation (d) and the work function of the electrodes (φe and φc for the emitting and collecting electrodes) [2–6], with both the engineering of low work-functions and uniform nano-meter separations over relatively large areas being simultaneous challenges to overcome. Recent advances in work function reduction of semiconductor surfaces, particularly of diamond [11, 12], and in parallel, the fabrication of nano-scale vacuum gap [8–10, 13] show that fabrication of such free-standing structures will be possible in near future. By applying a thermal gradient across the ultra-thin (a few nm) vacuum gap of a tunnel device, a large unidirectional flux of thermionic and thermo-tunnel currents between an emitter and collector, and a thermal potential difference can be achieved. Thermo-tunnel devices offer several advantages over conventional thermionic and thermoelectric devices. Since d in a thermo-tunnel device is extremely small, the negativespace-charge effects [1], which impede the flow of electrons from emitter to collector in a conventional thermionic converter, are negligible. For thermoelectric devices, in addition to a high Seebeck coefficient, high electrical but low thermal conductivity are necessary to achieve a high figure of merit [1]. In contrast, thermo-tunnel devices have no such requirement, although in practice the encapsulating material must have a low th
Data Loading...
