Abstract
Most attention to improving vacuum thermionic energy conversion device (TEC) technology has been on improving electron emission with little attention to collector optimization. A model was developed to characterize the output characteristics of a TEC where the collector features negative electron affinity. According to the model, there are certain conditions for which the space charge limitation can be reduced or eliminated. The model is applied to devices comprised materials reported in the literature, and predictions of output power and efficiency are made, targeting the sub-1000 K hot-side regime. By slightly lowering the collector barrier height, an output power of around 1 kW, at ≥20% efficiency for a reasonably sized device (∼0.1 m2 emission area) can be achieved.
Highlights
Producing more energy at high efficiency is a constant challenge facing humanity
Most attention to improving vacuum thermionic energy conversion device (TEC) technology has been on improving electron emission with little attention to collector optimization
The method of calculating the output current characteristic in the space charge limited mode is nearly identical to the Langmuir case, one difference being the space charge mode extends from the saturation point only to the virtual critical point when the collector exhibits negative electron affinity (NEA)
Summary
Producing more energy at high efficiency is a constant challenge facing humanity. Thermionic energy conversion has a role to play due to its many advantages: the design is simple, there are no moving mechanical parts, and there is no efficiency-reducing direct conduction of heat across the device.. The challenges have been dealing with the high temperatures required to achieve a reasonably good efficiency and having to overcome the effects of a space charged limited mode. The first challenge is to create a thermionic energy conversion device (TEC) which operates at a reasonably low temperature, but still outputs acceptable power at high efficiency. The output current of a TEC is limited by the negative space charge effect because electrons traversing the interelectrode space create a negative charge barrier which blocks lower energy electrons from reaching the collector. The electrodes should be close enough together to mitigate the negative space charge effect, bearing in mind that decreased spacing increases the engineering difficulty of device fabrication. Smith et al have predicted that an emitter which features a negative electron affinity (NEA), such as hydrogen terminated diamond, could mitigate or eliminate the negative space charge effect.. The model predicts that a reasonably sized device (
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