Improving efficiency of ID thermopower wave devices and studying 2D reaction waves

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 124-127).With growing energy consumption, current research in the field is focused on improving and developing alternatives for energy storage and conversion. Factors such as efficiency of energy conversion, usability of this converted form of energy, power density, energy density etc. help us in determining the right energy source or conversion technology for any specific application. The main aim of this thesis was to study self-propagating reaction waves as a means of converting chemical energy into electrical energy. We carried out numerical simulations to study these self-propagating reaction wave systems and their heat transfer properties. Our analysis shows that for certain specific system heat transfer properties, self-propagating reaction waves can sometimes lead to superadiabatic temperatures, which are temperatures higher than the predicted adiabatic reaction temperature. Having energy available at higher temperature has advantages in heat harvesting applications such as thermoelectricity and thermophotovoltaics. We calculated the improvement in efficiency of a modified thermophotovoltaics setup, when the input is a reaction wave, operating under superadiabatic conditions. Experimentally, we studied these self-propagating reaction waves by launching I D thermopower waves. We demonstrated improved chemical-to-electrical conversion efficiency of these devices (from about 10-⁴ % to 10-² %) by operating with newer fuels such as sodium azide and sucrose with potassium nitrate on single-walled carbon nanotube-based thermal conduits. The net efficiency of operation of the device was also improved to up to 1% by using external thermoelectric harvesters to capture the heat energy lost via convection and radiation. We proposed a model combining the ID reaction heat and mass balance equations with the theory of excess thermopower to predict the output voltage profiles of thermopower wave devices and extract useful data from the voltage plots obtained experimentally. This model allows us to quantify the impact of the device-to-device variation of the fuel and thermal conduit properties, and can guide us to a better choice of fuel-thermal conduit pairs to improve the efficiency of operation. Finally, we experimentally studied 2D reaction waves. These waves were launched with a nitrocellulose fuel layer atop an aluminum foil thermal conduit. A wave front characteristic, the shape of these wave fronts, was studied as a function of heat loss. Energy released by these reactions was again harvested using external thermoelectrics to convert heat energy into electricity. We demonstrated that such a setup of 2D reaction waves can be used to illuminate a light-emitting diode (LED).by Sayalee G. Mahajan.Ph. D

Excess Thermopower and the Theory of Thermopower Waves

Self propagating exothermic chemical reactions can generate electrical pulses when guided along a conductive conduit such as a carbon nanotube. However, these thermopower waves are not described bran existing theory to explain the origin of power generation or why its magnitude exceeds the predictions of the Seebeck effect In this work, we present a quantitative theory that describes the electrical dynamics of thermopower waves, showing that they produce an excess thermopower additive to the Seebeck prediction. Using synchronized, high-speed thermal, voltage, and wave velocity measurements, we link the additional power to the chemical potential gradient created by chemical reaction (up to 100 mV for picramide and sodium azide on carbon nanotubes). This theory accounts for the waves' unipolar voltage, their ability to propagate on good thermal conductors, and their high power, which Is up to 120% larger than conventional thermopower from a fiber of all-semiconducting SWNTs. These results underscore the potential to exceed, conventional figures of merit for thermoelectricity and allow us to bound the maximum power and efficiency attainable for such systems

Excess Thermopower and the Theory of Thermopower Waves

Self-propagating exothermic chemical reactions can generate electrical pulses when guided along a conductive conduit such as a carbon nanotube. However, these thermopower waves are not described by an existing theory to explain the origin of power generation or why its magnitude exceeds the predictions of the Seebeck effect. In this work, we present a quantitative theory that describes the electrical dynamics of thermopower waves, showing that they produce an excess thermopower additive to the Seebeck prediction. Using synchronized, high-speed thermal, voltage, and wave velocity measurements, we link the additional power to the chemical potential gradient created by chemical reaction (up to 100 mV for picramide and sodium azide on carbon nanotubes). This theory accounts for the waves’ unipolar voltage, their ability to propagate on good thermal conductors, and their high power, which is up to 120% larger than conventional thermopower from a fiber of all-semiconducting SWNTs. These results underscore the potential to exceed conventional figures of merit for thermoelectricity and allow us to bound the maximum power and efficiency attainable for such systems