Thermal Modelling of a Concept for a Hybrid Micro Combustion Generator

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Mobile electronics or remote devices like sensors for the Internet of Things (IoT) require energy storage with increasing energy densities, approaching the limits of the widespread lithium batteries. At the same time, lithium batteries are subject to growing criticism about their environmental impact due to their raw materials and their mining methods. Chemical energy seems to be a promising alternative and the development of micro combustion to make use of it is already for almost 30 years the topic of ongoing research. Recent studies show that compliant combustion engines are a good solution to mitigate some problems like leakage and friction and thus, achieve higher efficiencies. However, the most prevalent problem is heat losses through the walls because of the high surface-to-volume ratio in small-scale combustion. In this work, a design concept was proposed that not only insulates the compliant combustion chamber of the actuator for Flapping Wing Micro Air Vehicles (FWMAV) but also uses the waste heat to generate electricity. That was done by attaching Thermoelectric (TE) modules to a support structure and thermally connecting them with porous media to the oscillating combustor wall. The actuator uses the heat and pressure generated by the catalytic reaction of hydrogen peroxide and therewith, only emits steam and oxygen. With the help of a one-dimensional steady-state heat resistance model and a one-dimensional transient heat resistance-capacitance model, an optimum design was found. With that, it is possible to create a temperature difference sufficient enough to enable energy harvesting by TE modules. That was achieved by implementing an exhaust gas recirculation which
transfers heat to the modules and simultaneously serves as a heating blanket for the combustor. A downside of the design is the relatively long time it takes to reach its steady state and especially for FWMAV, the increase in weight.