Micropower generation using combustion: Issues and approaches

The push toward the miniaturization of electromechanical devices and the resulting need for micropower generation (milliwatts to watts) with low-weight, long-life devices has led to the recent development of the field of microscale combustion. The concept behind this new field is that since batteries have low specific energy and liquid hydrocarbon fuels have a very high specific energy, a miniaturized power-generating device, even with a relatively inefficient conversion of hydrocarbon fuels to power, would result in increased lifetime and/or reduced weight of an electronic or mechanical system that currently requires batteries for power. In addition to the interest in miniaturization, the field is also driven by the potential fabrication of the devices using microelectromechanical systems or rapid prototyping techniques, with their favorble characteristics for mass production and low cost. The micro power generation field is very young and still is, in most cases, in the feasibility stage. However, considering that it is a new frontier of technological development, and that only a few projects have been funded, it can be said that significant progress has been made to date. Currently, there is consensus, at least among those working in the field, that combustion in the microscale is possible with proper thermal and chemical management. Several mesoscale and microscale combustors have been developed that appear to operate with good combustion efficiency. Some of these combustors have been applied to energize thermoelectric systems to produce electrical power, although with low overall efficiency. Several turbines/engines have also been, or are being, developed, some of them currently producing positive power, also with low efficienty to data. Microrockets using solid or liquid fuels have been built and shown to produce thrust. Hydrogen-based microsize fuel cells have been successfully developed, and there is a need to develop reliable reformers (or direct-conversion fuel cells) for liquid hydrocarbons so that the fuel cells become competitive with batteries. In this work, some of the technological issues related to meso- and microscale combustion and the operation of thermochemical devices for power generation will be discussed. Some of the systems currently being developed will be presented and described.