Due to recent advances in microelectronics and micro electro-mechanical systems (MEMS), there is considerable interest in miniaturizing thermochemical systems for electrical power generation, air or space reconnaissance vehicles, pneumatic power for micro-robots, and high-energy-density electrical power sources for autonomous or hand-carried electronics. Such devices have numerous military, space, and commercial applications. However, all of the aforementioned systems require gas pressurization and/or vacuum pumping systems, which in turn usually require devices with moving parts that experience more difficulties with heat and friction losses due to higher surface to volume ratios than their macroscale counterparts. Additionally sealing, fabrication, and assembly are much more difficult at small scales because microfabrication processes have poorer relative precision than convectional silicon based macroscale processes. The difficulty of reducing the size and/or power consumption of the pressurization or vacuum systems required for a complete system in turn limits the portability and utility of miniature thermochemical systems.

We are combining the concept of catalytic combustion-driven thermal transpiration and single chamber solid oxide fuel cells (SOFCs) to build an ideal small-scale power generation system that has (a) means for pumping, i.e., supplying reactants and expelling products, with no moving parts; (b) use fuel, not electrical power, as the “energy feedstock” for pumping; (c) produce electrical power with no moving parts and (d) not require high-precision fabrication (unlike, for example, devices based on internal combustion engines.)

Fig. 1 shows the basic building block unit of a catalytic reaction driven Knudsen compressor. The reactants first flow through a low-temperature “thermal guard” consisting of a plate with microchannels (not nanoscale pores as in the transpiration membrane). The purpose of the thermal guard is to reduce the thermal resistance between the ambient-temperature reactants and the cold side of the transpiration membrane, thereby obtaining a larger temperature difference across the membrane, resulting in better pumping performance. The reactants then pass through a transpiration membrane without catalyst where the pumping occurs due to the temperature difference across the membrane. The reactants then pass through a catalytic high-temperature thermal guard where the reactants are catalytically combusted, resulting in heat production. The heat release in this region, combined with the low thermal conductivity of the nanoporous material used in transpiration membrane, sustains the temperature gradient and thus the pumping action is self-sustaining.

The device shown in Fig. 1 is viable if the pumped gas can be seeded with a reactant(s). This is the case in catalytic reaction-driven meso/micro-scale thermoelectric power generation devices and in micro fuel cell applications, where it is the reactant stream itself that needs to be pumped. In this case, as found from preliminary results, the transpiration pump does not need to burn all the fuel; therefore, fuel will be left for the SOFCs. Based on the above information, we built a 3-D design for catalytic combustion-driven thermal transpiration pump and SOFC power generator, shown in Fig. 2.

Figure 1: Schematic diagram of thermal transpiration pumps.

Figure 2. Schematic of the thermal transpiration device: (a) exploded view; (b) assembled view. Not shown for clarity: electrical connections for thermocouples and SOFC, and transpiration membrane on left side in part (a).