Thermally efficient micromachined device invention

Abstract: A micromachined device for efficient thermal processing at least one fluid stream includes at least one fluid conducting tube having at least a region with wall thickness of less than 50 μm. The device optionally includes one or more thermally conductive structures in thermal communication with first and second thermally insulating portions of the fluid conducting tube. The device also may include a thermally conductive region, and at least a portion of the fluid conducting tube is disposed within the region. A plurality of structures may be provided projecting from a wall of the fluid conducting tube into an inner volume of the tube. The structures enhance thermal conduction between a fluid within the tube and a wall of the tube. A method for fabricating, from a substrate, a micromachined device for processing a fluid stream allows the selective removal of portions of the substrate to provide desired structures integrated within the device. As an example, the micromachined device may be adapted to efficiently react fluid reactants to produce fuel for a fuel cell associated with the device, resulting in a system capable of conversion of chemical to electrical energy.

Work related to the present invention was supported in part under DARPA Contract No. F30602-99-2-0544. The government may have certain rights in the invention.

The present invention is generally directed to a micromachined device for efficient thermal processing of fluids. More particularly, the present invention is directed to a micromachined device for carrying out fluid processing with improved thermal efficiency. As used herein, fluid processing includes, for example, subjecting a stream of a fluid to chemical reactions, heating, cooling, filtration, adsorption, desorption, and/or phase changes (e.g., evaporation and condensation). The present invention may be used in, for example, the processing of chemicals for purposes of power generation.

The field of micromachined fluidic devices encompasses all systems that process components (e.g., gases, liquids, solid particles (e.g., beads), complex molecules (e.g., DNA), and mixtures thereof and contain small features (minimum feature sizes smaller than 500 μm). Such micromachined devices have been shown to be useful in many fields, including chemical and biological analyses (e.g., capillary electrophoretic separations), small-scale chemical synthesis, and measurement of reaction kinetics. The smaller dimensions inherent in micromachined devices enable significantly smaller fluid flow rates, reduced system size, and, in many cases, improved performance.

A variety of fluid-processing micromachined devices are disclosed in the prior art. For example, U.S. Pat. No. 6,192,596 provides an active microchannel fluid processing unit and a method of making the unit. Arrays of parallel microchannels separated by thermally conductive fins provide the mechanism for heat transfer to or from fluids moving through the microchannels.

U.S. Pat. No. 6,193,501 discloses a microcombustor of sub-millimeter dimensions. A preferred embodiment includes a wafer stack of at least three wafers, with the central wafer housing a combustion chamber. At least one inlet and one outlet are included for the insertion of reactants and the exhaust of a flame.

U.S. Pat. No. 4,516,632 discloses a microchannel crossflow fluid heat exchanger and a method for making the same. The heat exchanger is formed from a stack of thin metal sheets bonded together.

One possible application of fluid-processing micromachined devices is as portable electric generators. This application is promising because the energy density of chemical fuels exceeds that of presently available batteries by approximately two orders of magnitude. However, to take advantage of the high energy density of chemical fuels and compete with batteries for portable power applications, suitable efficient designs for micro-sized fuel processors/generators that convert chemical to electrical energy must be created. Fuel processors/generators usually require regions of high temperature in order to sustain the desired reaction. The power consumed in sustaining those temperatures reduces the overall efficiency of the system. As device dimensions become smaller, it becomes increasingly difficult to maintain the thermal gradients and thermal insulation required for efficient fuel processing. Often with existing micromachined fuel processing devices, significantly more power is required to maintain the temperature within the high temperature regions of the device than is contained in the fuel, precluding the device's use for portable power generation.

Thermal management is crucial to producing efficient devices designed to operate with separate features held at different temperatures. In particular, thermal isolation of the reaction zone in micromachined power generation systems is paramount. For micromachined non-fluidic devices, thermal management is achieved by simple thermal insulation using long, thin and/or non-conductive supports, often assisted by packaging in vacuum. Examples of non-fluidic micromachined devices providing thermal management include, for example, bolometers, such as those disclosed in U.S. Pat. Nos. 5,021,663 and 5,789,753. However, the present inventors have concluded that micromachined fluidic devices add three unique difficulties: the need for enclosed fluidic structures connecting the thermal regions; the potential of added heat flow through thermal convection; and, frequently, a desire to include regions where the walls of the fluid conducting tube are held isothermal. Thus, a successful thermal management scheme for a micromachined fluidic device must include, in addition to certain known inventions and techniques used in non-fluidic thermal devices, a means to provide fluid communication with high temperature regions without causing excessive heat flow either through the static structure or through the moving fluid. It is frequently desirable to include in this scheme means to ensure thermal uniformity in specific regions of the device.

Accordingly, it would be advantageous to provide a micromachined device capable of efficiently conducting a chemical process involving at least one fluid, wherein a high temperature reaction zone is thermally isolated from its environment. It also would be advantageous to provide a micromachined device for conducting a chemical reaction involving fluidic reactants and wherein operation of the device consumes substantially less energy than can be produced from the fluid reactants. Such a device could be used as part of a portable electric generator.

The present invention addresses the above-described needs by providing a micromachined device for thermal processing at least one fluid stream. The micromachined device includes at least one fluid conducting tube, and at least a region of the fluid conducting tube has a wall thickness of less than 50 μm.

The present invention further addresses the above-described needs by providing a micromachined device for processing at least one fluid stream, and wherein the device incorporates at least one fluid conducting tube and at least one thermally conductive structure. The thermally conductive structure is in thermal communication with a first thermally insulating portion of the fluid conducting tube and a second thermally insulating portion of the fluid conducting tube.

The present invention also addresses the above-described needs by providing a micromachined device for processing at least two fluid streams including a first fluid conducting tube, a second fluid conducting tube, and at least one thermally conductive structure. The thermally conductive structure is in thermal communication with a thermally insulating portion of the first fluid conducting tube and a thermally insulating portion of the second fluid conducting tube.

The present invention additionally offers a solution to the above-described needs by providing a micromachined device for processing at least one fluid stream, wherein the device includes a thermally conductive region and at least one fluid conducting tube having at least one thermally insulating portion. At least a portion of the fluid conducting tube is disposed within the thermally conductive region.

The present invention is further directed to a method for processing a fluid stream. The method includes providing a micromachined device having at least one fluid conducting tube having a thermally insulating inlet portion and a thermally insulating outlet portion, and at least one thermally conductive structure in thermal communication with the inlet and outlet portions. A stream of at least one fluid is introduced into the inlet portion of the fluid conducting tube, and the fluid stream is processed within the fluid conducting tube. The thermally conductive structure conducts thermal energy between the inlet and outlets portions of the tube.

The present invention also provides a portable power generator including a micromachined device constructed according to the present invention. The micromachined device includes at least one fluid conducting tube and at least one thermally conductive structure. The thermally conductive structure is in thermal communication with a first thermally insulating portion of a fluid conducting tube and a second thermally insulating portion of a fluid conducting tube. A fuel cell is in fluid communication with the fluid conducting tube. A fuel is produced in the fluid conducting tube and is conveyed to the fuel cell, where it is used to generate power.