Micro Energy and Chemical Systems

The objective of this section is to lay the groundwork for a deeper discussion of MECS fabrication techniques. Specifically an overview of MECS is provided including a discussion of important MECS applications. An overview is given of some of the fabrication challenges associated with the large-scale production of MECS devices. The section concludes with an introduction to microlamination.

Micro Energy and Chemical Systems (MECS) are bulk microfluidic devices, which rely on embedded microstructures for their function. The overall size of MECS devices range between microscale systems, such as microelectromechanical systems (MEMS), and macroscale systems such as automobile engines and vacuum pumps. These mesoscopic systems are expected to provide a number of important functions where a premium is placed on either mobility, compactness, or point-of-use synthesis. The benefits of decentralised and portable energy and chemical systems are realised by the enhancement in heat and mass transfer performance within high surface-to-volume ratio microchannels. Other benefits of microchannels include rapid temperature changes, excellent temperature control, fast mixing, and the opportunity of operating at elevated pressures (Ameel et al. 1997, 2000; Peterson 2001). Implementations of MECS in heat transfer and chemical applications include microelectronic cooling systems (Kawano et al. 1998; Little 1990), chemical reactors (Martin et al. 1999; Matson et al. 1999), fuel processing (Daymo et al. 2000; Tonkovich et al. 1998), and heat pumps (Drost and Friedrich 1997; Drost et al. 1999, 2000) among others.

MECS devices are primarily focused on taking advantage of the extremely high rates of heat and mass transfer available in microscale geometries to radically reduce the size of a wide range of energy, chemical and biological systems. Typically a MECS device will have dimensions on the order of 1 to 10 cm but it will include embedded microscale geometries such as arrays of parallel microchannels where individual channel dimensions are on the order of 100 microns in width by several mm in height. By taking advantage of the enhanced rates of heat and mass transfer in the embedded microscale geometries, researchers have typically been able to reduce the size of a variety of energy and chemical systems by a factor of 5 to 10 (Brooks et al. 1999; Tonkovich et al. 1998; Warren et al. 1999). Typically MECS devices are fabricated in metals, ceramics or polymers and therefore cannot depend on conventional microfabrication techniques. Microlamination has proved to be an economical and flexible method for fabricating MECS devices (Porter et al. 2002). MECS devices are occasionally referred to as mesomachines and in Europe, research in this area is identified as process intensification.

14.2.1 Heat and Mass Transfer in MECS Devices

Typical MECS devices include individual microscale geometries with dimensions on the order of 100 microns. Single-phase flow in a 100-micron wide channel will in almost all cases of interest be laminar, with Reynolds numbers typically between 1 and 100. In laminar flow, the residence time required to have the fluid reach thermal equilibrium with the walls of the channel decreases as D2 where D is the width of the channel. If we decrease the channel width by a factor of 10, we will decrease the required residence time by a factor of 100. A heat exchanger with 100 micron wide channels would be 1/100th the size of a heat exchanger with 1 mm channels for the same heat transfer rate and it can be shown that for laminar flow, the increased pressure drop associated with the smaller channels is exactly offset by the decreased length of the channel required to deliver the same heat transfer rates. In effect we get a theoretical factor of 100 reduction in size with no increase in pressure drop. In practice microchannel heat exchangers have demonstrated heat fluxes 3 to 5 times higher than conventional heat exchangers. Ultimately, by going to smaller channels we will reduce the diffusion barrier to the point where other processes such as axial conduction along the heat exchanger determine residence time for thermal equilibrium. At that point, further reductions in channel dimensions will not result in reduced residence time. While singlephase flow in microchannels is well understood, processes that involve wall effects such as phase change and surface tension are significantly different in microscale geometries. Boiling is an example. The limited experimental investigations of boiling in microchannels suggest that the process of phase change in a microchannel is different from macro scale devices. However, published performance for microchannel evaporators and condensers show a significant improvement in performance when compared to single-phase microchannel devices, suggesting that as our understanding of phase change in microchannels improves we can expect a further improvement in the performance of microchannel heat exchangers. Mass transfer is similar to heat transfer in microchannels. The residence time that reactants need to reach complete conversion in a catalytic gas phase reactor also decrease by D2 if the residence time is dominated by diffusion rather then reaction kinetics. Many important reactions such as steam reforming are catalytic gas phase reactions where the primary barrier to short residence time is the time it takes for the reactant to diffuse to the catalyst site and for the products to diffuse back into the bulk flow. Reductions in residence time and reactor size on the order of a factor of 20 have been reported in the literature. Ultimately, by going to smaller channels we will reduce the diffusion barrier to the point where reaction kinetics will determine residence time for complete chemical conversion.

14.2.2 Applications of MECS Technology

The combination of size reductions and improved process control associated with MECS and the promise of high volume mass production (using microlamination)

means that small modular energy and chemical systems will be available at a reasonable cost. The availability of compact and modular energy and chemical systems means that MECS can be applied to both portable and distributed applications.

For heat transfer applications, example technologies include compact, high performance heat exchangers for thermal management and waste heat recuperation. Microscale combustion systems have been developed which take advantage of microchannel heat transfer to produce extremely compact high flux combustion systems. This technology can be used within compact heating and cooling schemes or for compact power generation or propulsion. Mechanically-constrained ultra thin film gas absorption and desorption has been used to reduce the size of absorption-cycle heat pumps and gas absorbers by a factor of 5 to 10. The application of compact heat pumps to microelectronics cooling could significantly reduce heat dissipation needs (Kawano et al. 1998; Little 1990). It is estimated that distributed, compact industrial heat pumps could improve building heating and cooling costs by 25% by eliminating ducting and heat pump cycling losses. Compact heat-actuated heat pumps could provide cooling for an automobile while using exhaust heat as the main energy source. Efforts are currently being made to adapt these systems to man-portable cooling for chemical and biological warfare suits. Targets are to eliminate the need for batteries weighing less than a third of a conventional system.

For chemical reactions where reaction rates are limited by diffusion, microreaction technology has been demonstrated within hydrocarbon fuel processing systems for fuel cells and proposed for distributed environmental clean-up systems and distributed point-of-use chemical synthesis. Hydrogen production systems for automotive fuel cells can reduce the size of automotive fuel reformers by a factor of 10. Integrated fuel cells and fuel reformers will lead to high energy density battery replacements (Benson and Ponton 1993). It is estimated that these types of systems could provide a person with 10 We for one week in a system weighing less than 1 kg. It is also expected that microscale fuel cells and thermoelectric generators capable of generating 50 to 100 milliwatts of power for up to one year are possible with a weight that is between one third and one tenth of the weight of an electrochemical battery system.

MECS microreactor technologies provide opportunities for distributed environmental restoration, in-situ resource processing and CO2 remediation and sequestration. Examples of environmental restoration applications include catalytic reactor systems for the distributed destruction of PCBs and the in-situ cleanup or neutralisation of contaminated aquifers (Koeneman et al. 1997). MECS-based high flux carbon dioxide absorption units and chemical processing systems could be used for the production of rocket fuel on the moon and on Mars greatly reducing the payload and cost of manned space flight. High flux carbon dioxide absorption units could be used for the removal and sequestration of carbon dioxide from power plant exhaust. Finally, the application of MECS microrection technology to the distributed production of nanoparticles and designer molecules could avert health risks and safety concerns surrounding the distribution of nanotechnology for consumption. Microchannel technology can also facilitate the miniaturisation of biologically-based processing systems by accelerating the diffusion of cell media, oxygen and other nutrients and by providing excellent temperature control. This technology might be important for high throughput enzymatic reactors and life support systems for tissue-based sensors.

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