What is MECS ?

A great deal of interest has been focused on the emerging area of micro-scale devices. The popular press has reported numerous examples of Micro Electro Mechanical Systems (MEMS) including devices such as micro-scale motors, actuators, grippers, and sensors. Microtechnology-based Energy and Chemical Systems (MECS) are related to these devices in that they are based on micro-scale features. However, MECS deals with heat transfer, mass transfer and fluidic processes. These basic processes are used in energy, chemical, and biological systems, so the devices of interest include heat exchangers, flow controllers, reactors, mixing devices, and separating devices. Examples of devices that would result from the application of MECS technology include miniature heat pumps, chemical synthesis systems, waste cleanup devices, miniature power sources and bio-reactors.

What characteristics distinguish MECS devices ?

It is clear from the initial work that has been done on MECS that there are some inherent features and operating characteristics that are associated with energy and chemical conversions done at the micro-scale:

• It is possible to obtain extraordinarily high heat transfer rates in a small volume. By constructing heat exchangers from micro-channel arrays, the thermal diffusion path is very small, the total surface area per unit volume is very high, and the resulting heat transfer capability is very high. It has been reported, for example, that a heat transfer rate of 20 kW has been achieved in a volume of 1 cubic cm. This is the amount of energy required to heat an average home on a cold day in Corvallis, Oregon.

• As with heat transfer, micro-scale devices have high mass diffusion rates. This leads to very fast and complete mixing in very small volumes. Contrast this inherent feature with the extensive effort required to ensure good mixing in a large-scale chemical reactor.

• Many desirable chemical reactions and biological processes can be controlled by temperature. With flow-through micro-scale devices temperature gradients can be very large (on the order of 100,000 K over a distance of a few microns or a time period of a few microseconds).

• By virtue of scale, the stresses in micro-scale devices are lower for a given operating pressure. It is therefore practical to operate at much higher pressures than in conventional processes. This is advantageous for some types of processes and can increase efficiency substantially.

What are the applications for MECS ?

One important application area for MECS devices is in portable systems. For example, applying MECS technology to energy systems has the potential of reducing the size, weight, and cost of miniature power generating equipment and coolers. This latter application has created recent excitement due to the problems of working in hazardous environments. MECS refrigeration units would be small and light-weight and have the capability of cooling individual protective suits worn in biologically and chemically "hot" areas. In power production, combustion driven electrical generators could be packaged to the size of batteries with lifetimes exceeding that of batteries by many times. This could provide for long lasting sources of electricity, for portable communications and computing.

The use of MECS in high capacity applications such as industrial chemical plants would be based on individual MECS devices assembled in a massively parallel architecture. The principle advantage to this approach is the higher production efficiency associated with micro-scale processing. Also of value is the reliability inherent in parallel architecture; the failure of one, or even several individual devices would have a small effect on the overall production rate.

Modest size applications of MECS technologies are the most likely to be commercially attractive in the near future. There are some types of applications where MECS makes it possible to use a technology that would otherwise be impractical. For example, a MECS reformer to strip hydrogen atoms from hydrocarbon-based fuels will allow the practical use of fuel cells in automobiles (with the associated higher efficiencies and lower pollution). The other major use of MECS technology will be in using distributed energy and chemical processing to eliminate transportation costs. Instead of manufacturing chemicals centrally and then shipping them to the end users, they may be manufactured on location and just-in-time to minimize warehousing costs. Likewise, instead of shipping toxic chemicals to a central processing plant, local MECS based processors may be used to render the waste inert as fast as it is produced.

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