This article reviews the micro technology-based energy and chemical systems (MECS) that help miniaturizing and integrating the components necessary for advanced energy and chemical systems. Researchers in the field view this idea as a way of improving traditional energy systems while addressing the challenges of new applications. This aim is analogous to what occurred when discrete electronic components were integrated on the same substrate. MECS are typically mesoscopic, that is, approximately between the size of a sugar cube and a fist. Thus, MECS are systems made up of smaller components brought together to form a larger package. Miniature energy and chemical systems rely on the small scales because heat and mass transfer rates increase by the inverse square of the reduction in size; that is, going to smaller sizes greatly enhances both heat and mass transfer rates. As MECS technology matures, the ideas and concepts presented here may result in exciting new applications that could include fuel cell-powered cars that fill up on gasoline, or power cartridges for portable electronics having 10 times the life of current batteries.
Oregon State University researchers call it MECS, for "micro technology-based energy and chemical systems." Battelle's Pacific Northwest National Laboratory has a program called Micro-CATS, which is short for "micro- chemical and thermal systems." Other names and acronyms have also been used. Whatever the terminology, the goal of this work is to miniaturize and integrate the components necessary for advanced energy and chemical systems
Researchers in the field view this idea as a way of improving traditional energy systems while addressing the challenges of new applications. This aim is analogous to what occur red when discrete electronic components were integrated on the same substrate. When applied to energy systems and chemical processing, this idea results in a systemona chip framework for addressing a wide variety of problems, from battery replacement to hazardous waste detoxification. Miniaturization will be especially important in areas where portability, compactness, weight, or point application is the driving consideration.
Most engineers familiar with microscale systems know about the rapid progress made in microelectromechanical systems. MEMS are distinguished by a number of characteristics, including a feature size that can approach 1 micrometer. MEMS use the extensive knowledge base available for processing integrated circuits in silicon ; thus, most MEMS are fabricated using a number of IC processing steps, even though the end product can be made from something other than silicon.
Also inherent in MEMS, as the name implies, is an integration of mechanical and electrical features for sensing or actuation. The single largest MEMS application today is in acceleration sensors for automobile air bags. MEMS are a growing area with new uses emerging yearly. MECS can be distinguished from MEMS in several ways : in overall size, in feature size, and in the physical processes exploited to achieve the desired effect.
MECS are typically mesoscopic; that is, approximately between the size of a sugar cube and a fist. Thus, MECS are systems made up of smaller components brought together to form a larger package. Component size is larger than that typically found in MEMS and spans the range from about 25 microns upward to millimeters.
A Limit to Size Reduction
Miniature energy and chemical systems rely on these small scales because heat and mass transfer rates increase by the inverse square of the reduction in size; that is, going to smaller sizes greatly enhances both heat and mass transfer rates. However, size reduction cannot go on indefinitely because of the increasing difficulty of transporting fluids and maintaining temperature differences within a miniaturized system. Excessive pressure drops, parasitic internal heat conductances, and mechanical friction are just a few of the problems engineers must contend with in developing miniature energy systems. But researchers are finding fertile ground in the mesoscopic scale for developing a nun1.ber of intriguing concepts.
The term "energy systems" in this case generally involves energy generation, use, distribution, or conversion from one form to another.
These systems will be used in areas where a premium is placed on mobility, compactness, weight, or point application of the process. Reliability can also be improved by using large arrays of small, simple components to perform a particular task.
Two areas where this technology makes sense are smallscale power production and resource processing. For example, if the chemical energy stored in a liquid hydrocarbon can be converted to electricity with 20 percent or higher efficiency, stored energy densities for portable power generation could be 10 to 100 times that of current battery technology. This would require a powerstation- on-a-chip with all the necessary components integrated to make it work.
If a manned Mars mission is to become a reality, native materials must be processed on the planet's surface for fuel, air, and water. This type of resource processing would be most reliably done by large numbers of small devices working to generate the necessary power and performing the required chemical processing of the indigenous materials.
Realization of these two examples (as well as others) is still several years away, but the vision of bringing about their development is one of the driving forces in the MECS area.
MECS components are much smaller than those in traditional devices. Flow channels, heat exchangers, valves, and pumps are just a few of the components that must be reduced to the scale of a few millimeters or less.
Two pioneers in the energy miniaturization field, Robert Wegend and Kevin Drost, directed early work in the development of microchannel heat exchangers at Pacific Northwest National Laboratory. According to Drost, who is now heading Oregon State University's MECS Initiative, heat-transfer rates in heat exchangers where the characteristic length for heat transfer is held to several hundred microns can be an order of magnitude higher than in macroscopic systems. At this scale, pressure losses, streamwise heat conduction, and fabrication tolerances must be carefully controlled, but what results is a miniature device of impressive performance in which volumetric heat transfer rates can exceed 104 W / cm3 (albeit under extreme conditions).
Valves are another important area for MECS. Designs have spanned the range from flapper and ball valves to others where no moving parts are involved. In the pursuit of workable components at mesoscale dimensions, simplicity has often been chosen over performance when developing a practical concept.
Valves Like Diodes
In fact, poor performance often accompanies macroscopic designs that are scaled down to mesoscopic sizes. For valves, a flapper-based design may work fine in a household refrigerator compressor, but sealing the flapper in a submillimeter design becomes problematic because of adverse scaling laws governing the performance of the face seal.
Researchers at the University of Washington are developing fixed- geometry valves, which require no moving parts. Fred Forster, principal investigator on the project and an associate professor in the university's mechanical engineering department, has used the term "diodicity" for the performance of his small valves. According to Forster, the valves are essentially fluid diodes where channel patterns are used to minimize flow resistance in the forward direction but create interference in the reverse direction. Because of the reversibility of laminar flow, the valve also shows some performance loss at small sizes, but it has sufficient diodicity at microscale dimensions to form a working pump.
A small combustor is another necessary component in certain types of miniature energy systems because chemical energy in the form of a liquid hydro carbon has one of the highest known energy densities for conventional sources. Oregon State University has had a program in this area for the past few years to develop the smallest possible source of heat from common hydrocarbon fuels.
In a joint project, John Vanderhoff of the Army Research Laboratory in Aberdeen, Md., and the author developed a miniature catalytic combustor having subwatt heat release rates in volumes approaching 0.5 mm3. Research suggests that, for catalytic combustion, heat loss management is the key to developing these small-scale combustors. The researchers see the possibility of even smaller devices using aggressive thermal management techniques
Pursuing System Integration
Even though much work remains to be do ne at the component level, what really excites engineers is the potential for integrated systems for power production, chemical reforming, refrigeration, and many other specific applications. The Defense Advanced Research Projects Agency, a consistent supporter of work at both the component and system level, has great interest in seeing successful devices emerge from this activity.
William Warren of the Defense Sciences Office of DARPA in Arlington, Va., participated in ASME's 1999 International Mechanical Engineering Congress and Exposition during sessions on miniature and microscale energy systems. Warren 's view of the mesomachine area is that "it offers new opportunities to perform tasks faster, better, more efficiently, and less expensively." With such potential in both the military and commercial areas, it appears only a matter of time until breakthrough applications emerge.
In the view of man y, this breakthrough may come first from chemical reforming of gasoline and diesel to power fuel cells. The vision here is to use the small scales of micro channel arrays to enhance the catalytic reformation of fuel to hydrogen gas. If the catalyst is embedded in the surface of the microchannels, conversion rates in micro reformers may exceed by a factor of 10 the rates found in larger, traditional devices.
Combining a small combustor with the reformer, to drive the chemical shift reactions, is also feasible when the reformer is based on an integrated design. Because of the microchannel architecture and the overall small package configuration , special attention must be paid to both heat transfer and pressure. Through appropriate designs for the micro channel array, and with modern high- temperature super insulation, these difficulties are being solved. Entities such as Battelle Memorial Institute and Argonne National Laboratory are developing small fuel reformers.
Miniature Heat Engines
A number of research programs have also been exploring the development of miniature heat engines for power production and propulsion. Among the concepts being looked at are miniature gas turbines, Stirling cycle machines, Wankel rotary engines, and nontraditional heat engines.
The gas turbine has been a focus for a number of years at the Massachusetts Institute of Technology. The goal of researchers is to develop a gas turbine the size of a shirt button . A n o table aspect of the MIT work is the thorough exploration of problems facing the miniaturization of a traditional prime mover. Challenges such as bearings, fluid friction through small passages, close tolerance seals, and heat transfer within the engine will all have to be solved for practical designs to emerge.
At the University of California, Berkeley, a project headed by professor Carlos Fernandez-Pello is studying the miniaturization of a Wankel engine. According to Pello, the particular design of the Wankel permitsa two-dimensional fabrication process, thus simplifying the overall production of the device. Valvingis also simplified by the Wankel design. As with mostwork on heat engine miniaturization, problems with seals, friction, and internal heat transfer are challenging the group.
A project being supported by the National Science Foundation program for XYZ-on-a-chip is developing a heat engine that is truly designed from a miniaturization point of view. That is, it is not an attempt to take a traditional design for an engine and try to shrink its size. Professor Robert Richards is the lead on the project at Washington State University. The unique design uses flexible membranes, phase change of a working fluid, and the piezoelectric effect to create a miniature Carnot cycle heat engine.
Heat pumps, refrigerators, and cryocoolers have also been the focus of projects to reduce system size. One program that has met with some success is the Pacific Northwest Lab's effort to develop an absorption heat pump for man-portable cooling. The PNNL program was notable in its attempt to develop a cooling scheme driven by combustion. The system relied on mechanically constrained thin liquid films for absorption and desorption.
Making Compressors Smaller
The University of Illinois hosts a program to develop a cooling system for personal use, especially in chemical, biological, and fire protection suits. The goal is to develop a miniature vapor compressor, as well as other heat pump components, and integrate them into a patch that can be applied to the surface of a suit. The main component of the device is an electrostatically actuated compressor based on a thin flexible membrane.
For cryocooling, only a few projects have looked at reducing the size of the cooler down to the mesoscopic level.
One program in particular, led by Patrick Phelan at Arizona State University, is exploring the scaling of pulse tube coolers. Although Phelan has studied the performance of the pulse tube itself, he views the key to a successful mesoscale cryocooler as the development of a compressor suitable for use in the pulse tube concept, or other cooling schemes requiring gas compression. One possible alternative to the electrostatically actuated compressors studied to date is based on electrostriction. (Phelan is co-author of an article on superconductor projects and cryocoolers, elsewhere in this issue.)
The University of Central Florida is also exploring several different miniature energy systems under a program called Miniature Engineering Systems Initiative. Led by professors Louis Chow and Jay Kapat, the initiative has as one of its current projects a vapor compression refrigerator for portable cooling applications. This system, partly funded by Lockheed Martin Missile and Fire Control, has studied several different mesoscale compressors, including a centrifugal compressor, a reciprocating compressor driven by a linear comb drive, and a Wankel compressor
A centrifugal compressor with a 2-cm diameter for R134a refrigerant is expected to produce a pressure ratio of 1.35 while rotating at 100,000 rpm. Currently, the compressor is being redesigned to be an air blower for the air-side heat exchanger for the refrigeration system. Performance testing of the blower will start soon.
There are additional programs in place to develop miniature energy systems in fuel cells and other direct. energy conversion devices, as well as in solid state cooling. Each of these separate areas could warrant an article by themselves.
However, with the examples given here and the ideas presented, it is hoped that the reader has a better understanding of the activities taking place in energy systems miniaturization.
As MECS technology matures, the ideas and concepts presented here may result in exciting new applications that could include fuel cell-powered cars that fill up on gasoline, or power cartridges for portable electronics having 10 times the life of current batteries. These opportunities are but two of the many that await this new miniaturization technology.