Introduction
MEMS
Before the discussion of a MEMS-Based Rankine cycle can occur, it is crucial to understand what exactly “MEMS” is. Microelectromechanial Systems, or MEMS, are workable/movable devices built on very small scales. Typically, MEMS range in size from a micrometer to a millimeter. On such small scales, conventional manufacturing techniques do not work. The device is most commonly created by “etching” a series of patterns onto a Silicon substrate. Figure 1 allows the size of a MEMS device to be put into mind [6].
Figure 1: Scale of MEMS relative to a dust mite.
The Basic Rankine Cycle
A Rankine cycle is the thermodynamic cycle which converts heat energy into work energy. Typically, some source of combustible matter is utilized to add heat energy into the working fluid. In most cases, the working fluid is water. The water, at high energy, is processed through a turbine where shaft work is generated. After the water leaves the turbine, it gets condensed and pumped back to the point where heat is added to the fluid. Figure 2 shows what the schematic of a basic Rankine cycle [7].
Figure 2: The basic Rankine cycle schematic.
The Demand
In the highly advancing technical world, the market for MEMS devices is growing rapidly. However, there is one major drawback that limits the advancement of these devices. That limitation is an energy source, or motor, to operate these devices at such small scales. This is where the demand and research is for a MEMS-Based Rankine Cycle. By taking the basic knowledge of a Rankine cycle and miniaturizing it, a reliable and well known energy generation process can be utilized.
Why a MEMS-Based Rankine Cycle?
The design goal of a MEMS-based Rankine Cycle is to create a power source on a very small scale. The benefits of these miniaturized systems are that they can yield a high power density, are low in cost, and can be manufactured in large batches. Miniaturization of components affects the ability and efficiency of the components to change the state of the working fluid; however, current research is improving this problem [1]. When implemented, these systems can replace power sources (such as batteries) in practically any device.
The MEMS-Based Rankine Cycle
The MEMS-Based rankine cycle, like it's macro brethren, consists of a turbine, a condenser, a pump, and an evaporator ("boiler"). Specifically, the device consists of a steam turbine that operates a micropump and microgenerator. Integrated heat exchangers, utilizing two-phase flow, act as "boilers" and "condensers" [8]. These components implement the theory involved with a Rankine cycle in order to convert heat energy into work enrgy to generate "micro-power." In most cases, these MEMS Steam cycles produce roughly 1 to 12 Watts per chip. A chip is often 3mm thick by 1cm2.
Fabrication
Microfabrication is the main roadblock in creating MEMS-based systems. Condensing a large scale Rankine cycle down hundreds to thousands of times, as you can imagine, poses many challenges. Obsticles of their fabrication include the production of features hundreds of microns deep, fillets to reduce stress on highly loaded parts, electical properties for the generator, the excavation of volumes millimeters across and hundreds of microns deep, and assembly and packaging [2].
The typical way to fabricate these small devices is through a process called etching. Etching is used to chemically remove layers from the surface of the component during manufacturing until the desired shape is formed. Every component undergoes many etching steps before it is complete. To get the desired profile, part of the surface area is protected from the etching chemical by a "masking" material which resists removal, while the exposed area is removed [5].
Materials
Materials of MEMS-based power systems must exhibit high specific strength at high temperatures. High temperature operation also requires creep and oxidation resistance. Other properties to consider for MEMS components are fracture toughness and resistance to thermal shock [4]. Research in this field is not very mature at the moment, and testing continues.
Single crystal silicon (Si) is a commonly used material in micro-machinery because it exhibits many of the properties desired for MEMS-based systems. However, at temperatures below about 900K, Si is brittle, which can pose a problem in the Rankine system. Silicon Carbide (SiC), a possible alternative, is capable of reaching about 600K more, but knowledge in that area is not as solid. Currently, SiC costs about 100 times more than Si, and the etching rate is about 10 times slower [4].
The Heat Source for the MEMS Rankine Cycle
The heat source for these devices can come from a variety of different places. Due to their projected use in electronics, the MEMS "boiler" can obtain its heat from the waste heat generated by the electronic device. Also, more conventional heat addition from a combustible fuel can be used as well. This creates a lot of diversity when it comes to supplying the initial heat energy into the device.
The Device Configuration
Figure 3 shows the preliminary design for a MEMS Rankine Cycle. The device is comprised of a multi-wafer stack that encloses all the components [8].
Figure 3: The MEMS-Based Rankine Cycle Device Configuration.
- Turbine (see Table 1)
- 5 Stages (1 rotor) producing roughly 1 W per stage.
- Preliminary designs incorporate 5 rotors producing 3.8 to 8.4 W.
Table 1: Performance and Configuration Parameters for Single-rotor Microturbine. [8]
| Inlet/Exit Pressure | 0.60 to 0.18 MPa |
| Inlet/Exit Temp. | 400oC to 316oC |
| Inner/Outer Radius | 360 to 760 microns |
| Blade chord | Range: 20 to 50 microns |
| Flow exit angle (rel.) | 60o, 4th rotor: 55o, 5th rotor: 50o |
| Mass Flow | 24 mg/s |
| Rotational Speed | 4x105 rad/s |
| Total Power | 5W |
- Pump (see Table 2)
- Spiral groove viscous pump
- Polar array of shallow radial grooves inclined at a constant spiral angle that rotate parallel to a planar surface.
Table 2: Configurations for low and high pressure viscous pumps [8].
| delta P | 0.6MPa | 8MPa |
|---|---|---|
| Rotational Speed | 4x105 rad/s | 4x105 rad/s |
| Spiral Groove | 15o | 15o |
| Groove Depth, h1 | 4.6 micron | 3.1 micron |
| Clearance, h2 | 0.6 micron | 0.5 micron |
| Inner Radius, r1 | 214 micron | 243 micron |
| Outer Radius, r2 | 275 micron | 450 micron |
| Power req'd | 0.35 W | 4.60 W |
| Efficiency | 4.3% | 4.2% |
- Heat Exchangers
- Make up the evaporator ("boiler") and condenser
- Can remove 50-100 W of heat per cm2 by forced convection.
- Pressure drops <2% of the pump pressure rise.
- Channel length <1mm.
- Generator
- Based on microfabricated motors and generators.
- No specific design exists for Rankine cycle application.
- Conversion efficiencies are expected to be about 50%.
System Performance
The system performance for the MEMS-Based Rankine cycle will follow like that of a typical macro-scale Rankine cycle. Basically, losses incurred within the system (frictions, heat loss, seal windage, disk drag, etc.) will determine the overall performance in power generation. Figure 4 depicts 3 different cases to determine possible system outcomes [8].
Figure 4: The predicted system performance for a MEMS-Based Rankine Cycle.
Further Developement
A complete MEMS based Rankine cycle has yet to be created. Areas needing further devlopement are micro-combustors, electromagnetic micro-machinery, and other miscellaneous components such as micro-bearings. There is also a need for research in the areas of viscous flows of moderate Reynolds number, microsystem conjugate heat transfer, and micromachining techniques of high temperature materials. However, there are no unsurmountable barriers in the way of completing a MEMS based rankine cycle, and with the many applications possible it will be a welcome developement.
Bibliography
[1] Muller, Norbert. “Performance Analysis Of Brayton And Rankine Cycle Microsystems For Portable Power Generation”. <http://www.egr.msu.edu/mueller/NMReferences/MuellerN_FrechetteL_2002_ASME_MicrofabBraytonAndRankineCycle.pdf>
[2] Epstein, A.H., “Power MEMS and Microengines”. <http://mit.edu/jomur/www/thesis/Transducers1997.pdf>
[3] Cortes, Heriberto. "MEMS Rankine Engine". <http://www.eng.fsu.edu/tfml/student%20project%20files/Heriberto-MEMS%20Rankine%20Engine.pdf>
[4] Epstein, Alan. "Millimeter-Scale Gas Turbine Engines." <http://igti.asme.org/resources/articles/scholar_gt-2003-38866.pdf>
[5] "Etching (Microfabrication)". Wikipedia.org. <http://en.wikipedia.org/wiki/Etching_%28microfabrication%29>
[6] "Microelectromechanical systems." Wikipedia.org. <http://en.wikipedia.org/wiki/MEMS>
[7] "Rankine Cycle." Wikipedia.org. <http://en.wikipedia.org/wiki/Rankine_cycle>
[8] Frechette, Luc G. Preliminary Design of a MEMS Steam Turbine Power Plant-on-a-Chip. Columbia Univeristy. 5 Dec. 2003. <http://www.eureka.gme.usherb.ca/memslab/docs/PowerMEMS-Rankine-paper.pdf>
