The Sixth International Workshop on Micro and Nanotechnology for
Power Generation and Energy Conversion Applications, Nov. 29 - Dec. 1, 2006, Berkeley, U.S.A.
*
Dept. of Aerospace & Mechanical Engineering, University of Southern California, Los Angeles CA
Abstract
This work introduces the use of thermal transpiration for gas pumping in small-scale devices using hydrocarbon fuel as the energy feedstock; no electrical power is required. Thermal transpiration occurs in capillary tubes or porous material when (1) a temperature gradient is maintained across the tube or pore and (2) the tube or pore diameter is comparable to the mean free path of gas molecules. Under such conditions a pressure gradient is produced in the same direction as the temperature gradient. Thermal transpiration (thus pressurization) is accomplished using membranes made of nanoporous materials (aerogels and glass-fiber filters) which have pore sizes comparable to the mean free path of gas molecules and have very low thermal conductivity, thereby enabling temperature gradients to be sustained with minimal thermal power. Catalytic combustion of propane-air mixtures on the downstream side of the membrane provides the thermal power for self-sustaining pumping of the reactants. A novel tetrahedral configuration is used to minimize the effects of parasitic heat losses. As little as 6 Watts of fuel enthalpy would self-sustain the device. This proof-of-concept apparatus demonstrates the first-ever example of a gas pump with no moving parts that derives its energy for pumping from the gas itself with no external energy source.
Keywords: Thermal transpiration; catalytic combustion; gas pumping; microfluidics; nanoporous materials
1 - INTRODUCTION
Due to recent advances in microelectronics and micro electromechanical systems (MEMS), there is considerable interest in miniaturizing thermochemical systems for electrical power generation [ 1 , 2 ], gas chromatography [ 3 ], mass spectrometry
[ 4 , 5], optical spectrometers [ 6 ], separation and chemical reforming [ 7 , 8 ], pneumatic power for micro-robots [ 9 ] and even propulsion for micro air vehicles [ 10 ]. Such devices have numerous commercial, military and space applications.
For example, the use of hydrocarbon fuels for electrical power generation provides enormous advantages over conventional batteries both in terms of energy storage per unit mass and in terms of power generation per unit volume, even when the conversion efficiency from thermal energy to electrical energy is taken into account. For this reason automotive and aviation vehicles employ internal combustion engines for prime moving and electrical power generation almost entirely to the exclusion of batteries, even in vehicles whose mass may be less than 1 kg or more than 10 5 kg.
Micropumps such as the ones proposed here could be integrated with various microreactor systems or used as microreactors themselves for hydrogen production needed in
PEM fuel cells. Typical reactions include reforming, partial oxidation, water gas shift and selective catalytic removal of
CO. Chemical detectors could be used for autonomous monitoring of environmental pollutants, explosive atmospheres, toxic gases and chemical warfare agents.
Micropumps could also serve as active cooling systems for dense microelectronic devices. Micro air vehicles could be used for a wide variety of reconnaissance and surveillance missions. Moreover, in applications where the ultimate goal is to replace batteries, there is an environmental advantage in that disposal of empty fuel cartridges in landfills is preferable to disposal of used batteries containing toxic metals.
All of the aforementioned systems require gas pressurization and/or vacuum pumping systems that in turn usually require devices with moving parts that are difficult to miniaturize due to problems with friction losses, manufacturing tolerances, sealing, etc. In the case of microscale power generation, many ideas have been proposed, e.g.
[1, 2, 10] but most leave unresolved critical "balance of plant" issues, in particular pressurization for fuel and air. Even successful micromotors and micropumps are notoriously inefficient at converting electrical power into pumping power, meaning that a large fraction of the total system size and weight must be devoted to batteries. All miniaturized propulsion devices with moving parts, e.g. [10], experience more difficulties with heat and friction losses due to higher surface to volume ratios than their macroscale counterparts. The difficulty of reducing the size and/or power consumption of the pressurization or vacuum systems required for a complete system in turn limits the portability and utility of miniature thermochemical systems.
* Corresponding author: phone (213) 740-0490; email ronney@usc.edu
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The Sixth International Workshop on Micro and Nanotechnology for
Power Generation and Energy Conversion Applications, Nov. 29 - Dec. 1, 2006, Berkeley, U.S.A.
Low-temperature thermal guard (non-catalytic)
Reactants in
(low T, low P)
High-temperature thermal guard (catalytic)
Transpiration membrane
Figure 1 Schematic of catalytic combustion driven thermal transpiration pump.
Products out
(high T, high P)
Figure 2 - Images of thermal transpiration pump test fixture.
(outer diameter 2.5 cm). Top: assembled view; bottom: disassembled.
The proposed work focuses on gas pumps driven by thermal transpiration (also known as thermal creep) that occurs in gases in porous membranes or capillary tubes when two conditions are satisfied: (1) the mean free path ( λ ) of the gas molecules is comparable to the pore or tube diameter (d) and
(2) a temperature gradient is imposed in the solid phase along the length of the pore or tube. Under such conditions, a pressure gradient is induced in the gas, which can be used to cause a flow from the cold to hot end of the pores or tubes.
Thermal creep along the edge of the blades from the cold
(silvered) toward the hot (black) side is responsible for the spinning of a Crooke’s radiometer. Thermal transpiration can also be thought of as the opposite of thermophoresis, where small solid particles move from high to low temperature in a gas. A pumping device based on thermal transpiration is sometimes called a "Knudsen compressor" after M. Knudsen who first studied thermal creep in the early 1900's [ 11 ].
Requirement (1) indicates aerogels or nanostructured materials that is comparable to the mean free path of air molecules at ambient conditions (about 70 nm), thus they are potentially ideal for constructing Knudsen compressors operating at near-ambient pressures. Moreover, nanoporous materials have very low thermal conductivity k (often comparable to or lower than the gas filling the pores), thereby enabling temperature gradients to be sustained across the nanoporous material with minimal thermal power.
While studies of thermal transpiration in porous materials date back to Osborne Reynolds around 1870, only recently has this process been considered for small-scale pumping applications. Vargo and collaborators [ 12 ] fabricated and tested proof-of-concept thermal transpiration pumps using
500 µ m thick nanoporous silica (aerogel) membranes.
Silicon "thermal guards" having high thermal conductivity and micromachined to have many 10 µ m diameter holes were used to maintain uniform temperatures on either side of the aerogel membrane while allowing the gas to pass through the thermal guard. A gold film electrical resistive heater was patterned on one of the thermal guards to used to sustain the temperature difference across the aerogel. Thermal transpiration based pumping was observed for all gases and pressures tested. Perhaps most notably, the performance of the devices could be predicted based on previous models developed for single capillary tubes . By comparison of these models with experiments it was found that a wide range of nanoporous materials of mean pore radius L the same radius L r
and thickness L x exhibit nearly the same performance as a capillary tube with r
and tube length L x
. Thus, a single empirical constant of near unity seems to provide a bridge between a simple capillary tube and the complex geometry of nanoporous materials.
In this work we take the concept of thermal transpiration in nanoporous materials introduced by Vargo et a l. [12], a significant step further by using heat release from an exothermic surface (catalytic) chemical reaction instead of electrical heating to sustain the temperature gradient across the nanoporous material. Catalytic reaction is ideal for this application because it is localized on catalytic surfaces that can be located on the hot side of the Knudsen compressor, making the thermal resistance between the hot product gases and the hot side of the compressor negligible. (Fins may be added to the cold-side thermal guard to minimize the thermal resistance between the reactants and the thermal guard).
Figure 1 shows the basic building block unit of the catalytic reaction driven Knudsen compressor. As in Vargo et al.
[12], the reactants first flow through a low-temperature "thermal guard" consisting of a plate with microchannels (not nanoscale pores as in the transpiration membrane). The purpose of the thermal guard is to reduce the thermal
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The Sixth International Workshop on Micro and Nanotechnology for
Power Generation and Energy Conversion Applications, Nov. 29 - Dec. 1, 2006, Berkeley, U.S.A.
resistance between the ambient-temperature reactants and the cold side of the aerogel transpiration membrane, thereby obtaining a larger temperature difference across the membrane, resulting in better pumping performance. The thermal guard material should have a thermal conductivity much higher than that of the transpiration membrane and must be non-catalytic to the fuel, plus the thermal guard pore size must be much larger than the mean free path of the molecules (so that no thermal transpiration occurs in this region). The reactants then pass through a transpiration membrane without catalyst where the pumping occurs due to the temperature difference across the membrane. The reactants then pass through a catalytic high-temperature thermal guard where the reactants are converted into products, resulting in heat production. The heat release in this region, combined with the low thermal conductivity of the nanoporous material, sustains the temperature gradient and thus the pumping action is self-sustaining.
70
60
50
40
30
20
10
0
0
Glass fiber filter membrane
0.45 mm thick x 25 mm diameter
Maximum flow (
!
P = 0)
20 40 60
Theory
Experiment
80 100 120 140
Figure 3 - Volume flow rate (at zero pressure differential) vs. temperature differential for a glass fiber filter membrane.
2 - APPARATUS AND PROCEDURES
A proof-of-concept thermal transpiration / catalysis test fixture is shown in Figure 2. Two thermal transpiration membranes were tested: (1) a piece of commercial available silica aerogel (MarkeTech International, Port Townsend,
Washington) with advertised thermal conductivity in air of
0.016 W/m˚C and density 0.1 g/cm 3 that was machined using traditional techniques (after many failures resulting in broken aerogel pieces) into a disk 1 mm thick x 20 mm diameter and
(2) an Ahlstrom glass fiber microparticle filter with effective pore size ≈ 4.4 µ m. The former provides Kn ≈ 5, thus a high pressure rise, low flow but relatively high pumping efficiency whereas the latter provides Kn ≈ 0.012, thus significantly the outlet (exhaust) plenum is made of a high-temperature, low thermal conductivity polymer (PEEK-1000) to minimize heat losses. Thermocouples are attached to both thermal guards to measure the temperature differential across the aerogel. Pressure measurements are made with a differential capacitance type transducer. The temperature and pressure data are read and stored with a Labview application. Steadystate flow rates are measured using commercial mass flow controllers to control back pressure and measure flow rates.
3 - RESULTS higher flow at zero pressure differential, but very low pumping efficiency. The thermal guards were sealed into inlet and outlet plenums with o-rings. These membranes were sandwiched between two thermal guards with ≈ 100 holes drilled in them. The outlet-side (hot-side) thermal guard is a Pt wire mesh catalyst. The catalyst was specially prepared NH
3
-treated Pt that enables hydrocarbon combustion even at very low temperatures [ 13 ]. The inlet plenum is machined out of aluminum to maximize thermal conductivity and thus reduce thermal resistance to ambient
(i.e., to keep the inlet side at ambient temperature) whereas
Figure 4 - Schematic of catalytic combustion driven thermal transpiration pump Photographs of prototype combustion-driven thermal transpiration reactor. Left: assembled reactor; right: with plumbing, one face removed to show interior.
The simple test fixture shown in was found to produce steady pumping for at least several hours of continuous operation when a nichrome wire electrical heater wire. Figure 3 shows typical quantitative results, which are in reasonably good agreement with theoretical predictions by Varo et al.
[12] for both types of membranes. When tested aas a combustion device using hydrogen fuel, it was found that the device continued to draw in both fuel (at the flow rate preset by the mass flow controller) and air (at the rate controlled by thermal transpiration of the fuel/air mixture) for an indefinite period of time. A typical operating condition was 7 ml/min
H
2
, 30 ml/min air (as measured by a bubble meter) with a hot-side temperature of
100˚C, i.e. nearly the same flow as obtained with electrical resistive heating for the same temperature differential. However, no self-sustaining operation could be obtained using hydrocarbon fuels.
After consideration of the figuration shown in Fig. 2, along with a simple heat transfer analysis, it was concluded that heat losses to ambient from the hot
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The Sixth International Workshop on Micro and Nanotechnology for
Power Generation and Energy Conversion Applications, Nov. 29 - Dec. 1, 2006, Berkeley, U.S.A.
side of the pump fixture (i.e. the exhaust plenum) was substantial and prevented self-sustaining operation with hydrocarbon fuels. Since there are inevitable heat losses from the exhaust plenum, it was decided to exploit such losses by placing transpiration pumps on all sides. Hence, a tetrahedral configuration (Figure 4) was used to minimize parasitic heat losses. Each face acts as a chemical reaction driven thermal transpiration pump consisting of a lowtemperature aluminum thermal guard, a nanoporous transpiration membrane (aerogel or glass fiber filter) and catalyst/hot-temperature thernal guard. In order to have a
“fair” test of this device’s capabilities, the fuel inlet is open to ambient via a “T” fitting, thus the reactor must draw in its own air and fuel from ambient pressure ; it does not rely on pressurized fuel ( e.g.
as in a butane cigarette lighter.) The device shown in Figure 4 was found to operate for hours in a self-sustaining manner with a hot-side (catalyst) temperature of about 260˚C using butane fuel at a flow rate of 3.9 ml/min, corresponding to a thermal power of 6.1 watts. Device operation was found to be independent of the orientation with respect to gravity, thus buoyancy is not the driving force for the flow of fuel and air. Combustion products were measured using a gas chromatograph and found to contain no carbon monoxide or unburned hydrocarbons other than unreacted fuel.
4 - CONCLUSIONS
A proof-of-concept apparatus using thermal transpiration and catalytic combustion of propane-air mixtures demonstrated the first-ever example of a gas pump with no moving parts that derives its energy for pumping from the gas itself with no external energy source ( e.g.
, no electrical power). It is selfpumping for both air and fuel, thus it could in principle be used with liquid fuels having little or no vapor pressure. It operates with hydrocarbon fuels ( i.e
., not just hydrogen). The advantage of using chemical energy stored in fuels or other reactants rather than resistive electrical heating as the thermal source for thermal transpiration pumping cannot be overemphasized. The efficiency of fuel to electrical energy conversion in mobile power plants even at macroscales is
30% at best. For microscale systems there are no existing electrical power generation devices (though many such devices are under development.) The only current means to provide electrical power at small scales is via batteries that
(in the case of lithium ion batteries) have only about 1/100 the energy per unit mass of hydrocarbon fuels. Therefore, using fuels for thermal power generation provides a factor of about 100 mass savings over batteries.
ACKNOWLEDGEMENTS
This work was supported by the U. S. Defense Advanced
Research Projects Agency (DARPA) Microsystems
Technology Office under contract numbers DABT63-99-C-
0042 and SNWSC-N66001-01-8966.
REFERENCES
[1] Fu, K., Knobloch, A. J., Cooley B. A., Walter, D. C.,
Fernandez-Pello, C., Liepmann, D. and Miyaska, K., ASME 35 th
National Heat Transfer Conference, NHTC2001-20089 (2001).
[ 2 ] Weinberg, F. J., Rowe, D. M., Min, G., Ronney, P. D., "On thermoelectric power conversion from heat re-circulating combustion systems," Proceedings of the Combustion Institute ,
Vol. 29, pp. 941-947 (2002).
[3] Terry, S. C., Jerman, J. H., Angell, J. B., A Gas
Chromatographic Air Analyzer Fabricated on a Silicon Wafer.
IEEE Transactions on Electron Devices ED-26, 1880-1886,
(1979).
[4] Ferran, R. J., Boumsellek, S., High-Pressure Effects in
Miniature Arrays of Quadrupole Analysers for Residual Gas
Analysis from 10-9 to 10-2 Torr. Journal of Vacuum Science and
Technology A 14, 1258-1265 (1996).
[5] Orient, O., J., Chutjian, A., Garkanian, V., “Miniature, High-
Resolution, Quadropole Mass- Spectrometer Array”, Review of
Scientific Instruments, 68, 1393-1397, (1997).
[6] Blomberg, M., Rusanen, O., Keranen, K., Lehto, A., in The 9th
International Conference on Solid-State Sensors and Actuators –
Transducers ’97 1257-1258 (IEEE, Chicago, IL, 1997).
[7] P. Irving, L. Allen, Q. Ming, T. Healey, “Novel Catalytic Fuel
Reforming Using Micro-Technology with Advanced Separations
Technology, Proceedings of the 2002 U.S. DOE Hydrogen
Program Review , NREL/CP-610-32405, 2002.
[ 8 ] Brooks, K., et al.
, “Microchannel Fuel Reforming,” Fuel Cells for Transportation/Fuels for Fuel Cells 2002 Annual
Program/Lab R&D Review , Pacific Northwest National
Laboratories, May 6-10, 2002.
[9] “Development of Long Range Mesoscale Mobile Hopping
Platforms Based on Combustion of Hydrocarbon Fuels,” http://www.darpa.mil/dso/thrust/md/Mm/robot/snl1/index.htm.
[10.] Waitz, I. A., Gauba, G., and Tzeng, Y. Journal of Fluids
Engineering 120:109 (1998); Mehra, A., Ayon, A. A., Waitz, I. A. and Schmidt M. A., IEEE Journal of MEMS 8:152 (1999).
[11] M. Knudsen. Eine revision der gleichgewichtsbedingung der gase.Thermische molekularströmung. Ann. Physik 31:205–229
(1910); Ibid., Thermischer molekulardruck der gase in röhren.
Ann. Physik , 33:1435–1448 (1910).
[12] Vargo, S. E., Muntz, E. P., Shiflett, G. R. and Tang, W. C.,
“The Knudsen compressor as a micro-and macroscale vacuum pump without moving parts or fluids” . J. Vac. Sci. Technol. A 17,
2308-2313 (1999); Vargo, S. E. and Muntz, E. P., "Initial results from the first MEMS fabricated thermal transpiration-driven vacuum pump," 22
Dynamics, 2001. nd International Symposium on Rarefied Gas
[ 13 ] Ahn, J., Eastwood, C., Sitzki, L., Ronney, P. D., “Gas-phase and catalytic combustion in heat-recirculating burners,”
Proceedings of the Combustion Institute, Vol. 30, pp. 2463-2472
(2005).
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