Design, Fabrication, and Testing of a Prototype ... - CiteSeerX

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 13, NO. 5, OCTOBER 2004

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Design, Fabrication, and Testing of a Prototype Microthermophotovoltaic System Wenming Yang, Siaw Kiang Chou, Chang Shu, Hong Xue, and Zhiwang Li

Abstract—The design, fabrication and testing of a novel prototype microthermophotovoltaic (micro-TPV) system is first described in this paper. The system is made of a SiC emitter, a simple nine-layer dielectric filter and a GaSb Photovoltaic cell array. When the flow rate of hydrogen is 4.20 g/h and the 2 /air ratio is 0.9, the micro-TPV system is able to deliver an electrical 3 in power output of 1.02 W in a microcombustor of 0.113 volume. The open-circuit electrical voltage and short-circuit current are 2.28 V and 0.59 amp, respectively. This paper makes it possible for us to replace batteries with micro-TPV systems as the power of micro mechanical devices in near future. [1155]

here is in the order of cubic centimeter. For micro-TPV application, the desired output is a high and uniform temperature along the wall of the microcombustor. The high surface-to-volume ratio of microcombustor makes the study of micro-TPV systems particularly attractive in terms of output power density per unit volume. In this paper, the design, fabrication and testing of a prototype micro-TPV system is detailed.

Index Terms—Microcombuster, microthermophotovoltaic, SiC emitter.

A prototype micro-TPV power generator has been built-up and tested in NUS. The system mainly consists of: (1) a heat source; (2) a cylindrical SiC emitter (i.e., microcombustor); (3) a simple nine-layer dielectric filter; and (4) a GaSb photovoltaic cell array. Fig. 1 shows the schematic of the micro-TPV system . It should with no cooling fins. The overall volume is 3.19 be noted that the distance between the PV cells and the SiC Emitter is very large in present design. The glass and the copper base are also too thick (1 mm). Thereby, it is possible for the overall size to decrease to 8 mm (outer diameter) 18 mm in . One future design, corresponding to a volume of about 0.9 picture of the micro-TPV system is shown in Fig. 2, where the cylindrical SiC emitter is not incorporated into the system. The system does not involve any moving part. Its fabrication and assembly are relatively easy. As a result, it can be widely used in commercial electronics and microdevices. At microscale, a combustor is highly constrained by inadequate residence time for complete combustion. Therefore, hydrogen is chosen as the fuel because of its high heating value, short reaction time. /air mixture is burned in the microcylindrical SiC combustor. As the SiC emitter is heated to a high temperature, it emits a lot of photons. The filter is able to recycle part of photons with energies lower than the bandgap of PV cells to the emitter, and transmit most of photons with energies greater than the bandgap of PV cells. When those photons with high enough energies impinge on the GaSb PV cell array, they would evoke free electrons and produce electrical power output under the function of p-n junction.

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I. INTRODUCTION

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HERE has been a growing trend in the miniaturization of mechanical and electromechanical engineering devices during the past few years. All kinds of microdevices such as micropumps, micromotors, microrobots, microrovers, and microairplanes are being developed. However, the miniaturization of these devices is limited by the available power systems (batteries) that occupy significant fractions of both mass and volume of the entire devices. Typical portable mechanical devices also suffer from short operation cycles between charges or replacement. The need to reduce system weight, increase operational lifetimes urges the emergence of a new class of MEMS devices, micro power generators [1]. It is well known that hydrocarbon fuels have energy densities of 10–100 times greater than the best batteries. Therefore, taking advantage of the high energy density of chemical fuels to generate power becomes an attractive technological alternative to batteries. Microgas turbine engine [2], microrotary engine (Wankel-type) [3], microthermoelectric [4], and microfuel cell [5] are typical micropower generators being developed recently. The micro-TPV system [6] is another kind of micropower generators that is being developed in National University of Singapore (NUS). The system uses photovoltaic (PV) cells to convert heat radiation, e.g., from the combustion of fossil fuels, into electricity. Rumyantsev et al. [7] also developed a portable TPV generator, however, the overall size is typically in the order of 100 or even larger. The micro-TPV system described Manuscript received September 12, 2003; revised February 20, 2004. This work was supported by the National University of Singapore by Grant R-265000-114-112. Subject Editor G. Benjamin Hocker. W. M. Yang, S. K. Chou, C. Shu, and Z. Li are with the Department of Mechanical Engineering, National University of Singapore, Singapore 119260, Singapore (e-mail: [email protected]). H. Xue is with the Department of Mechanical Engineering, California State Polytechnic University, Pomona, CA 91768 USA. Digital Object Identifier 10.1109/JMEMS.2004.835759

II. DESIGN AND FABRICATION OF MICRO-TPV SYSTEM

A. A Microcylindrical SiC Emitter An emitter is the first key component of micro-TPV systems. There are two different types of emitters, namely broadband emitters and selective emitters. Blackbody is a typical broadband radiation materials, its emission behavior can be approximated by graphite or a soot-covered surface. However, a broadband radiating material of practical importance is silicon carbide (SiC) with emissivity . While a selective emitter exhibits a high emittance in the spectral range usable for the PV

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Fig. 1. Schematic of the micro-TPV system.

Fig. 3. A picture of microcombustion (H flow rate is 4.2 g/hr, H /air ratio is 0.9). Fig. 2.

A prototype micro-TPV system without microcombustor.

cells, and a low emittance elsewhere. For the last decade, several methods have been developed to fabricate selective emitters. One familiar way is to use oxides of rare earth materials such as erbia ( ) and ytterbia ( ) [8]. Another way is microstructuring the surface of the emitters [9]. Recently, a new thermally excited Co/Ni-doped MgO matched emitters [10] has been developed in University of Washington. This kind of emitters exhibits a better spectral efficiency. For the simplicity of fabrication and assembly, a SiC emitter is selected for our prototype micro-TPV system. The major challenge in the design of microcombustor is to maintain an optimal balance between sustaining stable combustion and maximizing heat radiation output. A high surface-to-volume ratio is very favorable to the output power density per unit volume. However, sustaining combustion will be greatly affected by the increased heat losses that tend to suppress ignition and quench the reaction. To testify the feasibility of microcombustion and optimize the design of microcombustor, a series of numerical simulation and experimental work were carried out. Results indicate that a micro cylindrical combustor with a backward facing step is one of the simplest but most effective structures for the micro-TPV application [11]. The backward facing step can facilitate recirculation along the wall and enhance the mixing process around the rim of the tube flow. Thereby, a high and uniform temperature distribution can be obtained along the wall of microcombustor. In a microcylindrical SiC combustor of 3 mm in diameter, 16 mm in length, and 0.3 mm in wall thickness, an average temperature of 1325 K has been achieved along the wall, and the biggest difference of temperature is less than 5%, when

Fig. 4. Outline of GaSb PV cell fabrication process.

the flow rate is 4.20 g/hr and the /air ratio is 0.9. The efficiency of the microcombustor is 19.5%. The SiC combustor is fabricated by casting. The volume of the microcombustor is 0.113 . Fig. 3 shows the picture of combustion under above conditions, it is taken by digital camera.

YANG et al.: TESTING OF A PROTOTYPE MICROTHERMOPHOTOVOLTAIC SYSTEM

Fig. 5.

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Quantum efficiency of GaSb PV cells.[13].

B. A GaSb PV Cell Circuit The second key component of the micro-TPV system is a low bandgap PV cell array. Compared to PV conversion of solar energy, the photons emitted from a heat source at 1000–1600 K are distributed at much lower energies and longer wavelengths. This necessitates the use of low bandgap semiconductors for the TPV energy conversion diode, in order to simultaneously maximize both the efficiency and the power density. Although the concept of TPV energy conversion was first proposed in 1960s [12], it was only in recent years that technological improvements in the field of low bandgap photovoltaic cells and high temperature materials have evoked a renewed interest in TPV generation of electricity [13]. GaSb [13], GaInAs [14] and InGaAsSb [15] are typical low bandgap PV cells developed recently for TPV applications. Corresponding with the filter to be introduced in the next section, a GaSb PV cell array is designed and fabricated for the micro-TPV system. This GaSb cell array responds out to 1.8 m. The technology used to form the pn junction is based on a Zn vapor diffusion process into an n-doped GaSb substrate [16]. Thus, expensive epitaxial growth of thin semiconductor layers is successfully avoided. The Zn vapor diffusion process is performed in a so-called “pseudoclosed” box. The diffusion source is a mixture of Zn and antimony. An n-type GaSb wafer doped with Te is first coated with a silicon-nitride diffusion barrier and standard photolithography is used to open holes in the dielectric. A pn junction is then formed by zinc diffusion through the mask opening. The diffusion creates a Zn-doped p-type GaSb emitter. The front patterned side of the wafer is then protected with photoresist, while the junction is removed from the back side of the wafer with a nonselective etch. The backside of the wafer is then metallized. The front-side metallization area is defined by standard liftoff photolithography and metal evaporation. Finally, etching of the emitter and deposition of an antireflection (AR) coating are performed to maximize photocurrent. Fig. 4 shows the outline of the GaSb PV cell fabrication process. The quantum efficiency of the GaSb cell is shown in Fig. 5 [13]. Fig. 6 shows the circuit board of the GaSb PV cell array, which is composed of six 4.5 mm 18 mm planar GaSb cells. The active area is 4.3 mm 15.5 mm for each cell. Fig. 7 shows

the hexagonal circuit bended from above PV cell array. The filters have been bonded on the top of the PV cells in this figure. The filter face-to-face distance is 8.419 mm. C. A Nine-Layer Dielectric Filter The third key component of the micro-TPV system is a simple nine-layer dielectric filter. The spectrums of SiC emitters operating at temperatures 1000–1600 K contain significant proportions of subband-gap photons with energies not sufficient enough to generate charge carriers in the PV cells. This portion of energy will be absorbed by the PV cells and results in a destructive heat load on the generator components, subsequently lowers the conversion efficiency of the system drastically. To improve the overall efficiency of the micro-TPV system, it is very important to recycle these photons. So a filter should be employed in the micro-TPV system. Ideally, the filter should be able to reflect all nonconvertible photons back to the emitter and transmit all convertible photons to the PV cell array. However, in practice, it is very hard to achieve. Here, a simple nine-layer dielectric filter is employed in the micro-TPV system. The filter is fabricated with alternating layers of silicon and silicon dioxide. It is deposited on a glass slide with a conventional electron beam evaporation system and bonded on the top of GaSb PV cells with silicone, see Fig. 7. This method makes the assembly of micro-TPV system very convenient. The filter is able to recycle the energy emitted in the 1.8 to 3.5 m midwavelength band [17], thereby improves the efficiency of the micro-TPV system. Fig. 8 shows the reflectance of the filter. It is measured with a custom optical test system that was built in-house at JX Crystals. The uncertainty is less than 3%. This has been verified by other laboratory measurements with a Perken-Elmer Lambda-9 scanning sprectrophotometer. III. RESULTS AND DISCUSSION The GaSb PV circuits with filters are first measured with a flash lamp solar simulator. By adjusting the light intensity can get a different current density for the PV cells. Table I summarizes the performance of five GaSb PV circuits incorporated with filters. Where, FF is the fill factor of the PV cells. It is defined as . is the maximum electrical power

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Fig. 6. Circuit board of GaSb PV cell array.

Fig. 7.

Hexagonal GaSb PV cell circuit with filters. TABLE I PERFORMANCE OF GASB PV CIRCUITS INCORPORATED WITH FILTERS

Fig. 8. Reflectance of a nine-layer dielectric filter.

output. voltage.

is the short-circuit current. is the open-circuit and are the current and voltage at the max-

imum electrical power output point respectively. In order to observe the performance of PV cells, a higher light intensity than that by SiC emitter at 1325 K is employed in the testing. The results indicate the GaSb PV circuits offer a very good electrical

YANG et al.: TESTING OF A PROTOTYPE MICROTHERMOPHOTOVOLTAIC SYSTEM

Fig. 9.

I-V Curve of the GaSb PV circuit with filters.

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GaSb PV cells from 15.5 to 22 mm. The volume of the micro. combustor will be 0.155 Furthermore, to further increase the output electrical power density and improve the efficiency of micro-TPV system, it is necessary to employ a selective emitter in future design of micro-TPV systems instead of SiC broadband emitters. If a thermally excited Co/Ni-doped MgO matched emitter [10] is employed in the micro-TPV system, by using the mathematical model developed in a previous paper [6], then an electrical power output of 5.5 W can be expected in a microcombustor of 0.155 in volume. IV. CONCLUSION

Fig. 10. Maximum electrical power output under different flow rates and H /air ratios.

conversion performance. The open-circuit voltage and shortcircuit current are greater than 2.78 V and 2.12 amp, respectively. The maximum electrical power outputs are also bigger than 4.42 W. Corresponding to an output power density bigger than 1.11 . The - curve of the second GaSb PV circuit with filters is shown in Fig. 9. It is measured with a flash lamp solar simulator. The uncertainty is less than 3%. The electrical power output of the prototype micro-TPV system incorporated with a SiC emitter is then measured for all kinds of flow rate and /air ratio. Fig. 10 shows the maximum electrical power output under different flow rates and different /air ratios. With the increase of the flow rate and the /air ratio, the maximum electrical power output increases drastically. This is because more fuels taking part in combustion. As the hydrogen flow rate is 4.20 g/h and the /air ratio is 0.9, an electrical power output of 1.02 W has been achieved for the second micro-TPV system. The temperature of the PV cells can be kept at about 60 by water or forced air convection. The open-circuit voltage and short-circuit current are 2.28 V and 0.59 amp respectively. The efficiency of the GaSb PV cells covered by filter is 3.7%, and the overall efficiency of the micro-TPV system is 0.72%. It should be noted, just like the other micropower system being developed around the world, the development of micro-TPV system is still at its early stage, so the efficiency is still low. However, compared to other prototype micropower systems, it is good. So far, the system has worked for more than 100 hours steadily. During experimenting, we found that the temperature along the wall of microcombustor remains near uniform, when the length of the microcombustor is increased to 22 mm, at the hydrogen flow rate of 4.20 g/h and the /air ratio of 0.9. This is due to the reflection of filter. Therefore, a 1.45-W electrical power output can be obtained if we increase the active length of

The design, fabrication, and testing of a novel prototype microthermophotovoltaic (micro-TPV) system, is first described in this paper. The system is made of a SiC emitter, a simple ninelayer dielectric filter, and a GaSb Photovoltaic cell array. When the flow rate of hydrogen is 4.20 g/h and the /air ratio is 0.9, the micro-TPV system is able to deliver an electrical power output in volume. The of 1.02 W in a microcombustor of 0.113 open-circuit electrical voltage and short-circuit current are 2.28 V and 0.59 amp respectively. The electrical power output can be further increased to 5.5 W in a microcombustor of 0.155 in volume, if a thermally excited Co/Ni-doped MgO matched emitter is employed in the micro-TPV system. This work makes it possible for us to replace batteries with micro-TPV systems as the power of micromechanical devices in near future. ACKNOWLEDGMENT The authors acknowledge JX Crystals for the fabrication of GaSb PV cells and filters. REFERENCES [1] A. H. Epstein and S. D. Senturia, “Macro power from micro machinery,” Science, vol. 276, p. 1211, 1997. [2] I. A. Waitz, G. Gauba, and S. T. Yang, “Combustors for micro-gas turbine engines,” J. Fluids Eng., vol. 120, pp. 109–117, 1998. [3] K. Fu, A. J. Knobloch, B. A. Cooley, D. C. Walther, C. Fernamdez-Pello, D. Liepmann, and K. Miyaska, “Microscale combustion research for applications to MEMS rotary IC engine,” in Proc. NHTC, 2001 National Heat Transfer Conf., Anaheim, CA, 2001, pp. 1–6. [4] L. Sitzki, K. Borer, E. Schuster, P. D. Ronney, and S. Wussow, “Combustion in microscale heat-recirculating burners,” in The Third Asia-Pacific Conf. Combustion, Seoul, Korea, 2001, pp. 1–4. [5] S. J. Lee, A. Chang-Chien, S. W. cha, R. O’Hayre, Y. I. Park, Y. Saito, and F. B. Prinz, “Design and fabrication of a micro fuel cell array with ’flip-flop’ interconnection,” J. Power Sources, vol. 112, pp. 410–418, 2002. [6] W. M. Yang, S. K. Chou, C. Shu, Z. W. Li, and H. Xue, “Development of microthermophotovoltaic system,” Appl. Phys. Lett., vol. 81, pp. 5255–5257, 2002. [7] V. D. Rumyantsev, V. M. Andreev, V. P. Khvostikov, S. V. Sorokina, and M. Z. Shvarts, “Thermophotovoltaic generators,” in IWRFRI’1999, St. Petersburg, Russia. [8] M. G. Krishna, M. Rajendran, D. R. Pyke, and A. K. Bhattacharya, “Spectral emissivity of ytterbium oxide-based materials for application as selective emitters in thermophotovoltaic devices,” Sol. Energy Material Sol. Cells, vol. 59, pp. 337–347, 1999. [9] A. Heinzel, V. Boerner, V. Gombert, V. Wittwer, and J. Luther, “Microstructured tungsten surfaces as selective emitters,” in Proc. Thermophotovoltaic Generation of Electricity 4th NREL Conf., Denver, CO, 1999, pp. 191–196. [10] L. G. Ferguson and F. Dogan, “A highly efficient NiO-Doped MgO matched emitter for thermophotovoltaic energy conversion,” Materials Sci. Eng., vol. B83, pp. 35–41, 2001.

856


[11] W. M. Yang, S. K. Chou, C. Shu, Z. W. Li, H. Xue, D. T. Li, and J. F. Pan, “Microscale combustion research for application to micro thermophotovoltaic systems,” Energy Convers. Manage., vol. 44, pp. 2625–2634, 2003. [12] D. C. White, B. D. Wedlock, and J. Blair, “Recent advance in thermal energy conversion,” in 15th Ann. Power Sources Conf., Atlantic, NJ, 1961, pp. 125–132. [13] L. G. Ferguson and L. M. Fraas, “Theroretical study of GaSb PV cell efficiency as a function of temperature,” Sol. Energy Materials Sol. Cells, vol. 39, pp. 11–18, 1995. [14] M. W. Wanlass, J. S. Ward, K. A. Emery, M. M. Al-Jassim, K. M. Jones, As thermophotovoltaic converters,” Sol. and T. J. Coutts, “ Energy Materials Sol. Cells, vol. 41, pp. 405–417, 1996. [15] C. A. Wang, H. K. Choi, S. L. Ransom, G. W. Charache, L. R. Danielson, and D. M. DePoy, “High-quantum-efficiency 0.5 eV GaInAsSb/GaSb thermophotovoltaic devices,” Appl. Phys. Lett., vol. 75, pp. 1305–1307, 1999. [16] L. M. Fraas, G. R. Girard, J. E. Avery, B. A. Arau, V. S. Sundaram, and J. M. Gee, “GaSb booster cells for over 30% efficient solar-cell stacks,” J. Appl. Phys., vol. 66, pp. 3866–3870, 1989. [17] L. M. Fraas, J. E. Samaras, H. X. Huang, L. M. Minkin, J. E. Avery, W. E. Daniels, and S. Hui, “TPV generators using the radiant tube burner configuration,” in 17th Europ. Photovoltaic Solar Energy Conf. Exhibition, Munich, Germany, 2001, pp. 1–4.

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Chang Shu was born in China in 1962. He received the B.S. and M.S. degrees from Nanjing University of Aeronautics and Astronautics, China, in 1983 and 1986, respectively. He received the Ph.D. degree from the University of Glasgow, Scotland, in 1991. From 1991 to 1992, he was a Postdoctoral Research Fellow with the University of Glasgow. He joined the National University of Singapore (NUS) from 1992 as a Research Scientist. He is currently an Associate Professor with NUS. His current research interests include efficient methods for solution of partial differential equations, computational fluid dynamics, and MEMS.

Wenming Yang received the B.S., M.S., and Ph.D. degrees in mechanical engineering from Jiangsu University of Science and Technology, China, in 1994, 1997, and 2000, respectively. His areas of research include diesel engine, combustion, and power MEMS. Since 2000, he has been with the National University of Singapore, where he is currently a Research Scientist, focusing on the study of power MEMS.

Hong Xue received the B.S. degree in mechanical engineering from Jiangsu University of Science and Technology, China, in 1982. He received the M.S. and Ph.D. degrees in mechanical engineering from the University of Tokyo, Japan, in 1988 and 1992, respectively. From 1992 to 2000, he was an Assistant Professor with the National University of Singapore. He is currently an Associate Professor with California State Polytechnic University, Pomona. His current research interests include MEMS and microfluid flow and heat transfer, and numerical thermal-fluids simulation. Dr. Xue is a member of the American Society of Mechanical Engineers and the Japan Society of Mechanical Engineers.

Siaw Kiang Chou received the B.S. degree in mechanical engineering from the National University of Singapore (NUS) in 1977 and the Docteur-Ingenieur degree from Ecole Nationale Superieure d’Arts et Metiers, Paris, France, in 1980. He is a Professor with the Department of Mechanical Engineering, NUS, and became head of department in 1998. His research interests include energy performance and analysis of buildings, including thermal performance of building envelopes and systems; drying technology and heat pump dryers; jet mixing flows and ejector systems for energy recovery; and microthermal processes and power generators. Dr. Chou is a fellow of the Institution of Engineers, Singapore, and chief editor of the ASEAN Journal on Science and Technology for Development.

Zhiwang Li received the B.S. and M.S. degrees in energy engineering from Harbin Institute of Technology, China, in 1987 and 1990, respectively. From 1990 to 1996, he was an engineer with Heilongjiang Electric Power Test and Research Institute. He joined the National University of Singapore (NUS) in 1996 where he began working as a Research Engineer. He is now a Ph.D. student with the NUS.