Effect of wall thickness of micro-combustor on the ...

Sensors and Actuators A 119 (2005) 441–445

Effect of wall thickness of micro-combustor on the performance of micro-thermophotovoltaic power generators Yang Wenminga,∗ , Chou Siawkianga , Shu Changa , Xue Hongb , Li Zhiwanga b

a MSTI, Department of Mechanical Engineering, National University of Singapore, Singapore 119260, Singapore Department of Mechanical Engineering, California State Polytechnic University, 3801 West Temple Avenue, Pomona, CA 91768, USA

Received 11 June 2004; received in revised form 21 September 2004; accepted 4 October 2004 Available online 5 November 2004

Abstract A prototype micro-thermophotovoltaic power generator is described in this work. The system is mainly composed of a micro-cylindrical SiC combustor, a simple nine layer dielectric filter and a GaSb photovoltaic cell array. Three kinds of micro-cylindrical SiC combustors with different wall thickness are fabricated and tested. The volume of the combustion chamber is 0.113 cm3 . The performance of the micro-TPV system is measured for all kinds of flow rates and H2 /air ratios. When the inner diameter of the micro-combustor is kept as constant, with the decrease of wall thickness of the micro-combustor, both the maximum electrical power output and short-circuit current increase drastically. As the flow rate of hydrogen is 4.20 g/h, and the H2 /air ratio is 0.9, the maximum electrical power outputs of the micro-TPV system are 0.92, 0.78 and 0.57 W for the wall thickness of 0.4, 0.6 and 0.8 mm, respectively. The performance of the micro-TPV power generator with a wall thickness of 0.4 mm is the best. © 2004 Elsevier B.V. All rights reserved. Keywords: Micro-thermophotovoltaic power generator; Micro-combustor; GaSb PV cells; SiC emitter

1. Introduction Recently, all kinds of micro-devices such as micropumps, micro-motors, micro-robots, micro-rovers and microairplanes are being developed. However, the miniaturization of these devices is bottlenecked by the weight of the available power systems (batteries) that occupy significant fractions of both mass and volume of the entire devices. To meet the increasing demand for compact power source, all kinds of micro-power generators such as micro-turbine engines [1], micro-rotary engines [2] and micro-free piston knock engines [3] are being developed around the world. The abovementioned micro-power generators feature a high speed moving part. To eliminate moving parts on micro-power sources, studies on direct energy conversion methods at microscale has also attracted many attentions recently. Various approaches ∗

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0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.10.005

include micro-thermoelectric generators [4], micro-fuel cells [5], micro-thermophotovoltaic (micro-TPV) systems [6,7]. TPV power generation is attracting attention due to a number of factors including advances in low band gap photovoltaic (PV) cells and design, as well as a widening appreciation of the large number of applications that can be addressed using TPV-based generators. The attractions also include the wide range of fuel sources and high reliability. During the past 10 years, more and more groups are focusing on the study of PV cells, selective emitters and TPV systems [8–12]. For TPV application, the desired output is a high and uniform temperature distribution along the wall of combustors. Compared to conventional TPV power generators, microTPV power generators produce greater output power density in terms of per unit volume due to the much higher surfaceto-volume ratio of micro-combustors, which makes the study of micro-TPV power generators particularly attractive. How-

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Y. Wenming et al. / Sensors and Actuators A 119 (2005) 441–445

ever, the development of micro-TPV power generators also experiences more challenges in organizing combustion and assembling system. In two early papers, the simulation and experimental results about micro-combustors were described [13,14]. Here, we will introduce the effect of wall thickness of microcombustor on the performance of micro-TPV power generators.

2. Structure and design of the micro-TPV power generator A prototype micro-TPV power generator has been built-up and tested in National University of Singapore. The system mainly consists of (1) a heat source, (2) a micro-cylindrical SiC emitter (i.e. micro-combustor), (3) a simple nine layer dielectric filter, and (4) a GaSb photovoltaic cell array, see Fig. 1, the schematic of the system. 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. H2 /air mixture is combusted in the cylindrical SiC microcombustor. As the wall of the micro-combustor (i.e. SiC emitter) is heated to a high enough temperature, it emits a lot of photons. When they impinge on the PV cell array, they would evoke free electrons and produce electrical power output under the function of P/N junction. Fig. 2 shows a picture of

Fig. 1. The schematic of the micro-TPV power generator.

Fig. 2. A prototype micro-TPV power generator without cooling fins.

Fig. 3. Reflectance of the nine layer dielectric filter [15].

the prototype micro-TPV power generator without cooling fins. The SiC is selected as the materials of micro-combustor because of its good emissivity and high temperature reliability. Furthermore, compared with other selective emitters such as micro-machining tungsten, rare-earth oxide, it is easier to be fabricated into a cylindrical shape. The SiC emitter is a typical broadband emitter. The spectrums of broadband emitters operating at temperatures 1000–1600 K contain significant proportions of sub-band gap photons with energies not sufficient to generate charge carriers in the PV cells. This portion of energy will be absorbed by the PV cells and result in a destructive heat load on the generator components, subsequently lower the conversion efficiency of the system. So a simple nine layer dielectric filter is employed in the micro-TPV system to recycle this portion of energy. The filter is fabricated with alternating layers of silicon and silicon dioxide, and is deposited on glass slide and bonded on the top of GaSb PV cells with silicone. The filter is able to recycle the photons emitted in the 1.8–3.5 ␮m mid-wavelength band. Fig. 3 shows the reflectance of the filter [15]. Corresponding with the filter, a GaSb PV cell array is employed. This GaSb cell array responds up to 1.8 ␮m. The process used to fabricate GaSb PV cells replicates the silicon solar cell fabrication process, using inexpensive diffusion steps with no toxic gases, in contrast with epitaxy. Because only planar PV cells can be fabricated, the cylindrical cell circuit is composed of six 4.5 mm × 18 mm planar GaSb PV cells. The active area is 4.3 mm × 15.5 mm for each cell. The GaSb PV cells circuit covered by filter is connected with the combustor only by a high heat conduction resistance ceramic panel at one end, furthermore, the ceramic panel is filled with pore so that the heat conduction rate is as small as possible, thereby, avoid overheating the PV cells. Three kinds of micro-cylindrical SiC combustors with different wall thickness are fabricated by moulding, followed by a polishing process to the outer surface. The inner diameter and length are 3 and 16 mm, respectively, for the three combustors, corresponding to a combustion chamber of 0.113 cm3 . Fig. 4 shows the specifications of the microcombustor. The outer diameter d is 3.8, 4.2 and 4.6 mm, respectively, corresponding to a wall thickness of 0.4, 0.6 and 0.8 mm.


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Fig. 4. The specifications of the micro-SiC combustor.

3. Results and discussion The performance of the planar GaSb PV circuit is first measured with a flash lamp solar simulator. The I–V curve of the circuit is shown in Fig. 5. The “light” is a relative number used to normalize intensity during flash testing, by adjusting the light intensity can get a different current density for the PV cells. FF is the fill factor of the PV cells. It is defined as Pmax /Isc /Voc . Pmax is the maximum electrical power output. Isc is the short-circuit current. Voc is the open-circuit voltage. Imax and Vmax are the current and voltage at the maximum electrical power output point, respectively. The results indicate the planar GaSb PV circuit offers a very good electrical conversion performance. The fill factor reaches 0.776. A maximum electrical power output of 3.64 W has been achieved, when the light is 0.376. The open-circuit voltage and short-circuit current are 2.80 V and 1.68 A, respectively. The electrical power output of the prototype micro-TPV system incorporated with three kinds of different SiC combustors are then measured for all kinds of flow rate and H2 /air ratio. Figs. 6–8 show the maximum electrical power output, short-circuit current and open-circuit voltage of the system under different wall thickness, when the flow rate at the inlet is kept as 12 m/s, and H2 /air ratio varies from 0.5 to 1.0. They clearly indicate: with a decrease in the wall thickness of the micro-SiC combustor, both the maximum electrical power output and the short-circuit current increase drastically, especially at low H2 /air ratio, while the increase in the open-circuit voltage is insignificant. For example, as the wall thickness decreases from 0.8 to 0.6 mm, the increase in the maximum electrical power output and short-circuit current are more than 36 and 34%, respectively. Especially, at

Fig. 6. Maximum electrical power output under different H2 /air ratio at 12 m/s.

Fig. 5. I–V curve of planar GaSb PV circuit.

Fig. 8. Open-circuit voltage under different H2 /air ratio at 12 m/s.

Fig. 7. Short-circuit current under different H2 /air ratio at 12 m/s.

H2 /air ratio of 0.5, the increase in the maximum electrical power output and short-circuit current are 68 and 65%, respectively. When the wall thickness further decreases from 0.6 to 0.4 mm, the increase both in the maximum electrical power output and short-circuit current are also evident but relatively lower. This variation is in correspondence with the variation rule of temperature along the wall of microcombustor. Fig. 9 shows the mean wall temperature curve under different H2 /air ratio. We can see that the mean wall temperature increases with the decrease of wall thickness, and it is more evident when the wall thickness decreases from 0.8 to 0.6 mm. However, the open-circuit voltage increases only about 3% when wall thickness decreases from 0.8 to 0.6 mm. This is because the open-circuit voltage of the PV cells is major determined by the materials characteristic and operating

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best, however, the micro-TPV power generator with a wall thickness of 0.8 mm is the most reliable.

4. Conclusions

Fig. 9. Mean wall temperature under different H2 /air ratio.

Fig. 10. The maximum electrical power output under different flow rates.

temperature rather than radiation density. Furthermore, from Figs. 6–9, we can also know that both the maximum electrical power output and short-circuit current under different wall thickness increase drastically when the H2 /air ratio increases from 0.5 to 0.9. This is due to more fuel taking part in combustion, subsequently increases the temperature along the wall of micro-combustor tremendously. However, when the H2 /air ratio increases further from 0.9 to 1.0, the temperature along the wall of micro-combustor increases very little, as a result, the increase in the maximum electrical power output is also negligible. Therefore, the micro-TPV system produces a better efficiency at H2 /air ratio of 0.9 than at 1.0. Fig. 10 shows the performance curves of the micro-TPV system with different wall thickness, when the H2 /air ratio is kept as 0.9, and flow rate changes from 8 to 12 m/s. With the increase of flow rate, the maximum electrical power output of the system under different wall thickness increases almost linearly. This is because the increase in flow rate enhances the heat release rate, subsequently increases the temperature along the wall of micro-combustor. Similarly, with the decrease of wall thickness, the maximum electrical power output increases drastically due to the increase of temperature along the wall. The increase is more significant in terms of absolute value when flow rate is high. When the flow rate of hydrogen is 4.20 g/h, and H2 /air ratio is 0.9, the maximum electrical power outputs of the micro-TPV system are 0.92, 0.78 and 0.57 W for the wall thickness of 0.4, 0.6 and 0.8 mm, respectively. From above analysis, we can know that the performance of the microTPV power generator with a wall thickness of 0.4 mm is the

A prototype micro-TPV power generator is described in this paper. The system mainly includes a micro-cylindrical SiC combustor, a simple nine layer dielectric filter and a GaSb photovoltaic cell array. Three kinds of micro-cylindrical SiC combustors with different wall thickness are fabricated and tested. The volume of the combustion chamber is 0.113 cm3 . The performance of the micro-TPV system is measured for all kinds of flow rates and H2 /air ratios. When the inner diameter of the micro-combustor is kept as constant, with the decrease of wall thickness, both the maximum electrical power output and short-circuit current increase. As the flow rate of hydrogen is 4.20 g/h, and H2 /air ratio is 0.9, the maximum electrical power outputs of the micro-TPV system are 0.92, 0.78 and 0.57 W for the wall thickness of 0.4, 0.6 and 0.8 mm, respectively. The performance of the micro-TPV power generator with a wall thickness of 0.4 mm is the best, but the generator with a wall thickness of 0.8 mm is the most reliable. Just like the other micro-power generators being developed around the world, the efficiency of the micro-TPV system is still low in present design, there are still a lot of works need to do before its commercial application. The measures to improve the efficiency of the micro-TPV system include: improving the design of micro-combustor, replacing the SiC broad emitter with a selective emitter, and employing catalytical combustion, etc.

Acknowledgement This work was supported by a National University of Singapore Grant No. R-265-000-114-112.

References [1] I.A. Waitz, G. Gauba, S.T. Yang, Combustors for micro-gas turbine engines, J. Fluids Eng. 120 (1998) 109–117. [2] K. Fu, A.J. Knobloch, B.A. Cooley, D.C. Walther, C. FernandezPello, D. Liepmann, K. Miyaska, Microscale combustion research for applications to MEMS rotary IC engine, in: Proceedings of NHTC, 2001 National Heat Transfer Conference, Anaheim, CA, 10–12 June, 2001, pp. 1–4. [3] W. Yang, MEMS free-piston knock engine, in: Proceedings of the 28th International Symposium on Combustion, Edinburgh, UK, 30 July–4 August, 2000, pp. 1–4. [4] L. Sitzki, K. Borer, E. Schuster, P.D. Ronney, S. Wussow, Combustion in microscale heat-recirculating burners, in: Proceedings of the Third Asia-Pacific Conference on Combustion, Seoul, 24–27 June, 2001, pp. 1–4. [5] S.J. Lee, A. Chang-Chien, S.W. Cha, R. O’Hayre, Y.I. Park, Y. Saito, F.B. Prinz, Design and fabrication of a micro fuel cell array with ‘flip-flop’ interconnection, J. Power Sources 112 (2002) 410–418.


445

[6] O.M. Nielsen, L.R. Arana, C.D. Baertsch, K.F. Jensen, M.A. Schmidt, A thermophotovoltaic micro-generator for portable power applications, in: Proceedings of the 12th International Conference on Solid State Sensors, Actuators and Micro Systems, Boston, 8–12 June, 2003, pp. 714–717. [7] W.M. Yang, S.K. Chou, C. Shu, Z.W. Li, H. Xue, Development of microthermophotovoltaic system, Appl. Phys. Lett. 81 (2002) 5255–5257. [8] L.G. Ferguson, L.M. Frass, Theoretical study of GaSb PV cells efficiency as a function of temperature, Solar Energy Mater. Solar Cells 39 (1995) 11–18. [9] P. Griffin, I. Ballard, K. Barnham, J. Nelson, A. Zachariou, J. Epler, G. Hill, C. Button, M. Pate, The application of quantum well solar cells to thermophotovoltaics, Solar Energy Mater. Solar Cells 50 (1998) 213–219. [10] T.J. Coutts, A review of progress in thermophotovoltaic generation of electricity, Renew. Sustain. Energy Rev. 3 (1999) 77–184. [11] L.G. Furguson, F. Dogan, A highly efficient NiO-doped MgO matched emitter for thermophotovoltaic energy conversion, Mater. Sci. Eng. B 83 (2002) 35–41. [12] W. Durisch, B. Bitnar, F.V. Roth, G. Palfinger, Small thermophotovoltaic prototype systems, Solar Energy 75 (2003) 11–15. [13] W.M. Yang, S.K. Chou, C. Shu, Z.W. Li, H. Xue, Combustion in micro-cylindrical combustors with and without a backward facing step, Appl. Therm. Eng. 22 (2002) 1777–1787. [14] W.M. Yang, S.K. Chou, C. Shu, Z.W. Li, H. Xue, D.T. Li, J.F. Pan, Microscale combustion research for application to micro thermophotovoltaic systems, Energy Conv. Manage. 44 (2003) 2625– 2634. [15] L.M. Fraas, J.E. Samaras, X. Huang, L.M. Minkin, W.E. Daniels, S. Hui, TPV generators using the radiant tube burner configuration, in: Proceedings of the 17th European Photovoltaic Solar Energy Conference and Exhibition, Munich, 22–26 October, 2001, pp. 1–4.

tional University of Singapore, where he is currently a research scientist, focusing on the study of Power MEMS.

Biographies

Li Zhiwang 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 in Heilongjiang electric power test & research institute. He joined National University of Singapore (NUS) from 1996 as a research engineer, he is now a Ph.D. student in NUS.

Yang Wenming 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 Na-

Chou Siaw Kiang received the B.S. degree in mechanical engineering from University of Singapore in 1977 and Docteur-Ingenieur from Ecole Nationale Superieure d’Arts et Metiers, Paris, in 1980. He is a professor of the Department of Mechanical Engineering, NUS, and became head of department in 1998. His research interests are: 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 micro thermal 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. Shu Chang 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 University of Glasgow in 1991. From 1991 to 1992, he was the post-doctoral research fellow in the University of Glasgow. He joined National University of Singapore (NUS) from 1992 as a research scientist. He is now an associate professor of NUS. The current research interests of Dr. Shu include efficient methods for solution of partial differential equations, computational fluid dynamics, and MEMS. Xue Hong received the B.S. degree in mechanical engineering from Jiangsu University of Science and Technology, China, in 1982. He received his M.S. and Ph.D. degrees in mechanical engineering from University of Tokyo, Japan, in 1988 and 1992, respectively. From 1992 to 2000, he was assistant professor in National University of Singapore. He is now associate professor in California State Polytechnic University. The current research interests of Dr. Xue include MEMS and micro fluid flow and heat transfer, numerical thermal-fluids simulation. Dr. Xue is a member of the American Society of Mechanical Engineers and Japan Society of Mechanical Engineers.