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Mar 17, 2004 - of the micro-heat engine, all kinds of micro-devices have been developed ... Micro-turbine engines, micro-rotary engine, micro-free-piston.
INSTITUTE OF PHYSICS PUBLISHING

JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 37 (2004) 1017–1020

PII: S0022-3727(04)71826-4

Development of a prototype micro-thermophotovoltaic power generator W M Yang1,3 , S K Chou1 , C Shu1 , H Xue2 and Z W Li1 1

Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260, Singapore 2 Department of Mechanical Engineering, California State Polytechnic University, 3801 West Temple Avenue, Pomona, CA 91768, USA

Received 13 November 2003 Published 17 March 2004 Online at stacks.iop.org/JPhysD/37/1017 (DOI: 10.1088/0022-3727/37/7/011) Abstract A prototype micro-thermophotovoltaic (micro-TPV) power generator is described in this paper. The system comprises a SiC emitter, a simple nine-layer dielectric filter and a GaSb PV cell array. When the flow rate of hydrogen is 4.20 g h−1 and the H2 /air ratio is 0.9, the micro-TPV system is able to deliver an electrical power output of 0.92 W in a micro-combustor of 0.113 cm3 in volume. The open-circuit electrical voltage and short-circuit current are 2.32 V and 0.52 A, respectively. If we replace the GaSb PV cells with GaInAsSb PV cells, then an electrical power output of 1.45 W can be expected.

1. Introduction The past few years have seen a growing trend towards the miniaturization of mechanical and electro-mechanical engineering devices [1]. All kinds of micro-devices, such as micro-pumps, micro-motors, micro-robots, micro-rovers and micro-airplanes are being developed. However, the miniaturization of these devices is limited by the weight of the available power systems (batteries). Recent developments in wireless micro-systems, which are laying the groundwork for the next generation sensing systems, are also bottlenecked by a tiny but powerful energy source. So it is of great interest to develop a micro-power generation system that has a high energy density. The concept behind this new field is to utilize the high specific energy of hydrocarbon fuels in combustion driven micro-devices to generate power. Since the 1990s, when Epstein [12] proposed the concept of the micro-heat engine, all kinds of micro-devices have been developed rapidly around the world. Generally, these microdevices can be grouped into three categories: rockets, direct energy conversion devices and indirect energy conversion devices. Solid propellant micro-rockets and bipropellant micro-rockets are typical micro-rockets being developed [3, 4]. Micro-rockets do not produce electrical power, but direct mechanical power. They are designed for orbit 3

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and station keeping of pico-satellites or micro-spacecraft. Micro-turbine engines, micro-rotary engine, micro-free-piston ‘knock’ engines and P3 micro-heat engine are typical indirect energy conversion devices [2, 5–7]. The basic principle of these kinds of micro-devices is the following: thermal power is first converted to mechanical power through the use of a proper thermodynamic cycle, and mechanical power is then converted to electrical power by an electrical generator. The micro-thermoelectric power generator is a typical direct energy conversion device [8]. These kind of micro-devices do not include any moving parts, and they convert thermal power to electrical power directly.

2. Micro-thermophotovoltaic power generator The micro-thermophotovoltaic (micro-TPV) system is another kind of direct energy conversion device [9]. The system uses PV cells to convert heat radiation, e.g. from the combustion of fossil fuels, into electricity. The micro-TPV system consists of four main parts: a heat source, a micro-flame tube combustor (the wall of the micro-combustor could be made of broadband materials such as SiC, or selective emitting materials such as Er3 Al5 O12 and Co-doped MgO), a filter and a PV array (see figure 1, the schematic of the micro-TPV system). A prototype micro-TPV system with no micro-combustor is shown in figure 2. The hydrogen/air mixture is combusted in

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Figure 1. Schematic of the micro-TPV system.

Figure 3. Temperature distribution on the axial plane of the flame tube with a backward facing step.

Figure 2. Micro-TPV power generator without micro-combustor.

the micro-combustor. When the wall of the micro-combustor (emitter) is heated to a sufficiently high temperature, it emits many photons. When photons with an energy greater than the bandgap of the PV cells impinge on the PV array, they produce free electrons and, consequently, electrical power. Compared to conventional combustors, a microcombustor is more highly constrained by inadequate residence time for complete combustion and high rates of heat transfer from the combustor. In this work, hydrogen is chosen as the fuel because of its original high heating value, fast diffusion velocity, short reaction time and high flame speed [10]. For micro-TPV applications, the desired output is a high and uniform temperature along the wall of the microcombustor. Compared to conventional macro-TPV systems, the micro-TPV systems feature a much higher power density per unit volume due to the high surface-to-volume ratio. For example, for a macro-TPV system with a cylindrical combustor of 50 cm in length and 20 cm in diameter, the surface and volume of the combustor are 3142 cm2 and 15708 cm3 . Assuming the radiation power density is 10 W cm−2 , the total radiation power from the combustor is 31420 W, corresponding to a volumetric power density of 2 W cm−3 . However, for a micro-TPV system with a cylindrical combustor 2 cm in length and 0.3 cm in diameter, the surface and volume of the combustor are 1.88 cm2 and 0.14 cm3 . Assuming the radiation power density remains 10 W cm−2 , the total radiation power is 18.8 W, corresponding to a volumetric power density of 133.3 W cm−3 . Therefore, it is of great interest for us to develop micro-TPV systems. The major challenge in micro-combustor design is to keep an optimum balance between sustaining combustion and maximizing the heat output. A high surface-to-volume ratio is very favourable to the output power density per unit volume. However, a high heat output will affect the stable combustion in the micro-combustor. To verify the feasibility of microcombustion and optimize the design of the micro-combustor, 1018

Figure 4. Temperature distribution along the wall of the micro-SiC flame tube combustor.

a series of numerical simulations and experimental studies were carried out. Results indicate that a micro-cylindrical flame tube with a backward facing step is one of the simplest, but all the same effective, structures for the micro-TPV application. The temperature distribution on the axial plane obtained by three-dimensional numerical simulation is shown in figure 3, where we can see a very interesting phenomenon. Because the backward facing step can facilitate recirculation along the wall and enhance the mixing process around the rim of the tube flow, the combustion takes place near the wall rather than at the centreline of the flame tube at the beginning stage. The part of the fuel/air mixture around the centreline is then heated and accelerated by combustion products surrounding it and flows quickly to near the end of the flame tube and combusts there, which inversely heats the gas near the wall, and therefore maintains a fairly uniform temperature along the wall. In a micro-SiC flame tube combustor 3 mm in diameter and 16 mm in length, an average temperature of 1305 K has been obtained along the wall in experimental testing, and the largest temperature difference is less than 6%, when the H2 flow rate is 4.2 g h−1 and the H2 /air ratio is 0.90 (see figure 4). The efficiency is 19.3%. The volume of the micro-combustor is 0.113 cm3 . The second key component of the micro-TPV system is a simple nine-layer dielectric filter. The SiC emitter is a

Prototype micro–TPV power generator

Figure 5. The reflectance of a simple nine-layer dielectric filter. Figure 6. Quantum efficiency of GaSb PV cells.

3. Results and discussion The performance of the planar GaSb PV circuit is measured with a flash lamp solar simulator. The results indicate that the

FF=0.776 VOC=2.80 Volts Isc=1.68 Amps Imax=1.54 Amps Vmax=2.37 Volts Pmax=3.64 Watts Light=0.376

Amperes

Figure 7. I –V curve of the planar GaSb PV circuit. Maximum electrical power output (W)

typical broadband emitter. The spectra of broadband emitters operating at temperatures in the range 1000–1600 K contain significant proportions of sub-band 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 result in a destructive heat load on the generator components, subsequently lowering the conversion efficiency of the system. To improve the 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. Here, a simple nine-layer dielectric filter is employed for the micro-TPV system [11]. The filter is able to recycle the energy emitted in the 1.8 to 3.5 µm mid-wavelength band. Figure 5 shows the reflectance of the filter. The third, key component of the micro-TPV system is a low band gap 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 band gap 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 the 1960s [12], it was only in recent years that technological improvements in the field of low band gap PV cells and high temperature materials have evoked a renewed interest in TPV generation of electricity. GaSb, GaInAs and InGaAsSb are typical low bandgap PV cells developed recently for TPV applications [13–15]. Corresponding to the filter, a GaSb PV cell array is employed for our micro-TPV system. This GaSb cell array responds to photons with wavelength less than 1.8 µm. The process used to fabricate these GaSb cells replicates the silicon solar cell fabrication process, using inexpensive diffusion steps with no toxic gases [16], in contrast with epitaxy. The quantum efficiency of the GaSb cell is shown in figure 6 [13]. Because only planar GaSb PV cells can be fabricated, the cylindrical array is composed of six 4.5 mm × 18 mm planar GaSb PV cells forming a cylindrical tube (see figure 2). The active area is 4.3 mm × 15.5 mm for each cell.

1 0.8 0.6 0.4 10m/s 12m/s 13m/s

0.2 0 0.4

0.5

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H2/air ratio

Figure 8. The maximum electrical power output under different H2 /air ratios and flow rates.

planar GaSb PV circuit offers a very good electrical conversion performance. The fill factor reaches 0.776. The I –V curve of the circuit is shown in figure 7. The ‘light’ is a relative number used to normalize intensity during flash testing, by adjusting the ‘light’ intensity to obtain a certain current density. The electrical power output of the prototype micro-TPV system incorporating a SiC emitter is then measured for various flow rates and H2 /air ratios. Figure 8 shows the maximum electrical power output for different flow rates and H2 /air ratios. With the increase in the flow rate and the H2 /air ratio, the maximum electrical power output increases drastically. This is because a larger amount of fuel is used in the combustion. As the hydrogen flow rate is 4.20 g h−1 and the H2 /air ratio is 0.9, an electrical power output of 0.92 W has been achieved for 1019

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0.92 W, corresponding to an open-circuit electrical voltage of 2.32 V and a short-circuit current of 0.52 A. An electrical power output of 1.45 W can be expected, if we replace the GaSb PV cells with GaInAsSb PV cells.

Acknowledgments We acknowledge JX Crystals for the fabrication of the GaSb PV cells and filters. This work was supported by the National University of Singapore Grant No R-265-000-114-112.

References Figure 9. Quantum efficiency of GaInAsSb PV cells.

the micro-TPV system, corresponding to an overall efficiency of 0.66%, which is eight times that obtained by Nielsen [17]. The open-circuit voltage and short-circuit current are 2.32 V and 0.52 A, respectively. In the micro-SiC flame tube combustor experiment discussed above, we noticed that the temperature along the wall of the micro-combustor is only 1305 K. At this temperature, if we replace the GaSb PV cells with GaInAsSb PV cells, the performance of the micro-TPV power generator can be further improved due to the lower band gap of the GaInAsSb PV cells. Figure 9 shows the quantum efficiency of GaInAsSb PV cells. Using the model developed in reference [18], an electrical power output of 1.45 W can be predicted, if a GaInAsSb PV cell array is employed in the design of micro-TPV power generators. The efficiency will be 1.04%. However, so far, the GaSb PV cell is the only commercial PV cell for TPV applications. That is why we did not employ GaInAsSb PV cells in this work. In order to further improve the efficiency of the micro-TPV system, it is necessary to employ a selective emitter in future designs instead of SiC broadband emitters, so that most of the photons emitted are located in the short wavelength range with energies greater than the bandgap of PV cells. Furthermore, there is still much room for improving the efficiency of the micro-combustor, since an efficiency of only about 19.3% has been achieved in this design.

4. Conclusion A novel prototype micro-TPV power generator is described in this paper. The system consists of a SiC emitter, a simple nine-layer dielectric filter and a GaSb PV cell array. In a micro-combustor of 0.113 cm3 in volume, when the flow rate of hydrogen is 4.20 g h−1 and the H2 /air ratio is 0.9, the microTPV system is able to deliver an electrical power output of

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[1] Carlos Fernandez-Pello A 2002 29th Int Symp. on Combustion (Sapporo, Japan, 2002) pp 1–45 [2] Epstein A H et al 1997 American Institute of Aeronautics and Astronautics AIAA 97-1773 1–12 [3] Lewis D H Jr, Janson S W, Cohen R B and Antonsson E K 2000 Sensors Actuators: Phys. 80 143–54 [4] London A P, Ayon A A, Epstein A H, Spearing S M, Harrison T, Peles Y and Kerrebrock J L 2001 Sensors Actuators: Phys. 92 351–7 [5] Kelvin Fu et al 2001 Proc. NHTC, 2001 National Heat Transfer Conf. (Anaheim, CA, 2001) pp 1–6 [6] Yang W 2000 28th International Symp. on Combustion (Edinburgh, UK, 2000) [7] Whalen S, Thompson M, Bahr D, Richards C and Richards R 2003 Sensors Actuators: Phys. 104 290–8 [8] Sitzki L et al 2001 The Third Asia–Pacific Conf. on Combustion (Seoul, Korea, 2001) pp 1–4 [9] Yang W M, Chou S K, Shu C, Li Z W and Xue H 2002 Appl. Phys. Lett. 81 5255–7 [10] Waitz I A, Gauba G and Yang S T 1998 ASME J. Fluids Eng. 120 109–17 [11] Fraas L M, Samaras J E, Huang H X, Minkin L M, Avery J E, Daniels W E and Hui S 2001 17th European Photovoltaic Solar Energy Conf. and Exhibition (Munich) pp 1–4 [12] White D C, Wedlock B D and Blair J 1961 15th Annual Power Sources Conf. (Atlantic, NJ, 1961) pp 125–32 [13] Ferguson L G and Fraas L M 1995 Sol. Energy Mater. Sol. Cells 39 11–8 [14] Wanlass M W, Ward J S, Emery K A, Al-Jassim M M, Jones K M and Coutts T J 1996 Sol. Energy Mater. Sol. Cells 41 405–17 [15] Wang C A, Choi H K, Ransom S L, Charache G W, Danielson L R and DePoy D M 1999 Appl. Phys. Lett. 75 1305–7 [16] Fraas L M, Girard G R, Avery J E, Arau B A, Sundaram V S and Gee J M 1989 GaSb booster cells for over 30% efficient solar-cell stacks J. Appl. Phys. 66 3866–70 [17] Nielsen O M, Arana L R, Baertsch C D, Jensen K F and Schmidt M A 2003 A thermophotovoltaic micro-generator for portable power applications 12th International Conf. on Solid State Sensors, Actuators and Micro Systems (Boston) pp 714–17 [18] Yang W M, Chou S K, Shu C, Li Z W and Xue H 2003 Research on micro-thermophotovoltaic power generators Sol. Energy Mater. Sol. Cells 80 95–104