A Promising Energy Source for Portable MEMS Devices

Transcription

1 A Promising Energy Source for Portable MEMS Devices S.K. Chou *, Yang Wenming, Li Zhiwang and Li Jun Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive, Singapore 7576 Abstract: The demand for energy sources that are compact, lightweight and powerful has significantly increased in recent years. Traditional chemical batteries which are highly developed are unable to meet the demand for high energy intensity. This gap is expected to widen in future as devices become more powerful and full featured. In this paper, we describe a promising new power source, the micro thermophotovoltaic (micro-tpv) power generator, for application in portable MEMS devices and personal systems. The micro-tpv system comprises a micro SiC combustor, a GaSb photovoltaic (PV) cell array, and a nine-layer dielectric filter, with a volume of about 3 cm 3. The system has no moving parts and its fabrication and assembly are relatively easy. As a result, the micro- TPV can be easily adapted for use in commercial electronics and micro devices. The power source of the micro-tpv is derived from burning hydrogen. When the flow rate of the H /O mixture is.3 l/s, and having a H /O ratio of.8, the system is able to deliver.5 W of electrical power, with an open-circuit voltage and short-circuit current of.3 V and.43 A, respectively. This output corresponds to a power density of about.8 W/cm 3. Considering the micro combustor is only.3 cm 3 in volume, there is great potential for further increasing the power density significantly. Keywords: Micro Combustor, Micro-TPV Power Generator, SiC Emitter, GaSb PV Cells, Dielectric Filter. INTRODUCTION Demand for clean and renewable energy sources has increased significantly in the past decade. Although the traditional rechargeable battery remains a good power source for applications that require large current supply, it has some weaknesses, namely, the recharging time is long, the operation time between recharging is short, it is environmental unfriendly, and its energy density is low. Recent rapid advancements in autonomous robots and mobile electronic devices are calling for better mobile power sources than batteries. This scenario presents an enormous opportunity for new power technologies and products. To meet the demand for smaller scale and higher energy density power sources, work on micro power devices or power MEMS has accelerated. MIT has been developing a micro gas turbine engine with dimensions of about. cm in diameter and.4 cm in height []. Figure shows a schematic of the micro engine. The engine is destined to produce an electrical power output of - W. UC Berkeley is developing a MEMS-sized rotary (Wankel-type) micro engine []. An electrical power output of 3.7 W has been obtained, although with low efficiency (.%). Princeton University and University of Southern California are developing micro-thermoelectric systems [3]. Figure shows the structure of the system. Other micro power generators being developed include free piston micro engines, micro piezoelectric engines, micro-fuel cells and micro thrusters [4-7]. Figure 3 shows a sketch of the micro thruster developed by National University of Singapore (NUS). Despite the proliferation of research activities on micro power generators, the relatively short development history presents enormous opportunity for exciting innovations to come. Fig. The MIT micro gas turbine engine Corresponding author: [email protected]

2 Fig. Micro thermoelectric system developed by University of Southern California Fig. 3 The NUS micro thruster In, we initiated a study of a new power MEMS concept, namely, the micro-tpv power generator [8, 9]. The system uses PV cells to convert heat radiation obtained. from the combustion of fossil fuels into electricity. For micro-tpv applications, the desired output is a high and uniform temperature along the wall of the micro combustor. The micro-tpv power generator fully utilizes the high surface-to-volume ratio of the micro combustor thus maximizing the output power density. According to the cubic-square law, with the decrease of the combustor size, the surface-to-volume ratio will increase drastically. At the same heat flux density per unit surface, the heat flux via the wall in terms of per unit volume will significantly increase for the micro combustor. This makes the development of micro-tpv power generators particularly attractive. In 3, Nielsen et al [] developed a planar microthermophotovoltaic power generator.. DESIGN OF MICRO-TPV POWER GENERATOR The micro-tpv power generator realized at the Department of Mechanical Engineering, NUS, comprises a cylindrical SiC emitter (i.e. micro combustor), a nine layer dielectric filter, and a GaSb photovoltaic cell array. Figure 4 shows the schematic of the micro-tpv system. H /O mixture is burned in the micro SiC combustor releasing heat. As the emitter is heated to a high temperature, it emits a stream of photons. The prototype micro-tpv power generator is shown in Fig.5, where the cylindrical SiC emitter and cooling fins are not incorporated into the system. The overall volume is 3. cm 3. The spectrum of the SiC emitter operating at a temperature of to 6 K contains a significant proportion of sub-band gap photons which serve no useful purpose other than to heat up the PV cells, thus reducing its life span and conversion efficiency. To improve the overall efficiency of the micro-tpv system, it is imperative that these redundant photons be turned back and recycled. The multi-layered dielectric filter is therefore employed to serve this purpose. Figure 6 shows the selective reflectance of the filter used in the present micro-tpv system. We use GaSb PV cells in our present design as they are technically proven and readily available. Earlier studies indicate that a micro cylindrical combustor with a backward facing step is one of the simplest but most effective structures for micro-tpv applications [, ]. It is believed that an appropriate backward facing step can facilitate recirculation along the wall and enhance the mixing process around the rim of the tube flow, therefore ensuring complete combustion.

3 An average temperature of about 3 K can be achieved along the wall of a micro cylindrical combustor with a backward facing step when H /air mixture was employed as fuel. However, the temperature is still too low to generate a high enough power density. In order to increase the output power density of the micro-tpv power generator, the micro combustor is modified and the H /air mixture is replaced by a H /O mixture. The step height to the inner diameter ratio is a major factor in the design of the micro combustor. Some of the fuel will be blown out before completion if the step height to the inner diameter ratio is too large. On the other hand, if the ratio is too small, the flame will flow back to the connection tube. Cooling fins Micro mixer PV cells covered with filters SiC em itter Combustion chamber Fig. 4 Schematic of a micro-tpv power generator Fig. 5 A prototype micro-tpv power generator without emitter and cooling fins 3. RESULTS AND DISCUSSION The performance of the micro-tpv power generator has been studied over a range of flow rates and H /O ratios. Two configurations of the micro combustor are tested. Figure 7 shows the basic design of the combustor-emitter. The first configuration is 3. mm in diameter, 6 mm in length, and the diameter of the connection tube, d, is.mm. This arrangement corresponds to a step height to inner diameter ratio of.5. The second configuration has the same dimensions as the first one, but with a connection tube diameter, d, of.7 mm, which corresponds to a step height to inner diameter ratio of.383. The volume of the micro combustor is.3 cm 3. The first configuration was initially designed for combustion using H /air mixture. The experiment shows that the micro combustor (the first configuration) with a low step height to inner diameter ratio is not suitable for H /O combustion. The flame cannot be maintained in the micro combustor, but travels back to the connection tube due to the high flame speed of the H /O mixture combustion. However, stable combustion can be achieved in the second micro combustor. This is because the connection tube in the second design has a much smaller diameter, resulting in a much higher flow speed in the connection tube than in the micro combustor. This helps to prevent flow back of the flame. Figure 8 shows the performance of the micro-tpv power generator when the flow rate of H /O mixture is.54 l/s. During the experiment, we found that combustion could not be sustained in the micro combustor when H /O ratio is., so the performance of the micro-tpv power generator at. is not shown in the figure. The best performance occurs at H /O ratio of.7, when an electrical power output of.58 W has been achieved. As the H /O ratio increases from. to.7, both the short circuit current and maximum output power increase rapidly. However, when the H /O ratio further increases from.7 to., the performance decreases. This is because the flame speed increases with an increase of the H /O ratio, resulting in partial flow back of the flame into the connection tube, thereby worsening the performance of the system. Similar trends can be observed under other flow rates. 3

4 Figure 9 shows the performance of the micro-tpv power generator at a fuel flow rate of.3 l/s. At this increased flow rate, however, the flow back of the flame occurs only at a H /O ratio of.9-.. Furthermore, the performance of the micro-tpv power generator only decreases slightly. The results indicate that we can prevent the flow back by increasing the flow rate. It can be observed from Fig. 9 that an electrical power output of.5 W can be achieved by the system when H /O ratio is.8, and the opencircuit voltage and short-circuit current are.3 V and.43 A, respectively, corresponding to a power density of about.8 W/cm 3. Fig. 6 Reflectance of the filter Fig. 7 Dimensions of the micro combustor-emitter Voltage (V) open circuit voltage.4 short circuit current maximum output power H/O ratio Current (A)/Power (W) Fig. 8 Performance of the micro-tpv power generator at a fuel flow rate of.54 l/s 4

5 .5 3 Voltage (V) open-circuit voltage.5 Short circuit current.5 maximum output power H/O ratio Current (A)/Power (W) Fig. 9 Performance of the micro-tpv power generator at a fuel flow rate of.3 l/s 4. CONCLUSION The micro-tpv power generator described in this paper holds promise as a reliable and versatile portable power source. Th e system, packed into a volume of about 3 cm 3, has no moving parts and its fabrication and assembly are relatively easy. As a result, t he new energy source can be more readily adapted for rapid deployment for personal and portable applications. Given that the micro combustor is only.3 cm 3 in volume, there is much scope for making further improvement to its power density. To fully exploit the potential of the micro-tpv, further work on the system is required. Firstly, the radiation efficiency of the micro combustor is still relatively low. To improve the efficiency of the micro-combustor, we envisage using a heat recuperator. In the present design, hydrogen is employed as the fuel. The storage and transport is a challenge. In the future, liquid fuel and natural gas could be alternatives. These present a challenge to the design of the micro combustor. In the present design, for simplicity of fabrication, SiC is employed as the emitter, which is a typical broadband emitting material. The spectrums of broadband emitters operating at a temperature of to 6 K contain significant amounts of useless sub-band gap photons. A proper selective emitter and a matched filter will need to be developed to reshape the radiation spectrum and to ensure an improved yield of the PV cells. 5. ACKNOWLEDGMENT This work was supported by a National University of Singapore Grant No. R REFERENCES [] Mehra. A. Xin, Z. Ayon, A. A., Waitz, I. A., Schmidt, M. A., and Spadaccini, C. M., () A six-wafer combustion system for a silicon micro gas turbine engine, Journal of Microelectromechanic Systems, 9, (4), pp [] Kelvin F. and A.J. Knobloch, () Microscale Combustion Research for Application to MEMS Rotary IC Engine, Proceedings of National Heat Transfer Conference, pp.-6. [3] Sitzki, L. and Borer, K. () Combustion in Microscale heat-recirculating burners, Proceedings of the Third Asia-Pacific Conference on Combustion, pp.-4. [4] Yang, W., () MEMS Free-Piston Knock Engine, Proceedings of 8 th international symposium on combustion, pp.-4. [5] Whalen, S., Thompson, M., Bahr, D., Richards, C., and Richards, C., (3) Design, fabrication and testing of the P 3 micro heat engine, Sensors and Actuators A 4, pp [6] Lim, C. and Wang, C. Y., (3) Development of high-power electrodes for a liquid-feed direct methanol fuel cell, Journal of Power Sources, 3, pp [7] Zhang, K. L., Chou, S. K., and Ang, S. S. (4) Development of a solid propellant microthruster with chamber and nozzle etched on a wafer surface, Journal of Micromechanics and Microengineering, 4, pp [8] Yang, W. M., Chou, S. K., Shu, C., Xue, H., and Li, Z. W., () Development of a micro thermophotovoltaic system, Applied Physics Letters, 8, pp [9] Yang, W. M., Chou, S. K. Shu, C., Xue, H., and Li, Z. W., (5) Research on micro-thermophotovoltaic power generators with different emitting materials, Journal of Micromechanics and Microengineering, 5, pp.s39-s4. [] Nielsen, O. M., Arana, L. R., Baertsch, C. D., Jensen, K. F., and Schmidt, M. A., (3) A thermophotovoltaic micro generator 5

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