Micro-thermophotovoltaic System Research Articles

Enhancement of thermal performance by converging-diverging channel in a micro tube combustor fueled by premixed hydrogen/air

As the core component of the micro thermophotovoltaic (MTPV) system, the micro combustor with a high and uniform wall temperature distribution is beneficial to improve the energy conversion efficiency. In this paper, a micro tube combustor with converging-diverging channel is proposed and the thermal performance is numerically investigated, compared with that of the micro combustor with cylindrical channel. The effects of inlet velocity of H2/air mixture, dimensionless position and diameter of throat, and solid material on the thermal performance are widely analyzed. Results show that the outer wall temperature and emitter efficiency of the micro combustor with converging-diverging channel are higher than that of the micro combustor with cylindrical channel, and the converging-diverging channel has more uniform temperature distribution. The converging-diverging micro combustor with dimensionless throat position l = 0.375 and dimensionless throat diameter β = 0.4 is more suitable for the application of MTPV system. When H2/air inlet velocity is 11 m/s and H2/air equivalence ratio is 1.0, the mean wall temperature is increased by 82.39 K and the emitter efficiency is increased by 6.59%, while the normalized temperature standard deviation is reduced by 65.85%. Additionally, the use of SiC as wall material can improve the thermal performance of the micro combustor. It is worth noting that this work will offer us significant guidelines for the optimized work of micro tube combustor.

The effect of embedded high thermal conductivity material on combustion performance of catalytic micro combustor

A novel micro combustor embedded with high thermal conductivity material has been proposed for micro-thermophotovoltaic system application. The effect of the materials, which is graphite and nickel with widths 1 mm and 3 mm on the thermal and radiant performance of the new micro combustor, was experimentally and numerically investigated. It was found that high and uniform temperature distribution along the walls of the combustor was obtained by embedding graphite with width of 1 mm because of the enhanced heat transfer, which is desirable for the micro-thermophotovoltaic system. The available radiation energy increased from 4.6 W for the conventional combustor to 5.2 W for the novel combustor when the inlet velocity was 1.0 m/s and the hydrogen/oxygen equivalence ratio was 0.5 under the experimental conditions. The available radiation efficiency increased from 8.9% to 9.8% and the effect of the embedded material could be enhanced by increasing the chemical energy input. Based on the combustor and fuel in this study, the conditions for good thermal performance are graphite with width of 1 mm, inlet velocity of 1.0 m/s and 0.5 equivalence ratio of hydrogen/oxygen by considering the reliability and durability of the combustor.

Numerical study on energy conversion performance of micro-thermophotovoltaic system adopting a heat recirculation micro-combustor

In order to evaluate the energy conversion performance of micro-thermophotovoltaic system (MTPV) effectively, a novel calculation method combining Fluent with MATLAB program is proposed, which has considered the non-uniformity of temperature distribution of radiant surface, the angular coefficient between the emitter and the PV cell and its cooling efficiency. In this paper, numerical study on hydrogen/air premixed combustion in micro-combustors integrated with MTPV system to produce electricity has been adopted. Results show that the overall efficiency of heat recirculation combstor is almost twice as large as obtained in straight channel combustor. Furthermore, dimensionless baffle length, a key parameter in designing micro-combustor with heat recirculation, has been investigated. It is concluded that the subsystem efficiencies of MTPV system are increasing first and then descending with an increase in dimensionless baffle length. However, as the dimensionless baffle length reaches a certain point, the working performance of subsystems changes little. Meanwhile, it is demonstrated that increasing the dimensionless channel gap reasonably will improve the combustion process. Finally, the energy conversion performances as a function of hydrogen volume flow rate are calculated and compared. Results indicate that choosing a proper inlet velocity is of great pivotal to gain a better energy conversion performance of MTPV system. Of all the investigated cases, the maximum power generated with the MTPV is 1.4014 W and the overall efficiency is 0.7092%.

Numerical Study on Heat Transfer Enhanced in a Microcombustor With Staggered Cylindrical Array for Micro-Thermophotovoltaic System

In consideration of high heat transfer efficiency and stable combustion, a new type of microplanar combustor for micro-thermophotovoltaic (micro-TPV) system is proposed, in which the heat transfer is enhanced by staggered cylindrical array. The numerical study results indicate that the temperature of radiation wall of cylindrical-array combustor is higher and more uniform comparing with the conventional-channel combustor, the application of cylindrical-array make the effective radiation of the combustor increase 34.55% and reach to 35.98 W. Moreover, with inlet velocity increase from 4 m/s to 16 m/s, the cylindrical-array combustor shows the better stability of combustion, which the position of the flame moves 4.8 mm in the cylindrical-array combustor and 9.1 mm in the conventional-channel combustor. However, the 0.5–4.5 equivalence ratio range for stable combustion is slightly narrower than 0.4–6.0 in the conventional-channel combustor. To extend the equivalence ratio range, one row of cylindrical array was canceled, and the distribution length of cylindrical array was reduced to 10 mm, After this improvement, the equivalence ratio range is extended to 0.3–5.5, and the negative effect on the flame stability of the cylindrical array is basically eliminated.

Numerical investigation of key parameters of the porous media combustion based Micro-Thermophotovoltaic system

Power generation with porous media driven Micro-Thermophotovoltaic (MTPV) was investigated and effects of changes to key parameters of the system investigated. The micro combustor had the dimensions of length–15 mm, width–10 mm, height–1 mm and wall thickness of 0.5 mm. The distance between the outside wall of combustor and the TPV cell was fixed at 1 mm. Variation in distance from 1 to 6 mm between the outside wall of the combustor and the thermophotovoltaic cell (TPV cell) caused a reduction of 13.75% and 1.4% in the radiation heat transfer efficiency and the TPV cell conversion efficiency respectively. An increase in the mixture flow rate from 300 mL/min to 1800 mL/min caused an increase in the radiation heat transfer efficiency, TPV cell conversion efficiency and the total system efficiency. As the flow rate increased, the system’s power output also increased. At 600 mL/min, the output power was 560 mW but rose to 3.2 W at the flow rate of 1800 mL/min. The cooling load of the system showed a linear growth as the flow rate increased. At 1800 mL/min the cooling load of the system was 12.4 W which is three times the cooling load at 900 mL/min.

Experimental and numerical study of premixed propane/air combustion in the micro-planar combustor with a cross-plate insert

Over the past decade, the micro-thermophotovoltaic (MTPV) system has aroused widely public attention. Micro-combustor is an important part, which can determine the working performance of this micro-power generator. In this paper, experimental investigations as well as a three-dimensional CFD simulation have been carried out to study the performance of propane/air premixed combustion in a new kind of cross-plate micro-planar combustor. Benefited from the heat transfer enhancement by the setting up of cross-plate, the average wall temperature of the new combustor is increased by more than 90 K, which results in the growth of radiation efficiency. Besides, the blowout limit is apparently extended in the cross-plate combustor. Compared to the single-channel combustor, the blowout limit of propane/air in the cross-plate combustor can be raised by 0.4 m/s at equivalence ratio 0.7. It is also found that the cross-plate length can significantly affect the flame shape in the micro-channel and temperature distribution of the external wall. In contrast, the dimensionless plate length of 5/9 is suggested as the optimal structure parameter for the micro-combustor, which is due to the highest radiation efficiency.

Numerical investigations on different configurations of a four-channel meso-scale planar combustor fueled by hydrogen/air mixture

Multiple single-channel meso-scale combustors assembling into a multi-channel meso-scale planar combustor is a good solution for elevating the output power of the micro-thermophotovoltaic system. Consequently, how to assembly a multi-channel meso-scale planar combustor for achieving higher output power comes to be an interesting issue. In this work, four representative four-channel meso-scale planar combustors, namely, combustor A, combustor B, combustor C and combustor D are designed. Extensive numerical investigations are conducted to compare the wall temperature of the four combustors under various hydrogen mass flow rates, hydrogen/air equivalence ratios and solid materials. Two variables, namely, mean wall temperature and nonuniformity coefficient of wall temperature are defined for comparing the wall temperature of the four meso-scale combustors. It is found that the purely counterflow four-channel meso-scale planar combustor D has more uniform and higher wall temperature compared with other combustors. Furthermore, the detailed heat transfer mechanisms of four different four-channel meso-scale planar combustors are analyzed and presented. In addition, some critical values are proposed for keeping high and uniform wall temperature on the purely counterflow four-channel meso-scale planar combustor. This work offers us great reference value for the design of multi-channel meso-scale planar combustor.

Numerical investigations on an improved counterflow double-channel micro combustor fueled with hydrogen for enhancing thermal performance

In this work, an improved counterflow double-channel micro combustor is designed. Computational Fluid Dynamics software Fluent is used to conduct numerical investigations on the thermal performance comparison between the old and improved counterflow double-channel micro combustors. It is found that the improved counterflow double-channel micro combustor has much higher and more uniform wall temperature compared that of the old one under various hydrogen mass flow rates, hydrogen/air equivalence ratios and solid materials. To make a quantitative comparison, the main results are presented as follows: (a) The improved combustor achieves the largest thermal enhancement at the hydrogen mass flow rate of 5.25 × 10−7 kg/s when the hydrogen mass flow rate is ranged from 5.25 × 10−7 kg/s to 9.8245 × 10−7 kg/s. Namely, the mean temperature of upper and right wall is improved by about 9.66 K and 13.53 K, respectively, and the mean nonuniformity of upper and right wall temperature is reduced by about 23.24% and 26.79%, respectively. (b) The improved combustor achieves the largest thermal enhancement at the hydrogen/air equivalence ratio of 0.6, when the hydrogen/air equivalence ratio is ranged from 0.9 to 0.5. Namely, the mean temperature of upper and right wall is improved by about 27.55 K and 29.55 K, respectively, and the mean nonuniformity of upper and right wall temperature is reduced by about 23.24% and 19.51%, respectively. (c) The improved combustor achieves the largest thermal enhancement at the solid material of silicon carbide when the solid material is changed from quartz to silicon carbide. Namely, the mean temperature of upper and right wall is improved by about 16.77 K and 18.38 K, respectively, and the mean nonuniformity of upper and right wall temperature is reduced by about 26.47% and 28.41%, respectively. Finally, some guidelines are proposed for applications of the improved counterflow double-channel micro combustor in the micro-thermophotovoltaic system.

Experimental investigation on combustion characteristics of premixed propane/air in a micro-planar heat recirculation combustor

With respect to micro-thermophotovoltaic (MTPV) system, achieving a higher wall temperature and wider range of flame stability in micro-combustor is pivotal, which may be the most effective method to improve the work efficiency of the device. On this basis, an improved combustor with heat recirculation is designed and fabricated. Experimental investigation has been conducted to study the effects of equivalence ratio and volume flow rate on combustion characteristics of premixed propane/air. Results show that the radiation efficiencies of external wall in heat recirculation combustor are significantly higher than the corresponding value in straight-channel combustor under the same conditions. In the meanwhile, the maximum blowout limit in heat recirculation combustor can be more than three times as that of straight-channel combustor. Furthermore, the baffle length is a critical parameter of the micro-combustor, which can determine the residence time and reaction zone of the propane/air mixture. Accordingly, studies under different baffle length cases have been adopted to further explore the most suitable length in heat recirculation combustor. By contrast, the 15mm baffle length is the most advantageous, in which case the radiation efficiency of the external wall is highest.

Numerical investigations on thermal performance of double-layer four-channel micro combustors for micro-thermophotovoltaic system

In order to extend the output power of the micro-thermophotovoltaic system, it is necessary to assemble multiple single-channel micro combustors into a multiple-channel micro combustor. As a result, how to assemble multiple single-channel micro combustors into a multiple-channel micro combustor comes to be an interesting issue. In this work, the counterflow combustion concept is used in a double-layer four-channel micro combustor, and three representative double-layer four-channel micro combustors are designed. Extensive numerical investigations are conducted to compare the thermal performance of the three different double-layer four-channel micro combustors under various hydrogen mass flow rates, hydrogen/air equivalence ratios and solid materials. Moreover, the purely counterflow double-layer four-channel micro combustor is compared with the counterflow single-layer double-channel micro combustor in our previous work under various H2 mass flow rates, hydrogen/air equivalence ratios and solid materials. It is found that the purely counterflow double-layer four-channel micro combustor can keep more uniform wall temperature. And the purely counterflow double-layer four-channel micro combustor has better thermal performance compared with that of the counterflow single-layer double-channel micro combustor. Moreover, some critical values are given for keeping the merit of the purely counterflow double-layer four-channel micro combustor.

Numerical investigations on a comparison between counterflow and coflow double-channel micro combustors for micro-thermophotovoltaic system

A high and uniform wall temperature on micro combustors is desirable for Micro-TPV system application. In this work, extensive numerical investigations of H2/air premixed combustion in the coflow and counterflow double-channel micro combustors are conducted under various H2 mass flow rates, H2/air equivalence ratios and solid materials. Moreover, in order to characterize the uniformity of wall temperature, a nonuniformity coefficient RT,w is introduced. Results show that the mean outer wall temperature of the coflow double-channel micro combustor is slightly higher than that of the counterflow double-channel micro combustor and the outer wall temperature nonuniformity of the counterflow double-channel micro combustor is greatly lower than that of the coflow double-channel micro combustor, meaning that the counterflow double-channel micro combustor has better thermal performance than the coflow double-channel micro combustor. More importantly, some critical values about the H2 mass flow rate, H2/air equivalence ratio and solid material are determined for keeping the better thermal performance of the counterflow double-channel micro combustor. This design offers us another way for assembling the single-channel micro combustor into the double-channel micro combustor.

Numerical investigations on thermal performance of a micro-cylindrical combustor with gradually reduced wall thickness

Micro combustor is the key component of the micro-thermophotovoltaic (MTPV) system. A high and uniform wall temperature distribution on the micro combustor is helpful for obtaining higher energy conversion efficiency. In this work, a micro-cylindrical combustor with gradually reduced wall thickness is developed. The micro-cylindrical combustors with gradually reduced wall thickness are widely investigated and compared with the micro-cylindrical combustors with a step under various conditions. Results suggest that the micro-cylindrical combustors with gradually reduced wall thickness has a higher and more uniform wall temperature distribution under various H2/air equivalence ratios, H2 mass flow rates and inlet/outlet diameter ratios compared with that of the micro-cylindrical combustors with a step. However, it should be noted that effects of solid material on the outer wall temperature of the two micro-cylindrical combustors are interesting. The thermal performance of micro-cylindrical combustor with gradually reduced wall thickness is better under the solid material of quartz whose thermal conductivity is low, while the positive effects brought by the micro-cylindrical combustor with gradually reduced wall thickness disappear under the solid material of silicon carbide whose thermal conductivity is high.

Numerical investigations on an improved micro-cylindrical combustor with rectangular rib for enhancing heat transfer

A high and uniform wall temperature on micro combustors is desirable for micro-thermophotovoltaic system application. In this work, an improved micro-cylindrical combustor with rectangular rib is developed. Numerical investigations on the combustors with and without rectangular rib are conducted under various H2 mass flow rates and H2/air equivalence ratios. Moreover, the effects of dimensionless rib positions and heights on the outer wall temperature of micro combustors are also widely investigated. Results suggest that the improved combustor has a higher and more uniform outer wall temperature distribution under the same inlet condition compared with that of the micro combustor without rectangular rib. It is concluded that the inserted rectangular rib makes a recirculation region, enhancing the heat transfer between the combustor inner wall and burned gas. In addition, among all the investigated geometrical models, the geometrical model with the dimensionless rib position of 5/9 and dimensionless rib height of 0.4 has the highest and most uniform outer wall temperature where the H2 mass flow rate and H2/air equivalence ratio are fixed at 5.25×10−7kg/s and 1.0, respectively. More importantly, the optimum dimensionless rib position is related with the inlet velocity of H2/air mixture and it is increased with the inlet velocity of H2/air mixture.

Numerical investigation on the combustion characteristics of non-premixed hydrogen-air in a novel micro-combustor

A good mixing of fuel/oxygen is important for the non-premixed combustion in micro combustor, and the thermal performance of outer wall is of significant influence to the application of micro thermo photovoltaic (MTPV) systems. The non-premixed hydrogen/air combustion has been carried out in a novel micro combustor, in addition, the mixing performance of hydrogen/oxygen, the combustion characteristics, working performance of micro combustors are also investigated. Results show that the mixing performance of hydrogen/oxygen can be improved by the adoption of four pipes in combustor, and the thermal performance of combustor affects flame velocity, reaction rate and combustion efficiency. Moreover, the cavity or backward-facing step contributes to the heat recirculation and the flame stabilization, and a high and uniform outer wall temperature distribution is desirable for the application of MTPV system, which can be obtained at the hydrogen volume flow rate 90mL/min and the equivalence ratio 1.0.

Numerical study on a multiple-channel micro combustor for a micro-thermophotovoltaic system

As a key component, the design of a micro combustor is critical when determining the performance of a micro-thermophotovoltaic (TPV) system. This study proposes a multiple-channel micro combustor for a micro-TPV system. A three-dimensional computational fluid dynamics (CFD) model with a skeletal reaction mechanism embedded is established for premixed hydrogen/air combustion in the micro combustors. The numerical simulation results indicate that the temperature distribution along the outer wall of the multiple-channel micro combustor is more uniform and higher than that of the traditional single-channel micro combustor, which is highly beneficial for the micro-TPV system. Moreover, the radiation energy and radiation efficiency of the multiple-channel combustor are higher than those of the single-channel combustor at the same volume flow rate. When the flow rate is 100cm3/s and H2/air equivalence ratio is 0.8, the mean temperature and the wall temperature difference on the cross-sectional direction for the multiple-channel combustor are 1294.9 and 86.6K, respectively, whereas those for the single-channel combustor are 1256.0 and 107.6K, respectively. The radiation energy and radiation efficiency are 53.57W and 18.84% for the multiple-channel combustor, respectively, and 47.40W and 16.67% for the single-channel combustor, respectively. Moreover, the high equivalence ratio assignment for the side channels can further improve the temperature uniformity of the multiple-channel combustor. Results show that the wall temperature difference on the cross-sectional direction can decrease from 86.6K to 68.8K.

Numerical investigation of a novel micro combustor with double-cavity for micro-thermophotovoltaic system

The micro combustor is a key component of a micro-thermophotovoltaic (TPV) system. In this study, a novel double-cavity micro combustor is proposed for a micro-TPV system. The thermal performance of the double-cavity micro combustor is numerically investigated. We found that the new combustor can obtain higher and more uniform temperature distribution along the wall because of the second high temperature zone formed in the downstream cavity at relatively high inlet velocity, which is desirable for the micro-TPV system. When the inlet velocity is 12m/s and the H2/air equivalence ratio is 0.3, the usable radiation energy is increased from 2.13W for the single-cavity combustor to 2.59W for the double-cavity combustor. The corresponding radiation efficiency increases from 1.25% to 1.53%. Meanwhile, the combustion characteristics of the two micro combustors are studied under different H2/air equivalence ratios at a fixed inlet velocity. The usable radiation energy and radiation efficiency of the double-cavity combustor are higher than that of the single-cavity combustor, and both values increased by increasing equivalence ratio. Furthermore, the influence of cavity gap length between the two cavities is examined at different inlet velocities. Results show that the optimum cavity gap length is determined by inlet velocity.

Effects of H2/CO blend ratio on radiated power of micro combustor/emitter

This study presents the effects of adding CO to H2 as a blend fuel on the radiated power and efficiency of a micro cylindrical emitter. The effects of emitter length and emitter wall thickness on the radiation energy and emitter efficiency are also examined numerically. The results show that the region with high radiated power appears far away from the emitter inlet with the increase of CO addition. Generally, the emitter efficiency decreases with the increase of CO addition. In the case of 2 m/s flow velocity, the emitter efficiency decreases slightly when the CO mass fraction increases from 5% to 10%. The emitter efficiency decreases with the increase of H2/CO/air mixture mass flow rate. In addition, the highest emitter efficiency is obtained when the tube length is 20 mm and wall thickness is 0.4 mm. The H2/CO could be a promising blend fuel for micro-thermophotovoltaic (TPV) system application.

Combustion characteristics and performance evaluation of premixed methane/air with hydrogen addition in a micro-planar combustor

Combustion performance of premixed methane/air with hydrogen addition in a micro-planar combustor is numerically investigated, aiming to extend the application of various fuels in a micro-thermophotovoltaic system. The simulation results show that hydrogen addition has significant effect on raising methane reaction rate and flame stability. With the increase of hydrogen mass fraction, the location of the flame shifts toward the combustor inlet gradually, and the temperature gets a steady growth. For the micro-combustor with a channel height of 3mm, it is found that the flame of pure methane is far away from inlet, which does not meet the basic working requirement of the micro-thermophotovoltaic system. However, the combustor wall temperature will be significantly improved when a small amount of hydrogen is added into the mixture, which is mainly due to the extension of flammability range. Along with the increase of H2 addition ratio, both the total radiant energy and the proportion of usable radiant energy to total input chemical energy increase dramatically, which brings a significant improving of electricity output and system efficiency.

Dynamic behaviors of premixed hydrogen–air flames in a planar micro-combustor filled with porous medium

The micro-combustor is a key component of the micro-thermophotovoltaic (TPV) system, and the flame stability limits are very important to its design and operation. An experimental study on the dynamic behaviors of premixed hydrogen–air flames in an H=1mm planar micro-combustor is undertaken. The combustor is partially filled with porous medium to enhance heat transfer and flame stabilities. Critical conditions (Φ1 for blow-off, Φ2 for flashback, and Φ3 for breaking through the porous medium) are experimentally identified for different combustor configurations. It is shown that the blow-off limits are nearly independent of the combustor configuration, while the flashback limits are strongly affected by the combustor configuration as well as the parameters of the inserted porous medium, including the position (Lout), the width (W) and the porosity (ε) of the porous medium. Most of the results can be explained by the classic boundary velocity gradient theory by von Elbe and Lewis in a qualitative sense. Based on the experimental findings, widening the operation window of the planar micro-combustor partially filled with porous medium is possible when the parameters are chosen properly.

Hyperlens makes thermal emission strongly super-Planckian

We suggest and theoretically explore a possibility to strongly enhance thermal radiation of hot bodies using an infrared hyperlens. The hyperbolic metamaterial of the hyperlens converts emitter's near fields into propagating waves which are efficiently irradiated from the hyperlens surface. Thus, with the hyperlens, emitter's spectral radiance goes well beyond the black-body limit for the same emitter in free space. Although the hyperlens can be kept at a much lower temperature than the emitter, the whole structure may radiate, in principle, as efficiently as a black body with the same size as that of the hyperlens and the same temperature as that of the emitter. We believe that this study can lead to a breakthrough in radiative cooling at microscale, which is crucial for microlasers and microthermophotovoltaic systems.