- Email: [email protected]
Mesoscale combustor-powered thermoelectric generator with enhanced heat collection
Energy Conversion and Management 205 (2020) 112403
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Mesoscale combustor-powered thermoelectric generator with enhanced heat collection
T
⁎
Guoneng Li , Dongya Zhu, Youqu Zheng, Wenwen Guo Department of Energy and Environment System Engineering, Zhejiang University of Science and Technology, Hangzhou 310023, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermoelectric generator Enhanced heat collection Overall efficiency System effectiveness
Small-scale power sources for portable electronic equipment have been attracting considerable attention in recent years. This paper presents the design and experimental tests of a mesoscale combustor-powered thermoelectric (TE) generator (TEG) with enhanced heat collection. The TE effect, combustion, heat collection efficiencies, and lean blowout (LBO) are investigated in detail. A parameter named system effectiveness (EFS), which is the ratio of overall efficiency to TE efficiency, is proposed to evaluate the performance of combustorpowered TEGs. The obtained electric power of 25.7 W, with an overall efficiency of 2.69%, is higher than previous results. The corresponding hot-end temperature changes from 236 °C to 152 °C along with the flue gas direction, indicating noticeable temperature variation despite the use of pure copper. EFS reaches 81.8%, which is higher than previous reports, and validates that the present TEG is well-designed. The LBO of the combustor is 0.63, and the pressure drop is 463 Pa when the input power is 957 W. Detailed discussions and comparisons with previous studies reveal that the enhanced heat collection design is essential for increasing the residence time of flue gases and EFS.
1. Introduction The development of portable small-scale power sources remains a challenge. Batteries with energy densities that are generally lower than 300 Wh/kg are unsuitable for many applications. Furthermore, the long recharging times and pollution problems of batteries are difficult to solve [1]. Thus, alternatives are continuously being developed. Hydrocarbons whose energy densities are hundreds of times those of batteries exhibit a considerable potential as small-scale power sources with the availability of advanced fuel-to-electricity technologies, such as micro gas turbines, micro fuel cells, and thermoelectric (TE) generators (TEGs). Although micro gas turbines and fuel cells perform efficiently, their maintenance hinders their application [2]. These problems include high-speed rotating parts, high-temperature bearings, and noise for micro gas turbines [3]; and humidification, hydrogen storage, and safety for fuel cells [4]. TEGs, which have been widely adopted in many applications and reviewed comprehensively [5], can be developed in portable form with hydrocarbons, using micro-/mesoscale combustors. TEGs are straightforward systems that require low maintenance because of their small moving parts. Material revolution has accelerated in the past few years, thereby creating excellent TE materials [6]. Thus, TEGs are promising. A micro-/mesoscale combustor-powered TEG (MCP-TEG) is ⁎
presented in this work. The working principle of TEGs is based on the Seebeck effect; that is, a temperature difference forces electrons to move in one direction between two metals or semiconductors, resulting in a continuous current through a complete circuit. The applications of MCP-TEGs include portable power sources, microelectronics, biomechanics and microfabrications. A literature review on MCP-TEGs is provided in the following section. Vican et al. [7] proposed an MCP-TEG in 2002. Their concept attracted considerable attention although the electric power was limited to 0.052 W. A group of scientists from the University of Delaware presented several prototypes of MCP-TEGs for military applications. The maximum electric power was approximately 1 W [8,9], and the overall efficiency was 1.1% [10]. Yoshida et al. [11] developed an MCP-TEG with an overall efficiency of 2.8% and limited electric power (0.185 W). The maximum electric power of Jiang’s MCP-TEG was augmented to 2 W [12], and the overall efficiency was 1.25%. Marton et al. [13] presented an MCP-TEG prototype that generated an electric power of 5.82 W and a relatively high overall efficiency of 2.53%. Hsu et al. [14] reported that an electricity power of 9.4 W was generated with their MCP-TEG and that the corresponding overall efficiency was 2.85%. Shimokuri et al. [15] established new records with their MCPTEGs, whose electric power and overall efficiency were 18.1 W and 3.01%, respectively; these values were the optimized results of their
Corresponding author. E-mail address: [email protected] (G. Li).
https://doi.org/10.1016/j.enconman.2019.112403 Received 25 September 2019; Received in revised form 10 December 2019; Accepted 11 December 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
Nomenclature A Aflue cp h I k L Lflue mburnt mfuel n P Pconv Pflue Pin Pld Pmax Prad PTE QAir QCH4 Qflue r Rld T Tatm Tave Tc Tf Th
Tin Tsurf Tout ΔP ΔT U w Z α ρ φ ε τ σ ηfuel ηheat ηsys ηTE
total area exposed to the atmosphere (m2) cross-sectional area of flue gas channel (m2) heat capacity (kJ/kg-K) convective heat transfer coefficient (W/m2-K) current (A) thermal conductivity (W/m-K) length of TE leg (mm) length of flue gas channel (m) combusted mass flow rate (kg/s) fuel mass flow rate (kg/s) electrical resistivity ratio (mm) electric power (W) heat loss rate through convection (W) heat flow rate of flue gases (W) input power (W) load power (W) maximum electric power (W) heat loss rate through thermal radiation (W) heat flow rate through TE modules (W) air flow rate (LPM) methane flow rate (LPM) volume flow rate (m3/s) thermal contact ratio (dimensionless) load resistance (Ω) temperature (°C) atmosphere temperature (°C) average temperature, (Th + Tc)/2 (°C) cold-end temperature (°C) combustion temperature (°C) hot-end temperature (°C)
inlet air temperature (°C) Surface temperature (°C) flue gas temperature (°C) pressure loss (Pa) temperature difference, ΔT = Th–Tc (°C) voltage (V) ratio of ceramic thickness to TE leg (dimensionless) TE figure-of-merit (1/K) Seebeck coefficient (V/K) electrical resistivity (Ωm) equivalent ratio (dimensionless) emissivity (dimensionless) residence time (ms) Stefan-Boltzmann constant (W/m2K4) combustion efficiency (%) heat collection efficiency (%) overall efficiency (%) TE efficiency (%)
Abbreviations DAQ data acquisition EFS system effectiveness HEX heat exchanger IR infrared MFC mass flow rate controller LBO lean blowout LPM liter per minute SPRF stagnation point reverse flow TE thermoelectric TEG TE generator MCP-TEG micro-/mesoscale combustor-powered TEG
considerably high overall efficiency of 2.5% could be reached in their air-cooled MCP-TEGs [20]. The overall efficiency of Singh’s MCP-TEG was only 1.42% [21], although fuel combustion was approximately complete. Several researchers from the Consiglio Nazionale delle
previous prototype [16]. A team from the Indian Institute of Technology Bombay designed several prototypes. They claimed that high overall efficiencies (4.6% in Ref. [17], 4.03% in Ref. [18], and 4.66% in Ref. [19]) could be obtained in their water-cooled MCP-TEGs and that a Table 1 Performance comparison of various MCP-TEGs. Authors
J. Vican, et al. [7] D.G. Norton, et al. [8] J.A. Federici, et al. [9] K. Yoshida, et al. [11] A.M. Karim, et al. [10] L.Q. Jiang, et al. [12] C.H. Marton, et al. [13] Y. Hsu, et al. [14] D. Shimokuri, et al. [16] S. Yadav, et al. [17] T. Singh, et al. [21] L. Merotto, et al. [22] D. Shimokuri, et al. [15] H. Abedi, et al. [24] B. Aravind, et al. [18] B. Aravind, et al. [19] C. Fanciulli, et al. [23] B. Aravind, et al. [20] H. Abedi, et al. [25] B.R. Guggilla, et al. [26] Present
Year
2002 2004 2006 2006 2008 2011 2011 2012 2015 2015 2016 2016 2017 2017 2018 2018 2018 2019 2019 2019 2019
Th
115.5 – 170 125 – 200 312 475 156 250 197 258 200 168 130 150 160 221 220 140 152–236
ΔT
92.5 253 85 70 160 130 266 400 126 – 88 200 165 150 99 115 128 124 200 62 105–188
Efficiency (%)
Pmax (W)
ηheat
ηsys
ηfuel
– – 19.3 30–44 – – – 41 – – – – 75.3 – 34.4 33.5 94 25.7 – – 87.2
0.57 0.85 0.51 2.8 1.1 1.25 2.53 2.85 2.23 4.6 1.42 2.36 3.01 2.27 4.03 4.66 1.1 2.5 2.5 0.1 2.69
100 100 100 99.5 100 100 100 99 100 – 100 96 86.9 96.4 – – 56 – 53 70 93.5
0.052 ~1 0.45 0.185 0.65 2 5.82 9.4 4.76 2.35 3.54 9.86 18.1 5.92 3.89 4.52 0.84 2.4 4.31 0.49 25.7
Reaction
TE
type†
material
CTC CTC CTC CTC CTC DTC CTC CTC DTC DTC DTC CTC DTC CTC DTC DTC CTC DTC CTC CTC DTC
Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3/PbTe Bi2Te3 Bi2Te3 Bi2Te3 Chalcogenides Bi2Te3 Chalcogenides Bi2Te3 Bi2Te3 Chalcogenides Bi2Te3 Chalcogenides Bi2Te3 Bi2Te3
Notes
Case: H2/air mixture at φ = 1.0 Case: 1.4 SLPM *Data from different cases
ηsys = 2.36% at Pmax = 8.1 W Case: 250 W input power ηsys = 2.85% at Pmax = 6.78 W
*Data from different cases *Data from different cases ηsys = 1.4% at Pmax = 0.6 W Case: 10 m/s, Fin + Fan at 3000 rpm ηsys = 3.4% at 1.2 Nl/min gas flow rate
– Denotes “not-found” or “not-studied”. *Denotes that a complete set of data cannot be found. Data from close cases were listed for comparison and discussion. Certain uncertainties may exist. †CTC and DTC denote catalytic-combustion and direct-combustion, respectively. 2
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
combustion, which is beyond the present non-premixed combustion. Nevertheless, Swiss-roll combustors can be integrated in MCP-TEGs provided that appropriate modifications are performed for non-premixed combustion. The cross-sectional dimensions of a flue gas channel, that is, 3 mm (width) × 20 mm (thickness), are designed such that the total area of flue gas channels is four times the total inlet areas of the air and fuel. This design helps minimize pressure drop and increase the residence time of flue gases. Three TE modules are sandwiched between the combustor and the multichannel water-cooled heat sink. Six TE modules and two watercooled heat sinks are assembled in the MCP-TEG, and all TE modules are wired in series. Commercially available Bi2Te3-based TE module (SAGREON Co., Ltd., China) measuring 40 mm (length) × 40 mm (width) × 3.3 mm (thickness) is selected. According to datasheets
Ricerche of Italy reported a 9.8 W maximum electric power [22] and an overall efficiency that varied from 1.1% [23] to 2.27% [24], 2.36% [22], and 3.4% [25]. Recently, Guggilla et al. [26] from Rowan University proposed an MCP-TEG and produced notable results despite yielding a limited overall efficiency. The detail characteristics and comparisons of abovementioned MCP-TEGs are presented chronologically in Table 1. Th, ΔT, ηheat, ηsys, ηfuel, and Pmax are the hot-end temperature, temperature difference, heat collection efficiency, overall efficiency, combustion efficiency, and the maximum electric power, respectively. Numerical studies on MCP-TEGs are also important because they show that the combination of thermo-photovoltaic and TEG is an interesting optimization direction [27] and that filtration combustion is another approach to improve TEG performance [28]. These works provided valuable results, but they employed limited TE modules, indicating that the flue gas temperature from micro-/mesoscale combustors is substantial and thus has potential to be utilized. The electric powers of the above MCP-TEGs are relatively low and limit the practicality of these equipments. In the current work, an MCPTEG with enhanced heat collection was designed and evaluated in terms of electric power and overall efficiency. Unlike previous generators, the present MCP-TEG offers a concrete method to augment electric power while maintaining a relatively high overall efficiency. The detailed comparisons of the experimental results with those from previous studies (Refs. [7–26] in Table 1) revealed that a new parameter, which is named system effectiveness (EFS) and comprehensively combines combustion completeness, heat collection, and TE conversion, should be used to evaluate the performance of MCP-TEGs. Finally, new insights are provided for the research community.
(a)
2. TEG configuration Fig. 1 shows the configuration of the proposed MCP-TEG. Fig. 1(a) and 1(b) shows a photograph and the schematic of the TEG. It comprises a mesoscale combustor, six TE modules, two water-cooled heat sinks, and two clamping plates. Two copper plates with multiple notch grooves are assembled to form the combustor, as shown in Fig. 1(c). These grooves are designed to form a stagnation-point reverse-flow (SPRF) combustor and a built-in heat exchanger (HEX) for absorbing heat from flue gases. SPRF combustors have a stable operation at lean fuel–air mixtures and have low nitrogen oxide emissions [29]. The combustor, which has overall dimensions of 120 mm (length) × 40 mm (width) × 24 mm (thickness), is designed in accordance with the dimensions of three TE modules on each side. The combustion chamber is in the middle of the two copper plates. As shown in Fig. 1(c), fuel and air are supplied separately, and partially mixed before being injected into the combustion chamber. Fuel–air mixtures are combusted in the chamber, which measures 22 mm (length) × 20 mm (width) × 20 mm (thickness). Flue gases are divided into two streams (left and right). Two other streams are formed for each stream, thereby increasing the residence time of flue gases. The cross sectional dimensions of the flue gas channels are 3 mm (width) × 20 mm (thickness). Given the gas expansion due to the combustion, the residence time of flue gases is estimated to be greater than 65 ms when the input power is less than 1000 W. The dimensions of the combustion chamber are set on the basis of a volumetric heat load of 7.2 × 107 W/m3, which is obtained in accordance with a previous work on an SPRF combustor [29] (case: equivalent ratio of 1.0 and reaction zone length of 80 mm). However, the volumetric heat load in the present work is set as 11.4 × 107 W/m3 because approximately one-third of the heat input is absorbed immediately through the walls of the combustion chamber. Many studies have demonstrated the superior performance of Swiss-roll combustors [30], which were proposed by Lloyd et al. in 1974 [31]. A Swiss-roll combustor inside a spiral HEX, which is similar to the present serpentine design, can serve as the combustor of an MCP-TEG. However, it features a super-adiabatic flame temperature and effective premixed
(b)
(c)
Fig. 1. TEG configuration. (a) Photograph of TEG. (b) Schematic of TEG. (c) Mesoscale SPRF combustor design and built-in HEX. 3
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
provided by the manufacturer, the TE module can typically generate 8.2 W of electric power at a temperature difference of 200 °C. This paper also presents the temperature-dependent properties of the TE material (Appendix A) that are used to theoretically calculate TE efficiency.
Table 2 Experimental cases.
3. Experimental setup Fig. 2 presents the experimental setup. Fuel and air were supplied with two Alicat mass flow controllers (MFCs). The accuracy of the MFCs was ± 0.8% of reading plus 1% of the full scale. Flue gas emissions (CO2, CO, NO) were measured with a Testo 340 gas analyzer. The accuracies of the CO2, CO, and NO measurements were 0.2 vol%, 10 vol %, and 5 ppm, respectively. Seven thermocouples with an accuracy of 0.5% were installed to measure the combustion, hot-end, cold-end, air inlet, and flue gas exit temperatures. The temperature signals were recorded with an Agilent-34970A data-acquisition (DAQ) instrument combined with a BenchLink data logger program. A CHY-130 differential pressure transducer, which had an accuracy of 0.5%, was installed to measure the pressure drop of the combustor. A Dali T8 infrared (IR) imager with 25 μm resolution was used to observe the temperature distribution of the entire MCP-TEG and to estimate the convection and thermal radiation heat losses. The IR camera was calibrated with type K thermocouples, and detailed information can be found in Appendix B. The power load feature was measured with a Prodigit 3311F electronic load, the measurement accuracy of which was ± 0.5%. Methane was utilized as fuel. The input power was set to 957 W, which corresponded to a methane mass flow rate of 1.6 L/min (LPM). Equivalent ratios of 0.9 and 1.0 were selected for this input power. For LBO, the air flow was fixed at 15.2 LPM, and the methane flow rate was gradually decreased. A water pump, a radiator, and three blowers were used to dissipate heat from the cold-end of the TE modules, thus maintaining a low cold-end temperature. The working conditions for the water pump and blowers were constant during the experiments. The water pump worked at 7.9 V and consumed 2.45 W of electricity. All blowers ran at 12 V and consumed 7.32 W of total electric power. Over 50 experimental cases were conducted, and they are listed in Table 2. QCH4, QAir, φ, Pin, and Rld are the methane flow rate, air flow rate, equivalent ratio, input power, and the load resistance, respectively. The total differential method was applied to estimate the uncertainty for the overall efficiency with Eq. (1), which was also used in our previous work [32]. The uncertainty for the overall efficiency was estimated to
No.
QCH4 (LPM)
QAir (LPM)
φ
Pin (W)
Rld (Ω)
1 2 3 4 5 6 7 8
1.6 1.6 1.5 1.4 1.3 1.2 1.1 1.0
16.8 15.2 15.2 15.2 15.2 15.2 15.2 15.2
0.9 1.0 0.94 0.88 0.81 0.75 0.69 0.63
957 957 897 837 777 717 658 598
5–150 5–150 23, 25, 23, 25, 23, 25, 23, 25, 23, 25, 23, 25,
δQCH 4 =
δU δI δQCH 4 + + U I QCH 4
27 27 27 27 27 27
be 5.6% at a 95% confidence level.
δηsys ηsys
= =
∂ηsys ∂P ∂ηsys ∂U
δP + δU +
∂ηsys ∂QCH 4 ∂ηsys ∂I
δQCH 4
δI +
∂ηsys ∂QCH 4
(1) 4. Data reduction The input power might not be fully available because fuel could not be completely combusted. Thus, combustion efficiency is an important parameter used to evaluate the performance of MCP-TEGs, and it is defined as follows:
ηfuel =
mburnt mfuel
(2)
where mburnt and mfuel are the combusted fuel (CH4) mass flow rate and fuel mass flow rate, respectively. mburnt can be calculated by measuring the CO2 concentration in the flue gas without consideration for CO concentration. This assumption is justified because the CO concentration is low (less than 3000 ppm). During the experiments, the equivalent ratio was used to control the combustion of methane; it is defined as follows,
φ=
( ) ( )
Fuel Air actual Fuel Air stoichiome tric
= 9.52 ×
QCH 4 Q Air
(3)
Heat is released through combustion, and part of this heat can be collected and passes through the TE modules. The heat collection
Fig. 2. Sketch of experimental setup. 4
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
ηsys = ηfuel ηheat ηTE
efficiency is defined as follows:
ηheat
PTE = ηfuel Pin
EFS, a parameter that could be used to evaluate the performance of an MCP-TEG, is expressed as follows:
(4)
where Pin is the input power calculated by multiplying the mass flow rate of methane and the corresponding lower heat value. PTE is the heat flow rate passing through the TE modules. The following equation is used to obtain PTE:
PTE = ηfuel Pin − Pflue − Pconv − Prad
EFS =
ηsys =
Pmax PTE
(5)
(6)
ηTE
(9)
−1
ηTE =
Pmax Pin
ηsys
This parameter presents the relationship between overall efficiency and TE efficiency. EFS indicates how closely an MCP-TEG running to the ideal working state (complete combustion; no heat loss though convection, radiation, and flue gases). Heat recovery efficiency is the ratio of recovered heat to exhausted heat [33], which has been studied widely in ventilation engineering. It has the same meaning as that of heat collection efficiency in the present work. For an MCP-TEG, EFS is the product of heat collection efficiency and combustion (reaction) efficiency. Therefore, EFS measures the design level of an MCP-TEG. Heat collection and combustion (reaction) completeness should be balanced to enhance the performance of an MCP-TEG. The TE efficiency can be theoretically predicted as follows [34]:
where Pflue, Pconv, and Prad are the heat loss rate in the flue gas, heat loss rate through convection, and heat loss rate through thermal radiation, respectively. The calculation of the heat flow rate of flue gases and the heat loss rates through natural convection and thermal radiation, are straightforward, and they are listed in Appendix B. The TE modules convert part of PTE to electricity. TE efficiency and overall efficiency can be defined as follows:
ηTE =
(8)
Th − Tc ⎧ T − Tc ⎞ ⎛ 4 ⎞ ⎛ 1 + n/ L ⎞ ⎤ ⎫ + (1 + 2rw )2 ⎡2 − 0.5 ⎛ h ⎢ Th ⎨ ⎝ Th ⎠ ⎝ ZTh ⎠ ⎝ 1 + 2rw ⎠ ⎥ ⎦⎬ ⎣ ⎭ ⎩ ⎜
⎟
⎜
⎟
(7)
(10)
Pmax represents the maximum electric power generated by the MCPTEG. As revealed in Eqs. (6) and (7), the overall efficiency must be smaller than the TE efficiency, and their relationship can be expressed as follows:
where Th and Tc are the hot and cold-end temperatures, respectively. The figure-of-merit Z is defined as α2/kρ, where α, k and ρ are the Seebeck coefficient, thermal conductivity, and electrical resistivity, respectively. r, w, n, and L are the thermal contact ratio, ratio of ceramic
(c)
Fig. 3. (a) Hot and cold-end temperatures under various load resistances when Pin = 957 W. (b) Temperature measuring locations. (c) IR image at φ = 0.9 when Pin = 957 W. 5
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
work.
thickness to the length of the TE leg, electrical resistivity ratio, and the length of the TE leg, respectively. r = 0.2, w = 0.516, n = 0.1 mm, and L = 1.5 mm. ZTave in the present experimental cases varied between 0.86 and 0.88, where Tave = 0.5(Th + Tc). The residence time (τ) of flue gases is estimated as follows:
τ=
5.2. Power load feature Fig. 4 presents the power load feature when the input power was 957 W. A maximum electric power of 25.7 W, which is greater than previous results, was obtained at 25 Ω when φ = 0.9 but decreased by 4.1% when the equivalent ratio increased to 1.0. This condition was caused by the lower hot-end temperatures at φ = 1.0 compared with those at φ = 0.9. The average electric power output for each TE module was 4.28 W. This value is smaller than those reported in Shimokuri’s [15] and Merotto’s works [22]. The reason is that the TE modules responsible for heat recovery worked at lower hot-end temperatures compared with the TE modules attached to the combustion chamber. As shown in Fig. 4(b), the load voltage increased with load resistance, and the load current contrarily varied. The intersection of the voltage and current curves was the optimized working condition, under which a maximum electric power could be obtained.
1000Aflue Lflue Qflue
(11)
where Qflue, Aflue, and Lflue are the volume flow rate of flue gases, cross sectional area of the flue gas channel, and the flue gas channel length, respectively. 5. Results and discussions 5.1. Temperature The hot and cold-end temperatures, their measuring locations and an IR image are shown in Fig. 3. The cold-end temperatures were close to each other; therefore, the average cold-end temperature was adopted. As shown in Fig. 3, the hot-end temperature noticeably decreased along the direction of the flue gases although materials with high thermal conductivity (copper) were used to construct the combustor. For example, the hot-end temperature for the middle TE modules reached 236 °C and decreased to 152 °C for the nearby TE modules. The hot-end temperatures slightly decreased with a decrease in load resistance. This phenomenon was caused by the well-known Peltier effect; that is, more heat was pumped from the hot-end to the cold-end as the electric current increased. The average hot-end temperatures at φ = 0.9 were approximately 9.6 °C higher than those at φ = 1.0 under the same input power due to differences in combustion efficiencies, which are discussed in Section 5.3. The temperature measurements obtained using the thermocouples and the IR imager at φ = 0.9 revealed that the heat flow rate of flue gases, and the heat loss rates through natural convection, and thermal radiation were 92.0, 14.0 and 8.9 W, respectively. A natural convection heat transfer coefficient of 10 W/m2-K and an emissivity of 0.4 were assumed for all surfaces during the above estimations. Given the magnitudes of convection and radiation heat losses, the differences between the abovementioned assumptions and their actual values had only a minor influence on the conclusions (see Appendix B for detailed information). The exhausted gas temperature was 199.4 °C at φ = 0.9 when Pin = 957 W; it was only 47.4 °C higher than the hot-end temperature (152 °C) of the TE module near the flue gas exit. This finding revealed that the heat flux of the exhausted gas was only 9.61% of the input power (957 W). Assuming that adding two other TE modules could capture 80% of the heat flux from the exhausted gas, the increased electric power will not exceed 2.0 W, which is not profitable enough. Therefore, adding TE modules was not necessary, and six TE modules were selected in the present
5.3. Efficiency The TE, combustion, heat collection efficiencies, and the EFS are shown in Fig. 5. An overall efficiency of 2.69% was obtained for the present MCP-TEG at φ = 0.9 when the input power was 957 W. The corresponding TE efficiency was measured to be 3.29%, which was consistent with the 3.44% predicted with Eq. (10). The experimental TE efficiency reached 95.6% of the theoretical prediction, which indicated that the TE modules employed in the present study worked normally. As shown in Fig. 5(b), the combustion efficiency at φ = 0.9 was 4.28% higher than that at φ = 1.0, whereas the heat collection efficiencies of these two cases were comparable. Therefore, the overall efficiency at φ = 0.9 was 4.66% higher than that at φ = 1.0. An essential issue behind the overall efficiency was the gap between the overall efficiency and TE efficiency, which was controlled by Eq. (8). The limit of the overall efficiency was the TE efficiency, which was controlled by the ZT value and working temperatures. The ZT value was fixed for a particular TE material working at a selected temperature range. Combustion and heat collection efficiencies should be optimized to narrow the gap between the overall efficiency and TE efficiency. The two efficiencies are shown in Fig. 5(b); the combustion and heat collection efficiencies reached 93.5% and 87.2%, respectively. The combustion of hydrocarbons has been extensively investigated, and a combustion efficiency exceeding 90% can be expected with minimal effort. The measured combustion efficiency of 93.5% is consistent with those in previous studies. The obtained heat collection efficiency was fairly high at 87.2% due to the serpentine channels that recover heat from flue gases. Detailed discussions related to heat collection are provided in Section 5.5. The EFS in Eq. (9) measures the gap between
Fig. 4. Power load test results when Pin = 957 W. (a) Load power. (b) Voltage and current. 6
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
Fig. 5. Efficiencies when the input power and equivalent ratio were set to 957 W and 0.9, respectively. (a) Comparison of overall, experimental TE, and predicted TE efficiencies. (b) Comparison of combustion efficiency, heat collection efficiency, and EFS.
when the equivalent ratio was between 0.8 and 1.0. For the equivalent ratio less than 0.8, the overall efficiency gradually decreased from 2.5% at φ = 0.8 to 1.9% at φ = 0.63. Notably, the present MCP-TEG had a wide operation range, and the load electric power and the aging of TE modules were balanced, as shown in Fig. 7. Concerning the bias error of the results mentioned in Figs. 3–7, T, P, U, I, and ΔP are directly measured parameters. Among the derived parameters (i.e. ηfuel, ηTE, ηheat, and ηsys), however, ηfuel was directly derived from the results of gas analyzer; ηTE was verified by the predicted result with Eq. (10) and Appendix A; ηheat was carefully examined with Eq. (5) and Appendix B. ηsys, the accuracy of which was estimated with Eq. (1), was thoroughly considered and emphasized in the discussion. Thus, the results reported in the present work are valid.
the overall efficiency and the TE efficiency, and it is the sole parameter to pursue when the TE material and working temperatures are fixed. As shown in Fig. 5 (b), the overall efficiency reached 81.8% of the experimental TE efficiency, indicating that the present MCP-TEG is welldesigned. However, room for further improvement remains, and an EFS of 90% should be targeted in future works. The heat absorbing area can be increased, and materials with high thermal conductivity can be used to construct a combustor to improve EFS. The overall and TE efficiencies presented in Fig. 5 are based on the maximum electric power output. Thus, the TE efficiency may not be the maximum efficiency of the TE module because of the temperature dependent properties. Nevertheless, maximum electric power output was targeted in the present work. For a TEG running under a constant heat flux mode (the MCP-TEG in this study), the maximum electric power and maximum efficiency can be obtained simultaneously [35], but an advanced electric power management system should be incorporated.
5.5. Discussion Overall efficiency is an essential parameter used to evaluate MCPTEGs. The literature review showed that only a team from the Indian Institute of Technology Bombay obtained an overall efficiency greater than 4.0% (4.6% in Ref. [17], 4.03% in Ref. [18], and 4.66% in Ref. [19]) for water-cooled MCP-TEGs. However, the heat collection efficiencies in the abovementioned studies were less than 40%, implying that the TE efficiencies in these studies should be larger than 10%. Thus, these values should be evaluated because they were for Bi2Te3based TE modules. The 2.69% overall efficiency in the present work is consistent with those in several previous studies (3.01% in Ref. [15],
5.4. LBO and pressure drop Flame stability and pressure drop are important factors used to characterize the performance of a micro-/mesoscale combustor. As shown in Fig. 6, the LBO of the SPRF combustor was 0.63, which indicated that the combustor operated with lean mixtures. Furthermore, the combustor operated at high input powers (large fuel flow rate), and the hot-end temperature exceeded the long-term working temperature of the TE module. Thus, only the experimental results at 957 W are shown in Fig. 6. Meanwhile, the combustion temperature in Fig. 6 represented the temperature at the central point of the combustion chamber only rather than the exact flame temperature. The actual flame front could be propagated to the upstream or downstream zones. As shown in Fig. 6, the combustion temperature decreased with the reduction in equivalent ratio, because of the decrease in the fuel injected into the combustor. During this process, the pressure drop was slightly affected; it decreased from 463 Pa at φ = 1.0 to 433 Pa at φ = 0.63. This minor influence was because the mass flow rate of air was maintained at 15.2 LPM, while the mass flow rate of methane varied from 1.6 LMP at φ = 1.0 to 1.0 LPM at φ = 0.63. This finding indicated that the mass flow rate of methane occupied only a small part of the total flow rate. The pressure drop of 463 Pa in the present MCPTEG is greater than the 246 Pa in Yoshida’s work [11] and 130 Pa in Marton’s work [13]. This condition was because the present MCP-TEG had longer flue gas channels than those in the abovementioned studies. Fig. 7 presents the hot-end temperature, cold-end temperature, load electric power and corresponding overall efficiencies. The hot-end temperature remarkably decreased with the decrease in equivalent ratio. This condition was because the input power decreased from 957 W to 598 W. The load electric power decreased from 24.3 W to 11.1 W. However, the overall efficiency remained larger than 2.5%
Fig. 6. Combustion temperature and pressure drop under various equivalent ratios when Qair = 15.2 LPM. 7
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
Table 3 Comparison of residence times in current and previous studies. Authors
Pin (W)
τ (ms)
J. Vican, et al. [7] K. Yoshida, et al. [11] D. Shimokuri, et al. [15] D. Shimokuri, et al. [16] Present
9.1 3.5 600 213 957
22.1* 55 less than30 less than10 66.7
*Results at 188 cm3/min.
which is higher than those in previous studies. An effective method for enhancing the heat collection efficiency of MCP-TEGs is to increase the residence time of flue gases. Table 3 compares the present residence time with those in previous studies. Few previous studies focused on residence time (Table 3). In the current study, the enhanced heat collection design (serpentine channels) prolonged the residence time. The residence time was calculated with Eq. (11). Methane is combusted such that no volume is created or lost before and after the reaction. Therefore, only the gas expansion due to temperature variation should be considered when calculating the volumetric flow rate of flue gas relative to those of reactants (air and fuel). The combustion and exhausted gas temperatures were 727 °C (Fig. 6) and 199.4 °C, respectively, when Pin = 957 W. Therefore, the averaged volumetric flow rate of flue gas was 6.92 × 10−4 m3/s. The crosssectional area of the flue gas was 2.4 × 10−4 m2, and the flue gas channel length was 0.1923 m. As a result, the residence time was 66.7 ms at the high input power of 957 W. However, the disadvantage of these serpentine channels was the pressure drop, which should be improved (although only ~450 Pa was found). The residence time is an indicator that characterizes the lifetime of reactant species from combustion to departure (from the exit). Therefore, it is critical for improving heat collection efficiency, which is directly linked to EFS. The electric powers presented in this work were under steady state conditions. The transient response during the addition of load to the MCP-TEG was not studied in this work. A previous work revealed this response is typical of a first-order system with temperature difference [36]. The reasons are the Peltier effect and the thermal inertia of the MCP-TEG [37]. When the load resistance is too small, a DC-DC converter cannot maintain a rated output voltage [38]. The load resistance ratio should be larger 1.0 (because of the thermal and electrical contact effects) to run the TEG under the optimum working condition for maximum electric power output [39]. Therefore, an advanced electric power management system needs to be developed for the field applications of MCP-TEGs. However, this task is beyond the scope of the present work. A previous work [40] showed that a cost-efficiency tradeoff study helps to reduce the cost of a TEG system and that TEG technology has the potential to be cost competitive with other technologies for power generation [41].
Fig. 7. MCP-TEG performance under various equivalent ratios when Qair = 15.2 SLPM. (a) Hot and cold-end temperatures. (b) Maximum electric power and corresponding overall efficiency.
2.80% in Ref. [11], and 2.53% in Ref. [13]). Other works cited in the Introduction obtained smaller overall efficiencies compared with the present result. The underlying mechanism that augmented the overall efficiency in the present work was the enhanced heat collection design, which augments the heat collection efficiency. Fig. 8 compares the heat collection efficiencies and EFS between the current and previous studies. A large heat collection efficiency and high combustion (reaction) efficiency are required to augment EFS. For example, the heat collection efficiency reached 94% in Fanciulli’s work [23], but its EFS was only 53%. In Shimokuri’s work [15], EFS reached 65.5%, although the heat collection efficiency of 75.3% was less than Fanciulli’s value (94%). In the present work, the EFS reached 81.8%,
Fig. 8. Comparisons of various MCP-TEGs. (a) Heat collection efficiency. (b) EFS. 8
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
6. Conclusions
Investigation, Methodology, Writing - original draft, Writing - review & editing. Dongya Zhu: Data curation, Investigation. Youqu Zheng: Funding acquisition, Methodology. Wenwen Guo: Validation.
An MCP-TEG was presented and evaluated in the present work. The TE, heat collection, and combustion efficiencies were investigated in detail. A maximum electric power of 25.7 W and an overall efficiency of 2.69% were obtained. Results showed that the heat collection and combustion efficiencies should be improved to obtain a high overall efficiency. The micro-/mesoscale combustor with an enhanced heat collection design was verified to be a potential solution due to its long residence time for flue gases. This feature provides a concrete way to improve MCP-TEG performance relative to those in previous studies. The use of a parameter named EFS, which is proposed as the sole parameter to pursue when the TE material and working temperatures are fixed, was developed to evaluate the performance of the MCP-TEGs. The EFS of the developed MCP-TEG was 81.8%, which is greater than those in previous studies.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments This work was partially supported by the key R&D plan of Zhejiang Province (Grant no. 2020C03115), National Natural Science Foundation of China (Grant nos. 51906220 and 51476145), and the Natural Science Foundation of Zhengjiang Province (Grant no. LQ19E060002).
CRediT authorship contribution statement Guoneng Li: Conceptualization, Validation, Funding acquisition, Appendix A
According to datasheets provided by the manufacturer, the physical properties of the TE material are dependent on temperature. Three order polynomial fittings were applied, and the results are as follows:
kp (T ) = tp1 + tp2 T + tp3 T 2 + tp4 T 3 kn (T ) = tn1 + tn2 T + tn3
T2
+ tn4
ρp (T ) = ep1 + ep2 T + ep3
T2
ρn (T ) = en1 + en2 T + en3
T2
(A-1)
T3
(A-2)
+ ep4
T3
(A-3)
+ en4
T3
(A-4)
αp (T ) = sp1 + sp2 T + sp3 T 2 + sp4 T 3
(A-5)
αn (T ) = sn1 + sn2 T + sn3
T2
+ sn4
T3
(A-6)
k, ρ and α are thermal conductivity, electrical resistivity and Seebeck coefficient, respectively. T is temperature. The subscripts p and n denote the P and N-type leg, respectively. Fig. 9 compares with present TE properties with those in previous reports [42–45]. The constants are as follows:
Fig. 9. Comparisons of present TE properties with those in previous reports. (a) k. (b) ρ. (c) α. 9
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
Table B1 Influence of h on convective heat loss and TE efficiency Case No.
h (W/m2-K)
Pconv (W)
ηTE (%)
1 2 3
5 10 15
7 14 21
3.27 3.30 3.33
Fig. 10. Verification of surface emissivity with IR camera set with ε = 0.4.
tp1 = 4.389, tp2 = −1.8168 × 10−2, tp3 = 2.437956 × 10−5, tp4 = 4.793196 × 10−10; tn1 = 4.09878, tn2 = −1.4976 × 10−2, tn3 = 1.799196 × 10−5, tn4 = 1.692996 × 10−9; ep1 = −6.7074 × 10−6, ep2 = 5.09 × 10−8, ep3 = 6.33243 × 10−11, ep4 = −5.31761 × 10−14; en1 = −1.51744 × 10−5, en2 = 1.142 × 10−7, en3 = −8.17056 × 10−11, en4 = −5.18487 × 10−15; sp1 = −1.0915819×10−4, sp2 = 1.67585 × 10−6, sp3 = −2.12 × 10−9, sp4 = 4.43743 × 10−14; sn1 = −4.3833365 × 10−4, sn2 = 2.90422 × 10−6, sn3 = −9.76 × 10−9, sn4 = 1.01202 × 10−11. Appendix B The heat flow rate of flue gases can be obtained as follows:
Pflue = mflue cp (Tin − Tout )
(B-1)
where the mass flow rate of flue gases, mflue, can be obtained directly with MFCs, and the flue gas heat capacity can be determined on the basis of the flue gas temperature and concentrations of flue gas species, which were measured with a gas analyzer. Tin and Tout are results measured with the thermocouples. Thus, the heat flow rate of flue gases can be calculated. In the present work, Pflue = 92 W when the input power was 957 W. The heat loss rate through natural convection can be obtained as follows: (B-2)
Pconv = hA (Tsurf − Tatm) 2
where the convective heat transfer coefficient, h, is assumed to be 10 W/m -K, and the total area exposed to the environment, A, can be measured. The average surface temperature can be measured with IR camera. In this work, Pconv = 14 W when the input power was 957 W. This heat loss rate is subjected to the risk of the assumption of h, which lies between 5 and 15 W/m2-K [46]. However, given that the magnitude of heat loss (~14 W) is only 1.46% of the total heat input, the above assumption of h is acceptable. Moreover, this assumption did not affect the overall efficiency result, which was obtained with Eq. (7). The influences of h on the convective heat transfer loss and TE efficiency are presented in Table B1. As shown in Table B1, the influence of the h assumption on the TE efficiency is within 0.06%; therefore, the uncertainty caused by this assumption is minimal. The heat loss rate through thermal radiation can be obtained as follows: 4 4 Prad = εσA (Tsurf − Tatm )
(B-3)
where the emissivity, ε, is assumed to be 0.4. In the present work, Prad = 8.9 W when the input power was 957 W. This heat loss rate is subjected to the risk of the assumption of ε, which lies between 0.23 and 0.68 in accordance with the materials used in the experimental setup. During the experiments, the copper surfaces exposed to the atmosphere were wrapped by a layer of fiberglass with a thickness of 20 mm. Then, aluminum foil was used to wrap the fiberglass. As a result, all the surfaces exposed to the atmosphere were alumina oxide. The assumption of ε = 0.4 is based on this material as verified by the measurements of a thermocouple and the IR camera (Fig. 10). Given that the magnitude of heat loss (~8.9 W) is only 0.93% of the total heat input, the above assumption of ε is acceptable.
10
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
References
on a catalytic premixed meso-scale combustor. Appl Energy 2016(162):346–53. [23] Fanciulli C, Abedi H, Merotto L, Dondè R, De Iuliis S, Passaretti F. Portable thermoelectric power generation based on catalytic combustor for low power electronic equipment. Appl Energy 2018;215:300–8. [24] Abedi H, Merotto L, Fanciulli C, Dondè R, Iuliis SD, Passaretti F. Study of the performances of a thermoelectric generator based on a catalytic meso-scale H2/ C3H8 fueled combustor. J Nanosci Nanotechnol 2017;17:1592–600. [25] Abedi H, Migliorini F, Dondè R, Iuliis SD, Passaretti F, Fanciulli C. Small size thermoelectric power supply for battery backup. Energy 2019;188:116061. [26] Guggilla BR, Alexander R, Bakrania S. Platinum nanoparticle catalysis of methanol for thermoelectric power generation. Appl Energy 2019;237:155–62. [27] Meng L, Li J, Li Q. A miniaturized power generation system cascade utilizing thermal energy of a micro-combustor: design and modeling. Int J Hydrogen Energy 2017;42:17275–83. [28] Bubnovich V, Martin PS, Henriquez L, Lemos MD. Filtration gas combustion in a porous ceramic annular burner for thermoelectric power conversion. Heat Transfer Eng 2019;40:1196–210. [29] Bobba MK, Gopalakrishnan P, Periagaram K, Seitzmann JM. Flame structure and stabilization mechanisms in a stagnation-point reverse-flow combustor. J Eng Gas Turb Power 2008;130:031505. [30] Chen C, Ronney P. Three-dimensional effects in counterflow heat-recirculating combustors. Proc Combust Inst 2011;33:3285–91. [31] Lloyd S, Weinberg F. A burner for mixtures of very low heat content. Nature 1974;251:47–9. [32] Li GN, Xu ZH, Zheng YQ, Guo WW, Dong C. Experimental study on convective heat transfer from a rectangular flat plate by multiple impinging jets in laminar cross flows. Int J Therm Sci 2016;108:123–31. [33] Juodis E. Extracted ventilation air heat recovery efficiency as a function of a building’s thermal properties. Energy Build 2006;38:568–73. [34] Rowe DM, Min Gao. Evaluation of thermoelectric modules for power generation. J Power Sources 1998;73:193–8. [35] Apertet Y, Ouerdane H, Goupil C, Lecoeur Ph. Influence of thermal environment on optimal working conditions of thermoelectric generators. J Appl Phys 2014;116:144901. [36] Torrecilla MC, Montecucco A, Siviter J, Strain A, Knox AR. Transient response of a thermoelectric generator to load steps under constant heat flux. Appl. Energ. 2018;212:293–303. [37] Freunek M, Müller M, Ungan T, Walker W, Reindl LM. New physical model for thermoelectric generators. J Electron Mater 2009;38:1214–20. [38] Li GN, Zhang S, Zheng YQ, Zhu LY, Guo WW. Experimental study on a stovepowered thermoelectric generator (STEG) with self starting fan cooling. Renew Energy 2018;121:502–12. [39] Gomez M, Reid R, Ohara B, Lee H. Influence of electrical current variance and thermal resistances on optimum working conditions and geometry for thermoelectric energy harvesting. J Appl Phys 2013;113:174908. [40] Yazawa K, Shakouri A. Cost-efficiency trade-off and the design of thermoelectric power generators. Environ Sci Technol 2014;45:7548–53. [41] Yee SK, Leblanc S, Goodson KE, Dames C. $ per W metrics for thermoelectric power generation: beyond ZT. Energy Environ Sci 2013;6:2561–71. [42] Riffat SB, Ma X, Wilson R. Performance simulation and experimental testing of a novel thermoelectric heat pump system. Appl Therm Eng 2006;26:494–501. [43] Pérez-Aparicio JL, Palma R, Taylor RL. Finite element analysis and material sensitivity of Peltier thermoelectric cells coolers. Int J Heat Mass Tran 2012;55:1363–74. [44] Liao M, He Z, Jiang C, Fan X, Li Y, Qi F. A three-dimensional model for thermoelectric generator and the influence of Peltier effect on the performance and heat transfer. Appl Therm Eng 2018;133:493–500. [45] Lan S, Yang Z, Chen R, Stobart R. A dynamic model for thermoelectric generator applied to vehicle waste heat recovery. Appl Energy 2018;210:327–38. [46] Holman JP. Heat transfer. tenth ed. U.S.A: Mc Graw Hill; 2008 Chapter 7, pp. 327–347.
[1] Sonoc A, Jeswiet J, Soo VK. Opportunities to improve recycling of automotive Lithium ion batteries. Proc. CIRR 2015;29:752–7. [2] Chou SK, Yang WM, Chua KJ, Li J, Zhang KL. Development of micro power generators – a review. Appl Energy 2011;88:1–16. [3] Xiao G, Yang T, Liu H, Ni D, Ferrari ML, Li M, et al. Recuperators for micro gas turbines: a review. Appl Energy 2017;197:83–99. [4] Zhang T, Wang P, Chen H, Pei P. A review of automotive proton exchange membrane fuel cell degradation under start-stop operating condition. Appl. Energy 2018;223:249–62. [5] Champier D. Thermoelectric generator: a review of applications. Energy Convers Manage 2017;140:167–81. [6] Hinterleitner B, Knapp I, Poneder M, Shi Y, Müller H, Eguchi G, et al. Thermoelectric performance of a metastable thin-film Heusler alloy. Nature 2019;576:85–90. [7] Vican J, Gajdeczko BF, Dryer FL, Milius DL, Aksay IA, Yetter RA. Development of a microreactor as a thermal source for microelectromechanical systems power generation. Proc. Combust. Inst. 2002;29:909–16. [8] D.G. Norton, K.W. Voit, T. Brüggemann, D.G. Vlachos, Portable power generation via integrated catalytic microcombustion-thermoelectric devices, in: Proc. Army Science (24th), 2005, Nov 29–Dec 2, Orlando, Florida. [9] Federici JA, Norton DG, Bruggemann T, Voit KW, Wetzel ED, Vlachos DG. Catalytic microcombustors with integrated thermoelectric elements for portable power production. J Power Sources 2006;161:1469–78. [10] Karim AM, Federici JA, Vlachos DG. Portable power production from methanol in an integrated thermoelectric/microreactor system. J Power Sources 2008;179:113–20. [11] Yoshida K, Tanaka S, Tomonari S, Satoh D, Esashi M. High-energy density miniature thermoelectric generator using catalytic combustion. J Microelectromech S 2006(15):195–203. [12] Jiang LQ, Zhao DQ, Guo CM, Wang XH. Experimental study of a plat-flame micro combustor burning DME for thermoelectric power generation. Energy Convers Manage 2011;52:596–602. [13] Marton CH, Haldeman GS, Jensen KF. Portable thermoelectric power generator based on a microfabricated silicon combustor with low resistance to flow. Ind Eng Chem Res 2011;50:8468–75. [14] Y. Hsu, I. Sapir, M. Azazzy, P. Ronney, Miniature power source with catalytic combustor and hybrid thermoelectric generator, in: Proc. PowerMEMS, 2012, Dec 2 – Dec 5, Atlanta, Georgia. [15] Shimokuri D, Taomoto Y, Matsumoto R. Development of a powerful miniature power system with a meso-scale vortex combustor. Proc Combust Inst 2017;36:4253–60. [16] Shimokuri D, Hara T, Matsumoto R. Development of a small-scale power system with meso-scale vortex combustor and thermo-electric device. J Micromech Microeng 2015;25:104004. [17] Yadav S, Yamasani P, Kumar S. Experimental studies on a micro power generator using thermo-electric modules mounted on a micro-combustor. Energy Convers Manage 2015;99:1–7. [18] Aravind B, Khandelwal B, Kumara S. Experimental investigations on a new high intensity dual microcombustor based thermoelectric micropower generator. Appl Energy 2018;228:1173–81. [19] Aravind B, Raghuram GKS, Kishore VR, Kumar S. Compact design of planar stepped micro combustor for portable thermoelectric power generation. Energy Convers Manage 2018;156:224–34. [20] Aravind B, Saini DK, Kumar S. Experimental investigations on the role of various heat sinks in developing an efficient combustion based micro power generator. Appl Therm Eng 2019;148:22–32. [21] Singh T, Marsh R, Min G. Development and investigation of a non-catalytic selfaspirating meso-scale premixed burner integrated thermoelectric power generator. Energy Convers Manage 2016;117:431–41. [22] Merotto L, Fanciulli C, Dondè R, Iuliis SD. Study of a thermoelectric generator based
11
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Mesoscale combustor-powered thermoelectric generator with enhanced heat collection
T
⁎
Guoneng Li , Dongya Zhu, Youqu Zheng, Wenwen Guo Department of Energy and Environment System Engineering, Zhejiang University of Science and Technology, Hangzhou 310023, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermoelectric generator Enhanced heat collection Overall efficiency System effectiveness
Small-scale power sources for portable electronic equipment have been attracting considerable attention in recent years. This paper presents the design and experimental tests of a mesoscale combustor-powered thermoelectric (TE) generator (TEG) with enhanced heat collection. The TE effect, combustion, heat collection efficiencies, and lean blowout (LBO) are investigated in detail. A parameter named system effectiveness (EFS), which is the ratio of overall efficiency to TE efficiency, is proposed to evaluate the performance of combustorpowered TEGs. The obtained electric power of 25.7 W, with an overall efficiency of 2.69%, is higher than previous results. The corresponding hot-end temperature changes from 236 °C to 152 °C along with the flue gas direction, indicating noticeable temperature variation despite the use of pure copper. EFS reaches 81.8%, which is higher than previous reports, and validates that the present TEG is well-designed. The LBO of the combustor is 0.63, and the pressure drop is 463 Pa when the input power is 957 W. Detailed discussions and comparisons with previous studies reveal that the enhanced heat collection design is essential for increasing the residence time of flue gases and EFS.
1. Introduction The development of portable small-scale power sources remains a challenge. Batteries with energy densities that are generally lower than 300 Wh/kg are unsuitable for many applications. Furthermore, the long recharging times and pollution problems of batteries are difficult to solve [1]. Thus, alternatives are continuously being developed. Hydrocarbons whose energy densities are hundreds of times those of batteries exhibit a considerable potential as small-scale power sources with the availability of advanced fuel-to-electricity technologies, such as micro gas turbines, micro fuel cells, and thermoelectric (TE) generators (TEGs). Although micro gas turbines and fuel cells perform efficiently, their maintenance hinders their application [2]. These problems include high-speed rotating parts, high-temperature bearings, and noise for micro gas turbines [3]; and humidification, hydrogen storage, and safety for fuel cells [4]. TEGs, which have been widely adopted in many applications and reviewed comprehensively [5], can be developed in portable form with hydrocarbons, using micro-/mesoscale combustors. TEGs are straightforward systems that require low maintenance because of their small moving parts. Material revolution has accelerated in the past few years, thereby creating excellent TE materials [6]. Thus, TEGs are promising. A micro-/mesoscale combustor-powered TEG (MCP-TEG) is ⁎
presented in this work. The working principle of TEGs is based on the Seebeck effect; that is, a temperature difference forces electrons to move in one direction between two metals or semiconductors, resulting in a continuous current through a complete circuit. The applications of MCP-TEGs include portable power sources, microelectronics, biomechanics and microfabrications. A literature review on MCP-TEGs is provided in the following section. Vican et al. [7] proposed an MCP-TEG in 2002. Their concept attracted considerable attention although the electric power was limited to 0.052 W. A group of scientists from the University of Delaware presented several prototypes of MCP-TEGs for military applications. The maximum electric power was approximately 1 W [8,9], and the overall efficiency was 1.1% [10]. Yoshida et al. [11] developed an MCP-TEG with an overall efficiency of 2.8% and limited electric power (0.185 W). The maximum electric power of Jiang’s MCP-TEG was augmented to 2 W [12], and the overall efficiency was 1.25%. Marton et al. [13] presented an MCP-TEG prototype that generated an electric power of 5.82 W and a relatively high overall efficiency of 2.53%. Hsu et al. [14] reported that an electricity power of 9.4 W was generated with their MCP-TEG and that the corresponding overall efficiency was 2.85%. Shimokuri et al. [15] established new records with their MCPTEGs, whose electric power and overall efficiency were 18.1 W and 3.01%, respectively; these values were the optimized results of their
Corresponding author. E-mail address: [email protected] (G. Li).
https://doi.org/10.1016/j.enconman.2019.112403 Received 25 September 2019; Received in revised form 10 December 2019; Accepted 11 December 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
Nomenclature A Aflue cp h I k L Lflue mburnt mfuel n P Pconv Pflue Pin Pld Pmax Prad PTE QAir QCH4 Qflue r Rld T Tatm Tave Tc Tf Th
Tin Tsurf Tout ΔP ΔT U w Z α ρ φ ε τ σ ηfuel ηheat ηsys ηTE
total area exposed to the atmosphere (m2) cross-sectional area of flue gas channel (m2) heat capacity (kJ/kg-K) convective heat transfer coefficient (W/m2-K) current (A) thermal conductivity (W/m-K) length of TE leg (mm) length of flue gas channel (m) combusted mass flow rate (kg/s) fuel mass flow rate (kg/s) electrical resistivity ratio (mm) electric power (W) heat loss rate through convection (W) heat flow rate of flue gases (W) input power (W) load power (W) maximum electric power (W) heat loss rate through thermal radiation (W) heat flow rate through TE modules (W) air flow rate (LPM) methane flow rate (LPM) volume flow rate (m3/s) thermal contact ratio (dimensionless) load resistance (Ω) temperature (°C) atmosphere temperature (°C) average temperature, (Th + Tc)/2 (°C) cold-end temperature (°C) combustion temperature (°C) hot-end temperature (°C)
inlet air temperature (°C) Surface temperature (°C) flue gas temperature (°C) pressure loss (Pa) temperature difference, ΔT = Th–Tc (°C) voltage (V) ratio of ceramic thickness to TE leg (dimensionless) TE figure-of-merit (1/K) Seebeck coefficient (V/K) electrical resistivity (Ωm) equivalent ratio (dimensionless) emissivity (dimensionless) residence time (ms) Stefan-Boltzmann constant (W/m2K4) combustion efficiency (%) heat collection efficiency (%) overall efficiency (%) TE efficiency (%)
Abbreviations DAQ data acquisition EFS system effectiveness HEX heat exchanger IR infrared MFC mass flow rate controller LBO lean blowout LPM liter per minute SPRF stagnation point reverse flow TE thermoelectric TEG TE generator MCP-TEG micro-/mesoscale combustor-powered TEG
considerably high overall efficiency of 2.5% could be reached in their air-cooled MCP-TEGs [20]. The overall efficiency of Singh’s MCP-TEG was only 1.42% [21], although fuel combustion was approximately complete. Several researchers from the Consiglio Nazionale delle
previous prototype [16]. A team from the Indian Institute of Technology Bombay designed several prototypes. They claimed that high overall efficiencies (4.6% in Ref. [17], 4.03% in Ref. [18], and 4.66% in Ref. [19]) could be obtained in their water-cooled MCP-TEGs and that a Table 1 Performance comparison of various MCP-TEGs. Authors
J. Vican, et al. [7] D.G. Norton, et al. [8] J.A. Federici, et al. [9] K. Yoshida, et al. [11] A.M. Karim, et al. [10] L.Q. Jiang, et al. [12] C.H. Marton, et al. [13] Y. Hsu, et al. [14] D. Shimokuri, et al. [16] S. Yadav, et al. [17] T. Singh, et al. [21] L. Merotto, et al. [22] D. Shimokuri, et al. [15] H. Abedi, et al. [24] B. Aravind, et al. [18] B. Aravind, et al. [19] C. Fanciulli, et al. [23] B. Aravind, et al. [20] H. Abedi, et al. [25] B.R. Guggilla, et al. [26] Present
Year
2002 2004 2006 2006 2008 2011 2011 2012 2015 2015 2016 2016 2017 2017 2018 2018 2018 2019 2019 2019 2019
Th
115.5 – 170 125 – 200 312 475 156 250 197 258 200 168 130 150 160 221 220 140 152–236
ΔT
92.5 253 85 70 160 130 266 400 126 – 88 200 165 150 99 115 128 124 200 62 105–188
Efficiency (%)
Pmax (W)
ηheat
ηsys
ηfuel
– – 19.3 30–44 – – – 41 – – – – 75.3 – 34.4 33.5 94 25.7 – – 87.2
0.57 0.85 0.51 2.8 1.1 1.25 2.53 2.85 2.23 4.6 1.42 2.36 3.01 2.27 4.03 4.66 1.1 2.5 2.5 0.1 2.69
100 100 100 99.5 100 100 100 99 100 – 100 96 86.9 96.4 – – 56 – 53 70 93.5
0.052 ~1 0.45 0.185 0.65 2 5.82 9.4 4.76 2.35 3.54 9.86 18.1 5.92 3.89 4.52 0.84 2.4 4.31 0.49 25.7
Reaction
TE
type†
material
CTC CTC CTC CTC CTC DTC CTC CTC DTC DTC DTC CTC DTC CTC DTC DTC CTC DTC CTC CTC DTC
Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3 Bi2Te3/PbTe Bi2Te3 Bi2Te3 Bi2Te3 Chalcogenides Bi2Te3 Chalcogenides Bi2Te3 Bi2Te3 Chalcogenides Bi2Te3 Chalcogenides Bi2Te3 Bi2Te3
Notes
Case: H2/air mixture at φ = 1.0 Case: 1.4 SLPM *Data from different cases
ηsys = 2.36% at Pmax = 8.1 W Case: 250 W input power ηsys = 2.85% at Pmax = 6.78 W
*Data from different cases *Data from different cases ηsys = 1.4% at Pmax = 0.6 W Case: 10 m/s, Fin + Fan at 3000 rpm ηsys = 3.4% at 1.2 Nl/min gas flow rate
– Denotes “not-found” or “not-studied”. *Denotes that a complete set of data cannot be found. Data from close cases were listed for comparison and discussion. Certain uncertainties may exist. †CTC and DTC denote catalytic-combustion and direct-combustion, respectively. 2
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
combustion, which is beyond the present non-premixed combustion. Nevertheless, Swiss-roll combustors can be integrated in MCP-TEGs provided that appropriate modifications are performed for non-premixed combustion. The cross-sectional dimensions of a flue gas channel, that is, 3 mm (width) × 20 mm (thickness), are designed such that the total area of flue gas channels is four times the total inlet areas of the air and fuel. This design helps minimize pressure drop and increase the residence time of flue gases. Three TE modules are sandwiched between the combustor and the multichannel water-cooled heat sink. Six TE modules and two watercooled heat sinks are assembled in the MCP-TEG, and all TE modules are wired in series. Commercially available Bi2Te3-based TE module (SAGREON Co., Ltd., China) measuring 40 mm (length) × 40 mm (width) × 3.3 mm (thickness) is selected. According to datasheets
Ricerche of Italy reported a 9.8 W maximum electric power [22] and an overall efficiency that varied from 1.1% [23] to 2.27% [24], 2.36% [22], and 3.4% [25]. Recently, Guggilla et al. [26] from Rowan University proposed an MCP-TEG and produced notable results despite yielding a limited overall efficiency. The detail characteristics and comparisons of abovementioned MCP-TEGs are presented chronologically in Table 1. Th, ΔT, ηheat, ηsys, ηfuel, and Pmax are the hot-end temperature, temperature difference, heat collection efficiency, overall efficiency, combustion efficiency, and the maximum electric power, respectively. Numerical studies on MCP-TEGs are also important because they show that the combination of thermo-photovoltaic and TEG is an interesting optimization direction [27] and that filtration combustion is another approach to improve TEG performance [28]. These works provided valuable results, but they employed limited TE modules, indicating that the flue gas temperature from micro-/mesoscale combustors is substantial and thus has potential to be utilized. The electric powers of the above MCP-TEGs are relatively low and limit the practicality of these equipments. In the current work, an MCPTEG with enhanced heat collection was designed and evaluated in terms of electric power and overall efficiency. Unlike previous generators, the present MCP-TEG offers a concrete method to augment electric power while maintaining a relatively high overall efficiency. The detailed comparisons of the experimental results with those from previous studies (Refs. [7–26] in Table 1) revealed that a new parameter, which is named system effectiveness (EFS) and comprehensively combines combustion completeness, heat collection, and TE conversion, should be used to evaluate the performance of MCP-TEGs. Finally, new insights are provided for the research community.
(a)
2. TEG configuration Fig. 1 shows the configuration of the proposed MCP-TEG. Fig. 1(a) and 1(b) shows a photograph and the schematic of the TEG. It comprises a mesoscale combustor, six TE modules, two water-cooled heat sinks, and two clamping plates. Two copper plates with multiple notch grooves are assembled to form the combustor, as shown in Fig. 1(c). These grooves are designed to form a stagnation-point reverse-flow (SPRF) combustor and a built-in heat exchanger (HEX) for absorbing heat from flue gases. SPRF combustors have a stable operation at lean fuel–air mixtures and have low nitrogen oxide emissions [29]. The combustor, which has overall dimensions of 120 mm (length) × 40 mm (width) × 24 mm (thickness), is designed in accordance with the dimensions of three TE modules on each side. The combustion chamber is in the middle of the two copper plates. As shown in Fig. 1(c), fuel and air are supplied separately, and partially mixed before being injected into the combustion chamber. Fuel–air mixtures are combusted in the chamber, which measures 22 mm (length) × 20 mm (width) × 20 mm (thickness). Flue gases are divided into two streams (left and right). Two other streams are formed for each stream, thereby increasing the residence time of flue gases. The cross sectional dimensions of the flue gas channels are 3 mm (width) × 20 mm (thickness). Given the gas expansion due to the combustion, the residence time of flue gases is estimated to be greater than 65 ms when the input power is less than 1000 W. The dimensions of the combustion chamber are set on the basis of a volumetric heat load of 7.2 × 107 W/m3, which is obtained in accordance with a previous work on an SPRF combustor [29] (case: equivalent ratio of 1.0 and reaction zone length of 80 mm). However, the volumetric heat load in the present work is set as 11.4 × 107 W/m3 because approximately one-third of the heat input is absorbed immediately through the walls of the combustion chamber. Many studies have demonstrated the superior performance of Swiss-roll combustors [30], which were proposed by Lloyd et al. in 1974 [31]. A Swiss-roll combustor inside a spiral HEX, which is similar to the present serpentine design, can serve as the combustor of an MCP-TEG. However, it features a super-adiabatic flame temperature and effective premixed
(b)
(c)
Fig. 1. TEG configuration. (a) Photograph of TEG. (b) Schematic of TEG. (c) Mesoscale SPRF combustor design and built-in HEX. 3
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
provided by the manufacturer, the TE module can typically generate 8.2 W of electric power at a temperature difference of 200 °C. This paper also presents the temperature-dependent properties of the TE material (Appendix A) that are used to theoretically calculate TE efficiency.
Table 2 Experimental cases.
3. Experimental setup Fig. 2 presents the experimental setup. Fuel and air were supplied with two Alicat mass flow controllers (MFCs). The accuracy of the MFCs was ± 0.8% of reading plus 1% of the full scale. Flue gas emissions (CO2, CO, NO) were measured with a Testo 340 gas analyzer. The accuracies of the CO2, CO, and NO measurements were 0.2 vol%, 10 vol %, and 5 ppm, respectively. Seven thermocouples with an accuracy of 0.5% were installed to measure the combustion, hot-end, cold-end, air inlet, and flue gas exit temperatures. The temperature signals were recorded with an Agilent-34970A data-acquisition (DAQ) instrument combined with a BenchLink data logger program. A CHY-130 differential pressure transducer, which had an accuracy of 0.5%, was installed to measure the pressure drop of the combustor. A Dali T8 infrared (IR) imager with 25 μm resolution was used to observe the temperature distribution of the entire MCP-TEG and to estimate the convection and thermal radiation heat losses. The IR camera was calibrated with type K thermocouples, and detailed information can be found in Appendix B. The power load feature was measured with a Prodigit 3311F electronic load, the measurement accuracy of which was ± 0.5%. Methane was utilized as fuel. The input power was set to 957 W, which corresponded to a methane mass flow rate of 1.6 L/min (LPM). Equivalent ratios of 0.9 and 1.0 were selected for this input power. For LBO, the air flow was fixed at 15.2 LPM, and the methane flow rate was gradually decreased. A water pump, a radiator, and three blowers were used to dissipate heat from the cold-end of the TE modules, thus maintaining a low cold-end temperature. The working conditions for the water pump and blowers were constant during the experiments. The water pump worked at 7.9 V and consumed 2.45 W of electricity. All blowers ran at 12 V and consumed 7.32 W of total electric power. Over 50 experimental cases were conducted, and they are listed in Table 2. QCH4, QAir, φ, Pin, and Rld are the methane flow rate, air flow rate, equivalent ratio, input power, and the load resistance, respectively. The total differential method was applied to estimate the uncertainty for the overall efficiency with Eq. (1), which was also used in our previous work [32]. The uncertainty for the overall efficiency was estimated to
No.
QCH4 (LPM)
QAir (LPM)
φ
Pin (W)
Rld (Ω)
1 2 3 4 5 6 7 8
1.6 1.6 1.5 1.4 1.3 1.2 1.1 1.0
16.8 15.2 15.2 15.2 15.2 15.2 15.2 15.2
0.9 1.0 0.94 0.88 0.81 0.75 0.69 0.63
957 957 897 837 777 717 658 598
5–150 5–150 23, 25, 23, 25, 23, 25, 23, 25, 23, 25, 23, 25,
δQCH 4 =
δU δI δQCH 4 + + U I QCH 4
27 27 27 27 27 27
be 5.6% at a 95% confidence level.
δηsys ηsys
= =
∂ηsys ∂P ∂ηsys ∂U
δP + δU +
∂ηsys ∂QCH 4 ∂ηsys ∂I
δQCH 4
δI +
∂ηsys ∂QCH 4
(1) 4. Data reduction The input power might not be fully available because fuel could not be completely combusted. Thus, combustion efficiency is an important parameter used to evaluate the performance of MCP-TEGs, and it is defined as follows:
ηfuel =
mburnt mfuel
(2)
where mburnt and mfuel are the combusted fuel (CH4) mass flow rate and fuel mass flow rate, respectively. mburnt can be calculated by measuring the CO2 concentration in the flue gas without consideration for CO concentration. This assumption is justified because the CO concentration is low (less than 3000 ppm). During the experiments, the equivalent ratio was used to control the combustion of methane; it is defined as follows,
φ=
( ) ( )
Fuel Air actual Fuel Air stoichiome tric
= 9.52 ×
QCH 4 Q Air
(3)
Heat is released through combustion, and part of this heat can be collected and passes through the TE modules. The heat collection
Fig. 2. Sketch of experimental setup. 4
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
ηsys = ηfuel ηheat ηTE
efficiency is defined as follows:
ηheat
PTE = ηfuel Pin
EFS, a parameter that could be used to evaluate the performance of an MCP-TEG, is expressed as follows:
(4)
where Pin is the input power calculated by multiplying the mass flow rate of methane and the corresponding lower heat value. PTE is the heat flow rate passing through the TE modules. The following equation is used to obtain PTE:
PTE = ηfuel Pin − Pflue − Pconv − Prad
EFS =
ηsys =
Pmax PTE
(5)
(6)
ηTE
(9)
−1
ηTE =
Pmax Pin
ηsys
This parameter presents the relationship between overall efficiency and TE efficiency. EFS indicates how closely an MCP-TEG running to the ideal working state (complete combustion; no heat loss though convection, radiation, and flue gases). Heat recovery efficiency is the ratio of recovered heat to exhausted heat [33], which has been studied widely in ventilation engineering. It has the same meaning as that of heat collection efficiency in the present work. For an MCP-TEG, EFS is the product of heat collection efficiency and combustion (reaction) efficiency. Therefore, EFS measures the design level of an MCP-TEG. Heat collection and combustion (reaction) completeness should be balanced to enhance the performance of an MCP-TEG. The TE efficiency can be theoretically predicted as follows [34]:
where Pflue, Pconv, and Prad are the heat loss rate in the flue gas, heat loss rate through convection, and heat loss rate through thermal radiation, respectively. The calculation of the heat flow rate of flue gases and the heat loss rates through natural convection and thermal radiation, are straightforward, and they are listed in Appendix B. The TE modules convert part of PTE to electricity. TE efficiency and overall efficiency can be defined as follows:
ηTE =
(8)
Th − Tc ⎧ T − Tc ⎞ ⎛ 4 ⎞ ⎛ 1 + n/ L ⎞ ⎤ ⎫ + (1 + 2rw )2 ⎡2 − 0.5 ⎛ h ⎢ Th ⎨ ⎝ Th ⎠ ⎝ ZTh ⎠ ⎝ 1 + 2rw ⎠ ⎥ ⎦⎬ ⎣ ⎭ ⎩ ⎜
⎟
⎜
⎟
(7)
(10)
Pmax represents the maximum electric power generated by the MCPTEG. As revealed in Eqs. (6) and (7), the overall efficiency must be smaller than the TE efficiency, and their relationship can be expressed as follows:
where Th and Tc are the hot and cold-end temperatures, respectively. The figure-of-merit Z is defined as α2/kρ, where α, k and ρ are the Seebeck coefficient, thermal conductivity, and electrical resistivity, respectively. r, w, n, and L are the thermal contact ratio, ratio of ceramic
(c)
Fig. 3. (a) Hot and cold-end temperatures under various load resistances when Pin = 957 W. (b) Temperature measuring locations. (c) IR image at φ = 0.9 when Pin = 957 W. 5
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
work.
thickness to the length of the TE leg, electrical resistivity ratio, and the length of the TE leg, respectively. r = 0.2, w = 0.516, n = 0.1 mm, and L = 1.5 mm. ZTave in the present experimental cases varied between 0.86 and 0.88, where Tave = 0.5(Th + Tc). The residence time (τ) of flue gases is estimated as follows:
τ=
5.2. Power load feature Fig. 4 presents the power load feature when the input power was 957 W. A maximum electric power of 25.7 W, which is greater than previous results, was obtained at 25 Ω when φ = 0.9 but decreased by 4.1% when the equivalent ratio increased to 1.0. This condition was caused by the lower hot-end temperatures at φ = 1.0 compared with those at φ = 0.9. The average electric power output for each TE module was 4.28 W. This value is smaller than those reported in Shimokuri’s [15] and Merotto’s works [22]. The reason is that the TE modules responsible for heat recovery worked at lower hot-end temperatures compared with the TE modules attached to the combustion chamber. As shown in Fig. 4(b), the load voltage increased with load resistance, and the load current contrarily varied. The intersection of the voltage and current curves was the optimized working condition, under which a maximum electric power could be obtained.
1000Aflue Lflue Qflue
(11)
where Qflue, Aflue, and Lflue are the volume flow rate of flue gases, cross sectional area of the flue gas channel, and the flue gas channel length, respectively. 5. Results and discussions 5.1. Temperature The hot and cold-end temperatures, their measuring locations and an IR image are shown in Fig. 3. The cold-end temperatures were close to each other; therefore, the average cold-end temperature was adopted. As shown in Fig. 3, the hot-end temperature noticeably decreased along the direction of the flue gases although materials with high thermal conductivity (copper) were used to construct the combustor. For example, the hot-end temperature for the middle TE modules reached 236 °C and decreased to 152 °C for the nearby TE modules. The hot-end temperatures slightly decreased with a decrease in load resistance. This phenomenon was caused by the well-known Peltier effect; that is, more heat was pumped from the hot-end to the cold-end as the electric current increased. The average hot-end temperatures at φ = 0.9 were approximately 9.6 °C higher than those at φ = 1.0 under the same input power due to differences in combustion efficiencies, which are discussed in Section 5.3. The temperature measurements obtained using the thermocouples and the IR imager at φ = 0.9 revealed that the heat flow rate of flue gases, and the heat loss rates through natural convection, and thermal radiation were 92.0, 14.0 and 8.9 W, respectively. A natural convection heat transfer coefficient of 10 W/m2-K and an emissivity of 0.4 were assumed for all surfaces during the above estimations. Given the magnitudes of convection and radiation heat losses, the differences between the abovementioned assumptions and their actual values had only a minor influence on the conclusions (see Appendix B for detailed information). The exhausted gas temperature was 199.4 °C at φ = 0.9 when Pin = 957 W; it was only 47.4 °C higher than the hot-end temperature (152 °C) of the TE module near the flue gas exit. This finding revealed that the heat flux of the exhausted gas was only 9.61% of the input power (957 W). Assuming that adding two other TE modules could capture 80% of the heat flux from the exhausted gas, the increased electric power will not exceed 2.0 W, which is not profitable enough. Therefore, adding TE modules was not necessary, and six TE modules were selected in the present
5.3. Efficiency The TE, combustion, heat collection efficiencies, and the EFS are shown in Fig. 5. An overall efficiency of 2.69% was obtained for the present MCP-TEG at φ = 0.9 when the input power was 957 W. The corresponding TE efficiency was measured to be 3.29%, which was consistent with the 3.44% predicted with Eq. (10). The experimental TE efficiency reached 95.6% of the theoretical prediction, which indicated that the TE modules employed in the present study worked normally. As shown in Fig. 5(b), the combustion efficiency at φ = 0.9 was 4.28% higher than that at φ = 1.0, whereas the heat collection efficiencies of these two cases were comparable. Therefore, the overall efficiency at φ = 0.9 was 4.66% higher than that at φ = 1.0. An essential issue behind the overall efficiency was the gap between the overall efficiency and TE efficiency, which was controlled by Eq. (8). The limit of the overall efficiency was the TE efficiency, which was controlled by the ZT value and working temperatures. The ZT value was fixed for a particular TE material working at a selected temperature range. Combustion and heat collection efficiencies should be optimized to narrow the gap between the overall efficiency and TE efficiency. The two efficiencies are shown in Fig. 5(b); the combustion and heat collection efficiencies reached 93.5% and 87.2%, respectively. The combustion of hydrocarbons has been extensively investigated, and a combustion efficiency exceeding 90% can be expected with minimal effort. The measured combustion efficiency of 93.5% is consistent with those in previous studies. The obtained heat collection efficiency was fairly high at 87.2% due to the serpentine channels that recover heat from flue gases. Detailed discussions related to heat collection are provided in Section 5.5. The EFS in Eq. (9) measures the gap between
Fig. 4. Power load test results when Pin = 957 W. (a) Load power. (b) Voltage and current. 6
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
Fig. 5. Efficiencies when the input power and equivalent ratio were set to 957 W and 0.9, respectively. (a) Comparison of overall, experimental TE, and predicted TE efficiencies. (b) Comparison of combustion efficiency, heat collection efficiency, and EFS.
when the equivalent ratio was between 0.8 and 1.0. For the equivalent ratio less than 0.8, the overall efficiency gradually decreased from 2.5% at φ = 0.8 to 1.9% at φ = 0.63. Notably, the present MCP-TEG had a wide operation range, and the load electric power and the aging of TE modules were balanced, as shown in Fig. 7. Concerning the bias error of the results mentioned in Figs. 3–7, T, P, U, I, and ΔP are directly measured parameters. Among the derived parameters (i.e. ηfuel, ηTE, ηheat, and ηsys), however, ηfuel was directly derived from the results of gas analyzer; ηTE was verified by the predicted result with Eq. (10) and Appendix A; ηheat was carefully examined with Eq. (5) and Appendix B. ηsys, the accuracy of which was estimated with Eq. (1), was thoroughly considered and emphasized in the discussion. Thus, the results reported in the present work are valid.
the overall efficiency and the TE efficiency, and it is the sole parameter to pursue when the TE material and working temperatures are fixed. As shown in Fig. 5 (b), the overall efficiency reached 81.8% of the experimental TE efficiency, indicating that the present MCP-TEG is welldesigned. However, room for further improvement remains, and an EFS of 90% should be targeted in future works. The heat absorbing area can be increased, and materials with high thermal conductivity can be used to construct a combustor to improve EFS. The overall and TE efficiencies presented in Fig. 5 are based on the maximum electric power output. Thus, the TE efficiency may not be the maximum efficiency of the TE module because of the temperature dependent properties. Nevertheless, maximum electric power output was targeted in the present work. For a TEG running under a constant heat flux mode (the MCP-TEG in this study), the maximum electric power and maximum efficiency can be obtained simultaneously [35], but an advanced electric power management system should be incorporated.
5.5. Discussion Overall efficiency is an essential parameter used to evaluate MCPTEGs. The literature review showed that only a team from the Indian Institute of Technology Bombay obtained an overall efficiency greater than 4.0% (4.6% in Ref. [17], 4.03% in Ref. [18], and 4.66% in Ref. [19]) for water-cooled MCP-TEGs. However, the heat collection efficiencies in the abovementioned studies were less than 40%, implying that the TE efficiencies in these studies should be larger than 10%. Thus, these values should be evaluated because they were for Bi2Te3based TE modules. The 2.69% overall efficiency in the present work is consistent with those in several previous studies (3.01% in Ref. [15],
5.4. LBO and pressure drop Flame stability and pressure drop are important factors used to characterize the performance of a micro-/mesoscale combustor. As shown in Fig. 6, the LBO of the SPRF combustor was 0.63, which indicated that the combustor operated with lean mixtures. Furthermore, the combustor operated at high input powers (large fuel flow rate), and the hot-end temperature exceeded the long-term working temperature of the TE module. Thus, only the experimental results at 957 W are shown in Fig. 6. Meanwhile, the combustion temperature in Fig. 6 represented the temperature at the central point of the combustion chamber only rather than the exact flame temperature. The actual flame front could be propagated to the upstream or downstream zones. As shown in Fig. 6, the combustion temperature decreased with the reduction in equivalent ratio, because of the decrease in the fuel injected into the combustor. During this process, the pressure drop was slightly affected; it decreased from 463 Pa at φ = 1.0 to 433 Pa at φ = 0.63. This minor influence was because the mass flow rate of air was maintained at 15.2 LPM, while the mass flow rate of methane varied from 1.6 LMP at φ = 1.0 to 1.0 LPM at φ = 0.63. This finding indicated that the mass flow rate of methane occupied only a small part of the total flow rate. The pressure drop of 463 Pa in the present MCPTEG is greater than the 246 Pa in Yoshida’s work [11] and 130 Pa in Marton’s work [13]. This condition was because the present MCP-TEG had longer flue gas channels than those in the abovementioned studies. Fig. 7 presents the hot-end temperature, cold-end temperature, load electric power and corresponding overall efficiencies. The hot-end temperature remarkably decreased with the decrease in equivalent ratio. This condition was because the input power decreased from 957 W to 598 W. The load electric power decreased from 24.3 W to 11.1 W. However, the overall efficiency remained larger than 2.5%
Fig. 6. Combustion temperature and pressure drop under various equivalent ratios when Qair = 15.2 LPM. 7
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
Table 3 Comparison of residence times in current and previous studies. Authors
Pin (W)
τ (ms)
J. Vican, et al. [7] K. Yoshida, et al. [11] D. Shimokuri, et al. [15] D. Shimokuri, et al. [16] Present
9.1 3.5 600 213 957
22.1* 55 less than30 less than10 66.7
*Results at 188 cm3/min.
which is higher than those in previous studies. An effective method for enhancing the heat collection efficiency of MCP-TEGs is to increase the residence time of flue gases. Table 3 compares the present residence time with those in previous studies. Few previous studies focused on residence time (Table 3). In the current study, the enhanced heat collection design (serpentine channels) prolonged the residence time. The residence time was calculated with Eq. (11). Methane is combusted such that no volume is created or lost before and after the reaction. Therefore, only the gas expansion due to temperature variation should be considered when calculating the volumetric flow rate of flue gas relative to those of reactants (air and fuel). The combustion and exhausted gas temperatures were 727 °C (Fig. 6) and 199.4 °C, respectively, when Pin = 957 W. Therefore, the averaged volumetric flow rate of flue gas was 6.92 × 10−4 m3/s. The crosssectional area of the flue gas was 2.4 × 10−4 m2, and the flue gas channel length was 0.1923 m. As a result, the residence time was 66.7 ms at the high input power of 957 W. However, the disadvantage of these serpentine channels was the pressure drop, which should be improved (although only ~450 Pa was found). The residence time is an indicator that characterizes the lifetime of reactant species from combustion to departure (from the exit). Therefore, it is critical for improving heat collection efficiency, which is directly linked to EFS. The electric powers presented in this work were under steady state conditions. The transient response during the addition of load to the MCP-TEG was not studied in this work. A previous work revealed this response is typical of a first-order system with temperature difference [36]. The reasons are the Peltier effect and the thermal inertia of the MCP-TEG [37]. When the load resistance is too small, a DC-DC converter cannot maintain a rated output voltage [38]. The load resistance ratio should be larger 1.0 (because of the thermal and electrical contact effects) to run the TEG under the optimum working condition for maximum electric power output [39]. Therefore, an advanced electric power management system needs to be developed for the field applications of MCP-TEGs. However, this task is beyond the scope of the present work. A previous work [40] showed that a cost-efficiency tradeoff study helps to reduce the cost of a TEG system and that TEG technology has the potential to be cost competitive with other technologies for power generation [41].
Fig. 7. MCP-TEG performance under various equivalent ratios when Qair = 15.2 SLPM. (a) Hot and cold-end temperatures. (b) Maximum electric power and corresponding overall efficiency.
2.80% in Ref. [11], and 2.53% in Ref. [13]). Other works cited in the Introduction obtained smaller overall efficiencies compared with the present result. The underlying mechanism that augmented the overall efficiency in the present work was the enhanced heat collection design, which augments the heat collection efficiency. Fig. 8 compares the heat collection efficiencies and EFS between the current and previous studies. A large heat collection efficiency and high combustion (reaction) efficiency are required to augment EFS. For example, the heat collection efficiency reached 94% in Fanciulli’s work [23], but its EFS was only 53%. In Shimokuri’s work [15], EFS reached 65.5%, although the heat collection efficiency of 75.3% was less than Fanciulli’s value (94%). In the present work, the EFS reached 81.8%,
Fig. 8. Comparisons of various MCP-TEGs. (a) Heat collection efficiency. (b) EFS. 8
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
6. Conclusions
Investigation, Methodology, Writing - original draft, Writing - review & editing. Dongya Zhu: Data curation, Investigation. Youqu Zheng: Funding acquisition, Methodology. Wenwen Guo: Validation.
An MCP-TEG was presented and evaluated in the present work. The TE, heat collection, and combustion efficiencies were investigated in detail. A maximum electric power of 25.7 W and an overall efficiency of 2.69% were obtained. Results showed that the heat collection and combustion efficiencies should be improved to obtain a high overall efficiency. The micro-/mesoscale combustor with an enhanced heat collection design was verified to be a potential solution due to its long residence time for flue gases. This feature provides a concrete way to improve MCP-TEG performance relative to those in previous studies. The use of a parameter named EFS, which is proposed as the sole parameter to pursue when the TE material and working temperatures are fixed, was developed to evaluate the performance of the MCP-TEGs. The EFS of the developed MCP-TEG was 81.8%, which is greater than those in previous studies.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments This work was partially supported by the key R&D plan of Zhejiang Province (Grant no. 2020C03115), National Natural Science Foundation of China (Grant nos. 51906220 and 51476145), and the Natural Science Foundation of Zhengjiang Province (Grant no. LQ19E060002).
CRediT authorship contribution statement Guoneng Li: Conceptualization, Validation, Funding acquisition, Appendix A
According to datasheets provided by the manufacturer, the physical properties of the TE material are dependent on temperature. Three order polynomial fittings were applied, and the results are as follows:
kp (T ) = tp1 + tp2 T + tp3 T 2 + tp4 T 3 kn (T ) = tn1 + tn2 T + tn3
T2
+ tn4
ρp (T ) = ep1 + ep2 T + ep3
T2
ρn (T ) = en1 + en2 T + en3
T2
(A-1)
T3
(A-2)
+ ep4
T3
(A-3)
+ en4
T3
(A-4)
αp (T ) = sp1 + sp2 T + sp3 T 2 + sp4 T 3
(A-5)
αn (T ) = sn1 + sn2 T + sn3
T2
+ sn4
T3
(A-6)
k, ρ and α are thermal conductivity, electrical resistivity and Seebeck coefficient, respectively. T is temperature. The subscripts p and n denote the P and N-type leg, respectively. Fig. 9 compares with present TE properties with those in previous reports [42–45]. The constants are as follows:
Fig. 9. Comparisons of present TE properties with those in previous reports. (a) k. (b) ρ. (c) α. 9
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
Table B1 Influence of h on convective heat loss and TE efficiency Case No.
h (W/m2-K)
Pconv (W)
ηTE (%)
1 2 3
5 10 15
7 14 21
3.27 3.30 3.33
Fig. 10. Verification of surface emissivity with IR camera set with ε = 0.4.
tp1 = 4.389, tp2 = −1.8168 × 10−2, tp3 = 2.437956 × 10−5, tp4 = 4.793196 × 10−10; tn1 = 4.09878, tn2 = −1.4976 × 10−2, tn3 = 1.799196 × 10−5, tn4 = 1.692996 × 10−9; ep1 = −6.7074 × 10−6, ep2 = 5.09 × 10−8, ep3 = 6.33243 × 10−11, ep4 = −5.31761 × 10−14; en1 = −1.51744 × 10−5, en2 = 1.142 × 10−7, en3 = −8.17056 × 10−11, en4 = −5.18487 × 10−15; sp1 = −1.0915819×10−4, sp2 = 1.67585 × 10−6, sp3 = −2.12 × 10−9, sp4 = 4.43743 × 10−14; sn1 = −4.3833365 × 10−4, sn2 = 2.90422 × 10−6, sn3 = −9.76 × 10−9, sn4 = 1.01202 × 10−11. Appendix B The heat flow rate of flue gases can be obtained as follows:
Pflue = mflue cp (Tin − Tout )
(B-1)
where the mass flow rate of flue gases, mflue, can be obtained directly with MFCs, and the flue gas heat capacity can be determined on the basis of the flue gas temperature and concentrations of flue gas species, which were measured with a gas analyzer. Tin and Tout are results measured with the thermocouples. Thus, the heat flow rate of flue gases can be calculated. In the present work, Pflue = 92 W when the input power was 957 W. The heat loss rate through natural convection can be obtained as follows: (B-2)
Pconv = hA (Tsurf − Tatm) 2
where the convective heat transfer coefficient, h, is assumed to be 10 W/m -K, and the total area exposed to the environment, A, can be measured. The average surface temperature can be measured with IR camera. In this work, Pconv = 14 W when the input power was 957 W. This heat loss rate is subjected to the risk of the assumption of h, which lies between 5 and 15 W/m2-K [46]. However, given that the magnitude of heat loss (~14 W) is only 1.46% of the total heat input, the above assumption of h is acceptable. Moreover, this assumption did not affect the overall efficiency result, which was obtained with Eq. (7). The influences of h on the convective heat transfer loss and TE efficiency are presented in Table B1. As shown in Table B1, the influence of the h assumption on the TE efficiency is within 0.06%; therefore, the uncertainty caused by this assumption is minimal. The heat loss rate through thermal radiation can be obtained as follows: 4 4 Prad = εσA (Tsurf − Tatm )
(B-3)
where the emissivity, ε, is assumed to be 0.4. In the present work, Prad = 8.9 W when the input power was 957 W. This heat loss rate is subjected to the risk of the assumption of ε, which lies between 0.23 and 0.68 in accordance with the materials used in the experimental setup. During the experiments, the copper surfaces exposed to the atmosphere were wrapped by a layer of fiberglass with a thickness of 20 mm. Then, aluminum foil was used to wrap the fiberglass. As a result, all the surfaces exposed to the atmosphere were alumina oxide. The assumption of ε = 0.4 is based on this material as verified by the measurements of a thermocouple and the IR camera (Fig. 10). Given that the magnitude of heat loss (~8.9 W) is only 0.93% of the total heat input, the above assumption of ε is acceptable.
10
Energy Conversion and Management 205 (2020) 112403
G. Li, et al.
References
on a catalytic premixed meso-scale combustor. Appl Energy 2016(162):346–53. [23] Fanciulli C, Abedi H, Merotto L, Dondè R, De Iuliis S, Passaretti F. Portable thermoelectric power generation based on catalytic combustor for low power electronic equipment. Appl Energy 2018;215:300–8. [24] Abedi H, Merotto L, Fanciulli C, Dondè R, Iuliis SD, Passaretti F. Study of the performances of a thermoelectric generator based on a catalytic meso-scale H2/ C3H8 fueled combustor. J Nanosci Nanotechnol 2017;17:1592–600. [25] Abedi H, Migliorini F, Dondè R, Iuliis SD, Passaretti F, Fanciulli C. Small size thermoelectric power supply for battery backup. Energy 2019;188:116061. [26] Guggilla BR, Alexander R, Bakrania S. Platinum nanoparticle catalysis of methanol for thermoelectric power generation. Appl Energy 2019;237:155–62. [27] Meng L, Li J, Li Q. A miniaturized power generation system cascade utilizing thermal energy of a micro-combustor: design and modeling. Int J Hydrogen Energy 2017;42:17275–83. [28] Bubnovich V, Martin PS, Henriquez L, Lemos MD. Filtration gas combustion in a porous ceramic annular burner for thermoelectric power conversion. Heat Transfer Eng 2019;40:1196–210. [29] Bobba MK, Gopalakrishnan P, Periagaram K, Seitzmann JM. Flame structure and stabilization mechanisms in a stagnation-point reverse-flow combustor. J Eng Gas Turb Power 2008;130:031505. [30] Chen C, Ronney P. Three-dimensional effects in counterflow heat-recirculating combustors. Proc Combust Inst 2011;33:3285–91. [31] Lloyd S, Weinberg F. A burner for mixtures of very low heat content. Nature 1974;251:47–9. [32] Li GN, Xu ZH, Zheng YQ, Guo WW, Dong C. Experimental study on convective heat transfer from a rectangular flat plate by multiple impinging jets in laminar cross flows. Int J Therm Sci 2016;108:123–31. [33] Juodis E. Extracted ventilation air heat recovery efficiency as a function of a building’s thermal properties. Energy Build 2006;38:568–73. [34] Rowe DM, Min Gao. Evaluation of thermoelectric modules for power generation. J Power Sources 1998;73:193–8. [35] Apertet Y, Ouerdane H, Goupil C, Lecoeur Ph. Influence of thermal environment on optimal working conditions of thermoelectric generators. J Appl Phys 2014;116:144901. [36] Torrecilla MC, Montecucco A, Siviter J, Strain A, Knox AR. Transient response of a thermoelectric generator to load steps under constant heat flux. Appl. Energ. 2018;212:293–303. [37] Freunek M, Müller M, Ungan T, Walker W, Reindl LM. New physical model for thermoelectric generators. J Electron Mater 2009;38:1214–20. [38] Li GN, Zhang S, Zheng YQ, Zhu LY, Guo WW. Experimental study on a stovepowered thermoelectric generator (STEG) with self starting fan cooling. Renew Energy 2018;121:502–12. [39] Gomez M, Reid R, Ohara B, Lee H. Influence of electrical current variance and thermal resistances on optimum working conditions and geometry for thermoelectric energy harvesting. J Appl Phys 2013;113:174908. [40] Yazawa K, Shakouri A. Cost-efficiency trade-off and the design of thermoelectric power generators. Environ Sci Technol 2014;45:7548–53. [41] Yee SK, Leblanc S, Goodson KE, Dames C. $ per W metrics for thermoelectric power generation: beyond ZT. Energy Environ Sci 2013;6:2561–71. [42] Riffat SB, Ma X, Wilson R. Performance simulation and experimental testing of a novel thermoelectric heat pump system. Appl Therm Eng 2006;26:494–501. [43] Pérez-Aparicio JL, Palma R, Taylor RL. Finite element analysis and material sensitivity of Peltier thermoelectric cells coolers. Int J Heat Mass Tran 2012;55:1363–74. [44] Liao M, He Z, Jiang C, Fan X, Li Y, Qi F. A three-dimensional model for thermoelectric generator and the influence of Peltier effect on the performance and heat transfer. Appl Therm Eng 2018;133:493–500. [45] Lan S, Yang Z, Chen R, Stobart R. A dynamic model for thermoelectric generator applied to vehicle waste heat recovery. Appl Energy 2018;210:327–38. [46] Holman JP. Heat transfer. tenth ed. U.S.A: Mc Graw Hill; 2008 Chapter 7, pp. 327–347.
[1] Sonoc A, Jeswiet J, Soo VK. Opportunities to improve recycling of automotive Lithium ion batteries. Proc. CIRR 2015;29:752–7. [2] Chou SK, Yang WM, Chua KJ, Li J, Zhang KL. Development of micro power generators – a review. Appl Energy 2011;88:1–16. [3] Xiao G, Yang T, Liu H, Ni D, Ferrari ML, Li M, et al. Recuperators for micro gas turbines: a review. Appl Energy 2017;197:83–99. [4] Zhang T, Wang P, Chen H, Pei P. A review of automotive proton exchange membrane fuel cell degradation under start-stop operating condition. Appl. Energy 2018;223:249–62. [5] Champier D. Thermoelectric generator: a review of applications. Energy Convers Manage 2017;140:167–81. [6] Hinterleitner B, Knapp I, Poneder M, Shi Y, Müller H, Eguchi G, et al. Thermoelectric performance of a metastable thin-film Heusler alloy. Nature 2019;576:85–90. [7] Vican J, Gajdeczko BF, Dryer FL, Milius DL, Aksay IA, Yetter RA. Development of a microreactor as a thermal source for microelectromechanical systems power generation. Proc. Combust. Inst. 2002;29:909–16. [8] D.G. Norton, K.W. Voit, T. Brüggemann, D.G. Vlachos, Portable power generation via integrated catalytic microcombustion-thermoelectric devices, in: Proc. Army Science (24th), 2005, Nov 29–Dec 2, Orlando, Florida. [9] Federici JA, Norton DG, Bruggemann T, Voit KW, Wetzel ED, Vlachos DG. Catalytic microcombustors with integrated thermoelectric elements for portable power production. J Power Sources 2006;161:1469–78. [10] Karim AM, Federici JA, Vlachos DG. Portable power production from methanol in an integrated thermoelectric/microreactor system. J Power Sources 2008;179:113–20. [11] Yoshida K, Tanaka S, Tomonari S, Satoh D, Esashi M. High-energy density miniature thermoelectric generator using catalytic combustion. J Microelectromech S 2006(15):195–203. [12] Jiang LQ, Zhao DQ, Guo CM, Wang XH. Experimental study of a plat-flame micro combustor burning DME for thermoelectric power generation. Energy Convers Manage 2011;52:596–602. [13] Marton CH, Haldeman GS, Jensen KF. Portable thermoelectric power generator based on a microfabricated silicon combustor with low resistance to flow. Ind Eng Chem Res 2011;50:8468–75. [14] Y. Hsu, I. Sapir, M. Azazzy, P. Ronney, Miniature power source with catalytic combustor and hybrid thermoelectric generator, in: Proc. PowerMEMS, 2012, Dec 2 – Dec 5, Atlanta, Georgia. [15] Shimokuri D, Taomoto Y, Matsumoto R. Development of a powerful miniature power system with a meso-scale vortex combustor. Proc Combust Inst 2017;36:4253–60. [16] Shimokuri D, Hara T, Matsumoto R. Development of a small-scale power system with meso-scale vortex combustor and thermo-electric device. J Micromech Microeng 2015;25:104004. [17] Yadav S, Yamasani P, Kumar S. Experimental studies on a micro power generator using thermo-electric modules mounted on a micro-combustor. Energy Convers Manage 2015;99:1–7. [18] Aravind B, Khandelwal B, Kumara S. Experimental investigations on a new high intensity dual microcombustor based thermoelectric micropower generator. Appl Energy 2018;228:1173–81. [19] Aravind B, Raghuram GKS, Kishore VR, Kumar S. Compact design of planar stepped micro combustor for portable thermoelectric power generation. Energy Convers Manage 2018;156:224–34. [20] Aravind B, Saini DK, Kumar S. Experimental investigations on the role of various heat sinks in developing an efficient combustion based micro power generator. Appl Therm Eng 2019;148:22–32. [21] Singh T, Marsh R, Min G. Development and investigation of a non-catalytic selfaspirating meso-scale premixed burner integrated thermoelectric power generator. Energy Convers Manage 2016;117:431–41. [22] Merotto L, Fanciulli C, Dondè R, Iuliis SD. Study of a thermoelectric generator based
11