Development of a numerical model for performance prediction of an integrated microcombustor-thermoelectric power generator

Highlights

• Developed a numerical model of a microcombustor-thermoelectric power generator.

• Similar volume to that of a conventional electrochemical battery.

• Thermal and power characteristics analyzed extensively.

• Maximum power output of 5.6 W with an efficiency of 6.86% reported.

Abstract

This paper presents the development of a numerical model for a thermoelectric generator integrated with a novel microcombustor to predict the thermoelectric performance of the system. The novelty of the work lies in the development of a numerical model to predict and analyze the system’s thermal characteristics and thermoelectric performance with numerical simulations. The system consists of a microscale combustor and two Bi₂Te₃ thermoelectric modules mounted on the combustor. The combustor has backward facing steps and a recirculating cup to enhance its flame stability and thermal characteristics over other combustors. Following the thermal analysis of the microcombustor, the model for predicting the power output from the thermoelectric modules is validated and then integrated by using separate sub-routine. A maximum open-circuit voltage of 8 V was reported for the integrated system. For a load resistance of 2.8 Ω, a power output of 5.6 W was obtained at a conversion efficiency of 6.8%. It can be concluded that the successful integration of a thermoelectric model into CFD software will provide great impetus to future studies on thermoelectric generators, while the optimized combustor, with a volume comparable to that of a dry cell, can serve as a viable replacement for electrochemical batteries.

Introduction

A high energy density and an environment-friendly nature are two essential prerequisites for any portable power source. The increasing demand for thermoelectric generators [1,2] as portable sources of power can be primarily attributed to the usage of micro/meso scale combustors as heating sources, powered using high energy density hydrocarbon fuels [3]. The energy density of typical hydrocarbon fuel is ∼20–100 times higher than that of lithium ion batteries [4]. The low conversion efficiency of the thermoelectric generator hinders its use in practical systems [5]. However, a hydrocarbon fuelled thermoelectric generator with an overall conversion efficiency of ∼5% is nearly six times better than that of the conventional chemical batteries [4]. Therefore, the increasing demand for thermoelectric generators has prompted researchers to investigate the viability of portable combustion-based systems. Several experimental studies on combustion-based thermoelectric generators have shown that the conversion efficiency of such systems can be further enhanced through proper thermal management techniques [6,7]. The applications of electrochemical batteries as possible sources of power for portable applications are limited due to their low energy density, bulky volume, and negative impact on the environment [8]. On the other hand, combustion-based power generators are preferred for such applications due to the significantly higher energy density of hydrocarbon fuels and light weight of the device [9]. However, coupling a combustor with a thermoelectric generator for miniature scale applications requires a thorough investigation to maximize the performance of the combustor. The various configurations of microscale and mesoscale combustors suitable for TEG and TPV applications have been analyzed numerically and experimentally for understanding the most efficient form of proper thermal management [10,11]. However, the aforementioned studies show that extracting the maximum heat input from microcombustors for TEGs is extremely difficult because the combustors are prone to flame stability issues due to their high surface to volume ratio [12]. These issues were partially resolved through the use of excess enthalpy and flow recirculation techniques such as cavity [13], bluff body [14], catalytic combustion [15,16], backward facing steps [17,18], porous media [19,20] etc.

Extensive research has been carried out in the field of micropower generation by using thermophotovoltaic cells integrated with microscale combustors [21,22]. Alipoor et al. [23] numerically modelled a 3D microcombustor integrated with a TPV cell. Combustion was carried out in a U-shaped microtube with a maximum efficiency of 6.3%. Ahn et al. [24] integrated a SOFC (solid-oxide fuel cell) with a Swiss-roll combustor and obtained a power density as high as 420 mW/cm² by using a BSCF cathode. Despite having higher efficiencies than thermoelectric systems, TPVs and fuel cells are not as viable as their TEG counterparts due to bulky volumes and shorter life span. However, the low conversion efficiencies of TEGs are being investigated extensively, with promising results having been reported recently [25,26].

The problems faced by TPVs and fuel cells and the development of highly efficient small-scale combustors have accelerated research in the field of thermoelectric power generation [27] for portable applications. Schaevitz et al. [28] fabricated a combustion-based TEG system for MEMS applications with a capacity of delivering electric power at 7 V and a total system efficiency of ∼0.02%. Qiu and Hayden [29] developed a novel cascading power generation system by using a TEG and TPV and obtained a thermoelectric power output of 306.2 W, with an overall efficiency of 5.2%. Liu et al. [30] focused on improving the compactness of the power generating system for large-scale power production. Their experiments demonstrated that a power output of 1 kW was obtained from a system of 6000 modules having a total volume of 0.01 m³. Fan et al. [31] and Zhang et al. [32] studied the impact of segmented thermoelectric generators and obtained the optimum length ratio for maximum power output. Merotto et al. [33] utilized catalytic combustion to obtain a power output of 9.8 W with an overall efficiency of 2.85%. Angeline et al. [34] reported an increase of ∼53% in the power output through the application of a hybrid TEG over a conventional TEG. Shimokuri et al. [35] used a vortex combustor and integrated it with a thermoelectric generator to obtain a load output power of 18.1 W with a conversion efficiency of 3.01%. This model was later improved [36] with a power output of 10 W for a duration of 5 h with a 250 g butane cartridge. Aravind et al. [37] prototyped a portable thermoelectric power generator by using a planar stepped combustor and obtained a power density of 0.12 W/mm³ and a maximum conversion efficiency of 4.03%. This system was further improved by the same group wherein a dual microcombustor configuration was employed as a heat source for the TEG [38]. A maximum power output of 4.52 W with a maximum overall efficiency of 4.66% was reported. Furthermore, to improve the portability of a combustion-based TEG, Fanciulli et al. [39] designed a catalytic combustion-based TEG with its volume comparable to that of an alkaline battery which operated at ambient temperatures (∼40 °C) with a power output of 1 W and a DC load voltage of 0.75 V.

During the last decade, the development of several numerical and analytical models to predict the performance of the thermoelectric generators has been reported [40,41]. Various factors affecting the system performance such as thermoelectric material properties, the orientation of individual p-n couples, and thermal management have been studied to enable the development of highly efficient modules at lower costs [42,43]. Meng et al. [44] numerically modelled a commercial thermoelectric generator and managed to account for the properties of the thermoelectric materials. They reported a maximum power output of 0.13 W with a maximum conversion efficiency of 0.87%. Chen et al. [45] simulated a three-dimensional thermoelectric generator in a CFD environment with the help of custom-defined scalars and functions. Zhou et al. [46] prototyped a new design for TEGs with a cylindrical shell and straight fins and obtained a maximum TEG efficiency of approximately 5.5%. Meng et al. [47] studied the numerical integration of a combustor with a TEG and a TPV without particular emphasis on flame dynamics and performance of the combustor. It is interesting to note that in most of these cases, constant hot and cold side temperatures are assumed for predicting the TEG performance to reduce the complexity of the problem. However, in a practical system, a non-uniform thermal gradient on the hot side can be expected due to varying flame dynamics in the combustor. This affects heat flow from the heat source, resulting in a significant change in the TEG performance. Variation in the temperature of the hot side due to flame dynamics in combustion-based heat sources is predominant due to the existence of unstable flames in such small combustors [17]. Although several experimental studies report the performance of micropower generators by using thermoelectric modules, numerical investigations of such integrated microcombustor-thermoelectric micropower generators are lacking and need to be explored to understand the effect of various parameters on the system’s overall performance.

The present study involves numerical modelling of a thermoelectric generator coupled with an optimized microcombustor. Though there are several numerical studies reported on micro/mesocombustors and thermoelectric generators separately, studies on an integrated system, which are expected to be of great interest from the practical viewpoint, are lacking. The novelty of the present study lies in the extensive numerical analysis of the flame dynamics of the combustor by using detailed chemical kinetics and simultaneously optimizing the performance of the thermoelectric generator for the integrated combustor-TEG power generation system. This is the first such attempt to numerically model an integrated combustor-thermoelectric generator system. The thermal characteristics of the combustor are studied for different operating conditions. A TEG model is then developed and successfully validated with the specification sheet provided by the manufacturer of the TEG modules. Furthermore, the TEG modules are coupled with a double stepped microcombustor and the performance of the integrated system is analyzed and validated successfully.