Research PaperAnalysis of the fins geometry of a hot-side heat exchanger on the performance parameters of a thermoelectric generation system
Introduction
Current progress in thermoelectric materials indicates that in the near future, thermoelectric generation could be a competitive technology for the improvement of energy efficiency. Thermoelectric phenomena – appearing as the Seebeck, Peltier and Thomson effects – occur in every material except superconductors. is a parameter that specifies the usefulness of thermoelectric materials for energy conversion. The greater the of the thermoelectric material, the higher efficiency of the thermoelectric device (e.g. thermoelectric generator, heat pump). The highest figure of merit has been shown recently for SnSe single crystals [1]. Good thermoelectric material should have a high value Seebeck coefficient () to generate high voltage, low electrical resistivity () to minimize Joule-Lenz heat, and low thermal conductivity () to minimize heat loss [2]. Selection criteria for thermoelectric materials used to generate modules for the TEG and thermoelectric coolers (Peltier modules) are discussed in [3]. Nowadays, Bi2Te3 – Sb2Te3 modules with – which is about 5–7% in terms of conversion efficiency – are considered to be the best candidates for most uses. Unfortunately, thermoelectric materials vaporize at temperatures above 400 °C, which limits their use in thermoelectric modules to temperatures of approximately 300 °C. Many TEG prototypes and conceptions have already been presented by researchers [4], [5], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Qui et al. [4] integrated a TEG with residential boiler to construct an autonomously-powered unit. A prototype was investigated and conclusions indicated that electricity generated at 214 °C was sufficient to supply the whole heating system. A holistic approach to mathematical modelling of automobile TEG, including heat, fuel, and electrical losses, is presented in [5]. A modeled generator can reach up to 820 W with 9% efficiency for a 106 kW engine.
A thermoelectric generator consists of a hot-side heat exchanger (HHX) to receive heat from a hot medium, thermoelectric modules, and a cold-side heat exchanger (CHX), which acts as a heat sink for cooling the modules (Fig. 1). To maximize overall performance of the device, all of these parts need to be carefully matched in terms of their thermodynamic parameters. Jang et al. [6] modeled a finned heat exchanger as a part of a thermoelectric generator embedded in the walls of a chimney and compared the results with conducted experiment. The radiation and convection effects of flue gases were considered in a three-dimensional numerical simulation. Esarte et al. [7] modeled three HX geometries (spiral, zig-zag, straight fins) for a TEG, taking into account pressure losses. They concluded that the pump’s electrical energy consumption is proportional to the pressure drop through the circuit in which the TEG is fitted. The design parameters should be those that provide the most appropriate pair of values for both and . A numerical model for a TEG with a parallel-plate HX was developed based on a one-dimensional, differential equations model representing energy conservation for the HX [8]. Numerical calculations revealed that variation in the temperature of the fluids in the TEG is almost linear. This is different from the logarithmic change in the ordinary parallel-plate HX. The net power density per HX volume in the TEG was estimated at 45 kW/m3 [9]. The model of TEG suited for central heating boilers was presented by Martinez et al. [10]. It was concluded that one of the best methods for optimizing the TEG is to determine the region in the space that leads to the highest electrical power with the smallest number of runs. Different heights and thicknesses of fins on the hot-side HHX were analyzed [10]. A parametric modeling of the HX for automotive applications was developed by Su et al. [11]. Response surface methodology and a multi-objective generic algorithm based on a central composite rotatable design was applied in optimization of the HX.
Most TEG studies have been done on automotive applications. Hsiao et al. [12] prepared the simulation of a TEG combined with a cooling system to enhance the efficiency of the engine. Yu et al. [13] proposed and constructed a thermoelectric waste heat recovery system, which uses a maximum power point tracking system for gasoline vehicles and hybrid electric vehicles. Wang et al. [14] investigated the simulation of TEGs using exhaust gases as a heat source, and changed the performance conditions to a convection heat transfer coefficient of the hot side. This showed that an increase in this coefficient can significantly increase power output, and that it is more efficient to increase it on the hot side than on the cold side.
Two methods of enhancement of heat transfer in a TEG were investigated by Chen et al. [15] using rectangular off-set strip fins and metal foams as porous media. The foam increased heat transfer more significantly than the fins, but it also had a much larger . When using HXs for waste heat recovery [16], it was concluded that fins are more appropriate than foams and porous materials due to their lower .
The design approach in the heat exchanger theory suggests that a uniform temperature difference field is the most effective. Analytical and numerical calculations presented by Zengyuan [17] shows the validity of this principle for a counter-flow HX. The smaller entropy generation due to heat transfer corresponds to the larger uniformity factor of the temperature difference field in the HX. An analytical model has been developed for predicting and optimizing the performance of bidirectional fin heat sinks in partially confined configurations [18]. A thermoelectric generator is composed of two heat exchangers for hot and cold sides, and thermoelectric modules. Heat exchangers designed for thermoelectric generators differ from standard types. In typical heat exchangers, thermal resistivity of the baffle between working fluids has to be minimized, while in TEG heat exchangers, it has to be maximized to achieve a high temperature difference between the hot and cold sides. The higher the temperature difference, the more output power is generated by thermoelectric modules. Temperature distribution in TEGs is addressed in [8]. This study presents a numerical model for predicting the performance of a thermoelectric generator with a parallel-plate HX.
Section snippets
Analysed model
The thermoelectric generator system which was investigated is shown in Fig. 1. The TEG consists of a gaseous HHX, thermoelectric modules, and a CHX, which is modeled in our work as an isothermal surface. To account for internal load in the entire system analysis, a heat source and blower were introduced.
In the TEG, both the HHX and CHX introduce additional into the installation in which the TEG is being used. To overcome these losses, the additional power should be allocated to auxiliary
Results
Due to the fact that many cases were analyzed, only the most significant values are presented for certain calculated parameters. To determine HHX effectiveness, two approaches are presented. The inlet-outlet temperature relationship and calculation of effectiveness using the Carnot formula indicate the usefulness of the proposed HHX with respect to heat transferred through the modules.
Summary
This paper presents the results of numerical studies on the influence of the thermal, flow, and geometrical parameters of the power generated in a TEG system. The analysis takes into account the influence of the TEG on the installation in which it is being used (TEG power needs). The design of the device is based on a hexagonal HHX and six different fin geometries. The results show that geometries TEG5_F0 and TEG7_F0 are able to generate up to 350 W of net electrical power. In these cases, the
Acknowledgements
This study has been supported by the grants: 11.11.210.216, 11.11.160.438, 15.11.210.379 and 15.11.210.396.
This research was supported in part by PL-Grid Infrastructure.
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