Waste heat recovery from a biomass heat engine for thermoelectric power generation using two-phase thermosyphons

Highlights

• A novel TEG based heat recovery system from biomass engine is proposed.

• Waste heat to electric power is realized through TEGs and two-phase thermosyphons.

• Optimum operating conditions for biomass engine and thermosyphons are proposed.

• Parametric studies along with battery charging time are carried out.

• A thermal resistance based model to predict temperature difference is developed.

Abstract

In this study, we propose a thermoelectric generator (TEG) based power generation system operated through waste heat of a biomass engine. Power generated by TEGs is utilized for recharging a 12 V uninterruptible power source (UPS) battery. Experiments are done to study the variation of power output, current and conversion efficiency with average flue gas temperature, output voltages, thermosyphon filling ratio (TFR) along with source and sink temperatures. Gasifier operation is optimized to identify the appropriate equivalence ratio (ER). The optimized ER for the present system is evaluated as 0.305 yielding a maximum flue gas temperature of 283 °C. Thereafter, experiments are conducted to study various performance parameters when 48 TEGs are provided on the two-phase octagonal-shaped thermosyphons. Experimental results indicate that the maximum open circuit voltage of the present system is 31.52 V (17.12 V at ΔT_max.,1 = 39 °C and 14.40 V at ΔT_max.,2 = 31 °C) at an optimum TFR of 0.496. A thermal resistance based model is finally developed from which the maximum temperature gradient across the TEG for two thermosyphons is found as 40.12 °C with a maximum relative error of 14.91% between model and experimental values. The total power generated from the system is found as 1.033 W, whereas, the maximum conversion efficiency is calculated as 2.218%.

Introduction

Thermoelectric generator $\left(TEG\right)$ modules work on the principle of Seebeck effect within semiconductor materials which directly convert available heat energy into electrical energy. $TEG\text{s}$ produce clean energy and serve as potential candidates for transforming low temperature waste heat into electrical power [1]. Considerable progress has been made in the recent past towards $TEG$ power generation through waste heat recovery from various sources. Nuwayhid et al. [2] proposed a low cost locally available $TEG$ design for power generation from wood and diesel-based stoves. For recovering waste heat from a stove burner, Nuwayhid et al. [3] further tested the performance of $TEG$ fitted to the hot side of a domestic woodstove, where the heat sink was cooled by air under natural convection. Borelli and de Oliveira [4] presented the performance and cost analyses of $TEG$ based power generator from combined gas turbine-steam turbine power plants. For utilizing the heat transfer between the hot and the cold sides of a heat exchanger, Crane et al. [5] developed and tested the performance of a $TEG$ system. A similar concept of heat extraction by $TEG\text{s}$ from plate heat exchangers was also demonstrated by Niu et al. [6]. Utilizing the heat generated on the surface of biomass cook stove, Champier et al. [7] experimentally studied the performance of $TEG\text{s}$ for electric power generation. Singh et al. [8] experimentally investigated the power generation from solar pond using $TEG\text{s}$ under different temperature gradients. Dai et al. [9] experimentally investigated the $TEG$ performances when combined with the electromagnetic pump for harvesting the waste heat from liquid metals. He et al. [10] experimentally investigated the cogeneration (heat and power) study of $TEG$ integrated with solar heat pipe, and their analytical model was validated with the experimental results. Zheng et al. [11] proposed the concept of thermoelectric cogeneration system using solar energy and domestic boiler assisted heat source. Rezania et al. [12] optimized the areas of n and p-type thermoelectric elements using FLUENT software incorporated finite element method. Date et al. [13] proposed a novel system in which $TEG$ was combined with a water desalination system for power generation and water purification using low grade thermal energy. Zhao et al. [14] proposed a hybrid system comprising a fuel cell, a $TEG$ and a regenerator to produce power using waste heat generated from fuel cells. Dai et al. [15] proposed a combined system of evacuated tube solar collector (for heating the water in the pipe), and $TEG$ (based on Bi₂Te₃ material) for effectively converting the excess solar heat into electricity. Zhu et al. [16] proposed a combined solar photovoltaic-$TEG$ system for increasing the thermal efficiency of the complete system. Ziapour et al. [17] used a combined solar pond and organic Rankine cycle assisted $TEG$ system where the heat contained by the organic working fluid from the turbine exhaust was utilized as heat source for $TEG$. Li et al. [18] experimentally studied the performance of a biomass stove integrated with eight $TEG\text{s}$. In order to maintain the temperature difference, the source heat was supplied through copper flat-plates with fan-based air cooling of the sink. Kim et al. [19] fabricated TEGs for generating power using human body as a heat source. They found a maximum power density of 2.28μW/cm² using naturally convective heat sink. Haiping et al. [20] presented a novel design in which $TEG\text{s}$ were connected to a microchannel heat pipe array to exploit the heat energy obtained from a solar photovoltaic-thermal hybrid system. Recently, Karthick et al. [21] investigated the effect of various parameters such as, roughness of surface, contact pressure, thermal conductivity of interfacial material and temperature of heat source on the voltage and power outputs of the $TEG$. Shittu et al. [22] numerically compared the performance of photovoltaic-$TEG$-heat pipe combined system with sole photovoltaic-thermoelectric and photovoltaic systems. The variation of output power and efficiency are studied at different wind speed, ambient temperature and solar concentration ratio. Li et al. [23] developed a new hybrid system consisting of a photovoltaic-$TEG$ system engaged with array of heat pipes. They obtained 14.0% higher efficiency with the new system as compared to the simple photovoltaic-$TEG$ system. Mahmoudinezhad et al. [24] studied two types of $TEG$ (made of Bi₂Te₃ and Zn₄Sb₃) to observe output parameters such as short circuit current, open circuit voltage and maximum power. Tappura et al. [25] fabricated thin film $TEG\text{s}$ from aluminium-doped zinc oxide material on the substrates having low cost with large area (0.33 m²). They evaluated the performances at temperature gradients below 50 K.

From the above discussion, it is apparent on one hand that for harvesting low temperature thermal energy for $TEG$ based power generation; the usage of two-phase thermosyphon is a potential concept [8,26]. For a two-phase thermosyphon at various operating conditions, Zhang et al. [27] developed a generalized model to analyze its thermal performance. They validated the simulation results with experimental values. Naresh and Balaji [28a] examined the performance of a two phase thermosyphon with six internally located fins using two working fluids (water and acetone) at various thermosyphon filling ratios $\left(TFR\right).$The maximum heat transfer was realized at an optimum $TFR$ of 0.5. On the other hand, the power production from biomass is an encouraging alternative where its availability is large, which otherwise is wasted in the landfill areas. Towards this, biomass gasification and anaerobic digestion processes are found suitable for energy conversion [28b]. Power generation through biomass gasification has many advantages such as its renewable and inexpensive nature, carbon dioxide neutrality and ease of availability [29]. However, while using it for power generation, a considerable portion of thermal energy from syngas is generally lost in the form of exhaust flue gases due to a fixed thermal efficiency and thermodynamic limitations of the engine. These exhaust flue gases possess sufficiently high temperature (550–950 K) that can be further processed for power generation [30].

It is evident that power production from $TEG\text{s}$ has been accomplished with several heat sources, but, power generation from $TEG\text{s}$ combined with thermosyphon for recovering waste heat from biomass heat engine is not yet studied. In view of this research gap, here we study the power generation potential of $TEG\text{s}$ integrated thermosyphon operated using exhaust gas of a biomass engine. Parametric study is done to identify the equivalence ratio $\left(ER\right)$ of the biomass gasifier at which power output from $TEG$ system will be maximum. In particular, parameters such as open circuit voltage, short circuit current, output power, heat conversion efficiency and figure of merit have been studied to optimize the system's performance. Subsequently, a theoretical comparison is also done against the experimental observations using a thermal resistance network model. Using waste heat driven $TEG$ power, a 12 V uninterruptible power source $\left(UPS\right)$ battery is successfully charged, and its utility for real life applications is demonstrated. Further details are discussed in the next section.