• No results found

A Numerical Study and Demonstration of Exhaust Gas Heat Recovery System Using Thermoelectric Generator

N/A
N/A
Protected

Academic year: 2020

Share "A Numerical Study and Demonstration of Exhaust Gas Heat Recovery System Using Thermoelectric Generator"

Copied!
9
0
0

Loading.... (view fulltext now)

Full text

(1)

73 Available online at www.ijiere.com

International Journal of Innovative and Emerging

Research in Engineering

e-ISSN: 2394 - 3343 p-ISSN: 2394 - 5494

A Numerical Study and Demonstration of Exhaust Gas Heat

Recovery System Using Thermoelectric Generator

Gokul Raj C R

a

, Nandhu Krishna

b

and Aneesh K Johny

c

a,b,cPG Scholar , Department of Mechanical Engineering, Mar Athanasius College of Engineering, Kerala, India

ABSTRACT:

Fossil fuels rule the energy sector for more than a century. About 60% of total energy generated in the world coming from fossil fuels like coal, natural gas, petroleum oils, etc. The main consumer of fossil fuel is internal combustion engines. The main problem of internal combustion engine is the thermal efficiency, only up to 30% of the total heat supply is converted to useful work. The rest of the heat energy is lost in the form of various losses. The main form of loss is exhaust gas, which can be recovered using various recovery methods like EGR, turbocharging etc. Scientists and engineers have been researching on the exhaust gas heat recovery and engine performance for many decades. Many succeeded to demonstrate different ways but none of them are effective to use commercially. This work detailed about designing and demonstrating a system that utilizes waste heat by using a thermoelectric generator (TEG). Also a numerical simulation will be conducted to verify the results obtained from demonstration.

Keywords: Exhaust gas, Heat recovery, Thermal Electric Generators, Waste Heat

I. INTRODUCTION

The recent trend about the best ways of using the deployable sources of energy into useful work in order to reduce the rate of consumption of fossil fuel as well as pollution. The internal combustion engines consume a major portion of fossil fuel around the globe. The engine converts only 30 to 40% of total heat supply into useful work [1]. Through the exhaust gasses and engine cooling systems, the remaining heat is expelled to the environment, resulting in the entropy rise and serious environmental pollution, so it is a necessity to utilize waste heat into useful work. The recovery of waste heat not only conserves fuel, but it also reduces the amount of waste heat and greenhouse gasses dumped to the environment. The Internal Combustion Engine has been a primary power source for automobiles over the past century. As the most widely used source of primary power of the engine for the transportation, construction and agricultural sectors, the internal combustion engines have consumed more than 60% of fossil oil. So energy conservation on the engine is one of best ways to deal with these problems since it can improve the energy utilization efficiency of the engine and reduces emissions. Lots of successful research have done on heat recovery by scientists and engineers aimed to improve engine efficiency, however, exhaust gas heat recovery is considered as one of the most effective in all energy saving technologies.

(2)

74 Figure 1. Total Fuel Energy Content in I. C. Engine

A. Objectives

The main objective of this project is to study the possibility of recovering the exhaust gas heat energy from the exhaust gasses which are normally given out using a thermoelectric generator. For this purpose, a new system has to be designed and installed on an existing engine setup. The design includes thermoelectric generator and a booster circuit. The silencer has to be modified so that the heat should flow from the silencer into a side of the thermoelectric generator by means of conduction and the other side of the thermoelectric generator has to be cold for the production of a potential difference. There must be a significant temperature difference between both sides of the thermoelectric generator for deterring performance from TEG. The thermoelectric generator has to be position in such a way that the silence for maximum heat availability.

B. Methodology

The methodology for this study includes finding out the maximum temperature available from the engine in the exhaust. Using this temperature a suitable thermoelectric generator is designed and selected. The step up will be created to demonstrate the conversion of energy, for that the TEG is connected along with a mobile phone to charge. The proper placement of the thermoelectric generator is necessary for optimum performance, an appropriate position is selected on the silencer. The part of the silencer is made to a square shape to fix the thermoelectric generator. The thermoelectric generator is fixed to the silencer in such a way that one side of the surface has maximum contact hence maximum heat transfer. The output of the generator is connected to the step-up booster for boosting of the voltage. After boosting it is connected to a mobile phone charger.

II.THEORETICAL BACKGROUND

A. Working Principle

In 1821, Thomas Johann Seebeck discovered that when two dissimilar conductors subjected to a thermal gradient produces a voltage. At the heart of the thermoelectric effect is the results of the diffusion of charge carriers due to the temperature gradient in a conducting material and heat flow. The voltage difference is the result of the flow of charge carriers between the hot and cold regions. The effect is then named after him. In 1834, the reverse effect is discovered by Jean Charles Athanase Peltier, that running an electric current through the junction of two dissimilar conductors could cause it to act as a heater or cooler depending upon the direction of charge flow. The reverse effect is then termed as Peltier effect. The fig. 2 explains the working of thermoelectric generators.

(3)

75 Figure 2. The working schematic of TEG [13]

The mathematical representation of Seebeck effect as follows, the potential difference due to thermal gradient is measured in terms of electromotive force (emf). Ohm’s law was modified by this electromotive force by generating currents even in the absence of voltage differences (or vice versa); the local current density J can be represented as:

𝐽 = 𝜎(−∆𝑉 + 𝐸𝑒𝑚𝑓) − − − − − − − − − −(1)

Where V the local voltage and σ is the local conductivity. In general, the Seebeck effect is described locally by the creation of an electromotive field:

𝐸𝑒𝑚𝑓= −𝑆Δ𝑇 − − − − − − − − − −(2)

Seebeck noted that the voltage is proportional to the temperature gradient between the hot and cold sides of the material. The Seebeck coefficient (S) varies for different material and different temperature of operation. The Seebeck coefficient is thus defined as:

𝑆 = −∆𝑉

∆𝑇 − − − − − − − − − − − (3)

In this equation ΔV, ΔT is the voltage difference and temperature difference respectively between the hot and cold sides. The negative sign comes from the direction of electron flow, which is opposite to the potential difference. When electron flow from hot side to the cold side, cause an electrical current and this current is depending upon the temperature gradient. The larger the temperature difference, electrical current produced will be more [2].

For a thermoelectric device working between two temperature limits, the lower limit being “Tl” and higher limit

being “Th”, the maximum efficiency is given by:

𝜂 = Δ𝑇 (√1 + 𝑍𝑇 − 1)

𝑇ℎ(√1 + 𝑍𝑇 + (𝑇𝑐 𝑇ℎ ⁄ ))

− − − − − − − − − −(4)

Where ∆𝑇 = 𝑇ℎ− 𝑇𝑙

B. Availability of Waste Heat From IC Engine

The exhaust gas is the main source of waste heat from the internal combustion engine and the heat content of exhaust is a function of temperature and the mass flow rate:

𝑄̇ = 𝓂̇𝐶𝑝∆𝑇 − − − − − − − − − − − (5)

(4)

76

III.DESIGN

A. Component Design and Specification

This work was done by modifying the exhaust of a Maruthi 800 engine. In order to prevent and utilize the heat energy loss through the exhaust pipe, the heat energy of the exhaust is converted into electrical energy by means of a thermoelectric generator (TEG). Here we use a TEG of size 40mm length 40mm width 3.7mm thickness. It can accommodate a temperature difference of 20°C to 100°C and produce an open voltage of 0.97V to 4.8V and it produces a current of 225mA to 669mA. The engine assembly is fitted on a specially designed frame with the starter mechanism. The exhaust pipe is reshaped so that it can easily be fixed with a thermoelectric generator on the surface. The curved surface of the exhaust pipe is flattened based on the dimension of the TEG. The TEG is fixed on the modified surface with high heat resistant adhesive. To maintain a temperature difference in the TEG a heat sink is attached to the top surface of TEG and the heat is transferred to the frame by a conductor which is attached to the heat sink to frame.

A booster circuit is designed and attached to the TEG to maintain a steady output. A USB connection is given with the booster circuit so that the output can be tapped from the TEG. A schematic diagram of the demonstration is given in fig. 3. The design is mainly aimed to modify the engine exhaust system so that the integration of the thermoelectric generator won’t affect the smooth operation of the engine. Fig. 4 represents the fully fabricated assembly of the demonstration.

(5)

77 Figure 4. Engine assembly

B. Thermoelectric Generators

Conversion of heat directly into electric energy is called thermoelectricity, or vice versa. The Joule's law states that a conductor carrying a current generates heat at a rate proportional to the product of the voltage (V) through the conductor and the current (I). A circuit of this type is called a thermocouple; a number of thermocouples connected in series are called a thermopile [5]. Thermoelectric generators take a temperature difference and turn it into electrical power. Amazingly, these materials can also be run in reverse [3]. If you put power into a thermoelectric generator you will create a temperature difference.

Figure 5. TEG [12]

Fig. 5 shows thermoelectric generator and its construction. It consists of a series of thermocouples sandwiched between two ceramic substrate and two output leads are connected to the end. Thermoelectric devices may potentially produce twice the efficiency as compared to other technologies in the current market [6]. The thermoelectric generator is used to convert thermal energy from different temperature gradients existing between hot and cold ends of a semiconductor into electric energy by the principle of Seebeck effect [4]. Fig. 6 represents an actual thermoelectric generator element in fig. 7 shows the modification of the engine exhaust system so that the heat energy from the exhaust can be utilized.

(6)

78 Figure 7. TEG fixed on exhaust pipe

IV.NUMERICAL ANALYSIS

A. Geometry and domain

The numerical study was conducted to find out the temperature difference that can be achieved using a standard fin. A fin design details are given in the fig 9. The fin has the same base area of the TEG. The fin is attached to improve the heat transfer rate. An array of rectangular fins protruding vertically upward were used. The number of fins used will be 5. The surrounding fluid enters the channel from the open sides and develops a vertical velocity component as the density of the working fluid, air, changes as it get heated up, a single chimney type flow pattern is expected. Due to the variation of temperature density of the air changers and this creates a density gradient which creates an upward flow of air in the fluid domain. This flow eventually takes out the heat from the fin surface and a proper temperature difference between both sides of the TEG can be achieved. All the dimensions given in the reference figures are in mm.

Figure 8. Geometry of the TEG and fins

The width and length of the fin base is taken as (W x L) 40mm x 40mm, same as that of the dimensions of TEG. The thickness of the fin is taken as 2 mm (t) and the spacing between the fins are taken as (S) 3 mm. The height of the fin (Hf)

is considered as 10 mm from the top surface of the base.

B. Governing Equations

The temperature and velocity fields in the domain between the fins are governed by conservation of mass momentum and energy equations of the fluid. These equations combined with 3D heat conduction equation for the fin array are given below [11].

The Cartesian coordinate system is used for the modelling, the basic governing equations are follows

Continuity Equation:

𝜕𝑈

𝜕𝑥+

𝜕𝑉

𝜕𝑦= 0 − − − − − − − − − − − − − − − − − −(6)

Momentum Equations:

𝜌 (𝑈𝜕𝑈

𝜕𝑥 + 𝑉

𝜕𝑉

𝜕𝑦) = −

𝜕𝑃

𝜕𝑥+ 𝜇∇

2𝑈 − − − − − − − − − (7)

𝜌 (𝑈𝜕𝑈

𝜕𝑥+ 𝑉

𝜕𝑉

𝜕𝑦) = −

𝜕𝑃

𝜕𝑦+ 𝜇∇

2𝑉 + 𝜌𝑔𝛽(𝑇 − 𝑇

(7)

79 Energy Equation:

𝜌𝐶𝑝(𝑢𝜕𝑇

𝜕𝑥+ 𝑣

𝜕𝑇

𝜕𝑦) = 𝑘∇

2𝑇 − − − − − − − − − − (9)

∇2= 𝜕 2

𝜕𝑥2+ 𝜕2

𝜕𝑦2 − − − − − − − − − − − − − − − (10)

Rayleigh Number:

𝑅𝑎 =𝑔𝛽𝑞𝐻

4

𝛼𝑣𝑘 − − − − − − − − − − − − − − − − (11)

Where,

g is the local gravitational acceleration µ is the dynamic viscosity.

β is the coefficient of thermal expansion, α is the thermal diffusivity

C. CFD simulation

The CFD package, ANSYS Fluent 16.2 was used in the three dimensional simulation of the experimental setup. To simulate the problem, the whole fin arrangement is considered in a large fluid domain. The domain is filled with a Newtonian fluid, air in this particular case. Both conduction and convention parameters are considered for the simulation. The inlet and outlet conditions were selected as pressure based, atmospheric pressure is given as the inlet and outlet boundary conditions. Four sides of the fluid domain and the bottom portion as well selected as inlet and the top side of the domain is selected as outlet. From heat transfer point of view the fin is modelled as heat source. A volumetric heat generation condition is given to solid heat source. An incompressible fluid condition is considered for the fluid.

Finite volume package of FLUENT 16.2 were used for finding the numerical solution of velocity and temperature fields. The semi implicit method for pressure linked equation (SIMPLE) and finite volume techniques are used to solve basic conservation equations. The partial differential equations are represented using finite volume method to evaluate it as algebraic equations. Values are calculated at discrete places on a meshed geometry, similar to the finite difference method. The small volume surrounding each node point on a mesh is termed as Finite Volume. In this method, using the divergence theorem, volume integrals are converted to surface integrals. In each finite volume, these terms are evaluated as surface fluxes. Because the flux leaving a given volume is equivalent to that entering the adjacent volume, these methods are conservative. Another advantage of the finite volume method is that it is easily formulated to allow for unstructured meshes. Partial differential equations are discretized into a set of algebraic equations. All algebraic solutions are then solved numerically to render the solution field.

V. RESULT AND DISCUSSION

A. CFD simulation

(8)

80 Figure 9. Temperature contour of the fins used in the demonstration

Figure 10. Variation of the temperature along the height from the center of the TEG

B. Results of the demonstration

The exhaust gas heat energy recovery system was installed in the engine. The engine started and as the gasses start flowing outside the engine as the silencer gets heated up as a result of this, one of the sides of a thermoelectric generator (T.E.G) gets heated up and the electrons start to flow from the heated end of a thermoelectric material to cold end. When the electrons go from the hot side to the cold side this causes an electrical current, which was taken by the Power Pot harnesses and delivered through the USB devices.

This eco-friendly power generation method can be implemented for domestic and commercial use at an affordable cost. The efficiency of the engine will not be affected because only the surface heat of the silencer is drawn out. The main objective of this project is to recover the surface exhaust heat to avoid the accidents (Burn-outs) caused by the overheated silencers and to convert the recovered heat to useful electric energy. This objective has been successfully accomplished in this paper. The output could be increased by connecting a number of TEGs in series so that the voltage gets added up leading to increased power. The energy produced from this system could be used to power any auxiliary devices in an automobile directly or it could be stored in a battery and then used later.

The output obtained from the thermoelectric generator is measured as 2.8V at 548mA, this energy recovery method is demonstrated by charging a mobile phone on site. A 12V output is necessary to charge the mobile phone and the electricity produced from a single TEG cell is insufficient, so the output is boosted up by using a 1.5A, 1.5MHz Step-Up DC/DC booster circuit LTC3529. This booster circuit receives the voltage of 2V from the TEG and steps it up to 12V. As a result of the boost, the mobile phone was charged successfully.

VI.CONCLUSION

(9)

81 energy produced from this system could be used to power any auxiliary devices in an automobile directly or it could be stored in a battery and then used later.

VII. SCOPE OF FUTURE WORK

This work only demonstrates the possibility of waste heat recovery from exhaust gas using TEG. This work is just a prototype, if we can make a large TEG with a voltage output of 12.6V at 20A, we can even charge the battery of the vehicle and save the power used by the alternator. Another way to increase the output voltage is by arranging a number TEGs in series and we can meet the required voltage to charge the battery. Also, this electric energy is enough for the working of cruise control system. If the shape of the TEG can be made into circular and which is inbuilt with the exhaust pipe, the space utilized by the TEG will not form any back pressure at the engine. A better fin can be used so that the colder side of the thermoelectric generator does not get heated up as the engine is running.

ACKNOWLEDGMENT

The equations are taken from the textbook “Heat and Mass Transfer Fundamentals and Applications” 5th Edition, 2015,

authored by Yunus A. Cengel and Afshin J. Ghajar.

REFERENCES

[1] J. S. Jadhao, D. G. Thombare, “Review on Exhaust Gas Heat Recovery for I.C. Engine” Volume 2, Issue 12, 2013

[2] Mohak Gupta, “Review on Heat Recovery Unit with Thermoelectric Generators” Volume 4, Issue 4, 2014.

[3] P. Mohamed Shameer, D. Christopher, “Design of Exhaust Heat Recovery Power Generation System Using Thermo-Electric Generator” Volume 4, Issue 1, 2015

[4] J. Vazquez “State of the Art of Thermoelectric Generators Based on Heat Recovered from the Exhaust Gases of

Automobiles”, Proceedings of the seventh European workshop on Thermo Electrics, 2002.

[5] M. Stordeur, Igno Stark, “Low Power Thermoelectric Generator-Self-Sufficient Energy Supply for Micro System”, Proceedings of XVI International Conference -on Thermoelectric, pp. 575-577, 1997.

[6] R. M. Ware, D. J McNeill, “Iron Disilicide as a Thermoelectric Generator Material”, Proceedings of the Institute of

Electrical Engineers, Volume 111, Issue 1, pp. 178-182, 1964.

[7] K. Ikom, M. Munekiyo, K. Furuya,”Thermoelectric Module and Generator for Gasoline Engine Vehicles”, Proceedings of XVII International Conference on Thermoelectrics, pp. 464-467, 1998.

[8] X. Niu, J. Yu, S. Wang, “Experimental Study on Low-Temperature Waste Heat Thermoelectric Generator”, Journal

of Power Sources, Volume 188, Issue 2, pp. 621-626, 2009.

[9] X. Gou, H. Xiao, S. Yang, “ Modeling, Experimental Study and Optimization on Low-Temperature Waste Heat

Thermoelectric Generator System”, Journal of Applied Energy, Volume 87, Issue 10, pp. 3131-3136, 2010.

[10] J. Chen,” Thermodynamic Analysis of a Solar Driven Thermoelectric Generator”, Journal of Applied physics,

Volume 79, pp. 1-6, 1996.

[11] Yunus A. Cengel - Heat and Mass Transfer Fundamentals and Applications 5th Edition, 2015

[12] http://theelectricenergy.com/thermoelectric-generator-teg/

[13] http://physics.stackexchange.com/questions/150295/why-does-a-thermoelectric-generator-need-both-p-and-n-elements

Figure

Figure 1. Total Fuel Energy Content in I. C. Engine
Figure 2. The working schematic of TEG [13]
Figure 3. Schematic diagram of exhaust heat energy recovery system
Figure 6. A typical TEG [14]
+3

References

Related documents

The Nominating Committee suggests for Chairman of District IV, the name of

Firstly, the BLDC motor modelling is composed with power factor correction (PFC) based integrated Cuk converter and BLDC speed is regulated using variable DC-Link

In this paper we propose new direction based measures for the trajectory data warehouse that efficiently analyze the trends and behavior of moving objects like direction

Delorme followed Guidi, even to the point of still adhering to the now-disproved theory that the Post-Herulian Wall was built in the 15th century by the

The current internet network is well set and huge in terms of topologies and its connections all over the world. The data and the control plane being together yet makes it

of £4.44/min, whereas time saved in the PACU has a value of £0.33/min, confirming that sugammadex 2 mg/kg (or 4 mg/kg) is cost-effective for reversal of moderate (or deep)

Using conditional β– convergence concept, the results show that the two groups of regions (Europe and Asia are able to form two different convergence clubs based on their