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

Journal Pre-proof Waste heat recovery from a biomass heat engine for thermoelectric power generation using two-phase thermosyphons Rohtash Goswami, Ranjan Das PII:

S0960-1481(19)31558-7

DOI:

https://doi.org/10.1016/j.renene.2019.10.067

Reference:

RENE 12436

To appear in:

Renewable Energy

Received Date: 6 June 2019 Revised Date:

25 September 2019

Accepted Date: 12 October 2019

Please cite this article as: Goswami R, Das R, Waste heat recovery from a biomass heat engine for thermoelectric power generation using two-phase thermosyphons, Renewable Energy (2019), doi: https://doi.org/10.1016/j.renene.2019.10.067. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

1

Waste heat recovery from a biomass heat engine for thermoelectric power

2

generation using two-phase thermosyphons

3

Rohtash Goswami, Ranjan Das1

4

Department of Mechanical Engineering

5

Indian Institute of Technology Ropar, Rupnagar, Punjab, India, 140001

6 7

Abstract

8

In this study, we propose a thermoelectric generator (TEG) based power generation system

9

operated through waste heat of a biomass engine. Power generated by TEGs is utilized for

10

recharging a 12V uninterruptible power source (UPS) battery. Experiments are done to study the

11

variation of power output, current and conversion efficiency with average flue gas temperature,

12

output voltages, thermosyphon filling ratio (TFR) along with source and sink temperatures.

13

Gasifier operation is optimized to identify the appropriate equivalence ratio (ER). The optimized

14

ER for the present system is evaluated as 0.305 yielding a maximum flue gas temperature of

15

283°C. Thereafter, experiments are conducted to study various performance parameters when 48

16

TEGs are provided on the two-phase octagonal-shaped thermosyphons. Experimental results

17

indicate that the maximum open circuit voltage of the present system is 31.52V (17.12V at

18

∆Tmax.,1=39°C and 14.40V at ∆Tmax.,2=31°C) at an optimum TFR of 0.496. A thermal resistance

19

based model is finally developed from which the maximum temperature gradient across the TEG

20

for two thermosyphons is found as 40.12ºC with a maximum relative error of 14.91% between

21

model and experimental values. The total power generated from the system is found as 1.033W,

22

whereas, the maximum conversion efficiency is calculated as 2.218%.

23 24

keywords: thermoelectric generator; waste heat recovery; thermosyphon; biomass engine;

25

resistance model

26 27

Nomenclature

28

A

cross-sectional area, m2

29

As

surface area, m2

1

Dr. Ranjan Das, Associate Professor, Department of Mechanical Engineering, Indian Institute of Technology Ropar, Punjab, 140001, India. Telephone: +91-1881-230122; Fax: +91-1881-223395

1

30

a

side of octagonal, m

31

c

specific heat at constant pressure, J/(kg ⋅ K)

32

D

inner diameter, m

33

d

density, kg/m3

34

ER

equivalence ratio

35

g

acceleration due to gravity, m/s2

36

hfg,wf

latent heat of evaporation for working fluid, J/kg

37

Iₒ

output current, A

38

Is

short circuit current, A

39

K

thermal conductance, W/K

40

k

thermal conductivity W/(m ⋅ K)

41

L

length, m

42

ms

mass of source water, kg

43

mwf

mass of working fluid, kg

44

ncouples

number of couples connected in series in a TEG

45

Patm.

atmospheric pressure, N/m2

46

Po

output power, W

47

Pvap.

vapour pressure, N/m2

48

Pv

vacuum pressure inside the thermosyphon, mm of Hg

49

Q

rate of heat energy supplied, W

50

RE

external load resistance, Ω

51

RT

total electrical resistance in a TEG, Ω

52

Rt

thermal resistance, K/W

53

TEG

thermoelectric generator

54

TFR

thermosyphon filling ratio

55

Tc

sink water temperature, ºC

56

Tf

average flue gas temperature, ºC

57

Th

hot side temperature of TEG, ºC

58

Ts

source water temperature, ºC

59

t

time, minutes

60

UPS

uninterruptible power source 2

61

V

open-circuit voltage, V

62

Vₒ

output voltage, V

63

x

result value of any parameter

64

x

mean of results value measured y times

65

Y

number of times result value is measured

66

Z

figure of merit, K-1

67

z

independent variable

dT dt

68

rate of rise in temperature, K/s

69

Greek symbols

70

α

Seebeck coefficient, (µ·V)/K

71

δ

thickness, m

72

∆T

temperature gradient across TEG (ºC)

73

φ

concluding result of a parameter

74

η

heat conversion efficiency, %

75

µ

dynamic viscosity of working fluid, N·s/m2

76

θ

figure of merit for boiling, K-1

77

ρ

electrical resistivity, µΩ ⋅ m

78

σest.

estimated population standard deviation

79

ξ

absolute uncertainty

80

ψ

aspect ratio, m-1

81

Subscripts

82

cond.

condenser section of thermosyphon

83

cont.

source container

84

cop.

copper material used for thermosyphon

85

evap.

evaporator section of thermosyphon

86

e

equivalent

87

fb

film boiling

88

fc

film condensation

89

l

liquid phase 3

90

max.

maximum value of any parameter

91

n

n-type semiconductor material

92

p

p-type semiconductor material

93

pb

pool boiling

94

q

number of independent variables

95

v

vapour phase

96

w

wall

97

1

thermosyphon 1

98

2

thermosyphon 2

99 100

1. Introduction

101

Thermoelectric generator (TEG ) modules work on the principle of Seebeck effect within

102

semiconductor materials which directly convert available heat energy into electrical energy.

103

TEGs produce clean energy and serve as potential candidates for transforming low temperature

104

waste heat into electrical power [1]. Considerable progress has been made in the recent past

105

towards TEG power generation through waste heat recovery from various sources. Nuwayhid et

106

al. [2] proposed a low cost locally available TEG design for power generation from wood and

107

diesel-based stoves. For recovering waste heat from a stove burner, Nuwayhid et al. [3] further

108

tested the performance of TEG fitted to the hot side of a domestic woodstove, where the heat

109

sink was cooled by air under natural convection. Borelli and de Oliveira [4] presented the

110

performance and cost analyses of TEG based power generator from combined gas turbine-steam

111

turbine power plants. For utilizing the heat transfer between the hot and the cold sides of a heat

112

exchanger, Crane et al. [5] developed and tested the performance of a TEG system. A similar

113

concept of heat extraction by TEGs from plate heat exchangers was also demonstrated by Niu et

114

al. [6]. Utilizing the heat generated on the surface of biomass cook stove, Champier et al. [7]

115

experimentally studied the performance of TEGs for electric power generation. Singh et al. [8]

116

experimentally investigated the power generation from solar pond using TEGs under different

117

temperature gradients. Dai et al. [9] experimentally investigated the TEG performances when

118

combined with the electromagnetic pump for harvesting the waste heat from liquid metals. He et

119

al. [10] experimentally investigated the cogeneration (heat and power) study of TEG integrated

120

with solar heat pipe, and their analytical model was validated with the experimental results. 4

121

Zheng et al. [11] proposed the concept of thermoelectric cogeneration system using solar energy

122

and domestic boiler assisted heat source. Rezania et al. [12] optimized the areas of n and p-type

123

thermoelectric elements using FLUENT software incorporated finite element method. Date et al.

124

[13] proposed a novel system in which TEG was combined with a water desalination system for

125

power generation and water purification using low grade thermal energy. Zhao et al. [14]

126

proposed a hybrid system comprising a fuel cell, a TEG and a regenerator to produce power

127

using waste heat generated from fuel cells. Dai et al. [15] proposed a combined system of

128

evacuated tube solar collector (for heating the water in the pipe), and TEG (based on Bi2Te3

129

material) for effectively converting the excess solar heat into electricity. Zhu et al. [16] proposed

130

a combined solar photovoltaic- TEG system for increasing the thermal efficiency of the complete

131

system. Ziapour et al. [17] used a combined solar pond and organic Rankine cycle assisted TEG

132

system where the heat contained by the organic working fluid from the turbine exhaust was

133

utilized as heat source for TEG . Li et al. [18] experimentally studied the performance of a

134

biomass stove integrated with eight TEGs . In order to maintain the temperature difference, the

135

source heat was supplied through copper flat-plates with fan-based air cooling of the sink. Kim et

136

al. [19] fabricated TEGs for generating power using human body as a heat source. They found a

137

maximum power density of 2.28µ W/cm2 using naturally convective heat sink. Haiping et al. [20]

138

presented a novel design in which TEGs were connected to a microchannel heat pipe array to

139

exploit the heat energy obtained from a solar photovoltaic-thermal hybrid system. Recently,

140

Karthick et al. [21] investigated the effect of various parameters such as, roughness of surface,

141

contact pressure, thermal conductivity of interfacial material and temperature of heat source on

142

the voltage and power outputs of the TEG . Shittu et al. [22] numerically compared the

143

performance of photovoltaic- TEG -heat pipe combined system with sole photovoltaic-

144

thermoelectric and photovoltaic systems. The variation of output power and efficiency are

145

studied at different wind speed, ambient temperature and solar concentration ratio. Li et al. [23]

146

developed a new hybrid system consisting of a photovoltaic- TEG system engaged with array of

147

heat pipes. They obtained 14.0% higher efficiency with the new system as compared to the

148

simple photovoltaic- TEG system. Mahmoudinezhad et al. [24] studied two types of TEG (made

149

of Bi2Te3 and Zn4Sb3) to observe output parameters such as short circuit current, open circuit

150

voltage and maximum power. Tappura et al. [25] fabricated thin film TEGs from aluminium-

5

151

doped zinc oxide material on the substrates having low cost with large area (0.33m2). They

152

evaluated the performances at temperature gradients below 50K.

153 154

From the above discussion, it is apparent on one hand that for harvesting low temperature

155

thermal energy for TEG based power generation; the usage of two-phase thermosyphon is a

156

potential concept [8, 26]. For a two-phase thermosyphon at various operating conditions, Zhang

157

et al. [27] developed a generalized model to analyze its thermal performance. They validated the

158

simulation results with experimental values. Naresh and Balaji [28] examined the performance of

159

a two phase thermosyphon with six internally located fins using two working fluids (water and

160

acetone) at various thermosyphon filling ratios (TFR ) . The maximum heat transfer was realized

161

at an optimum TFR of 0.5. On the other hand, the power production from biomass is an

162

encouraging alternative where its availability is large, which otherwise is wasted in the landfill

163

areas. Towards this, biomass gasification and anaerobic digestion processes are found suitable

164

for energy conversion [28]. Power generation through biomass gasification has many advantages

165

such as its renewable and inexpensive nature, carbon dioxide neutrality and ease of availability

166

[29]. However, while using it for power generation, a considerable portion of thermal energy

167

from syngas is generally lost in the form of exhaust flue gases due to a fixed thermal efficiency

168

and thermodynamic limitations of the engine. These exhaust flue gases possess sufficiently high

169

temperature (550-950K) that can be further processed for power generation [30].

170 171

It is evident that power production from TEGs has been accomplished with several heat sources,

172

but, power generation from TEGs combined with thermosyphon for recovering waste heat from

173

biomass heat engine is not yet studied. In view of this research gap, here we study the power

174

generation potential of TEGs integrated thermosyphon operated using exhaust gas of a biomass

175

engine. Parametric study is done to identify the equivalence ratio ( ER ) of the biomass gasifier at

176

which power output from TEG system will be maximum. In particular, parameters such as open

177

circuit voltage, short circuit current, output power, heat conversion efficiency and figure of merit

178

have been studied to optimize the system’s performance. Subsequently, a theoretical comparison

179

is also done against the experimental observations using a thermal resistance network model.

180

Using waste heat driven TEG power, a 12V uninterruptible power source (UPS ) battery is 6

181

successfully charged, and its utility for real life applications is demonstrated. Further details are

182

discussed in the next section.

183 184

2. Experimental setup

185

A TEG -based thermosyphon system (Fig. 1) operated using waste heat from a downdraft type

186

biomass gasifier (10kW capacity) is used. As revealed in the figure, two thermosyphons are used

187

to utilize the waste heat emerging out with the exhaust gas of a biomass engine. The setup

188

consists of (1) air blower, (2) resistance heater, (3) hopper, (4) cyclone filter (5) charcoal filter,

189

(6) cooling tower, (7) sawdust filter, (8) gas burner, (9) cotton filter, (10) gas analyzer, (11) gas

190

flow meter, (12) genset, (13) control panel, (14) inverter-battery system, (15) engine exhaust

191

pipe, (16) sealed plate, (17) evaporator section, (18) two-phase flow thermosyphon, (19) thermal

192

insulation, (20) condenser section, (21) pipe for creating vacuum, (22) water inserting port, (23)

193

48 TEGs (Model No.: SP1848-27145) connected in series, (24) output wires of TEG , (25)

194

submersible pump, (26) water circulation pipe, (27) vacuum pressure gauge, (28) vacuum pump,

195

(29) multimeter, (30) rheostat, (31) UPS battery, (32) temperature sensor, and (33) biomass

196

dryer. For convenience in understanding, a block diagram of the experimental facility is also

197

shown in Fig. 1.

198 199

Flue gases from biomass engine possess sufficient temperature to heat a plate that in turn

200

transfers the heat to two stainless steel containers (acting as evaporator sections for the

201

thermosyphon) attached to it. TEGs could not be directly attached to the hot plate, because the

202

plate vibrates during the operation of the engine. Not only this, difficulty also arises to cool the

203

other end of TEG due to very small thickness (3.9mm) between the hot and cold ends. Therefore,

204

source container/evaporator is necessary. To minimize heat loss, the sides of both source

205

containers are insulated by thermal insulation. The source water gets heated by the hot plate and

206

thermocouple is used to measure its temperature. Two octagonal thermosyphons fabricated using

207

copper sheets of 1.0mm thickness are used in source containers. Each thermosyphon is filled

208

with a pre-measured quantity of distilled water. Two ports are provided on the top of each

209

thermosyphon, the first one is for conveying the distilled water, whereas, the second one is

210

connected to the vacuum pump. 48 TEGs in series are fixed to the upper end (i.e., condenser

211

section) of the two thermosyphons. Two sink containers holding cold water are fixed to the upper 7

212

213 214

Figure 1: Details of experimental setup with block diagram 8

215

end of the thermosyphon. For each thermosyphon, one submersible water pump (20W having

216

discharge of 1.81 × 10 − 4 m3/s) is dipped inside each sink container to transfer warm water to a

217

water tank attached to the cooling tower. Further, another water pump of the same capacity is

218

supplied at the cooling tower water tank to circulate the cooled water to the condenser section.

219

This water circulation maintains nearly cold temperature inside the sink container. Before

220

operating the thermosyphon, adequate vacuum is created by vacuum pump (ultimate vacuum: -

221

734mm of Hg gauge, with maximum air displacement: 6.33 × 10-4 m3/s) to ensure water boiling at

222

temperatures in the range ( 75o C − 90o C ) . A multimeter is used to measure voltage and current

223

generated from TEG , whereas, a rheostat is used to create variable external load resistance.

224 225

3. Experimental procedure

226

Initially, a sample of biomass is measured on a weighing machine and the gate is closed after

227

feeding it into the hopper. A small quantity (100-150g) of dried biomass is supplied inside the

228

resistance heater port and resistance heater is switched on. Air velocity through the blower is

229

measured by a vane-type anemometer. Burned biomass is carried across various zones to get

230

converted into syngas that in turn is passed across various filters [31]. The clean gas runs the

231

engine of the genset to produce electricity.

232 233

Exhaust flue gas from the engine at high temperature heats a sealed flat plate and the heat is then

234

subsequently transferred from the plate to water inside the source tank, i.e., evaporator section.

235

Octagonal shaped thermosyphons are provided inside each source container, where heat from the

236

source water is transferred to the distilled water inside the thermosyphon. Since thermosyphon is

237

under vacuum pressure, distilled water inside the thermosyphon that is in contact with the source

238

tank boils at temperature lower than 100oC. The generated steam flows upwards to lose its latent

239

heat of vaporization to the upper zone of the thermosyphon, outer surface of which is water-

240

cooled. Thus, the inner surface of the condenser section is always maintained nearly at a uniform

241

temperature to keep one end of TEG under hot condition. The other surface of TEG is cooled by

242

water circulation from the tank of the cooling tower. This phenomenon results in a potential

243

difference by Seebeck effect. The condensed water falls down and collected again inside the

244

lower container (evaporator section) of the thermosyphon as shown in Fig. 2. The circuit is

245

completed by applying a variable external load resistance (rheostat) to the flow of current (Fig.

9

246

3). The power generated from TEG is used for charging a 12V UPS battery that stores the energy

247

and can be used for further application.

248

Figure 2: Schematic diagram of two-phase thermosyphon with resistances offered in heat flow

+

̶

v

+

2

+

v

+

v

̶

3

5 4

v

̶ ̶

1 1 24 TEG connected in series 2 Output wires of TEG

249

3 Voltmeter 4 Ammeter

5 Rheostat

Figure 3: Circuit diagram for measuring the current at variable external load resistance

250 251

4. Principle of thermoelectric generator

252

Thermoelectric generators are accomplished devices to directly transform heat energy into

253

electrical energy through Seebeck effect. Therefore, the thermo-physical parameters of 10

254

semiconductor material play a significant role in TEG -based electric power generation. Various

255

parameters like Seebeck coefficients α p and αn , electrical resistivities ρ p and ρn , total

256

electrical resistance

257

power of a TEG system. Figure 4 shows a schematic illustration of a TEG made up of “p” and

258

“n”-types of semiconductor materials. In the present work, bismuth telluride is used and its

259

properties are provided in Table 1. The charge carriers for p-type semiconductor materials are

260

positively-charged holes, whereas the same for n-type are the negatively-charged electrons.

261

Therefore, the Seebeck coefficient for p-type semiconductor materials is positive, whereas, it is

262

negative for n-type semiconductors. On the top side of TEG , p and n-type materials make

263

discrete junctions, above which ceramic substrates are affixed. On the bottom side of TEG , an

264

electrically conductive material is fixed separately below the p and n-type materials and the

265

ceramic substrate joins these materials. The supplied heat is provided at the top side of TEG ,

266

whereas heat is rejected from the bottom side of TEG . When heat is supplied, holes from the p-

267

type semiconductor transfer into the n-type semiconductor, whereas, electrons from the n-type

268

semiconductor travel into the p-type semiconductor. Therefore, a potential difference is setup

269

across the output wires and the external load resistance completes the circuit. Electrical power is

270

continuously generated as long as the temperature potential across the two ends of TEG is

271

maintained.

(

( RT ) and

)

(

)

the number of series-connected couples directly influence the

272

Table 1: Properties of bismuth telluride semiconductor material

273

αp

αn

(µV/K)

(µV/K)

Kim et

Yoo et

al. [32]

al. [33]

140.0

−188.5

ρp

ρn

αe × 103

(µ Ωm)

(µ Ωm)

(µV/K)

Kim et

Yoo et

al. [32]

al. [33]

6.0

29.5

41.1

274 275

11

kp

kn

RT

W/(m ⋅ K)

W/(m ⋅ K)

(Ω)

Kim et al.

Takashiri et

[32]

al. [34]

1.3

0.8

6.0

Ke (W/K) 0.2

(a)

(b)

1 Hot side of TEG 2 Cold side of TEG

(c)

3 p-type semiconductor material 4 n-type semiconductor material

5 Electric conductive material 6 Output wire of TEG

276

Figure 4: (a) Schematic diagram of TEG (b) top internal view and (c) side view; TEG SP1848-

277

27145

278

5. Evaluation of performance parameters

279

The power generated from TEG system is a function of the output current and the external load

280

resistance and is calculated by Eq. (1), whereas, the current in TEG circuit is calculated by Eq.

281

(2) [35]. The current depends upon the equivalent Seebeck coefficient

282

gradient of TEG (∆T), the internal electrical resistance

283

resistance ( RE ) .

( RT ) and

Po = Vo × I o = I o × RE = (α e × ∆T − I o × RT ) × I o 2

Io =

α e × ∆T RT + R E

(αe ) ,

temperature

the external electrical load

(1) (2)

284 12

285

The equivalent Seebeck coefficient (α e ) is dependent upon the individual Seebeck coefficients

286

of p- and n-type semiconductor materials along with the number of series-connected couples

287

(n

c o u p le s

) as given by Eq. (3) below [35],

α e = ncouples × (α p − α n )

(3)

288

The internal electric resistance, RT is again a function of the aspect ratio (ψ ) , resistivity of both

289

materials ρp and ρn , and series- connected couples ( n c o u p le s ) as shown in Eq. (4) [35],

(

)

RT = ncouples × ψ × ( ρ p + ρn ) where,ψ =

Lp A

=

(4)

Ln A

290

Next, dimensionless figure of merit ( ZT ) is the one of the comprehensively accepted

291

performance criteria of thermoelectric materials. It is described by

292

conductance ( K e ) and the average temperature (T ) of the cold and the hot sides of TEG, i.e., [35],

ZT = 293

RT × K e

RT , equivalent thermal

×T

(5)

In Eq. (5), Ke is calculated as [35],

Ke = 294

α e2

αe ,

where, kp and

ncouples

ψ

× ( k p + kn )

(6)

kn are thermal conductivity of p and n type materials, respectively.

295 296

6. Results and discussion

297

In the present study, electrical power is generated using TEGs by recovering waste heat from the

298

exhaust flue gases emerging out of a 10kW biomass generator. Two octagon-shaped

299

thermosyphons under vacuum (-700mm of Hg gauge) are used for this purpose to transfer the

300

heat from source water to the hot side of TEGs . The average temperature of flue gas is first

301

studied at various ER . For this, six experiments are done at different ERs to identify the

302

maximum temperature of the flue gas. Further, to determine the maximum voltage, V ., the

303

optimum TFR is determined by performing nine experiments at various TFR . At optimum TFR, 13

304

the transient variations of open circuit voltage, V with temperature difference, ∆T, source

305

temperature, Ts , and sink temperature, Tc are then studied. The performance of TEGs in terms of

306

power output, Po and efficiency, η is analyzed at various output voltage, Vo and the hot side

307

temperature, Th respectively. The figure of merit for the thermosyphon-based TEG is finally

308

studied. In the present study, the sensitivities associated with the multimeter for measuring the

309

voltage and current are 0.001V and 0.001A, respectively. Temperatures are measured by K-type

310

thermocouples with sensitivity of 1ºC. Performing one experiment for a particular set of

311

conditions doesn’t confirm the correctness of the outcome and random errors are always

312

included in the results. Therefore, to confirm the observed outcome, each experiment is

313

repeated/replicated three times and the average value of these replicates are studied. The

314

accuracy in the measurement is described by standard error which depends upon the number of

315

measured

316

x1, x 2, x3, ............, x y are the results value of any measured parameter, y times. Then, the

317

associated standard error is found by following equation [31],

values

( y ) and

estimated

population

Standard error = 318

standard

deviation

(σ est . ) . Suppose,

σ est. y

(7)

In the above, the value of σ est . is calculated as [31],

σ est .

(

)

2  y ∑ x j − x  j =1  =   y −1    

0.5

(8)

319

where, x indicates the mean of results value for any parameter that is measured y times. Finally,

320

the experimental results are compared with a theoretical thermal resistance network model.

321 322

6.1 Variation of average flue gas temperature

323

A biomass engine driven by syngas is used to produce primary electrical power. From

324

thermodynamics point of view, even under ideal conditions, an engine can’t convert the total

325

thermal energy of syngas into useful work. As pointed out earlier, that in practice, approximately

326

60%-70% of the available thermal energy at reasonably high temperatures (550K-950K) is lost 14

327

in the form of waste heat [30, 36]. Recovering this waste heat into useful power generation using

328

TEGs serves as the motivation for the present research. Figure 5 shows the variation of average

329

flue gas temperature, Tf at various ER. ER is defined as the fraction of the actual air-fuel ratio to

330

stoichiometric air-fuel ratio. The average flue gas temperature is studied at different ERs

331

ranging from 0.239 to 0.352. The maximum value of Tf was found as 283°C at optimum ER of

332

0.305. The energy content (i.e., calorific value) of the syngas directly affects the combustion

333

temperature of engine as more calorific value creates high temperature in the engine. The syngas

334

produced during the initial stages of ER has low calorific value and gradually increases to a

335

maximum at the optimum ER. Beyond this, calorific value of produced syngas again decreases.

336

For a given gasifier, at optimum ER, the syngas produced is always of the highest calorific value

337

that favours the optimum combustion of fuel-air mixture which consequently results in the

338

highest possible temperature of flue gas.

Average flue gas temperature, Tf (°C)

295 280 265 250 235 220 0.22

339 340

0.24

0.26

0.28

0.30

0.32

0.34

0.36

Equivalence ratio, ER

Figure 5: Average flue gas temperature at various equivalence ratios

341 342

6.2 Maximum open circuit voltage

343

The maximum open circuit voltages (Vmax,1 and Vmax,2 ) across 48 (24 with each thermosyphon)

344

series-connected TEGs obtained at various TFR are studied in Fig. 6. Here, TFR is defined as the

345

ratio of the volume of working fluid to the volume of the evaporator section. Vmax,1 and Vmax,2 are

346

the respective maximum open circuit voltages corresponding to thermosyphons 1 and 2 at

347

maximum source temperature, Ts as indicated. Under varying TFR (0.216-0.647), both

15

348

thermosyphons are operated at the same vacuum pressure of -700mm of Hg (gauge). It has been

349

envisioned that with increase in TFR, the maximum open circuit voltages (Vmax,1 and Vmax,2 ) also

350

increase which reach maximum at optimum TFR (0.496, for both thermosyphons). Beyond the

351

optimum level, further increase in TFR decreases the maximum open circuit voltages. This is due

352

to the reason that when TFR is gradually increased, more heat transfer occurs between the source

353

tank and the working fluid at the evaporator section. Consequently, there is more heat transfer at

354

the condensing section and more wall temperature attached at the hot-side of TEGs. However,

355

beyond a particular limit, thick liquid film is formed that offers thermal resistance to the heat

356

transport, thereby reducing the temperature difference between the two ends of the TEG. The

357

maximum values of Vmax,1 and Vmax,2 are obtained as 17.12V and 14.40V, respectively, which

358

yields a total of 31.52V. Under the same vacuum pressure and optimum TFR , thermosyphon 1

359

produces more open circuit voltage than that of thermosyphon 2. This is because, the source

360

container of thermosyphon 1 contains water at a higher temperature (87ºC), whereas the same for

361

thermosyphon 2 is at a low temperature (77ºC). Therefore, using thermosyphon 1, more heat is

362

transferred to the hot side of TEG that leads to a higher value of maximum temperature gradient

363

across TEGs ( ∆T = 39ºC) as compared to that of thermosyphon 2 ( ∆T = 31ºC).

364

Vm1 Vmax.1

Vm2 Vmax.2

Maxium open circuit voltage,Vmax (V)

20 ER = 0.305 Pᵥ = - 700 mm of Hg

17

Ts1 = 87 °C

14 11 Ts2 = 77 °C

8 5 0.2

0.27

0.34

0.41

0.48

0.55

0.62

0.69

Thermosyphon filling ratio, TFR

365 366

Figure 6: Variation of maximum open circuit voltage obtained at various TFR for

367

thermosyphons 1 and 2

368 369

6.3. Variation of source and sink temperatures 16

370

Hot gas exiting the engine heats a metallic plate, and ideally to utilize the maximum available

371

energy, the evaporator of the thermosyphon should be affixed to it. The source temperature, Ts is

372

the temperature of hot water contained inside the source container (i.e., lower container),

373

whereas the sink temperature, Tc is the cold water temperature contained within the sink

374

container (i.e., the upper container). Figure 7 shows the variation of temperatures (Ts and Tc )

375

with time, t for both thermosyphons. As highlighted, at optimum ER, the maximum Ts,1 and Ts,2

376

are found as, 87°C and 77°C, respectively. Since, the source container 1 is located closer to the

377

engine exhaust pipe than the source container 2, therefore, more heat is transferred to source

378

container 1 than container 2. Initially, Ts ,1 and Ts ,2 are low and they increase with t, but upto a

379

certain period and thereafter almost remain constant. This is because after a definite period of

380

time, the rate of heat addition to the source water becomes almost equal to the heat lost from the

381

surface of source hot water to the working fluid. However, both Tc,1 and Tc,2 remain almost

382

constant with t , because cooling tower yields a fixed temperature at its outlet. Tc1

Tc2

100

40 ER = 0.305, TFR = 0.496

90

35

Pᵥ = - 700 mm of Hg

80

30

70

25

60

20

50

15

40 30

10 0

383 384

Ts2

20

40

60

80

100

Sink temperature, Tc (°C)

Source temperature, Ts (°C)

Ts1

120

Time, t (minutes)

Figure 7: Transient variation of source and sink temperatures in thermosyphons

385 386

6.4 Variation of open circuit voltage and temperature gradient across TEG

387

The open circuit voltage is the maximum voltage obtained at zero current flow condition,

388

whereas, the short circuit current is the maximum current flow inside the circuit when voltage

389

across the resistance is zero. The variation of the open circuit voltage (V ) and short circuit

390

current ( I s ) with temperature gradient across TEG is studied in Fig. 8. The analysis has been 17

391

done for the optimum TFR = 0.496. It is revealed from the figure that the open circuit voltage of

392

TEG always increases with increase in the temperature difference, ∆T . As discussed above,

393

temperature of cold water remains almost constant, therefore, ∆T is invariably proportional to the

394

hot side temperature of the TEG. From the present study, the maximum open circuit voltage

395

observed from thermosyphons 1 and 2 is 17.12V ( ∆Tmax,1 = 39o C ) and 14.40V ( ∆Tmax,2 = 31o C ) ,

396

respectively. The short circuit current obtained corresponding to 17.12V and 14.40V are 0.152A

397

and 0.127A, respectively.

18

V1 Is1

V2 V2

Short circuit current, Is (A)

Open circuit voltage, V (V)

V1 V1 ER = 0.305, TFR = 0.496 Pᵥ = - 700 mm of Hg

15 12 9 6 3 0 0

5

10

15

20

25

30

35

40

398

ER = 0.305, TFR = 0.496 Pᵥ = - 700 mm of Hg

0.15 0.12 0.09 0.06 0.03 0 0

Temperature gradient across TEG, ∆T (°C)

V2 Is2

0.18

5

10

15

20

25

30

35

Temperature gradient across TEG, ∆T (°C)

Figure 8: Variation of open-circuit voltage and short circuit current for thermosyphons 1 and 2

399 400

6.5 Variation of power output and conversion efficiency of TEG

401

The output power, Po and heat conversion efficiency, η are among the main performance

402

characteristics of TEG. Figure 9 shows the variation of output power, Po and output current,

403

I o for thermosyphons 1 and 2 at optimum TFR of 0.496. As discussed above, the highest values

404

of Vmax,1 and Vmax,2 are found at the optimum value of TFR. Therefore, under similar conditions

405

of Tc and Pv , in both thermosyphons, the power generated from TEGs is governed only by ∆T .

406

The curves of power as shown in Fig. 9 are obtained by varying external load resistance

407

connected in the series with output voltage, Vo and current, I o being noted by voltmeter and

408

ammeter, respectively. For thermosyphon 1, maximum output power ( Po ,max,1 ) of 0.615W was

409

obtained at 7.98V of output voltage and 0.077A of output current when ∆Tmax,1 was maintained at

410

39°C as shown in Fig. 9. Similarly, for thermosyphon 2, the maximum output power, Po ,max,2

18

40

411

(0.418W) was found at 6.33V of output voltage and 0.066A of output current corresponding to

412

∆Tmax,2 = 31o C. For both thermosyphons, the output current linearly decreases with increase in

413

the external load resistance. IIₒ(A) (A)

PPₒ(W) (W) 0.6

0.12

0.5

0.09

0.4 0.3

0.06

0.2

0.03

0.1

0

Output current, Iₒ (A)

Pᵥ = - 700 mm of Hg

ER = 0.305, TFR = 0.496, ∆T = 31 °C

Output power, Pₒ (W)

Output current, Iₒ (A)

0.5

0.7

ER = 0.305, TFR = 0.496, ∆T = 39 °C

0.15

Pᵥ = - 700 mm of Hg

0.12

0.4

0.09

0.3

0.06

0.2

0.03

0.1

0.0

0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

0.0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Output voltage, Vₒ (V)

414

PPₒ(W) (W)

0.15

Output voltage, Vₒ (V)

Figure 9: Variation of output current and power for thermosyphons 1 and 2

415 416

Heat conversion efficiency, η is a function of the output power and the rate of heat energy

417

supplied to TEG ( QTEG ) . The power generated from TEG is the product of output voltage and

418

current as shown in Eq. (9), whereas the rate of heat energy supplied can be calculated by using

419

Eq. (10). Finally, the heat conversion efficiency is computed as the ratio of the output power to

420

the rate of heat energy supplied as shown in Eq. (11),

421

Po = Vo × I o

QTEG = mwf × cwf , l ×

η=

(9)

dT dt

Po × 100 QTEG

(10) working fluid

(11)

422

Figure 10 shows the variation of η with hot side temperature of TEG , Th . Since, Tc remains

423

almost invariant with time, therefore the efficiency, η of TEG is mainly governed by the source

424

temperature, Ts and the hot side temperature, Th . As expected, η proportionally increases with

19

Output power, Pₒ (W)

IIₒ(A) (A) 0.18

425

increase in Th for both thermosyphons. The maximum efficiency (η max. ) corresponding to

426

thermosyphons 1 and 2 are found as 2.218% at Th,max,1=59°C and 1.472% at Th,max,2=52°C,

427

respectively. Although conversion efficiency is low, but generating continuous power from

428

TEGs by recovering the waste heat can be an environment friendly and cost-effective

429

alternative.

η1 n1

η2 n2

3.0 ER = 0.305, TFR = 0.496

Heat conversion efficiency, η (%)

2.5

Pᵥ = - 700 mm of Hg

2.0 1.5 1.0 0.5 0.0 28

430 431

34 40 46 52 Hot side temperature of TEG, Th (°C)

58

Figure 10: Variation of heat conversion efficiency with hot side temperature of TEG

432 433

6.6 Variation of figure of merit

434

Dimensionless figure of merit ( ZT ) is the design criteria for thermoelectric materials. The

435

variation of ZT with ∆T is shown in Fig. 11. It has been found that ZT increases linearly with

436

∆T . The maximum values of dimensionless figure of merit ( ZTmax. ) for thermosyphons 1 and 2

437

are calculated as 0.456 and 0.451 corresponding to ∆Tmax. of 39ºC and 31ºC, respectively. For

438

high heat conversion efficiency, the high value of ZT is required. For the temperature range

439

studied here, material properties are considered independent of temperature, therefore, the value

440

of Z also remains constant over the studied values of ∆T . However, to attain high heat

441

conversion efficiency, the thermoelectric material should have high value of α e with low values

442

of RT and K e . Due to this reason, to derive maximum heat conversion efficiency, TEG should be

443

operated at the maximum available ∆T . It is highlighted from the present study that when the hot

444

side temperature increases from 45ºC ( ∆T =25ºC) to 59ºC (∆Tmax,=139ºC) with cold side 20

445

temperature remaining nearly at a constant value (20ºC-21ºC), the heat conversion efficiency

446

increases from 0.867% to 2.218% (Fig. 10). The increase in ZT with ∆T in Fig. 11 is mainly

447

attributed to increase in the hot side temperature, Th at the condenser section.

V1 ZT1

V2 ZT2

Dimensionless figure of merit, ZT

0.48 ER = 0.305, TFR = 0.496 Pᵥ = - 700 mm of Hg

0.46 0.44 0.42 0.40 0

448 449

5

10

15

20

25

30

35

40

Temperature gradient across TEG, ∆T (°C) Figure 11: Variation of dimensionless figure of merit with temperature gradient across TEG

450 451

6.7 Charging of a 12 V UPS battery

452

The output power generated from TEGs has been utilized for charging a 12V UPS battery. The

453

circuit as shown in Fig. 3 is used for charging the battery where the rheostat was replaced by a

454

UPS battery. Before charging the battery, the output voltage from UPS battery was measured

455

through voltmeter which was found to be 10.21V. After charging the battery for 20 minutes

456

under short circuit current of 0.152A, the output voltage was again measured which was 12.31V

457

that shows the fully charged condition of the battery. While charging UPS battery, the flow of

458

current in the circuit starts when short circuit current reaches upto a certain value. The minimum

459

short circuit current required for charging a 12V UPS battery was found to be 0.118A. As the

460

current increases above the given value, the rate of charging is found to increase. Therefore,

461

output energy from TEG can be stored in a battery which can be used for further various

462

applications. The fully charged UPS battery can be used for providing the backup power to the

463

computer system, lighting in remote areas as shown in Fig. 12.

21

UPS battery

UPS battery

464

UPS battery

465 466

Figure 12: Real life use of fully charged UPS battery for various applications

467 468

6.8 Comparison of theoretical and experimental parameters

469

During the flow of vapour from the evaporator to condenser section, working fluid suffers many

470

resistances which offer restriction to heat flow. Therefore, heat supplied by source water to the

471

thermosyphon working fluid reaching the hot side of TEG located at the condenser section

472

passes through these resistances. It is assumed here that source water is well-mixed and

473

possesses constant temperature throughout. Since, source water is stationary, thus convective

474

resistance offered by source water layer on thermosyphon surface at the evaporator section can 22

475

be neglected. Thus, at the evaporator section, thermal resistance consists of conduction resistance

476

due to thermosyphon wall along with, pool and film boiling heat transfer. In the condenser

477

section, resistance is offered by film condensation along with thermosyphon wall conduction.

478

Various parameters are calculated below in Eq. (12) indicated below [37, 38],

Rt , w, evap. =

δ cop.

Rt , pb, evap. =

d wf , l d wf , v

(1 2 )

(1 4 )

Qevap. = As , evap. ×

θ pb × g

× k wf , l

(3 10 )

× h fg , wf

(2 5 )

Qcont. , Acont.

Rt , fb , evap . =

where,

A s , evap . = 8 × a × L evap . 1

where,

θ pb = 0.325 ×

, where

kcop. × As , evap.

(1 5 )

× Qevap.

× c wf , l

× (As , evap. )

(3 5 ) ,

(7 10 )

× µ wf , l

Acont . =

(2 5 )

π 4

(1 10 )

pvap .

×

(12 b)

( 23 100 )

patm .

× Dcont . , Qcont . = ms × c × 2

0 .235 × Q evap .

dT dt

source water

(1 3 )

(12 c)

θ fb (4 3 ) × g (1 3 ) × De (4 3 ) × Levap .

θ fb =

(12 a)

(2 )

Lcond . × d wf , l × k wf , l

(3 )

14

µ wf , l

, De = 8× a π

if, Rt , pb, evap. > Rt , fb , evap. , then Rt , max = Rt , pb, evap. else, Rt , ma x = TFR × Rt , pb , evap . + 1 − TFR × Rt , fb , evap .

Rt , fc , cond . =

Rt , w, cond. =

0 .235 × Qevap .

(1 3 )

(12 d)

θ fb ( 4 3 ) × g (1 3 ) × De (4 3 ) × Lcond .

δ cop. kcop. × ( As , cond. )

, where A s , cond

479

23

.

= 8 × a × L cond

.

(12 e)

480

The equivalent thermal resistance between the source and the hot side temperature of TEG is

481

then calculated by the following expression,

Rt , e = Rt , w, evap. + Rt , max . + Rt , fc , cond . + Rt , w, cond .

(13)

482 483

Various properties of working fluid can be obtained from the literature [39]. While computing

484

the thermal resistance, interfacial resistances between vapour-liquid at evaporator and condenser

485

sections are considered negligible [37, 38]. In the mathematical model, the maximum

486

temperature gradient across the TEG (∆Tmax) is obtained using Eq. (14) [37, 38],

∆Tmax = maximum of Ts − Tc − Qevap. × Rt , e

(14)

487

For both thermosyphons 1 and 2, a study is made in Fig. 13 to compare the model and the

488

experimental values of the maximum temperature gradient across TEG at different values of

489

TFR. It has been envisaged that at the optimum TFR, ∆Tmax. for thermosyphon 1 and 2 is 40.12ºC

490

and 32.38ºC, respectively by resistance model, whereas the same is 39ºC and 31ºC from

491

experiments. The relative errors are also studied in the same figure. The maximum and the

492

minimum relative errors in ∆Tmax . are found as 14.91% and 2.79%, respectively. The relative

493

error is defined as the ratio of the absolute error to the model value as reported by Chen et al.

494

[40], i.e.

Absolute error = Model value − Experiment value Relative error =

Absolute error Model value

(15) (16)

495

In Table 2, a comparative assessment of the present output voltage obtained with a single TEG

496

per unit temperature difference is done against other published literatures. The reported studies

497

[3, 8, 38, 41-43] considered different quantities of TEGs powered through different heat sources,

498

thereby leading to different output voltages. For instance, woodstove and electrical heater

499

simulating a solar pond were employed in [3] and [8], respectively. Electrical heating was also

500

considered in [41, 43], whereas, a solar thermal collector was used in [42]. It is apparent from the

501

comparison that the present TEG system utilizing exhaust heat of a biomass engine performs in

502

accordance with the other systems.

24

25

40

20

35

15

30

10

25

5

20

0 0.2

0.27

0.34

0.41

0.48

0.55

0.62

relative error

ER = 0.305 Pᵥ = - 700 mm of Hg

35

30 25

32

20

29

15

26

10

23

5

20

0 0.2

0.27

0.34

0.41

0.48

0.55

0.62

Thermosyphon filling ratio, TFR

Thermosyphon filling ratio, TFR

503

experiment 4444444444

38

Relative error, (%)

45

model 444444

Maximum temperature gradient across TEG, ∆Tmax (°C)

ER = 0.305 Pᵥ = - 700 mm of Hg

30

Relative error, (%)

Maximum temperature gradient across TEG, ∆Tmax (°C)

experiment 4relative error model 4444444444 444444 50

Figure 13: Comparison of model and experiment results for thermosyphons 1 and 2

504 505

Table 2: Comparison of open circuit voltage per ∆T for one TEG with published literature Present

Nuwayhid

Parameter

work

et al. [3]

(V/ºC)

2 ×10−2

4 ×10−2

Singh

Tundee

Singh

Deng

Singh

et al.

et al.

et al.

et al.

et al.

[8]

[38]

[41]

[42]

[43]

6 ×10−2

2 ×10−2

2 ×10−2

3 ×10−2

2 ×10−2

506 507 508

6.9 Uncertainty analysis

509

Any experiment is not fully accurate and always involve errors due to equipments, procedure,

510

human error and so on. Thus, uncertainty analysis is important to gauge the confidence in the

511

experimental results. Let, φ be the concluding result of any parameter of interest which is a

512

function of q number of independent variables as shown in Eq. (17), i.e.,

φ = f ( z1 , z 2 , z3 ,................., z q )

(17)

513

Assuming, ξ z is the absolute uncertainty corresponding to an independent variable, z. Then, the

514

absolute uncertainty in φ (i.e. ξφ ) is calculated by using Eq. (18) as reported by Moffat [44], 2

2

2

 ∂φ   ∂φ   ∂φ  ξφ =  × ξz1  +  × ξz2  +  × ξz3  + ................... +  ∂z1   ∂z2   ∂z3  515

25

2 0.5

 ∂φ   × ξzq   ∂z   q 

(18)

Table 3: Uncertainty analysis of various parameters

516

ER = 0.305, TFR = 0.496, Pv = -700 mm of Hg Tf

Ts,1

Ts,2

Tc,1

Tc,2

Th,1

Th,2

V1

V2

Absolute

6 × 10−2 o C

8 × 10−2 o C

8 × 10−2 o C

8 × 10−2 o C

8 × 10−2 o C

1 × 10−1 oC

1 × 10−1 oC

1 × 10−4 V

1 × 10−4 V

Relative

2 × 10 −4

1 × 10−3

1 × 10−3

4 × 10−3

4 × 10−3

3 × 10−3

4 × 10−3

2 ×10−5

3 × 10−5

ER = 0.305, TFR = 0.496, Th,max,1 = 59.0 ± 0.3 ºC, Tc,1 = 20.0 ± 0.3 ºC, Th,max,2 = 52 ± 0.3 ºC, Tc,2 = 21.0 ± 0.3 ºC, Pv = -700 mm of Hg Vmax,1

Vmax,2

Io,max,1

Io,max,2

Po,max,1

Po,max,2

ηmax,1

ηmax,2

Absolute

3 × 10−4 V

3 × 10−4 V

3 × 10−4 A

3 × 10−4 A

2 × 10−3 W

2 × 10−3 W

2 × 10−2 %

2 × 10−2 %

Relative

2 × 10−5

2 × 10−5

2 × 10−3

2 × 10−3

4 × 10−3

4 × 10−3

9 × 10−3

1 × 10−2

ER = 0.305, TFR = 0.496, Th,max,1 = 59.0 ± 0.3 ºC, Tc,1 = 20.0 ± 0.3 ºC, Th,max,2 = 52 ± 0.3 ºC, Tc,2 = 21.0 ± 0.3 ºC, Pv = -700 mm of Hg Vo,1

Vo,2

Io,1

Io,2

Po,1

Po,2

Absolute

6× 10−5 V

6× 10−5 V

6× 10−5 A

6× 10−5 A

2 × 10−7 W

1 × 10−7 W

Relative

8 × 10−6

1 × 10−5

7 ×10−4

9 × 10−4

5 × 10−7

5 × 10−7

517

26

518

Subsequently, the ratio of absolute uncertainty to concluding result is known as relative

519

uncertainty and given by Eq. (19), i.e.,

Relative uncertainty = 520

ξφ

φ

(19)

The uncertainties associated with various parameters are shown in Table 3.

521 522

7. Conclusion

523

In this study, waste heat recovery from a biomass heat engine is accomplished for generating

524

electrical power through two-phase thermosyphon integrated thermoelectric generator (TEG )

525

system. At first, the operation of biomass gasifier is optimized to determine the equivalence ratio

526

(ER) resulting in the highest source temperature. Thereafter, corresponding to the optimum ER,

527

the maximum voltage obtained from the system is studied at different values of the TFR. Two

528

thermosyphons each containing 24 series-connected TEGs are used, and using the electrical

529

power, a 12V battery is successfully charged. Performance parameters in terms of output voltage,

530

current, power, figure of merit, conversion efficiency derived from the TEG system are studied.

531

Finally, a thermal resistance model for determining the temperature difference across the hot and

532

the cold ends of TEG system is developed. From the present study, the following conclusions are

533

made,

534

•

The optimum ER of the present gasifier resulting in the highest possible source

535

temperature is found to be 0.305, whereas, the optimum TFR of the thermosyphon is

536

obtained as 0.496. The performance of the TEG system is found maximum at the

537

optimum values of ER and TFR.

538

•

more. Both these phenomena affects the power generation from the TEG system.

539 540

At low TFR, formation of vapour is less, whereas, at high TFR, liquid film formation is

•

Figure of merit always increases nearly in a direct proportion to the temperature

541

differential across the TEGs. The maximum dimensionless figure of merit for

542

thermosyphons 1 and 2 is calculated as 0.456 and 0.451 respectively.

543

•

The flue gas temperature at the exit of the gasifier varies in the range 252°C-283°C.

544

However, the maximum source temperature (Ts) temperature are found as 87°C and 77°C

545

for thermosyphons 1 and 2, respectively.

27

546

•

With respect to the maximum temperature gradients across the TEG , thermosyphons 1

547

and 2 produced a maximum open circuit voltage of 17.12V and 14.40V, respectively. The

548

corresponding short circuit current is found as 0.152A and 0.127A, respectively.

549

•

The maximum output power obtained from thermosyphons 1 and 2 is 0.615W and

550

0.418W, respectively. For thermosyphon 1, the maximum output power is obtained when

551

output voltage and current are 7.98V and 0.077A, respectively. However, for

552

thermosyphon 2, the output voltage of 6.33V and current of 0.066A is found to yield the

553

maximum output power. Further, the maximum heat conversion efficiency was calculated

554

as 2.218% and 1.472% for thermosyphons 1 and 2, respectively.

555

•

The minimum short circuit current required for charging the 12V battery is found as

556

0.118A. Under the optimized condition, the time taken by the battery for full recharge is

557

found as 20 minutes.

558

•

The value of maximum temperature difference obtained from the thermal resistance

559

network model for thermosyphons 1 and 2 is found as 40.12ºC and 32.38ºC, respectively.

560

However, the same as obtained from the experiments is 39ºC and 31ºC, respectively. The

561

relative error in the maximum temperature gradient across TEGs with respect to the

562

experimental value is found as 14.91%.

563

The present study is concluded to provide cardinal guidelines for selecting a heat recovery

564

system to transform waste heat into electrical energy from a fully renewable energy source of

565

biomass power.

566 567

Acknowledgment

568

Financial support received from Science & Engineering Research Board (SERB), Govt. of India

569

for the project EEQ/2016/000073 titled “Design and Development of a Solar Pond and Biomass

570

Driven Thermoelectric Unit for Domestic Power Generation using Inverse Method” is thankfully

571

acknowledged.

572 573

References

574

[1] I. Garcia, J.V.M. Zorraquino, Energy and environmental optimization in thermoelectrical

575

generating processes-application of a carbon dioxide capture system, Energy 27(6) (2002) 607-

576

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32

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