Thermodynamic performance assessment of solar based Sulfur-Iodine thermochemical cycle for hydrogen generation

Accepted Manuscript Thermodynamic Performance Assessment of Solar Based Sulfur-Iodine Thermochemical Cycle for Hydrogen Generation

Fatih Yilmaz, Reşat Selbaş PII:

S0360-5442(17)31496-2

DOI:

10.1016/j.energy.2017.08.121

Reference:

EGY 11490

To appear in:

Energy

Received Date:

06 March 2017

Revised Date:

10 July 2017

Accepted Date:

31 August 2017

Please cite this article as: Fatih Yilmaz, Reşat Selbaş, Thermodynamic Performance Assessment of Solar Based Sulfur-Iodine Thermochemical Cycle for Hydrogen Generation, Energy (2017), doi: 10.1016/j.energy.2017.08.121

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ACCEPTED MANUSCRIPT A new solar based S-I thermochemical cycle for hydrogen generation is developed Energy and exergy analyses of each step of S-I cycle are performed A clean hydrogen generation method is proposed The overall energy and exergy efficiency of system are calculated as 32.76% and 34.56%, respectively

ACCEPTED MANUSCRIPT

THERMODYNAMIC PERFORMANCE ASSESSMENT OF SOLAR BASED

1 2 3

SULFUR-IODINE THERMOCHEMICAL CYCLE FOR HYDROGEN GENERATION Fatih YILMAZ a,, Reşat SELBAŞ b

4 5 6 7 8 9 10 11

Department of Electrical and Energy, Vocational School of Technical Science, Aksaray University, 68100, Aksaray/Turkey b Department of Energy Systems, Engineering Faculty of Technology, Suleyman Demirel University, 68100, Isparta/Turkey

a

ABSTRACT

12 13

Recent studies show that thermochemical cycles has a great potential for green hydrogen

14

generation. In this study, the thermodynamic performance assessment of a solar based Sulfur-

15

Iodine (S-I) thermochemical cycle for hydrogen generation is performed focusing on the energy

16

and exergy methods. Moreover, we investigated that various reference environment and reaction

17

temperatures effects on energy and exergy efficiencies of S-I cycle steps. The results of

18

thermodynamic analyses indicated that energy and exergy efficiency of S-I cycle are found to be

19

43.85% and 62.39%, respectively. In addition, the overall energy and exergy efficiencies of

20

cycle are computed as, 32.76% and 34.56%, respectively. It was concluded that the S-I

21

thermochemical cycle offers a feasible and a diverse option for hydrogen generation and seems to

22

be a promising cycle.

23 24 25 26 27

Keywords: Energy, Exergy, Hydrogen generation, Thermochemical cycle, Sulfur-Iodine,

28 29 30 

Corresponding author. Tel: +90-382-288-2504 E-mail address:[email protected](F. Yılmaz)

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31 32

1. Introduction

33 34

Nowadays, energy and energy conversion play an important role in our lives and have an impact

35

on every sector of the economy. The energy consumption of the world is increasing day by day

36

due to population growth and industrialization. Thus, the energy consumption is climbing rapidly.

37

The rise in energy consumption is primarily due to increase in fossil fuel usage. The non-

38

renewable energy sources are diminishing and worries about greenhouse gas emissions have

39

increased. Therefore, green and renewable energy resources have gained the importance [1].

40 41

Hydrogen is considered as one of the most promising energy carriers, with high energy content,

42

which can easily be used in fuel cells without any greenhouses gas emissions (GHGE) [3].

43

Hydrogen generation and its usage have come out as one of the paramount solutions to solve the

44

present-day environment problems such as acid rain, global warming and ozone layer depletion.

45

In this regard, the implementation of the hydrogen has numerous advantages to solve the above

46

mentioned problems. Nevertheless, at present, hydrogen is mostly generated from fossil fuels

47

such as steam reforming of natural gas. Therefore, carbon dioxide emissions are still rising

48

around the globe [2]. Subsequently, hydrogen generation is very important because of it does not

49

naturally exist by itself in environment and must be generated from compounds that contain it.

50 51

In the literature, there are several hydrogen generation methods such as steam methane reforming

52

(SMR), thermochemical cycles, and electrolysis. Among these methods, SMR and electrolysis

53

are widely used methods. However, these hydrogen generation methods are not sustainable and

54

environmental benign. Thus, many researchers and industrialists are trying to find new ways to

55

generate hydrogen energy from renewable and sustainable energy resources such as

56

environmentally friendly renewable energy based thermochemical cycles and water electrolysis.

57

Water dissociation through thermochemical cycles offer a significant potential for hydrogen

58

generation process. These cycles consist of a series of chemical reactions which either use water

59

and heat or heat and electricity. The water is decomposed into from oxygen and hydrogen at the

60

end of this process. The sustainable and environmental benign hydrogen generation requires

61

usage of renewable energy sources. So, the required heat for the cycle can be easily obtained

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62

from solar energy, which is a renewable energy source. In this regard, hydrogen generation

63

assisted with solar energy is becoming more attractive day by day for environmental friendly

64

technologies.

65 66

In the recent years, a number of studies has been conducted on different thermochemical cycle

67

for hydrogen generation [4-7]. Balta et al. [8] have been analyzed the geothermal based hydrogen

68

generation using four steps Cu-Cl thermochemical cycle. They have also calculated the energy

69

and exergy efficiencies of the considered thermochemical cycle as 21.67% and 19.35%,

70

respectively. Furthermore, several studies have been analyzed and reviewed on solar based

71

thermochemical hydrogen generation [9-11].

72 73

Balta et al. [12], have studied the thermodynamic analyses of Mg-Cl cycle, driven by solar

74

energy, for hydrogen generation. The energy and exergy efficiencies of the whole system was

75

found as 18.18% and 9.15%, respectively.

76 77

The hydrogen generation from nuclear reactor using sulfur iodine thermochemical cycle was

78

studied by Giraldi et al. [13]. They also investigated the GHGE from this process and compared

79

the obtained results from other hydrogen generation studies in the perspective of life cycle

80

analysis.

81 82

Lattin and Utgikar [14] evaluated the S-I thermochemical cycle and analyzed the variation level

83

of GHGE at different external power loads. The flowsheet study of the thermochemical S-I cycle

84

for hydrogen generation was conducted by Kasahara et al. [15]. In this study, research and

85

development program on the thermochemical S-I for hydrogen generation was performed and

86

reported in Japan Atomic Energy Agency. The energy efficiency of the HI process was estimated

87

as 57%.

88 89

Leybros et al. [16], performed the plant sizing as well as the cost analysis for hydrogen

90

generation using nuclear heat source and found that the hydrogen generation cost is

91

approximately 12 €/kg.

92

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93

Mawdsley et al. [17] investigated that sulfur trioxide (SO3) decomposition step in S-I

94

thermochemical and hybrid cycle which occur at low temperatures. The results of their study

95

showed that the conversion of SO3 can be obtained at 590 oC, as long as the oxygen is removed,

96

during the SO3 decomposition stage.

97 98

The hydrogen generation step in S-I thermochemical cycle was experimentally conducted by

99

Caple et al. [18]. They found that by increasing the initial concentration of water, the rate of

100

reaction substantially increases. They also suggested that the experimental study and modeling

101

of the S-I thermochemical water decomposition cycle can be further developed.

102 103

The S-I thermochemical cycle is a decomposition of water into hydrogen and oxygen, through

104

chemical reactions, using nuclear heat at high temperature. These studies have been proposed S-I

105

thermochemical cycle for hydrogen generation, which is a very promising solution in terms of

106

productivity and cost [19-21].

107 108

Thermodynamic performance and cost assessment of a new design S-I thermochemical for

109

hydrogen generation was conducted by Öztürk et al. [22]. In this study, they found the energy

110

and exergy efficiencies as 76.0% and 75.6%, respectively. The cost rate of SO2 was obtained as

111

2.2 $ per kmol.

112 113

Most of thermochemical cycles are required high heat temperature such as 800-900 oC. This

114

temperature can be obtain from nuclear and solar energy sources. The hydrogen generation from

115

water is a promising technology to achieve a carbonless energy system. S-I thermochemical cycle

116

is one of the best hydrogen generation cycles such as Cu-Cl and Mg-Cl [23].

117 118

As mentioned above, there are insufficient studies on the assessment of energy and exergy

119

efficiencies S-I thermochemical cycle assisted with solar energy. The main objective of this study

120

is devoted to examine the thermodynamic performance assessment of solar based S-I

121

thermochemical cycle for hydrogen generation. In order to evaluate the proposed cycle

122

performance, energy and exergy methods of the all steps are performed. A parametric study is

123

conducted for several parameters to evaluate the cycle such as operation conditions, state

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properties, and reference environment conditions. The effect of various parameters on the energy,

125

exergy efficiencies and exergy destructions of the cycle are also assessed and demonstrated. The

126

overall energy and exergy efficiencies of solar based S-I cycle are evaluated.

127 128 129 130 131

2. System description

132

generation is given in Fig.1. This cycle uses both heat and electricity for hydrogen generation

133

from water, at maximum step temperature of 850 oC. The S-I thermochemical cycle is composed

134

of two thermochemical reactions and one electrochemical reaction. The chemical reactions and

135

steps of the S-I cycle are given below;

The schematic flow diagram of the S-I thermochemical cycle with solar energy for hydrogen

136 137

(I) H2SO4 (g) →H2O (g) +SO2 (g) + ½ O2 (g)

(850 oC)

138

(II) I2 (l) +SO2 (g) +2H2O (l) →2HI (l) +H2SO4 (aq)

(120 oC)

139

(III) 2HI (l) → I2 (l) + H2 (g)

(450 oC)

140 141

It is possible to divide the cycle into three steps [22, 24]

142 143

I O2 production step at 850 oC

144 145

II HI production step

146 147

III H2 production step

148 149

In step I, which is the first step of S-I cycle, endothermic chemical reaction occurs at the highest

150

temperature of 850 oC. Furthermore, H2SO4 decomposition and O2 generation also take place.

151

The step I consists of two sections; firstly gaseous H2SO4 decomposition into H2O and SO3, at

152

400-500 oC. Secondly, SO3 decomposition into SO2 and O2 at 850 oC with solid catalyst. These

153

reactions take place at same time. H2SO4 enters step I and converts to gaseous O2, SO2, and H2O.

154

The step II, which can be called as Bunsen reaction. In this step, exothermic chemical reaction

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155

occurs at 120 oC, and I2 and H2O in liquid phases enter and react with SO2 gaseous and convert

156

into hydronic acid (HI) and H2SO4. In third (hydrogen production) step, the electrochemical

157

process takes place and then HI decomposition at 450 oC in liquid phase. The hydrogen

158

generation occurs in this step. The general schematic concept of S-I thermochemical cycle for

159

hydrogen generation is shown in Fig.2.

160 161

In short, in S-I hybrid thermochemical cycle, for hydrogen generation, H2O decomposes with

162

input heat and electricity and converts into H2 and O2 as shown below;

163 164 165 166 167 168 169

H2O (v) +Heat and Electricity → H2 (g) + ½ O2 (g)

170

the increasing energy demands [25]. In this regard, the thermodynamic performance analysis of

171

any energy consumption system plays a vital role. In this study, the energy and exergy analyses

172

of each step of S-I cycle for hydrogen generation are performed. For this study, calculations for

173

all the reactions of S-I thermochemical cycle are done according to 1 mole hydrogen generation.

3. Performance analyses The efficient utilization of energy sources is very important for any sustainable plan to confront

174 175

The thermodynamic performance analysis of the solar based S-I thermochemical cycle based on

176

mass, energy and exergy equations for control volume. During the analysis, we assumed that the

177

solar based S-I cycle runs at steady state flow conditions, the reference pressure and environment

178

temperature are taken as 100 kPa and 25 oC, respectively. The kinetic and potential energies are

179

assumed as zero.

180 181

The mass, energy and exergy balance equations, are to calculated work input, energy and exergy

182

efficiencies and exergy destruction rates.

183 184 185 186

For the steady-state mass rate balance as given below;

∑𝐦 = ∑𝐦 𝐢𝐧

𝐨𝐮𝐭

or

∑𝐦 = ∑𝐦 𝐑

𝐏

(1)

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187 188 189 190

Energy balance formulation can be written as; (2)

𝐄𝐢𝐧 ‒ 𝐄𝐨𝐮𝐭 = ∆𝐄𝐬 which becomes [26];

191 192

Q‒W=

∑m

outhout ‒

∑m

(3)

inhin

193 194

where Q and W denote heat and work, respectively. The general energy balance in a chemical

195

processes can be expressed as;

196

Q‒W=

197

where h°f and h° stands for specific enthalpy of formation and enthalpy at reference state,

198

respectively. h is specific enthalpy and its unit is kJ/mole. n, subscripts R and P stands for

199

number of moles, reactants and products, respectively.

200

The exergy balance for a S-I thermochemical cycle can be written as

201

∆Exsys =

202

where Exin, Exout and Exdest are the rate of net exergy transferred and exergy destruction,

203

respectively. ∆Exsys, is zero when the system is in a steady-state condition, so Eq. (5) becomes;

204

ExQ ‒ ExW + Exmass,in ‒ Exmass,out = Exdest

∑ n (h + h ‒ h ) ‒ ∑ n (h + h ‒ h ) P

° f

∑Ex ‒ ∑Ex in

°

P

R

° f

°

(4)

R

(5)

out ‒ Exdest

(6)

205 206

The exergy balance is related to physical and chemical processes, hence it can be written as:

207

ex = exph + exch

208

Finally, if kinetic and potential exergy is ignored, specific exergy formulation can be expressed

209

as:

210

ex = (h ‒ h0) ‒ T0(s ‒ s0) + exch

(7)

(8)

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211

where h is enthalpy, s is entropy, and the subscript zero indicates properties at the reference

212

(dead) state of P0 and T0.

213 214

The last step of S-I cycle, which is hydrogen production step and electrolysis, the electricity

215

demand is calculated by [27].

216 217 218 219

∆G =‒ nFE

(9)

where F is stands for Faraday’s constant and it is 96,485 C/mole, E is the cell potential and n is

220

the number of moles. Electrical energy input of the step III is defined as follows;

221 222

𝑊𝑒𝑙 =‒ ∆G

223 224

The S-I thermochemical cycle of standard chemical exergy values for compounds and elements

225

are taken from the literature [28] and given in the Table 1.

226 227

Shomate equations used to calculated enthalpy and entropy of the each step elements and

228

compounds in S-I thermochemical cycles [29].

229 230 231 232

T2 T3 T4 1 h ‒ h0 = AT + B + C + D ‒ E + F ‒ H 2 3 4 T T2 T3 1 s = Aln(T) + BT + C + D ‒ E 2 + G 2 3 2T

(10)

(11)

(12)

233 234

where T is reaction temperature (K) which is normalized to 1/1000 and constants (A - H) in the

235

formulation are given in Table 2 [29].

236 237

The exergy efficiency for the all steps can be written as follows;

238

Exout

(13)

239

ψstep =

240 241

where Exin the specific exergy is input of step, and Exout is the specific exergy output from step.

242

Exin

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243

The rate of heat received by heliostat can be described as [30];

244

Qs = IsA

245

where Is and A are solar radiation and heliostat field area, respectively. The exergy rate on the

246

heliostat can be expressed as;

(14)

247 248

Exs = Qs (1 ‒

T0 Tsun

)

(15)

249 250

where T0 and Tsun stands for ambient temperature and apparent sun temperature, respectively.

251

Additionally, Tsun is taken as 4500 K. The field efficiency of the heliostat can be taken as 75%

252

[30].

253 254

The absorbed heat rate of the receiver Qrec is expressed as,

255 256

Qrec = mms (Tout,rec ‒ Tin,rec)

(16)

257 258

Where mms is molten salt, Tout,rec is outlet temperature of receiver and Tin,rec inlet temperature

259

of receiver. The exergy rate on the receiver can be expressed as;

260 261

Exrec = Qrec (1 ‒

T0

) Tms

(17)

262 263

The S-I thermochemical and overall system energy efficiency can be written as;

264 265 266

ηS ‒ I =

LHVH2

∑Q + W

el

(18)

267 268

ηsystem =

LHVH2 Qs

(19)

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269 270

where Q is heat flow into cycle, the hydrogen lower heating value is taken as 240 kJ/mole Wel is

271

the electricity requirement of the last step.

272 273

The S-I thermochemical and overall system exergy efficiency can be formulated as

∑Ex

274

ψ𝑆 ‒ 𝐼 =

275

ψ𝑠𝑦𝑠𝑡𝑒𝑚 =

H2

(20)

Exin + Wel LHVH2

(21)

Exs

276 277 278 279 280 281 282

where Exin is energy input to the S-I cycle and ExH total exergy content of hydrogen generation. 2

4. Results and discussion The solar heliostat system was designed according to the heat demand of heat exchangers and

283

reactors of the S-I hybrid cycle. For hydrogen generation rate of 1 mole/s, the solar heliostat area

284

was calculated as 140 m2 with a solar radiation intensity of 800 W/m2. The properties of the solar

285

heliostat subsystem are listed in Table 3.

286 287

The exergy efficiencies of the subsystems, which consists of heliostats, receiver system, SGSS

288

and S-I cycle are calculated as 75%, 40.89%, 23.19% and 62.39% respectively, at a constant

289

ambient temperature of 25 oC. Total heat loss to the environment from the system is accepted as

290

20%. The total exergy destruction rate of the system is computed as 630.70 kJ/mole.

291 292

The heat requirement of step 1 at 850 oC is 170.4 kJ/mol H2. The correlation between exergy

293

destruction, reaction temperatures and environment reference temperatures for Step I, is

294

graphically illustrated in Fig.3. In this figure, the exergy destruction of the step I decreases with

295

increase in reaction temperature from 750 oC to 850 oC. Fig.3 also shows that with increase in

296

reference environment temperature the exergy destruction rate of the step I decreases. The

297

variation in the inlet and outlet exergy rates for step I for various ambient temperatures is shown

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298

in Fig.4. It can also be seen from Fig.4 that the inlet and outlet exergies of step I decrease by

299

increasing reference environment temperature, from 0 oC to 50 oC, at a reaction temperature 850

300

oC.

301

The change in exergy destruction rates for step II, with reaction and reference environment

302

temperatures are given in Fig.5. The exergy destruction rate increases with increase in reaction

303

temperature, from 80 oC to 120 oC. The inlet and outlet exergies, with different reference

304

temperatures, are illustrated in Fig.6. If the reference environment temperature increases, the inlet

305

and outlet exergies decreases, at a reaction temperature of 120 oC.

306 307

In the last step of S-I cycle, namely hydrogen generation step, an endothermic reaction was

308

occurred. The heat requirement of step III at 450 oC is 91.52 kJ/mol H2. The variation in exergy

309

destruction rate with different reaction and environment reference temperatures is shown in Fig.7.

310

It can be seen from Fig.7 that when the reaction temperature increases, the exergy destruction of

311

this step also increases. Fig.8 illustrates the relation between reference environment temperature

312

and the inlet and outlet exergies of step III, at a reaction temperature of 450 oC. The Fig.8 also

313

shows that by increasing the reference environment temperature the outlet exergy increases,

314

whereas the inlet exergy decreases.

315 316

The relationship with reference environment-temperature and energy and exergy efficiency of the

317

step I is illustrated in Fig.9. The figure shows that while increasing the reference environment

318

temperature from 0 oC to 50 oC, the energy efficiency does not change, but the exergy efficiency

319

increases. Fig.10, demonstrates the effects of the reference temperature on the energy and exergy

320

efficiency for step II. The exergy efficiency of step II increases nearly 9% with increase in

321

reference environment temperature, at 120 oC reaction temperature. The effect of reference

322

environment temperature on energy and exergy efficiency for step III is shown in the Fig.11. As

323

it can be seen from Fig.11, the energy efficiency is constant at 0.3317, while the exergy

324

efficiency increases, at standard step temperature of 450 oC.

325 326

Fig.12, shows that the exergy efficiency of S-I cycle increases by increasing the reference

327

environment temperature. The exergy efficiency of this cycle increases nearly 5%, when

328

reference environment temperature increases from 0 oC to 50 oC. Fig.13, illustrates the energy

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329

and exergy efficiencies of S-I cycle and the overall system. The energy and exergy efficiency of

330

the whole system are calculated as 32.76% and 34.56%, respectively. According to this results, it

331

can be said that the solar based S-I cycle is better than solar based Mg-Cl cycle.

332 333 334 335 336 337

5. Conclusion

338

thermochemical cycle for hydrogen generation. In addition, we presented the energy and exergy

339

efficiencies, inlet and outlet exergy rates as well as the exergy destruction for each step of a solar

340

based S-I thermochemical cycle. S-I thermochemical cycle energy and exergy efficiencies are

341

computed as 43.85% and 62.39, respectively. The energy and exergy efficiency of the whole

342

system are computed as 32.76% and 34.56%, respectively. The highest exergy destruction is

343

observed in the central receiver system. The total heat demand required for S-I cycle is calculated

344

as 456.1 kJ/mole. The total heat demand of heat exchangers calculated as 90.03 kJ/mole while the

345

total heat released from heat exchanger calculated as -65.04 kJ/mole. The electricity demand

346

required for hydrogen generation step is calculated as 104.203 kJ (e)/mole H2.

347

As a result, the S-I thermochemical cycle for hydrogen generation become prominent cycle in

348

terms of the energy and exergy efficiency and environmentally friendly hydrogen generation

349

ways. Additionally, the proposed study showed that S-I thermochemical cycle has reasonable

350

results at least as good as Mg-Cl and Cu-Cl cycles. In the near future, the use of thermochemical

351

cycles for hydrogen generation is expected to increase. It is envisaged that the offered cycle can

352

be used for hydrogen generation as a more environmentally and more feasible option.

This study investigated the detailed thermodynamic performance assessment of a solar based S-I

353 354 355 356 357

NOMENCLATURE A

area m2

358

𝐸

energy, kW

359

𝐸𝑥

exergy, kW

360

𝑒𝑥

specific exergy, kJ/kg

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361

exph

specific physical exergy, kJ/kg

362

exch

specific chemical exergy, kJ/kg

363

G

Gibbs function, kJ

364

G

gravity, m/s2

365

h

enthalpy, kJ/kg

366

h

enthalpy, kJ/mole

367

h0

specific enthalpy at reference point, kJ/mole

368

h0

formation enthalpy, kJ/mole

369

m

mass flow rate, kg/s

370

n

moles number, mole

371

s

specific entropy, kJ/kg K

372

T

temperature, K - oC

373

W

work, kW

374

Q

heat, kJ

375

Greek letters

376

𝜼

energy efficiency

377

𝝍

exergy efficiency

378

Subscripts

379

in

inlet

380

P

product

381

R

reactant

382

out

outlet

383

0

dead state point

384 385 386 387 388 389 390 391

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[19] Vitart Xavier, Carles Philippe, Anzieu Pascal. A general survey of the potential and the main issues associated with the sulfur-iodine thermochemical cycle for hydrogen generation using nuclear heat. Prog Nucl Energy 2008;50:402-10. [20] Zhang P, Chen SZ, Wang LJ, Yao TY, Xu JM. Study on a labscale hydrogen generation by closed cycle thermo-chemical iodine sulfur process. International Journal Hydrogen Energy 2010;35:10166-72. [21] Garcia, L., Gonzalez, D., Garcia, C., Garcia L., Brayner, C., Efficiency of the sulfureiodine thermochemical water splitting process for hydrogen production based on ADS (accelerator driven system) Energy 2013;57:469-477 [22] Öztürk I.T., Hammache A, Bilgen E. An improve process for H2SO4 decomposition steep of the sulfure iodine cycle, Energy Convers. Mgmt 1995 ;(36),1, 11-21 [23] Xinxin W. Kaoru O. Thermochemical water splitting for hydrogen generation utilizing nuclear heat from an HTGR, Tsinghua Science and Technology 2005;10: 270–276. [24] Norman J.H., Mysels K.J. Sharp S. Williamson D. Studies of the sulfur–iodine thermochemical water-splitting cycle. International Journal of Hydrogen Energy 1982;7 (7):545–556. [25] Orhan MF. Dincer İ. Rosen MA. Energy and exergy assessments of the hydrogen generation step of a copper–chlorine thermochemical water splitting cycle driven by nuclearbased heat, International Journal of Hydrogen Energy 2008;33: 6456–6466 [26] Cengel, Y.A. and Boles, M.A. Thermodynamics: An Engineering Approach, 6th edition, McGraw-Hill, NY 2008. [27] Barbir F. PEM fuel cells theory and practice. Elsevier; 2005 [28] The Exergoecology Portal, http://www.exergoecology.com/excalc/ (accessed 03.01.2016)

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[29]National

Enstitute

of

Standards

and

Technology,

(NIST).

http://webbook.nist.gov/chemistry/form-ser.html (accessed 03.01.2016)

[30] Xu C, Wang Z, Li X, Sun F. Energy and exergy analysis of solar power tower plants. Appl Therm Eng 2011;31:3904-13.

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Fig.1. A schematic flow diagram of solar S-I cycle

Fig.2. A general schematic concept of S-I cycle

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Fig. 3. Variation of the exergy destruction for the step I with reaction temperature, for several reference-environment temperatures. 168 T 0 = 25 o C T 0 = 15 o C

Exdest (kJ/mol H2 )

T 0= 5 oC

166

164

162 740

760

780

800

820

840

860

o

Treaction ( C)

Fig. 4. Inlet and outlet exergy rates of step I as a variation of reference-environment temperature at a reaction temperature of 850 oC. 320

Exergy Rate (kJ/mol)

300 Exout,SI

280

Exin,SI

260 240 220 200 0

10

20

30 T0 (o C)

40

50

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Fig. 5. Variation of the exergy destruction for the step II with reaction temperature, for several reference-environment temperatures. 220 T0 = 25 o C

200

T0 = 15 o C T0 = 5 o C

Exdest (kJ/mol H2 )

180 160 140 120 100 70

80

90

100

110

120

130

Treaction (o C)

Fig. 6. Inlet and outlet exergy rates of step II as a variation of reference-environment temperature at a reaction temperature of 120 oC.

575

Exergy Rate (kJ/mol)

570

Exout , SII Exin , SII

565

560

555

550 0

10

20 T0 (o C)

30

40

50

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Fig. 7. Effects of the reaction temperature on exergy destruction of step III, for several referenceenvironment temperatures 48 47 46

T0 = 25 o C T0 = 15 o C T0 = 5 o C

Exdest (kJ/mol)

45 44 43 42 41 40 39 400

420

440 Treaction

460

480

500

(o C)

Fig. 8. Effects of the reference environment temperature on inlet and outlet exergy rate of step III, for reaction temperature 450 oC 600

Exergy Rate (kJ/mol)

580 Exout, SIII

560

Exin, SIII

540

520

500 0

10

20 T0 (o C)

30

40

50

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Fig. 9. Effects of the reference environment temperature on energy and exergy efficiency of step I, for reaction temperature 850 oC 1.0

0.714

Treaction = 850 (o C) en,SI ex ,SI

0.712

0.710 0.90 0.708 0.85

Exergy Efficiecny

Energy Efficiecny

0.95

0.706

0.80

0

10

20

30

40

0.704

50

T0 (o C)

Fig. 10. Effects of the reference environment temperature on energy and exergy efficiency of step II, for reaction temperature 120 oC 0.52

0.80 Treaction = 120 (o C) en,SII ex,SII

0.78

0.76 0.48 0.74 0.46 0.72

0.44

0

10

20

T0 (o C)

30

40

50

0.70

Exergy Efficiecny

Energy Efficiecny

0.50

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Fig. 11. Effects of the reference environment temperature on energy and exergy efficiency of step III, for reaction temperature 450 oC 0.36

0.91 Treaction = 450 (o C) en, SIII ex, SIII

Energy Efficiency

0.90 0.32 0.90 0.30

0.28

Exergy Efficiecny

0.90

0.34

0.89

0

10

20

30

40

50

0.88

T0 (o C)

Fig. 12. Variation of exergy efficiency of S-I cycle with reference environment temperature

0.65

Exergy Efficiecny

0.64 0.63 0.62 0.61 0.6 0

10

20 T0 (o C)

30

40

50

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Fig. 13. Energy and exergy efficiency of S-I cycle and overall systems

ACCEPTED MANUSCRIPT Table 1. Standard chemical exergy of the compounds (adapted from ref [27]) 𝒆𝒙𝒄𝒉 (kJ/mol) 160.53 106.89 9.34 0.75 126.34 310.41 3.97 191.15 153.8 236.10

Chemical H2SO4 (aq) H2SO4 (g) H2O (g) H2O(l) SO3 SO2 O2 I2 HI H2

Table 1. Enthalpy of formation, reference entropy and Shomate constants for chemical compounds (adapted from ref. [28]) Compoun ds

ℎ0𝑓 (kJ/mol)

𝑠00 (kJ/mol* K)

A

B

C

D

E

F

G

H

H2SO4

-735.13

298.78

47.28924

190.3314

-148.1299

43.86631

-0.740016

-758.9525

301.2961

735.1288

H2O (850 oC)

-241.830

188.840

30.092

6.832514

6.793435

-2.53448

0.082139

-250.881

223.3967

-241.8264

-395.770

256.770

24.02503

119.4607

-94.38686

26.96237

-0.117517

-407.8526

253.5186

-395.7654

-296.84

248.21

21.43049

74.35094

-57.75217

16.35534

0.086731

-305.7688

254.8872

-296.8422

205.15

30.03235 0

8.772972

-0.398813

0.788313

-0.741599

11.32468 0

236.1663 00

0

SO3 SO2 O2

0

I2 (120 oC)

13.52

150.36

80.66919

6.855652 ×10-8

8.724352 ×10-8

3.723132 ×10-8

4.735829 ×10-10

-10.53

247.98

13.523

HI

26.36

206.59

26.0454

4.6897

4.911765

-2.654

0.1214

18.755

237.2

26.359

H2

0

130.68

33.06617 8

-11.36

11.43282

-2.773

-0.159

-9.981

172.71

0

I2 (450 oC)

62.42

260.69

37.798

0.225453

-0.913

1.0349

-0.084

50.869

305.92

62.4211

H2O (120 oC)

-285.83

69.95

203.6060 0

1523.290 00

3196.413 00

2474.455 00

3.85533

256.5478 0

488.7163 0

285.8304 0

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Table 3. Properties of the base case solar heliostat subsystem Subsystem Heliostat Field

Properties Solar irradiation Aperture area Field efficiency

Values 800 140 75%

Unit W/m2 m2 -

Receiver

Inlet temperature of molten salt Outlet temperature of molten salt Mass flow rate of molten salt Emissivity

565 293 0.1946 0.8

oC

Inlet temperature of water Outlet temperature of water Reference state temperature

230 552 25

oC

SGSS

oC

kg/s -

oC oC

Table 4. The results of exergy analysis of the considered system Subsystem

Inlet (kW)

Outlet (kW)

Heliostat field Central receiver SGSS S-I cycle

526.2 394.7 161.8 378.4

394.7 161.8 37.8 236.1

Exergy destruction rate (kW) 131.5 232.9 124 142.3

Efficiency 75 40.89 23.19 62.39