Experimental investigation on potential of a concentrated photovoltaic-thermoelectric system with phase change materials

Accepted Manuscript Experimental investigation on potential of a concentrated photovoltaicthermoelectric system with phase change materials

Tengfei Cui, Yimin Xuan, Ershuai Yin, Qiang Li, Dianhong Li PII:

S0360-5442(17)30087-7

DOI:

10.1016/j.energy.2017.01.087

Reference:

EGY 10222

To appear in:

Energy

Received Date:

06 October 2016

Revised Date:

16 January 2017

Accepted Date:

17 January 2017

Please cite this article as: Tengfei Cui, Yimin Xuan, Ershuai Yin, Qiang Li, Dianhong Li, Experimental investigation on potential of a concentrated photovoltaic-thermoelectric system with phase change materials, Energy (2017), doi: 10.1016/j.energy.2017.01.087

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

ACCEPTED MANUSCRIPT Research Highlights ►An experimental PV-PCM-TE device is constructed and compared to the pure PV system. ►The performances of hybrid system under practical working conditions are studied. ►The PV-PCM-TE system is proved to have higher efficiencies than the pure PV system. ► The cost and influence parameters of the PV-PCM-TE system are investigated.

ACCEPTED MANUSCRIPT 1 2 3

Experimental investigation on potential of a concentrated photovoltaic-thermoelectric system with phase change materials

4

Tengfei Cui1, Yimin Xuan*1,2, Ershuai Yin1, Qiang Li1, and Dianhong Li1

5 6 7

1School

8

2College

9 10

of Energy and Power Engineering, Nanjing University of Science & Technology, China of Energy and Power Engineering, Nanjing University of Aeronautics & Astronautics, China * Correspondent author. Tel: +86 025 84890688. E-mail: [email protected]

11 12

Abstract

13

Since the influence of temperature on the conversion efficiencies of photovoltaic (PV) cells

14

and thermoelectric (TE) generators are totally different and opposite, the system operating

15

temperature becomes a key parameter which significantly determines the utilization efficiency of

16

the common PV-TE system on solar energy. In order to make the PV-TE system obtain higher

17

energy utilization efficiency, phase change material (PCM) is incorporated to construct a novel

18

PV-PCM-TE hybrid system to maintain the system operating at the ideal working temperature.

19

The performance of such a novel hybrid system is experimentally studied corresponding to a

20

number of practical working conditions. The temperature, efficiency, and output power of the

21

hybrid system are compared with those of the pure PV system under the same circumstance. The

22

effects of the optical concentrations ratio and cooling approaches on the conversion efficiency of

23

the hybrid system are experimentally investigated. The whole conversion efficiencies of the

24

hybrid system incorporated with TE generators with different values of dimensionless

25

thermoelectric coefficient (ZT) are discussed. The present work reveals that such a hybrid system

26

possesses a promising potential on the full-spectrum utilization of solar energy.

27 28

Keywords: Solar energy; Phase change material; hybrid generation system; Photovoltaic cells;

29

Thermoelectric generator;

1

ACCEPTED MANUSCRIPT 31

1. Introduction

32

With the intensified problems of environment pollutions and the shortage of fossil energy, the

33

renewable energy sources, for example solar energy, become more and more attractive. In plenty

34

of renewable energy utilization technologies, the photovoltaic (PV) technology is one of the most

35

common approaches of making use of solar energy because that it can directly convert solar

36

energy into high quality electrical energy. However, due to the limit of the band-gap of the

37

semiconductor materials used in PV cells, there is a large portion of the solar spectrum that cannot

38

be utilized by PV cells, lots of solar energies are wasted as heat [1-3]. Therefore, the PV-TE

39

system combined by PV cell and thermoelectric (TE) generator becomes a promising approach of

40

realizing the utilization of full-spectrum solar energy.

41

Thermoelectric generator, as a solid-state device, can directly transform thermal energy into

42

electricity by the Seebeck effect. Therefore, it has been widely used in the utilization of thermal

43

energy, such as the hot exhaust in vehicles [4], the hot water [5] and the solar energy [6,7] and etc

44

[4]. Being compared with the pure PV system, the advantage of the PV-TE system lies in that the

45

TE generator can absorb the waste heat from the PV cell and convert it to electricity [8-10], so that

46

the PV-TE system can produce more output electricity than the pure PV system. So far, there are

47

many kinds of PV-TE systems, which were combined by different PV cells with TE generators,

48

were investigated [11-21], most of these investigation efforts indicated that the efficiencies of the

49

hybrid PV-TE systems were higher than that of the pure PV systems under the AM1.5 condition

50

[22-26]. However, there were also some inverse results: the efficiencies of the hybrid PV-TE

51

system were decreased [27, 28]. The main reason was that the influences of temperature on the

52

efficiencies of PV cells and the TE generators were not taken into account [29-31]. It is well

53

known that the PV cell efficiency decreases with the increase in the cell temperature [32, 33]. On

54

the other hand, the greater temperature drop between the hot-side and the cold-side of the TE

55

generator results in the higher thermoelectric conversion efficiency obtained by the TE generator

56

[34-36]. In the conventional PV-TE hybrid system, the TE generator is usually placed between the

57

PV cell and the heat sink. Therefore, it can be assumed that the hot-side temperature of TE

58

generator is equal to the temperature of the PV cell in the system, and the cold-side temperature of

59

TE generator is mainly depended on the cooling method and the temperature of environment.

60

Therefore, the selection of the working temperature has a vital influence on the total conversion 2

ACCEPTED MANUSCRIPT 61

efficiency of the hybrid PV-TE system. Figure 1 illustrates the efficiency curve of a PV-TE

62

system corresponding to different working temperatures [37]. The PV cell is the single-junction

63

GaAs cell, the dimensionless thermoelectric coefficient (ZT) of TE module is given to be 0.77,

64

and the cold-side temperature of the TE module is set to 300K. According to Fig.1, one can find

65

that, with the system working temperature increases, the system efficiency will increase first and

66

then decrease. There exist the highest efficiency and the according optimal working temperature

67

for PV-TE systems. This is because that, although the efficiency of TE generator grows faster than

68

the efficiency reduction of PV cell when the system working temperature augments, the heat loss

69

to the surroundings through radiation and convection is increased, the heat energy that can be used

70

by TE generator is decreased. So, when the heat loss increases to a limit value, which happens at

71

the optimal working temperature, the increased output power of TE generator equals to the

72

reduced output power of PV cell, the system efficiency reaches the highest value. Therefore, it is

73

quite meaningful to maintain the system operating at the optimal working temperature.

74

>>Figure 1<<

75

Proceeding from this idea, we propose a novel photovoltaic-phase change material-

76

thermoelectric (PV-PCM-TE) hybrid system in the previous work [37], in which PCM is used to

77

maintain the hybrid system operating on the optimal temperature. The theoretical computation

78

indicated that the efficiency of the hybrid system can surpass 1.02% over that of the pure PV cell

79

system. However, simulations are always based on a number of assumptions. It is still uncertain

80

whether the PV-PCM-TE system is indeed better than the pure PV system. Therefore, in order to

81

verify the theoretical results and help us to better understand and analyze performance of the PV-

82

PCM-TE system, we construct an experimental PV-PCM-TE device and a contrast pure PV

83

system in this paper. The temperature, efficiency, and output power of the hybrid system are

84

compared with those of the pure PV system under the same circumstance. The influences of the

85

optical concentration, cooling method, and the dimensionless thermoelectric coefficient on the

86

hybrid system are investigated.

87

2. Experimental device of PV-PCM-TE system

88

The schematics of the structure and the energy transport as well as the conversion process of

89

the PV-PCM-TE system are present in Fig. 2. The present system is constructed by a Fresnel lens,

90

a PV cell, PCM, a TE generator, and a heat sink. The concentration ratio (CR) is defined as the 3

ACCEPTED MANUSCRIPT 91

ratio of the Fresnel lens area Alens to the PV cell area APV, CR= Alens /APV. However, according to

92

the optical concentration efficiency ηlens of the Fresnel lens, the realistic optical concentration ratio

93

C can be expressed as C=ηlensCR. Once the solar radiation irradiates on the hybrid system, the

94

sunlight is firstly concentrated on the PV cell by the Fresnel lens. Then, a portion of solar

95

spectrum is utilized by the PV cell to generate electricity PPV, the rest solar spectrum is turned into

96

heat and absorbed by the PCM, which leads to the temperature rise and phase change of the PCM.

97

The ΔH is the heat energy absorbed by the PCM for phase changing. The remaining heat is

98

transferred into the TE modules to generate electricity PTE. Finally, the waste heat energy that

99

cannot be used by the TE generators is transferred to heat sink and be removed by water. As the

100

solar radiation reduces, the PCM will release the stored energy via reverse phase changing. The

101

output originated from the solar cell decreases, but the TE generator can continue outputting

102

electricity because of the stored energy in PCM. Therefore, the PCM plays the functions of storing

103

heat energy and maintaining the hybrid system operating at the optimal working temperature.

104

Certainly, there are some heat losses caused by the radiation and convection among the hybrid

105

system with the environment.

106

>>Fig.2<<

107

Figure 3 presents photographs of the experimental PV-PCM-TE device. The PV-PCM-TE

108

device is installed on a double-axis sunlight-trace system which can be observed in the Fig.3(a). A

109

sunlight tracer is applied to receive sunlight and output moving order to the double-axis stepper

110

motor to keep the PV-PCM-TE system perpendicular to the sunlight. In order to make sure that

111

the focused sunlight can be totally and uniformly impinged on the PV cell, a second prismatic

112

glass concentrator is placed on the PV cell. Besides, a pure PV system shown in Fig.3(b) is also

113

placed on the double-axis sunlight-trace system as a contrastive experiment. The pure PV system

114

is consisted by a Fresnel lens, a PV cell and a water/air cooled heat sink. The PV cell is connected

115

with the water/air cooled heat sink through thermal conductive adhesive. We also tried to compare

116

the present hybrid system with the traditional PV-TE system. However, the PV cell in the PV-TE

117

system was burned immediately when the PV cell is connected to the TE generators directly

118

through thermal conductive adhesive. This is because that the poor thermal conductance of the TE

119

generator causes heat cannot be spread away from the PV cell. The cell temperature exceeds the

120

maximum allowable value. Figure 3(c) is the photograph of the upside of hybrid system, the phase 4

ACCEPTED MANUSCRIPT 121

change material is filled in a copper-made container which is wrapped by thermal insulation

122

materials, the PV cell is connected to the PCM container by thermal conductive adhesive. Two

123

temperature sensors are situated at the edge of the PV solar cell to measure its temperature, TPV.

124

The Fig. 3 (d) and Fig.3 (e) are the photographs of the downside of the hybrid system with a water

125

cooling heat sink and an air cooling heat sink, respectively. The location of the TE generators,

126

which adhered through thermal conductive adhesive, is between the PCM and the water/air

127

cooling heat sink. The nethermost pressure sensors are applied to measure the connection

128

pressure. Several temperature sensors are applied to measure the upper side temperatures (TTE-hot)

129

and the down-side temperatures (TTE-cold) of the TE generators, respectively. A high precision

130

electronic load (N3302A, made by Keysight Technology Inc.) is implemented to measure the I-V

131

curves of the PV cells and the TE generators. The measurement accuracies of current and voltage

132

are both 0.1%. The pyrheliometer and pyranometer (PC-2-T, made by Jinzhou Sunshine

133

Technology Co. Ltd ) with the accuracy of 0.5% are applied to measure the local direct and the

134

total solar irradiances.

135

>> Fig. 3 <<

136

The PV cells used in experiments are the single-junction GaAs solar cells manufactured for

137

satellites. The size of the PV cell is 10mm*10mm*0.2mm. The open circuit voltage Voc of the cell

138

amounts to 1.01V, the short circuit current Isc is 21.8mA, and the energy conversion efficiency ηPV

139

is 18.8% at room temperature (25℃) under AM1.5 1000W/m2 irradiation. The TE generator

140

(TEP1-1264-1.5) with a length of 40 mm, width of 40mm, and the thickness of 4mm are currently

141

made of bulk bismuth telluride (Bi2Te3) alloys and two isolated ceramic layers. A total 126 Bi2Te3

142

thermocouples are arranged electrically in series and thermally in parallel in a TE module. The

143

size of isolated ceramic layers is 40mm*40mm*1mm, the cross section of Bi2Te3 thermocouples is

144

1.5*1.5mm2, the height of Bi2Te3 thermocouples is 1.8mm. The Seebeck coefficient of Bi2Te3

145

material is 1.15*10-4 V/K, and the electrical and the thermal conductivity of the Bi2Te3 material

146

are respectively 2.52*105 S/m and 1.39 W/mK at room temperature. The electrical resistance of

147

the TE generator is measured to be 1.62Ω at room temperature. According to the properties of the

148

single-junction GaAs PV cell and the Bi2Te3 TE generator, the paraffin PCM with the melting

149

temperature Tmelting of 60℃ is selected based on the optimal simulation of the hybrid system. The

150

latent heat, thermal conductivity, and specific heat capacity of the paraffin PCM are 182 kJ/kg, 0.2 5

ACCEPTED MANUSCRIPT 151

W/mK, and 2 kJ/kgK, respectively. Two types of Fresnel lenses with the size of

152

230mm*230mm*5mm and the concentration efficiency ηlens of 82.6%, and the size of

153

340mm*340mm*5mm and ηlens of 81.3%, are applied to study the optical concentration influences

154

on the hybrid system efficiency, respectively. Thus, the geometrical concentration ratios CR are

155

529 and 1156, but the actual optical concentration ratios C are 438 and 940, respectively. In the

156

present hybrid system, one practical design is that the PCM area APCM is equal to the Fresnel lens

157

area Alens, so that the PCMs in an arrayed system can be manufactured together just as the arrayed

158

Fresnel lens. Therefore, when the CR is 529, the size of the copper-made PCM container is

159

230mm*230mm*10mm according to our design. The mass of paraffin PCM is nearly 355 g. Two

160

temperature sensors are placed in the PCM at 40mm and 80mm from the center of the PCM to

161

measure the temperature distribution of PCM (TPCM-40mm and TPCM-80mm). Similarly, when the CR

162

is 1156, the size of the copper-made PCM container is 340mm*340mm*10mm. The mass of

163

paraffin PCM is nearly 800 g. Three temperature sensors are placed in the PCM at 40 mm, 80 mm,

164

and 120mm from the center of the PCM to measure the temperature distribution of PCM (TPCM-

165

40mm,

166

the same copper-made water cooling heat sinks. The size of the water cooled heat sink is 40

167

mm*40 mm*10 mm, the flow channel length is nearly 154 mm, and the cross area of the flow

168

channel is a circle with diameter of 8 mm. The temperature of inlet water Tinlet is 20 ℃, the

169

pressure drop between the entrance and the exit is 0.01MPa, and the volume flow of water is

170

0.3L/min. The air cooling heat sink for the present hybrid system is made by aluminum material.

171

The total surface area of the air cooled heat sink is approximately 0.02 m2, and the fin efficiency is

172

nearly 73%. In view of the realistic concentration PV system, an aluminum-made plate with the

173

size of 350mm*350mm*1mm is applied as the air cooled heat sink for the pure PV system, which

174

is different from the air cooled hybrid system. The detailed parameters and the uncertainties/errors

175

of the hybrid system are provided in Table 1.

176 177 178

TPCM-80mm, and TPCM-120mm). In addition, both the hybrid system and the pure PV system use

>>Table 1<< In order to evaluate the performance of the hybrid PV-PCM-TE system, the electrical efficiency ηe of the total system in a day is represented as

6

ACCEPTED MANUSCRIPT 24

179

e 

 (P t 0

PV

(t )  PTE (t )  Ppump (t ))t

24

 GCRG

d

t 0

(t ) APV t

,

(1)

180

where t is time, Gd(t) represents the direct solar irradiance, PPV(t) and PTE(t) are the maximum

181

output power calculated through the I-V curves of the PV cell and TE generator, respectively. The

182 183

required pump power Ppump is calculated by Ppump  pQv

,

(2)

184

where Δp is the pressure drop from the entrance and the exit through the heat sink, Qv is the

185

volume flow of water. The photoelectric conversion efficiency of PV cell ηPV is formulated as

186

187 188

189

 PV 

PPV (t ) GCRGd (t ) APV lens

.

(3)

The thermoelectric conversion efficiency of TE generator ηTE is formulated as

TE 

TTE  hot  TTE  cold TTE  hot

1  ZT  1 1  ZT  TTE  cold / TTE  hot

,

(4)

and the dimensionless thermoelectric coefficient ZT of the TE generator is defined as

190

ZT   2

 TTE  hot  TTE  cold k

2

,

(5)

191

where α indicates the Seebeck coefficient, σ signifies the electrical conductivity and k reveals

192

thermal conductivity. Therefore, one can get that the dimensionless thermoelectric coefficient ZT

193

of the TE generator is about 0.75 through the Eq.(5).

194 195

In addition, the mean thermal efficiency ηT of the hybrid PV-PCM-TE system in a day which is provided by hot-water can be calculated through 24

T  196

  mC t 0

p  water

[Toutlet (t )  Tinlet (t )]t

24

 CRG (t ) A t 0

d

PV

t ,

(6)

197

 is the mass flow rate of water, Cp-water is the specific heat of water, Toutlet and Tinlet are where m

198

respectively the outlet temperature and the inlet temperature of the coolant.

199

Based on the uncertainty transfer function in the ISO Guide: [38]

200

 n  Z  2  U z     Ui    i 1  xi    

7

,

(7)

ACCEPTED MANUSCRIPT 201

where UZ is the calculated uncertainty of parameter Z, xi is the influence parameters, Ui is

202

measured uncertainty of parameter xi, one can get that through the Eqs.(1) and (7), the uncertainty

203

of the system efficiency is about 0.618%, which is mainly caused by the uncertainty of measuring

204

the solar irradiance of 0.5%. The uncertainty of the output power is quite small that has no

205

significant effect on the calculation of the uncertainty of the total efficiency. For the similar

206

reasons, the uncertainty of the PV cell efficiency is about 2.512%, which is caused by the

207

uncertainty of the concentration efficiency of the Fresnel lens of 2% based on the Eqs.(3) and (7).

208

3. Results and discussion

209

All of experiments are carried on in sunny days at Nanjing city, China (Longitude/Latitude: E

210

118º42' / N 32º03'). Owing to the obstruction of high rises, the experimental device can only

211

accept sunlight during 7:00 to 16:30 in a day. Figure 4 indicates the temperatures of the present

212

hybrid system with different geometrical concentrations ratio and cooling methods. One can find

213

that the introduction of PCM benefits a lot on maintaining the working temperature of the hybrid

214

system. Figure 4(a) presents the temperature of the water cooled hybrid system. With the CR=529.

215

The highest TPV is 76.2℃ at 11:30 with the direct solar irradiance of 750 W/m2, the TPCM-40mm of

216

61.8℃, the TPCM-80mm of 58.1℃, the TTE-hot of 57.9℃, and the TTE-cold of 23.3℃. The PV cell

217

temperature has a significant relationship between the direct solar irradiance, not the total solar

218

irradiance. And it is much higher than the hot-side temperature of the TE generator, which is far

219

from the ideal situation. (The PV cell temperature approximately equals to the temperatures of

220

PCM and the hot-side of TE generator). This is because that the thermal resistance, which includes

221

the thermal contact resistance at contact-interfaces and the thermal resistance of PCM, between

222

the PV cell and the TE generator greatly hindered the heat transfer between the PV cell with the

223

TE generator. Besides that, one can find that the PCM near the center of the PCM is much easier

224

to get melting than the PCM far from the center of the PCM because of the low thermal

225

conductivity of PCM. Simultaneously, the highest temperature of the PV cell in the contrast water

226

cooled pure PV system is 32℃. Similar to the Fig.4(a), the Fig.4(b) is the temperatures of the

227

water cooled hybrid system with the CR=1156. The highest TPV is 89.6℃ at 13:00 with the direct

228

solar irradiance of 673 W/m2, the TPCM-40mm of 64.5℃, the TPCM-80mm of 61.1℃, the TPCM-120mm of

229

58.2℃, the TTE-hot of 61.1℃, and the TTE-cold of 25.4℃. Because of the increase of optical

230

concentration, the temperature drop between the PV cell and the phase change material caused by 8

ACCEPTED MANUSCRIPT 231

thermal contact resistance in Fig.4(b) much higher than that in Fig.4(a). Thus, reducing the

232

thermal contact resistances inside the hybrid system with high optical concentration is significant.

233

In addition, one can realize that the temperature of PCM located at 120 mm from the center of the

234

PCM has never reached the melting temperature during the whole day, but the PCMs at 40mm and

235

80mm from the center of the PCM are all melted. This is because that the low thermal

236

conductivity of PCM impedes the heat transfer in PCM. Therefore, improving the thermal

237

conductance of PCM is another crucial factor to upgrade the performance of the present hybrid

238

system. The highest PV cell temperature in the contrast water cooling pure PV system is 39℃.

239

Figure 4(c) is the temperature of the air cooled hybrid system with the CR=1156. At the time

240

12:30, the TPV reaches 88.1℃ with the direct solar irradiance of 610 W/m2, the TPCM-40mm of

241

68.8℃, the TPCM-80mm of 65.9℃, the TPCM-120mm of 62.4℃, the TTE-hot of 67.9℃, and the TTE-cold of

242

40.1℃. Compared to the Fig.4(b), the air cooling method lifts the cold-side temperature of the TE

243

generator which reduces the output of the TE generators. However, under the same environment

244

condition, the highest temperature of the PV cell in the air cooled pure PV system reaches 158 ℃,

245

which indicates the importance of the PCM in the hybrid system. >>Fig.4<<

246 247

The I-V curves of PV cells in the present hybrid system with different optical concentration

248

are presented in Fig. 5(a). At the same working temperature, the open voltage of the PV cell

249

(1.03V) with the CR of 1156 is higher than that (1.02V) with the CR of 529. However, due to the

250

peculiarity of single-junction GaAs PV cell manufactured for satellites, the photoelectric

251

conversion efficiency with the CR of 529 amounts to 19.8% at the temperature of 71℃, which

252

much smaller than that of 14.7% with the CR of 1156 and temperature of 71℃. Figure 5(b)

253

presents the I-V curve and the output power supplied by the TE generator. With the TTE-hot of 60℃

254

and the TTE-cold of 24℃, the open circuit voltage Voc of the TE generator reaches 1.7V and the

255

short circuit current Isc of the TE generator is 1.01A. The maximum output power of the TE

256

generator is 0.427W.

257

>>Fig. 5<<

258

The electric output power of the present hybrid system and the pure PV system in a day are

259

indicated in Fig.6. One can find that the power provided by the PV cells is fluctuant along the

260

direct solar irradiance, not the total solar irradiance. This is because that the Fresnel lens cannot 9

ACCEPTED MANUSCRIPT 261

concentrate the scattered sunlight. Besides, because of the various PV cell temperatures, it can be

262

easily found that the power provided by the PV cell in the hybrid system is much lower compared

263

to the water cooled pure PV system, but much higher than that in the air cooled pure PV system.

264

>>Fig.6<<

265

The total electric output energies and the required pump energies of the present hybrid

266

system and the pure PV system are illustrated in Fig.7. With the CR of 529, the electric output

267

energies of the PV cell and the TE generator in the water cooled hybrid system are 191.9 KJ and

268

14.6 KJ respectively, the required electric energy for pump is 2.07 KJ. The total direct solar

269

energy incident on the hybrid system is 1149.305KJ. Consequently, the realistic total electric

270

output energy of the present hybrid system amounts 205.81 KJ and its electric efficiency is

271

17.79%. Under the same condition, the total electric output energy of the pure PV system for

272

contrast is 205.9 KJ, the required pumping energy is 1.8 KJ. The electric efficiency of the pure PV

273

system is 17.75% which is 0.04% less than that of the hybrid system. Likewise, the total electric

274

output energies of the PV cells and the TE generators in the water cooled hybrid system with the

275

CR of 1156 are 261.01 KJ and 16.82 KJ, respectively. The required pumping energy is 2.25 KJ.

276

The total electric output energy of the hybrid system is 275.58 KJ. The overall electric output

277

energy of the water cooling pure PV system under the same condition is 275.11 KJ. The total

278

direct solar energy is 2048.548KJ. Thus, the electric efficiencies of the present hybrid system and

279

the pure PV system efficiency are 13.45% and 13.43%, respectively. Two contrastive experiments

280

both prove that the present hybrid system excels the pure PV system. Although the improved

281

electric energy efficiencies in experiments are very small, there are plenty of affairs that can

282

improve the hybrid system electric efficiency, such as improving the dimensionless thermoelectric

283

coefficient ZT of the TE generator, reducing the interfacial thermal contact resistance, increasing

284

the thermal conductivity of PCM, and etc. In the present experiments, the ZT of TE generator is

285

0.75. Actually, with development in nanotechnology [39,40], once novel technologies in solid

286

state physics and semiconductor physics [41,42] are employed to produce TE generators, the value

287

of ZT may be much higher. If the value of ZT is equal to 2, according to the temperature

288

distribution of the water cooling hybrid system with CR=1156, the total electric output energy of

289

TE generators can reach 32.21 KJ, the electric efficiency of the hybrid system will arrive 14.21%,

290

which surpass 0.78% over that of the water cooled pure PV system. And if the value of ZT reaches 10

ACCEPTED MANUSCRIPT 291

5, the electric efficiency of the hybrid system will promote to 15.08%, the system electric

292

efficiency promotion reaches to 1.65%. Therefore, the PV-PCM-TE hybrid system shows a

293

potential performance to improve the solar energy utilization efficiency. In addition, with the CR

294

of 1156, the air cooled hybrid system also provides electric energy of 264.39 KJ, and the received

295

total direct solar energy is 2014.214 KJ. The hybrid system electric efficiency is 13.12%. On the

296

contrary, under the same working environment, due to the high PV cell temperature, the total

297

electric output energy of the air cooling pure PV system is 204.89KJ. The electric efficiency of the

298

air cooled pure PV system is only 10.17% which is 2.95% less than that of the air cooled hybrid

299

system. This is because that the PCM can absorb much of heat through phase change process,

300

which maintains the PV cell working at a low temperature. By comparing the efficiencies of the

301

hybrid system with two different cooling methods, one can find that the hybrid system with air

302

cooling has a lower efficiency, but the cost of the hybrid system with air cooling will be much

303

cheaper and it will be much convenient to service the air cooled hybrid system. >>Fig.7<<

304 305

The thermal output power of the present hybrid system and the pure PV system within a day

306

are proposed in Fig.8. It is noteworthy that the coolant - water applied in the present hybrid system

307

is to mainly provide a steady cooling source for the TE module and the hot-water from the system

308

may be considered as a by-product during the system operation. In fact, a higher outlet

309

temperature of the coolant is not the main purpose of this work, but one can still find that a certain

310

magnitude of thermal energy is recovered by the coolant, which further promotes the utilization

311

efficiency of the whole system.

312

>>Fig.8<<

313

Table 2 indicates the electrical and thermal efficiencies of PV cells, TE generators, pumps,

314

hot-water in the PV-PCM-TE hybrid system. One can find that the efficiency of the TE modules

315

in the hybrid system with CR=529 and water-cooling is the highest. This is because that in the

316

hybrid system with CR=1156 and water-cooling, the larger area of PCM leads to the more heat

317

loss through convection which reduces the output power of the TE modules. At the meantime, the

318

larger hot loss also reduces the heat transferred to water, which leads to a lower thermal efficiency

319

of the hybrid system. In the hybrid system with CR=1156 and air-cooling, a higher cold-side

320

temperature of the TE generator makes the efficiency of the TE generator decrease. 11

ACCEPTED MANUSCRIPT >>Table 2<<

321 322

A cost estimation of the two mentioned above system is provided in Table 3. The cost

323

estimation is calculated based on the prices of the two systems in experiments that will be higher

324

than that in mass production. The total cost of the present hybrid system is 770 $/m2 which is

325

much higher than the total cost of the pure PV system of 415 $/m2. This is due to the fact that the

326

cost of TE generators occupies a large portion of the expense of the present hybrid system. The

327

cost of the PCM is not that so significant. >>Table 3<<

328 329

4. Conclusions and future work

330

An experimental PV-PCM-TE hybrid setup has been constructed to analyze its performance.

331

The hybrid system shows a privilege that the working temperature of the system can be

332

maintained at a desired level during a day. Compared to the pure PV system, the present hybrid

333

system has higher energy conversion efficiencies, which indicates a potential of improving the

334

solar energy utilization efficiency. The major conclusions are listed as follows:

335 336

(1) The scattered sunlight cannot be concentrated by the Fresnel lens. The temperature and the output power of the PV cell are significantly affected by the direct solar irradiance.

337

(2) The thermal contact resistance at interface and the poor thermal conductance of the PCM

338

will cause the high temperature deviations among the PV cell, the PCM, and the TE generator,

339

which is harmful to efficient utilization of solar energy for such hybrid systems.

340

(3) The efficiency of the present hybrid system with water cooling is higher than that with air

341

cooling under the same working environment. However, the cost of the hybrid system with air

342

cooling will be much cheaper than the hybrid system with water cooling, and it will be much

343

convenient to service the air cooled hybrid system.

344 345

(4) The high cost of TE generators which can be reduced in mass production is the main reason of high cost of present hybrid system. The cost of the PCM is very small.

346

The low dimensionless thermoelectric coefficient ZT of the TE generator is the most

347

important restriction on improving the hybrid system efficiency. If the dimensionless

348

thermoelectric coefficient of the TE generator can be much higher in the future, and the cost of the

349

TE generators is reduced, the PV-PCM-TE will have a great prospect of being applied in a wide

350

range. Also, there are other plenty of approaches that can improve the hybrid system efficiency, 12

ACCEPTED MANUSCRIPT 351

such as selecting the Fresnel lens with a higher optical concentration efficiency and the PV cells

352

with a higher efficiency, reducing the interfacial thermal contact resistance, increasing the thermal

353

conductivity of PCM, and optimizing the mass of the PCM. To turn such PV-PCM-TE into

354

practical application, much more research work is expected.

355 356 357 358

Acknowledgments It is grateful that this work is supported by the National Natural Science Foundation of China (Grant No. 51336003).

359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401

References [1] P. K. Nayak, G. Garcia-Belmonte, A. Kahn, J. Bisquert, and D. Cahen, Photovoltaic efficiency limits and material disorder, Energ. Environ. Sci., 5(2012), pp.6022-6039. [2] W. Sha, W. Choy, Y. Wu, and C. Weng, Optical and electrical study of organic solar cells with a 2D grating anode, Opt. Express, 20(2012), pp.2572-2580. [3] A. Kirk, M. J. Dinezza, S. Liu, X. Zhao, and Y. Zhang, CdTe vs. GaAs solar cells - A modeling case study with preliminary experimental results, 39th IEEE Photovoltaic Specialists Conference (PVSC), (2013), pp.2515-2517. [4] K. Koumoto, Y. Wang, R. Zhang, A. Kosuga, and R. Funahashi,. Oxide thermoelectric materials: a nanostructuring approach. Materials Research, 40(2010), pp:363-394. [5] H. Scherrer, L. Vikhor, B. Lenoir, A. Dauscher, and P. Poinas, Solar thermolectric generator based on skutterudites, J. Power Sources, 115(2003), Pp:141-148. [6] F. J. Lesage, R. Pelletier, L. Fournier, E. V. Sempels, Optimal electrical load for peak power of a thermoelectric module with a solar electric application, Energ. Convers. Manage., 74(2013), pp:51–59. [7] F. J. Lesage, P. Nicolas, Experimental analysis of peak power output of a thermoelectric liquid-to-liquid generator under an increasing electrical load resistance, Energ. Convers. Manage., 66(2013), pp:98-105. " [8] X. Gou, H. Xiao, and S. Yang, Modeling experimental study and optimization on lowtemperature waste heat thermoelectric generator system, Appl. Energy, 87(2010), 31313136. [9] D. Kraemer, K. McEnaney, M. Chiesa, and G. Chen, Modeling and optimization of solar thermoelectric generators for terrestrial applications, Sol. Energy, 86(2012), pp.13381350. [10] S. W. Van, Feasibility of photovoltaic–thermoelectric hybrid modules, Appl. Energy, 88(2011), pp.2785-2790. [11] J. Zhang, Y. Xuan, and L. Yang. Performance estimation of photovoltaic-thermoelectric hybrid systems, Energy, 78(2014), pp.895-903. [12] T. T. Chow, A review on photovoltaic/thermal hybrid solar technology, Appl. Energy, 87(2010), pp.365–379. [13] Y. Deng, W. Zhu, Y. Wang, and Y. Shi, Enhanced performance of solar-driven photovoltaic–thermoelectric hybrid system in an integrated design, Sol. Energy, 88(2013), pp.182-91. [14] K. T. Park, S. M. Shin, A. S. Tazebay, H. D. Um, J. Y. Jung, S. W. Jee, M. W. Oh, S. D. Park, B. Yoo, C. Yu, and J. H. Lee, Lossless hybridization between photovoltaic and thermoelectric devices, Scientific reports, 3(2013), pp.120. [15] F. Attivissimo, A. D. Nisio, A. M. L. Lanzolla, and M. Paul, Feasibility of a photovoltaic– thermoelectric generator: performance analysis and simulation results, IEEE T. Instrum. Meas., 64(2015), pp.1158-69. [16] W. Chen, C. Wang, C. Hung, C. Yang, and R. Juang, Modeling and simulation for the design of thermal-concentrated solar thermoelectric generator, Energy, 64(2014), pp.28797. 13

ACCEPTED MANUSCRIPT 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459

[17] M. Fisac, F. X. Villasevil, and A. M. López, High-efficiency photovoltaic technology including thermoelectric generation, J. Power Sources, 252(2014), pp.264-9. [18] A. Luque, and A. Martı́, Limiting efficiency of coupled thermal and photovoltaic converters, Sol. Energ. Mat. Sol. C., 58(1999), pp.147-65. [19] Y. Wu, S. Wu, and L. Xiao, Performance analysis of photovoltaic–thermoelectric hybrid system with and without glass cover, Energ. Convers. Manage., 93(2015), pp.151-9. [20] H. Chen, J. Ji, Y. Wang, W. Sun, G. Pei, and Z. Yu, Thermal analysis of a high concentration photovoltaic/thermal system, Sol. Energy, 107(2014), pp.372-9. [21] H. Najafi, and K. A. Woodbury, Modeling and analysis of a combined photovoltaicthermoelectric power generation system, J. Sol. Energ. Eng., 135(2013), pp.031013. [22] P. Bermel, K. Yazawa, J. L. Gray, X. Xu, A. Shakouri, Hybrid strategies and technologies for full spectrum solar conversion, Energ. Environ. Sci., DOI: 10.1039/c6ee01386d [23] N. Wang, L. Han, H. He, N. H. Park, K. Koumoto, A novel high performance photovoltaicthermoelectric hybrid device, Energ. Environ. Sci. 4(2011), pp.3676-3679. [24] T. Hsueh, J. Shieh, and Y. Yeh, Hybrid Cd-free CIGS solar cell/ TEG device with ZnO nanowires, Pro. Photovolt: Res. Appl. 23(2015), pp.507-512. [25] B. S. Dallan, J. Schumann, and F. J. Lesage, Performance evaluation of a photoelectricthermoelectric cogeneration hybrid system, Sol. Energy, (2015), pp.276–285. [26] D. Kraemer, B. Poudel, H. P. Feng, J. C. Caylor, B. Yu, X. Yan, Y. Ma, X. Wang, D. Wang, A. Muto, K. McEnaney, M. Chiesa, Z. Ren, and G. Chen, High-performance flat-panel solar thermoelectric generators with high thermal concentration, Nat. Mater. 10(2011), pp.532538. [27] R. Bjørk, and K. K. Nielsen, The performance of a combined solar photovoltaic (PV) and thermoelectric generator (TEG) system, Sol. Energy, 120(2015), pp.187-94. [28] Y. Vorobiev, J. González-Hernández, and A. Kribus, Analysis of potential conversion efficiency of a solar hybrid system with high-temperature stage, J. Sol. Energ. Eng., 128(2006), pp.258-60. [29] N. Khattab, and E. T. E. Shenawy, Optimal operation of thermoelectric cooler driven by solar thermoelectric generator, Energ. Convers. Manage., 47(2006), pp.407-26. [30] J. Lin, T. Liao, and B. Lin, Performance analysis and load matching of a photovoltaic– thermoelectric hybrid system, Energ. Convers. Manage., 105(2015), pp.891-9. [30] W. Zhu, Y. Deng, Y. Wang, S. Shen, and R. Gulfam, High-performance photovoltaicthermoelectric hybrid power generation system with optimized thermal management, Energy, 100(2016), pp.91-101. [31] H. Cotal, and R. Sherif, Temperature Dependence of the IV Parameters from Triple Junction GaInP/InGaAs/Ge Concentrator Solar Cells, IEEE 4th World Conference on Photovoltaic Energy Conversion, (2006), 845-848. [32] T. Sakuraia, H. Uehigashia, M.M. Islama, T. Miyazakia, S. Ishizukab, K. Sakuraib, A. Yamadab, K. Matsubarab, S. Nikib, and K. Akimotoa, Temperature dependence of photocapacitance spectrum of CIGS thin-film solar cell, Thin Solid Films, 517(2009), pp.2403–2406. [33] W. He, G. Zhang, X. Zhang, J. Ji, G. Li, and X. Zhao, Recent development and application of thermoelectric generator and cooler, Appl. Energy, 143(2015), pp.1-25. [34] R. O. Suzuki, and D.Tanaka, Mathematical simulation of thermoelectric power generation with the multi-panels, J Power Sources, 122(2003), pp:201-209. [35] J. Xiao, T. Yang, P. Li, P. Zhai, and Q. Zhang, Thermal design and management for performance optimization of solar thermoelectric generator, Appl. energy, 93(2012), pp.33-8. [36] W. Chen, C. Liao, C. Hung, and W. Huang. Experimental study on thermoelectric modules for power generation at various operating conditions, Energy, 45(2012), pp.87481. [37] T. Cui, Y. Xuan, and Q. Li, Design of a novel concentrating photovoltaic–thermoelectric system incorporated with phase change materials, Energ. Convers. Manage., 112(2016), pp.49-60. [38] P. Zhang, B. Shi, Y. Xuan, and Q. Li, A high-precision method to measure thermal conductivity of solids using reversible heat flux, Meas. Sci. Technol., 24(2013), pp.095004. [39] B. Poudel, B. Poudel, Q. Hao, Y. Ma, A. Minnich, A. Muto, Y.C. Lan, B. Yu, X. Yan, D.Z. Wang, D. Vashaee, X.Y. Chen, M.S. Dresselhaus, G. Chen, and Z.F. Ren, High14

ACCEPTED MANUSCRIPT 460 461 462 463 464 465 466 467 468 469

thermoelectric performance of nanostructured bismuth antimony telluride bulk alloys, Science 320(2008), pp:634–8. [40] A. I. Hochbaum, R. K. Chen, R. D. Delgado, W. Liang, E. C. Garnett, M. Najarian, A. Majumdar, and P. Yang, Enhanced thermoelectric performance of rough silicon nanowires, Nature 451(2008), pp:163–7. [41] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis and M. G. Kanatzidis, Cubic AgPbmSbTe2+m: bulk thermoelectric materials with high figure of merit, Science, 303(2004), pp:818–21. [42] R. Venkatasubramanian, E. Siivola, T. Colpitts, and B.O'Quinn, Thin-film thermoelectric devices with high room-temperature figures of merit, Nature, 413(2001), pp:597–602.

15

ACCEPTED MANUSCRIPT 471 472 473 474

Figure Captions

475

Fig.2 the schematics of the structure and the energy flow of the PV-PCM-TE hybrid system

476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497

Fig.1 The efficiency curve of a PV-TE system corresponding to different working temperatures

Fig.3 The photographs of the experimental PV-PCM-TE device (a) the photograph of the PVPCM-TE hybrid system and pure PV system (b) the photograph of a pure PV system (c) the photograph of the upside of PV-PCM-TE system (d) the photograph of the downside of PV-PCMTE system with water cooled heat sink (e) the photograph of the downside of PV-PCM-TE system with air cooled heat sink Fig. 4 The temperature of the PV-PCM-TE system in a day with (a) water cooling and CR=529 (b) water cooling and CR=1156 (c) air cooling and CR=1156 Fig. 5 (a) The I-V curves of PV cells at the temperature of 71℃ (b) The I-V curves and output power of TE generator with TTE-hot=60℃, TTE-cold=27℃ Fig.6 The output power of the PV-PCM-TE hybrid system with (a) water cooling and CR=529 (b) water cooling and CR=1156 (c) air cooling and CR=1156 Fig.7 The total output energies and the required pump energies of the PV-PCM-TE hybrid system and the pure PV system with different condition Fig.8 The thermal output power of the PV-PCM-TE hybrid system and pure PV system with (a) water cooling and CR=529 (b) water cooling and CR=1156

16

ACCEPTED MANUSCRIPT 499 500

List of Tables

501

Table 1. The parameters and uncertainties/errors of the PV-PCM-TE hybrid system

502 503 504 505 506 507

Table 2. The efficiencies of the PV-PCM-TE hybrid system with different conditions Table 3. The cost analysis of the PV-PCM-TE hybrid system and the pure PV system

508 509 510

17

ACCEPTED MANUSCRIPT 512 513 514

515 516 517 518 519 520 521

Fig.1 The efficiency curve of a PV-TE system corresponding to different working temperatures

18

ACCEPTED MANUSCRIPT 523 524 525

526 527 528 529 530

Fig.2 the schematics of the structure and the energy flow of the PV-PCM-TE hybrid system

531 532 533 534

19

ACCEPTED MANUSCRIPT 536 537 538

539 540 541 542 543 544 545 546 547 548

Fig.3 The photographs of the experimental PV-PCM-TE device (a) the photograph of the PVPCM-TE hybrid system and pure PV system (b) the photograph of a pure PV system (c) the photograph of the upside of PV-PCM-TE system (d) the photograph of the downside of PV-PCMTE system with water cooled heat sink (e) the photograph of the downside of PV-PCM-TE system with air cooled heat sink

549

20

ACCEPTED MANUSCRIPT

551

552

553 554 555 556 557

Fig. 4 The temperature of the PV-PCM-TE system in a day with (a) water cooling and CR=529 (b) water cooling and CR=1156 (c) air cooling and CR=1156

21

ACCEPTED MANUSCRIPT 559 560

561

562 563 564 565 566 567

Fig. 5 (a) The I-V curves of PV cells at the temperature of 71℃ (b) The I-V curves and output power of TE generator with TTE-hot=60℃, TTE-cold=24℃

22

ACCEPTED MANUSCRIPT

569

570

571 572 573 574 575

Fig.6 The electric output power of the PV-PCM-TE hybrid system and pure PV system with (a) water cooling and CR=529 (b) water cooling and CR=1156 (c) air cooling and CR=1156

23

ACCEPTED MANUSCRIPT 577 578

579 580 581 582 583 584

Fig.7 The total electric output energies and the required pump energies of the PV-PCM-TE hybrid system and the pure PV system with different condition

585

24

ACCEPTED MANUSCRIPT

587

588 589 590 591

Fig.8 The thermal output power of the PV-PCM-TE hybrid system and pure PV system with (a) water cooling and CR=529 (b) water cooling and CR=1156

592 593

25

ACCEPTED MANUSCRIPT 595 596 597

Table 1. The parameters and uncertainties/errors of the PV-PCM-TE hybrid system Parameters The size of the Fresnel lens (CR=529) The size of the Fresnel lens (CR=1156) The concentration efficiency (CR=529) The concentration efficiency (CR=1156) The size of the PV cell The size of the PCM container(CR=529) The size of the PCM container(CR=1156) The mass of the paraffin PCM(CR=529) The mass of the paraffin PCM(CR=1156) Melting temperature of PCM Latent heat of PCM Thermal conductivity of PCM Specific heat capacity of PCM The size of the Bi2Te3 TE generator The Seebeck coefficient of Bi2Te material The electrical conductivity of Bi2Te3 material The thermal conductivity of Bi2Te3 material The size of water cooled heat sink The pressure drop of the heat sink The temperature of inlet water The volume velocity of water The specific heat capacity of water The superficial area of the air cooled heat sink The efficiency of fins Pressure force Thermal conductivity of thermal conductive adhesive The root mean square roughness of surfaces Temperature Voltage Current Output Power Solar irradiance

Value 230mm*230mm*5mm 340mm*340mm*5mm 82.6% 81.3% 10mm*10mm*0.2mm 230mm*230mm*10mm 340mm*340mm*10mm 355g 800g 60℃ 182 kJ/kg 0.2 W/mK 2 kJ/kgK 40mm*40mm*4mm 1.15*10-4 V/K 2.52*105 S/m 1.39 W/mK 40mm*40mm*10mm 0.01MPa 20℃ 0.3 L/min 4.2 kJ/kgK 0.02m2 70% 100 N

Uncertainty/error ±0.2mm ±0.2mm ±2% ±2% ±0.1mm ±0.1mm ±0.1mm ±5g ±5g ±3℃

2 W/mK

±0.2 W/mK

1 μm

±0.1μm ±0.01℃ 0.1% 0.1% 0.14% 0.5%

598

26

±0.2mm

±0.2mm ±0.001MPa ±0.5℃ ±0.01L/min ±0.002 m2 ±4% ±5N

ACCEPTED MANUSCRIPT 600

Table 2. The efficiencies of the PV-PCM-TE hybrid system with different conditions Electric/Thermal efficiency PV cells TE generators Pumps Thermal efficiency (Hotwater)

CR=529, water cooling 16.70% 1.27% -0.18%

CR=1156, water cooling 12.74% 0.91% -0.20%

35.5%

28.5%

601 602 603 604

27

CR=1156, air cooling 12.54% 0.58%

ACCEPTED MANUSCRIPT 606 607 608

Table 3. The cost analysis of the PV-PCM-TE hybrid system and the pure PV system Items Fresnel lens second prismatic glass concentrator Single-junction GaAs PV cell The container of PCM PCM TE generator Water cooling system(including pump, heat sink, and etc) Sunlight tracer and packaging Total

609

28

Cost (PV-PCM-TE) 25 $/m2 30 $/m2 150 $/m2 20 $/m2 35 $/m2 300 $/m2

Cost (pure PV) 25 $/m2 30 $/m2 150 $/m2 -

10 $/m2

10 $/m2

200 $/m2

200 $/m2

770 $/m2

415 $/m2