Effect of high irradiation and cooling on power, energy and performance of a PVT system

Accepted Manuscript Effect of high irradiation and cooling on power, energy and performance of a pvt system

R. Nasrin, M. Hasanuzzaman, N.A. Rahim PII:

S0960-1481(17)30965-5

DOI:

10.1016/j.renene.2017.10.004

Reference:

RENE 9292

To appear in:

Renewable Energy

Received Date:

01 June 2017

Revised Date:

25 September 2017

Accepted Date:

01 October 2017

Please cite this article as: R. Nasrin, M. Hasanuzzaman, N.A. Rahim, Effect of high irradiation and cooling on power, energy and performance of a pvt system, Renewable Energy (2017), doi: 10.1016 /j.renene.2017.10.004

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ACCEPTED MANUSCRIPT

Highlights: 1. Concentrating solar irradiation has been extrapolated from cell to module level. 2. A new correlation is developed for cell temperature calculation. 3. Every 100 W/m2 increase in irradiation raises electrical power by 19.65 W. 4. Optimum cooling rate suitable for radiations as high as 5000 W/m2 is 180 L/h. 5. Every 10 L/h increase in coolant flow rate lower cell temperature by 1.1oC.

ACCEPTED MANUSCRIPT 1 2

EFFECT OF HIGH IRRADIATION AND COOLING ON POWER, ENERGY AND PERFORMANCE OF A PVT SYSTEM

3

R. Nasrin,a,b, M. Hasanuzzamana, N.A. Rahima,c aUM

4 5 6 7 8 9 10 11

Power Energy Dedicated Advanced Centre (UMPEDAC), Level 4, Wisma R&D, University of Malaya, 59990 Kuala Lumpur, Malaysia bDepartment of Mathematics, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh c Renewable Energy Research Group, King Abdulaziz University, Jeddah 21589, Saudi Arabia Abstract

12

Irradiation level is the key factor of photovoltaic power generation. Photovoltaic/thermal

13

systems are more effective at concentrating power in areas of high irradiation as compared to

14

traditional PV systems. High irradiation maintains the cell temperature and maximizes

15

electrical-thermal energy. An optimum cooling system is required to remove the extra heat

16

from a PVT system, leading to enhancement of overall performance. In this research, the

17

effect of different high irradiation levels and cooling fluid flow rate are investigated in terms

18

of cell temperature, outlet temperature, electrical-thermal energy and overall performance of

19

PVT system. Finite element based software COMSOL Multiphysics has been used to solve

20

the problem numerically in three-dimensional model. The numerical model has been

21

validated with available experimental and numerical results. It is found that overall efficiency

22

increases with increasing fluid flow rate and with an optimum cooling fluid flow rate of about

23

180 L/h. Electrical and thermal energy increase from 197 to 983 W and 1165 to 5387 W

24

respectively, for increasing irradiation from 1000 to 5000 W/m2 with an optimized flow rate

25

of 180 L/h. Electrical, thermal and overall efficiency are found to be about 10.6, 71 and

26

81.6% respectively, at the highest irradiation level of 5000 W/m2.

27

Keywords: PVT system; High irradiation; Cooling; Power; Energy; Performance.

28

Nomenclature

29

A

Area of PVT surface (m2)

30

Al

Area of each lens (m2)

31

Asc

Area of each solar cell (m2)

32

Cp

Specific heat at constant pressure (Jkg-1K-1)

33

Cr

Concentration ratio



Corresponding author. Email address: [email protected] (Dr. Rehena Nasrin)

1

ACCEPTED MANUSCRIPT 34

Ep

Electrical power (W)

35

Er

Received energy by PV (W)

36

Et

Thermal energy (W)

37

G

Solar radiation (Wm-2)

38

k

Thermal conductivity (Wm-1K-1)

39

m

Mass flow rate (kgs-1)

40

PV

Photovoltaic

41

PVT

Photovoltaic thermal

42

Psc

Packing factor of module

43

Tamb

Temperature of ambient (°C)

44

Tf

Temperature of water (°C)

45

Tg

Temperature of glass (°C)

46

The

Temperature of heat exchanger (°C)

47

Tin

Temperature of input water (°C)

48

Tout

Temperature of output water (°C)

49

Tr

Reference temperature (°C)

50

Tsc

Temperature of solar cell (°C)

51

Ttd

Temperature of tedlar (°C)

52

Uhea

Heat transfer coefficient from heat exchanger to ambient (Wm-2K-1)

53

Uhe

Heat transfer coefficient from heat exchanger to water (Wm-2K-1)

54

Uga

Heat transfer coefficient from glass to ambient (Wm-2K-1)

55

Ut

Heat transfer coefficient inside PV layers (Wm-2K-1)

56

Utd

Heat transfer coefficient from tedlar to heat exchanger (Wm-2K-1)

57

Vin

Input velocity of water (ms-1)

58

Greek Symbols

59

αg

Absorptivity of glass

60

αsc

Absorptivity of PV

61

αtd

Absorptivity of tedlar

62

τg

Glass transmitivity

63

εg

Glass emissivity

64

ρ

Density (kgm-3)

65

μsc

PV temperature coefficient (%/°C)

66

ηsc

Reference efficiency

67

ηe

Electrical efficiency of PV 2

ACCEPTED MANUSCRIPT 68

ηt

Thermal efficiency of PVT

69

ηo

Overall efficiency of PVT

70

1. Introduction

71

Solar energy is one of the most promising and abundant of renewable energy sources . It is of

72

great importance to develop reliable, cost-effective, and environmentally-friendly sources of

73

solar energy. Energy conversion through thermal receivers, photovoltaic (PV), and

74

photovoltaic-thermal (PVT) systems are most commonly used for solar systems. In a PVT

75

system, generally air or water is used to collect heat through a heat exchanger, which is

76

attached to the bottom tedlar surface of the PVT module [1-5]. Solar thermal collectors along

77

with PV modules combine to form the PVT system [6-10]. Numerical and experimental

78

investigations [11-13] have been conducted with various designs of PVT. In order to enhance

79

overall efficiency of PVT systems, [14-18] more focus should be given to operational factors

80

and the system design of PVT. The performance of different types of PV cell technologies

81

can vary due to different operating and weather conditions [19-22]. The PV operating

82

temperature has a significant effect on PV performance [23-26]. This performance is

83

enhanced due to reducing cell temperature. The solar spectrum was not considered to vary in

84

a relevant way in these studies.

85

The temperature of a solar cell enhances the unused energy in thegap area between cells

86

when a PV module is covered by glass at the top and tedlar at the bottom. The amount of heat

87

transfer from tedlar to ambient becomes lower for opaque PV modules than semitransparent

88

PV modules. Temperature plays an important role in PV performance and increasing

89

temperature decreases PV power by upto 7% [27-30]. The electrical power of a PV module

90

decreases with increasing temperature, while voltage reduces greatly. The impact of solar

91

radiation on the PV temperature needs to be investigated in greater detail. This investigation

92

has been performed based on the PV design with a converging lens and solar concentrator, as

93

shown in research by Avireni [31], Kerzmann and Schaefer [32], El –Sayed et al. [33] and

94

Xie et al. [34].

95

Experimental investigation of the PVT system [35] was conducted in Greece by applying a

96

dual heat extraction operation (either with water or with air circulation) for system

97

performance improvement. Numerical modelling and simulation of a hybrid PVT solar

98

energy system

99

phenomena at different locations such as Cyprus and the UK. A highly transparent low e-

[36-37] have been performed for both laminar and transient fluid flow

3

ACCEPTED MANUSCRIPT 100

coating based on silver, specifically optimized for the application in PVT collectors, was

101

developed by Laemmle et al. [38] based on experiment as well as simulation. A computer

102

simulation was performed to analyze the PVT system, facade-integrated photovoltaic/thermal

103

(BiPV/T) technology, with performance based on a dynamic model, with steady as well as

104

unsteady cases [39-40]. Based on experimental data and validated numerical models, a study

105

of the appropriateness of the glass cover on a thermosyphon-based water-heating PVT system

106

was carried out by Chow et al. [41].

107

It is found from the literature that PVT technology is a very promising field of solar energy

108

based power generation and energy conversion. Limited research has been conducted to find

109

better use of electric power generation and thermal energy from PVT modules, as well as

110

cooling systems[35-41]. However, there is a lot of scope in optimizing the outcomes of PVT

111

systems using high irradiation. In this research, a new model is developed to introduce a lens

112

(concentrator) panel on the top of an existing solar panel in order to get high irradiation. In

113

addition, a new design is implemented inside the heat exchanger using baffles to extract more

114

heat from the PVT system. The aims of the present research are to find out the effect of high

115

irradiation on PV power generation, to optimize the flow rate of cooling fluid, thermal

116

energy conversion and analysis of the overall performance of the PVT system operating

117

under high irradiation due to a lens concentrator.

118

2. Methodology

119

2.1 Experimental investigation

120 121 122 123 124 125 126 127 128 129 130 131 Figure 1: View of experimental set up where (1) Monitor, (2) Data taker, (3) MPPT, (4) Radiator, (5) Water tank, (6) Pump, (7)4 Flow meter, (8) Pyranometer, (9) Water inlet, (10) Water outlet and (11) PV module

ACCEPTED MANUSCRIPT 132

There exists an experimental setup of a SY-90M monocrystalline module in the Solar Park at

133

Level 3, Wisma R&D, UMPEDAC, University of Malaya, Malaysia. The specifications of

134

this PV module, outdoor experimental setup, instruments, test conditions, data acquisition

135

system are mentioned in Rahman et al. [2] in details. The experimental setup has been shown

136

in Figure 1, with data taken in February, 2017. The solar cell temperature was measured for

137

irradiation levels of 300 to 1000 W/m2, with a fixed cooling flow rate of 180 L/h and an

138

average inlet fluid temperature of 30°C. The obtained solar cell temperature for different

139

irradiation levels is shown in Figure 2.

140 141 142 143 144 145 146 147 148 149 150 151 152

Figure 2: Cell temperature against irradiation

153

2.2 Model development

154

The idea of using concentrator lenses is to generate electrical power more effectively using

155

the same size of solar cells [10]. In the present research, the concept of concentrating

156

irradiation has been extrapolated from the solar cell level to the PV module level. Figure 3(a-

157

c) is a schematic diagram of the PVT concentrator system in: a magnified view, full-view and

158

cross-sectional view, respectively. A lens panel (Fresnel lenses) has been implemented to

159

concentrate sunlight to every cell of the PV module. At the same time, the system usually has

160

a second optical element, namely glass, which is attached to the cell. Its mission, amongst

161

others, is to increase the numerical aperture of the set whilst at the same time ensuring a

162

uniform distribution of the energy from the cell, preventing the generation of hot spots on it.

163

Thermal paste has been used as a heat exchanger, attached to the back side of the PV module.

164

The traditional heat collection systems attached under PV cells are circular/square shaped 5

ACCEPTED MANUSCRIPT 165

parallel pipes [2] or empty box-shaped heat exchangers [42]. In traditional cases, cooling

166

fluid cannot take a high level of heat from tedlar due to it having less contact with the surface

167

of tedlar. In order to get more contact surface with cooling fluid and maximum thermal

168

energy, a new design has been developed by using sixteen baffles inside the heat exchanger.

169

In this case, water is considered as a cooling fluid. The baffle design and fluid flow path are

170

shown clearly in Figure 3(c). Heat exchangers, input-output pipes, and baffles are made of

171

aluminium metal. The four sides of the PVT module are bound by an aluminium frame and

172

the cooling system has two domains: an inlet-outlet ports-baffles of solid domain and a fluid

173

domain. In order to ensure maximum irradiation incident on the module surface, sun tracking

174

is necessary. However, in numerical simulation with COMSOL Multiphysics, the normal

175

incidence of irradiation is already given by default software settings. A large PV module of

176

72 cell-polycrystalline-silicon (with each cell area being 0.024 m2) is considered in this

177

investigation. The mean solar radiation is 1000 W/m2 in Malaysia. To get solar irradiation up

178

to 5000 W/m2, the area of each lens is 5*0.024 m2, bringing the total area of 72 lenses to

179

72*0.122 m2. The solar cell total area (1.75 m2) is considered the computational domain for

180

numerical simulation. The physical properties of different layers of the PVT module are

181

displayed in Table 1.

182

Table 1: The dimensions and properties of the PVT layers [2, 10, 42] PVT components Glass EVA Polycrystalline cell Tedlar Thermal paste Heat exchanger Inlet-outlet pipes Baffles Fluid

ρ (kg/m3) 2450 950 2329 1200 2600 237 237 237 998

Dimension (m) 2×1×0.003 2×1×0.0008 1.75×1×0.0001 2×1×0.00005 1.75×1×0.0003 1.75×1×0.012 0.05×0.01×0.01 0.978×0.001×0.01 1.75×1×0.01 (fluid region)

cp (J/kgK) 500 2090 700 1250 700 900 900 900 4200

k (W/mK) 2 0.311 148 0.15 1.9 2700 2700 2700 0.68

183

2.3 Numerical investigation

184

Considering all the outdoor experimental data (Figure 2) of a PVT system, a new correlation

185

is developed between Tsc and G:

186

where the coefficient of correlation is r2 = 97.32%. The cell temperature is calculated by

187

using the equation [2, 10]: Tsc 

Tsc = 2.9692*(G)0.3948 Psc G  g sc   sc   (U gaTamb  U tTtd )

U 6

ga

 Ut 

(1)

(2)

ACCEPTED MANUSCRIPT Sun

188 189 Converging lens

190 191 192

Irradiation (a)

193 Solar cell

194 195 196 197 198 199 200 201 202

(b)

203 204 205 206 207 208 209 210 211 212

Glass

213Aluminium 214 frame 215

EVA Solar Cell EVA

216

Thermal paste

(c)

Tedlar

217

Outflow

218 Inflow 219 220 221

Heat exchanger

Baffle container Figure 3: Schematic diagram of (a) Fresnel lens with cell, (b) lens panel with solar panel and (c) PVT system (cross sectional view upto 7 PV layers and top view in fluid flow layer)

ACCEPTED MANUSCRIPT 222

The amount of received energy by the PV cell is found from the following equation [10] Er   g sc Psc GA

223

(3)

224

A relation among concentration ratio, lens area and solar cell area [10, 33] is: Cr 

225

Table 2 represents the PVT properties.

226

Al Asc

(4)

Table 2: Properties of PVT system [2, 10, 42] PVT properties Glass emissivity Transmissivity of glass Absorptivity of PV Absorptivity of tedlar PV reference efficiency PV temperature coefficient Ambient temperature Input temperature Reference temperature Packing factor Each cell area PV area Number of cells Heat transfer coefficient inside PV layers Heat transfer coefficient from tedlar to heat exchanger Heat transfer coefficient from heat exchanger to ambient Heat transfer coefficient from heat exchanger to water

Value 0.04 0.96 0.9 0.5 0.13 -0.0045 32 30 25 20% 0.156*0.156 8.76 6*12 150 77 5.84 66

227

Networks with different hidden layers have been used for 3D numerical modeling and

228

performances have been evaluated. Wind velocity is not considered and all other assumptions

229

for this numerical simulation are mentioned in [10]. The heat transfer equation for PV layers

230

[43] and laminar flow equations for the fluid domain [9-10] of a PVT system are given

231

below:

232

For the glass

233

 k   cp 

234

For the cell

235

 k   cp 

236

For the tedlar

   2Tg  2Tg  2Tg   2  2  2 y z  g  x

 4 4    g G  U ga Tg  Tamb    g Tg  Ts  U t Tg  Tsc  





   2Tsc  2Tsc  2Tsc     2     sc g G  Ee  U t Tsc  Ttd   U t Tsc  Tg  y 2 z 2   sc  x

8

(5)

(6)

ACCEPTED MANUSCRIPT

237

 k   cp 

   2Ttd  2Ttd  2Ttd    2  2  x  y z 2  td

238

For the heat exchanger

239

 k   cp 

240

For the fluid domain

241

u v w   0 x y z

242

u u   u p  f u j  v j  w j     f y z  x  x

243

  f c pf   u

244

where j = 1, 2, 3 in the case of u, v, w components of the velocity vector, σ

245

1.5 = 5.670367×10−8 Wm−2K−4 (Stefan-Boltzmann value), Ts  0.0552Tamb is the sky temperature.

246

Also ρf, kf, cpf and  f are the density, thermal conductivity, specific heat at constant pressure

247

and dynamic viscosity of fluid, respectively. The boundary conditions of the numerical model

248

are as follows [10]:

   2The  2The  2The    2  2  x  y z 2   he

   U t Tsc  Ttd   U td Ttd  The  

(7)

   U td Ttd  The   U he The  T f   U hea The  Tamb  

(8)

T f T f   T f v w   kf y z   x

(9)

  2u j  2u j  2u j  2  2  2 y z  x

  2T f  2T f  2T f  2  2  2 y z  x

249

1. for side surfaces of PVT: n .  k T   0

250

2. for solid boundaries of fluid domain: u = v = w = 0

251

 T   T  3. for fluid-solid interface: k f    khe    n  f  n  he

252

4. for inlet: T  Tin , u = 0, v = Vin, w = 0

253

5. for outlet: p = 0

  

  

(10)

(11)

254

where n is the distance along x or y or z directions, acting normal to the surface.

255

The output electrical power and thermal energy of the PVT module can be calculated using

256

the following relations [10]:

257

E p   sc psc g sc GA 1   sc Tsc  Tr  

(12)

258

and Et  mc p Tout  Tin 

(13)

9

ACCEPTED MANUSCRIPT 259

The instantaneous electrical, thermal and overall efficiencies of the PVT system can be

260

found using the following relations, respectively [10]:

261 262 263

e 

Ep Er

,

t 

E  Et Et and o  p Er Er

(14)

The instantaneous thermal efficiency can be written another way:



FR APsc G  g sc   U t Tin  Tamb   APsc G mc p Tout  Ti n 

 FR  g sc   FR U t

Tin  Tamb  G

264

where FR 

265

value of the heat removal factor remains between 0.7 and 0.9 for a water collector system.

266

Thermal efficiency linearly depends on (Tin - Tamb) if all other terms are constant for a specific

267

PVT. Actually, the overall heat transfer coefficient, Ut, is not constant. It is a function of

268

inlet fluid and ambient air temperatures, thus it is chosen as FR U t  b  c Tin  Tamb  .

269

Then, the maximum useful energy can be rewritten as:

270 271 272 273 274

APsc G  g sc   U t Tin  Tamb  

is the PVT heat removal factor. Generally, the

Qu sfl  FR APsc G  g sc   bAPsc Tin  Tamb   cAPsc Tin  Tamb 

2

Thus, the derived quadratic form of thermal efficiency is:

FR APsc G  g sc   bAPsc Tin  Tamb   cAPsc Tin  Tamb  Q   usfl  AI APsc G

T  T  T  T   FR g sc  b in amb  c in amb G

G

2

2

 a  bT * cGT *2

(15)

275

where a = FR τg αsc and T* = (Tin - Tamb)/G.

276

2.4. Numerical Modeling

277

The finite element method (FEM) with Galerkin's weighted residual technique based on

278

software COMSOL Multiphysics is used for solving the 3D numerical model of PVT. It is a

279

finite element analysis solver and simulation software for various physics and engineering

280

applications, especially coupled phenomena, or multiphysics. This software is also used for

281

creating physics-based applications. It allows entering coupled systems of partial differential

282

equations (PDEs). The PDEs can be entered directly or using the so-called weak form. The

283

finite element technique [44] is applied in order to solve the system of Equations (5)-(11) for

284

the present simulation. The numerical technique is described in detail in Nasrin and Alim 10

ACCEPTED MANUSCRIPT 285

[45]. Tecplot and Microsoft-Excel are software used to plot the graphs by exporting

286

simulated data from COMSOL Multiphysics.

287

2.4.1 Meshing and grid test

288

The finite element meshing of the computational domain of a PVT module is displayed in

289

Figure 4. In the present numerical model, the subdomain and boundary elements have been

290

chosen as free tetrahedral and free triangular forms, respectively. At irradiation level of 1000

291

W/m2 and a flow rate of 180 L/h have been chosen, and a grid test has been conducted for the

292

PVT model. Different types of non-uniform grid systems are checked with elements:

293

2,68,882; 4,28,585; 8,13,304; 14,34,582; and 32,11,718. Supervising parameters are chosen

294

as cell temperature and outlet fluid temperature. It was noticed that there was no considerable

295

change in the value of cell and outlet temperatures between normal and fine meshing but time

296

intolerable. Thus, the PVT model with 14,34,582 domain elements is considered for

297

numerical analysis. The grid test result is depicted in Table 3.

298

Table 3: Grid test at irradiation 1000 W/m2 and volume flow rate 180 L/h Meshing type

Extra Coarse

Coarser

Coarse

Normal

Fine

No. of elements

2,68,882

4,28,585

8,13,304

14,34,582

32,11,718

Tsc (°C)

41.8535

41.3248

42.8751

43.0112

43.0118

Tout (°C)

34.0214

34.5321

35.0634

35.4521

35.4526

Time (s)

719

895

1081

1225

1831

299 300 301 302 303 304 305

Figure 4: Finite element meshing of PVT model

306

2.4.2. Simulated steady-state condition

307

The solar radiance, ambient temperature, wind velocity, etc. can be varied any time and

308

depend on the operating weather conditions. Thus, PVT operation is inherently dynamic. The 11

ACCEPTED MANUSCRIPT 309

steady-state condition is required in numerical simulation for producing accurate results and

310

this is explained in more detail in Nasrin et al. [10]. For this purpose, the numerical

311

simulation has been performed at fixed a inlet temperature of 30°C, with an irradiation level

312

of 1000 W/m2 and a flow rate of 180 L/h. Figure 5 expresses the values of cell and output

313

temperatures against operating time. The steady-state condition was reached after 1200 s.

314 315 316 317 318 319 320 321 322 323 324

Figure 5: Steady-state condition in 3D model simulation

325

2.4.3. Code validation

326

The validity of present 3D simulation has been conducted with experimental as well as

327

numerical results which are as follows:

328

2.4.3.1 Validity test with experimental findings

329

The values of the solar cell temperature at an irradiation level of 1000 W/m2 with various

330

flow rates (30, 60, 90 and 180 L/h) was obtained from the present numerical code, validated

331

with that of Rahman et al. [2]. The authors [2] used an SY-90M PV (1200 * 545 * 35 mm)

332

module of six layers, with 4*9 monocrystalline cells, a rectangular heat exchanger (950 * 420

333

mm) composed of seven copper tubes (each having a 22 mm diameter) and with an inlet

334

temperature of 35°C. Table 4 expresses this validation and represents a very good agreement

335

with experimental results [2].

336

2.4.3.2 Validity test with numerical findings

337

The values of surface temperature of the PVT system with an inlet fluid temperature of 34°C,

338

solar radiation of 1000 W/m2, and an inlet velocity of 0.0007 m/s has been obtained using the

339

present numerical code, validated with that of Nahar et al. [42]. Figure 6 displays the

12

ACCEPTED MANUSCRIPT 340

validation and a good match of results is found. This code validation is described in detail in

341

Nasrin et al. [10].

342

Table 4: Code validation of cell temperature against flow rate Mass flow rate (L/h) 30 60

Cell temperature (°C) Present research Rahman et al. [2] 51.11 52.88 48.04 50.23

Percentage of error 3.3% 4.3%

90

47.70

49.65

3.9%

180

45.76

47.76

4.2%

343 344

126

345 346 347 348 349 350 351

Present code Nahar et al. [42] Figure 6: Model validation of surface temperature of PVT system

352 353

3. Results and Discussion

354

The present research is conducted to analyze the effects of high irradiation and volume flow

355

rate on PVT performance. The variety of solar radiation and volume flow rate of inlet fluid

356

has been chosen as 1000 to 5000 W/m2 and from 30 to 210 L/h, respectively. The outcomes

357

of the different cases are presented in the following sections.

358

3.1 Effect of irradiation

359

Figure 7 depicts the solar cell surface temperature along the module length at the middle

360

section (width 500 mm) with varying levels of solar irradiance (1000, 2000, 3000, 4000 and

361

5000 W/m2). In this figure, the cooling system’s flow rate was 180 L/h. It is observed from

362

this figure that the solar cell temperature of the PVT module increases along the PV length at

363

the initial solar irradiation level of 1000 W/m2. The variation of the cell temperature of the

364

PV module becomes higher for increasing values of irradiation. Equation (2) shows that solar

365

cell temperature is linearly proportional to solar radiation. This is justified by the heat

366

transfer procedure of the PVT system. 13

ACCEPTED MANUSCRIPT 367 368 369 370 371 372 373 374 375

Figure 7: Cell temperature along module length for different irradiation

376 377

The streamlines have been plotted for the PVT system at different values of irradiation from

378

1000 to 5000 W/m2 with a flow rate of 180 L/h, as shown in Figure 8. The average

379

temperature of the water at the exit port becomes higher with increasing values of irradiation.

380

This is due to the fact that increasing solar radiation increases the surface temperature of the

381

PV module, resulting in more heat being produced by the solar cell. Consequently, more heat

382

is absorbed by the tedlar surface and the heat transfer system from the solar cell to the

383

cooling fluid through the heat exchanger is enhanced. As a result, the outlet fluid’s mean

384

temperature becomes higher.

385

3.2 Effect of flow rate

386

Figure 9 depicts the effect of flow rate from 30 to 210 L/h on the surface temperature plot for

387

the PVT system at an irradiation of 1000 W/m2. The minimum and maximum temperatures

388

for each surface temperature plot are represented by blue and red colors, respectively. This

389

figure shows that the rising volume flow rate accelerates thermal current activities through

390

the heat exchanger surface to the fluid flow domain. With increasing flow rate, the color of

391

the surface near the outlet port of the PVT module becomes lighter, whereas initially this

392

color becomes deeper. With the variation of flow rate from 30 to 210 L/h, the temperature

393

distribution becomes distorted, resulting in an increase in the overall heat transfer. It is found

394

that increasing the value of the inlet fluid flow rate results in the maximum temperature of the

395

PVT material dropping gradually. At the lowest flow rate (30 L/h), increasing temperature is

396

the maximum and at the highest flow rate (210 L/h), it is minimized. 14

ACCEPTED MANUSCRIPT 397 398 399

5000 W/m2

400 401 402 403 404 405 406 407

4000 W/m2

408 409 410 411 412 413 414

3000 W/m2

415 416 417 418 419 420 421

2000 W/m2

422 423 424 425 426 427 428 429

1000 W/m2

430 15various irradiation at flow rate 180 L/h Figure 8: Streamlines of water for

ACCEPTED MANUSCRIPT 431 432 433 434

210 L/h

435 436 437 438 439 440 441

180 L/h

442 443 444 445 446 447 448 449

90 L/h

450 451 452 453 454 455 456

60 L/h

457 458 459 460 461 462 463

30 L/h

464 16 Figure 9: PVT surface temperature for various flow rate at irradiation 1000 W/m2

ACCEPTED MANUSCRIPT 465 466 467 468

210 L/h

469 470 471 472 473 474 475

180 L/h

476 477 478 479 480 481 482

90 L/h

483 484 485 486 487 488 489

60 L/h

490 491 492 493 494 495 496 497

30 L/h

498 17

Figure 10: Streamlines of water for various flow rate at irradiation 1000 W/m2

ACCEPTED MANUSCRIPT 499

This illustrates the dominating behavior of fluid flow properties. Thus, conductive as well as

500

convective heat transfer processes run from the top glass surface to the outlet exit through the

501

heat exchanger of the PVT module.

502

The streamlines of water for different values of flow rate (30, 60, 90, 180 and 210 L/h) are

503

displayed in Figure 10. For this simulation, solar radiation is chosen as 1000 W/m2. Figure 10

504

shows the fluid flow distribution throughout the heat exchanger from inlet to outlet. When the

505

volume flow rate is low, the temperature of streamlines at the exit port becomes high. The

506

outlet fluid temperature reduces with increasing values of the flow rate. This result is

507

significant because rapidly flowing fluid is not capable of taking in more heat from the heat

508

exchanger. So, the volume flow rate of the inlet fluid is very important in the cooling system

509

of the PVT module. The pumping power for the volume flow rate at 210 L/h is higher than

510

other flow rates.

511

3.3 Solar cell temperature

512

Figure 11(a)-(b) depicts the solar cell temperature against various solar radiation levels and

513

fluid flow rates, respectively. Increasing the rate of cell temperature by 0.81% increases

514

irradiation from 1000 to 5000 W/m2 in the PVT system. Teo et al. [1], Rahman et al. [2],

515

Nasrin et al. [10] and Chandrasekar et al. [46] found a 1.4, 2.71, 1.85 and 1.4°C increase in

516

cell temperature for every 100 W/m2 increase in the irradiation level with the cooling system.

517

The operating irradiation level of Teo et al. [1], Rahman et al. [2], Nasrin et al. [10] and

518

Chandrasekar et al. [46] were 550 to 1050 W/m2, 240 to 979 W/m2, 1000 to 3000 W/m2 and

519

600 to 1300 W/m2, respectively. The present result shows that the cell temperature increases

520

about 0.9°C for each increase of irradiation 100 W/m2. The aim of this research is to maintain

521

the cell temperature under a certain range using the proper cooling system so that the PV

522

material may not be degraded. This result is better than that of Teo et al. [1], Rahman et al.

523

[2], Nasrin et al. [10] and Chandrasekar et al. [46]. This implies that the proper cooling

524

system is maintained in this PVT system.

525

Figure 11(b) shows that the average cell temperature, Tsc decreases with increasing inlet fluid

526

volume flow rate from 30 to 210 L/h at the irradiation level of 1000 W/m2, and with an inlet

527

fluid temperature of 30°C. As the inlet fluid flow rate increases, more heat is removed from

528

the module by convection which reduces the average cell temperature. It is found from this

529

figure that the cell mean temperature decreases rapidly with increasing flow rate. Initially, the

530

flow rate is 30 L/h, with a solar cell temperature of 67°C. The cell temperature decreases 18

ACCEPTED MANUSCRIPT 531

gradually as the flow rate increase up to 180 L/h. After that, at a flow rate of 210 L/h, it

532

reduces a little bit to 42.6°C, but the pumping power is higher. Thus, for this PVT system, the

533

optimum volume flow rate of cooling fluid is 180 L/h. The solar cell temperature decreases

534

by 1.1°C for each 10 L/h increment of volume flow rate of inlet fluid.

535 536 537 538 539 540 541 542 543 544 545 546 547

(b) (a) Figure 11: Solar cell temperature with the variation of (a) irradiation at flow rate 180 L/h and (b) flow rate at irradiation 1000 W/m2

548

Equation (1) shows the correlation of cell temperature and irradiation from the experimental

549

result for low irradiation (300 - 1000 W/m2). Another correlation was developed from the

550

result of the numerical simulation for high irradiation (1000 - 5000 W/m2) at the fixed

551

cooling fluid flow rate of 180 L/h. This is written as:

552

Tsc = 3.4913*(G)0.3641

(16)

553

where the coefficient of correlation is r2 = 99.02%. Equation (16) has a similar pattern as

554

Equation (1). This demonstrates numerical validity. The percentage of error between these

555

two correlations for the PV cell temperature at an irradiation of 1000 W/m2 is less than 5%.

556

3.4 Electrical power

557

The output power for the variation of irradiation (1000 - 5000 W/m2) and flow rate (30 - 210

558

L/h) is expressed in Figure 12(a)-(b). It is seen from Figure 12(a) that the initial irradiation

559

level is 1000 W/m2, the output power is 197 W, the irradiation level is 5000 W/m2 and the

560

electrical power is 983 W. For every 100 W/m2 increment in irradiation, there is an increase

561

of 19.65 W in electrical power of the PVT system at a flow rate of 180 L/h. Rahman et al.

562

[2], Nasrin et al. [10] and Nahar et al. [42] showed that the output power increased about 19

ACCEPTED MANUSCRIPT 563

3.88, 6.4 and 10 W, respectively, for every 100 W/m2 increase in irradiation level. So, the

564

present numerical result is consistent with these authors [42] but differs a little bit from

565

Rahman et al. [2] and Nasrin et al. [10]. This occurs because of the size of the PV module,

566

the packing factor, the reference temperature, operating conditions, poor installation of the

567

cooling system and the properties of materials used.

568

At a fixed irradiation level of 1000 W/m2, under different volume flow rates of fluid, the

569

initial flow rate is 30 L/h and the electrical power is 195 W. During the peak flow rate of 180

570

L/h, the output power becomes 197 W. But there is no significant increase in output power

571

(197.09 W) for further increases of water flow rate upto 210 L/h. The output power increased

572

by 2.09 W with an approximately 180 L/h increase in the volume flow rate of the fluid.

573

Under cooling conditions, the output power increased by 0.12 W as for each 10 L/h

574

increment of fluid flow rate.

575 576 577 578 579 580 581 582 583 584 585 586 587

(a)

(b)

Figure 12: Electrical power versus (a) irradiation at flow rate 180 L/h and (b) flow rate at irradiation 1000 W/m2

588

3.5 Electrical efficiency

589

The electrical efficiency of the PVT system as a function of the solar irradiance and volume

590

flow rate varies from 1000 to 5000 W/m2 and 30 to 180 L/h, respectively, as shown in Figure

591

13(a)-(b). The PV efficiency decreases with rising irradiation level at the flow rate of 180

592

L/h. The electrical efficiency devalues from 13 to 10.6% due to increasing solar radiation. So,

593

electrical efficiency decreases approximately 0.06% for each 100 W/m2 increment of

594

irradiation. Rahman et al. [2], Nasrin et al. [10] and Nahar et al. [42] showed that PV

20

ACCEPTED MANUSCRIPT 595

efficiency decreased about 0.87%, 0.09% and 0.16% for every 100 W/m2 increase of

596

irradiance.

597

Due to increasing inlet volume flow rate (30 - 210 L/h) of water, the cell average temperature

598

(Figure 11(b)) is reduced. Consequently, the PVT module’s current drops marginally with a

599

noticeable increase in PVT voltage which, in turn, increases the output power and electrical

600

efficiency. The maximum electrical efficiency was found to be around 13.05%. There wa no

601

noticeable increment observed for increasing the flow rate from 180 to 210 L/h. So, for the

602

cooling system of the PVT module, an inlet fluid flow rate not more than 180 L/h is

603

advantageous. Electrical efficiency increases by approximately 0.03% for each increase of

604

fluid flow rate by 10 L/h.

605 606 607 608 609 610 611 612 613 614 615 616

(b) (a) Figure 13: Electrical efficiency versus (a) irradiation at flow rate 180 L/h and (b) flow rate at irradiation 1000 W/m2

617 618

3.6 Outlet temperature of cooling fluid

619

Figure 14(a)-(b) shows the effect of G (1000 - 5000 W/m2) and flow rate (30 - 210 L/h) on

620

mean temperature of the outlet cooling fluid. Rising solar radiance enhances outlet

621

temperature. At the fixed volume flow rate of 180 L/h and irradiation of 1000 W/m2, the

622

outlet fluid temperature was found to be approximately 35.45oC. This outlet temperature

623

became 55.2oC at the highest irradiation level of 5000 W/m2. So, the temperature of the outlet

624

fluid was enhanced by 19.75oC for each 4000 W/m2 increment of irradiation. The outlet

625

temperature increases about 0.5oC for each 100 W/m2 increase of solar radiance.

626

The water outlet temperature also decreases with increasing inlet flow rate at G = 1000 W/m2,

627

as shown in Figure 14(b). This declining trend can also be attributed to the increasing 21

ACCEPTED MANUSCRIPT 628

convection heat transfer rate with increasing velocity. With the increase in flow velocity, the

629

rate of heat removal is also increased and less time is available for thermal accumulation,

630

thereby decreasing the water outlet temperature. Nahar et al. [42] represented the

631

experimental result where the graph of outlet fluid temperature against inlet velocity had a

632

similar trend, but different values were presented due to different inlet velocity and

633

conditions. The similarity in trend and proximity in magnitude between the present numerical

634

value and that of Nahar et al. [42] validates the present numerical model with quiet

635

confidence. This decreasing pattern changes significantly up to a 180 L/h flow rate of cooling

636

water. It slightly decreases as the flow rate increases and exceeds 180 L/h. Hence, 180 L/h is

637

the most suitable flow rate to enhance the efficiency of the PV module.

638 639 640 641 642 643 644 645 646 647 648 649 650

(a) (b) Figure 14: Outlet temperature against (a) irradiation at flow rate 180 L/h and (b) flow rate at irradiation 1000 W/m2

651

3.7 Thermal energy

652

The thermal energy with respect to various irradiation and volume flow rates is displayed in

653

Figure 15(a)-(b). At the flow rate 180 L/h and G = 1000 W/m2, the thermal energy was found

654

to be 1165 W. This energy became 5387 W for G = 5000 W/m2. The reason for this fact is

655

that there is conductive heat transfer inside the PVT module’s surfaces, as well as convective

656

heat transfer inside the fluid. Therefore, a high temperature gradient takes place between the

657

output and input fluids due to high irradiation. The thermal energy increases by 105.55 W for

658

every increase of irradiance by 100 W/m2. This huge amount of thermal energy can be used

659

for secondary purposes such as cooking, cleaning, bathing, drying, pain removal, space

22

ACCEPTED MANUSCRIPT 660

heating, building, ventilation etc. in households as well as in industries and agricultural

661

sectors.

662

At the irradiation level of 1000 W/m2, Figure 15(b) shows that the total amount of thermal

663

energy (1055 W) has to be collected from the system when the flow rate is 30 L/h. At the

664

flow rate of 180 L/h, thermal energy is found to be 1165 W and at the highest flow rate of

665

210 L/h, the amount of thermal energy becomes 1170 W. Only 5 W of extra thermal energy

666

is obtained for further increasing the flow rate up to 210 L/h. Actually, increasing the flow

667

rate of the cooling fluid from 30 to 60 L/h significantly affects the decreasing rate of outlet

668

fluid temperature as shown in Figure 14(b). After that, increasing the flow rate upto 180 L/h

669

decreases the outlet fluid temperature gradually. Nonetheless, at a flow rate of 210 L/h, the

670

output temperature decreases very slowly. Consequently, the extracted thermal energy from

671

the system enhances slowly when the cooling fluid’s flow rate exceeds 180 L/h. Thus, the

672

optimum flow rate of inlet fluid is 180 L/h for proper cooling of the PVT. For each 10 L/h

673

increment of volume flow rate, the extracted heat energy is 6.4 W.

674 675 676 677 678 679 680 681 682 683 684 685 686

(a) (b) Figure 15: Thermal energy against (a) irradiation at flow rate 180 L/h and (b) flow rate at irradiation 1000 W/m2

687

3.8 Thermal efficiency

688

The rate of thermal efficiency decreases for various values of irradiation from 1000 to 3000

689

W/m2 in (Figure 16(a)) because the total amount of receiving energy from the PVT system is

690

increased for escalating irradiation. The thermal efficiency of the PVT system devalues from

691

77% to 71% for increasing solar radiance. For every increase of 100 W/m2, irradiation

23

ACCEPTED MANUSCRIPT 692

thermal efficiency decreases to about 0.15%. Nasrin et al. [10] found a 0.3% decrement of

693

thermal efficiency for each 100 W/m2 increment of irradiation.

694

The thermal efficiency increase with increasing volume flow rate of the working fluid at a

695

fixed irradiation of 1000 W/m2 is shown in Figure 16(b). An increase in flow rate from 30 to

696

180 L/h enhances the convective heat transfer coefficient of the cooling fluid. So, under a

697

given temperature difference, more heat is transferred at higher velocities, thereby increasing

698

the thermal efficiency. The thermal efficiency increases from 70% to 77% for this numerical

699

simulation. The efficiency rate is high due to the variation of the fluid flow rate from 30 to

700

180 L/h. After that, at a flow rate of 210 L/h, a very small increment in thermal efficiency is

701

observed. The fact behind this trend is that increasing the volume flow rate (up to 180 L/h) of

702

cooling fluid decreases solar cell temperature gradually. The solar cell temperature slightly

703

decreases as the flow rate increases and exceeds 180 L/h. Hence, 180 L/h is the most suitable

704

flow rate to enhance the efficiency of the PVT system. About 0.43% thermal efficiency

705

increases due to each 10 L/h increase of volume flow rate of inlet water.

706 707 708 709 710 711 712 713 714 715

(b) (a) Figure 16: Thermal energy versus (a) irradiation at flow rate 180 L/h and (b) flow rate at irradiation 1000 W/m2

716

3.9 Overall efficiency

717

The PVT overall efficiency versus irradiation (1000 - 5000 W/m2) and flow rate (30 - 210

718

L/h) is expressed in Figure 17(a)-(b). It decreases from 90% to 81.6% due to rising irradiance

719

at the flow rate of 180 L/h. The overall efficiency of the PVT system devalues by 0.21% for

720

each increase of 100 W/m2 irradiation. Nasrin et al. [10] showed that the overall efficiency

721

decreased by 0.39% due to each increase of 100 W/m2 irradiation. 24

ACCEPTED MANUSCRIPT 722

As both the electrical and thermal efficiencies increase with increasing inlet fluid flow rate,

723

so the overall efficiency also increases. Accordingly, at lower flow rates, overall efficiency

724

was obtained as 82.5%. It increased significantly up to 90% with increasing flow rates up to

725

180 L/h. After that, the value of overall efficiency for the volume flow rate of 210 L/h is

726

approximately 90.85%. So, a 180 L/h flow rate is perfect for this PVT module. Overall,

727

efficiency increases about 0.46% for each 10 L/h increase of volume flow rate.

728 729 730 731 732 733 734 735 736 737 738 739 740

(a) (b) Figure 17: Overall efficiency against (a) irradiation at flow rate 180 L/h and (b) flow rate at irradiation 1000 W/m2

741

3.10 Comparison

742

3.10.1 PV temperature

743

The increasing rate of cell temperature for every 100 W/m2 increase in irradiation resulting

744

from various research such as Teo et al. [1], Rahman et al. [2], Nasrin et al. [10],

745

Chandrasekar et al. [46], Bahaidarah et al. [47] and the current work is shown in Table 5. A

746

good agreement was noticed between the present result and Teo et al. [1], Nasrin et al. [10],

747

Chandrasekar et al. [46], Bahaidarah et al. [47]. Nevertheless, the result is a little different

748

from Rahman et al. [2] because of the variation in the range of incident solar irradiation, the

749

range of the module’s operating temperature, wind velocity, ambient temperature, and

750

physical properties of each layer of the PVT module.

751

Table 5: Comparison of cell temperature increment per 100 W/m2 irradiance Investigations

G (W/m2) Starting Ending

Tsc (°C) Tsc increment per 100 Tamb W/m2 irradiance (°C) Starting Ending 25

ACCEPTED MANUSCRIPT [1] [2]

550 312

1050 995

41 31

48 50

1.4 2.71

35

[10] [46]

1000 600

3000 1300

48 40

85 50

1.85 1.4

32 37

[47]

240

979

21

35

1.9

21

Present research

1000

5000

43

78

0.9

32

752 753

3.10.2 Electrical efficiency

754

The obtained numerical results were compared for electrical efficiency with the variation of

755

flow rate with the experimental result of Rahman et al. [2]. For this comparison, input fluid

756

temperature was 33°C, ambient temperature was 35°C, and solar radiation was 996 W/m2.

757

These parameters were kept fixed. The experiment of Rahman et al. [2] was operated to find

758

the energy and efficiency of the PVT module in Malaysia. Figure 18 demonstrates the above

759

stated comparison. The percentage of error between numerical and experimental data lies

760

between 0 to 5%. This is due to the different operational factors at the time of experimental

761

investigation, such as ambient temperature, reference temperature, wind velocity, and the

762

effect of dust and other factors. In numerical investigations, the input and ambient

763

temperatures are fixed, but in outdoor experiments such as that of Rahman et al. [2], the

764

fluctuation of ambient and inlet water temperature andalso wind speed could not be

765

controlled.

766 767 768 769 770 771 772 773 774 775 776

Figure 18: Comparison for electrical efficiency between present result and [2]

777 778

3.10.3 Output temperature and thermal efficiency 26

ACCEPTED MANUSCRIPT 779

The variations of output fluid temperature and thermal efficiency for increasing values of

780

input fluid velocity are observed from this numerical simulation and then compared with the

781

experimental result of Nahar et al. [42]. The comparison is displayed in Figure 19(a)-(b). The

782

PVT model is solved numerically with water as the working fluid, the input fluid temperature

783

at 34°C, ambient temperature at 34°C, and irradiation at 1000 W/m2. The result of the present

784

investigation of a PVT system is little different from Nahar et al. [42] because of the

785

variation of module operating temperature, wind velocity, reference temperature, sky

786

temperature, ambient temperature and other conditions. The numerical investigation was

787

conducted with constant irradiation, reference temperature, sky temperature, ambient

788

temperature, etc. It was noticed that although the experimental curve followed the similar

789

trend of numerical results, the numerical values were somewhat higher than the

790

corresponding experimental values. This discrepancy between the numerical and

791

experimental values is due to the uncontrollable outdoor ambient conditions of the

792

experimental site where an inlet temperature of water could not be maintained at a certain

793

level as in the case of numerical simulation. So, the reason behind these incongruities is due

794

to the unrestrained ambient conditions that prevail in an outdoor experiment.

795 796 797 798 799 800 801 802 803 804 805 806 807

(a) (b) Figure 19: Comparison between numerical and experimental [42] data for (a) output temperature and (b) thermal efficiency

808 809

3.10.4 Quadratic regression equation

810

A quadratic form of the regression equation is developed for PVT thermal efficiency (ηt) and

811

reduced temperature T* (Km2/W) by using Equation (15). The regression equation is: 27

ACCEPTED MANUSCRIPT 812

ηt = - 0.0873 T*2 - 7.0209 T* + 0.6035

(17)

813

where the confidence coefficient is r² = 0.9805.

814

This quadratic regression equation is calculated in the range of reduced temperature

815

Tout  Tin   Tin  2 - 0.01 < T* < 0.055, where T *   G

   Tamb  .

816

The graphical form of this regression equation is depicted in Figure 20. Generally, the slope

817

becomes lower due to reduced temperatures as the electrical efficiency of PV cells decreases,

818

resulting in a relatively higher thermal efficiency of PVT module.

819 820 821 823 824

ηt

822

825 826 827 828 829

T* (Km2/W) Figure 20: Regression graph for thermal efficiency against reduced temperature

830 831

Bosanac et al. [48] conducted an experimental and numerical analysis of a PVT solar

832

collector in Denmark. The authors [48] used crystalline Si cells, with a solar irradiation level

833

of 800 W/m2, a variation of inlet fluid temperature from 10 to 50°C, a wind velocity of 5 m/s,

834

an ambient temperature of 20°C, a PV area of 2.2 m2, emittance of PV cells at 0.95, a PV

835

reference temperature of 25°C, with the power temperature coefficient of PV cells being

836

0.40%/K. The regression equation of [48] was measured as:

837

ηt = -18.338T*2 - 6.4965T* + 0.6193 with confidence coefficient r2 = 0.988.

838

The comparison for quadratic regression equations between the present result (Equation (17))

839

and Bosanac et al. [48] is expressed in Table 6. For this comparison, the present work is

840

conducted by considering solar irradiation levels of 800 W/m2, a variation of inlet fluid

841

temperature from 10 to 50°C, an ambient temperature of 20°C. At the reduced temperature,

28

ACCEPTED MANUSCRIPT 842

the computed error between the present numerical result and that of Bosanac et al. [48] is

843

4.4%.

844

Table 6: Comparison of regression equations Efficiency at T* = 0.025 Km2/W Error

Investigations

Quadratic regression equations

[48]

ηt = - 18.338T*2 - 6.4965T* + 0.6193

45%

Present work ηt = - 0.0873T*2 - 7.0209T* + 0.6035

43%

4.4%

845 846

3.10.5 Quadratic form of thermal efficiency

847

Figure 21 shows the comparison of thermal efficiency based on quadratic form (Equation

848

(15)) against reduced temperature, T* (Km2/W) between the present simulated numerical

849

result and that of Lammle et al. [38]. In their numerical model, they used c-Si cells, low-

850

emmisivity coatings on glass surface, a solar irradiation level of 1000 W/m2, a mass flow rate

851

of 90 Kg/h, a linear heat loss coefficient of 6.37 W/m2K, PV reference electrical efficiency at

852

11.5%, an inlet fluid temperature of 25°C, a wind velocity of 3 m/s, an ambient temperature

853

of 25°C, with emissivity of PV cells at 0.89, and with the temperature coefficient of PV cells

854

being 0.54%/K. The comparison was shown for without coatings and a good agreement

855

between the present result and that of Lammle et al. [38] is observed.

856 857 858 859 860 861 862 863 864 865 866 867 868

T* (Km2/W) Figure 21: Comparison for thermal efficiency against reduced temperature

869

29

ACCEPTED MANUSCRIPT 870

Again, the comparison of the percentage of thermal efficiency against reduced temperature,

871

T* (Km2/W) between current numerical result and that of Guarracino et al. [39] is displayed

872

in Figure 22. In their numerical model, they conducted electrical-thermal modelling of sheet-

873

tube hybrid photovoltaiv/thermal collectors. The comparison is conducted using single glazed

874

collector, solar irradiation level of 1000 W/m2, linear heat loss coefficient of 4.028 W/m2K,

875

PV reference electrical efficiency of 12.6%, 14 pipes, inlet fluid temperature of 20°C, wind

876

velocity of 1 m/s, nominal electrical power of 180 W, ambient temperature of 20°C, PV area

877

of 1.43 m2 with 72 cells, with emissivity of PV cells being 0.90, transmitivity of glazing

878

being 0.95, and the temperature coefficient of PV cells being 0.40%/K. A difference occurred

879

due to the novel design of the present PVT system, though the pattern of the graph of thermal

880

efficiency against reduced temperature is similar.

881 882 883 884 885 886 887 888 889 890 891 892 893

T* (Km2/W) Figure 22: Compared results for thermal efficiency against reduced temperature

894 895

3.11 Correlation

896

From the results of this numerical simulation, a correlation was developed among solar cell

897

temperature, irradiation, ambient temperature, cooling fluid input temperature and volume

898

flow rate of cooling fluid. The correlations were conducted for solar irradiation levels from

899

1000 to 5000 W/m2, cooling fluid volume flow rates from 30 to 210 L/h, and ambient air and

900

inlet fluid temperatures 32ºC and 30ºC respectively. This can be written as:

901

Tsc = (0.2491*Tamb + 0.12736*Tin) (G)0.3691 (V)-0.245

902

where the correlation coefficient is r2 = 0.9881. 30

(18)

ACCEPTED MANUSCRIPT 903

4. Conclusion

904

The investigation shows that high solar radiation and thevolume flow rate of cooling fluid

905

have significant effects on PVT performance. Various irradiation levels and flow rates have

906

been applied for getting high performances of PVT by maintaining optimum cooling systems.

907

The electrical power generation and thermal energy conversion of the PVT system increases

908

with increasing irradiation and varying cooling fluid flow rate. The following conclusions can

909

be drawn from the present research:

910

 The optimum cooling fluid flow rate is found to be 180 L/h for a PVT module exposed to high irradiation of up to 5000 W/m2.

911 912



For every 100 W/m2 increase in solar radiation, the cell temperature, outlet fluid

913

temperature, electrical power and thermal energy increase approximately 0.9, 0.5ºC and

914

19.65, 105.55 W, respectively.

915



for each 100 W/m2 increase of irradiation, respectively.

916 917

The electrical, thermal and overall efficiencies decrease about 0.06%, 0.15% and 0.21%



The cell temperature and outlet fluid temperature decrease approximately 1.1 and 1.4ºC

918

respectively; electrical power and thermal energy increase about 0.12 and 6.4 W,

919

respectively, for every 10 L/h fluid flow rate increase.

920 921



For every 10 L/h increment of

fluid flow rate, the electrical, thermal and overall

efficiencies increase about 0.03%, 0.43% and 0.46%, respectively.

922

In this present research, a novel model was developed by using the converging lens panel on

923

the solar PV panel as well as a new design of heat exchanger (aluminium metal) consisting

924

sixteen baffles is attached directly to the PV module. The proposed model is simple and

925

suitable for building integration, providing air/water depending on the season, and for the

926

thermal requirements of the building, etc. Using high irradiation, this model is able to

927

generate more electrical power and extract thermal energy. The present developed model was

928

found to perform well enough in enhancing the overall efficiency of the PVT system.

929

Acknowledgement

930

Using the economic support of UMPEDAC, HICoE grant, Ministry of Higher Education

931

(Project: UM.0000067/HME.OM, UMPEDAC - 2016) the present research has been done.

31

ACCEPTED MANUSCRIPT 932

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