Energy and exergy analysis of a solar dryer integrated with sodium sulfate decahydrate and sodium chloride as thermal storage medium

Accepted Manuscript Energy and Exergy analysis of a solar dryer integrated with sodium sulfate decahydrate and sodium chloride as thermal storage medium

M.C. Ndukwu, L. Bennamoun, F.I. Abam, A.B. Eke, D. Ukoha PII:

S0960-1481(17)30601-8

DOI:

10.1016/j.renene.2017.06.097

Reference:

RENE 8964

To appear in:

Renewable Energy

Received Date:

02 February 2017

Revised Date:

07 June 2017

Accepted Date:

28 June 2017

Please cite this article as: M.C. Ndukwu, L. Bennamoun, F.I. Abam, A.B. Eke, D. Ukoha, Energy and Exergy analysis of a solar dryer integrated with sodium sulfate decahydrate and sodium chloride as thermal storage medium, Renewable Energy (2017), doi: 10.1016/j.renene.2017.06.097

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ACCEPTED MANUSCRIPT 1

Energy and Exergy analysis of a solar dryer integrated with sodium sulfate decahydrate and

2

sodium chloride as thermal storage medium

3 4 5

Ndukwu M.C.a*, L. Bennamounb, F.I .Abamc, A.B.Ekea, D. Ukohaa aDepartment

of Agricultural and Bioresources Engineering, Michael Okpara University of

6 7

Agriculture, Umuahia, Nigeria bDepartment

of Mechanical Engineering, University of New Brunswick, 15 Dineen Drive, E3B 5A3

8 9

Fredericton, New Brunswick, Canada cDepartment

of Mechanical Engineering, Michael Okpara University of Agriculture, Umuahia,

10

Nigeria

11

*E-mail: [email protected]

12

Abstract

13

Energy and exergy-based performances of a natural-convective solar dryer (NCSDR) integrated with

14

sodium sulfate decahydrate (Na2SO4.10H2O) and sodium chloride (NaCl) as thermal storage medium

15

are presented. The NCSDR was operational in Nigerian climate and applied for red chilli. The objectives

16

of this study were to evaluate the thermal storage potential of Na2SO4.10H2O and NaCl with focus on

17

energy consumption and exergy-sustainability indicators. The performances were compared to control-

18

experiment conditions. The results showed that NCSDR integrated with Na2SO4.10H2O, NaCl, and the

19

control-experiment reduced the moisture content of red chilli from 72.27 % to 7.6, 10.1 and 10.3 %

20

respectively. While, the overall drying efficiency and energy consumption of the three scenarios varied

21

from 10.61-18.79% and 7.54-12.98 MJ respectively. The exergy efficiency for the drying system during

22

sunshine-hours ranged from 66.79 to 96.09 %. The exergy efficiency of drying using Na2SO4.10H2O

23

thru off-sunshine hours and overall exergy-efficiency of the entire drying process were 81.19 and 66.82

24

% respectively. Furthermore, the exergy-based sustainability indicators, waste-exergy ratio,

25

sustainability index and improvement potential for sunshine-hours ranged from 0.166 to 0.174, 3.01 to

26

8.15 and 1.285 to 1.295 W respectively. Approximately 602 tonnes/year of CO2 could be limited from

27

entering the air using Na2SO4.10H2O as thermal storage medium compared to diesel powered dryer.

28 1

ACCEPTED MANUSCRIPT 29

Keywords: Solar drying, thermal storage, exergy – based sustainability, Glauber’s salt, phase change

30

material, red chilli

Nomenclature

31

A

Area (m2)

32

Cp

Specific heat capacity of air (J/kg.K)

33

E

Emissive power, kJ/s,

34

F

Shape factor

35

Ff

Efficiency factor

36

FN

Heat removal factor

37

G, ma

Air mass flow rate (kg/s)

38

g

Gravitational acceleration (m/s2)

39

gc

Constant in Newton's law

40

I

Radiation intensity (W/m2)

41

J

Joule constant

42

l

Latent heat of vaporization of (kJ/kg)

43

M, m

Mass (kg)

44

N

Number of species

45

P

Pressure (kPa)

46

Q

Energy (J)

47

Rg

Gas constant in J/ kg K

48

S

Specific entropy, (kJ/kg K)

49

T

Temperature (ºC or K)

50

t

Time (s)

51

U

Heat loss (W/m2 ºC) or specific internal energy (kJ/kg)

52

V

Velocity (m/s)

53

v

Specific volume (m3/kg) 2

ACCEPTED MANUSCRIPT 54

Z

Altitude coordinate (m)

55 56

Greek symbols

57

ʎ

Latent heat of fusion (kJ/kg)

58

τ

Transmittance

59 60

1.0

Introduction

61

Solar drying offers a suitable alternative to drying crops considering the current instability in the

62

cost of fossil fuels, environmental concerns, and energy cost. Many designs of solar dryer exist in the

63

literature [1- 7]. Based on various proposed designs and local weather conditions, solar dryers had been

64

reported to save between 12.5 to 87 % of the drying time when compared with open sun drying [7-9].

65

Nevertheless, the efficiency of solar dryers is strongly depending on the weather conditions which make

66

the utilization of the solar dryers intermittent and applications characterized by the sunshine and off-

67

sunshine hours. Additionally, during the off -sunshine hours, temperature of the drying chamber drops

68

with probable increase in the humidity of the air which can lead to rewetting of the crop [10]. The latter

69

increases drying time and can affect the quality of the crop [11]. Consequently, researchers had

70

introduced several thermal storage materials to absorb energy during the sunshine hours and release it

71

during the off-sunshine hours [2, 12-14]. Some of these materials include Rocks, water, bricks, concrete

72

materials, phase change materials such as paraffin, non-paraffins, hydrated salts (desiccants) and

73

eutectics materials [2, 12, 14 - 15]. Similarly, studies had shown that desiccants with solid-liquid phase

74

present special attraction because of high latent heat storage density with phase change taken place at

75

narrow temperature range and small volume. Another advantage is that the process of moisture removal

76

is heat and mass transfer process followed by evaporation, it can be driven by either stored heat from

77

the desiccant or concentration difference even at a low temperature inside the drying chamber [10].

78

Sodium chloride (NaCl) has been used by local farmers probably as a cheap thermal storage material in

79

both sun and solar drying of different crops. However, up to now, its application to drying had not been

80

reported. One of the limitations, for developing countries farmers, to integrate hydrated salts in solar

81

drying is its availability and cost. Nonetheless, one of the cheap desiccants accessible is sodium sulphate 3

ACCEPTED MANUSCRIPT 82

decahydrate (Na2SO4.10H2O) known as Glauber’s salt. The literature review shows that there are

83

limited applications of this desiccant. One of the major reasons might be its incongruent melting and

84

poor nucleating properties if applied for a lengthy period which can be overcome by adding water and

85

other additives [16].

86

In addition, drying is an energy consuming process, and emphasis has been on efficient energy

87

utilization for moisture removal [17-19]. For this purpose, various mathematical models were developed

88

and several models were estimating the energy of the drying system based on the first law of

89

thermodynamics. The weakness of the first law of thermodynamics is that it does not give information

90

on losses or the quality of the energy moving across the thermal boundary and may give a false

91

impression about the efficiency of an energy conversion device [20]. This is because the first law of

92

thermodynamics does not provide a measure of how closely the performance of a system approaches

93

reality [21]. In recent times, there is an increasing attention in the combined utilization of the first and

94

second laws of thermodynamics, embodied in the concept of exergy. Exergy breakdown embroils the

95

estimation of the performance of energy conversion devices and processes, by observing the exergy at

96

different points in a sequence of energy conversion stages. With this information, efficiencies can be

97

estimated and the process steps having the largest losses identified thus providing a more realistic view

98

of the process. Since, these steps are not adequately defined by the first law of thermodynamics being

99

that energy is completely conserved, it is actually difficult to approach reality with the first law of

100

thermodynamics. Conversely, the later makes the application of exergy analysis more certain.

101

Therefore, exergy which is based on the second law of thermodynamics has been used to evaluate the

102

efficiency of drying systems [19, 21]. Conversely, exergy sustainability is now a burning issue due to

103

the rapid depletion of the energy resources and their environmental consequences [18, 22].

104

Sustainability implies the efficient supply of energy at minimal cost with less damage to the

105

environment [22]. Rosen et al. [23] have stated that exergy methods provide a platform for measuring

106

sustainability and understanding sustainability will advance ecological knowledge and enhance policy

107

decisions.

108

thermal storage [7, 14]. There is also no information on the exergy sustainability of solar drying of red

109

chilli integrated with Na2SO4.10H2O and NaCl as thermal storage.

Detailed literature survey revealed nothing on exergy analysis integrated with desiccant

4

ACCEPTED MANUSCRIPT 110

This work analysed the energy and exergy performances of solar drying of red chilli working in

111

natural convective mode and using Na2SO4.10H2O and NaCl as storage medium. The main objective

112

was to build cheap solar dryers with available local building materials. However, the study was limited

113

to a typical highly humid region in Nigeria with very low average radiation intensity and frequent

114

rainfall. Commonly, the research works dealing with drying are limited to study of the behaviour of

115

dried material or the drying system including its efficiency without introducing the environmental

116

sustainability of this process. Consequently, the environmental sustainability, by the means of

117

calculation of the sustainability index, waste exergy ratio and improvement potential, as suggested by

118

Rosen et al. [23], was developed for the evaluated solar dryers. This approach was used for several

119

thermal processes but not for solar drying.

120 121

2. Material and Methods

122

2.1 Sample preparation

123

Red chilli was collected from Ubani, in Umuahia, Abia state in the southeastern region of Nigeria.

124

The red chilli was carefully sorted to remove any damaged or decayed sample. Initially, several physical

125

properties of the red chilli relevant to the research analysis were determined and presented in Table 1.

126

For a batch drying run, 1 kg of red chilli was used. The initial moisture content of the red chilli was

127

determined by bone - drying 20 g of the red chilli in an oven at 105ºC. However, the moisture contents

128

of the dried chilli in the solar driers were determined by weighing initially the tray empty and

129

subsequently weighing the tray with the material every one hour. The moisture contents were then

130

calculated from the weight loss. This approach allowed us following the variation of the moisture

131

content of the red chilli during natural convective solar drying.

132

Table 1. Some physical properties of the red chilli

Physical properties

Samples

Average value

Standard deviation

1

Minor diameter (m)

100

0.01396

0.00162

2

Intermediate diameter (m)

100

0.01564

0.002475

5

ACCEPTED MANUSCRIPT 3

Length (m)

100

0.0688

0.004087

4

Geometric mean diameter(m)

100

0.024688

0.002486

5

Initial moisture content (% wb)

20

70.27

0.23200

133 134

2.2

Solar dryer system description

135

Figure 1 shows the three arrays of the prototype NCSDR. The dryer consisted mainly of a flat plate

136

solar collector with a double transparent polyethylene used as solar collector and a top cover measuring

137

500 mm x 500 mm. The absorber was a 1 mm steel sheet painted in black and insulated with particle

138

board at the sides and base. Air plenum of 70 mm was uniformly created across the solar collector. The

139

drying chamber section was of the same width with the collector section (500 mm) and 1000 mm long.

140

However, the drying chamber was deeper. It contained crop tray made of nylon net framed in wood,

141

mounted to create lower and upper plenum in the drying chamber. Also, a double transparent

142

polyethylene was used as top cover of the drying chamber, while the sides and the base were insulated

143

particle board sandwiched with an aluminum sheet to avoid contamination. The solar collector and the

144

drying chamber sections were connected to become a single unit, which represented the solar dryer.

145

The solar collector air plenum was aligned with the lower air plenum of the drying chamber. The solar

146

dryer was positioned on a wooden stand and inclined at an angle of 35.5o sloping downward south [24].

147

The inlet air and outlet air space were at the solar collector section and the drying chamber section

148

respectively. During the sunshine hours, the Na2SO4.10H2O pellets and NaCl in a stainless storage plate

149

was placed at the solar collector plenum to absorb heat from the solar radiation. Thermal energy was

150

transferred to the desiccant where it was absorbed with Na2SO4.10H2O pellets melting from solid to

151

liquid as phase change material (PCM). Sodium chloride (NaCl) was not used as PCM due to a high

152

melting temperature which was not reached in the process. Therefore, in the case of NaCl, only sensible

153

heat is involved while Na2SO4.10H2O pellet both latent and sensible heat is involved. During the off-

154

sunshine hours the liquid PCM and NaCl are brought into the upper plenum in the drying chamber with

155

the upper plenum isolated from the ambient air [10]. In the process of discharge the PCM releases the

156

latent heat of fusion and solidifies [25]. Owing to the incongruent melting of Na2SO4.10H2O pellet

157

which decreased its latent storage capacity was replaced with new one at the beginning of each day. Air 6

ACCEPTED MANUSCRIPT 158

temperatures were measured at four points with a K-type thermocouple connected through a USB to an

159

Omega data acquisition (HH1147; Omega, Stanford, USA). The inlet temperature of the drying

160

chamber was measured with a thermocouple. Hot air anemometer was used to measure the air flow rate

161

at the air vent. A micro weather station was set up to measure the solar intensities, wind speed, ambient

162

temperature, and humidity. The instruments involved are: pyranometer (Apogee MP-200, serial 1250,

163

USA), temperature and humidity clock (DTH-82; TLX, Guandong China) and airspeed sensor (AM-

164

4826; Landesk, Guangzhou, China). The weather data was calibrated with nearby weather data from

165

the weather station of the National Root Crop Research Institute (NRCRI) Umudike.

166 167

Fig. 1 schematics of the array of prototype single layer NCSDR

168

The mass of the red chilli and other parameters were constantly recorded as the drying progressed to

169

anticipated moisture content 8-10 % w.b. The three solar dryers were labeled experiment 1 (with

170

Na2SO4.10H2O pellets), 2 (with NaCl) and 3 (without desiccant). The choice of NaCl in the experiment

171

is to ascertain the effect if any as applied by the local farmers in drying.

172 173

2.3

Energy analysis

7

ACCEPTED MANUSCRIPT 174

The total useful energy consumed (Qu) during the drying process is the total radiation energy received

175

during the sunshine hours and the thermal energy released during the off-sunshine hours, and this is

176

given by

177

𝑄𝑢 = 𝑄𝑟 + 𝑄𝑇

178

Where Qr is the radiation energy, and QT is the thermal energy released

179

Qu is given by Duffie and Beckman [26] as follows

180

𝑄𝑢 = 𝐴𝑠𝐹𝑁[𝐼𝜏 ‒ 𝑈𝑂(𝑇𝑠𝑐 ‒ 𝑇𝑜𝑡)]

181

Where Uo is the overall heat loss (W/m2 ºC).

182

𝑈𝑜 = 𝑈𝑐 + 𝑈𝑏 + 𝑈𝑒 + 𝑈𝑟

183

Where Uc, Ub, Ue and Ur where the collector top, back, edge and radiation heat loss coefficient

184

respectively in W/m2. The heat removal factor (FN) is calculated using equation 4 [3] as follows 𝐺𝐶𝑝

[

1

‒

2

3

𝑓 𝐴𝑠𝑈𝑜𝐹 𝐺𝐶𝑝

]

185

𝐹𝑁 = 𝐴 𝑈 1 ‒ 𝑒

186

The Na2SO4.10H2O pellet was used as phase change material with a transition temperature of 32 oC

187

[27]. Therefore, the thermal energy was emboldened more in the latent heat of fusion. However, it was

188

assumed that the material operates in between two temperatures before and after off-sunshine hours that

189

includes the melting point. Thus, the sensible heat was also considered, and the thermal energy storage

190

was calculated as follows [15]

191

𝑇 𝑇 𝑄𝑇 = 𝑚 ∫ 𝑚𝐶𝑝𝑑𝑇 + ʎ + ∫ 𝑜 𝐶𝑝𝑑𝑇

192

Where m is the mass of the desiccant, Ti and To are the lower and upper temperature which the desiccant

193

operates, Tm is the melting temperature, Cp is the specific heat of the material (for Na2SO4.10H2O, Cp =

194

2.0934 kJkg-1K-1 below the melting point temperature and 3.35 kJkg-1K-1 above the melting point

195

temperature while it is 0.871 kJkg-1K-1 for NaCl) and ʎ is the latent heat of fusion of the material (for

196

Na2SO4.10H2O, ʎ = 252 kJ/kg).

197

Hence, the specific energy consumption (kWh/kg) and specific moisture extraction rate (kg/kWh) are

198

given by equations 6 and 7 respectively [7]

𝑠 𝑜

[{

𝑇𝑖

}

4

{

𝑇𝑚

}]

5

8

ACCEPTED MANUSCRIPT 𝑄𝑢

199

𝑆𝐸 =

6

200

𝑆𝑚𝑟 = 𝑄

201

Where w is the mass of moisture expelled from the red chilli (kg).

202

The drying efficiency (def) of the drying process is calculated using equation 8, as follows

203

𝑑𝑒𝑓 = 𝑄

204

2.4

205

The exergy analysis of the drying process was divided into two parts viz: the exergy analysis of drying

206

during the sunshine period without the thermal storage and the off-sunshine period (mostly in the night)

207

when the thermal storage material was introduced. Generally, the exergy flow rate of a system was

208

made up off the chemical exergy (ech), physical exergy (eh), kinetic exergy (ek) and potential exergy

209

(ept). The exergy balance was givens as follows [3]

210

𝑒 = eh + 𝑒𝑐ℎ + 𝑒𝑘 + 𝑒𝑝𝑡

211

Equation 9 can be expanded as follows

𝑤 𝑤

7

𝑢

𝑤𝑙

8

𝑢

Exergy analysis of the drying process

212 e = (U ‒ U∞) ‒ T∞(S ‒ S∞) + 4 3T4 ‒ T∞ ‒ 4T∞T3) (213

9

V2

P∞

g

∑( ) ( ) ( ) J v ‒ v∞ + 2gJ + Z ‒ Z∞ gcJ + c Uc ‒ U∞ Nc + + EnAnFn

10

214

Where ∞ is the reference state, c is chemical, n is the inlet. The exergy analysis is divided into the

215

sunshine hours and phase change martial (PCM).

216

2.4.1 Exergy analysis for drying during sunshine period

217

Neglecting the momentum and gravitational terms in equation 10 and also assuming that v v∞ The

218

general form of exergy stream applicable during the sunshine hours without the phase change martials

219

(PCM) was given by [17,20].

220

𝐸𝑥 = 𝑚𝑎𝐶𝑝 (𝑇 ‒ 𝑇𝑎) ‒ 𝑇𝑎𝐼𝑛𝑇

221

The input and output exergy to the drying chamber was calculated using equation 12 and 13 as follows

222

[7, 19]

223

𝐸𝑥𝑖𝑛 = 𝑚𝑎𝐶𝑝 (𝑇𝑖𝑛 ‒ 𝑇𝑎) ‒ 𝑇𝑎𝐼𝑛 𝑇

[

𝑇

[

𝑎

]

11

]

𝑇𝑖𝑛 𝑎

12

9

ACCEPTED MANUSCRIPT

[

]

𝑇𝑜𝑢𝑡

224

𝐸𝑥𝑜𝑢𝑡 = 𝑚𝑎𝐶𝑝 (𝑇𝑜𝑢𝑡 ‒ 𝑇𝑎) ‒ 𝑇𝑎𝐼𝑛 𝑇

225

ṁa is the air flow rate

226

The exergy loss for sunshine hours (ExLs) was calculated as

227

𝐸𝑥𝐿𝑠 = 𝐸𝑥𝑖𝑛 ‒ 𝐸𝑥𝑜𝑢𝑡

228

The exergy efficiency is given as

229

𝐸𝑥𝑒𝑓 = 1 ‒

𝑎

13

14

𝐸𝑥𝑙𝑜𝑠𝑠

15

𝐸𝑥𝑖𝑛

230 231

2.4.2

Exergy analysis for the thermal storage

232

The exergy analysis was carried out for only the PCM because the melting temperature of NaCl was

233

not reached therefore the exergy of drying with NaCl was limited to only the sunshine hours. There was

234

an entropy generation in the heat storage process when the PCM melts and also due to air flow across

235

in the case of heat exchanger. Li et al. [25] gave the entropy generated for PCM as

236

𝑆𝑔𝑒𝑛 = 𝑀(𝑆𝑙 ‒ 𝑆𝑠) + 𝑚𝑎(𝑆𝑎𝑜𝑢𝑡 ‒ 𝑆𝑎𝑖𝑛)𝑡𝑝

237

Where Sl and Ss are the specific entropy (J kg-1s-1) of the PCM in the liquid and solid phase respectively

238

while Sa is the specific entropy of air tp is the melting time (s), M (kg) is the mass of the PCM and ma

239

(kg.s-1) is the mass flow rate of air

240

El-Dessouky and Faisal [28] gave the entropy change from solid to liquid for PCM as

241

𝑀(𝑆𝑙 ‒ 𝑆𝑠) =

242

Where ʎ pcm is the latent heat of fusion (J K-1 ) of the PCM and Tm is the melting temperature

243

Also, Topić [29] gave the change in specific entropy of air passing the PCM as

244

𝑆𝑎𝑜𝑢𝑡 ‒ 𝑆𝑎𝑖𝑛 = 𝐶𝑝𝐼𝑛 𝑇 ‒ 𝑅𝑔𝐼𝑛 𝑃

245

Therefore, the exergy loss or the lost work potential (irreversibility) for the PCM is given as

246

𝐸𝑥𝐿𝑃𝐶𝑀 = 𝑇0𝑆𝑔𝑒𝑛 = 𝑇0

16

Mʎ𝑝𝑐𝑚

17

𝑇𝑚

𝑇𝑜𝑢𝑡

𝑃𝑜𝑢𝑡

𝑖𝑛

[

𝑀ʎ𝑝𝑐𝑚 𝑇𝑚

18

𝑖𝑛

(

𝑇𝑜𝑢𝑡

𝑃𝑜𝑢𝑡

+ 𝑚𝑎𝑡𝑝 𝐶𝑝𝐼𝑛 𝑇 ‒ 𝑅𝑔𝐼𝑛 𝑃 𝑖𝑛

10

𝑖𝑛

)]

19

ACCEPTED MANUSCRIPT 247

Where T0 is the reference temperature taken as 28.3oK which is the average of ten years ambient

248

temperature from the nearby weather data from the weather station of the National Root Crop Research

249

Institute (NRCRI) Umudike.

250

For this drying process, the PCM was added during the off-sunshine hours with the system isolated

251

from the ambient air [10]. Therefore, the air flow rate (ma) across the PCM at that time is assumed to

252

be zero. Consequently, equation 19 was reduced to

253

𝐸𝑥𝐿𝑃𝐶𝑀 = 𝑇0𝑆𝑔𝑒𝑛 = 𝑇0

254 255

( ) 𝑀ʎ𝑝𝑐𝑚

20

𝑇𝑚

The exergy efficiency (Exef) for the PCM was calculated as 𝐸𝑝𝑐𝑚𝑥𝑒𝑓 = 1 ‒

𝐸𝑥𝐿𝑃𝐶𝑀

22

𝐸𝑥𝑖𝑛

256 257

2.4.3 Exergy efficiency of drying with the solar dryer with PCM

258

The overall exergy efficiency (Eoxef) for the drying process for sunshine and off- sunshine hours (with

259

PCM) was written as follows [25]

260

𝐸𝑜𝑥𝑒𝑓 = 𝐸𝑥𝑒𝑓 × 𝐸𝑝𝑐𝑚𝑥𝑒𝑓

23

261 262

2.4.4 Exergy sustainability indicators

263

Dincer [18] introduced some sustainability indicators in his assessment of renewable energy approach

264

for sustainable growth. Based on some of the indicators he concluded that renewable energy was worthy

265

to be explored. However, some researchers have revealed that sustainability varies with temperatures

266

[30]. The exergy sustainability indicators assessed in this work was for sunshine hours only. The

267

indicators were: the improvement potential (IP), waste exergy ratio (WER) and sustainability index

268

(SI). They were calculated in equations 24-26 respectively as follows [22].

269

𝑊𝐸𝑅 = 𝐸

270

𝑆𝐼 = 1 ‒ 𝐸

271

Exef is the exergy efficiency in decimal

272

The improvement potential (IP) of the system was calculated as follows [31].

𝐸𝑥𝑙𝑠

24

𝑥𝑖𝑛

1

25

𝑥𝑒𝑓

11

ACCEPTED MANUSCRIPT 273

𝐼𝑃 = (1 ‒ 𝐸𝑥𝑟𝑓)𝐸𝑥𝑙𝑜𝑠𝑠

26

274 275

2.5

CO2 reduction using solar drying

276

The energy consumption of the solar dryer was compared with an artificial dryer powered by a diesel

277

generator. The energy produced by a diesel generator in kWh was expressed in Ould-Amrouche et al.

278

[32] as:

279

𝐺𝐸 = 𝑣𝑑𝑘𝑑𝜂𝑑

280

Where vd,kd and ηd are the volume of diesel generator, the heating value of diesel and efficiency of

281

diesel generator respectively. Assuming equal amount of diesel is to be burnt to produce the same

282

thermal energy to dry the red chilli; combination of equations 1 and 19 gives

283

𝑄𝑟 + 𝑄𝑇 = 𝑣𝑑𝑘𝑑𝜂𝑑

284

Consequently, the volume of diesel that will be produced equivalent energy is given by

285

𝑣𝑑 =

286

Ndukwu et al [22] gave the mass of CO2 produced for a given litre of fuel as

287

𝑚𝐶 = 𝑣𝑑𝑘𝑓

288

The values of kf,, kd and ηd were given by Ould-Amrouche et al. [32] as 2.63 kg/l, 10.08 kWh/l and,

289

30% respectively.

27

28

𝑄𝑟 + 𝑄𝑇

29

𝑘𝑑𝜂𝑑

30

290 291

2.6

292

The drying rate (dr) was deduced as a finite difference of mass of water removed from the wet red chilli

293

per kg of dried solid per unit time and expressed as in Ndukwu et al. [33]

294

𝑑𝑟 =

295

Where t is the drying time.

296

The effective diffusivity (de) was calculated with the method of slopes by plotting -ln(MR) versus time

297

[34,35]. The slope k was given by

298

𝑘=

Drying rate and effective diffusivity

𝑚𝑡 ‒ 𝑚𝑡 ‒ 1

31

𝑑𝑡

𝜋2𝑑𝑒

32

𝑟2

12

ACCEPTED MANUSCRIPT 299

Where r2 is the equivalent radius of the red chilli. The moisture ratio was given by Ndukwu et al [36]

300

as

301

𝑀𝑅 = 𝑚

302

Where mo is the moisture content at time t, me is the equilibrium moisture content, and mi is the initial

303

moisture content.

𝑚𝑜 ‒ 𝑚𝑒

33

𝑖 ‒ 𝑚𝑒

304 305

3.0

Results and discussion

306

This study was conducted in Umudike Abia state South Eastern Nigeria with a geographical location

307

of 5.53o N, 7.49oE during the period of 28th August – 4th September 2016.This period is marked with

308

high humidity occasioned by frequent rainfall and low solar radiation intensity. The experiment was

309

divided into sunshine hours and off-sunshine hours (including rainy period and night time). Fig. 2

310

depicts the ambient temperature, humidity, and solar intensity variation during the sunshine hours.

311

These values were taken continuously until the last day of drying without desiccant. Hence, the average

312

value might vary for solar dryers with thermal storage which take less time comparing to drying without

313

using desiccant. Performances of the tested solar dryers using diverse options are presented in Table 2.

314

The minimum and maximum temperature (for sun shine and off-sunshine hours), humidity (for sun

315

shine and off-sunshine hours) and solar radiation were 27.30 and 34.8 oC, 64.9 and 89.6 %, 100 and

316

795 W/m2 respectiely. Accordingly, the average value of the inlet and outlet temperature of the drying

317

chamber were 41.6 and 40.35oC, 43.13 and 41.83oC, 42.53 and 41.25oC respectively for solar drying

318

with integration of Na2SO4.10H2O (Exp 1), NaCl (Exp 2) and solar drying without desiccant (Exp 3).

319

The variation of the inlet and outlet temperature of the drying chambers with the time of the day

320

is shown in Fig. 3. Temperatures shown in the figure (Figure3) T1, T2 and T3 represent the inlet

321

temperatures of the drying chamber for Exp1, Exp2 and Exp3 respectively. Similarly, T21, T22 and

322

T23 represent the outlet temperatures of the drying chamber. In order to make a clear difference between

323

inlet and outlet temperatures we opt for using two axes with different scales. Figure 3 confirmed the

324

effect of solar radiation during solar drying, as we can see that both inlet and outlet temperatures follow

325

exactly the variation of the radiation. Accordingly, both temperatures increased with the radiation

13

ACCEPTED MANUSCRIPT 326

increase and decreased with the radiation decrease (i.e maximum temperatures are reached at maximum

327

radiation time and lowest temperatures were obtained at the lowest radiation time). However, we can

328

remark that maximum outlet temperatures were obtained after around 1 hour after maximum inlet

329

temperature were reached. This can probably due to the existence of a reaction time between the outlet

330

temperature and the inlet temperature. Similar results were obtained by Bennamoun et al. [37], where

331

they observed the same the existence of a reaction time during solar drying of food. It is also important

332

to mention that the solar collector increased the ambient temperature in low radiation times by around

333

5 degrees against 20 degrees for high radiation times. The humidity inside the drying chamber for both

334

the sunshine and off-sunshine hours is depicted in Fig. 4. The evolution of temperature using

335

NaSO4.10H2O during the off-sunshine hours helped Exp 1 maintained much lower humidity during the

336

night time compared to Exp 2 and Exp 3. The latter is obvious because naturally NaCl has humidity

337

above 70 % and needs high temperature to drive it down and creates moisture evaporation from the red

338

chilli. The average humidity of the drying chamber (both the sunshine and off sunshine hours) was

339

42.23 %, 59 % and 59.3 % for Exp 1, Exp 2 and Exp 3 respectively. It took 24.5 sunshine hours, 36.5

340

sunshine hours and 40.5 sunshine hours to reduce the moisture content of the red chilli from 72.27 %

341

w.b to 7.6 % w.b, 10.1 % w.b and 10.3 % w.b respectively for Exp 1, Exp 2 and Exp 3. There was no

342

significance difference between weight loss at the same drying time between Exp 2 and Exp 3 at 0.05

343

level of significance while difference exists between Exp 1 and Exp 2 or Exp 3. Therefore it is concluded

344

that application of NaCl by local farmers has no positive influence in the drying process as a thermal

345

storage material rather it will increase the cost of drying by its procurement.

14

Ambient Temperature(º C)

Ambient Relative Humidity (%)

Solar Radiation (W/m²)

100

900 800 700 600 500 400 300 200 100 0

80 60 40 20 0

Solar Rdiation (W/m²)

Temperature , humidity (º C, %)

ACCEPTED MANUSCRIPT

15 16 17 12 1315.316.317.310 12 14 9 13 15 10 12 14 12 15 17 11 13 15 11 13 15 Time of the day

346 347

Fig 2 Ambient Temperature, Relative Humidity and Solar Radiation intensity from 28 August – 4th September 2016.

T1(Na₂SO4.10H₂O)

T2(NaCl)

T3(Non desiccant)

T21(Na₂SO4.10H₂O)

T22(NaCl)

T23(Non desiccant)

70

60

65

Inlet temperatures

50

60

40

55

30

50

Outlet temperatures

45

20

40

10

Outlet drying chamber temperature (ºC)

Inlet drying chamber temperature (ºC)

348

35 13

11

18

15

12

19

16

Time of the day

12

18

9

12

13

15

12

9

17

15.3

12.3

18

16.3

30 15

0

349 Fig 3 Inlet and outlet temperature of the drying chamber for the solar dryers from 28th August – 4th

351 Sept. 352 2016. 353 354 355 356

Relative humidity inside the drying chamber (%)

350

Na₂SO4.10H₂O

NaCl

Non dessicant

80 70 60 50 40 30 20 10 0

15 1516.31819.3 12.3 15.317 19 22 10 13 20 9 14 20 9 12 18 21 12 16 19 11 14 17 20 13 Time of the day

ACCEPTED MANUSCRIPT 357 358 359 360 361 362 363 364

Fig 4 Humidity of the drying chamber for the solar dryers from 28th August – 4th September 2016.

365 366

The drying rate of the red chilli is shown in Fig. 5 for the three drying treatments. The drying rate

367

was higher at the beginning and decreased with time. Exp 1 dried faster than others due to lower

368

humidity in the night which provided better moisture gradients for moisture evaporation. This is

369

exhibited in their effective moisture diffusivity in Table 2 which varies from 1.227 x 10-10 m2/s for Exp

370

1, 9.262 x 10-11 m2/s for Exp 2 and 8.547 10-11 m2/s for Exp 3. This range of effective moisture diffusivity

371

was similar to other crops in literature [38]. The average overall drying efficiency of the three systems

372

lies between 10.61 – 18.79 % with Exp 1 showing the peak value. This value was within the range of

373

most available solar dryers in literature.

374 375 376 377

379 380 381 382 383 384

Drying rate (kg of water/kg of dried solid h

378 Na₂SO4.10H₂O

NaCl

Non dessicant

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1516.31819.312.315.317 19 22 10 13 20 Time of the day9 14 20 9 12 18 21 12 16 19 11 14 17 20 13

16

ACCEPTED MANUSCRIPT 385 386 387

Fig. 5 Drying rate of the red chilli for the three treatments 3.1

Energy performance

388

Table 2 shows the overall value of the utilized energy, specific energy consumption and specific

389

moisture extraction rate for the three drying experiments. The continuous single layer solar drying with

390

Na2SO4.10H2O expended 7.37 MJ of solar energy and 0.204 MJ of thermo-chemical energy to remove

391

0.627 kg of water in 24.5 h sunshine hours. Similarly, the solar dryer integrated with NaCl consumed

392

11.49 MJ and 0.011 MJ of solar and thermo- chemical energy respectively to expel 0.602 kg of water

393

in 36.5 h sunshine hours. The control which was solar dryer without integration of thermal storage used

394

up 12.84 MJ only to evaporate 0.600 kg of water from the red chilli in 40.5 h sunshine hours. However,

395

it was noted that bad weather conditions during the research also increased the drying time, but the

396

result revealed the effectiveness and continuous nature of solar drying integrated with Na2SO4.10H2O,

397

even at off sunshine hours. The effectiveness of the energy utilization was defined in the overall specific

398

energy consumption which was obtained as 3.34, 5.28 and 5.92 kWh/kg for Exp 1, 2 and 3 respectively.

399

This result indicated the effective utilization of energy by Exp 1 than others. Consequently, the specific

400

moisture extraction rate which is the energy required to evaporate 1 kg of water was of about 0.299,

401

0.189 and 0.169 kg/kW h respectively. According to Fudholi et al [7], the specific moisture extraction

402

rate showed the turnaround effects on energy utilization for drying.

403 404 405 406 407 408 409

Table 2. Performance parameters for solar drying of Nigerian red chilli with thermal storage materials Parameters

Unit

Na2SO4.10H2O

17

NaCl

No thermal storage

ACCEPTED MANUSCRIPT Total Energy gained by collector

MJ

7.37

Total Energy loss by collector

MJ

0.034

0.063

0.057

Total Thermal energy gained

MJ

0.204

0.011

0

Total useful energy consumed

MJ

7.54

11.44

12.78

Total Mass of water removed

Kg

0.627

0.602

0.600

Specific energy consumption

KWh/kg

3.340

5.281

5.916

Specific moisture extraction rate

Kg/kWh

0.299

0.189

0.169

%

18.79

11.89

10.61

o

C

28.25

28.6

28.9

of

o

C

41.6

43.13

42.53

Average outlet temperature of

o

C

40.35

41.84

41.25

%

42.21

h

24.5

36.5

40.5

m2/s

1.227E-10

9.262E-11

8.547E-11

Initial moisture content

(%wb)

70.27

70.27

70.27

Final moisture content

(%wb)

7.6

10.1

10.3

Initial mass

Kg

1.0

1.0

1.0

Final mass

Kg

0.373

0.398

0.400

Average solar radiation

W/m2

331.04

349.91

368.38

CO2 reduction per year

Tons

602.33

663.19

667.82

Overall Drying efficiency Average Ambient temperature Average

inlet

temperature

11.49

12.84

drying chamber

drying chamber Average

humidity

of

drying

58.81

59.55

chamber Total sunshine hours Effective moisture diffusivity

410 411 412

3.2

Exergy performance of the solar dryers in the sunshine hours

18

ACCEPTED MANUSCRIPT 413

The exergy performance analysis of the solar dryers was independent of the thermal storage

414

materials during the sunshine hours. Moreover, this was so because during this period the thermal

415

storage materials were not in the drying chamber and air movement was subject to vagaries of weather.

416

Figure 6 and 7 show the exergy flow at different times of the day and the effect of the interactions

417

between the exergy flow, ambient temperature and solar radiation for the three solar dryers. The flow

418

of exergy was affected by the variation of the daily weather conditions. The inlet (exin) and outlet

419

(exout) exergy flow for the three dryers ranged between 0.0259 ≤ exin ≤ 0.0662 kW and 0.0207 ≤ exout

420

≤ 0.0562 kW, 0.0253 ≤ exin ≤0.0843 kW and 0.0243 ≤ exout≤ 0.0729 kW, 0.0285 ≤ exin ≤ 0.0799 and

421

0.0266 ≤ exout ≤ 0.0688 kW for Exp 1,2 and 3 respectively. Frequently, Exp 1 exhibited lower exergy

422

loss than Exp 2 and 3 while the higher values were obtained in Exp 2. Maximum exergy loss was 0.0103

423

kW, 0.012 kW and 0.0111 kW for Exp 1, 2 and 3 respectively as shown in Fig 8 for the sunshine hours.

424

The exergy loss was higher at noon period and decreases towards the evening which reveals that high

425

solar radiation and inlet temperature result to higher exergy loss. During the solar drying process in the

426

sunshine hours the exergy efficiency varies from 66.79 to 96.09 % with average values of 82.3 %, 82.69

427

%, and 82.65 % respectively as depicted also in Fig 8. Table 3 also show the exergy efficiency of

428

Na2SO4.10H2O in the off-sunshine hours and overall exergy efficiency of the entire drying process with

429

Na2SO4.10H2O. The exergy efficiency and overall exergy efficiency was calculated as 81.19 and 66.82

430

% respectively.

Exergy (kW)

0.1

exin (Na₂SO4.10H₂O) exout(Na₂SO4.10H₂O)

exin (NaCl) exout(NaCl)

exin (Non desiccant) exout (Non desiccant)

0.08 0.06 0.04 0.02 0 15 16 17 12.313.3 16 17 18 10 12 14 10 14 9 11 13 18 12 15 11 13 15 17 12 14 Time of the day

431 432

Fig 6 Inlet and outlet Exergy for the solar dryers during the sunshine hours from 28 August – 4th

433

September 2016.

19

ACCEPTED MANUSCRIPT

0.16 Exergy (Kw)

0.14 0.12

exin (Na₂SO4.10H₂O)

exin (NaCl)

exin (Non desiccant)

exout(NaCl)

exout (Non desiccant)

solar radiation (W/m²)

exout(Na₂SO4.10H₂O)

0.1

0.08 0.06 0.04 0.02 0

1000 900 800 700 600 500 400 300 200 100 0

31.1 32.8 32.8 31.1 30.4 28.6 27.3 28.1 28.93028.8 30.9 30.1 30.3 29.8 29.1 31.1 30.7 30.9 31.2 30.4 30.5 27.3 28.6 31.2 33.1 31.6 26.8 28.7 27.4 26.9 25.8 27.6 29.3 31.1 30.93029.8 28.1 30.6 30.8 30.3 33.8 34.7 34.8 32.5 28.4 31.4 32.7 30.9 31.2

Ambient Temperature (o C)

434 435

Fig 7 Effect of ambient temperature and solar radiation on Inlet and outlet Exergy for the solar dry

436

during the sunshine hours from 28 August – 4th

0.05

ex loss (Na₂SO4.10H₂O)

ex loss(NaCl)

ex loss (Non desiccant)

0.04

ex eff(Na₂SO4.10H₂O)

ex eff(NaCl)

ex eff (Non desiccant)

1.6 1.4 1.2 1

0.03

0.8 0.02

0.6 0.4

0.01

0.2

0

0 15 16 17 12.313.3 16 17 18 10 12 14 10 14 9 11 13 18 12 15 11 13 15 17 12 14 16 Time of the day

438

Fig 8 Exergy loss and exergy efficiency for the solar dryers from 28 August – 4th September 2016.

439 440 441

20

Exergy efficiency (%)

Exergy loss in sunshine hours

437

Solar Radiation

3 13. 23. 23. 13. 02. 82. 72. 82. 8 . 2 83. 03. 03. 02. 92. 93. 13. 03. 03. 13. 03. 02. 72. 83. 13. 33. 12. 62. 82. 72. 62. 52. 72. 93. 13. 0 . 2 92. 83. 03. 03. 03. 33. 43. 43. 22. 83. 13. 23. 03. 1 . 1 8 8 1 4 6 3 1 9308 9 1 3 8 1 1 7 9 2 4 5 3 6 2 1 6 8 7 4 9 8 6 3 1 9308 1 6 8 3 8 7 8 5 4 4 7 9 2

ACCEPTED MANUSCRIPT 442

Table 3: Exergy parameters for Na2SO4.10H2O (PCM) in off -sunshine hours S/N

parameters

unit

Na2SO4.10H2O

1

Exergy loss

kW

0.0074

2

Exergy efficiency of PCM in off sunshine hours

%

81.19

3

Average Exergy efficiency of PCM in sunshine hours

%

82.30

4

Overall exergy efficiency for the drying process with PCM

%

66.82

443 444 445

3.3

Exergy sustainability Indicators

446

The results of the sustainability indicators are presented in Fig 9 and 10. The purpose of the solar

447

dryer was to increase the inlet air temperature with lower humidity and vapour pressure which should

448

create a moisture gradient to drive moisture out of the crops. In this process, exergy was lost into the

449

environment with the moisture. The extent of this loss in comparison to the exergy that enters the system

450

was expressed in waste exergy ratio. The average value of waste exergy ratio (WER) was calculated

451

with equation 24 and estimated at 0.172, 0.166 and 0.174 for the three dryers used in Exp 1, 2 and 3

452

respectively. The WER was higher in Exp. 3 and lower in Exp. 2. Also, the sustainability index which

453

is a function of exergy efficiency ranged from 3.01 ≤ SI ≤ 8.15 with average values of 5.76, 6.10 and

454

5.84 respectively. The average value of improvement potential which was deduced by the belief that

455

minimizing exergy loss will result in the improvement of the efficiency was calculated as 1.285, 1.292

456

and 1.295 W respectively for the three cases.

457 458

21

SI(Na₂SO4.10H₂O)

SI(NaCl)

SI(Non desiccant)

WER (Na₂SO4.10H₂O)

WER(NaCl)

WER (Non desiccant)

20

0.25 0.2

15

0.15

10

0.1

5

0.05

0

West exergy ratio (WER)

Sustainability index (SI)

ACCEPTED MANUSCRIPT

0 15 16 1712.313.316 17 18 10 12 14 10 14 9 11 13 18 12 15 11 13 15 17 12 14 16 Time of the day

459

Fig 9 Sustainability indicators for the solar dryers from 28 August – 4th September 2016.

460 Improvement potential ( W)

Na₂SO4.10H₂O

NaCl

Non dessicant

4 3.5 3 2.5 2 1.5 1 0.5 0 15 16 1712.313.316 17 18 10 12 14 10 14 9 11 13 18 12 15 11 13 15 17 12 14 16 Time of the day

461

Fig 10 Improvement potential for the solar dryers from 28 August – 4th September 2016.

462 463

3.4

Environmental impact analysis

464

The major advantage of renewable energy application is its potential to limit the emission of harmful

465

gaseous such as CO2. The analysis of CO2 reduction by utilization of the solar dryers was done by

466

quantifying and comparing the energy consumption with a dryer powered by a diesel generator. The

467

comparison was made in terms of volume of diesel used to power the diesel generator. The results show

468

that about 602 tonnes per year can be abated by using solar dryer with Na2SO4.10H2O while 663.19 and

469

667.82 tonnes/year can be abated by solar drying with NaCl and non-desiccants respectively.

470 22

ACCEPTED MANUSCRIPT 471

4. Conclusion

472

Performances of solar dryer integrated with Na2SO4.10H2O and NaCl as thermal energy storage for

473

drying red chilli was presented in this study. It took 24.5 sunshine hours, 36.5 sunshine hours and 40.5

474

sunshine hours of drying with solar dryers integrated with Na2SO4.10H2O, NaCl and the control

475

experiment respectively. The moisture content removal was 72.27 % for Exp1, 10.1 for Exp 2 and

476

10.3 % for Exp 3. The mean value of the humidity of the drying chamber (night and day) for the three

477

experiments were 42.23 %, 59 %, and 59.3 % respectively. The effective diffusivity was calculated at

478

1.227 x 10-10 m2/s for Na2SO4.10H2O, 9.262 x 10-11 m2/s for NaCl and 8.547 10-11m2/s for the control

479

experiment. The average overall drying efficiency of the three systems lies between 10.61 – 18.79 %.

480

The NaCl has no positive influence on the drying process. The total energy consumption varied from

481

7.54 – 12.98 MJ while the specific energy consumption ranged from 3.34 - 5.92 kWh/kg with solar

482

dryer integrated with NaSO4.10H2O having the least value. The exergy efficiency for the three dryers

483

lies between 66.79 – 96.09 % with average values of 82.3 %, 82.69 %, and 82.65 % respectively. The

484

exergy efficiency of drying with Na2SO4.10H2O in the off sunshine hours and overall exergy efficiency

485

of the entire drying process with Na2SO4.10H2O was calculated as 81.19 and 66.82 % respectively. The

486

west exergy ratio (WER) lies between 0.166 - 0.174 while the sustainability index (SI) and improvement

487

potential (IP) exists at 3.01 ≤ SI ≤ 8.15 and 1.285 ≤ IP ≤ 1.295 W. Using the solar dryers for drying can

488

save at least 602 tonnes of CO2 entering the atmosphere in a year. Economic study, cost and payback

489

of the three dryers compared to the dryer that uses diesel generator can be investigated more deeply.

490

We predict that it will reinforce our finding that using solar dryer with NaSO4.10H2O as thermal storage

491

material is the best option to be used and implemented.

492 493 494 495 496 497

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ACCEPTED MANUSCRIPT Highlights 

First time exergy of solar dryer with Na2SO4.10H2O & NaCl as heat storage were studied



Energy consumption was lower with Na2SO4.10H2O and higher with NaCl.



Na2SO4.10H2O proved to be more sustainable with higher drying efficiency.



NaCl has no positive effect on the drying process when compared with the control.



About 602 T/yr of CO2 could be limited from entering the air with Na2SO4.10H2O.