沙漠研究 19-1, 17- 20 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Temperature Stabilization and Value Addition in Solar Drying of Arid
Area Horticulture Products with Phase Change Materials
and High Heat Capacity Materials
Shilpa CHOUDHARY1, 2, Sridhar KUMAR1, 2, Sangeeta SINHA2, 3*, Sanjay KUMAR2, 3
Abstract: Arid and desert areas are home to priceless medicinal plants and horticulture produce.
Scattered production, absence of preservation technology and specialized transportation facility
has hindered economic exploitation of these produce. Family size solar dryer in the required
temperature range is best suited for these regions. Decentralized mode of solar drying ofhorticulture produce and easy transportation has immense potential to contribute in thedevelopment of the region. In this paper, a typical solar dryer is designed and experimentallyevaluated using a combination of High Heat Capacity Material (HHCM) and Phase Change
Material (PCM). The result show that high storage density, with small temperature swing of PCM
in the HHCM-PCM combine, allows the produce to dry in a single day without loss of vitamins
and/or changes in taste and odor.
Keywords: Arid horticulture, Latent heat storage system, Phase change material, Solar drying

1. Introduction
Aid areas are worst hit by global climate changes. Options of sustainable development have shrieked
at a faster pace than expected. Ecosystems are under stress and mass migration of local people has becomecommon. Arid areas are also home to priceless medicinal and horticulture produces, rich in vitamins,
essential minerals and medicinal values; and high solar insolation. Seasonal horticulture produces areeither consumed locally in the season itself or sent to the bigger market immediately after harvest causing
instant and short-lived slumps in prices. Transportation of these products is also difficult and a large partgets rotten before they reach market. Cost-effective preservation technology at local and small scale levels
for these produces is hardly visible at local level due to their scattered production.
Solar drying at very small scale (family size) has many advantages (Choudhary et al., 2005, 2006).
Decentralized drying in hygienic manner (without any application of insecticide) and value addition ispossible. Dried produces can be preserved and transported without fear of degradation. Single day drying
is most suitable for these produces. However, most of the horticulture produces need to be dried inparticular temperature range to preserve vitamins, color and taste. Controlling temperature variation byincreasing thermal mass increases drying time and therefore loss during off sunshine hours. Latent heatstorage in a Phase Change Material (PCM) is very attractive option to overcome this lacuna because of its
high storage density with small temperature swing.
Phase Change Materials (PCM) is latent heat storage material. When the source temperature rises, thechemical bonds within the PCM break up as the material changes phase from solid to liquid (as is the casefor solid-liquid PCMs which are of particular interest here). The phase change is a heat-seeking(endothermic) process. Upon storing heat in the storage material, the material begins to melt when thephase change temperature is reached. The temperature then stays constant until the melting process isfinished. The heat stored during the phase change process (melting process) of the material is called latentheat. The effect of latent heat storage has two main advantages; (a) It is possible to store large amounts ofheat with only small temperature changes and therefore to have a high storage density, (b) Because thechange of phase at a constant temperature takes some time to complete, it becomes possible to smoothtemperature variations. They store 5 to 14 times more heat per unit volume than sensible storage materials
such as water, masonry, or rock.
Phase change materials themselves cannot be used as heat transfer medium. Latent heat energy storage
1University Department of Physics, B.R.Ambedkar Bihar University, Muzaffarpur, Bihar India
Centre for Renewable Energy & Environmental Research, CREER, Muzaffarpur, India
3Chandragupt Institute of Management, Patna - 800001, Bihar, India; Ph. - +91-9431239820,*Corresponding Author: E-mail: creer@sancharnet.in2

18system must, therefore, posses at least three properties: (1) a suitable
PCM with its melting point in the desired temperature range; (2) asuitable heat exchange surface; and (3) a suitable containercompatible with the PCM. The theoretical process of heat exchange
in PCM is explained in the next section.

X=-L1
Liquid state
X=0Solid state
X=L22. Energy Balance Equations: Thermal Analysis
Fig. 1. Phase Change
Heat gain and loss from Phase Change Material (PCM) is a nonlinear and complex process. Let X= 0 be the heat retrieval plane with X = -L1 and X = L2 as liquid andsolid boundaries of the PCM respectively. Total heat exchange q is given by,
q = m& C (To − Ti ); m& in kg/sec
Heat gain by the phase change material gives rise to temperature gain at different boundaries, governed bythe following equation,∂T(ατ ) eff I (t ) = − K1 1+ U t (Tx =− L1 − TS )∂x x =− L1and,T1(X = 0) = TS (X = 0) = T0Where, (ατ)eff is effective gain of solar radiation I(t). Ut is heat loss coefficient from phase change material
to the environment and,∂TS
∂T−K 1= −K S+q∂x x =0∂x x =0∂T−K S= h2 (TS x = L2 − TS )∂x x = LThe average temperature TP of the PCM can be obtained as,TP =L2
⎡ 0
⎤
1T(x)
dxTS (t ) dx ⎥+⎢ ∫− L 1∫( L1 + L2 ) ⎣⎢ 10⎦⎥

The effective thermal property of the PCM can be given by,⎞⎛ L ⎞⎛HC P = C S + ⎜⎜ O − Ta ⎟⎟ ⎜⎜ 1 ⎟⎟⎠ ⎝ L2 ⎠⎝ TP
CP is effective specific heat of the PCM, CS is specific heat material in solid phase. Now energy balance
for the cabinet solar dryer with phase change and high heat capacity material inside the system is given by,dT(ατ ) eff I (t ) Ab = ( M r C r + M p C p ) CSD + U s (TCSD − Ta ) Ab
dtand,dTCSD
dTdT= M r Cr r + M P CP Pdt
dtdt
Ab is area of the base of the solar dryer. Mr, MP and Mf are the thermal mass of the high heat capacity
material, phase change material and thermal load kept for drying, respectively. Similarly, Cr, CP and Cf are
the specific heat capacity of the high heat capacity material (HHCM), phase change material (PCM) andthermal load kept for drying, respectively. TCSD and Tr are the transient temperatures of Cabinet SolarDryer (CSD) and HHCM kept inside the CSD.
M f Cf

3. Materials and Methods
Family size passive solar dryer was designed by optimizing various parameters for remote areas andscattered rural population of North Bihar. The dryer achieved 90 degree Celsius during peak summer anddried even high water and sugar content horticulture products in a single day (12-14 Hrs). One of the main
objectives was to make it simple for local production. Field experience showed that the design lacked at19

Table 1. List of PCMs for drying application.

Trymyristin
Arachic acidFatty Acid PCM
Myristic acid
Palmitic acid
Stearic acid
Salt Hydrates
Fe(NO3 )2 .9H2O
Na2 S2 O3.5H2O
MgSO4 .7H2 O
CH3 COONa. 3H2 O
LiC2 H3O2 .2H2 O
MgCl2. 4H2O
NaOH.H2 O
NaAl(SO4) 2.12H2O
FeSO4 .7H2 O
LiCH3 COO.2H2O
Na2 P2 O7.10H2O
Ba(OH)2 .8H2 O

55
62.1

213
331

---44
47.6
51
54
5659
61
64
65

249
234
255
238
257
236255
240
252

763
769
766
769
770
773
765-910765-910765-91045484850515255
33-57
76.5230218
218238213234213
201-213227851--842---862

49-51
64
69

205
185.4
202.5

861
850
848

47
48
48.4
58
5858
58
61
64
70
70
78

155-190209202
270-290251-377
178
272
181
200
150-251
186-230265-280

1684
1600
168
145
----1893--1937

Fig. 2. Mango pulp drying in prototype solar dryer.
100
90
80
70
60
50
4030
20
100

1400
Temperature (CSD)
Ambient temperature 1200
Solar Intensity
1000
800
600
400
200

Solar Intensity (W/m 2)

Density3(Kg/m )08

9

10 11 12 13 14 15 16 17 18 19 20 21 22Time (hours)

Fig. 3. Variation of solar intensity, ambient temperatureand temperature inside the dryer.
10090807060504030201001400
Temperature (CSD)
Ambient temperature 1200
Solar Intensity
1000800
600
400200

Solar Intensity (W/m 2)

Hypophosphoric acidL. heat(kJ/Kg)

Temperature ( oC )Inorganic PCM
H4 P2 O6SO3(γ)
Paraffins PCM
n- Docosane
n- Tricosane
n- Tetracosane
n- Pentacosane
n- Hexacosane
n- Heptacosane
n- Octacosane
n- Nonacosane
n- Triacotane
Non – Paraffins PCM
Methyl Eicosanate
3-Heptadecanone
2-Heptadecanone
Camphene
9-Heptadecanone
Methyl Behanate

M.P.
o( C)

Temperature ( oC )Material08910 11 12 13 14 15 16 17 18 19 20 21 22Time (hours)

Fig. 4. Variation of temperature inside the dryer withdifferent combinations of HHCM and PCM(NaOH. H2O).

three fronts, (1) temperature range control mechanism needed manual intervention with small PV operated
fans and/or opening the vents, which was unpopular; (2) Variation in temperature (>5 degree C) more thanrequired range for drying made products brittle, sticky and even changed their colour, taste, odour and foodvalue; and (3) Operation after off-sunshine hours increased loss of thermal energy due to local
construction. Strict control on heat loss cannot be possible by local construction. It also highlighted
reduction in the amount of HHCM for early temperature gain, which increased further temperature
variation. The situation worsened in partly cloudy sky.
Dryer with four compartments is designed (Fig. 2), which uses PCMs in combination with HHCMs fortemperature control. PCMs allow temperature inside the dryer to rise faster in the early sunshine hours ascompared to HHCMs. Table 1 show the list of PCMs, which are nontoxic and are in middle temperature
range (45-75 ˚C). Latent heat of fusion, Melting Point, Density and thermal conductivity are mostimportant parameters in selection of PCMs (Sharma et al., 2004; Grigoriov and Mulikhov, 1995).

20Experiments on drying of mangoes, Litchi, Aonla, Jamun and plants were conducted with Sodium
Hydroxide (NaOH.H2O), Sodium Thiosulphate Penta Hydrate (Na2S2O3.5H2O), and Acetamide, which areeasily available and have low price. Numerical analysis and a series of experiments were undertaken tostabilize temperature inside the dryer for different temperature ranges.

Weight (g)100
4. Results and discussion
6 0 kg HHCM
It allows longer period of slow and continuous
4 5 kg HHCM + 15 kg P CM
80
drying, suitable for bulky fruits. Fig. 3 shows the60
variation of solar intensity, ambient temperature
40and temperature inside the dryer without any
HHCM and PCM thermal mass. A large variation
20in temperature is found with maximum
0temperature rising to 90˚C. This is not good for
810121416
1820
22Time (hours)
horticulture drying. Fig. 4 shows a typical result
Fig. 5. Variation of dried weight of aegle marmelosof HHCM (boulders) and PCM (NaOH.H2O)
with PCM
combine.
Pick of the temperature curve iscurtailed and thermal energy is preserved for longer period of time with increase in PCM. At the sametime, initial rise in temperature improves with PCM use. Fig. 5 shows the variation in weight with time foraegle marmelos. It clearly shows that PCM application increases initial rate of drying and therefore,reduces thermal energy loss in off sun-shine hours.
Sodium Hydroxide is best suited for temperature stabilization at 58˚C and good for medium size fruits.
Na2S2O3.5H2O is found good for slow drying process of fish and fruit pulp. Fast drying changes color of
the produce. Acetamide (stabilization at 81˚C) is good for quick drying of small fruits. The whole drying
process requires no chemicals or any pesticides for preservation up to six months. The system is simple tooperate and convenient for rural people particularly in developing countries.

5. Conclusion
Appropriate solar drying designs with options of a combination of high heat capacity material andPCMs can be one of the main planks of rural development, economic prosperity in remote areas as well as
support food security. Results indicate that a combination of high heat capacity material combined with
PCMs can stabilize temperature in the desired range. The CSD design can be used for drying ofhorticulture products in (a) desired range of temperature and (b) single day optimized drying.
Acknowledgement
Authors are grateful to Nalanda Open University, Bihar and Department of Sc. & Technology, Govt.
of India for financial support to carry out the research work.
References
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