Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314

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Renewable and Sustainable Energy Reviews

journal homepage: www.elsevier.com/locate/rser

Solar dryer with thermal energy storage systems for drying agricultural food
products: A review
Lalit M. Bal *, Santosh Satya, S.N. Naik
Centre for Rural Development and Technology, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India

A R T I C L E I N F O A B S T R A C T

Article history: Developing efficient and cost effective solar dryer with thermal energy storage system for continuous
Received 26 June 2009 drying of agricultural food products at steady state and moderate temperature (40–75 8C) has become
Accepted 8 February 2010 potentially a viable substitute for fossil fuel in much of the developing world. Solar energy storage can
reduce the time between energy supply and energy demand, thereby playing a vital role in energy
Keywords: conservation. The rural and urban populations, depend mainly, on non-commercial fuels to meet their
Solar energy energy needs. Solar drying is one possible solution but its acceptance has been limited partially due to
Thermal energy storage
some barriers. A great deal of experimental work over the last few decades has already demonstrated
Solar dryer
Phase change material
that agricultural products can be satisfactorily dehydrated using solar energy. Various designs of small-
Latent heat scale solar dryers having thermal energy storage have been developed in the recent past, mainly for
Sensible heat drying agricultural food products. Therefore, in this review paper, an attempt has been taken to
summarize the past and current research in the field of thermal energy storage technology in materials
as sensible and latent heat in solar dryers for drying of agricultural food products. With the storage unit,
agricultural food materials can be dried at late evening, while late evening drying was not possible with a
normal solar dryer. So that, solar dryer with storage unit is very beneficial for the humans and as well as
for the energy conservation.
ß 2010 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2299
2. Thermal energy storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2300
2.1. Sensible heat storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2300
2.2. Latent heat storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2300
2.3. Thermo-chemical energy storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2301
3. Benefits of LHS systems in comparison with SHS systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2301
4. Storage of latent heat in phase change materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2302
4.1. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2303
4.2. Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2303
4.3. Kinetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2303
4.4. Chemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2303
4.5. Economic criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2303
5. Classification of PCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2303
5.1. Organic phase change materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2304
5.1.1. Paraffins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2304
5.1.2. Non-paraffins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2304
5.2. Inorganic phase change materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2304
5.2.1. Salt hydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2305
5.2.2. Metallics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2306
5.3. Eutectics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2306

* Corresponding author. Fax: +91 11 26591121.

E-mail address: lalit.bal@gmail.com (L.M. Bal).

1364-0321/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.rser.2010.04.014
 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314 2299

6. Solar dryers with thermal heat storage materials: a review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2307

7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2313
8. Future vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2313
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2313
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2313

also either unavailable, unreliable or, for many farmers, too

Nomenclature expensive. In such areas, crop-drying systems that employ
electrically operated fans, heaters and other accessories are
am fraction melted inappropriate. The large capital and running costs of fossil fuel-
ar fraction reacted powered dryers are often not affordable for small farmers.
Cap average specific heat between Ti and Tf (J/kg K) According to the International Energy Outlook 2006 [2], total
Clp average specific heat between Tm and Tf (J/kg K) World marketed energy consumption grows from 421 quadrillion
Cp specific heat (J/kg K) British thermal units (Btu) in 2003 to 563 quadrillion Btu in 2015
Csp average specific heat between Ti and Tm (kJ/kg K) and 722 quadrillion Btu in 2030 on an average by 2.0% per year
shown in Fig. 1 and India is the fifth largest energy consumer, Fig. 2
Dhm heat of fusion per unit mass (J/kg)
[3]. Energy consumption for drying in developing countries is a
Dhr endothermic heat of reaction
major component of the total energy consumption, including
m mass of heat storage medium (kg)
commercial and non-commercial energy sources. Utilization of
Q quantity of heat stored (J) solar energy for thermal applications, like cooking, heating and
T temperature (8C) drying, is well recognized in tropical and semitropical regions.
Tf final temperature (8C) Harnessing solar energy for drying offers significant potential to
Ti initial temperature (8C) dry agricultural products such as food grains, fruits, vegetables
Tm melting temperature (8C) and medicinal plants, thereby eliminating many of the problems
experienced with open-sun drying and industrial drying, while
saving huge quantities of fossil fuels. Various drying techniques
are employed to dry different food products. Each technique has
1. Introduction its own advantages and limitations. Industrial drying offers
quality drying whereas its high cost limits its use. Open-sun
Drying is an essential process in the preservation of agricultural drying suffers from quality considerations though it enjoys cost
products. Food products, especially fruits and vegetables require advantage. A solar air heater provides the hot air with a large
hot air in the temperature range of 45–60 8C for safe drying. Drying variation in the temperature to the dryer only during sunshine
under controlled conditions of temperature and humidity helps hours. Whereas, drying of many agricultural products (e.g. cereals,
the agricultural food products to dry reasonably rapidly to a safe pulses, foods and vegetables) are performed at the steady and
moisture content and to ensure a superior quality of the product moderate temperature and continuously for few days. In such a
[1]. Controlled drying is practiced mostly in industrial drying case, the thermal storage is required with a solar air heater for
processes. Hot air for industrial drying is usually provided by continuous drying so that possibility of drying during partial
burning fossil fuels, and large quantities of fuels are used clouds and/or in late evening hours continuously for few days and
worldwide for this purpose. High cost of fossil fuels, gradual hence, the storage will increase the utility and reliability of the
depletion of its reserve and environmental impacts of their use solar dryers. A thermal storage unit integrated with the solar air
have put severe constraints on their consumption. Many rural heater may be charged during the peak sunshine hours and
locations of developing countries suffer from non-access to grid utilized (discharged) during off-sunshine hours for supplying the
electricity; supplies of other non-renewable sources of energy are hot air to the dryer. The performance of solar air heaters has been
simulated, designed, tested and suggested by many researchers
for crop-drying purposes [4–9]. Choosing the right drying system
is thus important in the process of drying agricultural products.
Especially, in the tropical regions, where some crops have to be
dried during rainy season, special care must be taken in choosing
the drying system.
India is blessed with good sunshine. Most parts of the country
receive mean daily solar radiation in the range of 5–7 kWh m2,
and have more than 275 sunny days in a year [10]. Hence, solar
drying has a high potential of diffusion in the country, and offers a
viable option in the domestic sector. It is identified as an
appropriate technology for Indian masses, and has numerous
advantages such as no recurring costs, potential to reduce
drudgery, high nutritional value of food, high durability, etc. In
spite of these advantages, the main hurdles in its dissemination are
reluctance to acceptance as it is a novel technology, intermittent
nature of sunshine, limited space availability in urban areas, higher
initial costs and convenience issues [11].
Solar energy is free, environmentally clean, and therefore is
Fig. 1. Showing World marketed energy consumption, 1980–2030.Source: IEO2006 recognized as one of the most promising alternative energy
[2]. recourses options. In near future, the large-scale introduction of
2300 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314

Fig. 2. Showing India’s energy scenario.Source: BP States [3].

solar energy systems, directly converting solar radiation into heat, combination of these. An overview of major technique of storage of
can be looked forward. However, solar energy is intermittent by its solar thermal energy is shown in Fig. 3 [13].
nature; there is no sun at night. Its total available value is seasonal
and is dependent on the meteorological conditions of the location. 2.1. Sensible heat storage
Unreliability is the biggest retarding factor for extensive solar
energy utilization. Of course, reliability of solar energy can be In sensible heat storage (SHS), thermal energy is stored by
increased by storing its portion when it is in excess of the load and raising the temperature of a solid or liquid, utilizing the heat
using the stored energy whenever needed. Energy storage is, capacity and change in temperature of the material during the
therefore, essential to any system that depends largely on solar process of charging and discharging. The amount of heat stored
energy. It adjusts temporal mismatches between the load and the depends on the specific heat of the medium, the temperature
intermittent or variable energy source, thereby improving the change and the amount of storage material [14].
system operability and utility. Solar radiation cannot be stored as Z
such, so first of all an energy conversion has to be brought about Q¼ mC p dT ¼ mC ap ðT f  T i Þ (1)
and, depending on this conversion, a storage device is needed. Solar
energy can be stored by thermal, electrical, chemical, and The sensible heat storage capacity of some selected solid–liquid
mechanical methods. materials is shown in Table 1 [15]. Water appears to be the best
SHS liquid available because it is inexpensive and has a high
2. Thermal energy storage specific heat. However molten salts, oils and liquid metals, etc. are
used above 100 8C. Rock bed type storage materials are used for air
Energy storage is a key issue to be addressed to allow heating applications.
intermittent energy sources, typically renewable sources, to match
energy supply with demand. There are numerous technologies for 2.2. Latent heat storage
storing energy in various forms including mechanical, electrical
and thermal energy [12]. Thermal energy can be stored in well- Latent heat storage (LHS) is the heat absorption or release when
insulated fluids or solids as a change in internal energy of a a storage material undergoes a change of phase from solid to liquid
material as sensible heat, latent heat and thermo-chemical or or liquid to gas or vice versa at more or less constant temperature

Fig. 3. Different types of thermal storage of solar energy.

 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314 2301

Table 1
A list of selected solid–liquid materials for sensible heat storage.

Medium Fluid type Temperature range (8C) Density (kg/m3) Specific heat (J/kg K)

Rock 20 2560 879

Brick 20 1600 840
Concrete 20 1900–2300 880
Water 0–100 1000 4190
Caloriea HT43 Oil 12–260 867 2200
Engine oil Oil Up to 160 888 1880
Ethanol Organic liquid Up to 78 790 2400
Propanol Organic liquid Up to 97 800 2500
Butanol Organic liquid Up to 118 809 2400
Isobutanol Organic liquid Up to 100 808 3000
Isopentanol Organic liquid Up to 148 831 2200
Octane Organic liquid Up to 126 704 2400
Source: Sharma et al. [15].

which is presented graphically in Fig. 4 [16]. The storage capacity of (iii) PCMs absorb and emit heat while maintaining a nearly
the LHS system with a phase change material (PCM) medium [14] constant temperature.
is given by (iv) They store 5–14 times more heat per unit volume than
sensible storage materials such as water, masonry, or rock
Tm
Z ZT f (Fig. 5 [17] and Fig. 6).
Q¼ mC p dT þ mam Dhm þ mC p dT (2) (v) Thermal storage capacity per unit mass and unit volume for
Ti Tm small temperature differences is high.
(vi) Thermal gradients during charging and discharging are small.
Q ¼ m½C SP ðT m  T i Þ þ am Dhm þ C lP ðT f  T m Þ (3) (vii) Simultaneous charging and discharging is possible with
appropriate selection of heat exchanger.
2.3. Thermo-chemical energy storage
Phase change can be solid–solid, solid–liquid, solid–gas, liquid–
Thermo-chemical systems rely on the energy absorbed and gas and vice versa. In solid–solid transitions, heat is stored as the
released in breaking and reforming molecular bonds in a material is transformed from one crystalline to another. These
completely reversible chemical reaction. In this case, the heat transitions generally have small latent heat and small volume
stored depends on the amount of storage material, the endother- changes than solid–liquid transitions. Solid–solid PCMs offer the
mic heat of reaction, and the extent of conversion.

Q ¼ ar mDhr (4)

Amongst above thermal heat storage techniques, latent heat

thermal energy storage is particularly attractive due to its ability to
provide high-energy storage density per unit mass and per unit
volume in a more or less isothermal process, i.e. store heat at
constant temperature corresponding to the phase-transition
temperature of phase change material.

3. Benefits of LHS systems in comparison with SHS systems

(i) In LHS systems the temperature of the medium remains more

or less constant since it undergoes a phase transformation.
(ii) Phase change storages with higher energy densities are more
attractive for small storage.

Fig. 4. Changes in temperature and heat during the change in phase.Source: Nuckols Fig. 5. Performance comparison of PCM, water and rock based storage
[16]. system.Source: Kaygusuz [17].
2302 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314

The volume changes of the PCMs on melting would also necessitate

special volume design of the containers to wholes PCM. It should
be able to absorb these volume changes and should also be
compatible with the PCM used. Any latent heat energy storage
system therefore, possess at least following three components:

(i) A suitable PCM with its melting point in the desired

temperature range.
(ii) A suitable heat exchange surface.
(iii) A suitable container compatible with the PCM.

Fig. 6. Comparative volumes for the same amount of heat storage using three
different storage materials. The development of a latent heat thermal energy storage
system hence, involves the understanding of three essential
subjects: phase change materials, containers materials and heat
advantages of less stringent container requirements and greater exchangers. A wide range of technical options available for storing
design flexibility [18]. Most promising materials are organic solid low-temperature thermal energy is shown in Fig. 7 [20].
solution of pentaerythritol (m.p. 188 8C, latent heat of fusion
323 kJ/kg), pentaglycerine (m.p. 81 8C, latent heat of fusion 216 kJ/ 4. Storage of latent heat in phase change materials
kg), Li2SO4 (m.p. 578 8C, latent heat of fusion 214 kJ/kg) and KHF2
(m.p. 196 8C, latent heat of fusion 135 kJ/kg) [19]. Phase change materials are ‘‘Latent’’ heat storage materials.
Solid–gas and liquid–gas transition through have higher latent They use chemical bonds to store and release heat. The thermal
heat of phase transition but their large volume changes on phase energy transfer occurs when the chemical bonds with the material
transition are associated with the containment problems and rule break up as the PCM changes from a solid to a liquid, or from a
out their potential utility in thermal-storage systems. Large liquid to a solid. This is called a change in state, or ‘‘phase’’. Initially,
changes in volume make the system complex and impractical these solid–liquid PCMs perform like conventional storage
[20]. Solid–liquid transformations have comparatively smaller materials; their temperature rises as they absorb heat. Unlike
latent heat than liquid–gas. However, these transformations conventional (sensible) storage materials, when PCMs reach the
involve only a small change (of order of 10% or less) in volume. temperature at which they change phase (their melting point) they
Solid–liquid transitions have proved to be economically attractive absorb large amounts of heat without getting hotter. The
for use in thermal energy storage systems. These are available in a temperature then stays constant until the melting process is
range of heats of fusion and transition temperatures. PCMs finished. The heat stored during the phase change process of the
themselves cannot be used as heat transfer medium. A separate material is called latent heat. The effect of latent heat storage has
heat transfer medium must me employed with heat exchanger in two main advantages: (i) it is possible to store large amounts of
between to transfer energy from the source to the PCM and from heat with only small temperature changes and therefore to have a
PCM to the load. The heat exchanger to be used has to be designed high storage density. (ii) Because the change of phase at a constant
specially, in view of the low thermal diffusivity of PCMs in general. temperature takes some time to complete, it becomes possible to

Fig. 7. Flow chart showing different stages involved in the development of latent heat storage system.
 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314 2303

smooth temperature variations. When the ambient temperature (iv) High thermal conductivity of both solid and liquid phases to
in the space around the PCM material drops, the PCM solidifies, assist the charging and discharging of energy of the storage
releasing its stored latent heat. They store 5–14 times more heat systems.
per unit volume than sensible storage materials such as water,
masonry, or rock [21]. PCM take advantage of latent heat that 4.2. Physical properties
can be stored or released from a material over a narrow
temperature range. PCM possesses the ability to change their (i) High density, so that a smaller container volume holds the
state with a certain temperature range. These materials absorb material.
energy during the heating process as phase change takes place (ii) Small volume changes on phase transformation.
and release energy to the environment in the phase change (iii) Low vapor pressure at operating temperatures to reduce the
range during a reverse cooling process. Basically, there are three containment problem.
methods of storing thermal energy: sensible, latent and thermo- (iv) Congruent melting (phase stability) of the phase change
chemical heat or cold storage. Thermal energy storage in solid- material for a constant storage capacity of the material with
to-liquid phase change employing phase change materials has each freezing/melting cycle.
attracted much interest in solar systems due to the follow
advantages: 4.3. Kinetic properties

(i) It involves PCMs that have high latent heat storage capacity. (i) High nucleation rate to avoid super cooling of the liquid phase.
(ii) The PCMs melt and solidify at a nearly constant temperature. (ii) High rate of crystal growth, so that the system can meet
(iii) A small volume is required for a latent heat storage system, demands of heat recovery from the storage system.
thereby the heat losses from the system maintains in
a reasonable level during the charging and discharging 4.4. Chemical properties
of heat.
(i)Long-term chemical stability.
A large number of PCMs are known to melt with a heat of fusion (ii)Complete reversible freeze/melt cycle.
in any required range. However, for their employment as latent (iii)No degradation after a large number of freeze/melt cycle.
heat storage materials these materials must exhibit certain (iv) Compatibility (non-corrosiveness) with materials of construc-
desirable thermodynamic, kinetic and chemical properties. More- tion.
over, economic considerations and large-scale easy availability of (v) Non-toxic, non-flammable and non-explosive materials for
the phase change materials is also very important. The PCM to be safety.
used in the design of thermal-storage systems should possess
desirable thermophysical, kinetics and chemical properties which 4.5. Economic criteria
are as follows [15,22–25].
(i) Large-scale availability.
4.1. Thermal properties (ii) Cost effective.

(i) Suitable phase-transition temperature (melting temperature) in 5. Classification of PCMs

the desired operating temperature range.
(ii) High sensitive heat capacity and latent heat of fusion per unit A large number of phase change materials (organic, inorganic
volume to minimize the physical size of the heat storage and eutectic) are available in any required temperature range from
container. 0 to 150 8C that is interesting for solar applications. A classification
(iii) High specific heat to provide for additional significant sensible of PCMs is given in Fig. 8. There are a large number of organic and
heat storage. inorganic chemical materials, which can be identified as PCM from

Fig. 8. Classification of PCMs.

2304 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314

the point of view melting temperature and latent heat of fusion. Table 2b
Melting point and latent heat of fusion of paraffins.
However, except for the melting point in the operating range,
majority of phase change materials does not satisfy the criteria No. of Melting Latent heat Groupa
required for an adequate storage media. As no single material can carbon atoms point (8C) of fusion (kJ/kg)
have all the required properties for an ideal thermal-storage media, 14 5.5 228 I
one has to use the available materials and try to make up for the 15 10 205 II
poor physical property by an adequate system design. For example 16 16.7 237.1 I
17 21.7 213 II
metallic fins can be used to increase the thermal conductivity of
18 28.0 244 I
PCMs, supercooling may be suppressed by introducing a nucleat- 19 32.0 222 II
ing agent or a ‘cold finger’ in the storage material and incongruent 20 36.7 246 I
melting can be inhibited by use of suitable thickness. 21 40.2 200 II
22 44.0 249 II
In general inorganic compounds have almost double volumetric
23 47.5 232 II
latent heat storage capacity (250–400 kg/dm3) than the organic 24 50.6 255 II
compounds (128–200 kg/dm3). For their very different thermal 25 49.4 238 II
and chemical behavior, the properties of each subgroup which 26 56.3 256 II
affects the design of latent heat thermal energy storage systems 27 58.8 236 II
28 61.6 253 II
using PCMs of that subgroup are discussed in detail below.
29 63.4 240 II
30 65.4 251 II
5.1. Organic phase change materials 31 68.0 242 II
32 69.5 170 II
33 73.9 268 II
Organic materials are further described as paraffin and non-
34 75.9 269 II
paraffins. Organic materials include congruent melting (the
Source: Sharma et al. [15].
material should melt completely so that the liquid and solid
a
phases are identical in composition. Otherwise, the difference in Group I, most promising; and Group II, promising.
densities between solid and liquid cause segregation resulting in
changes in the chemical composition of the material) means melt
and freeze repeatedly without phase segregation and consequent some undesirable properties such as: (i) low thermal conductivity,
degradation of their latent heat of fusion, self nucleation means (ii) non-compatibility with the plastic container and (iii) moderate
they crystallize with little or no supercooling and usually non- flammability. All these undesirable effects can be partly eliminated
corrosiveness. by slightly modifying the wax and the storage unit. Some selected
paraffins are shown in Table 2b along-with their melting point,
5.1.1. Paraffins latent heat of fusion and groups. PCMs are categorized as: (i) group
Paraffin wax consists of a mixture of mostly straight chain n- I, most promising; (ii) group II, promising; and (iii) group III, less
alkanes CH3–(CH2)–CH3. The crystallization of the (CH3)– chain promising.
release a large amount of latent heat. Both the melting point and
latent heat of fusion increase with chain length. Paraffin qualifies 5.1.2. Non-paraffins
as heat of fusion storage materials due to their availability in a The non-paraffin organics are the most numerous of the phase
large temperature range. Due to cost consideration, however, only change materials with highly varied properties. Each of these
technical grade paraffins may be used as PCMs in latent heat materials will have its own properties unlike the paraffins, which
storage systems. Paraffin is safe, reliable, predictable, less have very similar properties. This is the largest category of
expensive and non-corrosive. They are chemically inert and stable candidate’s materials for phase change storage. Abhat [26] and
below 500 8C, show little volume changes on melting and have low Buddhi and Sawhney [25] have conducted an extensive survey of
vapor pressure in the melt form. For these properties of the organic materials and identified a number of esters, fatty acids,
paraffins, system-using paraffins usually have very long freeze– alcohol’s and glycol’s suitable for energy storage. These organic
melt cycle. Table 2a lists thermal properties of some technical materials are further subgroups as fatty acids and other non-
grade paraffins, which are essentially, paraffin mixtures and are paraffin organic. These materials are flammable and should not be
not completely refined oil [21]. The melting point of alkane exposed to excessively high temperature, flames or oxidizing
increases with the increasing number of carbon atoms. Apart from agents. Few non-paraffins are tabulated in Table 3.
some several favorable characteristics of paraffins, such as Some of the features of these organic materials are as follows:
congruent melting and good nucleating properties, they show (i) high heat of fusion, (ii) inflammability, (iii) low thermal
conductivity, (iv) low flash points, (v) varying level of toxicity, and
(vi) instability at high temperatures.
Table 2a Fatty acids have high heat of fusion values comparable to that of
Thermal properties of some paraffins.
paraffins. Fatty acids also show reproducible melting and freezing
Paraffina Freezing point/range (8C) Heat of fusion (kJ/kg) Groupb behavior and freeze with no supercooling [27,28]. The general
6106 42–44 189 I formula describing all the fatty acid is given by CH3(CH2)2nCOOH
P116c 45–48 210 I and hence, qualify as good PCMs. Their major drawback, however,
5838 48–50 189 I is their cost, which is 2–2.5 times greater than that of technical
6035 58–60 189 I
grade paraffins. They are also mild corrosive. Some fatty acids of
6403 62–64 189 I
6499 66–68 189 I low temperature latent heat storage applications are tabulated in
Source: Sharma et al. [15].
Table 4.
a
Manufacturer of technical Grade Paraffin’s 6106, 5838, 6035, 6403, and 6499: 5.2. Inorganic phase change materials
Ter Hell Paraffin Hamburg, FRG.
b
Group I, most promising; group II, promising; group III, less promising; –
insufficient data. Inorganic materials are further classified as salt hydrate and
c
Manufacturer of Paraffin’s P116: Sun Company, USA. metallics. These phase change materials do not supercool
 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314 2305

Table 3 Table 4
Melting point and latent heat of fusion of non-paraffins. Melting point and latent heat of fusion of fatty acids.

Material Melting Latent Groupa Material Formula Melting Latent heat Groupa
point (8C) heat (kJ/kg) point (8C) (kJ/kg)

Formic acid 7.8 247 III Acetic acid CH3COOH 16.7 184 I
Caprylic acid 16.3 149 – Polyethylene H(OC2H2)nOH 20–25 146 I
Glycerin 17.9 198.7 III glycol 600
D-Lactic acid 26 184 I Capric acid CH3(CH2)8COOH 36 152 –
Methyl palmitate 29 205 II Elaidic acid C8H7C9H16COOH 47 218 I
Camphenilone 39 205 II Lauric acid CH3(CH2)10COOH 49 178 II
Docosyl bromide 40 201 II Pentadecanoic acid CH3(CH2)13COOH 52.5 178 –
Caprylone 40 259 II Tristearin (C17H35COO)C3H5 56 191 I
Phenol 41 120 III Myristic acid CH3(CH2)12COOH 58 199 I
Heptadecanone 41 201 II Palmitic acid CH3(CH2)14COOH 55 163 I
1-Cyclohexyl Octadecane 41 218 II Stearic acid CH3(CH2)16COOH 69.4 199 I
4-Heptadecanone 41 197 II Acetamide CH3CONH2 81 241 I
p-Toluidine 43.3 167 – Methyl fumarate (CHCO2NH3)2 102 242 I
Cyanamide 44 209 II Source: Sharma et al. [15].
Methyl eicosanoate 45 230 II
a
3-Heptadecanone 48 218 II Group I, most promising; Group II, promising; and –, insufficient data.
2-Heptadecanone 48 218 II
Hydrocinnamic acid 48.0 118 –
Cetyl alcohol 49.3 141 – hydrates usually melts to either to a salt hydrate with fewer moles
a-Nepthylamine 50.0 93 –
of water, i.e.
Camphene 50 238 III
O-Nitroaniline 50.0 93 –
AB  nH2 O ! AB  mH2 O þ ðn  mÞH2 O (5)
9-Heptadecanone 51 213 II
Thymol 51.5 115 –
Methyl behenate 52 234 II or to its anhydrous form
Diphenylamine 52.9 107 –
p-Dichlorobenzene 53.1 121 – AB  nH2 O ! AB þ nH2 O (6)
Oxalate 54.3 178 –
Hypophosphoric acid 55 213 II At the melting point the hydrate crystals breakup into
O-Xylene dichloride 55.0 121 – anhydrous salt and water, or into a lower hydrate and water.
b-Chloroacetic acid 56.0 147 III One problem with most salt hydrates is that of incongruent
Chloroacetic acid 56 130 III
Nitronaphthalene 56.7 103 –
melting caused by the fact that the released water of crystallization
Trimyristin 33–57 201–213 I is not sufficient to dissolve all the solid phase present. Due to
Heptadecanoic acid 60.6 189 II density difference, the lower hydrate (or anhydrous salt) settles
a-Chloroacetic acid 61.2 130 – down at the bottom of the container. Most salt hydrates also have
Beeswax 61.8 177 II
poor nucleating properties resulting in supercooling of the liquid
Glycolic acid 63 109 –
p-Bromophenol 63.5 86 – before crystallization begins. One solution to this problem is to add
Azobenzene 67.1 121 – a nucleating agent, which provides the nucleon for initiation of
Acrylic acid 68.0 115 – crystal formation. Another possibility is to retain some crystals, in a
Dinitrotoluene (2,4) 70.0 111 – small cold region, to serve as nuclei. Salt hydrates are the most
Phenylacetic acid 76.7 102 –
Thiosinamine 77.0 140 –
important group of PCMs, which have been extensively studied for
Bromocamphor 77 174 – their use in latent heat thermal energy storage systems. The most
Durene 79.3 156 – attractive properties of salt hydrates are: (i) high latent heat of
Benzylamine 78.0 174 – fusion per unit volume, (ii) relatively high thermal conductivity
Methyl bromobenzoate 81 126 –
(almost double of the paraffins), and (iii) small volume changes on
Alpha naphthol 96 163 –
Glutaric acid 97.5 156 – melting. They are not very corrosive, compatible with plastics and
p-Xylene dichloride 100 138.7 – only slightly toxic. Many salt hydrates are sufficiently inexpensive
Catechol 104.3 207 III for the use in storage [29]. Three types of the behavior of the
Quinone 115 171 II melted salts can be identified: congruent, incongruent and semi-
Acetanilide 118.9 222 II
Succinic anhydride 119 204 II
congruent melting.
Benzoic acid 121.7 142.8 III
Stilbene 124 167 – (i) Congruent melting occurs when the anhydrous salt is
Benzamide 127.2 169.4 II completely soluble in its water of hydration at the melting
Source: Sharma et al. [15]. temperature.
a
Group I, most promising; Group II, promising; Group III, less promising; and –, (ii) Incongruent melting occurs when the salt is not entirely
insufficient data. soluble in its water of hydration at the melting point.
(iii) Semi-congruent melting the liquid and solid phases in
equilibrium during a phase transition is of different melting
appreciably and their heats of fusion do not degrade with composition because of conversion of the hydrate to a lower-
cycling. hydrated material through loss of water.

5.2.1. Salt hydrates The major problem in using salt hydrates, as PCMs is the most of
Salt hydrates may be regarded as alloys of inorganic salts and them, which are judged suitable for use in thermal storage, melts
water forming a typical crystalline solid of general formula incongruently. As n moles of water of hydration are not sufficient
ABnH2O. The solid–liquid transformation of salt hydrates is to dissolves one mole of salt, the resulting solution is supersatu-
actually a dehydration of hydration of the salt, although this rated at the melting temperature. The solid salt, due to its higher
process resembles melting or freezing thermodynamically. A salt density, settles down at the bottom of the container and is
2306 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314

unavailable for recombination with water during the reverse Table 5

Melting point and latent heat of fusion of salt hydrates.
process of freezing. This results in an irreversible melting–freezing
of the salt hydrate goes on decreasing with each charge–discharge Material Material point (8C) Latent heat ((kJ/kg) Groupa
cycle. Another important problem common to salt hydrates is that K2HPO46H2O 14.0 109 II
of supercooling. At the fusion temperature, the rate of nucleation is FeBr36H2O 21.0 105 II
generally very low. To achieve a reasonable rate of nucleation, the Mn(NO3)26H2O 25.5 148 II
solution has to be supercooled and hence energy instead of being FeBr36H2O 27.0 105 II
CaCl212H2O 29.8 174 I
discharged at fusion temperature is discharged at much lower
LiNO32H2O 30.0 296 I
temperature. Other problem faced with salt hydrates is the LiNO33H2O 30 189 I
spontaneous of salt hydrates with lower number of water moles Na2CO310H2O 32.0 267 II
during the discharge process. Adding chemicals can prevent the Na2SO410H2O 32.4 241 II
KFe(SO4)212H2O 33 173 I
nucleation of lower salt hydrates, which preferentially increases
CaBr26H2O 34 138 II
the solubility of lower salt hydrates over the original salt hydrates LiBr22H2O 34 124 I
with higher number of water moles. The problem of incongruent Zn(NO3)26H2O 36.1 134 III
melting can be tackled by one of the following means: (i) by FeCl36H2O 37.0 223 I
mechanical stirring [29], (ii) by encapsulating the PCM to reduce Mn(NO3)24H2O 37.1 115 II
Na2HPO412H2O 40.0 279 II
separation [30], (iii) by adding of the thickening agents which
CaSO47H2O 40.7 170 I
prevent setting of the solid salts by holding it in suspension [31], KF2H2O 42 162 III
(iv) by use of excess of water so that melted crystals do not produce MgI28H2O 42 133 III
supersaturated solution [32], and (v) by modifying the chemical CaI26H2O 42 162 III
K2HPO47H2O 45.0 145 II
composition of the system and making incongruent material
Zn(NO3)24H2O 45 110 III
congruent [33,34]. Mg(NO3)24H2O 47.0 142 II
To overcome the problem of salt segregation and supercooling Ca(NO3)24H2O 47.0 153 I
of salt hydrates, scientists of General Electric Co., NY [35] suggested Fe(NO3)29H2O 47 155 I
a rolling cylinder heat storage system. The system consists of a Na2SiO34H2O 48 168 II
K2HPO43H2O 48 99 II
cylindrical vessel mounted horizontally with two sets of rollers. A
Na2S2O35H2O 48.5 210 II
rotation rate of 3 rpm produced sufficient motion of the solid MgSO47H2O 48.5 202 II
content (i) to create effective chemical equilibrium, (ii) to prevent Ca(NO3)23H2O 51 104 I
nucleation of solid crystals on the walls, and (iii) to assume rapid Zn(NO3)22H2O 55 68 III
FeCl32H2O 56 90 I
attainment of axial equilibrium in long cylinders.
Ni(NO3)26H2O 57.0 169 II
Some of the advantages of the rolling cylinder method as listed MnCl24H2O 58.0 151 II
by [36] are: (i) complete phase change, (ii) latent heat released was MgCl24H2O 58.0 178 II
in the range of 90–100% of the theoretical latent heat, (iii) CH3COONa3H2O 58.0 265 II
repeatable performance over 200 cycles, (iv) high internal heat Fe(NO3)26H2O 60.5 126 –
NaAl(SO4)210H2O 61.0 181 I
transfer rates, and (v) freezing occurred uniformly. A list of salt
NaOHH2O 64.3 273 I
hydrates is given in Table 5. Na3PO412H2O 65.0 190 –
LiCH3COO2H2O 70 150 II
5.2.2. Metallics Al(NO3)29H2O 72 155 I
This category includes the low melting metals and metal Ba(OH)28H2O 78 265 II
Mg(NO3)26H2O 89.9 167 II
eutectics. These metallics have not yet been seriously considered KAl(SO4)212H2O 91 184 II
for PCM technology because of weight penalties. However, when MgCl26H2O 117 167 I
volume is a consideration, they are likely candidates because of the Source: Sharma et al. [15].
high heat of fusion per unit volume. They have high thermal a
Group I, most promising; Group II, promising; Group III, less promising; and –,
conductivities, so fillers with added weight penalties are not
insufficient data.
required. The use of metallics poses a number of unusual
engineering problems. A major difference between the metallics
and other PCMs is their high thermal conductivity. A list of some incorrectly called eutectics, since they are minimum melting.
selected metallics is given in Table 6. Some of the features of these Because of the components undergoes a peritectic reaction during
materials are as follows: phase transition, however, they should more properly be termed
peritectics [38]. The eutectic point of laboratory grade hexadecane
(i) Low heat of fusion per unit weight.
(ii) High heat of fusion per unit volume. Table 6
(iii) High thermal conductivity. Melting point and latent heat of fusion of metallics.

(iv) Low specific heat. Material Melting Latent heat Groupa

(v) Relatively low vapor pressure. point (8C) (kJ/kg)

Gallium–gallium 29.8 – –
5.3. Eutectics antimony eutectic
Gallium 30.0 80.3 I
Cerrolow eutectic 58 90.9 –
A eutectic is a minimum-melting composition of two or more
Bi–Cd–In eutectic 61 25 –
components, each of which melts and freezes congruently forming Cerrobend eutectic 70 32.6 I
a mixture of the component crystals during crystallization [37]. Bi–Pb–In eutectic 70 29 –
Eutectic nearly always melts and freezes without segregation since Bi–In eutectic 72 25 –
they freeze to an intimate mixture of crystals, leaving little Bi–Pb–tin eutectic 96 – –
Bi–Pb eutectic 125 – –
opportunity for the components to separate. On melting both
Source: Sharma et al. [15].
components liquefy simultaneously, again with separation unlike-
ly. Some segregation PCM compositions have sometimes been a
Group I, most promising; and –, insufficient data.
 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314 2307

Table 7
List of organic and inorganic eutectics.

Material Composition (wt.%) Melting point (8C) Latent heat (kJ/kg) Groupa

CaCl26H2O + CaBr26H2O 45 + 55 14.7 140 –

Triethylolethane + water + urea 38.5 + 31.5 + 30 13.4 160 I
C14H28O2 + C10H20O2 34 + 66 24 147.7 –
CaCl2 + MgCl26H2O 50 + 50 25 95 II
CH3CONH2 + NH2CONH2 50 + 50 27 163 II
Triethylolethane + urea 62.5 + 37.5 29.8 218 I
Ca(NO3)4H2O + Mg(NO3)36H2O 47 + 53 30 136 –
CH3COONa3H2O + NH2CONH2 40 + 60 30 200.5 I
NH2CONH2 + NH4NO3 53 + 47 46 95 II
Mg(NO3)36H2O + NH4NO3 61.5 + 38.5 52 125.5 I
Mg(NO3)36H2O + MgCl26H2O 58.7 + 41.3 59 132.2 I
Mg(NO3)36H2O + MgCl26H2O 50 + 50 29.1 144 –
Mg(NO3)36H2O + Al(NO3)29H2O 53 + 47 61 148 –
CH3CONH2 + C17H35COOH 50 + 50 65 218 –
Mg(NO3)36H2O + MgBr26H2O 59 + 41 66 168 I
Naphthalene + benzoic acid 67.1 + 32.9 67 123.4 –
NH2CONH2 + NH4Br 66.6 + 33.4 76 151 II
LiNO3 + NH4NO3 + NaNO3 25 + 65 + 10 80.5 113 –
LiNO3 + NH4NO3 + KNO3 26.4 + 58.7 + 14.9 81.5 116 –
LiNO3 + NH4NO3 + NH4Cl 27 + 68 + 5 81.6 108 –
Source: Sharma et al. [15].
a
Group I, most promising; Group II, promising; and –, insufficient data.

(m.p. 5.3 8C) and tetradecane (m.p. 17.9 8C) mixture occurs at taken in open-air (sun) drying. The drying process is controlled by
approximately 91.67% of tetradecane, and its phase change initial removal of ‘free’ water followed by removal of ‘bound’ water.
temperature is approximately 1.7 8C. A list of eutectic is given in Cassava chips of length l dry according to t = (12l + 60)h; which
Table 7. predicts that for samples of negligible dimensions (e.g. rice and
A large number of solid–liquid PCMs have been investigated for maize) a minimum drying period of 60 h may be required to
heating and cooling applications [19,24,39–44]. Recently, the achieve an equilibrium moisture content of 14% (wet basis) in the
incorporation of heat storage system in solar dryers has grown solar dryer. The efficiency of the solar collector is 22%, and the rock
interest to the researcher. Heat storage system using PCM review storage system stores 1.1 kWh1 to enhance drying.
article are available for any one application except solar dryers for Tiwari et al. [48] worked on experimental simulation of a grain
drying of agricultural food products. Therefore, in this paper, an drying for wheat crop having sensible heat storage using rocks
attempt has been taken to summarize the investigation of the solar (average size 5–8 cm diameter, density I750 kg/ml and specific
drying system having sensible heat storage and latent heat storage heat 0.81 kJ/kg K). The experimental observations have been used
with PCMs. This review will help to find the design, development of
suitable heat storage unit for solar dryers.

6. Solar dryers with thermal heat storage materials: a review

Butler and Troeger [45] have experimentally evaluated a solar

collector-cum-rockbed storage system for peanut drying. The
drying time ranged from 22 to 25 h to reduce the moisture content
from 20% to the safe storage moisture level with an air flow rate of
4.9 m3/s.
Garg et al. [46] experimentally investigated inexpensive solar
collector cum storage system, i.e. a solar air heater with an
augmented integral rock system for agricultural uses [Fig. 9(a–c)].
For a given rock bed thickness and small value of mass flow rate,
where appreciable rise of temperature above the ambient occurs,
the use of two glass covers is recommended as the use of single or
double glass covers depends on the compromise of optical and
thermal losses. Storage in the integrated rock storage and
collection system is effective normally up to 3.30 pm irrespective
of mass flow rate. The performance of the system is promising
showing a satisfactory overall efficiency improvement as com-
pared either to commonly used conventional solar air heater or
with the integrated rock storage and collection system.
A non-mechanical solar dryer (Fig. 10) based on convective heat
and mass transfer and with energy storage has been constructed
and tested to investigate the drying characteristics of various
tropical products (cassava leaves, cassava chips, pepper and fish)
by Ayensu and Asiedu-Bondzie [47]. The solar collector is capable
of transferring 118 W m2 to the drying air at a temperature of Fig. 9. (a) Flat plate air heater. (b) Integrated rock storage and collection system. (c)
32 8C. The drying time for solar drying of a sample is half the time Augmented integrated rock system.
2308 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314

Fig. 10. Solar dryer showing drying chamber (CEJK), chimney (FGHI) and plenum
chamber (ABCKL) with rock storage.
Fig. 11. Schematic view of the drying bin-cum-air-heater-cum-rockbed storage for
deep-bed drying of coriander.

to evaluate the drying time for wheat crop for given moisture
content. It is observed that the fluctuation in temperature is by 1 h for each increment of 50 kg/h m2 in air mass velocity, i.e. the
significantly reduced due to the storage effect. On the basis of drying time can be set at 14 off-sunshine hours for an air flow
experimental simulation, the following conclusions have been velocity of 200 kg/h m2 and 12 off-sunshine hours for an air flow
drawn: (1) the steady state condition for drying the wheat crop velocity of 300 kg/h m2. Hence, the heat stored in the rockbed can
with and without thermal storage is reached after about 2 h for a be used effectively for heating the inlet (ambient) air for off-
given storage capacity and I kg of wheat grain (drying material), (2) sunshine drying of agricultural products.
the moisture content of the drying material decreases with Ayensu [50] designed a solar drying system [Fig. 12(a, b)]
increase in time for a given temperature, (3) the drying rate is having rock storage system on the principles of convective heat
reduced with the decrease of moisture content, (4) the steady state flow. The dryer was constructed from local materials (wood,
condition will take a larger time to achieve for high thermal metals and glass sheets) and used to dry food crops (cassava,
capacity of the rock bed thermal storage, and (5) by using thermal pepper, okro, groundnuts, etc.). The solar collector could transfer
storage, the maximum temperature of the drying material is 118 W m2 thermal power to the drying air. Ambient air at 32 8C
reduced within a safe range, thereby improving the quality of the and 80% relative humidity (RH) could be heated to 45 8C at 40% RH
agricultural procedure. for drying. The crops were dried to a final moisture content of <14%
Chauhan et al. [49] compared drying characteristics of (wet basis) and were preserved for a period of 1 year without
coriander in a stationary 0.5 tonne/batch capacity deep-bed dryer deterioration. The low-temperature drying system ensured the
coupled to a solar air heater and a solar air-heater-cum-rockbed viability of the seeds for planting. The drying process can be
storage unit to receive hot air during sunshine and off-sunshine represented by an empirical equation of the form
hours, respectively (Fig. 11). The drying bed was assumed to M(t) = M0 exp(kt) or dM/dt = kM, where M0 is the initial
consist of a number of thin layers of grains stacked upon each moisture content, M(t) is the moisture content at time t, and k
other. The theoretical investigation was made by writing the is the drying constant. Under identical conditions, a high value of k
energy and mass balance equations for different components of the was correlated with a shorter drying period. The drying process
dryer-cum-air-heater-cum-storage and by adopting a finite takes place in two phases: constant rate and falling rate periods,
difference approach for simulation. The results revealed that for and the drying equation was solved to predict the total drying time.
reducing the moisture content from 28.2% (db) to 11.4% (db) the The mechanisms for the dehydration are the removal of unbound
solar air heater takes 27 cumulative sunshine hours, i.e. about 3 ‘‘free’’ water in the cell cavities and of ‘‘bound’’ water (water films)
sunshine days, whereas the solar air heater during sunshine hours trapped within cells or chemically bound with solids as water of
and the rockbed storage during off-sunshine hours, combined take crystallization. It took nearly 2 times longer to dehydrate crops by
31 cumulative hours (18 sunshine and 13 off-sunshine hours), i.e. open-air sun-drying compared to the solar dryer.
about 2 days and 2 nights at an air mass velocity of 250 kg/hm2. Devahastin et al. [51] proposed via numerical simulation the
During sunshine drying, the effect of grain bed depths on drying use of latent heat storage to store energy from the exhausted gas of
performance of coriander is observed to be remarkable, while the a modified spouted bed grain dryer and saving energy up to 15%.
air mass velocity has no significant effect on the moisture content Ziegler et al. [52] analyzed sorption storage of solar heat using a
reduction rate. However, off-sunshine drying time can be reduced layer of wheat as the desiccant by means of a deep-bed drying
 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314 2309

Fig. 13. Multiple use of solar roof and desiccant storage for tandem-arranged drying
processes (schematic): SR: solar roof, DS: desiccant storage, MC: mixing chamber,
VF: ventilation fan, GD: grain drying, HD: hay drying, ED: energy crops drying.

conditions in East Germany, a collector area of about 5 m2 per m2

ventilated area is sufficient to avoid economic losses due to over
drying and to reduce the danger of decay to a minimum even at
unfavorable weather conditions. The holding container of the
desiccant grain may be constructed similar to conventional mixed
flow dryers and alternatively be operated in continuous-flow. Due
to its airflow resistance, such storage of solar drying potential is
mainly suitable for ventilation drying of bulk materials. The
comparatively small mass of the desiccant grain which temporarily
can be exposed to collector-outlet temperatures of more than 80 8C
may be used as animal feed after the drying period. The economic
efficiency of the whole process will considerably depend on the
multiple use of the collector-storage-unit for the tandem-arranged
drying processes. Intended to be applied to solar-assisted in-
storage drying of agricultural bulk materials, the probability of the
persistence of unfavorable weather periods was quantified
statistically for Potsdam for the month of August, as an example.
Simulation results demonstrate that a relative humidity of the
drying air of 65% can be maintained day and night for weeks
without combustion of fossil fuels. Using a simple strategy of
control, periods with insufficient solar radiation can be bridged
over. Simple solar air heaters can be used to avoid economic losses
due to over drying and to reduce the danger of decay to a minimum
even at unfavorable climatic conditions.
Aboul-Enein et al. [9] reported a parametric study of a solar air
heater with and without thermal storage for solar drying
applications (Fig. 14). Three kinds of material for thermal storage
were used, i.e. water, stones and sand. The average temperature of
flowing air increases with the increase of the collector length and
width up to typical values for these parameters. The outlet
Fig. 12. (a) Schematic diagram of fixed bed dryer with solar collector, plenum
chamber, drying chamber and chimney. (b) General view of the dryer. temperature of flowing air was found to decrease with an increase
of the airflow channel spacing and mass flow rate. The thermal
performance of the air heater with sensible storage materials is
model (Fig. 13). Sorption storage of solar drying potential offers a
promising possibility for gentle, uninterrupted in-storage drying of
different agricultural bulk materials such as grain, hay or wood
chips without combustion of fossil fuels. The use of grain as the
desiccant has decisive economic and processing advantages
compared with, e.g. Silica Gel. Grain is not endangered by dust.
The required strategy of control is based on the mixing of ambient
air and solar heated air that has also flown though the desiccant
bulk. A relative humidity of the drying air of 65% can be maintained
day and night except for those hours when the relative humidity of
ambient air is below 65%. Mold growth inside the storage is
avoided as a matter of principle. The required regeneration
temperatures can he easily achieved using simple collectors that
are operated temporarily in a low-flow mode. At typical climatic Fig. 14. Schematic view of the air heater.
2310 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314

Fig. 15. Inclined multi-pass air heater with in-built thermal storage attached with deep-bed drying system.

considerably higher than that without the storage. An optimal for evaluating the thermal performance of a flat plate solar air
thickness of the storage material of about 0.12 m was found to be heater for the grain drying applications. It is also useful to predict
convenient for drying various agriculture products. In addition, the the moisture content, grain temperature, humidity of drying air
proposed mathematical model may be used for estimating of the and drying rate in the grain bed.
thermal performance of flat plate solar air heater with and without Jain [54] studied a periodical analysis of multi-tray crop drying
thermal storage. attached to an inclined multi-pass solar air heater with in-built
Jain and Jain [53] evaluated performance of a tilted multi-pass thermal storage for drying of the paddy crop (Fig. 16). The crop
solar air heater with in-built thermal storage and attached with the temperature increases with the increase in collector length,
deep-bed dryer for drying the paddy crop using an appropriate breadth and tilt angle up to typical value of these parameters.
deep-bed drying model (Fig. 15). The grain temperature increases The thin layer drying equation has been used to study the drying
with the increase of collector length, breadth and tilt angle up to rate and hourly reduction in moisture content in the different
typical value of these parameters. The thermal energy storage also trays. It has been observed that the crop moisture content
affect during the off-sunshine hours is very pertinent for crop- decreases with the drying time of the day. Different drying rate
drying applications. The proposed mathematical model is useful has been observed in different drying trays due to the variation in

Fig. 16. Multi-tray crop dryer with inclined multi-pass air heater with in-built thermal storage.
 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314 2311

Fig. 17. (a) Schematic diagram of the experiment set-up with attached drying chamber. (b) A detailed sketch of the LHS vessel.

crop temperatures. The thermal efficiency of the drying increases dryer. It is also useful for predicting the crop temperature,
with increase in mass of the crop. moisture content and drying rate of the crop.
Devahastin and Pitaksuriyarat [55] investigated the feasibility Madhlopa and Ngwalo [57] designed, constructed and evaluat-
of using latent heat storage with paraffin wax [Fig. 17(a, b)] as a ed an indirect type natural convection solar dryer with integrated
phase change material to conserve excess solar energy during collector-storage solar and biomass-backup heaters for drying of
drying and release it when the energy availability is inadequate or
not available and its effect on drying kinetics of a food product
(sweet potato). Heat transfer characteristics, temperature profiles
as well as the effects of the inlet air temperature and velocity on
the charge and discharge periods were investigated. It was found
that melting was dominated by heat conduction followed by free
convection; melting took place from the center of the LHS to a point
far away in the radial direction and took place from top to bottom
points in the axial direction. However, only heat conduction was
dominant in the solidification process. PCM froze from an outer to
an inner of the LHS tank due to heat loss to the surrounding. Charge
time decreased with an increase of the inlet air temperature and air
velocity. The amount of extractable energy per unit mass flow rate
of inlet ambient air was 1920 and 1386 kJ min kg1 when using
inlet air velocity of 1 and 2 ms1, respectively. This LHS could save
thermal energy during drying of sweet potato by approximately
40% and 34% when using inlet air velocity of 1 and 2 ms1,
respectively.
Jain [56] presented a transient analytical model to study the
new concept of a solar crop dryer having reversed absorber plate
type collector and thermal storage with natural airflow (Fig. 18).
The performance of this crop dryer with packed bed was carried
out for drying onions in trays. The crop temperature depends on
width of the air flowing channel and height of packed bed. The
thermal energy storage affects drying during the non-sunshine
hours and is very pertinent in reducing the fluctuation in
temperature for drying. The proposed mathematical model is
useful for evaluating the performance of reversed absorber type Fig. 18. Schematic view of reversed absorber with thermal storage natural
collector and thermal storage with natural convective solar crop convective solar crop dryer.
2312 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314

Fig. 19. Cross-sectional view of the solar dryer through the burner, collector, drying
chamber and solar chimney.

fresh pineapple (Ananas comosus). The major components of the

dryer are biomass burner (with a rectangular duct and flue gas
chimney), collector-storage thermal mass and drying chamber
(with a conventional solar chimney) are shown in Fig. 19. The
thermal mass was placed in the top part of the biomass burner
enclosure, stored part of the heat from both solar and biomass air
heaters, thereby moderating temperature fluctuations in the
drying chamber and reducing wastage of energy. It was possible
to dry a batch of pineapples using solar energy only on clear days.
In this operational mode, the dryer reduced the moisture content
of pineapple slices from about 669–11% (db) final moisture content
which is within acceptable limits for safe storage and yielded a
nutritious dried product. The solar mode of operation was slowest
in drying the samples, with the solar–biomass mode being fastest
under the prevailing meteorological conditions. Drying proceeded
successfully even under very bad weather conditions in the solar–
biomass mode of operation. The rate of drying was not uniform
across the trays. Consequently, there is need for interchanging
them during drying to achieve a uniformly dried product. Reverse
thermo-siphoning was observed in the solar chimney during Fig. 20. (a) Schematic of the desiccant integrated solar dryer. 1. Blower, 2. Flat plate
nocturnal drying, which reveals the need to re-visit the design of solar air collector, 3. Drying chamber, 4. Insulation, 5. Absorber plate, 6. Bottom
solar chimneys fitted on solar dryers developed for both diurnal plate, 7. Transparent cover, 8. Desiccant bed, 9. Plywood, 10. Air inlet, 11. Duct for air
and nocturnal free-convection drying of crops. exit, 12. Drying trays, 13. Reversible fan, 14. Valve, 15. Plywood. (b) Pictorial view of
the experimental set-up.
Shanmugam and Natarajan [58] investigated the performance
of an indirect forced convection and desiccant integrated solar
dryer [Fig. 20(a, b)] for drying of green peas and pineapple slices designed, simulated, developed and investigated experimentally
with and without the reflective mirror. The system is operated in for agricultural products such as chillies and fenugreek leaves by
two modes, sunshine hours and off-sunshine hours. During Potdukhe and Thombre [59]. Thermic oil was used as a storage
sunshine hours the hot air from the flat plate collector is forced material for reducing the drying period and enhancing the quality
to the drying chamber for drying the product and simultaneously of dried products. The novel absorber utilizing thermic fluid as in-
the desiccant bed receives solar radiation directly and through the built storage had helped to attain higher drying air temperatures in
reflected mirror. In the off-sunshine hours, the dryer is operated by the drying chamber around 65  3 8C with the length of absorber as
circulating the air inside the drying chamber through the desiccant 0.826 m. It can maintain uniform drying air temperature for longer
bed by a reversible fan. The inclusion of reflective mirror on the period, which is a noteworthy feature of dryer non-existent in the
desiccant bed increases the drying potential by 20%. The useful natural convection solar dryers. The length of operation of solar dryer
temperature rise of about 10 8C was achieved with mirror, which is increased by 1–2 h depending on the solar radiation on the
reduced the drying time by 2 and 4 h for green peas and pineapple, particular day. This will ensure higher drying rate without damage to
respectively. Also, the pick-up efficiency, drying rate and average agricultural products. This also exhibits excellent control over the
dryer thermal efficiency were relatively higher, when compared to airflow rate and the drying rates by the dryer. The model developed
solar drying and desiccant integrated drying. Uniform drying in all can be used to optimize absorber plate for any size of solar dryer. It
the trays were achieved with good quality in terms of colour and can also predict the temperatures at various locations without the
microbiological decay, when compared to solar drying. Taste of the need for experimentation on the prototype. This was the first attempt
dried pineapple is satisfactory. The desiccant material is stable in the development of comprehensive analytical model that has
even after continuous operation for more than a year. The dryer can yielded best results. The drying efficiency of the new type of solar
be used for drying various agricultural products. It can reduce dryer for chillies is 21%, which is higher than reported. The collection
drying time and improve quality of the dried product. efficiency of the new type of solar dryer for chillies is 34%, which is
A new type of solar dryer (Fig. 21), i.e. in-built thermal-storage also higher than reported. Higher percentage weight reduction
agro solar dryer, was conceived, mathematically modelled, occurred for the first tray in all the tests. Higher percentage weight
 L.M. Bal et al. / Renewable and Sustainable Energy Reviews 14 (2010) 2298–2314 2313

Fig. 21. Schematic of experimental set-up.

reduction (47.3%) occurred when the spacing between the two trays consolidated and missing information needs to be obtained
was 46 cm. The drying period in the solar dryer was reduced by 75 through further research.
and 40% compared with open-sun drying and using conventional
dryer, respectively. The colour value of dried chillies powder from the 8. Future vision
new type of solar dryer has been considerably high, about 4 times
compared with the open-sun drying sample. The striking feature of There is need to focus on the hybrid solar drying with thermal
the dryer is that all parts were detachable and were built with locally storage (latent heat storage and sensible heat storage) for
available materials and skills. The dryer is most suitable for continuous drying of agricultural/food products at steady and
agricultural products that are sensitive to direct exposure to solar moderate temperature of 40–75 8C. As only using wax as latent
radiation such as chillies, fenugreek leaves, onion, grapes, sweet heat storage material qualifies as energy storage materials but a
potatoes and mint. No external power is required for operating the major drawback of paraffins is the low thermal conductivity. This
solar dryer since it operates on the combined effect of thermal problem is addressed through an increase of the surface area of
buoyancy and chimney. Hence, the maintenance and operating costs heat transfer between the heat transfer fluid (HTF) and the PCM.
for such dryer are negligible. So this drier provides desired drying air The use of finned tubes as well as metal fiber and metal matrix or
temperature for a longer period. The length of operation of the solar sand, for example, resulted in an increase of one-to five fold of the
air heater and the efficiency of the dryer were increased, and better effective thermal conductivity of the PCM and hence the rate of
quality of agricultural products in terms of colour value were heat transfer. Also using bamboo in solar dryer may reduce cost as
obtained compared with open-sun drying. it has good thermal insulation and mechanical strength compared
to metal. Furthermore heat loss is a main problem of the back panel
7. Conclusion of solar dryer can be corrected by adding a coating of polyurethane
foam or PU foam and dry grass stems. So, this research gap also
The above discussion emphasizes the fact that the advantages needs adequate attention in future studies.
and drawbacks of various designs of solar dryer having heat
storage systems for drying of agricultural food products. As drying
Acknowledgement
energy plays an important role in sustainable energy management
in Indian as well as worldwide, effective utilization of solar energy
One of the authors (Lalit Mohan Bal) gratefully acknowledges
holds the key to future’s non-exhaustive energy source thereby
Council of Scientific and Industrial Research (Government of India),
reduce the time between energy supply and energy demand. A
New Delhi for providing financial assistance in terms of Senior
great deal of experimental work over the last few decades has
Research Fellowship.
already demonstrated for drying of agricultural food products
using solar dryer having solar thermal energy storage in the form of
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