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Experimental investigation of an indirect solar dryer integrated with phase change material for drying valeriana jatamansi (medicinal herb)
A.K. Bhardwaj, Ranchan Chauhan, Raj Kumar, Muneesh Sethi
CASE STUDIES IN THERMAL ENGINEERING
www.elsevier.coïï/bcate/csite
PII: S2214- 157X(17)30078-3
DOI: http ://dx. doi.org/ 10.1016/j. csite.2017.07.009
Reference: CSITE205
To appear in: Case Studies in Thermal Engineering
Received date: 14 March 2017 Revised date: 17 July 2017 Accepted date: 31 July 2017
Cite this article as: A.K. Bhardwaj, Ranchan Chauhan, Raj Kumar and Munees] Sethi, Experimental investigation of an indirect solar dryer integrated with phas change material for drying valeriana jatamansi (medicinal herb), Case Studies i Thermal Engineering, http://dx.doi.org/10.1016/j.csite.2017.07.009
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Experimental investigation of an indirect solar dryer integrated with phase change material for drying valeriana jatamansi (medicinal herb)
A.K. Bhardwaj, Ranchan Chauhan, Raj Kumar, Muneesh Sethi
Faculty of Engineering and Technology, Shoolini University, Solan, H.P, (India)
ABSRTACT
In this study, an experimental investigation of an indirect solar dryer integrated with phase change material has been carried out for drying Valeriana Jatamansi. The experimentation has been performed under the climatic conditions of Himalayan region, Solan (latitude - 30.91° N, longitude - 77.09° E), Himachal Pradesh (India) in the month of October-November 2016. Paraffin RT-42 has been used as a phase change material in the dryer. Using this system, the moisture content of rhizomes reduced from 89% to 9% in 5 days as compared to heat pump drying and shade drying, which took 8 days and 14 days, respectively. Results of present study infer that the drying time using phase change material in this setup has reduced by 37.50% and 64.29% when compared to heat pump drying and shade drying, respectively. The dried rhizomes obtained are of superior quality in terms of colour, texture, aroma and bio-medical constituents. Analyses show that by using present setup, total valepotriates obtained were 3.47% as compared to traditional shade drying which yield 3.31%.
Keywords
Dehydration; Forced Circulation Mode; Medicinal Herb; Moisture Content; Thermal Energy Storage.
Nomenclature
Ac Area of collector, (m )
I Solar intensity, (W/ m2)
LHS Latent heat storage, (J)
M Mass, (kg)
ma Mass flow rate of air, (kg/s)
mw Moisture evaporated in time t, (kg/s)
Pf Energy consumption of blower, (kWh)
PCM Phase change material
PU Polyurethane
Qc Energy absorbed by the collector, (kWh)
RC Rehydration capacity
SHS Sensible heat storage, (J)
SMER Specific moisture extraction rate, (kg/kWh)
SR Shrinkage ratio
T Temperature, (°C)
Drying time, (sec) Dry matter content, (%)
Subscripts:
collector
initial
original (before drying)
rehydrated
thermal
wet basis
Greek:
efficiency, (%)
1. Introduction
Drying is considered as the oldest technique to preserve agricultural based products and medicinal herbs. In this process, moisture content is reduced to its saturation level. Heated air is utilized by natural or artificial means and moisture concentration gradient thus created causes the movement of moisture from inside to outer surface of the product. Temperature more than the acceptable limit causes both physical and chemical changes and ultimately deteriorates the quality of the dried product. Air supplied at controlled temperature enhances their storage life, minimizes loss and saves transportation cost as most of the water contents are dehydrated [1-3]. Dehydration of such products is necessary to avoid bacterial and fungal growth.
Belessiotis et al. [4] investigated solar drying of agricultural based products and analyzed that drying under controlled limits of temperature allows the product to dry rapidly to safe moisture level and ensures the product of superior quality. Ekechukwu et al. [5] studied the solar energy drying system and analyzed that the acceptable temperature of hot air for safe drying of product depends on its composition. Pangavhane et al. [6] investigated the performance of a natural convection solar dryer and analyzed that the energy requirement for drying products depends on the amount of moisture to be removed. High cost of coal and fossil fuels, ecological impacts and gradual diminishing trends of their reserve have imposed serious constraints on their use and have emphasized the use of some other form of energy which is renewable, abundant, eco-friendly and has less adverse impact on the environment. Purohit et al. [7] compared the performance of solar drying methods with the traditional drying and evaluated the financial aspects.
Kant et al. [8] studied the contributions made in the field of solar drying system based on the thermal energy storage medium, capable of storing heat as sensible and latent heat. EI-Khadraoui et al. [9] designed and investigated the feasibility of solar air heater with PCM to store solar energy during the day time, and release it at night. Esakkimuthu et al. [10] investigated the
feasibility of latent heat storage (LHS) unit with an HS 58 (inorganic salt based phase change material) to store the excess thermal solar energy and release it overnight as well as during poor weather conditions. Shalaby et al. [11] studied the applications of paraffin wax as a thermal storage medium and noticed that PCM reduces the heat loss and improves the efficiency of the system. Teng-yne et al. [12] utilized lauric acid as a PCM and investigated the collector charging and discharging time of thermal storage device through different air volume flow rates. Rabha et al. [13] studied the performance of drying of chilli in a forced convection solar dryer integrated with paraffin wax as a latent heat storage medium and found improvement in drying efficiency. Agarwal et al. [14] investigated the suitability of paraffin wax as a latent heat storage material for solar drying applications and evaluated on the basis of the results. Sharma et al. [15] investigated phase change materials (PCMs) for low temperature solar thermal applications and observed improvement in quality of the product.
Soysal et al. [16] studied the effect of drying techniques on the medicinal herbs and found the dehydration process as a critical threshold to preserve the product for a longer time. Khalid [17] et al. investigated the influence of drying temperature, humidity and drying time on the medicinal herbs and observed that these parameters greatly affect the essential oil present in the product. Agah et al. [18] studied the effect of drying temperature, humidity and drying time on the medicinal herbs and found that these parameters significantly affect the essential oil present in the product. Fathi et al. [19] investigated the influence of drying techniques on important ingredients of essential oil and observed that all active constituents are retained using drying techniques in the shade. Rocha et al. [20] studied the influence of drying process on the medicinal plants and analyzed that volatile compounds are sensitive to temperature difference. Choudhary et al. [21] experimented solar drying of horticulture products with PCM and found this technology effective for preservation of medicinal plants. Jain et al. [22] developed an indirect solar crop dryer with phase change material to maintain continuity of drying herbs for their colour and flavor vulnerability.
Some of the medicinal plants are considered as heat sensitive and require drying under controlled conditions; otherwise the quality will deteriorate [23]. Literature review of solar drying of medicinal plants and herbs is presented in Table 1.
Table 1: Solar drying of medicinal plants and herbs
Authors
Product
Moisture content (%)
Max. allowable temp. (°C)
Drying time (hours)
Bala et al. [24]
Initial Final Reduction in
moisture content
Adhatoda 74 3
vasica nees
Traditional Type of solar dryer Saving drying (time) (%)
Hybrid solar Dryer (8)
Srisitti et al. Andographic 75 7 68 75 168 Solar tunnel 71.43
[25] paniculata Dryer (48)
Muller et al. Chamomilla 60 12 48 50 48 Greenhouse type 81.25
[26] recutita solar dryer (9)
Banout et al. Jerky 72 30 42 50 96 Double-pass solar 89.58
[27] dryer (10)
EI-Sebali et Mint 85 11 74 45 288 Indirect forced 75.00
al. [28] convection solar
dryer (with thermal
storage) (75)
Shalaby et Nerium 65 14 51 50 96 Indirect forced 85.41
al. [29] oleander convection solar
dryer (with thermal
storage) (14)
Slama et al. Orange peels 76 13 63 50 96 Indirect forced 75.00
[30] convection solar
dryer (24)
Janjai et al. Rosella 90 18 72 45 120 Roof integrated solar 40.00
[31] flower dryer (72)
Baladin et Rose petals 66 25 41 30 96 Solar wire basket 83.33
al. [32] dryer (16)
Baladin et Thymus 95 10 85 45 66 Indirect forced 48.48
al.[33] vulgaris convection solar
dryer (with thermal
storage) (34)
Hossain et Valeriana 89 9 80 37 336 Heat pump solar 42.85
al.[34] officinalis Dryer (192)
Aritesty et Wild ginger 83 11 72 47 68 Rack type 59.56
al.[35] greenhouse solar
dryer (27.5)
Baladin et Zingiber 80 10 70 40 300 Basket solar crop 76.00
al.[36] officinale dryer (72
Medicine is an essential requirement of all human beings. Valeriana Jatamansi Jones is an aromatic as well as medicinal crop. Miyasaka et al. [37] experimented valeriana in the treatment of nervous state and anxiety-induced sleep disturbances. Grunwald [38] studied the use of valerian extracts as dietary supplements. These supplements are primarily composed of dried roots or its extracts, formulated into tablets or soft gelatin capsules. Each dose contains approximately 50 mg of dried root or its extract. Spinella [39] analyzed the treatment of epilepsy using valerian and recommended it for use. Muller et al. [40] observed that valerian tends to sedate the agitated person, stimulates the fatigued person and brings about a balancing effect on the system. Mathela et al. [41] studied its commercial importance and found it as a substitute for Valeriana Officinalis in India, Nepal and in the Himalayan region. Kaur et al. [42] investigated
the composition of its fresh underground parts for medicinal use and analyzed that rhizome and root yield essential oil, which is important in world trade. Woerdenbag et al. [43] examined the influence of post harvesting process on the quantity and quality of important active ingredients in the dried product and observed that it greatly affects the production chain. The herb is required to process locally due to its perishable nature, scattered plantation, means of preservation and difficulty in transportation to nearby markets. Fennel et al. [44] investigated the influence of dehydration rate on the safe storage of herb and found drying as an essential part which aims at decreasing moisture content, avoiding enzymatic and microbial activity, and as a result preserving the product to extend shelf life. Calixto et al. [45] studied the positive consequences of drying process and observed that effective dehydrating method contribute to a regular supply and facilitate the marketing of medicinal product by reducing the weight and volume of the herb.
The unstable nature of valepotriates due to their sensitivity to light, heat and humid conditions affects their concentration. So, collecting the raw material from the production areas and then processing the same at nearby sites helps in avoiding enzymatic and microbial activity, and preserving the important constituents. For this purpose, adequate drying techniques are needed using controlled temperature, velocity and humidity values. Hence, research on optimal combination of dryer design, operational method and type of energy in use along with quality of the product is required.
Literature reveals that limited study has undergone on medicinal herb drying, using solar energy. Valeriana Jatamansi roots contain high moisture content (about 89%). In order to prevent microorganism's growth and degradation in quality, the moisture content of rhizomes is required to dehydrate immediately up to 9% [34]. Drying near the production sites also helps in reducing its weight and volume and minimizing storage and transportation costs. Therefore, the main objectives of this study are:
(i) To investigate the performance of an indirect solar dryer integrated with phase change material (PCM) for drying Valeriana Jatamansi rhizomes.
(ii) To improve the performance of dried product by improving drying rate and reducing drying time.
(iii) To retain the bio-medical constituents (valepotriates) in the dried product by providing isothermal heating under optimum temperature.
The phase change material (PCM) has the ability to store large amount of excess thermal energy during its melting process and benefit of it under constant temperature later, makes it excellent device to enhance thermal performance of the solar drying system. In this work, for the first time, Valeriana rhizomes are dried at its prescribed drying temperature (40°C) in an indirect solar dryer, in which the PCM storage unit is located at the bottom of the drying compartment to provide isothermal heating.
2. Experimental Study
The whole study is carried out in four stages as discussed below: 2.1. Post-harvest processing of material
Sequence of operations, preparing Valeriana rhizomes for drying is presented in Table 2.
Table 2: Various stages of Valeriana rhizomes before drying
References Process Processing stage Description
Mathela et al. Valeriana [41] plantation
• It is a medicinal crop.
• It is commercially used in the Himalayan region.
Kaur et al. [42] uprooting
• Highest level of essential oil is found in autumn harvested crops.
• Best harvesting stage is October-November.
• Material is procured from department of Forest Product, University of Horticulture and Forestry, Nauni, Solan, India.
Washing
Woerdenbag et rootstock al. [43]
• Herb is required to process locally due to its perishable nature, scattered plantation and difficulty in transportation to nearby markets.
• Rootstock is washed with water to remove the soil particles.
Cleaning rootstock
Mathela et al. [41]
rMïiÊM
Its underground parts (rhizome and root) are rich in bio-chemical constituents and yield essential oil, which is important in world trade.
Rootstock is cleaned with muslin cloth.
Segregating Woerdenbag et rootstocks al. [43]
• Fresh underground parts (rhizome and root) are rich in biomedical constituents.
• Plant produces valepotriartes, mostly from underground parts, having therapeutic importance.
• Rootstocks are segregated.
Calixto et al. [45]
Segregating rhizomes
Most of the valepotriates concentrated in rhizome.
Valepotriates in Valeriana Jatamansi are effective against leprosy, insomnia, restlessness and tension.
Rhizomes are collected on clean cloth for next operation (drying).
2.2. Experimental set up
The setup consists of an indirect solar dryer in forced convection mode with a flat plate solar collector as shown in Fig. 1. A mixture of gravel and iron scrap along with engine oil as a thermal storage medium in the collector and paraffin RT-42 as a phase change (PCM) material in the dryer, are used. Heated air is passed between the absorber plate and the insulation. The solar collector is placed on a supporting structure inclined at an angle of 30° with the horizontal and is oriented to south direction to maximize the exposure of solar radiation. To one side of the collector, blower is connected by providing a PVC tube of diameter 0.035 m. The drying cabinet is constructed from galvanized iron plate having size 1.00 m x 0.60 m x 0.60 m. Valeriana rhizomes are placed on the trays inside the drying cabinet through an insulated door. Three trays each of size 0.85 m x 0.45 m are provided. Trays are made of galvanized wire mesh of thickness
0.005 m and a perforated aluminum sheet. Vertical, top, bottom walls and door are insulated using polyurethane (PU) foam of thickness 0.060 m. The distance of the bottom tray from the base of the drying cabinet is kept 0.18 m and a gap of 0.12 m is maintained between the each tray.
Data logger
Copper tube
All dimension in cms
Fig. 1 (a): Schematic diagram of experimental setup.
Fig. 1 (b): Experimental setup.
Drying chamber
Airoutlet
Drying product
Phase change material
I ron stand
All dimension in cms
Fig. 1 (c): Schematic diagram of drying chamber.
2.3. Experimental instrumentations
Experiments were conducted by using following instruments as shown in Table 3. Table 3: Instruments used during experimentations
Parameter
Equipment/ Instrument
Accuracy
Solar intensity Pyranometer
Hygrometer air Data logger
Relative humidity Dryer temperature Air mass flow Van rate anemometer Weight_Digital balance
Model CMP-3, Kipp and Zonen BV, ± 0.5W/m2
Rontgenweg, Holland (10 W/m2
(1% - 99%)
± 0.2%
Model 2700, Keithley Instruments, ± 0.1 ^V Cleveland, Ohio, USA type Model AV6, 100 mm Hg, Air Flow ± 0.01 m/s) Instruments, England: Range: 0 - 30 m/s (0 - 500) g_± 0.01g
2.4. Experimental procedure
Experiments were conducted during October-November 2016. Fresh Valeriana Jatamansi rhizomes were procured from department of Forest Product, University of Horticulture and Forestry, Nauni, Solan, India, for the purpose of experiments. Rhizomes of about same size were segregated to ensure better drying effect. Three samples of rhizomes with initial weight were taken by an electronic balance (accuracy: ± 0.01g) and then kept in the hot air oven (accuracy: ± 0.5°C) maintaining temperature of 135°C till it attained a constant weight. Initial moisture level was then calculated and found 89% (wet basis). Experiments were performed by adopting four drying methods; traditional shade drying, forced convection without thermal storage system, forced convection with sensible heat storage (SHS - mixture of gravel and iron scrap in the collector along with engine oil in the copper tube) only and forced convection with sensible heat storage (SHS) system in the collector along with phase change material (PCM) in the drying chamber. Fresh Valeriana rhizomes were taken and then distributed 0.5 kg in each tray for every set of experiments. Experiment was continued till the desired level of moisture achieved (9%). The samples of dried rhizomes were collected and sealed in the plastic bag for further investigation of quality attributes.
3. Parameters of performance evaluation
3.1. Determination of moisture content
Drying rate is considered as an important feature and it helps in analyzing the performance of the system. Mathematically, it is expressed as given by Eq. (1) [46].
3.2. Determination of drying rate
Performance estimation of any solar drying system is analyzed by drying rate and is the most important characteristic. Mathematically, drying rate is proportional to difference in moisture content between the material being dried and the equilibrium moisture content at the drying air state and is expressed by Eq. (2) [47].
Mwb = [(Mt - Md)/ Mt] x100
DR = dM/dt = - k (Mt - Me)
dM/ (M - Me) = - k dt
3.3. Determination of moisture ratio On integrating the Eq. (3)
(Mt - Me)/ (Mo - Me) = c exp. (- kt)
Moisture ratio is defined as:
MR = (Mt - Me)/ (Mo - Me)
The moisture ratio (MR) is simplified, because the moisture content at any time Mt and initial moisture content Mo are comparatively higher than equilibrium moisture content Me and is considered significantly less than the moisture content Mo [48]. Therefore, Eq. (3) becomes
MR = (Mt / (Mo) (6)
MR = c exp. (- kt) (7)
log (MR) = - kt + c (8)
The mass shrinkage ratio (SR) is defined as [49]
SR = mt/mo (9)
3.4. Determination of specific moisture extraction rate
The specific moisture extraction rate, which is the energy needed for removing one kg of water, is calculated by using Eq. (10)
SMER = md/Pd (10)
3.5. Determination of dryer thermal efficiency
The thermal efficiency of solar dryer is estimated using Eq. (11) [50]
nd = mL/IavA (11)
4. Results and Discussion
4.1 Analyses of ambient parameters
Figure 2 shows variation of solar intensity, ambient temperature and dryer air temperature with and without thermal storage system against time. It was observed that ambient temperature increased to reach 31.1°C at 1.00 pm, which was considered the maximum ambient temperature during the day time, whereas the solar insolation reached to 886 W/m . The temperature was
70 60 50
• Ambient air temp.
• Dryer air temp. (without storage)
—Dryer air temp. (with storage system) —"—Solar intensity_
9.00 am 10.00 am11.00 am12.00 pm 1.00 pm 2.00 pm 3.00 pm 4.00 pm 5.00
Time (hours)
1000 900 800 700 600 500 400 300 200 100 0
Fig. 2: Variation of solar intensity, ambient temperature and dryer air temperature with and without thermal storage
system v/s time
relatively low at the beginning and the end of the day. With the increase in solar intensity, the temperature of the ambient air reached to maximum value in the afternoon and then started to decrease again. The relative humidity has a reverse trend to that of temperature. For effective drying, higher temperature and lower relative humidity are the important parameters. It was also observed that with the introduction of thermal storage system, average temperature of the drying air increased significantly as shown in Fig. 2. It happened due to the fact that thermal storage material absorbed the excess heat of the day time and released the same during the low sunshine hours or night hours.
The variations of solar intensity, ambient temperature and humidity during the month of October were analyzed critically and minimum, maximum and average values of these parameters are presented in Table 4. During the experiments, the solar irradiance ranged from
301 W/m to 886 W/m . The temperature and relative humidity of ambient air ranged from 19.7 °C to 31.1 °C and 36.75% to 51.25%, respectively. It is found that with the increase in ambient temperature, the relative humidity of the air decreased to a minimum value of 36.75% at 1.00 pm.
Table 4: Parametric values of ambient air
| Parameter Experimental observations
Minimum value Maximum value Average value
Solar intensity (W/m2) 301 886 564.22
Ambient temperature 19.7 31.1 25.04
Ambient air relative 36.75 51.25 42.31
humidity (%)
4.2 Performance of solar collectors
Performance of solar collector was examined with and without thermal storage materials. To study the effectiveness of thermal storage materials with the solar heater, five different combinations of sensible heat storage materials such as (1) sand, (2) gravel, (3) sand with iron scrap, (4) gravel with iron scrap and (5) gravel with iron scrap and engine oil in the copper tube were investigated for thermal performance. It was observed that using mixture of gravel and iron scrap along with engine oil carried in the copper tube showed better thermal performance in comparison to the use of other combinations of storage materials. Using this combination as a thermal storage material, average collector outlet air temperature enhancement was noticed as 26.18°C above the ambient average temperature which ensured higher moisture dehydration rate during the process. Temperature enhancement of hot air with thermal storage material was analyzed as 43.11 % more than without using such materials as presented in Table 5. It happened due to the fact that gravel with iron scrap acted as a sensible heat storage medium and absorbed excess heat of the day time and released during the off-sunshine hours. Further, engine oil
carried in copper tube also helped in temperature enhancement due to the high conductivity of copper and high specific heat of engine oil.
Table 5: Parametric values of collector air temperature
| Parameter Experimental observations Average percentage |
Minimum Maximum Average improvement in
value (°C) value (°C) value (°C) collector outlet air
Ambient temperature 19.7 31.1 25.04 temperature
Collector outlet air temp. without 21.2 46.3 35.79 -
thermal storage
Collector outlet air temp. with 22.6 49.3 38.27 6.93 |
storage-1, (sand) 1
Collector outlet air temp. with 23.4 52.6 40.3 12.60 f
storage-2, (gravel) 1 A
Collector outlet air temp. with 24.8 58.1 43.9 22.66 J
storage-3, (sand with iron scrap) 1
Collector outlet air temp. with 26.1 60.7 47.1 31.60 |
storage-4, (gravel with iron scrap) 1
Collector outlet air temp. with 26.3 64.2 51.22 43.11 f
storage-5, (gravel with iron scrap and 1
engine oil in the copper tube)
4.3 Performance of solar dryer
Hot air received from the collector outlet was passed from bottom of drying chamber to the trays kept inside the insulated enclosure. Thermal performance of hot air inside the drying chamber was examined with and without loading the phase change material. Temperature of the hot air experienced by drying chamber was slightly lower than the collector outlet air temperature when phase change material was not loaded. Also, among the different trays, slightly fall in temperature was noticed from bottom to top. It happened because some part of energy is utilized to carry the hot air from collector outlet to drying chamber inlet and subsequently from bottom to top trays.
Experiments were conducted by adopting four drying methods; traditional shade drying, forced convection solar drying without thermal storage system, forced convection with sensible heat storage (SHS) system in the collector and forced convection with sensible heat storage (SHS) system in the collector along with phase change material (PCM) in the drying chamber.
However, by loading phase change material at the bottom of the drying cabinet, almost constant temperature was noticed inside the enclosure. It was due to the fact that phase change material absorbed excess heat of the day and supplied it to the trays when there was fall in drying air temperature, by changing its phase and helped in maintaining constant temperature.
4.4 Drying of Valeriana Jatamansi herb
For safe storage of Valeriana rhizome, its moisture content is required to reduce from 89% to 9% [43]. Using forced convection solar dryer integrated with phase change material, it took 5 days to dry rhizomes whereas traditional shade drying needed 14 days. The variation of moisture content with time by adopting different drying techniques are presented in Fig.3. Using phase change material, temperature of drying air inside the dryer chamber was sufficiently high as compared to the atmospheric temperature and corresponding humidity was lower than the ambient air. So, drying rate of rhizomes in a forced convection mode using phase change material (PCM) was faster than the other drying techniques applied. By adopting this technique, phase change material permitted hot air at optimum temperature to rhizomes placed on the trays and did not allow fall in air temperature. Phase change material absorbed excess heat above the optimum temperature (40°C) and released the same when there was fall in drying air temperature.
100 90 80 70 ^ 60 50 40 -30 -20 -10 0
Traditional shade drying
Solar drying forced convection (without storage)
Solar drying forced convection with SHS (collector) only
Solar drying forced convection with SHS (collector) and PCM (drying chamber)
—i—
—i— 150
200 250
Drying time (hours)
Fig. 3: Influence of drying rate on moisture content using different drying techniques.
Fig.4 indicates that the rate of moisture loss decreased with the increase in drying time, until the rhizomes approach the equilibrium moisture content. The drying mainly occurs in the falling rate period which indicates a diffusion-controlled type mechanism of drying. A similar trend has been reported by Midilli [49]. Initially, weight loss of rhizomes was faster because of the free
0.8 0.6 0.4 0.2 0
■ ♦
■ ♦
A X ■ A X : i
i 100 i 150
♦ Traditional shade drying
■ Solar drying forced convection (without storage)
A Solar drying forced convection with SHS (collector) only
X Solar drying forced convection with SHS (collector) and PCM (drying chamber)
♦ ♦
♦ ♦
♦ ♦
200 250 Drying time (hours)
Fig. 4: Variation of moisture ratio with time
moisture content on the outer surface and then became slower and slower due to internal moisture migration from inner layers to the surface, and finally gets saturated as illustrated in Fig.4. Plot shows the influence of drying time on moisture ratio. The moisture ratio decreased significantly with the progress of drying time.
Traditional shade drying
Solar drying forced convection (without storage)
Solar drying forced convection with SHS (collector) only
Solar drying forced convection with SHS (collector) and PCM (drying chamber)
♦—.
8 10 Drying time (days)
Fig. 5: Variation of drying rate with time
Initially, the drying rate was observed faster in all cases as illustrated in Fig.5. Analyses show that drying rate was highest using phase change material in the drying unit. Drying time has reduced by 28.57%, 37.50% and 64.28% as compared to indirect solar drying with sensible storage system (SHS) only, without thermal storage system in forced convection mode and shade drying, respectively as presented in Fig. 5 and 6. It happened because using phase change material (PCM), drying process gets elongated by 6-7 hours per day after sunshine hours as the drying air remained 8-10°C higher than the ambient temperature up to mid night. It helped in utilizing the stored energy during the night hours effectively and helped in increasing the drying rate.
♦ Traditional shade drying
■ Solar drying (forced convection without storage)
▲ Solar drying forced convection with SHS only
X Solar drying forced convection with SHS and PCM
—I—
30 40 50
Moisture ratio (%)
—I—
Fig. 6: Variation of drying rate with moisture ratio
Whereas using sensible heat storage medium only, drying of material during off sunshine hours was noticed for 2-3 hours per day. Without using sensible heat storage medium and phase change material (without thermal storage), drying after the sunshine hours was not found possible and therefore, required more drying hours. Shade drying was observed as the slowest process. This was because dehydration was carried under the shade and product was not exposed to hot air. Ambient temperature (average 25.04°C) was very much below the optimum required temperature (40°C) and hence, slowed down the dehydration process.
4.5 Quality of dried Valeriana rhizomes
Drying affected the physical properties of the rhizomes and resulted change in size, shape, colour, and texture. Many chemical and enzymatic conversions also took places which were very
important from the health point of view. So, comparison of quality parameters of dried product by using different drying techniques became necessary as it depends on drying air temperature, humidity, dryer design, drying time, dehydration rate and other parameters. During this study, dried rhizomes were assessed for quality attributes by considering sensory, rehydration and biomedical parameters.
4.5.1 Sensory evaluation
Valeriana rhizomes dried without thermal storage system in the solar dryer was found significantly of poor colour value. Adopting this technique, dryer provided hot air at a temperature higher than the optimum value and resulted in light brown colour. Moreover, longer drying period using traditional shade drying contributed in producing dull colour. The dried product obtained using solar dryer with phase change material (PCM) was noticed brown, which was reasonably well in terms of colour. It happened due to the controlled drying, which took place below the optimum temperature and retained the important ingredients.
Texture of dehydrated rhizomes using phase change material was found better. Loss of texture was observed in dried product when dehydrating without thermal storage system. It was due to the development of rupture of cells at undesirable (higher) drying air temperature. Herb is heat sensitive and retaining its original value of valepotriates was really a challenge during drying. Using phase change material moisture dehydrated continuously, faster and uniformly at about 37°C which is below the optimum temperature (40°C) and retained important ingredients without volatile losses and hence, resulted in better texture.
4.5.2 Rehydration evaluation
It is the ability of dehydrated product to recover its appearance, texture and aroma when soaked in water. The closure the product goes to its original value or the initial moisture levels, the better will be its characteristics and rehydration capacity. Tissues damaged during drying affect the rehydration capacity and assess the rupture of cells. Using phase change material, rehydration capacity of rhizomes was 61.10% which was appreciably high. In comparison to this, rehydration capacity of rhizomes was 52.80%, 43.20% and 55.30% when drying was carried with SHS only, without thermal storage and under the shade, respectively. This was because dehydration took place continuously, uniformly and isothermally when drying was carried with solar dryer integrated phase change material and did not disturb the internal geometry much. But, by adopting drying without thermal storage and shade drying, moisture was removed discontinuously (variation in day and night temperature) and non-uniformly, and caused rupture of cells.
4.5.3 Bio-medical evaluation
Valepotriates of Valeriana Jatamansi rhizome are heat and moisture sensitive and are affected by the dehydration process. Volatile constituents are destroyed by heat, humidity and
longer drying duration. From the medicinal point of view, valepotriates were considered for analyses. Analyses showed that results of the solar dryer integrating with phase change material was better in comparison to other three drying techniques adopted as shown in the Table 6. It happened because using this method, drying was carried faster at controlled and isothermal temperature. Otherwise, temperature higher than the optimum value, higher humidity and longer drying duration helped in degrading quality level along with decomposition of volatile substances. Table 6 presents the comparison of essential bio-medical constituents. The future perspectives of the present investigation aim to identify the optimized process parameters using multi-criteria decision making methods such as VIKOR [51, 52], GRA [53], TOPSIS [54], PSI [55, 56].
Table 6: Qualitative analyses of Valeriana rhizomes dried by adopting different drying methods
Drying techniques Traditional Indirect solar Indirect solar Indirect solar drying
-^ shade drying drying forced drying forced forced convection (with
convection convection SHS in the collector and
i Valepotriates (%) (without thermal (with SHS in the PCM in the drying
i storage) collector) chamber)
Valtrate (%) 2.34 1.39 1.48 2.39
Didro-valtrate (%) 0.25 0.13 0.19 0.29
Ace-valtrate (%) 0.37 0.22 0.26 0.41
IVHD valtrate (%) 0.35 0.17 0.23 0.38
Total valepotriates (%) 3.31 1.91 2.16 3.47
5. Conclusions
During the present study, effect of drying air temperature, relative humidity, solar intensity and use of thermal storage materials on the drying characteristics of Valeriana Jatamansi rhizomes has been investigated. The performance of an indirect solar dryer in forced convection mode, integrated with phase change material has been found indispensable. The present set up enabled to maintain consistent air temperature inside the dryer. The rhizomes were dried from initial moisture content of 89% to saturated level of 9%. Following observations are made:
• Using present set up, average collector outlet air temperature was observed 51.22°C which remained 26.18°C above the average ambient temperature. So, collector supplied hot air to drying chamber at a sufficiently higher temperature and ensured higher moisture dehydration rate.
• Inclusion of phase material helped in supplying hot air till midnight, maintaining temperature 8-10°C above the ambient temperature and elongated the drying time after sunset by about 7 hours (5.00 pm - 11.30 pm) per day.
• Use of phase change material (paraffin RT-42) in the drying chamber helped in supplying hot air continuously to the rhizomes, below the optimum temperature of Valeriana (40°C). Indirect and isothermal drying below optimum temperature helped in retaining volatile substances without any loss.
• Analyses shows that using phase change material, drying time reduced by 37.50 %, and 64.29 % as compared to heat pump drying and traditional shade drying, respectively.
• Using phase change material higher drying rate, continuous heating and controlled temperature resulted in dried rhizomes of superior quality in terms of colour, texture, aroma and biomedical constituents. Best results of valepotriates were obtained with minimum drying hours. Total valepotriates achieved were 3.47%, whereas the same were 3.31% and 1.91% by adopting shade drying and solar drying without thermal storage, respectively.
• Rehydration capacity of rhizomes using phase change material was 61.10% in comparison to drying carried with SHS only, without thermal storage and under the shade, which were 52.80%, 43.20% and 55.30%, respectively.
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