Accepted Manuscript Solar PV powered mixed-mode tunnel dryer for drying potato chips
Mohamed A. Eltawil, Mostafa M. Azam, Abdulrahman O. Alghannam PII:
S0960-1481(17)30968-0
DOI:
10.1016/j.renene.2017.10.007
Reference:
RENE 9295
To appear in:
Renewable Energy
Received Date:
17 November 2016
Revised Date:
09 August 2017
Accepted Date:
02 October 2017
Please cite this article as: Mohamed A. Eltawil, Mostafa M. Azam, Abdulrahman O. Alghannam, Solar PV powered mixed-mode tunnel dryer for drying potato chips, Renewable Energy (2017), doi: 10.1016/j.renene.2017.10.007
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ACCEPTED MANUSCRIPT
Highlights
Solar PV powered mixed mode solar tunnel for drying potato chips. Effect of using and without using black thermal curtain above potato chips. Verification of different drying models for potato chips. Effect of different pretreatments and drying conditions on potato chips. Enhancing quality and safety of potato chips by solar PV powered tunnel dryer.
ACCEPTED MANUSCRIPT 1 1
Solar PV powered mixed-mode tunnel dryer for drying potato chips
2
Mohamed A. Eltawil1,3; Mostafa M. Azam1,4 and Abdulrahman O. Alghannam2 1Department
of Agriculture Systems Engineering, College of Agricultural and Food Sciences, King Faisal
University, P.O.Box 420 Al-Hofuf, Al-Ahsa, 31982, Saudi Arabia.
3 4
2 Bio-Environmental
Engineering, Natural Resources and Environmental Research Institute, King Abdulaziz City for Science and Technology, Saudi Arabia. 3Agriculture Engineering Department, Faculty of Agriculture, Kafrelsheikh University, Kafr ElSheikh Egypt. 4Agriculture
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Engineering Department, Faculty of Agriculture, Minoufiya University, Shibin El-Kom, Egypt.
Abstract: Solar PV system powered mixed-mode solar tunnel dryer (STD) for drying potato chips was studied. The STD was equipped with axial dc fan and flat plate solar air collector to enhance the thermal performance by maintaining a reasonable high temperature inside the drying chamber. The STD performance was evaluated without load and with load; and without and with using thermal curtain above potato slices during sunny days. Different airflow rates (2.1, 3.12 and 4.18 m3/min) and pre-treatments for potato slices were investigated. The PV powered STD exhibited the ability to produce chips with safe moisture level within 6 and 7 h for without and with using thermal curtain, respectively at airflow rate of 3.12 m3/min. The frying time of potato chips was shortened to be only 15 s. The best chips color was achieved with 1% sodium meta-bi-sulphite with using black thermal curtain above slices. Predicted and experimental moisture ratio of chips using developed STD were compared through several thin-layer drying models. The highest drying efficiency of 28.49 and 34.29% was recorded at airflow of 0.0786 kg/s in case of without and with using thermal curtain, respectively. The developed STD provides chips in good quality and suitable for rural areas. Keywords: drying efficiency, drying tunnel, modeling, photovoltaic system, potato chips, solar air collector.
1. Introduction Drying of agricultural food products by solar energy is cost-effective application. Industrial drying consumes large quantities of traditional fuels for providing hot air. Solar dryer is a simple and cheap device, run by renewable solar energy. Solar dryer has a significant potential in the agricultural sector, where it used for drying vegetables, fruits and medicinal plants. Thereby minimize dependency on sun drying and industrial drying, hence save huge quantities of fossil fuels [1].
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Corresponding author: E-mail:
[email protected];
[email protected] (M.A. Eltawil). Tel.: +966558956094; fax: +966 135801778.
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Nomenclature STD Acoll Adryer APV Cpa D dM/dt DR Efan Einput.coll. Einput.dryer Inscoll(t) Inshor(t) Imax InsPV Isc k Lv m (t) M M0 Me Mf Mi Mp mr MRexp,i MRpre,i Mt N Outputcoll Outputdryer Pfan r t Tcoll Tin Vmax Voc
solar tunnel dryer the solar collector area, m2
STD area, m2 area of solar module, m2 air specific heat, J/kg. °C the moisture dependent diffusivity, m2/s drying rate of potato slices at any time of drying, kg water/ kg dry matter. h drying rate of potato slices, kg water/ kg dry matter. h dc fan energy consumption, MJ insolation input on the collector, MJ incident solar energy on the STD, MJ the insolation on the collector at time t, W/m2 insolation on the horizontal surface at time t, W/m2 maximum PV current (A) for power insolation in the same plane of PV module, W/m2 current of short circuit, A drying constant, 1/s vaporization latent heat of moisture, kJ/kg airflow rate at t time, kg/s the moisture content, kg water/kg dry solids initial moisture content of potato, kg water/kg dry matter equilibrium moisture content of potato slices, kg water/kg dry matter final moisture content of dried potato, kg water/kg dry matter mass of potato slices before drying initial mass of potato slices to be dried, kg moisture removed from potato slices, kg moisture ratio measured experimentally, dimensionless predicted moisture ratio, dimensionless moisture content of potato at any time of drying, kg water/kg dry matter observations number collector output, MJ dryer output, MJ dc fan power, W the diffusion path, m time, s air temperature at the outlet of solar collector, °C air temperature at the inlet of solar collector, °C. PV voltage (V) for maximum power voltage of open circuit, V
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W0 Wd Wwet z
mass of potato sample at t = 0, kg mass of potato dry matter, kg mass of wet potato slices after drying in a STD, kg constants number
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Drying is a method used for food preservation, since it provides an extension of shelf life, compact (less space) and easy for transportation because of lighter weight [2-4]. The purpose of drying process is to produce dried products in good quality with minimum with minimum cost and maximum throughput, and to optimize these factors consistently [3]. Drying process improves the food stability, because it reduces moisture content to a safe level, reduces microbiological activity and keeps chemical and physical changes of the dried materials at minimum level during storage [5]. However, the long drying time is undesirable for economic reasons and because of the danger of contamination and spoilage of the product exposed to the open environment [6;7].
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The pretreatments of agricultural products before drying led to reduce undesired changes in color and texture, reduce drying time and produce good quality dried food. The commercially used pretreatments are potassium-metabisulphate, potassium carbonate, potassium and sodium hydroxide, ascorbic acid, methyl and ethyl ester emulsions [7; 8-11]. Blanching of fruits and vegetables by steam or hot water is used before thermal processing, drying and freezing in order to prevent changes in flavor during storage [11-14].
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Many solar drying systems have been developed, designed and evaluated as alternatives to the open sun drying in tropical and subtropical climates. These dryers are classified in a variety of ways such as [15]: direct dryers (natural convection), indirect dryers, forced circulation dryers and mixed mode. Greenhouse dryer is classified as direct solar drying and also sometimes mixed mode drying. Greenhouse crop drying have been reported by several studies [16-18].
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Potatoes ranks the fourth important vegetable product for human nutrition in the world [19]. Drying potato is considered a suitable method for storage, since it reduces the microbiological and physicochemical degradative reactions [20].
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Total potato production in Kingdom of Saudi Arabia is about 404679 Ton from total area of 16296 ha [21]. Kingdom of Saudi Arabia is characterized by hot-dry weather, therefore potato tubers either produced or imported is deteriorating fast. Hence drying plays an important role in keeping the tubers in the form of dried chips.
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Deep-fat frying that followed in preparation of food in either industries or home, should be avoid to get healthy fried snacks. During frying, the oil replaces the evaporated water from raw material and constitutes about 40% of fried product. The acrylamides that formed during frying is harmful and can be avoid by lowering the frying temperature [22].
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To optimize the drying process, mathematical modeling and simulation were carried out on agricultural products by several investigators to obtain the drying curves such as okra, carrot, pea, mushroom, jujubes, strawberry, white radish, apple pomace and red pepper [4;7; 13; 23-29]. Several researches are investigated the modeling of potato drying [19-20;30], blanching [11;31], and drying and rehydration characteristics [5;12].
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This study is devoted to develop, construction and evaluate the performance of an active mixed mode solar tunnel dryer (STD) powered by stand-alone solar photovoltaic (PV) system for small scale farms. The performance of dehydration system is evaluated without load (without product) and with load (single layer potato slices), different of airflow rates, and without and with using black thermal curtain above potato slices. The pretreatments, moisture content and color parameters of potato slices were used to evaluate the effect of various drying conditions on potato chips. The drying constant for potato chips was determine through several thin-layer drying models.
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2. Materials and Methods
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The experimental work was done at College of Agricultural and Food Sciences, King Faisal University, Al Ahsa (25°18' N Latitude, 49° 29' E Longitude), Saudi Arabia. The elevation is about 179 m above sea level. The climate of Al Ahsa is arid mostly characterized by hot and dry summer with cool and slightly wet winter. The Kingdom of Saudi Arabia lies entirely within arid and semi-arid dry land, with an annual rainfall ranging from 0 to 100 mm/annum.
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The solar PV system was used to operate active mixed mode STD for drying potato chips under different treatments and operating conditions. 2.1. Description of solar tunnel dryer
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The experimental setup consists of the following components: solar PV module, charge controller, battery, speed controller, DC axial fan, solar tunnel and flat plate solar collector. The design concept of the STD is to collect the solar thermal energy by a solar collector and direct insolation into the drying tunnel. The forced convection is used to pass the hot mass of air from solar collector into drying chamber. The potato slices (Diamont potato tubers) are heated up by direct and indirect absorption of insolation. The PV system is used to force the hot mass of air from the flat plate solar collector to the SDT through an insulated tube, hence the indirect heat is transferred to the potato slices to be dried. The experimental setup of PV powered STD is sketched as shown in Fig. 1.
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The PV module of ASEM (100 W, 18 V, 5.55 A) was mounted on inclined metal frame with 30º from the horizontal plane (Near by the latitude angle of the location). The PV module was used to charge a 12 V battery of 17 A.h through a charge controller (model: KT1220, 12 V/24 V, and rated charge & load currents 20 A). The charge controller was used to protect battery from over charging or deep discharging rate. The PV generated energy was stored to the battery in order to meet the load requirements. The battery was connected to a DC fan (12 V, 80 W) through a DC
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speed controller to control the drying airflow rate. Fig. 2a and b shows the complete experimental setup of solar PV powered STD.
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The DC fan was connected to one side of the even span type greenhouse which used as drying chamber. The drying tunnel (1 m x 2 m) has a perforated stainless-steel mesh which mounted at 25 cm from the bottom to carry the product to be dried.
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The even span tunnel dryer was made of transparent plexiglas sheets of 2 mm thick. The two roof slopes are of equal pitch and width. The longitudinal axis of tunnel dryer was oriented North-South and the roof slopes (equal pitch and width) were about 30 towards East and 30 towards West.
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54
58
3
0 12
0 20
15
10 0
50
25
10 0
30
120
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Fig. 1. Schematic diagram of the PV powered tunnel dryer.
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To investigate the effect of direct solar radiation on the drying time and quality of dried chips, a black thermal curtain was installed under the roof and above the product mesh to make a shade on potato slices throughout the drying tunnel as shown in Fig. 2b.
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The developed tunnel dryer is loaded by about 5 kg of raw potato slices. The initial total mass of potato tubers used in the experiments were divided between different treatments as 833 ±6 g. Potato slices covered about 92% of the perforated tray. The perforated tray porosity is 5.45mm and the space between slices is about 10 ± 5 mm. It should be noted that, there are some gapes kept between different treatments to avoid overlaps. By selecting the best treatment for drying these gapes can be reduced and the tunnel dryer can carry upto 6 kg of fresh peeled potato slices.
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To enhance the tunnel dryer performance a flat plate solar collector (1.2 m × 1.0 m × 0.20 m) was constructed and connected to the tunnel dryer to assist the drying air with an auxiliary heat. The absorber was made of galvanized corrugated sheet (1.5 mm thick) and painted with matt black paint. Wood shavings of 10 cm thickness was used to insulate the absorber plate from backside. A colorless glass sheet of 4 mm thickness was used to cover the absorber plate.
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A gap of 10 cm was kept between the absorber plate and glass cover to allow the air passage. The collector was inclined with 30º on horizontal and oriented towards south. The air intake into the solar collector through circular holes of 1.0 cm each formed in the front side casing of the collector. The heated air in the solar collector was passed to the tunnel dryer through an insulated tube of 15 cm diameter which connected to the tunnel dryer from backside. The DC fan was used to assist the forced convection between solar collector and drying tunnel. The PV module and solar collector were oriented due south to get the highest solar radiation. The tunnel dryer works as a mixed mode type, since solar radiation fall on the drying tunnel (direct heating) and hot air coming from solar collector (indirect heating). It should be mentioned that, the inlet air to the drying tunnel comes from solar collector through one circular opening made above the product drying net, while the suction fan was kept below the drying net. Therefore, the drying air flow mostly in one direction from inlet through drying net to suction fan.
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Drying of the potato chips was done in batch mode. The dehydration process starts at 8.00 am and continued until the reduction in weight of labeled samples had almost ceased. Three labeled samples were used to monitor the drying data.
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Temp. sensors
PV module Potato slices on the drying net
DC fan
(a)
Battery
Solar air collector Solar tunnel dryer
Speed controller
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PV module
Temp. sensors
Potato slices on the drying net
(b)
Battery
DC fan Speed controller
Black thermal curtain
Solar air collector
Solar tunnel dryer
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Fig. 2. Experimental setup of solar PV powered enhanced tunnel dryer for potato chips. a) without shading the potato chips, b) with shading the potato chips (black thermal curtain).
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2.2. Tunnel dryer performance (Experimental parameters)
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The developed experimental set up was placed outdoors to evaluate its performance. The STD was tested during sunny days without potato slices and with potato slices, without using thermal curtain and with using curtain above potato chips, three different air flow rates (2.1, 3.12 and 4.18m3/min) and different pre-treatments for potato slices.
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2.3. Preparation of potato slices The freshly potato cultivar (Diamont) was manually peeled and sliced into 1.2 ± 0.1 mm using an industrial slicer and treated as follows (Adapted from Eltawil et al.[32]):
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i. Treat. A: Blanching in boiling water containing 0.5% salt for 4 min, then dipping in cold water. ii. Treat. B: Blanching in plain boiling water for 4 min, then dipping in 1% sodium meta-bisulphite for 10 min. iii. Treat. C: Dipping in 1% sodium meta-bi-sulphite for 30 min. iv. Treat. D: Blanching in boiling water containing 2% vinegar for 4 min, then dipping in cold water. v. Treat. E: Blanching in plain boiling water for 4 min, then dipping in cold water. vi. Treat. Control: Washing with cold water thrice.
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To avoid error, all potato treatments were dried inside the dryer at the same conditions and in the same time. After dehydration process and to avoid rehydration before frying, all dried slices were kept in tightly closed polyethylene plastic sacks.
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2.4. Measurements All measurements were recorded at hourly interval.
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2.4.1. Insolation (W/m2): it was measured by pyranometer with handheld read-out unit (LP02 LI19) which set horizontally (INSh) on the same plane of tunnel dryer and incline in the same plane of PV module (INSPV) as well as solar collector.
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2.4.2. Temperature (ºC): it was measured by LM35 precision centigrade temperature sensors with the help of digital temperature indicators. The measured temperatures were as follow: ambient air temperature, Tamb (inlet to solar collector); collector outlet, Tcoll.exit (inlet to tunnel dryer); air below product mesh, Tbpm (average of three points); air above product mesh, Tapm (average of three points); product mesh, Tpm (average of three points) and tunnel dryer exit, Td.exit. Wet bulb temperatures were measured by calibrated copper-constantan thermocouples covered with wet cotton cloth outside the tunnel dryer, rhamb; inlet of dryer, rh d.in and outlet of the dryer, rhd.exit. Wet and dry bulb temperatures were measured and used to calculate the air relative humidity with the help of psychrometric chart. 2.4.3. Air flow rate (m3/min): it was measured with the help of Kane KM4003 airflow meter/Hot wire anemometer), also it was used to measure ambient wind speed (m/s). 2.4.4. Massing and moisture content of potato slices: The moisture content of the of potato slices during drying process was determined at 60min /30 min time interval by the difference in mass resulting from the weighing sample. Weighing the sample was performed using an electronic balance (1 kg capacity with resolution of 0.001g), reducing the moisture content was recorded by measuring weight during the drying process. The moisture content was expressed as percent wet basis, and then converted to gram water per gram of dry matter. Determination of initial and final water balance is achieved by using an electric air convection oven at 70 ±1°C until a constant mass [33].
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The moisture content was calculated on dry bases as follows:
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Initial moisture content
196
Final moisture content 𝑀𝑓 =
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At time interval 't', the moisture content 'Mt' of potato slices on wet bases is expressed as follows [34]:
𝑀0 =
𝑊0 ‒ 𝑊𝑑 𝑊𝑑
𝑊𝑤𝑒𝑡 ‒ 𝑊𝑑 𝑊𝑑
(1) (2)
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199
𝑀𝑡 =
(𝑊𝑡 ‒ 𝑊𝑑)
(3)
𝑊𝑡
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2.5. Drying rate of potato slices and overall efficiency of the developed dryer
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The difference in moisture content between potato slices and the equilibrium moisture content is an indicator to drying rate [35;36]. Experiments were conducted in single layer drying as expressed in the following equation.
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𝐷𝑅 =
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Energy input on the solar air collector is computed as [37;38]:
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𝑡 𝐸𝑖𝑛𝑝𝑢𝑡.𝑐𝑜𝑙𝑙 = 10 ‒ 6 × 𝐴𝑐𝑜𝑙𝑙∫0𝐼𝑛𝑠𝑐𝑜𝑙𝑙(𝑡)𝑑𝑡
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𝑑𝑀 𝑑𝑡
(4)
= ‒ 𝑘(𝑀𝑡 ‒ 𝑀𝑒)
Energy output from solar air collector (MJ) is given as: 𝑡 𝑂𝑢𝑡𝑝𝑢𝑡𝑐𝑜𝑙𝑙 = 10 ‒ 6 × ∫ 𝑚(𝑡) × 𝐶𝑝𝑎(𝑇𝑐𝑜𝑙𝑙 ‒ 𝑇𝑖𝑛)𝑑𝑡 0
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Thus, collector efficiency (ηcoll) is given as:
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𝜂𝑐𝑜𝑙𝑙 =
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𝑂𝑢𝑡𝑝𝑢𝑡𝑐𝑜𝑙𝑙
(5)
(6)
(7)
𝐸𝑖𝑛𝑝𝑢𝑡.𝑐𝑜𝑙𝑙
Solar energy input on the tunnel drying is given by: 𝑡 𝐸𝑖𝑛𝑝𝑢𝑡.𝑑𝑟𝑦𝑒𝑟 = 10 ‒ 6 × 𝐴𝑑𝑟𝑦𝑒𝑟∫ 𝐼𝑛𝑠ℎ𝑜𝑟(𝑡)𝑑𝑡 0
(8)
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Energy output from tunnel dryer is:
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𝑂𝑢𝑡𝑝𝑢𝑡𝑑𝑟𝑦𝑒𝑟 = 10 ‒ 3 × 𝑚𝑟 × 𝐿𝑣
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The amount of moisture (kg) removed (mr) from the potato slices were calculated by:
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𝑚𝑟 =
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The efficiency of the dryer (ηdryer) is
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𝜂𝑑𝑟𝑦𝑒𝑟 =
𝑀𝑝(𝑀𝑖 ‒ 𝑀𝑓) (100 ‒ 𝑀𝑓)
𝑂𝑢𝑡𝑝𝑢𝑡𝑑𝑟𝑦𝑒𝑟 𝐸𝑖𝑛𝑝𝑢𝑡.𝑑𝑟𝑦𝑒𝑟 + 𝑂𝑢𝑡𝑝𝑢𝑡𝑐𝑜𝑙𝑙
(9)
(10)
(11)
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The energy consumed by the DC fan is given by:
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Efan = 10-6 x Pfan x t
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The overall efficiency (ηoverall) of PV powered solar tunnel dryer is expressed as follows:
(12)
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𝜂𝑜𝑣𝑒𝑟𝑎𝑙𝑙 =
𝑂𝑢𝑡𝑝𝑢𝑡𝑑𝑟𝑦𝑒𝑟
(13)
𝐸𝑖𝑛𝑝𝑢𝑡.𝑐𝑜𝑙𝑙 + 𝐸𝑖𝑛𝑝𝑢𝑡.𝑑𝑟𝑦𝑒𝑟 + 𝐸𝑓𝑎𝑛
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The PV conversion efficiency (ηPV, %) is expressed as the ratio of PV maximum power (Pmax, W) to the total input solar power (Pin, W) at a given panel temperature. 𝑃𝑚𝑎𝑥
[
𝐼𝑚𝑎𝑥 𝑥 𝑉𝑚𝑎𝑥
]x 100
(14)
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𝜂𝑃𝑉 =
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The power output (Poutput, W) of PV module is given by: 𝑃𝑜𝑢𝑡𝑝𝑢𝑡 = 𝑉𝑜𝑐 × 𝐼𝑠𝑐
𝑃𝑖𝑛
=
𝐼𝑛𝑠𝑃𝑉 𝑥 𝐴𝑃𝑉
(15)
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The PV power output (Voc, V and Isc, A), as well as power consumption of DC fan [load voltage (VL, V) and load current (IL, A)] were measured by digital multimeter, Model of M890C+.
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2.6. Thin-layer drying models
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The kinetics of the drying process is described by thin-layer drying models. These models are presented in Table 1. The coefficients of the selected models are estimated by nonlinear estimation package (Statistica 6.0, Statsoft Inc., Tulsa, OK). The different models are verified by experimental data through determination coefficient (R2), mean relative percent error (P), reduced chi-square (χ2) and root means square error (RMSE). These statistic criteria are expressed as: Table 1 The studied models for thin layer drying curve. Model name Mathematical expression Newton MR = exp(-kt) Henderson and Pabis MR = a exp(-kt) Page MR = exp(-ktn) Logarithmic MR = a exp(-kt) + c Two-term MR = a exp(-k0t) + b exp(-k1t) Wang and Singh MR = 1 + at + bt2 Parabolic MR = a + bt + ct2
Reference [39] [40] [41;42] [6;43] [25;44] [26] [45]
248
MR N
249
R2 1
250
P
i 1
MRexp,i
2
pre ,i
_____ MR pre MRexp,i i 1 N
100 N MRexp,i MR pre,i MR N i 1 exp,i
2
(16)
(17)
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MR N
251
2
252
1 RMSE N
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i 1
MR pre,i
2
exp,i
(18)
Nz
MR N
i 1
pre ,i
MRexp,i
2
1
2
(19)
Based on highest R2, the least P, reduced χ2 and RMSE, the best model describing the drying behavior was selected. 2.7. Frying chips and color measurements The dried slices were fried for 15s in hot oil at 180 °C ±1 [46]. The Minolta colorimeter model of CR320 was used to determine and calculate the color parameters of chips with the help of CIE Lab (L*, a*, b* and h*). Where L* is the degree of slices lightness (black = 0, white = 100). The a* value is the chromatic redness parameter [ Positive (+) = red color, negative (-) = green color]. The b* value is yellowness chromatic parameter [Positive (+) = yellow color, negative (-) = blue color]. Hue angle h* is the angle in theta that a line joining the point in the hunter space with the origin makes with the horizontal axis and its value equal to 0, 90 and 180 would be red, yellow and green, respectively [47]. A hue value shift from 0 to 90 shows color change from red into yellow whilst a shift from 90 to 180 shows a color change from yellow into green. Values in between these represent blends of colors.
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The potato slices were placed in a standard light and the color parameters of chips (L*, a*, and b*) values were recorded. The Minolta colorimeter was standardized using a white calibration plate. Three sample slices of each treatment were taken and three observations were recorded at different positions of each chip, then the average value was considered.
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The hue angle (h*) value, chroma (C*) value, and the total color difference (ΔE) were calculated as:
274
b* h tan * a
275
C * a * b*
276
E L* L*o a * ao* b* bo*
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1
*
2
2
(20)
1
2
2
(21)
2
2
1
2
(22)
2.8. Experimental uncertainty: During the experiments, several parameters were measured in order to evaluate the developed drying system performance. Typical measuring errors were considered which may affect the accuracy of results. Errors and uncertainties in the experiments can arise from instrument selection,
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condition, calibration, environment, observation, reading and test planning. In drying experiments in solar dryer of the potato slices, the temperatures, velocity of drying air, relative humidity of drying air, the initial and final moisture content of potato slices, mass losses and solar radiation were measured with appropriate instruments. During the measurements of the parameters, the uncertainties that occurred by considering the individual factors were estimated according to [48] and presented in Table 2.
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Table 2.
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Uncertainties of the parameters during drying experiment of potato chips. Parameter Uncertainty in the temperature measurement Solar collector inlet temperature (Arises from LM35, digital temperature indicators, thermocouples and reading errors) Solar collector temperature (Arises from LM35, digital temperature indicators, thermocouples and reading errors) Solar drying tunnel inlet temperature (Arises from LM35, digital temperature indicators, thermocouples and reading errors) Solar drying tunnel outlet temperature (Arises from LM35, digital temperature indicators, thermocouples and reading errors) Ambient air temperature Uncertainty in the time measurement Mass loss values Temperature values Uncertainty in the mass loss measurement (Digital balance and reading) Uncertainty in the air velocity measurement (Anemometer and reading) Uncertainty of the measurement of relative humidity of air (thermocouples and reading) Uncertainty in the measurement of insolation (pyranometer and reading) PV module tolerance Volts Amps Uncertainty in PV module temperature Uncertainty in PV module efficiency
Unit
°C °C
Value Solar Dryer ±0.38 to ±0.58 ±0.38 to ±0.58 ±0.38 to ±0.58
°C ±0.38 to ±0.58 °C °C
±0.38
min min g
±0.1 ±0.1 ±0.14
ms-1
±0.14
RH
±0.14
V/(W/m2 ) % % % % %
±0.24 x 10-6
±0.3 0.011 0.01
±3.11 3.8
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Data analyses were carried out using SPSS (statistical package). Mathematical modeling of drying rates was calculated by the computer program (Data-Fit 8.1).
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3. Results and discussion
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3.1.
PV performance
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The PV module performance in the field (outdoor) is quite different from the performance at standard conditions. Fig. 3 shows the effect of ambient conditions on the PV module power output, PVoutput and its conversion efficiency with respect to time of the day during April 2014. It was found that, the PV power output and module temperature was directly proportional to incident solar radiation and ambient temperature. The daily average solar intensity on horizontal surface, Ins Hor, and on the PV module surface, Ins 30 deg, were 5.58 and 6.034 kWh/m2/d, respectively. The daily average PV module temperature and ambient temperature were 51.2 and 34.4 ºC, respectively.
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Temperature, 0C
T amb
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T PV
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PV efficiency, %
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PV output Fan P.cons 1 Fan P.cons 2 Fan P.cons 3
Load power at 2.10 m3/min air flow rates 3.12 m3/min 4.18 m3/min
Fan power consumption, W
140
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Wind speed, m/s
294 295 296 297 298 299 300 301
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Fig. 3. Variation of PV module input, output, its conversion efficiency; and power needed to run the DC fan at three air flow rates during sunny days in April 2014.
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The daily average energy output of PV module was 0.803 kWh/d, while daily average energy required to operate the DC fan was 0.274, 0.353 and 0.452 kWh/d at flow rates of 2.1, 3.12 and 4.18 m3/min, respectively.
308 309 310 311 312 313
The maximum power generated by PV module was recorded at noon time, and this may be due to increase of short circuit current with high rate and small drop in voltage. It should be noticed that, increasing PV module temperature causes an increase in the short circuit current, Isc and a decrease in the open circuit voltage, VOC. This increase in PV module temperature may occur due to high solar radiation heating, high ambient temperature and low wind speed; and consequently, low heat transfer from the PV module to the ambient.
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Due to thermal effect, the PV conversion efficiency (ηPV) is relatively high in the morning and afternoon compared to midday as shown in Fig. 3. This means, the ηPV is inversely proportional to module temperature. The daily average PV efficiency (ηPV) of 9.67% at average TPV of 51.2ºC was recorded for tested days. It was found that the energy generated by solar module was sufficient to operate the load (dc fan) during day time, and the excess energy saved in the battery to run the fan at little insolation.
320 321 322 323 324 325 326 327 328 329 330 331 332 333 334
3.2. Experimental drying curves and drying rate
335 336 337 338
Fig. 4 shows the moisture content of potato slices versus drying time. It is clear that, moisture content (drying rate) decreases by increasing time. Results of drying and data analysis indicated that, the drying process of potato slices takes place in two different stages. In the first stage (constant drying rate), the moisture removal is rapid because of the excess of moisture on the surface of the potato chips. Some of the heat transferred from the flowing drying air to the chips surface is used to evaporate the moisture from the surface of the chips. The other remaining heat is transferred to the interior of the chips causing an increase in temperature. The moisture from the interior of chips is transferred to their surface to replace the losses of moisture there. In the second stage (falling drying rate), after a specific drying time, the drying rate is reduced and the temperature of the chips is thereby increases. This behaviour strongly suggested an internal mass transfer type drying with moisture diffusion as the controlling phenomenon. The obtained results are in agreement with Akpinar et al. [30]. Hence, the experimental results can be interpreted by using Ficks diffusion model; 𝑑𝑀 𝑑𝑡
=𝐷
𝑑2𝑀 𝑑𝑟2
(23)
For potato chips, the first boundary condition indicated that the moisture is initially uniformly throughout the potato slices. The second implies that the mass transfer is symmetric with respect to the center of the slices. The third condition indicated that, the surface moisture content of the
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chips instantaneously reaches equilibrium with the conditions of the surrounding air. Similar results were obtained by several authors during drying various agricultural products [6;23;29;30].
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The initial moisture content values based on w.b. (d.b) of potato slices which dried with black curtain were 80.11% (402.76%), 80.56% (414.40%), 83.30% (498.80%), 82.40% (468.18%), 81.44% (438.79%) and 82.80 % (481.39%) for treatments A, B, C, D, E and control, respectively (Fig.4). While values of initial moisture content of potato slices which dried without black curtain were 82.89% (488.23%), 80.44% (411.25%), 80.17% (404.28%), 83.17% (494.17%), 81.87% (451.57%) and 83.00% (488.24%) for treatments A, B, C, D, E and control, respectively.
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The differences in initial moisture content of potato slices may refer to absorption of moisture during pretreatment process. The moisture content of potato slices decreased by increasing drying time and the dehydration process terminated when the moisture loss ceased. The final moisture content values based on w.b. (d.b) of potato slices which dried with black curtain were 8.3% (9.05%), 8.0% (8.7%), 8.85% (9.71%), 9.08% (9.99%), 8.2% (8.93%) and 9.04% (9.94%) for treatments A, B, C, D, E and control, respectively (Fig.4). While values of final moisture content of potato slices which dried without black curtain were 8.52% (9.31%), 7.21% (7.8%), 8% (8.7), 8.5% (9.29%), 8.28% (9.03%) and 9.12% (10.03%) for treatments A, B, C, D, E and control, respectively.
356 90
70
Treat. B
60
Treat. C
50
Treat. D
40
Treat. E
30
Control
20
Without black thermal curtain
80
Treat. A
Moisture content (w.b) %
Moisture content (w.b), %
90
With black thermal curtain
80
10
Treat. A
70
Treat. B
60 50
Treat. C
40
Treat. D
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Treat. E
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Control
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Drying time, h
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Drying time, h
Fig. 4. Effect of different treatments of potato slices and drying modes on the moisture content. 357 358 359 360 361 362 363 364 365 366 367
The average drying time required for potato slices in the developed drier was varied from 300 to 390 min to reach safe moisture content (equilibrium moisture content) in case of without and with using thermal curtain. As shown in Fig. 4, the Treat. B was reached the equilibrium moisture content (EMC) at 300 min from the beginning of the system operation; treatments A, D and E were reached the EMC at 330 min, while treatments C and control were reached the EMC at 360 min in case of drying without using black thermal curtain. The same trend was followed in case of using black thermal curtain, but the EMC was varied from 330 min to 390 min. It should be mentioned that, there is no specific set point, because the drying behaviour inside the drying tunnel varied according to several parameters such as ambient weather conditions and pretreatments of potato slices etc.
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Fig. 5 indicated that when the moisture content was high the drying rate was highest for all treatments, then decreased rapidly until all reached similar rate after 300 min of drying. This was due to the unbound moisture near the surface of potato slices which removed early in the dehydration process. The initial drying rate of slices blanched treatment was higher than that of the un blanched slices. This indicates that blanching has effect on the drying rate of potatoes due to the effect of starch gelatinization during blanching. Therefore, blanching treatment caused a reduction in the drying time when compared with the untreated or control samples. According to the results in Figs. 4 and 5, pretreatments have a dominant effect on the drying time. The samples of treatment B were found to have a shorter drying time compared to another samples and control. The longest drying time was recorded for treatments C and control (un blanched samples). This behavior may be because blanching treatment leads softening the potato slices, which facilitated the moisture removed. Similar findings were reported in the dehydration process of potatoes by Severini et al. [49].
With black curtain
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20
Treat. A
20
Drying rate, (kg/kg.h)
Without black curtain
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18 16
Treat. B
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Treat. C
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Treat. D
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Treat. E
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Control
10 8
Drying rate, (kg/kg.h)
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Fig. 5. Drying rate of potato slices versus solar drying time in case of with and without black thermal curtain. 384 385 386 387 388 389 390 391 392 393 394
3.3. Drying rate modeling The computer program of Data-Fit 8.1 was used to calculate coefficient of determination (R2) and root mean square error (RMER) and the results are given in Table 3. Data of moisture content for the different pretreated potato slices were converted to a moisture ratio then fitted against the drying time. As shown in Table, the Two Term model, Henderson & Papis and Page models provided the best R2 value and lower 2 for potato slices. Therefore, they were selected to represent the solar drying behavior for thin-layer drying of potato slices. Multiple regression analysis was carried out using Data-Fit 8.1 and SPSS to study the effect of drying variables on the Two Term and Henderson & Papis models constants. The constants k (min-1) and n (dimensionless) were regressed against both of drying air temperature and air
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velocity. Different combinations of drying variables (parameters) were tested through regression analysis, and the best combinations which gave the highest R2 were finally included in the model.
Table 3 Empirical models, coefficients, determination coefficient (R2), chi-square (2) and root mean square error (RMSE) for different treatments of dried potato. Modal
Newton
Page
Henderson & Pabis
Two term
Wang and Singh
Parabolic
Logarithmic
403
Pretreatment A B C D E Control A B C D E Control A B C D E Control A B C D E Control A B C D E Control A B C D E Control A B C D E Control
n
a
b
Coefficients c
0.9395 0.7826 0.8464 0.8410 0.9336 1.0311 0.9900 0.9520 0.9714 0.9695 0.9858 1.0067 1.0039 0.2842 0.6663 0.1277 0.1351 0.5034 -0.1862 -0.2147 -0.1724 -0.1852 -0.1579 -0.1695 0.9845 0.9388 0.9762 0.9730 0.9761 0.9832 -49.89 0.8697 0.8544 0.8635 0.9355 1.0290
0.0004 0.7216 0.3481 0.8834 0.8675 0.5030 0.0105 0.0135 0.0097 0.0108 0.0079 0.0084 -0.1803 0.01006 -0.1915 0.0117 -0.1634 0.0090 -0.1750 0.0100 -0.1489 0.0072 -0.1632 0.0079 50.73 0.1316 0.1567 0.1481 0.0618 -0.0273
k 0.2092 0.2488 0.1817 0.1985 0.1750 0.2016 0.2304 0.3442 0.2345 0.2567 0.1958 0.1916 0.2068 0.2350 0.1751 0.1911 0.1719 0.2031
K0
0.2202 0.0724 0.3222 -0.0124 0.5038 0.2034
-0.0016 0.3573 0.2642 0.2872 0.1971 0.1916
K1
-0.5005 0.4286 0.0627 0.2807 0.1534 0.2031
R2
2
RMSE
0.992 0.974 0.981 0.981 0.997 0.996 0.994 0.995 0.992 0.992 0.999 0.997 0.993 0.978 0.983 0.983 0.998 0.997 0.998 0.999 0.995 0.997 0.999 0.997 0.996 0.969 0.985 0.987 0.994 0.995 0.997 0.977 0.987 0.989 0.995 0.996 0.886 0.999 0.995 0.998 0.999 0.996
0.0006 0.0020 0.0013 0.0013 0.0018 0.0003 0.0005 0.0004 0.0006 0.0006 0.0020 0.0003 0.0006 0.0018 0.0013 0.0013 0.0019 0.0003 0.0002 0.0001 0.0004 0.0002 0.0030 0.0004 0.0003 0.0025 0.0011 0.0010 0.0027 0.0004 0.0003 0.0053 0.0011 0.0009 0.0029 0.0004 0.0112 0.0001 0.0004 0.0002 0.0026 0.0003
0.0224 0.0422 0.0343 0.0349 0.0400 0.0161 0.0202 0.0191 0.0228 0.0223 0.0405 0.0153 0.0221 0.0383 0.0324 0.0328 0.0396 0.0158 0.0265 0.0077 0.0167 0.0123 0.0439 0.0158 0.0148 0.0458 0.0299 0.0283 0.0470 0.0190 0.0135 0.0623 0.0284 0.0262 0.0461 0.0178 0.0904 0.0093 0.0171 0.0123 0.0438 0.0153
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Based on the multiple regression analysis done, the accepted model constants of Two Term and Henderson & Papis were as follows: MR (TT) = a. exp (-k0.t) + b exp (-k1 t) R2=0.989 (24) 2 MR (HP) = a. exp (-k. t) R =0.997 (25) The Two Term, Henderson & Papis and Page models were validated by comparing the estimated and predicted moisture ratio (MR) at any particular drying condition. The validation of the Two Term, Henderson & Papis and Page models for different treatment are shown in Fig. 6. As shown, the predicted data banded around the straight line which indicate the suitability of the Two Term, Henderson & Papis and Page models in describing the dehydration behavior of the potato chips. The effect of pretreatments of slices on the models’ constants were analyzed using multiple regression and included in Table 3. The different parameters (configurations) that gave the higher R2 were included in the best models. Therefore, with the help of experimental boundary conditions, the moisture content of the potato chips at any time during the dehydration process could be estimated. The non-linear regressions were used to fit drying curves to the data based on the seven drying models, namely, the simple (Newton); Page; Henderson & Pabis; Logarithmic; the Two-Term; Wang & Singh and Parabolic models. Table 3 represents the coefficients R2, the RMSE and the 2 for the seven models. Except Newton, logarithmic and Wang & Singh models, all fitted curves matched well with the experimental data and R2 values were higher than 0.97. However, the R2, RMSE and 2 for these models were always significantly different to the corresponding values for the other models. Hence, these models were not suitable to describe the drying curves of chips for all treatments. Based on these results, the Two-Term, Henderson & Pabis and page were chosen as the best models to represent the dehydration process of potato chips. Mathematical modeling of drying is crucial for the optimization of operating variables and performance enhancements of the drying systems. For all treatments, Henderson & Pabis, Two-Term and Page models predicted moisture contents well and matched the experimental values. These results were similar to those of Ceylan et al. [51] for tropical fruits. 3.4. Thermal and drying efficiencies Fig. 6 shows the variations of thermal and drying efficiencies of the collector and tunnel dryer with air mass flow rate in case of without and with using black thermal curtain. The highest values of air temperature inside the drying tunnel and at outlet of solar collector were recorded with lower airflow rate of 0.0572 kg/s, while minimum values of temperature were recorded with the higher airflow rate of 0.0998 kg/s. It is clear that the efficiency of the collector and tunnel dryer are dependent on the airflow rate and this refer to decrease of thermal losses to the environment. The best drying efficiency was recorded at 0.0786 kg/s in case of using and without thermal curtain.
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Thermal efficiency
Drying efficiency
0.0998
0.0572
60
Efficiency, %
50 40 30 20 10 0 0.0572
445 446 447 448 449 450 451 452 453 454 455 456 457
0.0786
0.0998
Without black thermal curtain With black thermal curtain Air flow rate, kgs -1
Fig. 6. Variations of thermal and drying efficiencies with airflow rate. Table 4 shows the thermal, drying and overall efficiencies of the developed drying system at different airflow rates. The highest drying efficiency of 28.49 and 34.29% was recorded at 0.0786 kg/s in case of without and with using thermal curtain, respectively. While, the highest overall efficiency of the developed drying system was 16.71 and 19.07% at the same mentioned conditions. Table 4 The calculated thermal efficiency, drying efficiency and overall efficiency of developed dying system at different airflow rates.
With thermal curtain
Without thermal curtain
Treatment
458 459 460
0.0786
Air flow rate (kgs-1)
Collector Energy input (MJ)
Energy output (MJ)
0.0572
15.12
0.0786
Dryer
Overall Efficiency (%)
Efficiency (%)
Energy input (MJ)
Energy output (MJ)
Efficiency (%)
6.23
41.20
22.32
6.77
30.33
17.82
17.71
8.57
48.39
23.04
7.90
34.29
19.07
0.0998
18.14
9.43
51.98
24.84
7.97
32.08
18.16
0.0572
15.03
5.40
35.93
22.46
5.87
26.14
15.65
0.0786
15.94
7.14
44.79
23.76
6.77
28.49
16.71
0.0998
20.22
11.60
52.20
28.51
7.00
24.55
14.05
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3.5.
Appearance and Color
Color is a dominant quality attribute in dried potato chips. In order to facilitate the comparison of color and appearances of the chips, a digital picture was taken for each dried chip. Fig. 7 shows the pictures of different treatments after drying. The quality of the potato slices dried by using black thermal curtain is higher than that dried without black curtain as the natural color and the appearance are maintained more under shading. The slices treated with sodium meta-bi-sulphite solution (treat. B) provided better color for drying. Generally, the appearance and color of the dried chips were preserved by treating with sodium meta-bi-sulphite solution in comparison with the other treatments. For measuring color of foods, the L*, a*, b*, h* color space was used because of the uniform distribution of colors as well as very close to human perception of color [51]. The values of L* for all dried potato treatments before frying ranged between 43.65 (Treat. B) to 51.19 (Control), but after frying the values showed a large changing comparing before frying treatment for all drying treatments where ranged between 43.08 (Treat. B) to 57.45 (Control) in case of using black curtain. Control samples were recorded the highest value of L* at all drying conditions, while samples of treatment B recorded the lowest value at all drying conditions as shown in Table 5. The a* values for potato chips under different treatments were low which indicating a tendency of slices to have more of a greenish color rather than red. The overcooked potato chips had a red color. The higher values of a* mean that more Maillard reactions are occurring [52].
With using black thermal curtain
Without using black thermal curtain
Fig. 7. Color differences referred to different treatments of potato slices after drying with and without using black thermal curtain. 481
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The yellowness of potato chips is desirable after frying and is specified by the parameter b* in color measurement. The higher b* values give more yellow potato chips [53]. Table 5 shows that all values of b* after frying were higher than before frying. These results are in agreement with those obtained by Abdulla et al. [54]. The treatment B recorded the highest values of b* (more yellowness) before and after drying as well as before and after frying compared to other treatments under the two drying modes. Table 5
489
Variation of potato chips color parameters which dried in the tunnel dryer with and without using
490
thermal curtain. Drying with black thermal curtain Treatment
491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511
Before frying
Drying without black thermal curtain
After frying
Before frying
After frying
L*
a*
b*
h*
L*
a*
b*
h*
L*
a*
b*
h*
L*
a*
b*
h*
A
46.96
7.76
19.05
68.41
45.78
10.46
25.89
71.52
42.04
4.62
19.71
69.75
56.63
9.08
27.42
70.76
B
43.65
7.01
20.03
69.75
43.08
12.51
29.30
68.43
35.57
6.81
22.19
75.48
51.70
7.49
29.74
69.72
C
47.03
7.54
16.66
68.13
57.21
9.75
23.47
67.46
49.78
22.22
72.02
6.66
18.68
68.93
48.81
8.24
24.72
71.77
43.97
67.58 69.48
7.79
43.7
15.02 17.38
64.56
D
10.89 5.28
60.03
8.72
26.97
72.64
E
44.65
6.46
19.72
68.96
45.71
10.32
26.11
67.88
39.53
12.55
20.77
68.63
52.03
8.76
28.52
72.70
Control
51.19
7.33
15.87
67.30
57.45
7.78
23.06
66.88
50.70
7.28
13.30
65.33
77.61
9.49
25.68
76.06
Hue angle (h*) values, of all the potato chips samples were above 67o, showing a very clear transition from red into yellow. This indicates the development of golden brown color which is highly preferred by fried potato chips consumers [55]. 4. Conclusions The tunnel dryer performance was enhanced by using solar PV power and flat plate solar collector. Adding a small dc fan powered by a PV system improved the airflow which enhanced drying rates. The developed dehydration system was used for drying potato chips in order to improve the chips quality and stability, since it reduces the moisture content, reduces microbiological activity; and reduces chemical and physical changes during its storage. Different pretreatments were carried out on potato slices before drying process. Blanched potato slices had higher drying rate compared to un blanched samples and control. The dehydration process carried out with and without (mixed mode) using black thermal curtain above potato slices. The quality of the dried potato chips with using black curtain and sodium meta-bi-sulphite solution is high in terms of color and appearance as compared to chips dried without using black curtain and other treatments. The highest drying efficiency of 28.49 and 34.29% was recorded at air flow rate of 0.0786 kg/s in case of without and with using thermal curtain, respectively. A suitable drying model was developed by combining the effect of studied variables on potato dehydration process. To interpret the drying behavior of potato chips, seven different thin-layer dehydration models were compared according to their R2, 2 and RMSE values. According to the results of thin-layer modeling, Henderson and Pabis, page, two-term and logarithmic models have
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shown a better fit to the practical data as compared with the other models and it would be a useful engineering application in designing and optimization of the PV powered tunnel dryers. The PV system operated solar tunnel dryer has the advantage that it can be operated independent of electrical grid. Such kind of dryer can be used for a small-scale industrial production of quality dried chips. Also, it can be used to dry fruits, vegetables and medicinal plants. The physicochemical properties of potato chips under different pretreatments, drying mode, packaging and storability will be discussed in another publication.
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Acknowledgement The authors acknowledge the Deanship of scientific research at King Faisal University for the financial support under grant 150018.
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