Comparison of energy parameters in various dryers

Energy Conversion and Management 87 (2014) 711–725

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Energy Conversion and Management

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Comparison of energy parameters in various dryers

http://dx.doi.org/10.1016/j.enconman.2014.07.0120196-8904/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +98 9121304520; fax: +98 21 4892200.E-mail address: [email protected] (S. Minaei).

Ali Motevali, Saeid Minaei ⇑, Ahmad Banakar, Barat Ghobadian, Mohammad Hadi KhoshtaghazaDepartment of Agricultural Machinery Engineering, Agricultural Faculty, Tarbiat Modares University, Tehran, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 March 2014Accepted 2 July 2014

Keywords:Energy efficiencySpecific energy consumptionDrying efficiencyThermal efficiencyRoman chamomile

This study aimed at investigating the energy output, thermal output, drying efficiency and specific energy invarious drying methods for drying of chamomile. These methods included convective, infrared, convective–infrared, microwave, microwave–convective, microwave–vacuum, vacuum, and hybrid photovoltaic–thermal solar (with/without heat pump). Results of data analysis showed that the highest energy outputof 49.99% belonged to the microwave dryer, while the lowest 1.41% belonged to the vacuum dryer.Moreover, the maximum and minimum thermal outputs being 78.22% and 2.68% were associated withthe vacuum–microwave and vacuum dryers, respectively. In the hot-air-related dryers, drying efficiencyincreased with temperature. In microwave-related dryers, however, drying efficiency first increased(up to a microwave power of 300 W) and then decreased with further increase in microwave power.Assessment of the specific energy requirement in various dryers showed that the highest and lowest valuesbelonged to the vacuum and microwave methods with 318.42 and 4.32 MJ/kgw, respectively. Additionally,results of analysis indicated that adding a heat pump to the photovoltaic solar dryer increases dryingefficiency, energy output and thermal output, while it reduces the required specific energy.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Many countries have embarked on conducting research toclearly identify their own flora and medicinal plants from othercountries [1]. An important medicinal plant indigenous to Iranis the Roman chamomile (Chamaemelum nobile). This plant isantipyretic, analgesic, energizer, anti-insomnia, anti-anorexia,and anti-enteritis, as well as a medicine for anemia, gastrointesti-nal helminthes and so forth [2]. Energy consumption and dryerefficiency are among the most important criteria for the dryingof agricultural crops and food products as well as medicinal plants[3,4]. Therefore, it is essential to apply economical and high-qualitydrying practices for such materials, especially medicinal plantswhich are sensitive products.

Challenges to the direct use of sunlight for drying purposes haveforced practitioners to replace traditional methods with industrialdryers. However, industrial dryers are characterized as highlyenergy-consuming [3,4]. Alternative methods need to be evaluatedfor drying agriculture material in an efficient manner. Beforedescribing the experimental procedure using various dryingmethods, an overview of the previous applications of thesemethods is presented below.

Hot air drying is one of the most common methods in drying ofagricultural products, food stuff and medicinal plants. Disadvan-tages of using hot air drying include considerable losses of thermalenergy and low thermal conductivity, case hardening of thematerial, relatively long drying times as well as poor quality andshrinkage of the dried product. The most important limitation ofhot air drying systems is their low drying efficiency due to convec-tive losses in heating and transportation of the working fluid [5,6].Such disadvantages highlight the need to use alternative technolo-gies for drying agricultural materials.

Infrared radiation is a form of electromagnetic radiation absorp-tion of which causes heat vibration in foodstuff and agriculturalproduce. IR drying is one of the best methods for thin layer dryingof agricultural material [3,4]. Distinct advantages of this methodsover conventional hot air drying include the production of highquality products, high energy efficiency, high heat transfer ratesand reduced drying time [7,8]. Researchers have stated some ofthese advantages as: low energy consumption and improveddrying efficiency [9,10], equipment used is compact [10–12], highefficiency of heat transfer through air or vacuum [10–12], andimproved hygiene [13]. IR rays penetrate the foodstuff and byheating it overheating and remove moisture from the material.However, penetration depth of infrared rays in the body of food-stuff is limited. Disadvantages of using IR drying are high operatingcosts and drying of material with low thickness like outer layersand coatings on foodstuffs. In order to resolve this problem, a

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Nomenclature

A tray area (m2)Ca specific heat (kJ/kg �C)Cm material specific heat (kJ/kg K)cosh impact powerCp air specific heat at constant pressure (kJ/kg K)D weight density (kg/m2)E energy (kJ)Efan energy consumption of fan (kJ)Eheat pump energy consumption of heat pump (kJ)EUmec mechanical energy consumption (kJ)EUter thermal energy consumption (kJ)hfg latent heat of vaporization (kJ/kg)I electric current (A)K lamp power (W)L nominal pump power (kW)M0 final product moisture content (w.b.%)Mi initial products moisture content (w.b.%)Mp particle moisture content, dry basis (kg water/kg

solid)Mw weight of loss water (kg)P microwave output power (kW)Pvs saturated vapor pressure of air (kPa)

Qm energy for the material heating (kJ)Qw energy for the moisture evaporation (kJ)RH relative humidity (%)SEC specific energy consumption (kW h/kg)T temperature (�K)t total time for drying each sample (h)Tm1 inlet material temperature (K)Tm2 outlet material temperature (K)U voltage (v)v velocity (m/s)W weight loss (kg)w humidity ratioWair drying air flow rate (m3/s)Wd weight of dry material (kg)Ww weight of moist material (kg)DP different pressure (mbar)DT temperature difference (�C)qa air density (kg/m3)wR uncertainty in the resultw1, w2, . . . , wn uncertainties in the independent variablesx1, x2, x3, . . . , xn independent variables

712 A. Motevali et al. / Energy Conversion and Management 87 (2014) 711–725

combination of infrared radiation and hot air flow has been pro-posed [14]. Here, heat transfer takes place simultaneously throughhot air flow and infrared radiation. In other words, while heatingthe sample surface by infrared radiation, hot air flow increasesthe heat transfer and removes the evaporated moisture [3,4].

More than 40 years ago Ginsburg reported that the penetrationability of hot airflow and IR radiation into agricultural produce andfoods is quite lower than that of microwaves. Thus, the use ofmicrowave technology in drying of food products has been devel-oped thanks to its better penetration capability and higher speedas well as lower energy consumption [15]. Microwave drying com-pared to conventional hot air drying and infrared-radiation dryingis performed in a shorter time and drying operations can be under-taken with lower energy expenditure [16,17]. Unlike conventionalheating systems, due to the penetration of microwave intofoodstuff, heat is spread throughout the whole food causing thetemperature to increase. Also, energy transfer in this method isnot affected by transmission constraints particularly for viscousmaterials [18]. That is why the microwave method is fasterthan other methods. Use of microwaves has problems such asnon-uniform heating and tissue damage. To alleviate this problem,microwave can be combined with other heating methods.

In vacuum drying, due to the lack of oxygen in the chamber andreduction of undesirable reactions in food and agricultural prod-ucts, quality of dried food will be higher than other methods[3,4]. Applying vacuum to the drying process provides expansionof air and water vapor through the material and this causes puffingof the foodstuff. These factors lead to swelling of foodstuff due toincreased volume and surface which causes higher heat transferbetween matter and drying air [19]. One of the disadvantages ofdrying agricultural products and foodstuffs under vacuum is highenergy consumption due to which, drying under vacuum is mainlyused for sensitive and high value-added products [3,4].

Microwave–vacuum method is regarded as having all theadvantages of dielectric heating due to applied vacuum and worksat a temperature lower than the microwave method. This combina-tion method has the advantages of both vacuum drying andmicrowave drying methods and can improve energy efficiencyand product quality [20,21].

Microwave–convective dryer uses the penetration capability ofmicrowaves to increase the pressure gradient and thereforefacilitates moisture movement from inside the product towardsits surface. With moisture leaving the matter, the hot airflowpassing over the product displaces moisture increasing the dryingrate. In this method, the hot airflow acts to remove moisture andminimize the effect of boundary layers, as the heating or coolingagent (depending on the temperature of the hot inlet air).

In order to prevent moisture congestion and to increasemoisture conveyance capacity around the drying product, it isnecessary to have hot air [22–26].

Drying is a process with high energy consumption compared toother production processes. Statistics [27] show that in Canada,USA, France and UK, drying consumes 10–15% of the total nationalindustrial energy demand as well as 20–25% of the total nationalindustrial energy demand for Denmark and Germany.

Energy crisis is a major concern of human societies, making itessential to find a solution for using everlasting, sustainable, safeand renewable energy sources. Solar dryers are gaining interest,especially, wherever abundant solar energy is available. Like manyother medicines and chemicals, medicinal plants are sensitive todirect light and heat, and should not be dried using direct sunlightor high temperatures. High drying temperature and direct sunlightinflict negative effects on dried products [28]. Therefore, quickdrying and constant temperature are essential in temperature-sensitive materials, such as chamomile [29]. Thus, a hybrid photo-voltaic/thermal solar dryer equipped with a heat pump wascompared with other dryers.

Solar dryers can save more energy compared to other industrialdryers since they use the available solar energy. Moreover, lowerdrying times and costs, space-efficiency, higher product quality,environmental-friendliness, less CO2 emission, and higherefficiency are among their advantages [30–32].

Various investigations have been conducted to study the spe-cific energy consumption, as well as drying and thermal efficien-cies of different drying methods. These include: carrots slices[33–35], agar gel and Gelidium seaweeds [36] garlic cloves [37],pistachios [25], food and non-food [23], porous media [38], nettleleaves [39], paper pulp [40], banana slices [41,42], moist particles

A. Motevali et al. / Energy Conversion and Management 87 (2014) 711–725 713

[43], berberis fruit [44], pomegranate arils [3], solar cabinet dryer[45], heat pump dryer [46], mushroom slices [4,47,48], red pepper[49], corn [50], coroba Slices [51], solar heat pump dryer [52,53],fruit leather [52–54], paddy drying [55], barely [56], organicpigments [57], peach [6], lactose powder [58], porous medium[59], vegetables [60], longan [61], apple slices [62] and saffron[63]. However few studies have focused on the use of different dry-ing methods and their effect on the specific energy consumption,dryer efficiency, energy efficiency and thermal efficiency of thedrying process.

The main objective of this research was the comparison ofspecific energy consumption, dryer efficiency, energy efficiencyand thermal efficiency in drying of Roman chamomile usingvarious drying methods including: hot air, IR, hot air–IR, micro-wave, microwave–vacuum, microwave–hot air, vacuum and solar.Four drying technologies of hot air convection, infrared radiation,vacuum and convection–infrared were applied.

2. Materials and methods

All the required plant materials for drying experiments wereacquired from a local farm and the flowers were separated fromstem. Then, the samples were packed inside separate plastic bagsand were refrigerator at 4 ± 1 �C. In this state the total moisturecontent of the flowers was about 83% on wet weight basis. Duringthe drying experiments, mean range of ambient temperaturevariation was 30 ± 2 �C and mean relative humidity was 28 ± 3%.

Air parameters were adjusted by measuring temperature andvelocity using a thermometer (Lutron, Taiwan), anemometer(Anemometer, Lutron-YK, Taiwan) and humidity meter (Testo650, 05366501, German). A digital balance (AND, model EK600i,Japan, ±0.01 g) was used for weighing the samples. Also, a solarpower meter (TES 1333R, Taiwan) measured the solar radiationintensity on the collector surface, during the tests. A pressuregauge (PVR 0606A81, Italy) was used to measure the innerpressure of the oven (vacuum and microwave–vacuum dryer)and calibrated with vacuum measuring device, PVR.

Experiments were conducted with Roman chamomile in themicrowave dryer (ME 3410W, Samsung, Thailand) were performedat ten power levels (100, 200, . . . , 1000 W) and in the vacuumdryer (Memmert GMBH D-91126, Germany) at four temperatures(40, 50, 60 and 70 �C) and four absolute pressure (25, 250, 500and 750 mbar). In the microwave–vacuum dryer four levels ofmicrowave power (130, 260, 380 and 450 W) and four levels ofabsolute air pressure (25, 250, 500 and 750 mbar) were used. Alsoexperiments in the microwave–convective dryer were performedat constant air velocity (1 m/s), 3 power levels (100, 200 and300 W) and two air temperature (40 and 50 �C).

Experiments were conducted in the infrared dryer at constantair temperature (30 �C), three radiation intensity levels of 0.49,0.31 and 0.22 W/cm2, and three levels of air velocity (0.5, 1 and1.5 m/s); in the hot air dryer at three temperatures (40, 50,60 �C) and three air velocities (0.5, 1 and 1.5 m/s). The experimentswere also performed in combined convective–IR dryer at threelevels of radiation intensity (0.49, 0.31 and 0.22 W/cm2), three airtemperatures of 40, 50 and 60 �C, and three air velocity levels(0.5, 1 and 1.5 m/s).

In the hybrid photovoltaic/thermal solar dryer, three levels ofair temperature (40, 50, 60 �C), three levels of air velocity (0.5, 1and 1.5 m/s), with and without the heat pump were used.

A laboratory microwave–vacuum dryer (consisting of a KawakeAirvac JP-120 h vacuum pump, and an AEG Micromat 725microwave oven (Germany) having 1200 W nominal power,2450 MHz frequency and internal chamber dimensions of23 � 32 � 36 cm) was used for microwave–vacuum drying. All

dryer are shown in Fig. 1. System absolute pressure was measuredusing a PVR VT1 NP vacuum gauge (Italy). Finally, for rotating thesample chamber inside within the microwave device, a BUCHIRE120 Rotary (Switzerland) was used for uniform dispersion ofwaves, condensation of vapor and prevention of increase in cham-ber pressure. A digital balance (AND, model EK600i, Japan, ±0.01 g)was used for weighing the samples.

Drying of medicinal plants is a temperature-sensitive process.Direct use of sunlight and high temperature of drying process leadsto the destruction of plant essential oils. Thus drying treatments indifferent dryer were determined using references number [2].

2.1. Energy consumption

2.1.1. Convective dryerEnergy consumption in the convective dryer is obtained from

the sum of energy consumed by the heaters (thermal energy)and the blower (mechanical energy). Thermal energy consumedby the heaters was calculated using Eq. (1) [3,4,40]:

EUter ¼ ðA � m � qa � Ca � DTÞ � 3600 ð1Þ

where qa was calculated as [64,65]:

qa ¼101:3250:287T

ð2Þ

Specific heat capacity of the inlet air in the convective dryer wascalculated using Eq. (3) [65]:

Ca ¼ 1009:26� 0:0040403ðT � 273:16Þ

þ 0:00061759ðT � 273:16Þ2

� 0:0000004097ðT � 273:16Þ3 ð3Þ

Eq. (4) was generally used to convert the relative humidity tohumidity ratio of the inlet air [65,66].

RH ¼ 101:3w0:62189Pvs þwPvs

ð4Þ

Pvs ¼ 0:1 exp 27:0214� 6887T� 5:31 ln

T273:16

� �� ð5Þ

The mechanical energy consumed by the blower was calculatedusing Eq. (6) [40]:

EUmec ¼ DP �Mair � t ð6Þ

2.1.2. IR and convective–IR dryerThese dryers exploit three energy sources to meet their energy

demands, including energy from the IR-lamps, heaters (thermalenergy), and energy consumed by the blowers (mechanicalenergy). Thermal energy consumed by the heaters and the IR lampswas calculated using Eq. (7) [3,4]:

EUter ¼ ðA � m � qa � Ca � DT þ K � tÞ � 3600 ð7Þ

Also, energy consumption by the blower during drying can becalculated using Eq. (6).

2.1.3. Microwave dryerThe energy consumption (thermal energy) of the microwave

dryer was calculated using Eq. (8) [67,68]:

EUter ¼ P � t � 3600 ð8Þ

2.1.4. Microwave–vacuum dryerTotal energy consumption in the microwave–vacuum dryer is

obtained from the sum of energy consumption by the microwaveoven (thermal energy) and the vacuum pump (mechanical energy).

Fig. 1. Setup for the (a) hybrid IR–convective, (b) hybrid PV/T solar dryer, (c) microwave–vacuum, (d) vacuum and (e) microwave–convective dryer.


Thermal energy consumed by the microwave oven was calculatedusing Eq. (8) while the mechanical energy consumed by thevacuum pump was calculated using Eq. (9) [3,4]:

EUmec ¼ L � t � 3600 ð9Þ

2.1.5. Microwave–convective dryerTotal energy (thermal and mechanical) consumed during

microwave–convective drying was calculated by summingEqs. (1) and (8). Thermal energy consumed in the convective–microwave dryer was calculated using Eq. (10) [40,67].


EUter ¼ ðA � m � qa � Ca � DT þ P � tÞ � 3600 ð10Þ

2.1.6. Vacuum dryerTotal energy consumption (thermal and mechanical) in the

vacuum dryer is the sum of the energy consumed by the heaters(thermal energy) and the vacuum pump (mechanical energy).Thermal energy consumption of the heaters was calculated basedon Eq. (10).

EUter ¼ U � I � cos h � t � 3:6 ð11Þ

U, I and cosh in Eq. (11) were determined using a power analyzer(Lutron, DW-6090). Also, mechanical energy consumption in thevacuum dryer was calculated using Eq. (9).

2.1.7. Solar dryerIn order to determine the energy level required for heating the

dryer inlet air Eq. (1) can be used. Furthermore, the requiredenergy to operate the fan during the drying process was deter-mined using Eq. (6). A wattmeter (model TM-1510 made by Tika,Iran) was employed to measure the power consumption of the heatpump during the drying process. The total mechanical energy con-sumed for drying with and without the heat pump was determinedusing Eqs. (12a) and (12b), respectively.

EUðmecÞ ¼ Efan ð12aÞ

EUðmecÞ ¼ Efan þ Eheat pump ð12bÞ

2.1.8. Specific energy consumptionSpecific energy consumption in drying of Roman chamomile is

calculated using Eq. (13) [44,67].

SEC ¼ EUðmecþterÞ

Mwð13Þ

2.2. Thermal efficiency

This efficiency is defined as the ratio of latent moistureevaporation heat of the sample to the amount of energy requiredto evaporate moisture from free water. Eq. (14) was used todetermine the thermal utilization efficiency [60].

TE ¼ D � A � hfg � ðMi �MoÞ3600Z � t � ð100�MoÞ

ð14Þ

Initial moisture content of the samples (Mi) was 83 (w.b.%) onthe average which were dried to a final moisture content (Mo) of12 (w.b.%). Since water makes up more than 83% (w.b.%) of Romanchamomile flowers, the latent vaporization heat of the sampleswas considered equal to the latent heat at ambient pressure forhot air, microwave, microwave–convective, solar, infrared andconvective–infrared combination dryers. The latent heat of evapo-ration at different levels of the experiment was also consideredequal for vacuum and microwave–vacuum dryers.

2.3. Energy efficiency

This efficiency is defined as the ratio of the energy used forevaporation of moisture from the sample to the total energy con-sumption. The energy used for moisture evaporation was calcu-lated using Eq. (15) which is valid for materials with highmoisture content [40].

Q w ¼ hfg �Mw;eV ð15Þ

Heat of vaporization as a function of the drying air temperatureis given as [69];

hfg ¼ 2:503� 106 � 2:386� 103ðT � 273:16Þ273:16 6 T ð�KÞ 6 338:72 ð16aÞ

hfg ¼ ð7:33� 1012 � 1:60� 107T2Þ0:5

338:72 6 T ð�KÞ 6 533:16 ð16bÞ

Energy efficiency of different dryers based on the First Law ofThermodynamics can be derived by using the energy balanceequation. Energy efficiency was calculated using Eq. (17) [40]:

Energy Efficiency ¼ Q w

EUðter þmecÞ ð17Þ

2.4. Drying efficiency

Drying efficiency is defined as the ratio of energy utilized toheating the product (sample) for moisture evaporation, to the totalenergy consumption [40].

Drying Efficiency ¼ Qw þ Q m

EUðter þmecÞ ð18Þ

In Eq. (18), Qw was calculated using Eq. (15) while Qm for convec-tive, IR, IR–convective, Solar and vacuum dryers was calculatedaccording to Eq. (19).

Qm ¼WdcmðTm2 � Tm1Þ ð19Þ

Specific heat of high-moisture materials is strongly dependenton the moisture content. Specific heat of the sample was calculatedusing Eq. (20).

cm ¼ 1465:0þ 3560:0Mp

1þMp

� �ð20aÞ

Mp ¼Ww �Wd

Wd

� �ð20bÞ

The energy required to heat the material (Qm) placed in amicrowave, microwave–vacuum or microwave–convective dryerwas calculated by Eq. (21) [70,71];

Qm ¼4:18 �W � Cp � DP

tð21Þ

2.5. Experimental uncertainty

The analysis of uncertainties in experimental measurement is apowerful tool, particularly when it is used in the planning anddesign of experiments. Uncertainties analyses are given withEq. (22) [72]:

WR ¼dRdx1

W1

� �2

þ dRdx2

W2

� �2

þ � � � þ dRdx3

Wn

� �2" #1=2

ð22Þ

During the experiments, total uncertainties of the measuredparameters and calculated experimental parameters were pre-sented at Table 1.

3. Results and discussion

3.1. Compassion of energy parameters in various dryers

3.1.1. Convective dryerResults of data analyses for drying of chamomile medicinal

plant using a convective dryer showed low energy efficiency.According to Fig. 2, the maximum energy efficiency (6.76%) wasachieved at 60 �C and 0.5 m/s air velocity, while the minimum

Table 1Uncertainties of the parameters drying experiment.

Parameter Unit Comment

Uncertainty of the measurement of relative humidity of air RH ±0.1Uncertainty in the measurement of moisture quantity g ±0.001Inlet temperature in convective dryer �C ±0.21Outlet temperature in convective dryer �C ±0.21Ambient air temperature �C ±0.21Microwave power W ±4.13Air velocity m/s ±0.33Mass loss measurement g ±0.5Temperature in vacuum dryer �C ±0.1Uncertainty of the measurement of absolute pressure mbar ±4.6Collector inlet temperature �C ±0.21Collector outlet temperature �C ±0.21Drying cabinet inlet temperature �C ±0.21Drying cabinet outlet temperature �C ±0.21Ambient air temperature �C ±0.21Uncertainty in the measurement of solar energy w/m2 ±0.17


value (1.91%) was at 40 �C and 1.5 m/s. Additionally, results shownin Fig. 2 indicate that increasing temperature decreased thespecific energy consumption. However, increasing the airflow ratealso increased the specific energy consumption. The maximumspecific energy value belonged to 40 �C and 1.5 m/s treatment with251.87 MJ/kgw, while the minimum was obtained at 60 �C and0.5 m/s with 69.76 MJ/kgw. As the temperature increases, capacityof the drying air (temperature difference between product anddrying air) to remove moisture increases and the drying timedecreases. At the same time, increasing the airflow rate acceleratesmoisture removal from the drying environment and prevents airsaturation inside the drying chamber. Normally, drying of agricul-tural materials occurs during the descending period, while, theproduct loses most of its moisture during the initial phases ofdrying, requiring more time to lose the remaining moisture. Sinceconvective drying uses the convection phenomenon for heattransfer, it is unable to raise the temperature inside the material.Therefore, the dryer must operate longer to remove a smalleramount of moisture, which may be a reason for low energy effi-ciency and high specific energy consumption of convective dryers.These results support those reported by other researchers forvarious agricultural products [3,4,44,45,46].

3.1.2. IR dryingThe energy radiated from the IR lamps is the drying agent in the

IR drying method. The heat generated by the IR lamps develops atemperature and partial vapor pressure gradient which, facilitatesmoisture evaporation and its removal by the fair flow.

0

2

4

6

8

10

40 °C, 0.5 m/s

40 °C, 1 m/s

40 °C, 1.5 m/s

50 °C, 0.5 m/s

50 °C,

Ene

gy E

ffici

ency

(%)

Energy Efficiency (%)

Fig. 2. Specific energy consumption and energy efficie

The results from applying the IR energy to drying of chamomileshowed that energy efficiency decreases as the airflow rate passingover the material is increased. Moreover, increasing the IR-radia-tion intensity from 0.22 to 49 W/m2 elevates the energy efficiencylevel. The highest energy efficiency (12.75%) was achieved at the49 W/m2 radiation intensity and airflow rate of 0.5 m/s. On theother hand, the minimum energy efficiency (3.58%) was at22 W/m2 and 1.5 m/s (Fig. 3). According to Fig. 3, specific energyincreased with decreasing radiation intensity and decreased asthe airflow rate increased. This might be due to an increase inthe internal temperature of the material with increasing radiationintensity, which produces a higher thermal gradient between theproduct and the ambient air. This, subsequently, accelerates mois-ture removal from the product, minimizing the drying time andenergy consumption. However, increasing the airflow rate coolsdown the product’s surface which slows down the moistureremoval process and increases the drying duration and energyconsumption. Other researches [56,60,37] reported similar results.

3.1.3. IR–convective dryingResults of data analysis showed that a combination of convec-

tive and IR dryers can contribute to increasing the energy and spe-cific energy efficiencies. From Figs. 4a and 6a and comparing themwith Figs. 2 and 3, it was found that the energy consumption in thishybrid dryer is higher and the required specific energy level islower. The maximum energy efficiency (16.15%) was obtained withthe hybrid dryer (IR–40 �C/0.5 m/s hot air) at 0.49 W/cm2 radiationwhile the lowest efficiency 3.37% was observed at 40 �C, 1.5 m/sand 0.22 W/cm2. The minimum value (3.37%) of this efficiencywas associated with a radiation intensity of 0.22 W/cm2 and anairflow rate of 1.5 m/s. In terms of the specific energy consumptionefficiency, the maximum was obtained at 0.22 W/cm2 and 1.5 m/swith 143.04 MJ/kgw, and the minimum specific energy efficiencywas found at 0.49 W/cm2 and 0.5 m/s with 29.85 MJ/kgw.Comparison of the hybrid dryer (IR with 40 �C air temperature)with both IR and convective dryers indicated that the hybrid dryerimproved the energy and specific energy efficiencies by 26.55% and27.81% (compared to the IR dryer), and 138.45% and 133.73%(compared the convective dryer), respectively.

Results of operating the hybrid dryer at 50 �C air temperaturefor drying of chamomile showed that the lowest required specificenergy (27.02 MJ/kgw) and the highest energy efficiency (17.65%)occurred at 0.49 W/cm2 and 0.5 m/s. However, the highest specificenergy consumption (152.42 MJ/kgw) and the lowest energyefficiency occurred at 0.22 W/cm2 and 1.5 m/s (Fig. 5). Using thehybrid dryer, the energy efficiency was improved by 160.78% and38.41% compared to the convective and IR dryers, respectively. This

1 m/s

50 °C, 1.5 m/s

60 °C, 0.5 m/s

60 °C, 1 m/s

60 °C, 1.5 m/s

0

50

100

150

200

250

300

SEC

(MJ/

kg w

)

SEC (MJ/kg w)

ncy under various convective drying treatments.

0

5

10

15

20

25

0.49 W/cm2, 0.5 m/s

0.49 W/cm2, 1 m/s

0.49 W/cm2, 1.5 m/s

0.31 W/cm2, 0.5 m/s

0.31 W/cm2, 1 m/s

0.31 W/cm2, 1.5 m/s

0.22 W/cm2, 0.5 m/s

0.22 W/cm2, 1 m/s

0.22 W/cm2, 1.5 m/s

Ener

gy E

ffici

ency

(%)

0

40

80

120

160

200

SEC

(MJ/

kg w

)

Energy Efficiency (%) SEC (MJ/kg w)

Fig. 3. Effect of radiation intensity on energy efficiency and specific energy consumption during IR drying at various air velocities.

Combine IR-Convective drying

0

5

10

15

20

25

0.49 W/cm2, 0.5 m/s

0.49 W/cm2, 1 m/s

0.49 W/cm2, 1.5 m/s

0.31 W/cm2, 0.5 m/s

0.31 W/cm2, 1 m/s

0.31 W/cm2, 1.5 m/s

0.22 W/cm2, 0.5 m/s

0.22 W/cm2, 1 m/s

0.22 W/cm2, 1.5 m/s

Ene

rgy

Effi

cien

cy (%

)

0

40

80

120

160

200

SEC

(MJ/

kg w

)


Fig. 4. Variations in energy efficiency and specific energy consumption at air temperature of 40 �C.


method also improved the required specific energy by 158.71% and41.18% compared to the convective and IR dryers, respectively.

Results from Fig. 6 show that the highest energy efficiency(20.40%) is associated with 0.49 W/cm2 and 0.5 m/s treatmentand the lowest value for this parameter was 3.02% which occurredat 0.22 W/cm2 and 1.5 m/s. On the other hand, the maximum spe-cific energy was obtained to be 160.97 MJ/kgw at 0.22 W/cm2 and1.5 ms, while its minimum value (23.14 MJ/kgw) was at 0.49 W/cm2 and 0.5 m/s.

Comparison of Figs. 4–6 showed that, as the temperature risesin the hybrid dryer, the energy efficiency increases while the spe-cific energy decreases. In IR dryers, the IR radiation energy pene-trates the material and converts into heat, which in turn, raisesthe internal temperature of the product. This causes the moistureto move outwards. At the same time, the air passing over the

Combine IR-Coa

0

5

10

15

20

25

0.49 W/cm2, 0.5 m/s

0.49 W/cm2, 1 m/s

0.49 W/cm2, 1.5 m/s

0.31 W/cm2, 0.5 m/s

0.31 W

Ene

rgy

Effi

cien

cy (%

)


Fig. 5. Energy efficiency and specific energy c

material facilitates the surface evaporation process and, along withthe IR radiation energy, accelerates the evaporation rate from theproduct’s surface. As the temperature of the passing air increases,its enthalpy also increases, further facilitating moisture removalfrom the surface. Therefore, the drying time and energy consump-tion decrease. These results agree with those reported by otherresearchers [3,4,37,56,60].

3.1.4. Microwave dryingAccording to Fig. 7, during the drying process, energy efficiency

had an increasing trend throughout the 100–300 W range, whilethe trend was declining during the 300–1000 W range. The maxi-mum energy efficiency (49.99%) was achieved at the 300 W powerlevel while, the minimum (35.83%) occurred at 1000 W. On thecontrary, specific energy decreased in the 100–300 W range and

nvective drying

/cm2, 1 m/s

0.31 W/cm2, 1.5 m/s

0.22 W/cm2, 0.5 m/s

0.22 W/cm2, 1 m/s

0.22 W/cm2, 1.5 m/s

0

40

80

120

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SEC

(MJ/

kg w

)

SEC (MJ/kg w)

onsumption at air temperature of 50 �C.

Combine IR-Convective drying

0

5

10

15

20

25

0.49 W/cm2, 0.5 m/s

0.49 W/cm2, 1 m/s

0.49 W/cm2, 1.5 m/s

0.31 W/cm2, 0.5 m/s

0.31 W/cm2, 1 m/s

0.31 W/cm2, 1.5 m/s

0.22 W/cm2, 0.5 m/s

0.22 W/cm2, 1 m/s

0.22 W/cm2, 1.5 m/s

Ene

rgy

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cy (%

)0

40

80

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SEC

(MJ/

kg w

)


Fig. 6. Energy efficiency and specific energy consumption at air temperature of 60 �C.


exhibited an increasing trend in the 300–1000 W range. Theminimum and maximum values were 4.32 and 5.52 MJ/kgw,associated with 300 and 1000 W power levels, respectively.

3.1.5. Microwave–vacuum dryingAccording to Fig. 8, the maximum required specific energy

(20.74 MJ/kgw) was observed at 130 W and 750 mbar of absolutepressure while its minimum value (5.02 MJ/kgw) occurred at380 W and 25 mbar. Moreover, the maximum energy efficiencywas 42.81%, respectively, which were achieved at the 380 W and25 mbar condition (Fig. 8). The minimum was 11.31%, occurringat the 130 W and 750 mbar condition (Fig. 8).

Advantages such as shorter drying times and lower energyconsumption were the key drivers for the further development ofthe microwave drying technique. Its limitations (e.g. non-uniformdistribution of internal temperature and moisture, tissue degrada-tion, and limited penetration), however, causes a qualitativedecline in the dried products [73]. When comparing the resultsof the hybrid microwave–vacuum dryer (Fig. 8) with those ofFig. 7, it is found that the specific energy of drying in the hybriddryer is higher than that of the microwave dryer. This increase inthe required specific energy and the decreased drying, thermaland energy efficiencies are due to the energy-intensive vacuumpump used to create vacuum in the drying chamber. Meanwhile,the results of other studies show that a lack of oxygen during thedrying process can enhance the quality of the dried mattercompared to the microwave dryer [3,4,16]. Therefore, the hybridvacuum–microwave dryer is recommended for drying of productsin which preservation of the active ingredients, quality andnutritional value are important.

Results of Fig. 9 showed that maximum energy efficiency was24.20% while the minimum was 12.81%. Additionally, the highestand lowest specific energy values for the hybrid ‘‘convective–microwave’’ dryer occurred at airflow rate of 1 m/s and 40, 50and 60 �C being 17.15, 10.47 and 7.14 times higher than those ofthe convective dryer. The maximum energy efficiency was also

0

20

40

60

80

100

100 200 300 400 500

Ener

gy E

ffici

ency

(%)


Fig. 7. Specific energy consumption and energy efficiency

achieved at the 100 W and 40 �C condition with 12.81%, respec-tively (Fig. 9). It was also found that the energy efficiency washigher than the convective dryer and the required specific energywas lower. This could be explained by the fact that in convectivedrying, a hard outer layer forms on the surface of the materialwhich impedes moisture removal, leaving a moist internal section.This hard layer slows down the evaporation rate and increasesdrying time and energy consumption. Microwave radiation, onthe other hand, penetrates the matter and converts IR energy intothermal energy, which in turn, raises internal temperature andincreases the internal vapor pressure. This accelerates masstransfer from inside the product, which is called moisturepumping. Similar results have been reported in other studies[23,25,26,36,58,66,70].

3.1.6. Vacuum dryingComparing the results for the vacuum drying of chamomile

with other dryers, it was found that the maximum requiredspecific energy of 318.42 MJ/kgw belonged to this dryer. Accordingto Fig. 10, the maximum specific energy was associated with the70 �C and 25 mbar treatment and its minimum was observed atthe 40 �C and 750 mbar condition. Analysis also showed that thehighest value for energy efficiency was 6.53% at 70 �C and 25 mbarwhile the minimum value was 1.42% at 40 �C and 750 mbar(Fig. 10).

Fig. 10 shows that the required specific energy of the vacuumdryer decreases with increasing temperature. This is due to thedevelopment of a thermal gradient between inside and outsidethe material caused by the increased temperature. Therefore,the evaporation rate increases and, consequently, drying timedecreases. Moreover, as the ambient absolute pressure isdecreased, the product puffs up easing the moisture removal pro-cess. This, in turn, reduces the drying time and the required specificenergy as well. Similar results have been reported in other studies[3,4,19,39].

600 700 800 900 10000

2

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10

SEC

(MJ/

kg w

)

SEC (MJ/kg w)

for drying Roman chamomile in the microwave dryer.

a

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100

130 W, 25 mbar

130 W, 250 mbar

130 W, 500 mbar

130 W, 750 mbar

260 W, 25 mbar

260 W, 250 mbar

260 W, 500 mbar

260 W, 750 mbar

380 W, 25 mbar

380 W, 250 mbar

380 W, 500 mbar

380 W, 750 mbar

450 W, 25 mbar

450 W, 250 mbar

450 W, 500 mbar

450 W, 750 mbar

Ene

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Fig. 8. Specific energy consumption and energy efficiency for drying Roman chamomile in the microwave–vacuum dryer.

0

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32

40

100 W- 40 °C

200 W- 40 °C

300 W- 40 °C

400 W- 40 °C

100 W- 50 °C

200 W- 50 °C

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400 W- 50 °C

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200 W- 60 °C

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Energy Effecincy (%) SEC (MJ/kg w)

Fig. 9. Specific energy consumption and energy efficiency for drying Roman chamomile in the microwave–convective dryer.

0

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6

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10

40 °C, 25 mbar

50 °C, 25 mbar

60 °C, 25 mbar

70 °C, 25 mbar

40 °C, 250 mbar

50 °C, 250 mbar

60 °C, 250 mbar

70 °C, 250 mbar

40 °C, 500 mbar

50 °C, 500 mbar

60 °C, 500 mbar

70 °C, 500 mbar

40 °C, 750 mbar

50 °C, 750 mbar

60 °C, 750 mbar

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Eneg

y Ef

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Fig. 10. Specific energy consumption and energy efficiency for drying Roman chamomile in the vacuum dryer.


3.1.7. Solar dryingResults of using the hybrid photovoltaic/thermal solar dryer

(with and without heat pump) are presented in Figs. 11 and 12.Generally, these results show that the energy efficiencies for bothmodes (with/without heat pump) increased with drying tempera-ture. These efficiencies decreased with increasing airflow rate.Moreover, a comparison of Figs. 11 and 12 shows that maximumenergy efficiency was achieved using the heat pump, their valuesbeing 17.56% while the minimum value, also with the pump, was3.63%. As for the specific energy consumption, it decreased withany increase in temperature or decrease in the airflow rate, whereits minimum value (26.88 MJ/kgw) occurred at 60 �C and 0.5 m/swith the heat pump, and the maximum value (132.56 MJ/kgw)was found at 40 �C and 1.5 m/s without the pump. Effects ofdeploying the pump on efficiencies and specific energy can be

determined by comparing Figs. 11 and 12. Accordingly, using theheat pump increases efficiency and decreases specific energy use.This is because the dry vapor pressure with the heat pump wouldbe lower than that without it. This increases the surface evapora-tion rate which decreases drying time and energy consumption.These results are in line with those obtained in other studies[62,63].

3.2. Compassion of thermodynamic parameters at various dryer

3.2.1. Convective dryerThermal and drying efficiencies presented in Table 2 indicate

that any increase in the airflow rate decreases both efficiencieswhile any increase in temperature increases both. The highestthermal and drying efficiencies were achieved at 60 �C and

0

5

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15

20

40 °C, 0.5 m/s

40 °C, 1 m/s

40 °C, 1.5 m/s

50 °C, 0.5 m/s

50 °C, 1 m/s

50 °C, 1.5 m/s

60 °C, 0.5 m/s

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Fig. 11. Specific energy consumption and energy efficiency for drying Roman chamomile in the solar dryer without heat pump.

0

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40 °C, 0.5 m/s

40 °C, 1 m/s

40 °C, 1.5 m/s

50 °C, 0.5 m/s

50 °C, 1 m/s

50 °C, 1.5 m/s

60 °C, 0.5 m/s

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Eneg

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Fig. 12. Specific energy consumption and energy efficiency for drying Roman chamomile in the solar dryer with heat pump.


0.5 m/s with 6.87% and 9.32%, respectively, while the lowest wereobtained at 40 �C and 1.5 m/s with 2.12% and 2.21%, respectively.Convective dryers allocate a large portion of energy consumptionto heating up the air (thermal energy) and mechanical energy con-sumption (energy required for pumping the air) is significantlylower than that of thermal energy. Therefore, value of the thermalefficiency and energy efficiency are very close. However, dryingefficiency is higher than thermal and energy efficiencies since partof the energy is spent for raising the product’s temperature (Qm).These results are similar to those reported in the literatures[40,70].

3.2.2. IR dryingThe highest drying and thermal efficiencies were 13.98% and

12.96%, respectively associated with the 0.49 W/cm2 radiationintensity and the 0.5 m/s airflow rate. The minimum values forboth parameters were recorded at 0.22 W/cm2 and 1.5 m/s with3.71% and 4.03%, respectively. Table 3 shows that the differencebetween the thermal and drying efficiencies decreases as the hotairflow rate is increased from 0.5 to 1 m/s. With further increase,

Table 2Values of the thermodynamic parameters and drying time in the convective dryer.

Drying method Temperature (�C) Air velocity (m/s) D

Convicting drying 40 0.5 71 51.5 4

50 0.5 21 21.5 1

60 0.5 11 11.5 1

from 1 to 1.5 m/s, the thermal efficiency becomes higher thanthe drying efficiency. This is because of the fact that increased air-flow rate slows down the drying process in the IR dryers; that is,longer drying time followed by excessive pump operation (highermechanical energy consumption). Based on the results for the spe-cific energy as well as the drying, energy and thermal efficiencies;higher radiation intensities and lower airflow rates are recom-mended for IR dryers.

3.2.3. IR–convective dryingAdditionally, comparing Table 4 with Tables 2 and 3, it was

found that drying efficiency is higher in the hybrid dryer than inthe convective and IR dryers. As shown in Fig. 3 and Table 3, vari-ations in the thermal, energy and drying efficiencies as well as thespecific energy are in line with variations in the IR dryer, and notwith the convective dryer. This is due to increased airflow ratewhich cools down the material surface and therefore slows downmoisture removal.

The maximum drying efficiency of the hybrid dryer was 18.03%(at 40 �C air temperature) obtained at 0.49 W/cm2 and 0.5 m/s

rying time (min) Drying efficiency (%) Thermal efficiency (%)

20 4.63 4.1560 2.89 2.6770 2.21 2.1285 7.78 6.2810 5.18 4.2695 3.63 3.0690 9.33 6.8750 5.83 4.3525 4.57 3.48

Table 3Values of the thermodynamic parameter and drying time in the infrared dryer.

Drying method Radiation intensity (W/cm2) Air velocity (m/s) Drying time (min) Drying efficiency (%) Thermal efficiency (%)

IR drying 0.49 0.5 110 13.97 12.961 140 9.31 9.091.5 200 5.45 5.74

0.31 0.5 130 11.62 10.961 170 7.55 7.481.5 230 4.66 4.99

0.22 0.5 170 8.71 8.381 210 6.01 6.061.5 285 3.71 4.03


(Table 4). On the other hand, the minimum value of this efficiencywas 3.51% obtained at 0.22 W/cm2 and 1.5 m/s. The increase indrying efficiency with IR–convective drying compared to separateconvective and IR methods was 93.34% and 29.03%, respectively.Thermal efficiency of the hybrid dryer (at 40 �C air temperature)indicated that the highest efficiency (16.35%) could be achievedby increasing the radiation intensity and decreasing the air flowrate; while the minimum efficiency (3.64%) is observed by decreas-ing the radiation intensity and increasing the air flow rate.

Furthermore, Table 5 shows that the thermal and drying effi-ciencies of the convective dryer (at 50 �C air temperature) arehigher than those of the convective and IR dryers alone. Dryingand thermal efficiencies increased by 113.75% and 1599.49%,respectively (compared to the convective dryer). However,compared to the IR dryer, the increases in drying and thermalefficiencies were 42.64% and 37.59%, respectively.

Results from Table 6 show that the highest thermal and dryingefficiencies 20.61% and 23.34% respectively, were obtained at0.49 W/cm2 and 0.5 m/s. The lowest values for these two parame-ters were 3.14% and 3.20%, respectively which occurred at0.22 W/cm2 and 1.5 m/s. Moreover, compared to the convectivedryer, the energy, thermal and drying efficiencies of the hybriddryer (at 60 �C air temperature) increased by 241.98%, 199.40%and 150.23%, respectively; compared to the IR dryer, in whichthese values were 87.00%, 58.75% and 66.99%, respectively.Comparison of Tables 4–6 showed that, as the temperature risesin the hybrid dryer, the thermal and drying efficiencies increaseand the specific energy decreases.

3.2.4. Microwave dryingAs shown in Table 7, the thermal and drying efficiencies show

increasing trends in the 100–300 W range and decreasing trendsin the 300–1000 W range. The maximum values for thermal effi-ciency (49.99%) and drying efficiency (65.25%) were found at the300 W power level; while their minimums occurred at 1000 Wwith 35.83% and 53.80%, respectively. In microwave drying,mechanical energy consumption is negligible compared to thethermal energy, therefore the energy efficiency and thermalefficiency are equal. Results of applying microwaves for drying

Table 4Values of the thermodynamic parameter and drying time in the hot air–infrared combina

Drying method Radiation intensity (W/cm2) Air velocity (m

Combine IR–convective drying 0.49 0.511.5

0.31 0.511.5

0.22 0.511.5

chamomile proved this to be a highly energy efficient method. Thisis while losses of the volatile substances of the product increase inthe convective dryer due to longer drying times. Microwave energypenetrates into the product and produces polarity in its watermolecules, creating heat inside the material. This addresses thechallenges caused by the low thermal conductivity of thesubstance. Thus, thanks to its high thermal conductivity factor,applying microwaves for drying of food and agricultural productscan enhance the evaporation rate and therefore decrease therequired time and energy. Similar results have been reported inother studies [22,41,58,68,71].

3.2.5. Microwave–vacuum dryingAccording to Table 8, the maximum drying and thermal effi-

ciencies were 50.42%, 78.21%, respectively, which were achievedat the 380 W and 25 mbar condition (Table 8). Their minimumswere 12.41%, 37.96%, respectively, occurring at the 130 W and750 mbar condition (Table 8). Results of energy and thermal anal-yses for the hybrid microwave–vacuum dryer showed that energyefficiency is lower than that of the microwave method, while ther-mal efficiency is relatively higher. This could be due to the highershare of mechanical energy (energy consumed by the pump)compared to thermal energy (energy consumed by the microwavegenerator) according to Eqs. (14) and (17), which reduced energyefficiency and increased thermal efficiency.

3.2.6. Microwave–convective dryingCombining convective drying with the microwave power

improves the efficiency of the former. Comparison of Table 9 withTable 2 and Fig. 9 with Fig. 2 indicates that adding microwaves tothe convective dryer not only increases the energy, thermal anddrying efficiencies, but also decreases the specific energy requiredfor drying of chamomile (the experiments were conducted at air-flow rate of 1 m/s). Results also showed that maximum dryingand thermal efficiencies were 33.47% and 24.69%, respectively,while their minimums were 14.06% and 13.37%, respectively. Themaximum thermal and drying efficiencies were also achieved atthe 100 W and 40 �C condition with 14.06% and 13.38%, respec-tively (Table 9). It was also found that these two efficiencies were

tion dryer at 40 �C air temperature.

/s) Drying time (min) Drying efficiency (%) Thermal efficiency (%)

70 18.03 16.35100 9.37 8.89130 5.59 5.59

80 15.34 14.30110 8.35 8.08160 4.46 4.54100 11.98 11.44140 6.41 6.35200 3.51 3.63

Table 5Values of the thermodynamic parameter and drying time in the hot air–infrared combination dryer at 50 �C air temperature.


Combine IR–convective drying 0.49 0.5 50 19.93 17.831 70 9.53 8.851.5 95 5.17 4.99

0.31 0.5 60 16.06 14.861 85 7.67 7.291.5 120 4.01 3.95

0.22 0.5 75 12.62 11.891 100 6.37 6.191.5 150 3.15 3.16

Table 6Values of the thermodynamic parameter and drying time in the hot air–infrared combination dryer at 60 �C air temperature.


Combine IR–convective drying 0.49 0.5 35 23.34 20.571 50 10.21 9.341.5 75 4.89 4.60

0.31 0.5 55 14.38 13.091 70 7.16 6.671.5 95 3.76 3.63

0.22 0.5 65 11.83 11.071 80 6.09 5.831.5 110 3.19 3.14

Table 7Values of the thermodynamic parameter and drying time in the microwave dryer.

Drying method Microwave power (W) Drying time (min) Drying efficiency (%) Thermal efficiency (%)

Microwave drying 100 88 48.51 44.54200 40 55.85 47.38300 24 65.24 49.99400 19 62.55 46.61500 16.1 59.48 43.54600 14 57.46 41.21700 12.2 56.95 40.01800 11 55.67 38.29900 10 54.71 37.04

1000 9.2 53.79 35.83

Table 8Values of the thermodynamic parameter and drying time in the microwave–vacuum combination dryer.

Drying method Microwave power (W) Absolute pressure (mbar) Drying time (min) Drying efficiency (%) Thermal efficiency (%)

Combine microwave–vacuum drying 130 25 51 17.86 54.66250 58 15.40 47.13500 66 13.54 41.41750 72 12.41 37.96

260 25 17 41.93 75.31250 19.5 36.76 69.52500 21 33.26 62.90750 25 27.93 52.83

380 25 12.5 50.43 78.21250 15 38.49 57.08500 16 36.09 53.52750 18 32.08 47.57

450 25 10 47.94 64.28250 12.5 40.56 54.39500 14 37.67 50.51750 15.5 35.17 47.14


higher than those of the convective dryer and the required specificenergy was lower.

3.2.7. Vacuum dryingAnalysis showed that the highest values for thermal and drying

efficiencies were 11.95% and 10.24% at 70 �C and 25 mbar whiletheir minimum values were 2.68% and 1.65% at 40 �C and 750 mbar

(Table 10). Based on the structure of the vacuum dryer, a large por-tion of the total energy consumption belongs to the vacuum pumpused for creating vacuum in the drying chamber, which elevatesthe thermal efficiency more than the other two efficiencies. Asshown in Table 10 and Fig. 10, using a vacuum dryer at low tem-peratures is not cost-effective due to its low efficiency and highspecific energy requirement.

Table 9Values of thermodynamic parameter and drying time in the microwave–convective dryer.

Drying method Microwave power (W) Temperature (�C) Drying time (min) Drying efficiency (%) Thermal efficiency (%)

Combine microwave–convective drying 100 40 36 14.05 13.3750 23 14.91 13.7560 17 15.64 14.28

200 40 28 18.02 15.7650 16 20.88 17.6360 11 23.96 19.87

300 40 18 29.37 21.9650 11.5 31.04 22.2860 7.5 33.47 24.66

400 40 17 27.61 21.0450 10 29.52 21.3060 7 31.75 22.70

Table 10Values of the thermodynamic parameter and drying time the combine microwave–vacuum dryer.

Drying method Absolute pressure (mbar) Temperature (�C) Drying time (min) Drying efficiency (%) Thermal efficiency (%)

Combine microwave–vacuum drying 25 40 1740 1.76 3.4150 1020 3.32 5.0560 540 6.55 8.6770 360 10.24 11.94

250 40 1860 1.83 3.1950 1140 3.07 4.5260 600 6.09 7.8070 420 9.05 10.24

500 40 2040 1.74 2.9150 1320 2.77 3.9160 660 5.76 7.0970 420 9.40 10.24

750 40 2220 1.65 2.6750 1580 2.38 3.2660 780 5.01 6.0170 480 8.44 8.96

Table 11Values of the thermodynamic parameter and drying time in the solar dryer.

Drying method Air velocity (m/s) Temperature (�C) Drying time (min) Drying efficiency (%) Thermal efficiency (%)

Solar drying without heat pump 40 0.5 780 10.47 10.171 580 6.58 6.841.5 490 4.14 4.41

50 0.5 310 14.83 12.401 230 10.04 8.821.5 190 6.73 5.82

60 0.5 220 17.11 13.171 170 10.77 8.521.5 140 8.41 6.90

Solar drying with heat pump 40 0.5 520 14.42 14.461 370 9.89 10.161.5 310 6.54 6.98

50 0.5 220 20.89 17.471 150 14.52 12.481.5 130 10.15 8.92

60 0.5 160 23.52 18.111 120 15.26 12.071.5 105 11.21 9.20


3.2.8. Solar dryingResults of using the hybrid photovoltaic/thermal solar dryer

(with and without heat pump) are presented in Table 11.Generally, these results show that the thermal, drying efficienciesfor both modes (with/without heat pump) increased with dryingtemperature. These efficiencies decreased with increasing airflowrate. Moreover, the results of Table 11 shows that maximum ther-mal and drying efficiencies were achieved using the heat pump,their values being 23.51% and 18.12%, respectively, while the

minimum values, also with the pump, were 4.14% and 4.42%,respectively (Table 11).

4. Conclusions

This study examined and compared seven widely-used dryingpractices applied to drying of Roman chamomile. In the convectivemethod, energy, thermal and drying efficiencies – unlike the


specific energy consumption – were directly associated withtemperature and airflow rate. In the IR and hybrid IR–convectivedryers with increasing radiation ray, the thermal and dryingefficiency increase and specific energy decrease. Moreover, in bothdryers, increasing airflow rate decreased the specific energyrequirement and increased the efficiency. The highest energy,thermal and drying efficiencies and the lowest required specificenergy were associated with microwave drying. Adding hot airflowto the microwave dryer (hybrid microwave–convective dryer),however, resulted in lower efficiency and higher specific energythan the microwave dryer alone. Vacuum drying is highlyenergy-intensive, and the lowest energy, thermal and dryingefficiencies and the highest required specific energy belonged tothis method of drying. A combination of vacuum and microwavedryers (the hybrid microwave–vacuum dryer) increased energy,thermal and drying efficiencies while decreasing the requiredspecific energy, compared to the vacuum dryer. However, ascompared to the microwave dryer, the required energy increasedwhile the efficiency decreased. The photovoltaic solar dryerexhibited good efficiency and energy savings. Results showed thatadding a heat pump to the solar dryer can increase the efficiencywhile decreasing the required specific energy.

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