Journal Pre-proof Performance assessment of passive indirect solar dryer comparing without and with heat storage unit by investigating the drying kinetics of carrot Mulatu C. Gilago , Vishnuvardhan Reddy Mugi , V.P. Chandramohan PII: DOI: Reference: S2772-4271(23)00008-6 https://doi.org/10.1016/j.nexus.2023.100178 NEXUS 100178 To appear in: Energy Nexus Received date: Revised date: Accepted date: 19 November 2022 20 January 2023 8 February 2023 Please cite this article as: Mulatu C. Gilago , Vishnuvardhan Reddy Mugi , V.P. Chandramohan , Performance assessment of passive indirect solar dryer comparing without and with heat storage unit by investigating the drying kinetics of carrot, Energy Nexus (2023), doi: https://doi.org/10.1016/j.nexus.2023.100178 This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2023 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Highlights An indirect solar dryer without and with thermal storage system (setups-I and II) was developed. Drying parameters and thermal performance of the dryers were estimated. The thermal efficiency and actual heat supply of the solar heater were 56.84 & 60.79% and 705.64 & 730.45 W, respectively. Surface transfer coefficients of carrot with drying air were calculated for both dryers. The specific energy consumptions for setups-I and II were 3.5 and 0.28 kg/kWh, respectively. 1 Performance assessment of passive indirect solar dryer comparing without and with heat storage unit by investigating the drying kinetics of carrot Mulatu C. Gilagoa,b, Vishnuvardhan Reddy Mugia, V.P. Chandramohana,* a Mechanical Engineering Department, National Institute of Technology Warangal, Warangal, Telangana, India, 506004 b Mechanical Engineering Department, Wachemo University, Post Box 667, Hosaena City, Ethiopia * Corresponding author: E-mail: vpcm80@nitw.ac.in, Mob: +91 8332969329, ORCID: 0000- 0002-8680-8363 2 Abstract The present study examined the drying kinetics of carrots and the effectiveness of passive indirect solar dryer without (setup-I) and with (setup-II) thermal energy storage (TES). A module containing paraffin wax as a TES unit was incorporated after the test was completed in setup-I to create setup-II. The sample slices were dried in setup-I and setup-II at a drying rate of 0.5 and 0.59 kg/h, respectively. In setup-I, the sample dried from 9.13 to 0.478 dry basis (db) in 16 h, whereas, setup-II took 15 h. The thermal efficiency of the solar heater and actual heat supply for the same were 56.84 & 60.79% and 705.64 & 730.45 W, respectively. Similarly, the average heat and mass transfer coefficients (h and hm) were 6.32 & 7.08 W/m2 K and 0.0055 & 0.0062 m/s, and average effective diffusivity (De) for the same setups were 6.7 × 10-9 and 7.24 × 10-9 m2/s, respectively. Furthermore, the specific energy consumption and specific moisture extraction rate for the same were 3.5 & 0.28 kWh/kg and 0.29 & 3.62 kg/kWh, respectively. Setups I and II had an average drying efficiency of 7.5 and 10.25%, respectively. The activation energy was evaluated for both setups. The De, h and hm were correlated with moisture content for both setups. In addition to helping the drying process to be completed in a day, setup-II performed better in all the evaluated parameters than setup-I. To ensure the reliability of the results, an experimental uncertainty analysis was conducted. Keywords Passive indirect solar dryer; paraffin wax; drying performance; surface transfer coefficients; thermal energy storage 3 Nomenclature cp Specific heat of air A Area Dab Moisture diffusivity of air db Dry base De Effective moisture diffusion coefficient Do Pre-exponential factor Eac Average activation energy h Heat transfer coefficient hm Mass transfer coefficient Ia Average solar radiation Is Solar intensity PCM Phase change material k Thermal conductivity L Thickness Le Lewis number Lw Latent heat of water MC Moisture content m Mass ṁ Mass flow rate MR Moisture ratio Q Heat supply R Universal gas constant 2 R Correlation coefficient SEC Specific energy consumption SAC Solar air collector SMER Specific moisture extraction rate T Temperature TES Thermal energy storage G Uncertainty V Volume wb Wet based xi Uncertainty of a measured parameter Y Calculated parameter Greek symbols α Thermal diffusivity η Efficiency Subscripts c Collector d Dryer e Effective f Final i Initial, inlet ic Input of collector o Output w Water 4 (kJ/kg K) (m2) (m2/s) (m2/s) (m2/s) (kJ/mol) (W/m2 K) (m/s) (W/m2) (W/m2) (W/m K) (m) (kJ/kg) (db) (kg ) (kg/s) (%) (W) (J/mol K) (%) (kW-h/kg) (kg/kW-h) (℃) (m3) (m2/s) (%) - 1. INTRODUCTION The agenda of energy saving together with utilizing a cleaner source is supposed to be a sensitive issue for the global community. It should be the front-line itinerary of all policymakers, researchers, scientists, industries, and organizations because life on the earth is totally dependent on energy. The energy demand is sloping up at an alarming rate as the population and energy dependent industries are growing very fast. Striving to exploit the reserved potential of solar energy by any means may put a step forward to a solution of the energy related challenges. It is essential to mention that solar energy is one of the most attractive methods of utilizing renewable sources. Due to its environmental friendly characteristics, accessibility and free availability, it is getting special attention in the present day. Food drying is another major energy intensive process and the usage of solar energy for food drying is a hot research area for the past few decades. Food drying can prevent microbial decomposition by removing moisture from them and the energy needed for the same can be utilized from solar energy [1] instead of wasting commercial energy. A careful choice of drying techniques and controlling of drying parameters as well as making the process continuous and short are to be achieved while drying agricultural food products in order to ensure the required quality. Continuous drying using solar energy may not be possible because of fluctuations in sunlight but this issue can be overcome by providing thermal energy storage (TES) unit with the solar dryer. The TES unit can absorb the excess energy during day time and deliver the same when the sunlight is off so the food gets more continuous drying tenure and the possibility of degradation may be avoided. Evaluating the drying kinetics has a major impact on the optimization of drying parameters while doing drying experiments [2]. There are a good number of studies existing for confirming the above facts. Lakshmi et al. [3] experimentally investigated the performance of an indirect type solar (ISD) by drying black turmeric. They applied paraffin wax as phase change material (PCM) in the TES unit. Accordingly, they reported the overall efficiency of the collector and drying were 25.6 and 12%, respectively. The condition of the final product was comparatively good in the setup with TES compared to open sun drying (OSD). Drying of green onion has been done by 5 Hadalgo et al. [4] with both passive and active solar dryers. They concluded that passive convection had a thermal efficiency (ηc) of 38.3 % and a specific energy consumption (SEC) of 16.39 kW/kg, while active convection had the same parameters of 34.2 % and 18.3 kWh/kg, respectively. Tajudin et al. [5] performed an experimental study of drying Roselle calyx in solar and convective heat pump dryers to analyze the impact of temperature variation and mass of the sample on the drying kinetics. They reported that higher drying temperatures and lower mass of the sample fostered faster drying. Additionally, as the temperature varied from 40 to 60 ºC, the effective diffusivity (De) enhanced from 0.787 to 2.04 × 10−9 m2/s. Gilago and Chandramohan [6] investigated the performance of natural and forced convection ISD using PCM that was kept inside the drying chamber while drying ivy gourd. They evaluated the impact of TES unit on drying. Accordingly, they reported that both setups effectively worked till mid-night so the drying process completed within a day. The drying kinetics of ivy gourd produced a logarithmic nature of variation on De and surface transfer coefficients over moisture content. Mohammad et al. [7] made hybrid passive and active ISDs to experimentally investigate the performance during drying pineapple in Uganda (East Africa). Additionally, they assessed the economic importance of the system and compared it with the OSD method. They presented that the average drying air temperature was 31.9 ºC for the proposed system and 27.6 ºC for the OSD. ISD dried the sample for 10 h while OSD took 30 h. Halaby and Bek [8] conducted drying experiments to investigate the drying kinetics of herbal leaves as well as to identify the impact of integrating the TES system on the performance of active ISD (AISD). Moreover, they observed the effect of mass flow rate (0.067–0.218 kg/s) on different study parameters. Accordingly, they found that in the dryer without TES (setup-I), the samples dried in 12 h, and it took 18 h in the dryer with TES unit (setup-II) with drying rates of 0.124 and 0.09 kg/s, respectively. The drying was completed continuously in one day in setup-II. An experimental study with a TES unit to determine the drying performance parameters of an ISD was studied by Aboul-Enein et al. [9]. Additionally, they examined the domination of the design details of the dryer on the drying performance. The researchers found that the TES system and the dryer design parameters considerably influenced the drying characteristics of 6 agricultural products. During an experimental investigation on the importance of TES unit with solar dryers, Shukla [10] demonstrated that the final products quality was improved. Different TES systems such as latent, sensible, and chemical energy storage provisions improved the quality of end products in dryers as the drying process was completed in one day without any interruption in drying. For their experimental investigation of drying black pepper, Lakshmi et al. [11] fabricated a dryer with a TES unit. The moisture content (MC) decreased from 3.46 - 0.1 dry basis (db) within 14 h in a mixed-type dryer and to 0.12 (db) in an AISD dryer after 23 hours. The report also presented that the dried product quality was improved and that dryer was more cost-effective compared to OSD drying. Safri et al. [12] surveyed the attainment of different solar-assisted dryers and drying kinetics of various agricultural food products. They reported that drying agriproducts in solar-assisted dryers were more hygienic, nutritional, and healthy than other types of dryers. A drying experiment of jackfruit leather was analyzed by Chowdhury et al. [13] to identify the drying kinetics of solar tunnel dryers. As a result, they concluded that the samples dried from a mean MC of 3.17 - 0.14% (db) with the solar air collector (SAC) and drying efficiencies of 27.45 - 42.5% and 32.34 - 65.30%, respectively, within two days. The recent advancements in drying agricultural food products were reviewed by Lamidi et al. [14] indicating that ISDs coupled with TES systems are effective for drying agricultural products. The above review indicates that some studies focus on solar dryers and their drying performances [15–17], some on drying kinetics, and some address the advantage of solar dryers while drying farm food items [14, 18]. But a comprehensive examination of the parameters of setup-II during drying agriproducts is not reported yet. Various works contributed to the study of the solar drying of green onion [4], ivy gourd [19], muskmelon [20], banana and bitter gourd [21], green chili and okra [22], apple and watermelon slices [23], citrus Aurantium [24]. No data was found on carrot slices dried in an ISD supported with TES. Few studies reported on the effect of adding TES unit to solar dryers and the merits of drying agriproducts using solar dryers [14, 16]. The drying kinetics of carrot without and with the TES unit was not found and such data is necessary to know the impact of the TES unit. Some 7 data on the drying kinetics of agricultural produce such as heat supplied to the drying section [9, 23, 24], heat and mass transfer coefficients [8, 25], diffusivity coefficient [4, 26], and rate of drying [4, 27] have been reported. No studies reported such drying mechanics for both setups as it needs to be done so that one can study the variation of these parameters without and with TES unit. In the literature, only a few studies addressed the investigation of activation energy (Ea), drying efficiency [2, 19], specific moisture extraction rate (SMER) [19, 30], specific energy consumption (SEC) [29] and efficiency of the collector of the setup without TES (setup-I). However, there is little data available concerning how the drying performance parameters are analyzed for the setup without TES (setup-II) during carrot drying. Furthermore, a comparative investigation of the setup without TES (setups-I) and with TES (setup-II) is not found. The MC vs De, heat transfer and mass transfer coefficients (h and hm) correlations were not developed in any studies as they are necessary data after such expensive experiments. The following objectives were rooted and achieved in this study based on the gaps found in the assessment of literature: i) to estimate and analyze the parameters of drying performance such as thermal efficiency, heat supplied to the drying section, and efficiency of the dryer during drying carrot in setups-I and II, ii) to evaluate the drying kinetics of carrot slices such as De, rate of drying, h and hm for both dryers, iii) to tabulate a comparative chart of performance and drying parameters of both dryers, iv) to evaluate Ea, SEC, and SMER for both dryers and v) to correlate the relationship between MC and De, h and hm during drying in setups-I and II. 2. EXPERIMENTAL TECHNIQUES 2.1. Procedure Carrots purchased from Warangal local market were sliced into 5 mm thick cylindrical shapes after making sure that the outer surface is cleaned by dry fabric. Initially, the tests were performed in setup-I (without TES unit) which is constructed at NITW in Telangana, India (17° 58′ 50.88′′ N, 79° 31′ 58.08′′ E). The setup-I was updated with a rectangular framed TES unit that was placed just below the first tray and inside the drying section called setup-II. It is made of 50 numbers of transparent polycarbonate tubes with concentric aluminium fins filled by paraffin wax (34 kg, EC No. : 232-315-6, no caking, liquefaction 8 temperature 56-60 ºC, latent heat type, CAS No.: 8002-74-2, IMEDIA, India). Fig. 1 (a) diagrammatically demonstrates its components and Fig. 1 (b) gives a single polycarbonate pipe and its dimensions. The dimensions of the drying chamber and the SAC are 350 mm × 850 mm × 700 mm and 2000 mm × 1050 mm × 700 mm, respectively. In setup-I, the experiments were performed in the day time as it has no TES unit and hence drying took two consecutive days. In setup-II, the drying process started at morning and continued even after sunset (after 6:00 p.m.). Since, the setup-II has the TES unit, the drying continued up to midnight but within the same day. Initial MC was estimated from the mass variation data using a hot air oven (India, PPI-Unix96). OHAUS weighing balance (USA, 8-1415VAC, PAG24, 50/60 Hz, readability-0.0019) was used to measure the mass of the samples. Indian made RTD sensors and humidity transmitter (Testo635) were used to measure relative humidity and temperature. More details are presented in the previous study of the authors [32]. Figure 2 (a) illustrates the inside view of the experimental setup. 0.8 kg of carrot slices (0.2 kg × 4) were placed on four parallel trays (Fig. 2 b). Mass variation and drying parameters data were found for both cases. Fig. 2 (c) shows the snapshot of the final dried sample. A hot air oven was used for 24 h at 105 °C to determine the initial moisture content (MCi) of slices. (a) 9 (b) Paraffin wax All dimensions are in mm Fig. 1. Schematic representation of (a) overall setup-II and (b) paraffin wax holding unit 10 2.2. Moisture content The MCi was estimated from five random out of 12 samples located and dried in the hot air oven by: (1) Accordingly, the MCi of carrot was determined to be 9.13 (db) or 90.13 (wb), where, m represents mass of the slices, i stands for initial, and f represents final. Chimney Dried slices Slices Trays Trays TES Trays Data logger TES Fig. 2. Photo snapshot of (a) inside view of setup-II, (b) sliced carrot slices and (c) dried carrot slices 2.3. Estimation of energy involved in the experiment To estimate the energy associated with drying, the principles of mass conservation have been engaged, which states that the mass entered and the mass left in a specific control volume are equal [25]. This can be determined by. ∑ ̇ ∑ ̇ (2) Energy that took part in the SAC can be evaluated as, (3) ̇ (4) Where, Qic is input heat (useful heat input), Qoc is collector heat output (actual heat supplied), Is gives solar radiation (W/m2) and Ac represents area of SAC. ̇ (5) 11 Equation (5) is used to estimate the collector’s efficiency (ηc). Evaluation of the energy inside the drying cabinet can be done using, (6) (7a) (7b) ̇ Ia is the mean solar intensity for overall duration (W/m2), td represents the gross time used for drying samples (h), mw gives the amount of eliminated water (kg), and Lw is latent heat (kJ/kg), Ein is input energy (kWh), As represents area of sample exhibited to radiation (m2); Ti,PCM and To,PCM are temperature of air at the inlet and outlet of the TES unit, respectively. The SEC (kWh/kg) and SMER (kg/kWh) [32] can be evaluated by, (8) (9) 2.4. Determining the drying kinetics Using experimental mass drop data, De [10] was determined by, ( ) (10) Where, L (m) is the thickness of the slab and MR is the moisture ratio. Energy of activation (Eac) can be estimated using the Arrhenius equation [22]. ( ) (11) Where, Do (m2 /s) and R (J/mol K) are pre-exponential factor and universal gas constant, respectively. MR [35, 36] is evaluated using, (12) Where, the subscript tn stands for instantaneous. (13) Where, the subscript t stands for time and dt represents change in time. The coefficient of mass transfer (hm) [37] can be calculated using, 12 (14) Where, V (m3) and At (m2) represent the volume and the overall surface area, respectively. (15) (16) Heat transfer coefficient (h) is evaluated using Eq. (15), where, Dab, α (m2/s), k (W/m K) and Le stand for diffusivity of MC in air, drying air, thermal conductivity of air, and Lewis number, respectively. 3.5. Analysis of uncertainty In order to appraise the reliability of the results of the experiment, an experimental uncertainty analysis was conducted. The root-sum square method can be used to evaluate the uncertainty of dependent and independent variables [38]. The uncertainty evaluated for the parameters is illustrated in Table 1, where G represents the degree of uncertainty, Y represents a parameter's calculated value, and xi represents the degree of uncertainty for the measured variable. [( ) ( ) ( ) ( ) ] Table 1. Uncertainties in the experiment Variable Uncertainty T ± 1 oC v ± 0.040 m/s m ± 0.0002 g I ± 10 W/m2 MC ±0.047 kg per kg db RH ± 2% De ±1.46% MR ±0.031 ηc ±1.35% Ea ±0.0674 kJ/mol hm ±3.68×10-5 m/s h ±0.029 W/m2K 13 (17) Qoc ±24.02 W ηd ±1.32% 3. ANALYSIS AND DISCUSSION 3.1. Data of solar radiation Figure 3 presents the solar radiation data during carrot drying in setups-I and II. In setup-I, the solar radiation was documented between 8:00 a.m. and 6:00 p.m., whereas, in setup-II, it was recorded from 8:00 a.m. to midnight as it has a TES unit. Setup-I achieved a peak and average solar radiation of 963 and 648.74 W/m2, and the same for setup-II were 975 and 616.27 W/m2, respectively. The values are almost the same because the tests were executed on two successive days and also with the same weather. Therefore, it didn’t affect the drying parameters significantly. 1200 Setup-I Setup-II Solar radiation (W/m2) 1000 800 600 400 200 0 8 10 12 14 16 18 20 Time (h) 22 24 26 28 Fig. 3. Solar flux noted for setup-I and setup-II 3.2. Heat supplied to the drying section (actual heat supply) Equation (4) has been used to evaluate heat supplied to the drying section (Qoc). Fig. 4 shows Qoc variation with time. The Qoc varies according to the solar radiation and hence it looks like the same as Fig. 3. The average and maximum values of Qoc for setup-1 were 705.64 and 1056.86 W, while the same for setup-II were 730.45 and 1218.56 W, respectively. The 14 difference in average Qoc was 24.81 W (3.4%) between setup-I and setup-II, which may be the result of a change in temperature and mass flow rate during testing which is comparatively negligible. 1400 Setup-I Actual heat supply (W) 1200 Setup-II 1000 800 600 400 200 0 8 10 12 14 16 18 20 Time (h) 22 24 26 28 Fig. 4. Supply of actual heat versus time for carrot drying experiment in setup-I and setup II 3.3. Thermal efficiency of the collector The data obtained during carrot drying in setup-I and setup-II was used to evaluate the thermal efficiency of the SAC (ηc). Fig. 5 describes the variation of ηc over time. The ηc changes with solar radiation and hence it increases and reaches its maximum at midday. Setup-I and setup-II had average and maximum ηc of 56.84 and 60.79% & 76.48 and 75.88%, respectively. The difference in average ηc was 6.95%. Mugi and Chandramohan [31] reported an average ηc of 63.3% during drying green chilli and Gilago and Chandramohan [6] reported 66.5% during drying ivy gourd and these values were formidable acceptance with the outcomes of the current study. 15 90 Setup-I 80 Setup-II 70 ηc (%) 60 50 40 30 20 10 0 8 10 12 14 16 18 20 Time (h) 22 24 26 28 Fig. 5. Thermal efficiency of the collector during drying carrot in setups-I and II 3.4. Drying efficiency The ηd was estimated while drying carrot slabs setup-I and setup-II and plotted in Fig. 6. The ηd is the corollary of the amount of MC removed during drying and the average solar radiation during the whole drying process [32] as mentioned in Eq. (7). As shown in Fig. 6 that the ηd values increase at a faster rate before midday for both setups, then decreased similarly at afternoon. The variation of ηd was higher in setup-I compared to setup-II because, in setup-II, the temperature of drying air is low due to heat absorption taking place in the TES unit and hence MC removal is low. For setup-I, the average and maximum values of ηd were 7.5 and 23.66%, and the same for setup-II were 10.35 and 20.9%, respectively. There was an enhancement of 38% in the average ηd of setup-II than setup-I. According to the study of Caesar et al. [39], the ηd of 11% was noticed while drying pears which is in agreement with the present study. 16 25 ηd (%) 20 Setup-I Setup-II 15 10 5 0 8 10 12 14 16 Time (h) 18 20 22 24 Fig. 6. ηd variation during drying carrot in setup-I and setup-II 3.5. Temperature recordings Figure 7 presents the recorded data of temperature during experiments in setups-I and II. Fig. 7 (a) gives the temperatures of outside air (Ta), the inlet of the collector (Tci), the outlet of the collector (Tco), trays 1, 2, 3, and 4, respectively are denoted by T1, T2, T3, and T4 for setup-I. Accordingly, the maximum Ta, Tci, Tco, T1, T2, T3, and T4 of setup-I were 41.6, 43, 75, 69, 65, 64, and 57 °C, respectively, and their corresponding averages were 35.8, 37.6, 61.2, 55.8, 51.4, 49.6, and 47.1 °C. 17 Similarly, the temperature records of setup-II are displayed in Fig. 7 (b). The average values of Ta, Tci, Tco, T1, T2, T3, and T4 in setup-II were 33.6, 34.2, 45.5, 41.1, 40.5, 40, and 39.1℃, respectively, while the maximum values of the same were 38.2, 41, 71, 52, 51, 51, and 51 ℃. Overall, setup-I had higher average and maximum temperature values than setup-II. The difference in temperature could have been due to the TES unit as it decreased the temperature inside the drying section (on the trays) because of heat stored in TES in setup II. (a) 80 Temperature (oC) 70 60 50 40 30 Ta T1 T4 20 Tci T2 Tco T3 10 8 10 12 14 16 Time (h) 18 20 22 24 (b) 80 Ta T1 T4 Tempeerature (oC) 70 Tci T2 Tco T3 60 50 40 30 20 10 8 10 12 14 16 Time (h) 18 20 22 24 Fig. 7. Data of temperature recorded during drying carrot slabs in (a) setups-I and (b) II 3.6. Kinetics of drying for carrot 3.6.1. Moisture content 18 To determine the MC, the mass loss of carrot during the drying experiment was recorded in both setups. Fig. 8 shows how the MC varies with time in both setups. The MC started to decrease slowly in the initial stage and a rapid drop was noticed after a certain period. This indicates that the early phase of the removal of moisture is initiation and warming. As the slopes of the graphs increased in the second phase, it was evident that there was a greater amount of moisture being lost from the outer and interior surfaces. Towards the end of the process, more moisture could be removed from the internal pores of the sample, but it requires more energy, and thus the slope tends to be flattened for both setups. Throughout the drying period, setup-I shows a steeper slope than setup-II. Setup-I and setup-II dried the slabs from 9.13 to 0.478 (db) within 16 h and 15 h, respectively where setup-I required two sunshine days but setup-II took only one day. 10 MC (db) 9 8 Setup-I 7 Setup-II 6 5 4 3 2 1 0 8 10 12 14 16 Time (h) 18 20 22 24 Fig. 8. The momentary MC (db) of carrot in setup-I and setup-II 3.6.2. Moisture ratio (MR) It is the ratio between the MC available in the drying slab at a particular moment to the MCi of the drying sample. Eq. (10) is used to estimate the same and is shown in Fig. 9. The characteristic graph of MR with time is the same nature as MC reported in Fig. 8 because it is a function of MC. 19 1.2 1 Setup-I Setup-II MR 0.8 0.6 0.4 0.2 0 8 10 12 14 16 Time (h) 18 20 22 24 Fig. 9. MR variation with time of carrot during drying in setups-I and II 3.6.3. Drying rate In this study, the rate of drying (DR) of carrot slices in setup-I and setup-II has been plotted (Fig. 10) and analyzed. It can be seen that DR increased at a steady rate until it reached its maximum at noon. DR of setup-I increased at a higher rate than setup-II due to lower drying temperature in setup-II (since TES unit is installed) than setup-I. The DR of setups-I and II decreased at a faster rate after reaching their maximum value. For setup-I and setup-II, the average and maximum DRs were 0.497 and 0.588 kg/h, and 1.38 and 1.24 kg/h, respectively. The average DR in setup-II improved by 18.3% compared to setup-1. Vijayan et al. [2] dried bitter gourds in a solar dryer at a maximum DR of 2.87 kg/h which is almost the same as the present analysis. 20 DR (kg/h) 1.6 1.4 Setup-I 1.2 Setup-II 1 0.8 0.6 0.4 0.2 0 8 10 12 14 16 Time (h) 18 20 22 24 Fig. 10. DR during drying carrot in setup-I and setup-II 3.6.4. Coefficient of moisture diffusion (De) The De for carrot slices dried in setup-I and setup-II have been evaluated from the observations and plotted in Fig. 11. The De increased at a faster rate for the first phase as a lot of MC is eliminated in the first phase. The interior MC needs more energy to be removed as it is trapped in the complex pores of carrot slabs and therefore, the De is increased at a slower rate in the second phase compared to first phase of drying. The differences in the rate of variation for De between the two setups match with the variations in MC. The average De for setup-I and setup-II was determined to be 6.7 and 7.24 × 10-09 m2/s and their values were 0.229 - 1.07 × 10-08 m2/s and 2.29 × 10-09 - 1.00 × 10-09 m2/s, respectively. Existing literature (Tagnamas et al. [1]) reported the De values those are almost in a similar range (1.2 × 10-9 4.2 ×10-9 m2/s). 21 1.2E-08 Setup-I 1E-08 Setup-II De (m2/s) 8E-09 6E-09 4E-09 2E-09 0 8 10 12 14 16 18 Time (h) 20 22 24 Fig. 11. De variation during drying carrot in setup-I and setup-II 3.6.5. Coefficient of heat transfer (h) Using Eq. (15), the h of carrot slices dried in hot air in setup-I and setup-II is calculated and its characteristic graph is depicted in Fig. 12. The curve of h follows the same trend as De as mentioned in Fig. 12. The mean values of h were 6.32 and 7.08 W/m2K for setup-I and setupII, respectively. In setup-II, there was an improvement of 12.03% in h in comparison with setup-I. The current outcomes are in favorable consensus with those of the existing data [40] while drying watermelon in a passive solar dryer as the reported range was 0.52 - 5.039 and 0.158 - 3.19 W/m2 K for apple and watermelon, respectively. 3.6.6. Coefficient of mass transfer (hm) Equation (14) has been used to determine the hm of carrot slices dried in setup-I and setup-II. It is mentioned in Fig. 13. The style of variation of hm is similar to those of h and De. Setup-I had an average hm of 0.0055 m/s and setup-II had an average hm of 0.0062 m/s. There was a 12.73% improvement in average hm noticed in setup-II. The evaluated hm range was between 0-0.011 m/s. The published hm data of watermelon and apple was 0.00052 – 0.005 m/s and 0.00016 – 0.0032 m/s [40], respectively, which is almost similar to the present study. 22 14 Setup-I 12 Setup-II h (W/m2 K) 10 8 6 4 2 0 8 10 12 14 16 Time (h) 18 20 22 24 Fig. 12. Coefficient of heat transfer during drying carrot in setup-I and setup-II 0.012 Setup-I 0.01 Setup-II hm (m/s) 0.008 0.006 0.004 0.002 0 8 10 12 14 16 Time (h) 18 20 22 24 Fig. 13. Coefficient of mass transfer during carrot drying in setup-I and setup-II 3.7 Describing the relationship of MC with h, hm and h The relationship of De, h, and hm with MC during the carrot drying experiment is represented in Figs. 14, 15, and 16, respectively. A trending function of correlation was established by tracing their variations. De, h, and hm showed significant correlations with the MC in the logarithmic function F(MC) = b1 ln (MC) + b2, where, F represents the variables, b1 and b2 23 are the correlation constants. Table 2 summarizes the values for all variables, constants, and coefficient of correlation (R2). Figure 14 illustrates the variation in De with MC (db) of carrot slabs dried in setup-I and setup-II. As the MC was decreased, De gradually increased for both setups. When the free bound MC is removed from carrot slabs, more amount of unbound moisture is diffused from the complex pores per unit area and hence such inverse variation is noticed between MC and De. For both setups, hm (Fig. 15) and h (Fig. 16) change in the same manner as De (Fig. 14). Findings from this study are in accordance with those from existing literature [4, 31]. 1.2E-08 1E-08 Setup-I De (m2/s) 8E-09 Setup-II 6E-09 4E-09 2E-09 0 0 1 2 3 4 5 6 7 Moisture content (db)) 8 9 10 Fig. 14. De variation over MC (db) of carrot slices dried in setup-I and setup-II 0.012 Setup-I 0.01 Setup-II hm (m/s) 0.008 0.006 0.004 0.002 0 0 1 2 3 4 5 6 MC (db) 7 8 9 10 Fig. 15. The hm variation over MC (db) of carrot slices in setup-I and setup-II 24 14 Setup-I Setup-II 12 h (W/m2K) 10 8 6 4 2 0 0 1 2 3 4 5 6 MC (db) 7 8 9 10 Fig. 16. The h variation over MC (db) of carrot slices in setup-I and setup-II 3.8. SEC, SMER and Ea The SEC refers to the specific energy consumed while removing a kg of MC from the drying sample. Eq. (8) was used to determine the SEC of carrot slices dried in setup-I and II. According to the calculations, setups-I and II had an average SEC of 3.5 and 0.28 kWh/kg, respectively. In comparison, setup-II exhibited 92% less SEC than setup-I. Additionally, the amount of MC removed while applying 1 kWh of energy during the drying of a sample is known as SMER. Accordingly, the average SMER of carrot slices dried in setup-I was calculated to be 0.29 kg/kWh, and in setup-II, it was 3.6 kg/kWh. In comparison to setup-I, setup-II showed an improvement of 3.31 kg/kWh of SMER. The results of Mugi and Chandramohan [31] also showed a similar SEC and SMER and they were approximately 1.783 kWh/kg and 0.6526 kg/kWh while drying green chilli in a passive solar dryer, confirming that the values of the present work are matching. Moreover, the Eac for the carrot drying experiments was appraised for both dryers. Accordingly, the average Eac of carrot slices was 41.25 kJ/mol in setup-I, and 39.5 kJ/mol in setup-II. In comparison with setup-I, Eac improved by 4.73% in setup-II. During drying carob kernels (Ceratonia Siliqua L.) in a solar dryer, Tagnamas et al. [1] observed an Eac of 41.46 kJ/mol which agrees well with the current work. 25 Table 2. Correlation details between De, h, and hm with MC F(MC)= b1 ln (MC) + b2 Variable (Y) De h hm Constants N Setup R2 b1 b2 I -3×10-9 8×10-9 0.9942 II -3×10-9 8×10-9 0.9849 I -4.033 8.5518 0.9891 II -3.857 8.3415 0.9937 I -0.004 0.0077 0.981 II -0.003 0.0073 0.9934 4. Conclusions Experimental research has been conducted to distinguish the drying kinetics of carrot in a passive indirect solar dryer without (setup-I) and with thermal energy storage (TES) (setupII). The following main conclusions are retrieved for both setups. o Having recorded an average solar radiation of 648.75 and 616.27 W/m2 for the drying days of setup-I and II, the average thermal efficiency of the collector was 56.84 and 60.79%, respectively. The mean actual heat supply for the same was evaluated to be 705.64 and 730.45 W, respectively. o Setup-II showed a 38% improvement in drying efficiency compared to setup-I, while setup-I and setup-II had respective averages of 7.5 and 10.35%. o The average drying rate was 0.498 and 0.588 kg/h for setup-I and setup-II, respectively, indicating setup-II achieved an 18.3% improvement over setup-I. o In setup-I and setup-II, the effective moisture diffusion coefficients (De) were 6.7 and 7.24× 10-9 m2/s, respectively. And the coefficient of heat transfer (h) was 6.32 and 7.08 W/m2K, respectively. Additionally, setup-I and setup-II showed an average mass transfer coefficient (hm) of 0.00549 and 0.00615 m/s, respectively. Moreover, the activation energy was 41.25 kJ/mol for setup-I and 39.5 kJ/mol for setup-II, respectively. 26 o Based on the analysis of the specific energy consumption for setup-I and setup-II, 3.5 and 0.28 kWh/kg were estimated, respectively, while setup-II showed a reduction of 92% in energy consumption. Moreover, for Setup-II, the specific moisture extraction rate was 3.31 kg/kWh higher compared to Setup-I, which had a specific moisture extraction rate of 0.29 kg/kWh and 3.6 kg/kWh, respectively. o As the moisture content decreased, the De, h, and hm were observed to be increasing in a logarithmic trend, while they were also increasing over time. o The sample dried from 9.13 to 0.478 (db) in 16 h (setup-I) and 15 h (setup-II). SetupII performed well, as the drying lasted for one day and was more efficient than setupI. The overall results of the evaluation suggest that setup-II would be reasonably recommended for subsequent large applications if it is further studied with an economic and computational analysis. Also, numerical optimization studies are necessary for the dryer dimensions and TES unit to achieve the maximum performance of the dryers. 27 Acknowledgements The authors acknowledge Department of Mechanical Engineering, NIT Warangal, India for financing the research work. The reference number is, NITW/MED/Head/2015/408 dated 3rd Dec. 2015. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 28 References [1] Z. Tagnamas, M. Kouhila, Y. Bahammou, H. Lamsyehe, H. Moussaoui, A. Idlimam, A. Lamharrar, Drying kinetics and energy analysis of carob seeds (Ceratonia siliqua L.) convective solar drying, J. Therm. Anal. Calorim. (2021). https://doi.org/10.1007/s10973-021-10632-6. [2] S. Vijayan, T. V. Arjunan, A. Kumar, Mathematical modeling and performance analysis of thin layer drying of bitter gourd in sensible storage based indirect solar dryer, Innov. Food Sci. Emerg. Technol. 36 (2016) 59–67. https://doi.org/10.1016/j.ifset.2016.05.014. [3] D.V.N. Lakshmi, P. Muthukumar, A. Layek, P.K. Nayak, Drying kinetics and quality analysis of black turmeric (Curcuma caesia) drying in a mixed mode forced convection solar dryer integrated with thermal energy storage, Renew. Energy; 120 (2018):23–34. https://doi.org/10.1016/j.renene.2017.12.053. [4] L.F. Hidalgo, M.N. Candido, K. Nishioka, J.T. Freire, G.N.A. Vieira, Natural and forced air convection operation in a direct solar dryer assisted by photovoltaic module for drying of green onion, Sol. Energy. 220 (2021) 24–34. https://doi.org/10.1016/j.solener.2021.02.061. [5] N.H.A. Tajudin, S.M. Tasirin, W.L. Ang, M.I. Rosli, L.C. Lim, Comparison of drying kinetics and product quality from convective heat pump and solar drying of Roselle calyx, Food Bioprod. Process. 118 (2019) 40–49. https://doi.org/10.1016/j.fbp.2019.08.012. [6] M.C. Gilago, V.P. Chandramohan, Performance parameters evaluation and comparison of passive and active indirect type solar dryers supported by phase change material during drying ivy gourd, Energy 252 (2022) 123998. https://doi.org/10.1016/j.energy.2022.123998. [7] S. Mohammed, N. Fatumah, N. Shadia, Drying performance and economic analysis of novel hybrid passive-mode and active-mode solar dryers for drying fruits in East Africa, J. Stored Prod. https://doi.org/10.101+6/j.jspr.2020.101634. 29 Res. 88 (2020) 101634. [8] S.M. Shalaby, M.A. Bek, Experimental investigation of a novel indirect solar dryer implementing PCM as energy storage medium, Energy Convers. Manag. 83 (2014) 1– 8. https://doi.org/10.1016/j.enconman.2014.03.043. [9] S. Aboul-Enein, A.A. El-Sebaii, M.R.I. Ramadan, H.G. El-Gohary, Parametric study of a solar air heater with and without thermal storage for solar drying applications, Renew. Energy 21 (2000) 505–522. https://doi.org/10.1016/S0960-1481(00)00092-6. [10] K.N. Shukla, Thermal energy storage for solar power generation: State of the art, Heat Transf. Eng. 3 (1981) 62–72. https://doi.org/10.1080/01457638108939581. [11] D.V.N. Lakshmi, P. Muthukumar, P.K. Nayak, Experimental investigations on active solar dryers integrated with thermal storage for drying of black pepper, Renew. Energy 167 (2021) 728–739. https://doi.org/10.1016/j.renene.2020.11.144. [12] N.A. Mhd Safri, Z. Zainuddin, M.S. Mohd Azmi, I. Zulkifle, A. Fudholi, M.H. Ruslan, K. Sopian, Current status of solar-assisted greenhouse drying systems for drying industry (food materials and agricultural crops), Trends Food Sci. Technol. 114 (2021) 633–657. https://doi.org/10.1016/j.tifs.2021.05.035. [13] M.M.I. Chowdhury, B.K. Bala, M.A. Haque, Energy and exergy analysis of the solar drying of jackfruit leather, Biosyst. Eng. 110 (2011) 222–229. https://doi.org/10.1016/j.biosystemseng.2011.08.011. [14] R.O. Lamidi, L. Jiang, P.B. Pathare, Y.D. Wang, A.P. Roskilly, Recent advances in sustainable drying of agricultural produce: A review, Appl. Energy 233–234 (2019) 367–385. https://doi.org/10.1016/j.apenergy.2018.10.044. [15] A.E. Kabeel, A. Khalil, S.M. Shalaby, M.E. Zayed, Improvement of thermal performance of the finned plate solar air heater by using latent heat thermal storage, Appl. Therm. Eng. 123 (2017) 546–553. https://doi.org/10.1016/j.applthermaleng.2017.05.126. [16] Z. Alimohammadi, H. Samimi Akhijahani, P. Salami, Thermal analysis of a solar dryer equipped with PTSC and PCM using experimental and numerical methods, Sol. Energy 201 (2020) 157–177. https://doi.org/10.1016/j.solener.2020.02.079. 30 [17] R. Moradi, A. Kianifar, S. Wongwises, Optimization of a solar air heater with phase change materials: Experimental and numerical study, Exp. Therm. Fluid Sci. 89 (2017) 41–49. https://doi.org/10.1016/j.expthermflusci.2017.07.011. [18] A.K. Bhardwaj, R. Kumar, S. Kumar, B. Goel, R. Chauhan, Energy and exergy analyses of drying medicinal herb in a novel forced convection solar dryer integrated with SHSM and PCM, Sustain. Energy Technol. Assessments 45 (2021) 101119. https://doi.org/10.1016/j.seta.2021.101119. [19] M.C. Gilago, V.P. Chandramohan, Effect of Phase Change Materials on the Performance of Natural Convection Indirect Type Solar Dryer during Drying Ivy Gourd, Heat Trans. Eng. (2022) 1–13. https://doi.org/10.1080/01457632.2022.2079045. [20] V. Reddy, M.C. Gilago, V.P. Chandramohan, Energy and exergy investigation of indirect solar dryer under natural and forced convection while drying muskmelon slices, Energy Nexus 8 (2022) 100153. https://doi.org/10.1016/j.nexus.2022.100153. [21] K.R. Arun, G. Kunal, M. Srinivas, C.S.S. Kumar, M. Mohanraj, S. Jayaraj, Drying of untreated Musa nendra and Momordica charantia in a forced convection solar cabinet dryer with thermal storage, Energy 192 (2020) 116697. https://doi.org/10.1016/j.energy.2019.116697. [22] M. Goud, M.V.V. Reddy, C. V.P., S. S., A novel indirect solar dryer with inlet fans powered by solar PV panels: Drying kinetics of Capsicum Annum and Abelmoschus esculentus with dryer performance, Sol. Energy 194 (2019) 871–885. https://doi.org/10.1016/j.solener.2019.11.031. [23] A. Lingayat, V.P. Chandramohan, V.R.K. Raju, A. Kumar, Development of indirect type solar dryer and experiments for estimation of drying parameters of apple and watermelon: Indirect type solar dryer for drying apple and watermelon, Therm. Sci. Eng. Prog. 16 (2020). https://doi.org/10.1016/j.tsep.2020.100477. [24] L.A. Mohamed, M. Kouhila, A. Jamali, S. Lahsasni, N. Kechaou, M. Mahrouz, Single layer solar drying behaviour of citrus aurantium leaves under forced convection, Energy Convers. Manag. https://doi.org/10.1016/j.enconman.2004.08.001. 31 46 (2005) 1473–1483. [25] V. Reddy Mugi, V.P. Chandramohan, Energy, exergy and economic analysis of an indirect type solar dryer using green chilli: A comparative assessment of forced and natural convection, Therm. Sci. Eng. Prog. 24 (2021) 100950. https://doi.org/10.1016/j.tsep.2021.100950. [26] A.K. Babu, G. Kumaresan, V.A.A. Raj, R. Velraj, Review of leaf drying: Mechanism and influencing parameters, drying methods, nutrient preservation, and mathematical models, Renew. Sustain. Energy Rev. 90 (2018) 536–556. https://doi.org/10.1016/j.rser.2018.04.002. [27] N. Wang, J.G. Brennan, A mathematical model of simultaneous heat and moisture transfer during drying of potato, J. Food Eng. 24 (1995) 47–60. https://doi.org/10.1016/0260-8774(94)P1607-Y. [28] A. Reyes, A. Mahn, F. Vásquez, Mushrooms dehydration in a hybrid-solar dryer, using a phase change material, Energy Convers. Manag. 83 (2014) 241–248. https://doi.org/10.1016/j.enconman.2014.03.077. [29] V.P. Chandra Mohan, P. Talukdar, Three dimensional numerical modeling of simultaneous heat and moisture transfer in a moist object subjected to convective drying, Int. J. Heat Mass Transf. 53 (2010) 4638–4650. https://doi.org/10.1016/j.ijheatmasstransfer.2010.06.029. [30] A. Kaya, O. Aydin, An experimental study on drying kinetics of some herbal leaves, Energy Convers. Manag. 50 (2009) 118–124. https://doi.org/10.1016/j.enconman.2008.08.024. [31] V. Reddy Mugi, V.P. Chandramohan, Energy, exergy and economic analysis of an indirect type solar dryer using green chilli: A comparative assessment of forced and natural convection, Therm. Sci. Eng. Prog. 24 (2021) 100950. https://doi.org/10.1016/j.tsep.2021.100950. [32] M.C. Gilago, V.P. Chandramohan, Performance evaluation of natural and forced convection indirect type solar dryers during drying ivy gourd: An experimental study, Renew. Energy 182 (2021). https://doi.org/10.1016/j.renene.2021.11.038. 32 [33] M.A. Karim, M.N.A. Hawlader, Mathematical modelling and experimental investigation of tropical fruits drying, Int. J. Heat Mass Transf. 48 (2005) 4914–4925. https://doi.org/10.1016/j.ijheatmasstransfer.2005.04.035. [34] Z. Tagnamas, Y. Bahammou, M. Kouhila, S. Hilali, A. Idlimam, A. Lamharrar, Conservation of Moroccan truffle (Terfezia boudieri) using solar drying method, Renew. Energy 146 (2020) 16–24. https://doi.org/10.1016/j.renene.2019.06.107. [35] Sunil, Varun, N. Sharma, Experimental investigation of the performance of an indirectmode natural convection solar dryer for drying fenugreek leaves, J. Therm. Anal. Calorim. 118 (2014) 523–531. https://doi.org/10.1007/s10973-014-3949-2. [36] M.R. Patel, N.L. Panwar, Drying kinetics , quality assessment and socio environmental evaluation of solar dried underutilized arid vegetable Cucumis callosus, Energy Nexus. 7 (2022) 100128. https://doi.org/10.1016/j.nexus.2022.100128. [37] E.L. Cussler, Fundamentals of Mass Transfer, Part III- Mass Transfer. (2012) 237–273. http://calliope.dem.uniud.it/CLASS/IMP-CHIM/C8-Cussler.pdf. [38] S. Yadav, V.P. Chandramohan, Performance comparison of thermal energy storage system for indirect solar dryer with and without finned copper tube, Sustain. Energy Technol. Assessments 37 (2020) 100609. https://doi.org/10.1016/j.seta.2019.100609. [39] L.V. Erick César, C.M. Ana Lilia, G.V. Octavio, S.S. Orlando, D.N. Alfredo, Energy and exergy analyses of a mixed-mode solar dryer of pear slices (Pyrus communis L), Energy. 220 (2021). https://doi.org/10.1016/j.energy.2020.119740. [40] A. Lingayat, V.P. Chandramohan, V.R.K. Raju, A. Kumar, Development of indirect type solar dryer and experiments for estimation of drying parameters of apple and watermelon: Indirect type solar dryer for drying apple and watermelon, Therm. Sci. Eng. Prog. 16 (2020). https://doi.org/10.1016/j.tsep.2020.100477. 33