Drying Solanum Iycopersicum Tomatoes in greenhouse solar dryer An eco-environmental study

Abstract

Keywords: Agriculture; Carbon Emission; Embodied Energy; Greenhouse; Life-cost; Payback; Solar Dryer

Introduction

Constant food production is not possible for various reasons, but the food can be produced in adequate quantity and stored for future demand. Still, the extra amount of the food produced starts to deteriorate as soon as it is harvested. Almost 1/3rd of the harvested agro products are wasted in India due to a lack of storage facilities [1]. Drying food products is very popular for the storage of food. The

conventional drying processes are heavily energy-driven, a significant concern as the traditional energy sources add huge amounts of carbon footprints [2]. The increase in carbon footprint will directly affect the environmental cycle. Solanum lycopersicum (Tomatoes) are a great source of vitamin A and one of the most consumed vegetables in the Indian market [3]. The overproduction of tomatoes in the harvesting season and their abundance in the market results

*Corresponding author. *E-mail address: pringalsvnitphd006@gmail.com This paper was recommended for publication in revised form by Editor-in-Chief Ahmet Selim Dalkılıç Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

in a decrease in the price of tomatoes. The post-harvesting losses can be significantly reduced by the drying of the tomatoes. Dried tomatoes have a large economic and application market with use in various salads, pizzas, ketchup, soups and other dishes [4].The various scientific research on the drying of tomatoes suggests that heat treatment of the tomatoes does not affect their nutritional value while sometimes also increasing the level of lycopene content and antioxidant property [5]. The major problem with free sun drying is that it is prone to dust contaminations, bird excrement, and rainfall, which would indefinitely deteriorate the product’s quality and appearance [6]. Chamber-based solar drying and storage systems evolved from these primary problems. The concept of chamber-based or greenhouse-based solar drying of food particles is suitable for developing nations, as farmers can utilize it directly without requiring any technical knowledge [7]. The material used in the drying arrangement is also readily available in the surrounding market at cost-effective rates. The significant advantages of a greenhouse drying system are that it consists of no moving parts and it has an almost negligible maintenance cost, and the initial cost of development is also low [8,9] dried 80 kg of chillies in a tunnel-type solar dryer, and the results suggest that the solar-dried chillies had a premium colour and astringency consistency compared to open sun-dried chillies. The pre-treatment of the products helped in shortening the drying time. The same tunnel dryer was also used to understand the drying kinetics of meat and peppermint plants [10]. The investigation of drying tomato pomace through a solar tunnel greenhouse dryer was carried out in a chamber built as a half cylinder with a plastic lid. The continuous use of exhaust fans increased the drying rate in the dryer [11]. The greenhouse solar dryer with polycarbonate sheets with 70% transmittance covered on the ceiling and three sides of the chamber with an angled roof and side loading entrance is used to dry out tomato pomace. The rear wall of the chamber was insulated with black polystyrene, and the product trays were kept inside the dryer; it took around 5 hours to dry the product at the temperature range of 40-to-58 degrees Celsius [12]. offered a somewhat alternative greenhouse solar dryer design. The concrete base of the dryer was replaced with two layers of solar collector. A solar module was provided to operate the exhaust fan. The coefficient of convective heat transfer increased due to the addition of the solar collector in the base, and the analytical finding suggests that the heat consumption factor, energy and exergy efficiency of the dryer is also more compared to the dryer without the solar collector. The dried is proposed to use for crop drying at temperatures ranging from 40oC to 70oC. A large-sized parabolic drying chamber greenhouse solar dryer was fabricated to dry 300 to 1000 kg of chilli. It consisted of nine ventilation fans which are operated with three 50 W solar cell modules. The drying time for 500 kg chilli was decreased from 5 days to 3 days when compared to open sun drying, with the chilli having initial moisture of 74% (wet basis) [13]. The

performance comparison was carried out in the greenhouse drier with thermal storage and without thermal storage for drying red pepper in the drying system. The drying time was reduced to 30 hrs for the dryer with thermal storage, which was found always higher about 5-19oC compared to ambient conditions. The drying time reduces to 55 hrs in the dryer without thermal storage when compared to 75 hrs of open sun drying [14]. The performance analysis of the thermal storage material calcium hexahydrate as PCM was investigated, which suggest that varying the thickness of the PCM in the north wall from 2-5 cm varies the amount of heat stored in the drying chamber. The thickness of 4 cm was found best with the increased temperature of around 6-12oC inside the dryer in the nighttime while used in passive mode [15]. the solar air heater was coupled with the greenhouse drier to power the PCM unit inside the dryer, the average exergy and energy efficiency were found to be 4.2% and 74.3% respectively [16]. The combination of various salt was used as PCM for minimizing the formation of frost in the greenhouse dryer during cold climatic conditions; two different heat exchangers were used to keep the floor and air inside the dryer heated up. The heat exchange with sub-surface configuration was used to raise the temperature of the floor, and a water-air heat exchanger was used for the air inside the greenhouse dryer. An average increment of 4–6 degrees Celsius was reported compared to the atmospheric condition. The combination of thermal storage systems allows the drying of both during day and night for the products [17,18]. Various researchers have been working on the development of various types of solar greenhouse dryer technology to increase productivity and decrease the drying time for various types of agriculture products; the active and hybrid systems show up great potential and can be scaled up for farmers. The various application of thermal energy storage is also applied to making the dryer work for around 24 hours. The scaling up of various potential designs of dryers would lead us to practical and edible agro products in the market. The actual and marketable agro products or dried products need to be homogenous in consistency. The big batches of products need to be dried up equally with the same drying rate to achieve such consistency. The application of medium and large-scale dryers will lead us to non-homogenous drying around various points in the drying chamber. As the utilization of chamber-based drying system is increasing exponentially in developing countries, more research needs to be focused on the spatial variation of the drying rate in the chamber, which affect the quality of the final product. The objective of the current study is to analyze the thermal performance of the greenhouse solar dryer for drying tomatoes and a novel approach to depict the spatial drying performance of the drying chamber. Estimate the embodied energy and carbon credits for the current greenhouse drying system. Analyze the life cycle cost of the current system and its economic sustainability for tomato drying.

Material And Methods

Process The solar radiation penetrates the transparent sheet of polycarbonate and is absorbed by the food products. The reflected infrared radiations are trapped due to the polycarbonate sheet, which creates a greenhouse effect. Due to the greenhouse effect, the content in the dryer is heated up, and the air inside is circulated through the product due to density difference; thus, the drying of the product is carried out. Through the outlet aperture, wet air emerges, taking the moisture the product released with it. The removal of this moisture is the primary phenomenon involved in drying. Agro products consist of diverse nutrient combinations, with varying degrees of heat resistance in their constituent bond chains. Consequently, it is crucial to apply heat for moisture removal within specific limits to avoid nutrient damage, thereby ensuring the preservation of the dried product’s quality [19]. The phenomenon of drying can be bifurcated into three parts, which would include pre-drying process, The drying process, and Post-drying process. In the pre-drying section, the material is processed through some treatment, which helps them to retain some nutrient properties or may help in the actual drying process. In the second part of drying comes the use of the equipment and variables for controlling the amount, direction, and time for which the product is exposed to heat energy used for removing the moisture content. The post-drying process is followed up at last to bring the product into thermodynamic equilibrium or consumable state [20]. The evaluation of the drying system is characterized by various parameters like thermal efficiency, energy efficiency and specific heat consumption. Generally, specific heat consumption and energy efficiency have been mainly analyzed for dryer performance. Figure 1 shows how the analysis of energy consumption is performed in an indirect convective dryer system [21]. Solar radiation, the ambient temperature, and the specific humidity are also a matter of concern while designing the solar drying system,

Figure 1. Schematic representation of an indirect drying process.

and all the environmental variables will directly affect the performance of the dryer [22, 23]. Experimental Apparatus The experimental setup consists of a greenhouse solar dryer (GHSD) which was developed on the premises of the company “727 Solar Food Products” at Navsari, Surat, Gujarat, with the global co-ordinate of 20.76oN, 73.05oE. The greenhouse solar dryer is a structure that directly traps solar energy in a restricted environment to transform it into thermal energy [24]. As shown in Figure 2, the developed GHSD was constructed using a galvanized iron (GI) box pipe with a foundation support of hardwood plywood with black colour thermal paint to absorb the maximum amount of heat from the surrounding [25]. The dryer’s structure consists of four walls and a roof that is angled at a 21o angle. The sides are covered with two mm-thick, clear polycarbonate sheets with a 70% transmittance value to allow the maximum amount of radiation to pass through, and they are also lightweight which makes the system much more accessible [26, 27]. The polycarbonate sheet was chosen to be the dryer’s transparent cover because it has a high transmittance of shortwave solar radiation and a low transmittance of infrared radiation, which causes the dryer to produce a greenhouse effect [28, 29]. The moist air was evacuated using the fan positioned on the upper

Figure 3. Schematic diagram of the dryer with measurement details.

side of one wall. The dryer is operated under force convection to increase its effectiveness [30]. The placement of exhaust fans is kept on the top of the chamber as the hot air always rises due to a decrease in density, and it is necessary to remove the saturated hot air for proper circulation and homogenous drying. Another reason for placing the exhaust fan at the top is to avoid dust particles and contamination from the surrounding, which generally settle down near the ground. The detailed specifications of the dryer are shown in the schematic diagram (Fig. 3). The product to be dried is placed in the three carts; each cart consists of two trays one placed at an upper level and the other one at the lower level. The size of each tray is 80 cm by 40 cm and is made of food-grade aluminium material.

were measured using the RTD sensors connected with the data logger cum indicator. The manual readings of the temperatures were also recorded every 15 minutes to verify any instrumentation errors. The relative humidity was measured using a digital thermometer and humidity meter inside and outside the dryer at 15-minute intervals. The solar radiation inside the drying chamber and in the outside atmosphere was measured using a solar meter at an interval of 10 min during the experiment. The wind velocity was measured at a similar instance using the digital anemometer at the exhaust of the drying chamber and outside atmosphere. The detailed specifications of the different instruments used are in Table 1.

Data Collection The various parameters like the temperature at the surface of various trays, atmospheric temperature, wind velocity, exhaust fan velocity, solar radiation and relative humidity were measured in various instances to relate the effect of each with the drying phenomenon. The temperature at the upper layer and lower layer trays in the trolley along with the temperature outside the dryer

Drying Kinetics The observation of the mass of the drying tomatoes is one of the important parameters to understand the drying rates. During the drying operations, the mass of the tomatoes was measured every hour in different trays. The moisture content was calculated using the expression given below [31],

Table 1. Specification of the instruments used in the experiment Instruments

Eight-channel universal temperature scanner logger +/- 0.1 oC for PT100

(1) Where Wi denotes the product’s starting weight, Wd denotes its bone-dry weight at time t, and Mt denotes its moisture content (%dB) at that same time. The moisture ratio (MR) can be used to indicate the moisture content (%dB) as a function of drying time. (2) Where MR is the moisture ratio, Mt is the moisture content at time t, Mo is the initial moisture content (%dB) and Me is the equilibrium moisture content at time t which can be determined using the relative humidity around the surrounding. The equation can be simplified as below to eliminate the effect of frequently changing relative humidity levels in the surrounding. (3) The moisture ratio was monitored to understand the drying dynamics of the product. The difference in the moisture ratio for the product at various location points was analyzed to understand the effect of shading and air flow circulation on the drying rates. The drying rates for various trays were compared to find out best location for optimum drying in the drying chamber. Experimental Procedure Fresh tomatoes were purchased from the local market to ensure the best quality and freshness. The experiments were conducted in April 2021 at Navsari, Gujarat, India. The drying of tomatoes started at 8:00 am. The drying was stopped at around 5:00 pm for the day. The evaluation of the moisture content removed from the products was carried out from the weight loss analysis. The tomatoes were weighed and sliced over the tray in a thin layer through

which air could pass in order to increase the drying rate. The tomatoes were distributed into six trays which weighed around 1 kg in each tray, to analyze the effect of the position and location of drying product in the dryer. The final combined mass of the product and tray was noted down before putting the tray in the dryer to monitor the water removal rate of the product accurately. The mass of the tray was measured every hour with a weight balance having an accuracy of 1 g.

Results And Discussion

Solar Radiation The solar radiation intensity over the greenhouse chamber and in the atmosphere directly affects the drying of the product (Tomatoes). The direct solar radiation intensity measurements were recorded just outside the solar dryer and inside the drying chamber to mark the difference of direct solar radiation falling on the drying product after passing through the polycarbonate sheet. The measurement was noted every 10 mins during the experimentation (9 hr). The maximum intensity measured was around 1185 W/m2 at 12:30 pm in the atmosphere and 925 W/m2 inside the drying chamber. The proceeding of solar intensity throughout the experiment is shown in Figure 4. The variation in the solar radiation intensity at some points may be due to clouds covering the area at that time. The average solar radiation is found to be 689 W/m2 for the day. Temperature Profile The temperature measurement at various locations was noted down in the drying chamber during the experiment. The specific temperature profile was created for the drying tray placed at six different locations to understand the dynamics of drying in the chamber. The arrangement of various trolleys in the GHSD is shown in Figure 5. The temperature profile for the various trays at different locations in the drying chamber is plotted in Figure 6. The temperature profile generally follows the solar radiation data patterns

Figure 6. Temperature profile at various locations in the drying system. Figure 5. Arrangement of trolleys in the drying chamber.

as it has directly affected the amount of heat concentrated at a location in the drying chamber. To prevent objects from overheating, it is necessary to monitor any process that involves heat dissipation and temperature distribution using advanced probe sensors [32]. The abrupt changes in the data points during the experiments result from clouds covering the atmosphere at different times. The variation in the temperature profile at various locations of the tray is caused due to airflow variation in the drying chamber and, in addition to that the varying location of the sun in the open sky. The difference in localized temperature is a major concern while drying out large batches of product, as the drying time is fixed for a batch. However, we still get different quality of dried product at each location of the drying chamber and different moisture ratios at different drying levels. The difference in the temperature over different trolleys at the upper-level and lower-level trays is plotted down in Figure 7. The plots indicating the variation in the temperature during the experiment in the upper and lower layer depict that the upper layer of the drying tray is always exposed to higher levels of solar radiation. The difference in temperature between the two layers also varies according to the location of the trolley; the difference between the two layers of the tray tends to be more in the trolley (TR-1) compared with the trolley (TR-2) and trolley (TR-3). The different temperature level suggests the importance of the location of the drying trays inside the drying chamber. The trolley (TR-1) is near to north side wall and due to the shading effect of the upper tray, the amount of heat gained at the lower level has a greater difference. The trolley (TR-3) is kept in the due south wall, so it is exposed to an adequate amount of solar radiation throughout the day; thus,

the difference in the temperature gain is pretty less but still, both trays have a slight difference due to shading effect of the upper tray on the lower tray. Moisture Ratio The actual drying process is a combination of multiple simultaneous processes involved between the different layers of the drying product and the atmosphere. The two major processes are heat and mass transfer which evaporates the moisture content from the tomatoes. The generalized form of the drying curve consists of 3 parts; the first phase of drying is the warming-up phase which is followed by the constant drying rate phase, after which it is concluded by the decreasing drying rate phase as the moisture content comes to an equilibrium state [33]. The moisture ratio reduction for various drying locations according to drying trays placed in the various trolleys is plotted in Figure 8. After a single day of drying experiments, the final moisture content reduces to around 20% to 7%. The moisture ratio reduction is directly proportional to the amount of temperature attained at the location during the experiment. The drying rate is higher in the upper level of drying trays than in the lower ones. The shading due to the upper tray is a possible reason for the effect. The upper drying tray in trolley-3 shows a maximum moisture reduction from 95% to 7%, while the lower tray in trolley-1 shows a minimum reduction from 95% to 20%. The different drying curve for individual trolleys is shown in Figure 9. The difference in the drying rate can be easily spotted in the graph according to various drying locations in the drying chamber. The results show that the effect of saturation of incident solar radiation and the effect of layering in the drying chamber can be a major factor affecting the drying phenomenon.

Figure 7. Variation in temperature at different levels with time for (a) trolley -1, (b) trolley-2 and (c) trolley-3.

surface properties of the materials. This parameter plays a crucial role in comprehending the underlying physics of the drying process. Employing moisture ratio (MR), the hm values at different locations of the drying chamber of various products can be calculated, enabling an assessment of their drying characteristics [34]. (4)

Figure 8. Plots for moisture ratio v/s time at various locations in the drying chamber.

Convective Mass Transfer Coefficient (hm) The estimation of drying behaviour often relies on a significant parameter known as hm, which represents the

In Eq. (4), V is the volume of the material, Am is the surface area of the material. The shape of tomato slices is cylindrical. The average size of the tomato slices was 4.5 cm in diameter and 3 cm thick, and the moisture ratio was calculated for different trays as per Eq. (3). The mass transfer coefficient for various trays during the experimental process is shown in Figure 10. The convective mass transfer coefficient for the upper trays in all the locations always seems to have a greater value than the lower-level tray. The mass transfer coefficient values keep on increasing as time increases. The average mass transfer coefficient for the drying chamber during the experiments is found to be 0.005 m/s.

Figure 9. Variation in moisture ratio at different levels with time for (a) trolley -1, (b) trolley-2 and (c) trolley-3.

Figure 10. Mass transfer coefficient (hm) for various trays locations during experiments.

Dryer Performance Efficiency variables can be used to describe dryer performance. The solar greenhouse dryer’s daily drying efficiency can be written as [35]: (5) Where Mev is the quantity of evaporated moisture (in kilogrammes), λ is the latent heat of evaporation (in joules per kilogramme), I(t) is the average solar radiation incident on the dryer (in watts per square metre), A is drying area used in the dryer (in square metres). From the 5.8 kg of tomatoes drying in 1 day, the removal of 5.1 kg moisture was obtained. Hence, the dryer’s average daily thermal output can be determined as follows: (Qu) = moisture evaporated (kg) x Latent heat of evaporation x (J/kg) Hence, the thermal output of the dryer annually [35], (6)

viability of the dryer needs to be studied before deployment of the dryer in the practical application. The economic analysis of the solar dryer depends upon the final product obtained from the dryer. The equations (7-19) are derived from [36]. The current study focuses on tomato drying, a popular and viable product after drying. The economic analysis is performed by considering the total mass of dried tomatoes in the dryer annually (Mpa) can be found out by: (8) Where Db is the time taken to dry one batch of product, D is the number of operation days, and Mpd is the amount of tomatoes dried in one batch. The total capacity of the current solar dryer is around 6 kg per batch and the drying time concluded from the experiment is one day per batch (9 h). Each batch’s fresh product mass is indicated by Mf, and Cfp is the average cost for procurement of 1 kg of fresh product. The cost of fresh product per kg of dried can be calculated by:

Input Energy (Ein)[35] = (I(t)* No. of sunshine hours * Drying time * Area * 10-3) kWh Thus, daily drying efficiency[35],

The cost of drying the product using the dryer (Cud) can be obtained by estimating the yearly cost of the dryer (Can) which can be found using the formula:

Thus, with a drying tray area of 1.94 m2 with a median solar intensity of 689 W/m2, the daily drying efficiency for the dryer mentioned above over one day (9 hours of sunshine) was calculated as 26.66% which is in the range of the previous research reported by [31, 36]. The various data used and calculated are shown in Table 2. Economic Analysis of Greenhouse Solar Dryer The solar dryer can become an essential tool for the agricultural community and the farmers. The economic

Table 2. Thermal analysis data for greenhouse solar dryer Available Data

Conclusion

The thermal performance of the small greenhouse solar dryer was evaluated to understand the various factors affecting the drying phenomenon. The drying process was evaluated by drying tomatoes inside the dryer. The economic and environmental effect of the solar drying inside the dryer was estimated using Embodied energy analysis.

The life cycle cost analysis of drying tomatoes was estimated. The following points are derived from the study: • The trays placed due south were exposed to proper drying compared to those in the north direction. • The lower-level tray in the various trolley is always less exposed to direct sun radiation due to the shading effect of the upper tray. • The convective mass transfer coefficient for the lower-level trays is always lower than the upper-level tray. The average convective mass transfer coefficient is 0.0005 m/s for the drying system. • Depending on the location of the drying tray in the chamber, the moisture content was reduced to 20% and 7%. The average moisture reduction all over the dryer was around 81%. • The maximum amount of moisture reduction is seen in the tray located on the upper level placed near the south wall, and the minimum reduction in the moisture was obtained in the tray placed in the lower level placed in the trolley around the north wall. • The thermal performance of the solar dryer was calculated using the various energy input and the output in terms of drying product output. The thermal efficiency for the day was around 26.66%. • The embodied energy analysis was also performed to assess the environmental effect of the solar dryer throughout its working life cycle. The Embodied energy for the dryer was estimated to be 3154.71 kWh. • The amount of CO2 emission for the system is estimated to be 6.62 Tons. The net carbon mitigation for 25 years of the life cycle was estimated to be 41.62 Tons which can earn 46766 INR to 62355 INR based on the carbon credit rates in the future. • The economic analysis for the GHSD was carried out based on the amount of daily useful product delivered and it was estimated to have Rs 81056 annual earning which concludes the payback period for the system to

Nomenclature

Drying area (m2) Annual operation Cost of Fan (INR) Annual Capital Cost (INR) Annual Cost (INR)

Ccc Cfd Cmt Cu Cud Cue d D Db Eaout Eei Ein Eout Fcp Fmc GHSD Imc I(t) La Ltd Me Mev Mo Mpa Mpd MR Mt N Nf Pb Pf Qu Rif Sb Sd Sk Skg SPdp Sv Wd Wi

Capital Cost (INR) Cost of Fresh Product (INR) Annual Maintenance Cost (INR) Drying cost for 1 kg material (INR) Cost of 1 kg of product dried inside the dryer (INR) Electricity Charge for 1 Unit Interest Rate (%) Number of Drying Days per year (days) No. of Days taken for drying one batch (days) Annual Thermal Output (kWh) Embodied Energy Input (kWh) Input Energy (kWh) Output Energy (kWh) Capital Recovery Factor Final Moisture Content (%) Green House Solar Dryer Initial Moisture Content (%) Average incident Solar radiation (W/m2) Losses due to Appliances Losses due Transmission and Distribution Equilibrium Moisture Content Moisture Evaporated (kg) Initial Moisture Content (%) Total Mass Dried Annually (kg) Amount of Product dried in 1 Batch (kg) Moisture Ratio Moisture content at time t No. of Sunshine Hours per Day No. of Hours Fan Runs Payback Time (years) Rated Power of Fan (W) Daily Thermal Output (kW) Inflation Rate (%) Saving per Batch (INR) Saving per Day (INR) Annual savings in Kth year (INR) Saving from drying 1 Kg of Product (INR) Selling Price of Dried Material (INR) Salvage Rate (%) The bone-dry weight (kg) Initial weight of the product (kg)

Greek Letters λ Latent Heat of Evaporation (J/kg) Daily Drying Efficiency (%) ηdi Lifetime of the System (years) ηsys

Authorship Contributions

Pringal M. Patel: Investigation, Data Curation, Formal analysis, Methodology, Writing - Original Draft, Visualization. Vikram P. Rathod: Conceptualization, Supervision. Divyesh Patel: Resources, Data Curation.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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