Experimental studies on drying characteristics of green chilies in a solar dryer

Abstract

Keywords: Selective Coating; Critical Radius of Insulation; Solar Collector; Solar Drying; Maximum Heat Transfer

Introduction

Solar energy is free and clean energy of source, and hence it is acknowledged as a most promising alternative energy source. Solar drying for agriculture products is an essential process in long time food preservation. Food

drying is a process of removing moisture using a heat and mass transfer technique. It is a classical method for preservations of food products like fruits and vegetables. Generally, a traditional and oldest open sun drying method

*Corresponding author. *E-mail address: sanjay.salve@pccoepune.org This paper was recommended for publication in revised form by Regional Editor Muslum Arici 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/).

is used to preserve the agriculture foodstuffs like grains and vegetables in many developing countries. But such conditions can severe losses in quantity (extra weight) and quality of the dried products [1]. Some regularly used food products, especially foods and vegetables require a 45-60 °C temperature range for safe drying. For storing and preserving the perishable agricultural produce for prolonged period, it must be dried effectively up to 5 % to 8 % moisture content [2]. A controlled condition of humidity and temperature for drying agriculture products always helps to maintain superior quality of the products [3]. In other way, there are chances of food contamination by surrounding dust, dirt, insects, birds, and animals in an open sun drying method. Hence, there is a need of solar dryers in many developing countries to reduce the chances of crop degradation and improving the quality of dried products. There are four methods used for solar drying to remove the moisture; 1) Sun drying: the material is directly placed under solar radiations in ambient conditions, 2) Direct solar dryers: Products can be placed inside a transparent enclosure or cover, 3) Indirect solar dryers: an external pre-heated air from solar dryer can be used for drying products, and 4) mixed-type solar dryer: a combined method of direct and indirect solar dryer. Several solar drying techniques have been designed and developed as a substitute option for open sun drying method. Several studies have been reported on the direct and indirect type solar drying system [4, 5]. The simple wooden hot box solar cabinet type direct solar dryer has been studied by Fudholi et al. [6]. A transparent polyethylene cover has been used at the upper side of the dryer while a wood and metal sheet were placed at the bottom. Sharma et al. [7] has achieved the highest plate temperature of 80-85° for no load conditions, while it is 45-50° for 20 kg of wheat food. The main drawbacks of such open type direct solar dryers are: 1) small in size, hence cannot be used on large scale crops, 2) high drying time, 3) overheating of the crop may take place [4]. Hence, the researchers have been attracted towards the design and development of indirect solar drying system. Three main indirect type dryers viz. chamber type dryers, wind ventilated dryers and chimney type dryers are used for drying food crop. A solar collector plays key role in indirect type dryer for enhancing the air temperature for a required level. Hence, the development in the solar collector with respect to higher efficiency is a key attraction of the today’s researchers. There are two types of solar collectors viz. 1) concentrated type, 2) flat plate has been used for drying system. The concentric collectors are very costly, difficult in manufacturing and high maintenance cost. In conventional type FPC, the absorbing plate is generally made up of high thermal conductivity metal. The selective coating materials (like black paint) of lower thermal conductivity are coated on the top surface of the absorbing plate, which has high absorptivity and less emissivity. The low thermal conductivity coating

materials create the thermal resistance for decreasing the heat transfer rate between coating material to the air. Many studies are reported on the indirect type solar drying system. In the year of 1986, Maroulis and Saravacos [8] did an experimental study for drying agriculture products. They developed a thermal storage bed for agriculture product heating using a heated air from flat plate collector. Bolaji [9] did a study on indirect type solar dryer using a box type solar collector. A black absorber plate has been employed at the collector for maximum absorption of heat. He found a maximum of 60.5 % efficiency of box type absorber system. He reported 64° C of average temperature inside the collector while he reached up to 57° C maximum average temperature inside the drying chamber. Pangavhane et al. [10] developed multipurpose natural convection based solar air heater with finned absorber (painted with matte black) for drying products. An aluminum matrix foil type fins are attached with a U-shaped aluminum duct through which air was passed. Fudholi et al. [11] did an experimental study of a double pass solar air dryer to investigate the performance of air dryer for palm oil fronds. They did a study for 22 h of solar drying study for 100 kg of palm oil fronds from 60% moisture content to 10 % moisture content. They observed a minimum and maximum collector efficiency of about 9-48% with 0.29 kg/kWh moisture extraction rate. One of the reasons of increasing surface area and thermal conductivity of the absorbing plate is it helps to improve the efficiency of the dryer. Different types of extended surfaces like corrugated structure [12], V-groove type [13], truncated cone [14] and plate finned [15] was employed as an absorbing plate FPC by the various researchers. A review-based study on a different solar air flat plate collector was presented by Fudholi and Sopian [16]. They majorly focused on various analysis methods used for assessment of solar collector overall performance. They mentioned a range of 28-62% energy efficiency and 30-57% exergy efficiency of solar flat plate collector. Yahya [17] designed and developed solar dryer with fluidized bed and finned solar collector. A total 12 kg of Paddy product has been dried to the moisture content from 20% to 14% in 0.66 h. He found a maximum of 60% collector efficiency for 900 W/m2 solar radiation. Karsli [18] presented a performance analysis of finned collector with varying angle. The first and second law efficiency for four different types of collector were calculated. He found better performance for the finned collector than the base collector with a maximum air temperature of 75° C. Karim and Hawlader [19] presented a comparative performance analysis for flat plate, finned and v-corrugated type solar collector. They found a v-corrugated fin type collector was most efficient collector than finned and plate collector. Karwa and Chitoshiya [20] performed an experimental study for v-down discrete ribs type solar collector. The enhancement in the thermal efficiency due to roughness of the rib was studied and found 12.5 to 20 % higher for different air flow rate. They also developed a mathematical

model for evaluation of performance analysis. Pakdaman et al. [21] investigated a performance analysis of rectangular finned collector for natural convection heat transfer. A longitudinal rectangular fin array was used for heat transfer enhancement in solar collector. An empirical model was developed and studied for various dimensionless variables, efficiency, and air temperatures. From the above discussion of the studies on solar dryers and type of solar collector for enhancing the heat transfer, it is inferred that, the development on the solar collector can help to enhance the overall performance of the solar dryer system. The main objective of this study is to investigate the performance of a newly developed flat plate solar collector from a waste aluminum can of beverages. A solar air flat plate collector is designed and developed in the laboratory as per IS 1933, 2003 standard. The performance of the system is evaluated on the basis of three different mass flow rates for drying green chilies. Green chili is selected for the experimentation because India is the second largest producer of green chili in the world and it has shelf life of only 3 to 4 days. But in practice other agriculture produce such as onion, slices of potato, beetroot can also be dried. The characteristic of the dried chilies is observed by evaluating a moisture content removed in a day. Outlet air temperature is measured, and collector efficiency is also calculated.

produce. Hence concept of Critical Radius of Insulation (CRI) is used for effectively transferring more heat from selective coating to the air. A photograph of the assembly of solar collector is shown in figure 2 (b).

Experimental Setup

Figure 3 shows the schematic of the lab scale experimental setup developed in Pune, India (altitude 1840 ft above sea level, latitude 18.32º N and Longitude 73.85 º E). It consists of flat plate collector made from a waste aluminum cans, wooden cabin with insulation for drying agriculture products, air blower, pyrometer, and measuring equipment’s. A flat plate collector is kept facing south with an inclination angle of 33.54°. A pyrometer made by Tempsens Instruments Ltd. with an accuracy of ± 1 W/m2 is used for measuring the solar radiation data with time interval of 30 mins. Six copper-Constantine (K-type) type thermocouples with an accuracy of ±0.1 °C are used for temperature recording. An air blower is connected at the bottom of collector to

Material And Experimental Setup

Flat Plate Collector A solar air collector is the main part in the solar drying system. A flat plate solar collector has been developed using a waste aluminum can of soft drinks. The top surface of the empty can is removed while the bottom surface is cut in the shape of a star and twisted in inward direction to create turbulence. All the cans are connected in series using silicon material to make it as a straight through pipe. The can has an outer diameter of 60 mm, and thickness of 0.3 mm, and has a lower thermal resistance. All collected cans were connected to each other to make a pipe through which air passes as shown in figure 2 (a). The surface area of the FPC must be increased by adopting circular surface area like hollow pipe. Higher the surface area, more is the absorption of solar radiation. The selective coating material has high absorptivity, very less emissivity and less thermal conductivity. It offers a thermal resistance to transfer the heat from selective coating surface to the air flowing through the pipe. Hence, the critical radius of insulation principle is used for deposition of selective coating on cylindrical curve surface. The paste formed by mixing selective coating (SC) Activated charcoal with black board paint is deposited over the surface of the absorbing pipe that has absorptivity of

Experimental Procedure

Before starting the experiments, initial readings of the ambient conditions are taken and a mass flow rate for a particular case is set. The solar radiation is recorded by pyrometer at an equal interval of 30 mins. The radiation at the start of experiment at 08:00 A.M. was 343 W/m2 and it reached a maximum level of 891 W/m2 at 12:00 P.M. While an average radiation was found to be 577.72 W/m2 for a total time of 10.5 hours from 8 A.M. to 6.30 P.M. for a mass flow rate 0.01 kg/s. Similarly, for the next two days the average solar radiation data recorded were 567.77 W/m2 and 571.5 W/m2 for the mass flow rate of 0.008 and 0.006 kg/s respectively. The ambient air temperature for all the cases is found to be in the range of 30 ºC to 33 ºC. All the experiments were performed in the month of March 2019 at Pune, India. The experimental tests were done to investigate the thermal performance of the solar collector. The air inlet and outlet temperature of the solar collector is recorded with respect to time. The setup is placed in such a way that, the

maximum surface area of the tubes is exposed to the solar radiations. The relative humidity inside the dryer chamber is recorded. For, three different mass flow rates the testing procedure were repeated for three consecutive days, so that the deviation between the solar radiations will be minimum. A time dependent radiation plot for three consecutive days is shown in figure 4. This setup is used for the application of drying the agriculture products.

Analysis Of The System

Critical Radius Of Insulation From the Fourier’s law and thermal electrical analogy, Q=

The equation (1) shows that rate of heat transfer is inversely proportional to equivalent resistance of the system. Obviously, the amount of change in value of ∑R will decide whether the heat flow will increase, decrease or remains unchanged [17] Consider a pipeline coated with selective coating (Mixture of Activated charcoal and Black board paint). If r1is inner radius of pipe, r2 is the outer radius of the pipes and r3 is the radius of selective coating deposited on the pipe then as per electrical analogy the equivalent thermal resistances is given by, r  r  ln  3  ln  2  (2)  r1  r  1 1 ∑ R = Q.2 πr l.h + Q.2 π lk + Q.2π2lk + Q.2π r l.h 1 1 1 2 3 2 To find Maximum thermal resistance equation 2 is differentiated with respect to r3 and equated to 0., dΣR dr3

Figure 3. (a) Schematic of the experimental test setup, (b) photograph of actual test setup.

 1 1 1  −1  Q.2 π l.k . r  +  Q.2 π lh . r 2  = 0   2 3 2 3 

has maximum or minimum value. To determine whether ∑R is maximum or minimum, the second derivative is evaluated hence, d2 Σ R dr3 d2 Σ R dr3

Where, I is total incident radiation. Hence, the thermal efficiency of the collector can be calculated using equation, η=

Usefull heat gain (Qu ) Incident radiationoncollector surface

Determination Of Moisture Content Moisture content of chilies can be assessed either on wet basis or dry basis. The percentage of moisture content removed from the green chilies can be evaluated using following equation,  M − Md  M removed =  w × 100  Mw 

Results And Discussion

Useful heat gain (Qu) is the heat gained due to an increase in temperature of air in a flat plate collector because of solar radiation and is calculated as,

A novel design of solar collector from the waste aluminum beverage cans has been manufactured and tested. A thermal performance of solar collector has been investigated for different mass flow rates of 0.006, 0.008 and

Characterization Of Dried Chilies

The weight of green chilies reduces with the reduction of moisture content. Figure 9 shows, the graph of reduced weight of the green chilies with respect to time. The

4. Maximum thermal efficiency of the solar collector is

50.62% at the end of the day. The average efficiency of the collector is 14.27% for the mass flow rate of 0.006 kg/s. The thermal efficiency of the collector reduces with increase in the mass flow rate.

5. There is a significant impact of the mass flow rate on

the moisture removal rate. The moisture removal rate can be increased with increase in mass flow rate of outlet air. 6. The total cost incurred to manufacture the experimental setup including all the accessories is 61000 Indian Rupees. The commercially available vacuum tube collector in the market for same capacity costs196000 Indian Rupees Hence for the same capacity there is an overall saving of almost 135000 Indian Rupees that is almost twice the cost of experimental setup developed.

Figure 10. Percentage moisture content removed for chilies for three different mass flow rates of air. temperature of the outlet air is 89.8 °C for the mass flow rate of 0.008 kg/s, that helps to reduce the moisture content in the green chilies. The weight of the green chilies reduced to

Conclusion

A solar dryer with a solar collector made of waste aluminum cans of beverage is designed and developed. of. A lab scale setup is manufacture with IS 1933, 2003 standard. Experiments are performed for three different mass flow rates namely 0.006 kg/s, 0.008 kg/s and 0.01 kg/s. The inlet and outlet temperature of the air is recorded and the overall performance is evaluated on the basis of moisture removal rate and dryer efficiency. The experimental investigation concludes the following, 1. The tube made from waste aluminum cans of beverage has the potential to increase the outlet air temperature of the solar collector. The star shape of each cans bottom helps to enhance the heat transfer due to turbulence in the air flow.

2. The outlet temperature of solar collector increases

with decrease in the mass flow rate of air. The maximum difference between the inlet and outlet temperature of air is observed to be 60 ºC for a mass flow rate of 0.006 kg/s.

Acknowledgements

We would like to thank the Savitribai Phule Pune University, BCUD Section, Pune, Maharashtra, for funding this project

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.

References

1. The article references can be accessed from the .pdf file.

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