An Experimental Study and Analysis on Solar Drying of Bitter Gourd Using an Evacuated Tube Air Collector in Thanjavur , Tamil Nadu , India

The objective of this research work is to design and develop a forced convection solar dryer using evacuated tube air collector and study its performance on bitter gourd in Thanjavur District, Tamilnadu, India. The designed solar dryer consists of a drying chamber, evacuated tube air collector, a blower, and a chimney. Drying parameters, moisture ratio and drying rates, are calculated and their performance is compared with natural sun drying. The results of the present study show that the proposed solar dryer has greater efficiency, and the moisture content of bitter gourd is reduced from 91% to 6.25% in 6 hours as compared to 10 hours in natural sun drying. In this solar dryer, the products are uniformly dried, and the moisture content of the sample is controlled. It is found that the quality of the dried bitter gourd using solar dryer is higher than the natural sun-dried bitter gourd.


Introduction
The global crises faced by mankind nowadays are increase in population, food scarcity, and depleting energy reserves.Also, most of the agricultural products are seasonal.Drying of products is traditionally done by natural sun, drying to reduce the loss of product due to fungi and microbial attacks and make it available throughout the year [1].However there are many disadvantages in the conventional sun drying.It takes longer duration to reach the desired moisture content.During long drying, there are chances of contamination of products due to fungi and pathogenic bacteria.Other disadvantages include the intermittent nature of solar energy throughout the day, exposure to rain and wind, and contamination by dirt, rodent, and birds.Also, the products are not uniformly dried, and the moisture content of the sample cannot be controlled.
From the literature, it is found that most of the solar drying systems use flat plate air collectors.Ayyappan and Mayilsamy have developed and made experimental investigation on a natural convection solar tunnel dryer for drying bitter gourd under the meteorological conditions of Pollachi, India, in which the quality of bitter gourd obtained is good as compared to open sun drying [2].Parikh and Agrawal constructed a double-shelf cabinet dryer coupled with flat plate solar air heater for drying green chillies and potato chips in hot and dry climate of Jaipur, India, and have observed higher hot air temperature rise and efficiency with short drying period for glass glazing as compared to polycarbonate sheet [3].Lamnatou et al. have performed experimental investigation and thermodynamic performance analysis of a solar dryer using an evacuated tube air collector for apples, carrots, and apricots [4].Yadav and Bajpai have observed that downflow configuration is more effective than upflow condition at all flow rates due to lesser losses in downflow in the experimental study made on evacuated tube solar collector for heating of air in India [5].A pilot study was done with an evacuated tube solar water heater in Thanjavur, and its performance was compared with flat plate water heater.The efficiency of evacuated tube solar water heater is found to be greater than the flat plate water heater [6].
Thus, in this experimental study, a novel forced convection solar active dryer with three trays is designed and developed with an evacuated tube air collector, and its performance is studied on bitter gourd and compared with natural sun drying in Thanjavur, Tamilnadu, India.

Experimental Setup
The solar drying system mainly consists of a drying chamber, evacuated tube collector (ETC), a blower, and a chimney.The schematic diagram of the experimental setup is shown in Figure 1.The size of the drying chamber used for the study is 18  × 18  × 18  which is made of stainless steel sheets of thickness 25 mm and insulated on all sides with rock wool slab of thickness 50 mm compressed to 40 mm to prevent the loss of heat.The chamber consists of three aluminium perforated trays to place the product for drying.It consists of six evacuated tube air collectors with a copper header for heat exchange.The twin glass evacuated tube collector is made of borosilicate of 1.6 mm thickness, and the gap between the glass tubes is evacuated.The inner tube of the collector is coated with a three-layer magnetron sputter coating (SS-Al N/Cu).Heat loss due to convection, conduction, and radiation is thus minimized, and it can withstand high temperature due to this technology.The length, inner diameter, and outer diameter of each tube are found to be 1500 mm, 37 mm, and 47 mm, respectively.The collector is placed at optimum tilt in accordance with the latitude of Thanjavur district (10 ∘ 45  N), Tamilnadu, along N-S direction, facing south so as to track maximum solar radiation throughout the day.This collector which is used as a heat source is connected to the drying chamber with the help of EPDM (ethylene propylene diene monomer) rubber hose.
A blower motor of 0.335 KW, 1300 rpm with a regulator to control the rate of flow of air is attached at the inlet of the solar collector to blow air into the collector.A chimney of height 100 cm made of SWG (Standard wire gauge) Gl (Galvanised) sheet is used at the top of the chamber which increases the air flow rate inside the chamber under the convective principle of hot air rising up.

Measuring Instruments and Devices
Temperature at various locations (inlet and outlet temperatures of the collector, temperature of all the trays inside the chamber, and temperature of the chimney) is measured with the help of RTD (resistance temperature detector) PT100 sensor (6 nos.) connected to SELEC 303 temperature controller and display unit.The ambient temperature, relative humidity, and wind speed are measured using a digital anemometer (MASTECH MS 6252B).Solar insolation is measured using a solar power meter (TES-1333).A digital electronic balance is used for weighing the samples (D-sonic digital scale: ±0.1 g accuracy).

Experimental Procedure
Solar drying and natural sun drying experiments are carried out for bitter gourd.Fresh bitter gourd is cut into thin slices, and the initial moisture content is measured by oven-drying method, maintained at a temperature of 105 ∘ C for 24 hours by taking 200 g sample.Bitter gourd is then spread uniformly on three trays for solar drying and one tray for natural sun drying.The blower motor is then switched on.The air that is passed through the evacuated tube collector gets heated up and is made to flow into the drying chamber, where bitter gourd is loaded in three trays.During the experiment, ambient temperature, relative humidity and wind velocity, solar insolation, inlet and outlet temperatures of the collector, and temperature of all the trays inside the chamber, temperature of the chimney are recorded on hourly basis from 9.00 am to 5.00 pm.During the experiment, all the drying trays are weighed on hourly basis until the product acquires constant weight, that is, equilibrium moisture content.(1)

Determination of Moisture Ratio.
Moisture ratio is given as  where  is the moisture content at any time,   is the equilibrium moisture content, and  0 is the initial moisture content of bitter gourd.

Results and Discussion
Hourly variations of solar insolation, wind velocity, and temperature are recorded for three days, and the mean is given in Table 1.From the observation, it is found that the temperature outlet of the collector (ETC out) and temperature inside the drying chamber (Tray 1, Tray 2, and Tray 3) are much higher than the ambient temperature.This indicates that the performance of solar dryer is better than the performance of natural drying.
Variation of the average solar insolation with respect to the time of the day is shown in Figure 2. It is found that the average solar insolation is higher at noon.The maximum and minimum solar insolations recorded during these days are 1166 W/m 2 and 120 W/m 2 , respectively.Also, the dryer is hot about midday when the sun is overhead.
Variation of moisture content with drying time for solar drying and sun drying is shown in Figure 3.The time taken by the solar dryer to reduce the moisture content of bitter gourd from 91% to 6.25% is 6 hours as compared to 10 hours in natural sun drying.Variation of moisture ratio with drying time is shown in Figure 4.It is seen that moisture removal is high initially and then gets reduced exponentially.This is because of the removal of moisture from the surface first followed by the movement of moisture from internal part of the product to its surface.

Conclusion
The dryer used in the present study reduces the drying period of bitter gourd considerably.Solar drying of bitter gourd takes nearly half the time as compared to natural sun drying.The minimum drying period of 6 hours is required for bitter gourd to achieve equilibrium in solar dryer, whereas the time taken by sun drying is 10 hours.The drying process is controlled in solar drying.It can be used to dry various products simultaneously and products that cannot be dried in natural sun drying.The most important advantage of using this dryer is that it can be used to dry products even during no sunshine and winter season as it makes use of evacuated tube collector.

5. 1 .
Determination of Moisture Content.The initial mass (  ) and the final mass (  ) of the sample are recorded at an interval of 1 hour till the end of drying using the balance.The moisture content on wet basis ( wb ) is given as  wb =   −     .

2 )Figure 2 :
Figure 2: Variation of average solar insolation versus Time of the day.

Figure 3 :
Figure 3: Variation of Moisture Content versus Time for Solar drying and Sun drying.

Figure 4 :
Figure 4: Variation of moisture ratio versus time for solar drying.

Table 1 :
Hourly variation of solar insolation, wind velocity, and temperature for bitter gourd (average of 3 days).