Chapter 3 DESIGN OF THE STUDY

Transcription

1 Chapter 3 DESIGN OF THE STUDY This chapter introduces overall design of the study, which includes the methodology adopted for carrying out the work and various phases of this research. The details of the work done in each phase along with the tools, techniques and models used have also been covered in detail here. 3.1 INTRODUCTION Solar drying of food products can be most successfully employed as a cost effective drying technique. It has got several advantages over Open Sun Drying Process., the solar energy is available at free cost and can be harnessed in the site itself. It is also possible to have controlled drying by using this method and it enhances the organoleptic qualities of dried product as well. Hence solar drying systems must be properly designed so that one can achieve particular drying conditions of specific crops and to give satisfactory performance with respect to energy requirements. Hence to design the dryer system, initial information which is required is, drying load and the amount of solar energy available. This data is essential to calculate the solar collector area. The energy gained in the collector should be sufficient to dry the product kept in the dryer. Literature shows that use of evacuated tubes in solar collector could give higher thermal efficiency than Flat plate collector. So ETC solar system is accordingly selected and designed for this study. 3.2 METHODOLOGY This study has been carried out with the purpose of developing an effective, economical and efficient solar dryer for drying food products in the rural areas of India. The study begins by working for selecting a drying batch size and variety of grapes to be dried by using non conventional source of energy. It is observed that Thompson seedless grapes are used for raisins production due to physical properties of this variety of grape. Subsequently, the solar system has been designed by calculating the total drying load and accordingly the solar collector area is calculated. The literature study reveals that ETC solar collector has better performance amongst 41 P a g e

2 all types available. Hence solar collector is selected as ETC solar system and accordingly the collector parameters, like total exposed solar collector area, mass flow rate, etc. are designed. The solar dryer chamber is designed based on this information and its efficiency is calculated based on the performance. The experimental analysis and simulations were employed to ensure an effective and cost efficient design. The designed components were then fabricated and tested under uncontrolled environmental condition for drying the grapes. The experimental drying behaviour of the Thompson seedless grapes was analyzed to fit the best suitable mathematical model and the analytically calculated data of solar ETC system is validated with experimental data. 3.3 SOLAR ETC DRYER SYSTEM The selection of an appropriate solar dryer for grapes is dependent on its characteristics, quality requirements and relevant economic factors (Kandpal et al., 2006). Additionally, consideration should be given to material selection used for constructing the system, which directly impacts the quality of the product. Attention is given here to identify adequate evaluation procedures of the product, dryer, and performance parameters that provide an overview of the necessary components of analysis, which help in the selection of appropriate dryer systems. It is found that, many of the researchers have experimentally proven that, Solar ETC collector has better performance. Normally in case of the Solar ETC system a heat pipe is used to gain the heat but in this current study working fluid is directly passed through the evacuated tubes to collect more amount of heat. A synopsis for evaluation procedure is then outlined from the preparation of dryer, solar ETC collector and instrumentation; to the actual experimentation. When it is required to select materials for dryer fabrication, consideration must be given to the available resources, including equipment or tools, as well as the level of employable craftsmanship. Additionally, all materials should be resistant to heat, light and relative humidity, to improve the lifetime of the ETC system. Specifically, collectors and drying chambers should be watertight. 42 P a g e

3 3.3.1 Dryer System Evaluation Sufficient testing of solar ETC dryer systems is necessary to evaluate technical performance and establishes a basis for comparison to other dryer designs. Such analysis can assist in the selection of appropriate dryer designs, given specific conditions which must be met. Furthermore, adequate evaluation can provide an indication of a solar ETC dryer s performance in conditions different from those tested in. Leon et al. (2002) proposed a comprehensive procedure for the evaluation of solar dryer performance, which provided methodology and test conditions in order to develop a standard practice of analysis. The procedure provides an explicit evaluation of solar dryer performance, while facilitating in the comparison of different solar food dryers. The parameters reviewed here provide a basis for comparison to different dryers and can assist in dryer selection to meet specific needs. Analysis of the product quality involves evaluation of rehydration, sensory elements and chemical procedures. Evaluation of dryer parameters should include, consideration taking dimensions and sizes, solar collector, construction requirements, drying temperature, relative humidity and the airflow rate. Performance parameters include drying time, efficiency, maximum drying temperature and cost-associated factors which are all detailed in this study. The total area of the Solar Collector is an important consideration in the estimation of Solar Collector Efficiency. In Solar ETC dryers, it should include the collector area and the spread area of the product receiving hot air from collector. Apart from the size and physical characteristics of the collector, the system is largely dependent on the angle of inclination with the sun. In fact, the highest yield is attained with the collector oriented perpendicular to the sun and as a general rule; the optimum angle of tilt is equal to the degree of latitude of the site (Leon et al., 2002). Alternatively, Adegoke and Bolaji (2000) recommended an inclination of 15 more than the local geographical latitude. Furthermore, consideration must be given to actual construction parameters and limitations that could restrict or prohibit the implementation of a specific dryer 43 P a g e

4 3.3.2 Raisins Evaluation The physical properties of the dried raisins like changes in size, shape, colour, texture, chemical and enzymatic conversions, depends greatly upon the method of drying. The discoloration is often prevented with careful control of temperature and moisture parameters, particularly in more sophisticated drying systems. However, simple solar drying systems do not necessarily provide a significant level of control for these parameters. For this reason, sulphites and other preservatives are frequently employed to control the enzymatic browning of grapes. The assessment of dried product quality is necessary to establish a basis of comparison between drying systems. However, as Kader (2005) noted, it is far more challenging to measure qualitative losses than quantitative. The analysis of quality characteristics such as consumer acceptability, edibility, caloric and nutritive value are often neglected in studies due to the challenge in developing and understanding adequate evaluation parameters. Although flavour loss of dried products is often due to volatile losses, chemical reactions such as oxidation and browning also contribute considerably. The size, shape, uniformity and absence of defects are all important in assessing product quality. While the evaluation of these parameters is relatively straightforward, evaluation of colour, aroma and taste are more difficult. The change of colour can influence a consumer s perception and may affect other attributes such as the flavour of the product. Measurement of product colour implies either visual matching paired against standard colours or expression in terms of numerical dimensions using hue, saturation and lightness. 3.4 OVERVIEW OF THE STUDY The present study comprises of four major parts namely, Design of the solar dryer system, which includes design of dryer and solar ETC collector and testing of ETC system. Experimental study/analysis of the solar collector at no-load and load conditions and solar dryer under uncontrolled environmental conditions. Analytical study of solar ETC system. Drying kinetics of the Thompson seedless grapes. 44 P a g e

5 Designing of solar system, deals with developing a solar ETC system and a suitable solar dryer to dry the grapes. The experimental study of the solar dryer means analysis of the collected data of the solar collector and dryer to find its performance and relations of every element of the system on each other. The research work has been carried out in following four phases, Phase I Literature review for clarifying the context; Phase II Design, developing and experimentation of solar dryer for grape drying; Phase III Developing the dynamic model for the solar ETC system and validating it; Phase IV Studying the drying kinetics of the Thompson seedless grape drying; 3.5 DESIGN OF THE SOLAR COLLECTOR AND DRYER Specific design parameters like area of the solar collector, mass flow rate and heat gained by working fluid, etc. were determined based on the series of engineering/ mathematical calculations. This adapted model took into account a variety of factors including the physical properties of grapes, the characteristics of previously reported solar systems in literature, environmental conditions, and the local production capabilities of small villages. These properties served as input for the proposed model and provided a preliminary standard for which the actual design was expected to meet and/or exceed in terms of efficiency. Specific characteristics of the solar dryer were then determined from the mathematical calculations and schematics were developed based on this information Mathematical Procedure to Design the Solar Dryer System Moisture Content (MC) is the amount of water found within a material at any given time. If is total mass of grapes (kg) to be dried with Initial Moisture Content to Final Moisture Content on wet basis then Mass of Water to be evaporated from the grapes is given by (Belessiotis, 2011), = (3.1) To design the solar dryer system it is essential to know how much moisture has to be removed from the grapes. For analytical and experimental study of the grape drying grape sample size is selected as 10 kg per test (Pangavhane, 2002). To calculate the amount of moisture to be removed from 10 kg of Thompson seedless grapes, from 45 P a g e

6 Initial Moisture Content 78-80% to Final Moisture Content up to 14-15%, which is recommended moisture content for producing good quality raisins, is calculated by as follows; = = The amount of heat required to evaporate the 7.67 kg of water from the grapes in a given time of 40 working hours is given by the equation (Sundaria, 2013), = =. =.. Drying rate is defined as, the amount of moisture removed from the grapes per unit time. It depends on the total mass of water to be evaporated to the time required in seconds as (Pangavhane, 2002), = =. = 5.33x10-5 (3.2) It is observed in the literature that the collector surface to the dryer volume ratio ( must be more than 3 to have efficient utilization of the system (Sundari 2013). This is expressed as follows, = > 3 (3.3) Where, SA is the collector surface area in m 2 and is the volume of the dryer chamber. 46 P a g e

7 3.5.2 Mathematical Procedure to Design the Solar Collector System An unsaturated hot air flow is required to remove the moisture from the grapes under Forced Air Convection Mode. The heat possessed by the hot air is sources of energy to supply energy equal to latent heat of the moisture, to evaporate. To evaporate mass water from grapes, it requires volume of hot air with, temperature (solar collector outlet temperature), and is the temperature of air leaving dryer. Then the energy balance equation can be written as (Shah, 2007), = ( ) (3.4) The density of air is considered at constant pressure and at mean temperature, with moderate temperature difference and is the specific heat of the air in J/kg 0 C. To design the dryer, it is important to determine suitable values of and. The temperature of hot air should not be too high; otherwise it deteriorates the quality and color of the grapes. The skin of the raisin becomes hard and brownish for drying the grapes above the 70 0 C for longer time, reducing the market value of the raisins (Pangavhane, 2002). The total heat absorbed by the Evacuated Tube Collector System is given by (Aed et. al., 2014), = (3.5) Where, is the optical efficiency of the evacuated tube which is 0.88 and, is solar radiation falling on the surface of the collector. Ten evacuated tubes are arranged at an angle of 45 o to horizontal with facing due south. Hence the total exposed area of the collector (Ac) is (Mahesh, 2012), = (3.6) Where, Diameter and length of the tube in m, and is number of tubes respectively. The total heat absorbed by the absorber cannot be utilized completely to heat up the working fluid. Some part of the heat is going to lose due to reflection of the incident 47 P a g e

8 rays from tube surface, conduction of the heat by the tube to the fixture and convection loss due to atmospheric air flowing over the outer tube. Let is the total heat loss in which is expressed as (Babagana, 2012), = + + (3.7) Hence total useful heat or net heat gain by the collector is expressed as (Yadav, 2011), = (3.8) This useful heat is going to heat up the working fluid that is air from ambient temperature to collector outlet temperature. with the help of the DC fans put up on open end of the evacuated tube, which causes the forced circulation of air through the system. The mass flow rate of air is in / flowing through the all evacuated tubes and centrally it is collected in a stainless pipe of diameter 60mm. Then the heat gain by fluid is given by the following equation (Aed et. al., 2014; Lamnatou, 2012), = (3.9) This much heat is collected by the solar ETC system and will be supplied to the dryer to remove the moisture content from the grapes. In most of the previous researches it is found that the same amount of heat is considered as an input to the dryer chamber. But in actual practice the temperature of hot air entering to the dryer chamber is always less than the temperature at the outlet of the collector though proper insulation is provided to the passage in between solar collector and dryer. The discharge flow rate of the hot air is expressed as (Amedorme, 2013), = / (3.10) Also the area of the fan giving forced air is calculated with air velocity / as, = / (3.11) The non dimensional parameter Moisture Ratio (MR) is calculated as (Ayyappan, 2010), = (3.12) Where, 48 P a g e

9 is the mass of the product at any time t and is equilibrium moisture content of the product. Solar collector efficiency is the measure of how effectively the energy available in the solar radiation is transferred to the flowing air within the system. This parameter was determined by assuming steady state conditions. In this work the Solar Evacuated Tube Collector Thermal Efficiency is defined as the amount of heat gained by the working fluid to the product of solar radiation, following on collector surface and its exposed area. This can be calculated by using Forced Convection is written as (Tchaya, 2014), h = % (3.13) The average solar radiation in the month of April and May is 870 W/m 2 to 950 W/m 2 respectively in Pune (India) (Auti, 2015). The optical efficiency of the ETC system is So effective solar radiation absorbed by inner tube is 740 W/m 2. The total overall efficiency of the solar dryer using ETC system is 9% to 11% (by considering the thermal efficiency of the solar ETC collector as 30% and dryer heat utilization efficiency as 35%) is observed as per the literature. So area required for the solar ETC system is given by the equation, = =. =. Selecting the standard length of ETC tube, available in the market as 1.8 m, the number of ETC tubes ( required to make the solar collector assembly is given by rearranging the equation (3.6) and. =.. =. ~ So the solar collector system is constructed by selecting the ten ETC arranged in the parallel. Hence for ten tubes the exposed area becomes; =.. 49 P a g e

10 =. m 2 From equation (3.5) the heat available at the ETC system is calculated as, =.. =. Heat gained by the working fluid (air) or heat absorbed by solar ETC is given by Equation (3.9), = =. The amount of heat loss in the solar collector is the difference of the heat absorbed by the collector and the heat utilized by it or heat gained by the working fluid. This is given by rearranging the Equation (3.8); = = = W Instantaneous thermal efficiency of the solar ETC collector is found out by using the equation (3.13), h =.. % h =27.2% The efficiency of the drier can be calculated in two ways as follows, 1- The dryer heat utilization efficiency is a practical evaluation of the amount of moisture evaporated from the product. This parameter essentially measures the effectiveness of the heated air to absorb the evaporated moisture. Thus a comparison is made between the actual absorbance of moisture to the capacity of moisture absorbance by the heated air. It can be defined as the amount of heat utilized by the dryer to evaporate the moisture from the grapes, to the amount of heat supplied by the collector. This is termed as Heat utilization efficiency of the dryer. Mathematically it is given by, = * 100 % (3.14) 50 P a g e

11 For the particular test the dryer heat utilization efficiency is calculated as follows; = 8.. * 100 % = 29.71% The system efficiency of a solar dryer is the measure of how effectively the solar energy input is used in drying the product. The Overall Efficiency of the dryer is defined as the amount of heat utilized by the dryer to evaporate the moisture from the grapes to the amount of heat fallowing on the collector surface or heat supplied by solar radiation (Pangavhane, 2002), = (3.15) This Overall Dryer Efficiency for the same test is calculated as;. =. = % The cross-sectional area of the drying chamber directly corresponds to the area of each tray. Although Buchinger and Weiss (2002) proposed a cross-sectional area with 10 kg of product for every square meter of tray, a more conservative spatial arrangement of one-third of this estimate was assumed for application mathematical procedures. Furthermore, a vertical distance of twelve inches between the trays was maintained, to allow clearance between the loaded trays and to permit easy access to each tray for better air passage. An overall design of the dryer chamber was expected to create enough density gradients to establish airflow within the dryer without creating a larger, unmanageable 51 P a g e

12 system. Of course, greater heights would increase this gradient, but this comes at the expense of additional material, labour and maintenance associated with a taller system. The number of tray serves as an input to these mathematical calculations and as such, a total of three trays were placed for this dryer in order to increase the product throughput. The vertical orientation of successive trays was expected to increase the pressure gradient, as vertical height was extended accordingly. Establishing greater vertical height was thus given more consideration than simply enlarging the tray area to hold more grapes. It is assumed that negligible pressure resistance would result from the inhibited air flow through the trays. From the equation 3.3 the calculated R ratio for this dryer is 4.4 where it is desired to have at least more than 3 (Sundari, 2013). 3.6 CONSTRUCTING THE SOLAR COLLECTOR Evacuated Tube Collector Evacuated tubes are composed of two coaxial borosilicate glass tubes joined at the top and sealed at the bottom which contain a vacuum. The outer tube is a transparent tube of 58mm diameter and 1800 mm in length, called as cover tube. Whereas, the inner tube is of 47mm diameter and 1750 mm in length, which is called as Absorber Tube. The thickness of Inner Tube and Outer Tube is 1.6 and 2.00 mm respectively. The inner tube contains the working fluid to be solar heated, which is air. The outer part of inner tube is coated with a suitably dark absorbing material (Aluminium Nitrite) for collecting the incident solar radiation and transmitting it to working fluid passing through it. The closed volume between the outer and the inner tube being evacuated works as a thermal insulator, preventing heat loss primarily due to convection and conduction. Thus the trapped solar energy absorbed and transmitted to working fluid, gets prevented from escaping backward to the environment (green house phenomena). Based on the design calculation it requires the area of solar collector as 1.51 m 2 to collect the solar energy which is sufficient enough to dry 10 kg of Thompson seedless grapes (Equation 3.5). According to the selected size of the evacuated tube (Table 3.1), it requires 9.28 tubes to make 1.51 m 2 exposed area of the collector. 52 P a g e

13 Fig. 3.1 Actual Solar ETC system Hence ten evacuated tube of mentioned dimensions (Table 3.1) are arranged parallel to each other as shown in figure 3.1. Output of individual tube is collected centrally to a tube arranged at the top of all evacuated tube. This central tube is of stainless steel material of 60 mm diameter. It is further connected to the bottom of the dryer to give the output hot air to dryer chamber. From the literature it is found that the angle of inclination is kept longitudinal value of location (Ahmad and Tiwari, 2009; Duffie and Beckman, 1988). In this setup array of tubes is inclined 45 0 to horizontal on steel frame to gain maximum solar beam radiation and absorb diffused radiation. From equation 3.5 the calculated collector exposed area to direct sun light, at any instant is measured as 1.61 m 2. The point of contact of the evacuated tube to frame is insulated with polyethylene foam sheet with thermal conductivity of 0.04 W/m k to minimize the heat loss (Sundari, 2014). Forced air is provided by 12 V/1A five fans which run on the 15 watt solar panel (Tang et al. 2010; Nebbali et al., 2014) Dryer Chamber The dryer chamber is made up of 0.6 mm thick Aluminum alloy sheet. On the bottom and top of rectangular chamber, divergent and convergent sections are made to observe uniform flow of the hot air. To minimize the heat loss by the dryer to 53 P a g e

14 surrounding this dryer chamber is also insulated with 12 mm polyethylene foam sheet (Rajagopal, 2014). The drying chamber consists of three stainless steel mesh trays to load the grapes. Fig. 3.2 Schematic view of ETC Solar dryer Figure 3.2 shows the schematic view of the complete system. In this research work the solar collector is designed based on the dryer load. While calculating the dryer load it is considered that 10 Kg of grapes from Initial Moisture Content of 80% wet basis to Final Moisture Content of 14% wet basis is dried in total 40 hours. In actual setup based on these design considerations 10 kg of grapes are dried in 36 hours from initial to final moisture content of 76% to 15% wet basis. In the literature it is found that the collector outlet air temperature is considered as dryer inlet temperature. But in practice it differs and it is observed 1 0 C -2 0 C less. 54 P a g e

15 Fig. 3.3 ETC Solar Collector(CAD Drawing) Fig. 3.4 Solar Dryer (CAD Drawing) Fig. 3.5 ETC Solar Collector (Actual ) Fig. 3.6 Solar Dryer (Actual Inside View) This leads to generate some errors in calculating thermal efficiency of the system. So in this work, collector outlet and dryer inlet temperatures are separately measured and considered while calculating the performance. The loss of the air velocity in the passage from collector to dryer is very small and hence neglected. The detail standard and calculated dimensions of the solar ETC system and the solar dryer are mentioned in the Table 3.1. Figure 3.3 and 3.4 are representing the CAD models of the solar ETC system and dryer chamber whereas, figure 3.5 and 3.6 represents the actual systems. 55 P a g e

16 Table 3.1 Dimensional Specification of ETC System and Solar dryer Sr. No. Particular Size/ Dimensions 1 Length of the outer and inner tube 1.8 m and 1.75 m 2 Thickness of inner and outer tube 1.6x10-3 and 2x10-3 m 3 Collector exposed area to solar radiation 1.61 m 2 4 Dryer chamber size 1x0.64x0.64 m 3 5 Area of single dryer tray 0.6x0.6 m 2 6 Number of trays and vertical distance 03 nos. and 0.25m between each tray in dryer chamber 7 Convergent and divergent angle to the dryer MEASURING INSTRUMENTS AND DEVICES Diurnal variation of the relative humidity, temperature of ambient air and air temperature on each tray of dryer is monitored on every hourly basis for entire test time. Sample bunches from each tray are weighed after every hour with an electronic weighing balance of accuracy ± 1grams from morning 9 am to evening 6 pm (Pangavhane, 2002). RTD Pt-100 sensors (accuracy ± 0.1 C) with microcontroller reading facility, with 12 channel selector switches are used to record the temperature reading. Various temperatures like ambient condition, air outlet to collector, inlet to dryer chamber are recorded. The two thermocouple sensors are put on each tray of dryer chamber and dryer outlet temperature is also monitored per hour. Air velocity is measured for all reading with anemometer (accuracy 0.1m/s) in dryer and at the exit of dryer. Solar radiation is measured with the help of solar Pyranometer having the accuracy +3%. All the equipments are calibrated with the standard calibrating agency. The details of the uncertainties of the measuring instruments are shown in the Table EXPERIMENTAL UNCERTAINTIES The collected data consists of several uncertainty sources during measurements such as solar radiation measurement, temperature measurement at various locations in collector and dryer, air velocity, relative humidity, current and voltage 56 P a g e

17 measurements, etc. In current experimentation, it is assumed that there is negligible heat loss from dryer chamber to the surrounding. Further the heat losses due to convection and radiation from evacuated tubes are considerably very small, hence are not calculated separately. The pressure drop of the working fluid in the collector is also not considered as the air velocity is measured at inlet and outlet of dryer. Successive measurements under identical operating conditions give different results with small change. This variation is of the order of 2 5% which is acceptable. Table 3.2 Uncertainties in the Measurements 3.9 EXPERIMENTAL PROCEDURE Based on design of studies, various experimental tests were carried out in the month of April to first week of June during the year 2013 to 2015 for checking the performance of the solar collector and the behavior of the grape drying. Thompson 57 P a g e

18 seedless grapes are selected for drying in the specially designed and fabricated solar dryer. Air is used as working fluid to remove the moisture from the grapes. Batch size of the grapes to be dried in every experiment is selected as 10 kg of Thompson seedless grapes purchased from the market in Pune (India). The initial moisture content of grapes is determined by oven dry method. For maintaining the physical properties similar, grapes from same lot are selected for experimentation as well as for moisture determination. 10 kg of fresh Thompson seedless grapes are selected and sorted for infected berries. These are washed with tap water to remove dirt and dust. Conventional alkaline pre-treatment is done to increase the water permeability. Dipping oil 2.5% and 2% Na 2 CO 3 is used as agents (Azad, 2008; Fadhel, 2005; Pangavhane, 2002). Grape bunches are dipped for two minutes and equally spread on the three trays in dryer chamber made up of stainless steel mesh. Experimental data is recorded from morning 9.00am to evening 6.00 pm. In the trial runs, which were carried out earlier to the actual experiments it was observed that in the evening time after 5.00 pm, temperature of working fluid is still higher than atmospheric air temperature. This is because of specific heat stored in the metallic component of the collector. Hence though the solar radiation is low at evening time still observations are taken up to 6.00 pm and then the inlet and outlet of the dryer is closed till observations are started to be recorded next day morning 9.00 am. Various parameters are recorded with measuring instruments on hourly basis for full test time CONCLUDING REMARKS In this chapter the methodology adopted along with the step-by-step approach employed, for carrying out the research work has been described with the sample calculations. The experimental setup and the measuring instruments used for collecting the experimental data are also explained in this chapter. The uncertainty analysis in the measuring instruments while gathering the observations is essential which is explained here. The next chapter which covers the detailed analysis of the collected data of solar collector and solar dryer is described according to the methodology. 58 P a g e