V.R.Badgujar * * Department of Mechanical Engineering,Pimpri Chinchwad College Of Engineering,

Transcription

1 An experimental investigation of solar dryer with regenerative desiccant material for multicrops. V.R.Badgujar * * Department of Mechanical Engineering,Pimpri Chinchwad College Of Engineering, Pune, India Abstract A forced convection with desiccant integrated solar dryer has been built and tested. The main parts are: two flat plate solar air collectors, a drying chamber, desiccant bed and a centrifugal blower. The system is operated in two modes, sunshine hours and off sunshine hours. During sun shine hours the hot air from the flat plate collectors is forced to the drying chamber for drying the product and simultaneously the desiccant bed receives solar radiation directly and through the reflected mirror. In the off sunshine hours, the dryer is operated by circulating the air inside the drying chamber through the desiccant bed by a reversible fan. The dryer is used to dry 20 kg of green peas and pineapple slices. Drying experiments were conducted with and without the integration of desiccant unit. The effect of reflective mirror on the drying potential of desiccant unit was also investigated. With the inclusion of reflective mirror, the drying potential of the desiccant material is increased by 25% and the drying time is reduced. The drying efficiency of the system varies between 48% and 59% and the pick-up efficiency varies between 25% and 60%, respectively. Approximately in all the drying experiments 68% of moisture is removed by air heated using solar energy and the remainder by the desiccant. The inclusion of reflective mirror on the desiccant bed makes faster regeneration of the desiccant material. Keywords- Forced convection; Solar dryer; Desiccant drying; Pick-up efficiency; Thermal efficiency I. Introduction : Solar drying is the most preferred method for drying agricultural products. Though well developed, and still a good deal of work is continuing in this direction throughout the world. For commercial producers, the ability to process continuously throughout the day is very important to dry the produces to its safe storage level. Various designs of solar dryers are developed and tested for their performance. Each differs in design and is developed for a specific product. Passive type of solar crop dryers is well realized and it overcomes the problems exist in open sun dryer and cabinet type of dryers [1]. Normally thermal storage systems are employed to store thermal energy, which includes sensible heat storage, chemical energy storage and latent heat storage [2]. Common systems include water tanks or gravel beds, sand, granite, concrete, etc. [3] where energy is stored in the form of sensible heat. Latent heat storage is also an efficient and suitable heat storage means, it include hydrated salts, paraffin s, non paraffin [4] and fatty acids, but the drawback is its low thermal conductivity. Solar drying systems with sensible and latent heat storage are successfully demonstrated by many researchers [5 8]. Generally the equipment required to store and exchange heat by sensible and latent heat storage is not practical for solar drying. The drying potential in a regenerated desiccant material represents one of the most promising mechanisms of thermal storage for the purpose of solar drying [9]. In this process, the moisture removal from the drying air could be realized by adsorption in a desiccant unit regenerated by solar energy. The heat generated during such exothermic adsorption is nearly equivalent to the latent heat of vaporization of the removed moisture. The desiccant bed serves as an open adsorption desorption cycle where solar energy is stored during the desorption stage as sensible heat. It is recovered later during the adsorption stage where a relatively humid and cold air is drawn through the adsorbent bed and the exiting warm and dry air can be used for drying. The sensible heat storage in the packed beds is also shown to be a sufficiently efficient system [10]. Several studies were conducted using regenerated solid desiccants such as silica gel, but it requires high temperature typically above 150 C for regeneration and high cost of desiccant material. Though silica gel has got high moisture sorption capacity its dust particles have shown to be carcinogenic and make it unsuitable for direct food processing applications. Thoruwa et al. [11] developed and studied the efficacy of various CaCl2 based solar regenerative solid desiccant material and showed that 60% bentonite, 10% CaCl 2, 20% vermiculite and 10% cement gave a maximum moisture sorption of 45% dry weight basis (dwb) and is suitable for grain drying applications. The purpose of this work is to study the possibility of using CaCl 2 based solid desiccant for grain drying applications for the typical climatic conditions of Pune, Maharashtra, India. Experiments have been conducted on the fabricated forced convection and desiccant integrated solar dryer for drying green peas and pineapple. In order to increase the intensity of solar radiation on the 3144 P a g e

2 desiccant bed a reflective mirror is incorporated and its effects were studied. Nomenclature :- A cross sectional area of pipe connecting drying chamber (m2) C a specific heat capacity of air (kj/kg K) C d specific heat capacity of desiccant (kj/kg K) dm/dt drying rate at any time of drying (kg water/ kg dry matter.min) h as adiabatic saturation humidity of air entering the chamber (kg water/kg dry air) h i absolute humidity of air entering the chamber(kg water/kg dry air) h fg latent heat of vaporization of water (kj/kg) I solar insolation (W/m2) k drying constant (s - 1) m a mass flow rate of air (kg/s) m d mass of desiccant (kg) m w mass of moisture evaporated in time t (kg) M e equilibrium moisture content (kg water/kg drymatter) M f final moisture content (kg water/kg dry matter) M o initial moisture content (kg water/kg dry matter) M t moisture content at any time of drying (kgwater/kg dry matter) M R moisture ratio P f blower power (W) t time (s) T a ambient temperature ( C) T co collector outlet temperature ( C) T di initial temperature of desiccant ( C) T df final temperature of desiccant ( C) T fi collector air inlet temperature ( C) W d mass of dry matter (kg) W 0 weight of sample at t = 0 (kg) W t weight of sample at any time t (kg) W wet mass of wet matter after drying in a solar dryer(kg) η p pick-up efficiency drier thermal efficiency η d II.Experimental Set Up: The experimental set up consists of an indirect forced convection solar dryer with two solar flat plate air collector, 0.2 kw centrifugal blower with an air flow rate up to 400 m 3 /h, drying cabinet consisting solid desiccant material stacked at the top, a reflective mirror to increase the concentration of solar radiation on the desiccant bed and a 0.02 kw reversible fan to circulate the drying air inside the drying chamber in the night. The schematic of the experimental setup is shown in Fig. 1. Fig. 1 : Schematic of the desiccant integrated solar dryer. The solar air collectors had dimensions of 1.2 m 2.4 m. A 0.9 mm thick copper sheet painted black was used as an absorber plate for incident solar radiation. It was oriented southward with a tilt angle of 30. A 6 mm plain window glass is used as a transparent cover for the air collector to prevent the top heat losses. The frame is made of thick wood. For insulation, fine saw dust is used at the sides and bottom of the collector. The drying cabinet was constructed with insulated wooden walls of dimensions 1.2 m 1.2 m cross sectional area and 2 m height consisting of 15 shelves for holding the products. At the top of the drying chamber, a double glazing with an air gap of 50 mm was provided with an inclination as that of collector to absorb the incident solar radiation. A perforated tray is provided just below the double glassing to stack 100 kg of solid desiccant material. The desiccant material is a mixture of 60% bentonite, 10% calcium chloride, 20% vermiculite and 10% cement and is molded in the shape of cylinders and is processed in a vacuum furnace. Provision is also made to sandwich a thick insulated board(plywood) just below the perforated tray during solar drying and above the desiccant bed during desiccant drying. III.Experiments And Calculation: Drying experiments were carried out outdoors for drying green peas and pineapple slices to study the drying characteristics and the dryer performance under three modes: (a) Desiccant drying with reflective mirror; (b) Desiccant drying without reflective mirror and (c) Solar drying without the integration of desiccant unit. The solar radiation intensity on the collector surface is measured using a calibrated solar meter (day solar meter) with an accuracy of ±3% at 1000 W/m 2. Copper constantan thermocouples coupled to a 22 channel temperature scanner with sensitivity 0.5 C is used to measure the temperatures at locations in the system. The moisture content of the drying product was measured at an interval of 30 min by taking 100 g of sample from the trays using an electronic balance with an accuracy of g. The relative humidity of ambient air and drying air are measured using thermo hygrometer (Testo 625, range from 0% to 100% RH, accuracy ±2.5%). The initial and final moisture contents of 3145 P a g e

3 the samples were determined by the drying oven method Moisture Ratio :- whose temperature is fixed at 105 C. The difference of mass before and after drying in the oven gives the moisture content. For practical applications the optimum flow rate of (4) air through the flat plate collector should be in the range of The main characteristics, which are generally used kg/m2 s [12]. for performance estimation of any solar drying system, are In this study the flow rate of air through the two drying rate, dryer thermal efficiency and pick-up efficiency solar flat plate collector is 0.02 kg/m2 s and kg/s [15]. The drying rate should be proportional to the difference inside the drying chamber during the off sunshine hours. in moisture content between the material to be dried and the Experimental solar drying runs were conducted for 20 kg of equilibrium moisture content. Mathematically, it can be green peas and pineapple slices. The pineapple was pealed, expressed as thin layer equation cored and sliced at 10 mm thickness. The Line diagram of desiccant integrated solar dryer is shown in Fig. 2. Drying rate :- (5) Pick-up efficiency determines the efficiency of moisture removal by the drying air from the product. Dryer thermal efficiency is the ratio of the energy used to evaporate the moisture from the product to the energy supplied to the dryer and can be expressed as [13] Pick-up efficiency :- (6) dryer Fig. 2: Line diagram of desiccant integrated solar Dryer thermal efficiency :- The system performance and the drying characteristics were calculated using the following equations. The moisture content on dry basis is the weight of moisture present in the product per unit weight of dry matter in the product and is expressed as Initial moisture content :- (7) IV.Results And Discussion: The variation of solar radiation, ambient temperature and collector outlet temperature for a typical experimental run are shown in Fig. 3. (1) Final moisture content :- (2) The instantaneous moisture content Mt at any given time t ondry basis is computed using the following expression [13] (3) Weller and Bunn [14] used the weight change over time to calculate the moisture change over time. A drying rate constant k was derived by fitting moisture content and time to a thin layer drying equation of the form Fig. 3 : Variation of solar insolation, ambient and collector outlet air temperature with time of day for a typical experimental run P a g e

4 The variation of temperature profile for the desiccant material over a day with and without a reflective mirror is shown in Fig. 4. Fig. 6: Variation of moisture content with drying time for pineapple slices under drying conditions Fig. 4 : Variation of temperature profile of the desiccant over a typical day drying run with and without a reflective mirror. The average temperature of the desiccant material with the reflective mirror reaches a maximum of 80 C at the noon, which is about 10 C higher than that of without reflective mirror. This shows that the drying potential of the desiccant material is increased and maintains the temperature level well above the ambient temperature even on the next day in the morning. The decrease in temperature of desiccant after the peak is mainly due to the fall in solar radiation and heat losses to the ambient air circulated through the desiccant. The variation of moisture content with the drying time for green peas and pineapple slices are shown in Figs. 5 and 6, respectively. It can be seen that the moisture content of the sample decreases exponentially with the drying time. In the beginning of drying cycle, the rate of moisture evaporation loss is very high and decreases as the drying proceeds. The moisture content of onion slices reached to 5% from 80% in 19 h of drying in desiccant integrated solar drying with reflective mirror, while it took 21 h of drying to bring down the moisture content without the use of convection solar drying with and without reflective mirror. But it took 2 sunshine days (34 cumulative hours) to bring it to the same moisture level. From the curves it is clear that the moisture change during the first 8 hours were almost similar and exhibits a difference, when the desiccant unit is incorporated in the drying chamber. The integration of desiccant chamber continued drying in the off sunshine hours and the equilibrium moisture level reached well before the next day. The inclusion of reflective mirror in the desiccant unit advanced the drying process by 2 h for green peas and 4 h for pineapple slices. The variation of pick-up efficiency for green peas and pineapple slices under drying conditions are plotted against the drying time in Figs. 7 and 8, Fig. 5: Variation of moisture content with drying time for green peas under drying conditions. Fig. 7: Variation of pick-up efficiency with drying time at drying conditions for green peas. respectively P a g e

5 It can be seen that the pick-up efficiency varies between 15% and 45% for green peas and between 52% and 62% for pineapple slices respectively. Fig. 10 : Variation of drying rate with drying time at drying conditions of pineapple Fig. 8. Variation of pick-up efficiency with drying time at drying conditions for pineapple slices It is obvious that the increase in pick-up efficiency is due to the faster evaporation of free moisture in the product outer surface and also due to the increase in collector outlet air temperature. During desiccant drying, the decrease in pick-up efficiency is due to the saturation of the desiccant material. In early stages of the drying applications of the moist products, drying was relatively easier and therefore high pick-up efficiency values were obtained. The variation of drying rate with drying time and moisture content are shown in Figs Fig. 11 : Variation of drying rate with moisture content at drying conditions for greenpeas Fig. 9 : Variation of drying rate with drying time at drying conditions for green peas. Fig. 12 : Variation of drying rate with moisture content at drying conditions for pineapple slices P a g e

6 The constant rate drying period is absent for both [2] D. Jain, R.K. Jain, Performance evaluation of an the cases and the entire drying process took place only in the inclined multi-pass solar air heater with in-built falling rate period. The drying rate in the initial stage is very thermal storage on deep-bed drying application, high for both the products and decreases as the moisture Journal of Food Engineering 65 (2004) content of the product reaches to a certain level. The drying [3] S. Aboul-Enein, A.A. El-Sebaii, M.R.I. Ramadan, rate at the start of drying for green peas and pineapple slices H.G. El-Gohary, Parametric study of a solar air heater were 2.3 and 10 kg water/kg dry matter-min was due to the with and without thermal storage for solar drying free surface moisture evaporation and later the drying rate is applications, Renewable Energy 21 (2000) controlled by diffusion of moisture from the interior of the [4] M.N.A. Hawlader, M.S. Uddin, M.M. Khin, Microencapsulated product to the surface. The higher drying rate of pineapple PCM thermal energy storage system, slices at the initial stage of drying is due to the high initial Applied Energy 74 (2003) free surface moisture content. The thermal efficiency over an [5] H.P. Garg, V.K. Sharma, R.B. Mahajan, A.K. entire drying run is the ratio of the energy used to evaporate Bhargave, Experimental study of an inexpensive solar the moisture from the product to the energy supplied to the collector cum storage system for agricultural users, dryer. In this dryer, the energy is obtained through solar Solar Energy 35 (1985) radiation, blower work and through the desiccant material. [6] P.W. Niles, E.J. Carnegie, J.G. Pohl, J.M. Cherne, The mean dryer thermal efficiency was little higher for green Design and performance of an air collector for peas and pineapple slices, when reflective mirror is industrial crop dehydration, Solar Energy 20 (1978) incorporated with the desiccant unit, which is about 48 and %, which is 5% greater than without the reflective mirror. [7] P.M. Chauhan, C. Choudhury, H.P. Garg, The efficiency of the dryer, when using solar energy alone Comparative performance of coriander dryer coupled was calculated to be 43% for green peas and 49% for to solar air heater and solar air-heatercum- rock bed pineapple. The average thermal efficiency of the solar flat storage, Applied Thermal Engineering 16 (1996) plate collector was calculated as 43% at 0.02 kg/m 2 s. of colour and microbiological decay, when compared to solar drying. Taste of the dried pineapple is satisfactory. The desiccant material is stable even after continuous operation for more than a year. The dryer can be used for drying various agricultural products. It can reduce drying time and improve quality of the dried product. V.Conclusions: The performance of an indirect forced convection and desiccant integrated solar dryer has been investigated for drying green peas and pineapple slices with and without the reflective mirror. The inclusion of reflective mirror on the desiccant bed increases the drying potential considerably. The useful temperature rise of about 15 C was achieved with mirror, which reduced the drying time by 3 h and 4 h for green peas and pineapple, respectively. Also, the pick-up efficiency, drying rate and average dryer thermal efficiency were relatively higher, when compared to solar drying and desiccant integrated drying. Uniform drying in all the trays were achieved with good quality in terms of color and microbiological decay, when compared to solar drying. Taste of the dried pineapple is satisfactory. The desiccant material is stable even after continuous operation for more than a year. The dryer can be used for drying various agricultural products. It can reduce drying time and improve quality of the dried product. 6.References: [1] G.N. Tiwari, P.S. Bhatia, A.K. Singh, R.K. Goyal, Analytical studies of crop drying cum water heating system, Energy Conversion and Management 38 (1997) [8] S.O. Enibe, Thermal analysis of a natural circulation solar air heater with phase change material energy storage, Renewable Energy 28 (2003) [9] R. Hodali, J. Bougard, Integration of desiccant unit in crops solar drying installation: optimization by numerical simulation, Energy Conversion and Management 42 (2001) [10] A. Ahmad, J.S. Saini, H.K. Varma, Thermo-hydraulic performance of packed bed solar air heaters, Solar Energy 47 (2) (1991) [11] T.F.N. Thoruwa, C.M. John stone, A.D. Grant, J.E. Smith, Novel, low cost CaCl2 based desiccants for crop drying applications, Renewable Energy 19 (2000) [12] A.A. Hegazy, Optimum channel geometry for solar air heaters of conventional design and constant flow operation, Energy Conversion and Management 40 (1999) [13] J. Mumba, Design and development of a solar grain dryer incorporating photovoltaic powered air circulation, Energy Conversion and Management 37 (1996) [14] C.L. Weller, J.M. Bunn, Drying rate constants for yellow dent corn as affected by fatty acid ester treatments, American Society of Agriculture Engineers 36 (1993) [15] M.A. Leon, S. Kumar, S.C. Bhattacharya, A comprehensive procedure for performance evaluation of solar food dryers, Renewable and Sustainable Energy Reviews 6 (2002) P a g e