PERFORMANCE OF FORCED CONVECTION EVACUATED TUBE SOLAR GRAIN DRYER. BOOKER ONYANGO OSODO B. Ed.(TECH); M. Phil (Tech. Ed) (J98/25749/2011)

Transcription

1 PERFORMANCE OF FORCED CONVECTION EVACUATED TUBE SOLAR GRAIN DRYER BOOKER ONYANGO OSODO B. Ed.(TECH); M. Phil (Tech. Ed) (J98/25749/2011) A research Proposal Submitted in Partial Fulfillment of the Requirements for the Award of the Degree of Doctor of Philosophy in Sustainable Energy Technology in the School of Engineering and Technology, Kenyatta University

2 DECLARATION This proposal is my original work and has not been presented for award of any degree in any other University. Signature o,:-~ Date... J:.-(J..~.!.~Y BOOKER ONYANGO OSODO (REG NO: J /2011) This research proposal has been submitted for examination with our approval as the University supervisors. Signature...~ Dr. Jeremiah K. Kiplagat... Date... L!!()J?:D.Ii Energy Engineering Department, Kenyatta University Signature.. ~... Date..~~II':t Prof. Daudi M. Nyaanga Department of Agricultural Engineering, Egerton University ii

3 ABSTRACT Open sun drying leads to loss of grain as a result of birds and other pests that feed on it. The quality of the grain is also compromised due to contamination by dust, animal droppings and other contaminants as well as due to cracking and discoloration of the grain. Solar grain dryers may be used to address these problems. Direct solar dryers protect the grain from adverse weather, but have limitations, among them being cracking and discoloration of the grain. Natural convection solar dryers, though convenient at places with no mains electricity, perform poorly due to poor ventilation. Forced convection solar dryers, utilizing solar driven fans, may be used to improve ventilation, and to achieve an optimum drying air velocity. However, it is necessary to determine the optimum air velocity for a given dryer. The most common solar air heaters used in crop dryers are the flat plate collectors, whose thermal efficiency is limited due to convective heat loss from the absorber plate to the glass cover. This means that such dryers are only appropriate for small scale drying of maize and are inadequate for large scale farmers. This problem may be resolved by applying evacuated tube solar collectors, in which the only mechanism for heat loss is radiation, leading to greater thermal efficiency. However, few studies have been carried out to investigate the performance of such solar collectors when used in crop dryers. This study aims at simulating and optimizing the design of a forced convection evacuated tube solar grain dryer for large scale drying of maize. Thereafter a prototype of the dryer will be fabricated and tested, with a view to upscaling it for use in large scale drying of maize. The dryer, consisting of three main components: two variablespeed'blowers, an evacuated tube solar collector and drying chamber, shall utilise the solar air heater for pre-heating the air before it is forced into the drying chamber. The optimum air velocity required to maintain a drying air temperature below 60 degrees Celcius, essential for preventing detereoration of grain quality, will be determined. The effects of selected drying process parameters, namely drying air velocity and grain layer thickness and grain moisture content on dryer performance will be determined, both for the simulated solar dryer, and the prototype. The performance of the dryer will be evaluated on the basis of Energy Utilisation Ratio, drying rate and total drying time, and on whether the grains have been discolored and/or cracked after drying or not. In addition, the relationship between grain moisture content on one hand, and drying rate and time will investigated. The drying model that best describes the relationship between moisture content and drying time for the grain will be selected and the model used to predict drying time. iii

4 TABLE OF CONTENTS DECLARATION ii ABSTRACT iii TABLE OF CONTENTS iv LIST OF TABLES vi LIST OF FIGURES vii LIST OF S"YMBOLS viii ABBREVIATIONS AND ACRONYMS.x CHAPTER ONE: INTRODUCTION Background of the Study Statement of the Problem : Significance of the Study Objectives of the Study Scope and Limitations 4 CHAPTER TWO : LITERATURE REVIEW Introduction.: Modeling, Modeling Process Modeling of Solar Drying Curves Solar Drying Simulation ANSYS Software Post Harvest Food Loss The Drying Process Drying Systems Open Sun Drying Types of Solar Crop Dryers Evacuated Tube Solar Collectors Solar Dryers with Back up heaters 19 iv

5 2.6.6 Review of Past Works on Crop Dryers Analysis of the Drying Process 22 CHAPTER THREE: MATERIALS AND METHODS INTRODUCTION Simulation and Design Optimisation The Physical Solar Grain Dryer Operation of the System Parametric Analysis of the Simulated dryer Achievement of Optimum Drying Air Temperature Effects of Selected Drying Process Parameters on Prototype performance Grain layer thickness Air Velocity Grain Moisture Content Drying Model and Computer Program Fabrication of the Prototype Testing of the prototype Data Analysis Expected Results/Outputs ~ 32 REFERENCES 33 APPENDIX I : WORK PLAN. ~ 38 APPENDIX II: BUDGET 39 v

6 LIST OF TABLES Table 1: Mathematical models for drying curves 9 vi

7 LIST OF FIGURES Figure 1 Chart showing Stages of Modeling Process 7 Figure 2 Cross-sectional View of Sanyo Evacuated Tube Collector 17 Figure 3 Top Elevation of Corning Evacuated Tube Collector 18 Figure 4 Phillips(Germany) Evacuated Tube Collector(Top Elevation and Cross-sectional View).18 Figure 5 Side View of Solar Grain Dryer 28 vii

8 LIST OF SYMBOLS I g = Rate oftota1 radiation incident on the absorber surface (W/m2) Ac = Collector area (m") T ci = Temperature of air at collector inlet (K) T co = Temperature of air at collector outlet (K) c pa = Specific heat capacity of air (kj kg" K- I ) m = mass of heated air (kg) L = Length of evacuated tube (m) D= Diameter of external tube of evacuated tube (m) L, = Latent heat of vaporization (Jkg-I) 'ilz= mass flow rate of heated air (kg s-i) h = Specific enthalpy (Jkg- I ) = Dryer efficiency W= Weight of water evaporated from product (kg) L= latent heat of evaporation of water at exit temperature (kjkg- 1 ) I = Hourly average solar radiation on aperture surface (W m- 2 H- 1 ) Mb = Mass of fuel (kg) PI' = Energy consumed by fanlblower t = Drying time x = moisture content of wet material (kg of water /kg of dry matter) Xo = Initial moisture content (kg ofwaterlkg of dry matter) Xeq Kcr = Equilibrium moisture content (kg of waterlkg of dry matter) = Critical moisture content (kg of water/kg of dry matter) m, = Mass of air leaving dryer per unit time (kgs") mw= Mass of the water evaporated from the material dried (kg) viii

9 M, = initial moisture fraction M, = final moisture fraction Rg = Gas constant for water vapour (kjkg-lk I ) T, = Ambient air temperature (K) He= Latent heat of vaporization of water kjkg- 1 dt = Time interval ( see ) T I = Initial temperature of air (K) T2= Final temperature of air (K) m, = Mass of drying air (kg) R, = Specific gas constant (kjkg-1k 1 ) hl= layer drying bed thickness (m). 3 I V= Volume flow rate (m s' ) P ;, = density of air (kg m- 3 ) Vc = Average air velocity at exit of air heater (ms") Y= Measured /Experimental value Y- = Model/Simulated value Subscripts Dr Drying chamber o Outlet iinlet c collector wb Wet basis ix

10 OSD DSD Open sun drying Direct solar drying ABBREVIATIONS AND ACRONYMS ISD EUR LeV Indirect solar drying Energy Utilisation Ratio Lower Calorific Value of a fuel x