DESIGN, DEVELOPMENT AND PERFORMANCE EVALUATION OF AN ACTIVE SOLAR DRYER FOR BIOMATERIAL PROCESSING IKEJIOFOR, MAUREEN. C (PG/M.

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1 DESIGN, DEVELOPMENT AND PERFORMANCE EVALUATION OF AN ACTIVE SOLAR DRYER FOR BIOMATERIAL PROCESSING BY IKEJIOFOR, MAUREEN. C (PG/M.ENG/06/41126) DEPARTMENT OF AGRICULTURAL & BIORESOURCES ENGINEERING FACULTY OF ENGINEERING UNIVERSITY OF NIGERIA, NSUKKA A PROJECT REPORT SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF MASTER S DEGREE (M.ENG) IN AGRICULTURAL & BIORESOURCES ENGINEERING DECEMBER, 2010

2 2 DEDICATION Dedicated to the Almighty God for his numerous blessings and also, for granting me a meaningful future.

3 3 CERTIFICATION AND APPROVAL This is to certify that Ikejiofor, Maureen C. who is a postgraduate student of the Department of Agricultural and Bioresources Engineering, has satisfactorily completed The requirements leading to the award of Masters of Engineering in Agricultural and Bioresources Engineering. This work is original and has not been submitted in part or full for any other diploma or degree of this or any other University. We do hereby approve the project work... Engr. Dr. W.I. Okonkwo Supervisor Date.. Engr. Dr. B.O. Ugwuishiwu Ag. Head of Department Date

4 4.. Engr. Prof. S.I. Oluka External Examiner Date ACKNOWLEDGEMENT I wish to express my immense appreciation to all those who have helped me throughout my career and postgraduate programme, especially my project supervisor, Dr. W.I. Okonkwo for his thorough supervision and guidance on the project work. My sincere thanks also go to the Head of Department: Engr. Dr. B.O. Ugwuishiwu and my lecturers: Prof. (Engr.) C.O. Akubuo; Prof. (Engr.) E.U Odigboh; Prof. (Engr.) G.O. Chukwuma and Prof. I.U Obi, for enhancing my knowledge. I am also indebted to my employer: Dr. K.I. Nwosu, the Executive Director, National Root Crops Research Institute, Umudike, for his encouragement and support. I also wish to thank the Workshop Engineer and the technicians from the Research Institute s Engineering Workshop for their assistance. Special thanks are also due to my husband and family members for their prayers, encouragement and support.

5 5 Finally, I wish to express my gratitude to the Almighty God for his goodness, mercy and kindness to all of us. ABSTRACT An active solar dryer was designed, developed and the performance evaluation undertaken. The dryer has 3 main sections: The solar collector, heat storage unit and drying chamber with the following dimensions 160cm x 95cm x 10cm, 85cm x 65cm x 35cm and 95cm x 75cm x 70cm respectively. The dryer is incorporated with a suction fan that enhances air flow in the chamber. Tests were conducted during the dry and wet seasons in March and June respectively to indicate if the suction fan used has effect in both seasons. The different drying methods: open sun drying, solar drying without suction fan and solar drying with suction fan were used for the test. Also, different root / tuber crop products: cassava, yam, sweet potato, cocoyam, ginger and turmeric were used. Statistical analysis using ANOVA was used to determine the effect of the different drying methods and crop types on the drying rates. Results from the dry season test show that solar drying with suction fan gave highest drying rate of kg/hr when used to dry yam. Also, the least drying rate was obtained when open sun drying was used to dry turmeric. Also, results from the wet season test show that solar drying with suction gave highest drying rate of

6 kg/hr when used to dry yam. The least drying rate was obtained when open sun drying was used to dry cassava. The use of suction fan reduces drying period and therefore enhances drying of agricultural products. Indication showed that the value of the relative humidity within the drying chamber was lower when the suction fan was in use. The dryer has actual capacity of 15kg / batch. The collector has maximum heat gain / efficiency of W and 94.4% respectively. The products dried with the solar dryer were of better quality than those dried with open sun drying method because they retained their colour and also free from mould after storage for about 3 months. LIST OF TABLE IN APPENDIX Table 1: Properties of Sensible Heat Storage Materials Table 2: Properties of Selected Salt Hydrates Table 3: Temperature of Collector on the test dates for March, 2010 Table 4: Temperature of Collector on the test dates for June, 2010 Table 5: Collector Heat Gain (W/m 2 ) / Efficiency: 24 th March, 2010 Table 6: Collector Heat Gain (W/m 2 ) / Efficiency: 25 th March, 2010 Table 7: Collector Heat Gain (W/m 2 ) / Efficiency: 26 th March, 2010 Table 8: Collector Heat Gain (W/m 2 ) / Efficiency: 27 th March, 2010 Table 9: Collector Heat Gain (W/m 2 ) / Efficiency: 29 th March, 2010 Table 10: Collector Heat Gain (W/m 2 ) / Efficiency: 30 th March, 2010 Table 11: Collector Heat Gain (W/m 2 ) / Efficiency: 22 nd June, 2010 Table 12: Collector Heat Gain (W/m 2 ) / Efficiency: 23 rd June, 2010 Table 13: Collector Heat Gain (W/m 2 ) / Efficiency: 24 th June, 2010

7 7 Table 14: Hourly Total Radiation in ml on the test dates for March, 2010 Table 15: Hourly Total Radiation in W/m 2 on the test dates for March, 2010 Table 16: Hourly Total Radiation in ml on the test dates for June, 2010 Table 17: Hourly Total Radiation in W/m 2 on the test dates for June, 2010 Table 18: Maximum Temperature of the Drying Chamber on the test dates, March 2010 Table 19: Maximum Temperature of the Drying Chamber on the test dates, June 2010 Table 20: Average Relative Humidity of the Drying Chamber on the test dates, March, 2010 Table 21: Average Relative Humidity of the Drying Chamber on the test dates, June, 2010 Table 22: Drying Rates of Root / Tuber Crops and Drying Methods for test in March, 2010 Table 23: Drying Rates of Root / Tuber Crops and Drying Methods for test in June, 2010 Table 24: Ambient Temperature on the test dates for March, 2010 Table 25: Ambient Temperature on the test dates for June, 2010 Table 26: Ambient Relative Humidity on the test dates for March, 2010 Table 27: Ambient Relative Humidity on the test dates for March, 2010 Table 28: Data obtained for the Drying rates (kg/hr) during dry season test Table 29: ANOVA table for the Drying rates during dry season test Table 30: Data obtained for the Drying rates (kg/hr) during wet season test Table 31: ANOVA table for the Drying rates during wet season test

8 8 LIST OF FIGURE Page Fig. 1: Mean Annual Solar Map of Nigeria 20 Fig. 2: Design of Active Solar Dryer 45 Fig. 3: Omega RS- 232 Data Logging Thermometer used 51 Fig. 4: Collector / Average Ambient Temperature versus Solar Time on a 56 typical dry period Fig. 5: Collector / Average Ambient Temperature versus Solar Time on a 56 typical rainy period Fig. 6: Efficiency of the Collector on a typical dry period 57 Fig. 7: Efficiency of the Collector on a typical rainy period 57 Fig. 8: Hourly Total Radiation in W/m 2 on a typical dry period 58 Fig. 9: Hourly Total Radiation in W/m 2 on a typical rainy period 58 Fig.10: Maximum Temperature of the Drying Chamber / Average Ambient 59 Temperature versus Solar Time for March, 2010 Fig.11: Maximum Temperature of the Drying Chamber / Average Ambient 60

9 9 Temperature versus Solar Time for June Fig.12: Average Relative Humidity of the Drying Chamber / Average 60 Ambient Relative Humidity versus Solar Time for March, 2010 Fig.13: Average Relative Humidity of the Drying Chamber / Average 61 Ambient Relative Humidity versus Solar Time for June, 2010 Fig.14: Bar Chart representation of the Drying Period versus Method used 63 for March, 2010 Fig.15: Bar Chart representation of the Drying Period versus Method used 63 For June, 2010 Fig.16: Ambient Temperature versus Solar Time for March, Fig.17: Ambient Temperature versus Solar Time for June, Fig.18: Ambient Relative Humidity versus Solar Time for March, Fig.19: Ambient Relative Humidity versus Solar Time for June, Fig.20: Active Solar Dryer (side view) 66 Fig.21: Active Solar Dryer (front view) 67 Fig.22: Testing of the Active Solar Dryer 68 Fig.23: Products dried with the Solar Dryer (Sweet Potato, Cocoyam 69 and Turmeric) Fig.24: Products dried with the Solar Dryer (Yam, Cassava and Ginger) 69 Fig.25: Products dried with Open Air (Ginger, Yam and Cocoyam) 70 Fig.26: Products dried with Open Air (Sweet Potato, Cassava and Turmeric) 70

10 10 TABLE OF CONTENTS Title Title page Dedication Certification Acknowledgement Table of Contents List of Tables List of Figures Abstract Page i ii iii iv x vi viii v

11 11 CHAPTER 1: INTRODUCTION Drying of Agricultural Products Objectives Scope of the study 2 CHAPTER 2: LITERATURE REVIEW Technology of Drying Concept of Drying Theory of Crop Drying Principles of Air Movement in Crop Drying Importance of Crop Drying Types of Crop Drying System High Temperature Drying Energy Sources for Crop Drying Conventional Energy Sources in Crop Drying Systems Non Conventional Energy Sources in Crop Drying Systems Solar Radiation Solar Energy Collection Storage of Solar Energy Storage in Sensible Heat Materials Solar Energy Applications in Crop Drying Types of Solar Dryers Active Solar Dryer Passive Solar Dryer Advantages of Solar Energy Applications in Crop Drying Limitations of Solar Energy Applications in Crop Drying 38 CHAPTER 3: MATERIALS AND METHODS 39 CHAPTER 4: DESIGN AND DESCRIPTION OF THE SOLAR DRYER Design Considerations 41

12 Design Assumptions Description of the Solar Dryer Solar Collector Drying Chamber Heat Storage Unit Design Calculations Determination of Collector Area Collector Energy Gain 48 CHAPTER 5: PERFORMANCE EVALUATION EXPERIMENTAL PROCEDURES Weather Conditions on the test dates Hourly Solar Radiation Temperature Rainfall and Wind speed Analysis and Presentation of Results Solar Collector Drying Chamber Open Sun Drying 61 CHAPTER 6: CONCLUSION AND RECOMMENDATIONS Conclusion and Recommendations 71 REFERENCES 72 APPENDICES

13 13 CHAPTER 1 INRODUCTION 1.1 Drying of Agricultural Products Drying refers to the removal of moisture from agricultural materials until the moisture content of the product is in equilibrium with the surrounding air usually 12 to 14 percent moisture, wet basis. Successful drying depends on enough heat to draw out moisture, dry air to absorb the released moisture and adequate air circulation to carry off the moisture. The key factor in drying is to remove moisture as quickly as possible at a temperature that does not seriously affect the flavor, texture and colour of the product. If the temperature is too low in the beginning, microorganisms may grow before the product is adequately dried, but if the temperature is too high and the humidity too low, the product may harden on the surface. This makes it more difficult for moisture to escape and the product does not dry properly. Dried foods are tasty, nutritious, light weight, easy-to- prepare, and easy-tostore and use (David, 2000). The energy input is less than what is needed to freeze or can, and the storage space is minimal. The nutritional value of food is only minimally affected by drying. Vitamin A is retained during drying, however it is light sensitive and food containing it should be stored in dark places (Herringshaw, 1997). Vitamin C is destroyed by exposure to heat, although pretreating foods with lemon, orange, or pineapple juice increases vitamin C content (David, 2000). Dried foods are high in fibre and carbohydrates and low in fat, making them healthy food choices (Kerr Barbara, 1999). Dried Foods that are not completely dried are susceptible to mold. Microorganisms are effectively killed when the internal temperature of food reaches 145 o F (Herringshaw Doris, 1997) According to Kerr Barbara (1999), nutritionally dried food is ranked by the United State Food and Drug Agency as better than canning and freezing. She states that the tastes are related to the food, but there is some uniqueness in their flavour

14 14 and texture. Dennis Scanlin (1997) also found that by reducing the moisture content of the food to between 10 and 20%, yeast, bacteria, mold, and enzymes are all prevented from spoiling it. The flavour and most of the nutritional value are preserved and concentrated. Drying permits long-term storage of agricultural products without deterioration, since extended storage periods are becoming increasingly important with the large amount of crop being stored and carried over through another storage year by the government and industry. 1.2 Objectives i. To design an active solar dryer with heat storage unit. ii. To evaluate the effect of suction fan in solar crop drying system. iii. To undertake the performance evaluation of the dryer using different drying methods / crops. iv. To determine the drying rate of different root / tuber crops using the solar dryer. 1.3 Scope of the Study 1. The design and development of an active solar dryer with both direct and indirect mode of solar collection. 2. The study evaluates the performance of the dryer with respect to drying rate and dried quality of various biomaterials. 3. It also evaluates the effect of the suction fan on the drying rate of crops. 4. The study determined the drying rate of the following root crops: cassava, yam, sweet potato, ginger, cocoyam and turmeric. 5. The study examined the temperature and relative humidity variations within the drying chamber. CHAPTER 2

15 15 LITERATURE REVIEW 2.1 CROP DRYING TECHNOLOGY The technology of crop drying involves processes of temperature increase of the product, the drying air, and also moisture removal from the product. During the first part of the drying process, the air temperature can be relatively high, that is about 60 o C to 70 o C (Driscoll and Adamczak, 1988), so that moisture can evaporate quickly from the product. As soon as surface moisture is lost from the product, the outside begins to feel dry, and rate of evaporation slows down. The temperature should be high enough to evaporate moisture from the product, but not high enough to dehydrate the product. The higher the temperature and the lower the humidity the more rapid the rate of drying will be. Humid air slows down evaporation. If drying takes place too fast, case hardening will occur. This means that the cells on the outside of the product give up moisture faster than the cells on the inside. The surface becomes hard, preventing the escape of moisture from inside. Moisture in the product escapes by evaporating into the air. As soon as trapped air within drying area takes as much moisture as it can hold and then drying can no longer takes place. For this reason, it is necessary to ensure that there is proper air circulation to enhance moisture removal from the product. 2.2 CONCEPT OF DRYING Drying refers to the removal of moisture from agricultural products until the moisture content of the product is in equilibrium with the surrounding air, usually 12 to 14 percent moisture, wet basis. Reducing the moisture content can be either drying or dehydrating. Dehydrating is the removal of the moisture to very low moisture content, nearly bone dry condition. Bone-dry is the removal of all the moisture from a material. The moisture content is zero (Hall Carl, 1957). Crop drying is the removal of some of the moisture from crops by mechanically moving air through them after being harvested.

16 16 Drying may be accomplished by using either heated or unheated air. Crop in the field dries naturally as it matures, giving up moisture to the air until the crop moisture is in equilibrium with the moisture in the air (equilibrium moisture content). Conditions become less favorable for crops to dry while in the field to moisture content considered safe for storage, as the harvest is delayed and also crops are exposed to danger of pests, diseases and insects attack (Kenneth and Hellenvang 1994). 2.3 THEORY OF CROP DRYING Crop drying methods improve as theories of drying are advanced. The mechanisms resisting moisture movement during drying of agricultural products needs to be studied. The vapour pressure theory of drying should be considered as the temperature of the product increases while the moisture content is maintained. The flow of moisture is from points of high to low vapour pressure and is approximately proportional to the difference between the product and the surrounding atmosphere. It is only approximately proportional because the resistance to the movement of moisture on the surface is different with the resistance in the interior of the product (Barre, 1938). Farm crops differs from many other products, such as dust, paper, sand, stone chemical, etc RATE PERIODS OF DRYING The rate at which moisture is evaporated is determined largely by the material from which moisture is being evaporated (Driscoll and Adamczak, 1988). In the constant rate period, drying takes place from the surface of the product and is similar to evaporation of moisture from a free water surface. The two major periods of drying are the constant rate period and the falling rate period. The point marking the end of the constant rate period occurs when the rate of moisture diffusion within the product decreases below that necessary to replenish the

17 17 moisture at the surface. Most of the drying of sand, washed seed takes place in the constant rate period. The constant drying rate period is short in duration for farm crops. The magnitude of the rate of drying during this period is dependent upon; 1. Area exposed 2. Difference in humidity between air stream and wet surface 3. The coefficient of mass transfer and 4. Velocity of the drying air. The movement of moisture during drying or drying rate is represented by the following equation: M/ T = -K (M - M c ) (1) Integrating the equation M/M-M c = K t (2) M - M c = e -kt (3) M o M c Where M/ T = rate of drying per hr M = moisture constant (dry basis) of product at any time, t M o = original moisture content (d.b) M c = equilibrium moisture content t = time hr K = drying constant M - M c = moisture ratio (4) M o - M c The falling rate period is entered after the constant rate period. The critical moisture

18 18 content occurs between the constant rate and the falling rate periods. The critical moisture content is the minimum moisture content of the product that will sustain a rate of flow of free water to its surface, which is equal to the maximum rate of removal of water vapour from the product under the drying conditions. The falling rate interval is the most important period of drying for most agricultural products. Even when the constant rate period is in effect at the start of drying, it is often neglected because of its short duration and small amount of moisture to be removed before entering the falling rate period. According to Abdul and Kaddus (1993), drying in the falling rate period involves two processes: 1. Movement of moisture within the material to the surface by liquid diffusion. 2. Removal of the moisture from the surface. Falling rate period of drying can be divided into two stages: a. Unsaturated surface drying and b. Drying where the rate of water diffusion within the product is slow and is the controlling factor. 2.4 PRINCIPLES OF AIR MOVEMENT IN CROP DRYING The principles involved in the flow of air through crops during drying are same for natural ventilation, forced ventilation and heated air drying. Basically, a drying system consists of a means of moving the air through the product, the wind for natural ventilation or a fan for mechanical ventilation. The relationship among the volume of air moved through the product, the static pressure developed, and the depth of product over the air distribution system is important. A fan may be used to move a small volume of air against large pressure or a large volume of air against low pressure or a large volume of air against large pressure or a large volume of air against low pressure. Generally, all types of fans can be made to operate under various conditions. Fans may be placed into one of two general classes, those that operate by centrifugal force, or radial flow, where the

19 19 flow is perpendicular to the axis of rotation; and those that operate by axial flow where the flow is parallel with the axis of rotation of the fan blade. 2.5 IMPORTANCE OF CROP DRYING The drying of farm crops is important for the following reasons: 1. Drying permits early harvest by reducing field loss of products from storm and natural shattering. 2. It permits planning the harvest season to make better use of labour. Farm crops can be harvested when drying conditions are favorable. 3. Drying permits long- time storage without deterioration. Extended storage periods are becoming increasingly important with the large amount of crop being stored and carried over thorough another storage year by the government and industry. 4. Drying permits farmers to take advantage of higher price a few months after harvest. 5. Drying also permits maintenance of viability of seeds over long periods. 6. It permits the farmer to sell better quality products since reducing the moisture content of crop through drying prevents it from molds, enzymes action and insects damage. 7. Drying permits use of waste products since waste products can be converted to useful products, for example livestock feed from fruit pulp and cassava peel. 8. Products with greater economic value are produced, example dried fruits and vegetables. 9. Drying also facilitates production operation for products such as cotton and corn.

20 TYPES OF CROP DRYING SYSTEMS Dryers can be categorized in different ways. There are low temperature, sun and high temperature dryers which include the following: The bin, continuous gravity flow, rotary, tray, spray, pneumatic conveying, spin- flash, tunnel, and the band dryers. Dryers can also be classified according to the direction of airflow through the products; such as cross-flow, counter- flow and concurrent flow dryers Low- Temperature Drying Low temperature or ambient air drying is possible if the initial moisture content of the product is not in excess, the average daily relative humidity of the ambient- air is not too high, and the air flow rate is sufficient. In ambient-air layer drying, a bin is filled in stages. A wet layer is added only after the lower layers have been partially dried. In stir drying, one or more vertical stirring augers slowly move the dried grain from the button of the bin to the top, and the top layers to the bottom of the bin. Layer and stir drying require lower airflow rates and permit higher initial moisture contents than ambient-air drying(kenneth and Hellenvang,1994). In periods of zero drying potential during rain or high humidity, the relative humidity of the air can be decreased to the desired level ( 60%- 70%) by increasing the air temperature. This process of in-bin is called low- temperature drying (Kenneth and Hellenvang, 1994). Stirrers are frequently employed in an instore/low- temperature drying system Sun Drying In many tropical and sub-tropical regions, sun drying remains the preferred method of drying mostly for economic reasons. The products are spread on mats or paved ground in layers of 5 to 15cm thickness for exposure to the sun. However, sun drying is an unreliable process because it is weather-dependent. Also the solar radiation changes with the season and the time of day. Notwithstanding its

21 21 disadvantages, it is possible to produce dried grain of superior quality if sun drying is practiced competently. It is important to consider proper selection of the maximum layer thickness and the initial moisture content of the product and the recognition that during certain periods of the year (the wet season) adequate sun drying is not feasible Solar Drying Solar drying is an improvement to the traditional sun drying method. It can be classified into three main groups, according to the type of energy used for drying and the equipment employed (Imre, 1988a). These include: a) Solar natural dryers b) Semi- artificial solar dryers c) Solar- assisted dryers Solar Natural Dryer: These devices use only ambient energy and have no active elements. The airflow if there is any, is maintained by natural convection or, in some cases, by thermosphon effects induced by a chimney. Solar natural dryers are mainly used as substitutes for traditional open- air drying methods in areas where no other source of energy is available. In contrast to these traditional methods, however, losses and damage to the product caused by rain, dust, insects, birds and other animals as well as the pollutions from the atmosphere are avoided by design configuration (cabinet tent type arrangements) Semi- Artificial Solar Dryers: These usually feature a solar collector and a fan for maintaining a special airflow through the drying space. The use of semi artificial solar dryer is justified by their unsophisticated and fairly cheap construction. They can be recommended for drying materials that are not sensitive to change in the drying conditions caused by periodic character of the solar irradiation and by the changing atmospheric conditions.

22 Solar Assisted Dryer: These are convectional dryers having a solar collector. They are generally fitted with a heat storage system and also with an auxiliary energy source, for use in situation where the solar energy collected is insufficient for drying. Also, they are normally fitted with modern control systems. 2.7 HIGH TEMPERATURE DRYING High- temperature dryers are employed if high drying capacities are required. These include bin, continuous gravity flow, rotary, tray, spray, pneumatic conveying, spin- flash, tunnel, band, cross- flow, counter-flow and concurrent flow dryers The Bin Dryers There are two main types of bin dryers which include batch-in bin dryer and the recirculation/ continuous flow bin dryer. Batch-in bin dryer: Batch-in bin drying process involves using a bin containing about to 1.219m deep layer of material to be dried in which heated air is forced through the mass. Drying begins at the floor and progresses upward. Product at the floor of the bin becomes excessively dry, while the top layer of the batch remains fairly wet. A stirring device can be added to provide more uniform drying and moisture content and to increase the capacity of the bin dryer. Research conducted at the IOWA state university indicates that with a stirring device, there is less than 1 percentage point moisture variation between upper and lower of a batch of product. There is also resistance to airflow, and tendency for the fine materials to migrate to the bin floor as the stirring device is in operation (Kenneth, and Hellevang, 1994). The systems are simple, moderate inexpensive and serve as storage units after drying is completed Recirculation/Continuous Bin Dryer: This dryer incorporates a tapered sweep auger which removes the product from the bottom of the bin as it dries. The

23 23 sweep auger may be controlled by temperature or moisture sensors. When the desired condition is reached, sensors start the sweep auger, which removes layer of the product. After one complete revolution around the bin, the sweep auger stops until the sensor determines that another layer is dried The Continuous Gravity Flow Dryer (Column Dryer) Granular material that flow readily, permits air to flow through them, and are not damaged in handling, can be dried in a gravity flow dryer such as column dryer. The wet material is placed in the hopper and flows by gravity between the perforated retaining wall and is discharged at the bottom by a continuous operating meter valve. Heated air is forces across the column at right angles to the direction of grain motion. High building is usually required to house it because height is required to get appreciable capacity. Labour costs are low since handling is completely mechanical. The capacity is directly proportional to the column width and material movement rate through the column which varies from 80 to 1000 bushels. Column widths are normally from 10 to 20 inches. Typical operation sequence is fill- drycool-unload. The retaining time in the column is the drying time for the material as defined by its drying indices, the required moisture reduction, and state factors of the drying air. Control of the drying sequence can be either manual or automatic The Rotary Dryer Materials that neither are not free flowing nor damaged by continuous handling are usually dried in rotary dryer. The dryer has high initial cost and requires more floors per unit of capacity than either the batch or column dryer. It is widely used to dry relatively large through- puts of granular products and byproducts in a number of industries. Like many other types of dryer, they are available in several different forms. Of particular interest to the food industry are the cascading, rotary-louvre, and steam- tube type. Rotary dryers are characterized

24 24 by a slowly rotating cylindered drum, which is normally inclined at a small angle (0-5 o ) to the horizontal. The internals of the different classes of rotary dryer are however, quite different, as the methods used to transfer heat to the drying materials. They can handle free flowing solids of virtually any size or shape The Tray Dryer Materials that cannot be dried by any of the previously discussed methods are dried on trays; fruits and vegetables are best examples. The product is spread in thin layers on tray supported by angle slide mounted on the side of the drying cabinet. The product is placed in thin layers, usually at kgm -2, though higher tray loadings can be used if the material is re-spread or turned during the drying process. Tray dryers are very flexible in their operations. The velocity, temperature and humidity of the air, and the drying time can all be varied widely to suit the characteristics of any particular product. The principle drawback is the high man power requirement for filling and emptying the trays, loading them into the dryer and, if necessary, turning the product at intermediate moisture content The Spray Dryer Spray dryers remove the water from solutions or suspensions and dry the resulting powder to a moisture content that approaches equilibrium with the exhaust drying air. Spray dryers are used extensively in food, chemical, and pharmaceutical industries. Design varies from rectangular chamber fitted with spray jets, through which the drying air passes to continuous large volume systems. These procedures are used for breaking the material into fine drops. i. High- pressure atomization: The liquid is forced through a nozzle under high pressure. Mixing with the drying air and the spray pattern can be controlled. Drop size and gradation are difficult to predict. ii. Centrifugal: The liquid is fed at low pressure onto a horizontal disc or cup turning at speeds up to 20,000rpm or more. The materials break up into

25 25 small drops as it leaves the edge of the rotor. The drops are of more uniform size The Pneumatic Conveying Dryer In pneumatic-conveying dryer, a suitable particulate feed is conveyed through the dryer by a fast- moving steam of hot drying air. In the conveying process, heat from the air is transferred to the wet materials, causing the moisture to be flashed off. Thus, the drying process is very rapid. The basic type of pneumatic conveying dryer, often termed a flash dryer, consists normally of a straight vertical tube of circular square cross section. The air and the wet solids are introduced at the bottom of this tube. The dried solids which are collected in a suitable particle- separate device, and the exhaust air, both leaves at the top. The ring dryer is a development of the flash dryer in which the drying tube consists of loop or ring, which enables larger, slower drying particles to be recirculated through the drying zone, thereby permitting more versatile operation. Pneumatic-conveying dryers are popular in certain parts of the food industry for their ability to dry powdered materials rapidly. They are particularly suitable for processing heat sensitive materials because of the short residence time within the dryer. Pneumatic conveying dryers are extremely versatile and have the suitable key features as: - They are suitable for drying materials in range mm. -The short residence time ensures minimum heat damage with drying time of 2 to 6o seconds. -The low hold up of materials in the dryer ensures maximum safety of operations and less of fire or expulsion. -The dryer can accept wet powder, filter cakes or slurries as feed. -The dryer exhibits a high drying efficiency and product handling is gentle, with a throughput of 10kg to 25tons/hr solids. -The dryer has a small foot print and fits easily into a restricted space.

26 The Spin Flash Dryer The spin flash dryer is essentially an agitated fluidized bed that can be used to dry pastes, filter cakes, sludge and high viscosity liquids on a continuous basis. It provides a useful alternative to the spray dryer in cases where the feed cannot be pumped and atomized. The feed materials are dropped into the feed hopper in which it is broken up into cakes of uniform consistency by means of a low speed agitator. It is then transferred into the body of the dryers by means of a screw feeder. As lumps of the feed material enter the drying chamber, they became coated with dried powder and fall into the fluidized bed at the bottom of the chamber where they are kept in motion by means of the rotor, and heat is supplied through the heater. The light, dry particles are entrained by the rising air stream and travel up the wall of the drying chamber, at the top of which they are forced to pass through a classification orifice. This permits the finer material to exit the drying chamber, while the larger particles are retained in the dryer and tend to fall back into the fluidized bed, where they eventually breakup. The orifice can be sized to suit the requirement product size. Spin-flash dryers are available in a number of standard sizes with quoted throughputs ranging up to 10tons/ hr The Tunnel Dryer Tunnel dryers are normally designed on the basis of drying curves obtained in simple convective ovens. Proper allowance must be made, however, for the changing drying conditions within the tunnel. Sokhansang and Wood (1991) designed a tunnel dryer to dry baled forage from 3o % to 12% moisture content at 110 o C at the nominal throughput of 6 tonnes of dried material per hour. The dryer consists of three drying zones and one cooling zone with a total length of about 16m. Each zone was 3.67m wide and 3.67m long. Various airflow systems are employed. Parallel airflow gives a fast

27 27 initial drying rate, while counter-flow gives faster drying at the drying end of the tunnel. Parallel flow is seldom used because of its poor drying ability at the dry end of the tunnel. Combination tunnels utilize the advantages of both parallel flow and counter-flow, but the initial cost is greater and control is more difficult. Counterflows are the most extensively used. The air rate must be high enough so that the relative humidity of the discharged air is below the equilibrium relative humidity of the material at the point where the material discharges The Band Dryer: A band dryer consist essentially of a moving, perforated conveyor on which a layer of the drying solids rests. The band passes through a drying enclosure which heated air is directed through the layer of solids. The airflow rate is often lower than that employed in other types of dryer, for example, fluidized beds. Their versatile design and construction enable them to process a wide range of different products. In food industry, these are normally granular particles or extrudates. However, band dryers are also used to dry products as diverse as boards for use in building trade. Band dryers are well-suited to drying medium to high throughputs of products on a continuous basis. Integral coolers are frequently fitted. Depending upon the specific requirements, band dryers can be of the single-pass or multi-pass type. 2.8 DRYER TYPES ACCORDING TO THE DIRECTION OF AIRFLOW These include the following: The cross-flow, counter-flow, mixed- flow and concurrent flow dryers. The air and grain move in perpendicular directions in cross flow dryers, in the same direction in concurrent- flow dryers, in opposite direction in counter flow, and in a combination of cross-flow, concurrent-flow, and counter flow directions in mixed-flow dryers.

28 ENERGY SOURCES FOR CROP DRYING The energy sources for crop drying are in 2 main groups. These are conventional and non-conventional energy sources. The conventional includes the natural gas, coal, oil, electric, nuclear and hydro, while the non-conventional includes the biomass, geothermal, solar/sun and wind. The heat required, E to evaporate the moisture is based on the principles of heat transfer (Karlekar and Desmond, 1982). E = m c C c T +M w L (5) Where m c is mass of product in kg, C c is specific heat of product in KJ/Kg, M w is mass of moisture removed in Kg, L is latent heat of water vaporization in KJ/Kg (2256KJ/Kg) from Liley, (1997), and T, change in temperature o C The Conventional Energy Sources The fossil fuels Air heated by any of the fossil fuels can be a good source of energy for crop drying. In some commercial installations, the air is heated by steam formed by these fuels. The coal is a good source of heat for crop drying. Also, Natural gas is a relatively clean burning fossil fuel, which is used in turbines to produce electricity Electricity This is the major source of energy for crop drying, especially for the heated air crop dryers with fans and heaters which are operated by electricity. But, this energy source is rarely available in the rural areas where majority of the farmers dwell Nuclear Power Nuclear power harnesses the heat of radioactive materials to produce steam for power generation, which can be used for crop drying. This source of energy is not easily available.

29 Hydro Power This uses the force of moving water to produce electricity. Hydro power is one of the main suppliers of electricity in the world, but most often in the form of large dams. A better approach is the use of small, run of the river hydro plant Non- conventional energy sources Biomass Energy Energy from plants is a rich source of carbon and hydrogen. Fast growing plants, such as switch grass, willow and poplar tress can be harvested as Power Crops. Biomass wastes, including forest residues, lumber and paper mill wastes, crop wastes, garbage, landfill and sewage gas can be used to produce heat and electricity for crop drying Geothermal Energy This energy source taps the heat under the earth s crust to boil water. The hot water is then used to drive electric turbines which can generate energy for crop drying Solar Energy In several developing countries, sun drying of harvested crops is the general practice and the sun is the source of energy for drying. Generally, sun drying applies to spreading the crop in the sun on a suitable surface (such as road sides and mats), hanging crops from the eaves of building, trees, etc, and drying on the stalk by standing in stocks or bundles. In Nigeria, over 80% of harvested agricultural products are sun dried. Solar crop drying also relies on the sun as its major source of energy. However, it differs from sun drying in that a simple structure, such as a flat-plate solar collector, is used to enhance the effect of isolation and minimize loss of the collected sun energy to the surrounding. Solar drying is a good alternative to sun drying, especially for farmers in developing countries. In comparison with sun

30 30 drying, solar dryers can generate higher air temperatures and consequently lower air relative humidity, both of which are conducive to improved drying rates and lower final moisture contents of the dried product. This advantage reduces the risk of spoilage both during the actual drying process and in storage. The higher temperatures attainable are also a deferent to insect and microbial infestation, and protection against dust, insects, and animals is enhanced by drying in an enclosed structure Wind Power Advanced aerodynamics research has developed wind turbines that can produce electricity at a lower cost. The electricity can then be used to operate fans and heaters in a crop dryer Solar Radiation Solar radiation is the sun s energy that is reaching the earth s surface.the mean intensity of normal solar radiation (solar constant) outside the earth s atmosphere is 1353W/M 2. As this radiation passes through the atmosphere, it is partly scattered, reflected, and absorbed by the atmospheric particles so that only a portion reaches the surface of the earth. The total solar radiation reaching the earth s surface has both direct (beam) and diffuse components. The available total solar radiation on the earth s surface is intermittent and seasonal in nature. The solar energy received by a flat collector is at maximium (Ghana et al. 2007) if the inclination angle of the collector to the horizontal is such that: (L-10 o )<β< (L+10 o ), Where β is the angle of inclination and L is the latitude of the location. The most important factors that determine the amount of solar radiation reaching the earth s surface are: -Atmospheric composition.

31 31 - Atmospheric air mass that the radiation must pass through before reaching the surface. -Time of day or solar hour angle, maximum collection occurs at solar noon. -Latitude of the location. -Earth s rotation that determine the seasonal and daily variation in the amount of solar energy. The annual available total horizontal solar radiation in Nigeria determined from mean annual sunshine hours, varies from 5000 MJ/M 2, or 45% of the maximum possible value outside the earth s atmosphere, in the humid Niger Delta to over 9400 MJ/M 2, or 70%, in the extreme northeast of Nigeria (Arinze and Obi, 1984b), as shown in fig.1 which represents the annual solar map of Nigeria. Generally from January to April are the months with greatest solar energy availability and July to September have the lowest availability. This period is also the wettest.

32 32 Fig.1 Annual Solar Map of Nigeria (Arinze, 1986) Radiation is emitted from the sun with an energy distribution fairly similar to that of a black body, or a perfect radiator, at a temperature of 6,000 o k. Therefore, the emission, according to Stefan Boltzmann law of thermal radiation (Welty, 1974) is; q = A T 4 (6) Where q = radiant emission, W

33 33 A = area of the emitting surface, m 2 = Stefan Boltzmann constant, numerically equal to x 10-8 W/M 2 - o K T = absolute temperature, o k From Equation (6) above, 2 4 q 4 x R T a (7) Where Ra= radius of the sun = 6.96x10 8 m, substituting values of Ra, and T in the equation (2.7), then q = 4.478x10 23 KW. The total energy radiated by the sun into space, the earth receives only a very small fraction. There are two main reasons for this: a) The energy received from the sun decreases inversely as the square of the distance of the sun from the earth. b) The sun s elevation and the rotation of the earth (responsible for day and night variation) determine the amount of energy received at any particular point on the earth s surface. Solar radiation reaching any surface of the earth from outside the earth s surface is subjected to attenuation, reflection and scattering; hence two types of radiation are received on the surface of the earth, namely; beam or direct and diffused or scattered radiation. i) Beam or direct radiation is the solar radiation which travels from the sun through the atmosphere without any changes in direction. This is the solar flux arriving at the collector without having suffered any scattering in traversal of the atmosphere. ii) Diffused or scattered radiation is the solar radiation whose direction has been changed by scattering and reflection. This solar flux produced by atmospheric scattering through Raleigh scattering, clouds and dust, and aerosol scattering. It cannot be focused by any optical system, but it does

34 34 contribute flux to flat-plate and other non-focusing collectors (Welty, 1974), hence. I = I e + I d sin (8) I = Solar flux intensity, W/m 2 I d = direct component of solar radiation I e = scatted solar flux and = solar attitude above the horizontal Solar Energy Collection Natural collection of solar energy occurs in the Earth s atmosphere, oceans, and plant life. Interactions between the sun s energy, the oceans, and the atmosphere, for example, produce the wind. Modern applications of wind energy use strong, light, weather-resistant, aerodynamically designed wind turbines that when attached to generators, produce electricity for local, specialized use or as part of a community or regional network of electric power distribution. Approximately 30 percent of the solar energy reaching the outer edge of the atmosphere is consumed in the hydrological cycle, which produces rainfall and the potential energy of water in mountain streams and rivers. Through the process of photosynthesis, solar energy contributes to the growth of plant life (biomass) that can be used as fuel, including wood and the fossil fuels that are derived from geologically ancient plant life. Fuels such as alcohol or methane can also be extracted from biomass. The oceans also represent a form of natural collection of solar energy. As a result of the absorption of solar energy in the ocean and ocean currents, temperature gradients occur in the ocean. In some locations, these vertical variations approach 20 o C (36 o F) over a distance of a few hundred meters. When larger masses exist at different temperatures, thermodynamic

35 35 principles predict that a power generating cycle can be created to remove energy from the high-temperature mass and transfer a lesser amount of energy to a lowtemperature mass. The difference in these two heat energies manifests itself as mechanical energy (for example, output from a turbine), which can be linked with a generator to produce electricity. Such systems, called ocean thermal energy conversion (OTEC) systems, require enormous heat exchangers and other hardware in the ocean to produce electricity in the MW range. Solar energy is radiant energy produced in the sun as a result of nuclear fusion reactions. It is transmitted to the Earth through space by electromagnetic radiation in quanta of energy called photons, which interact with the Earth s atmosphere and surface. The strength of solar radiation at the outer edge of the Earth s atmosphere when the Earth is taken to be at its average distance from the sun is called the solar constant, the mean value of which is 1.37KW/m 2. The intensity is not constant, however; it appears to vary by about 0.2 percent in 30 years. The intensity of energy actually available at the Earth s surface is less than the solar constant because of absorption and scattering of radiant energy as photons interact with the atmosphere. The strength of the solar energy available at any point on the Earth depends, in a complicated but predicable way, on the day of the year, the time of the day, and the latitude of the collection point. Furthermore, the amount of solar energy that can be collected depends on the orientation of the collecting object. 2.9 Solar Energy Storage As solar radiation is intermittent, the storage of solar energy is necessary in order to realize a sustainable solar contribution to total energy needs. Solar energy or the product of solar processes can be stored as electrical, chemical mechanical or thermal energy. Solar electromagnetic energy can be converted by photovoltaic or other processes into electricity, which can then be stored in capacitors or

36 36 inductors, or more continently, in electric batteries as chemical energy. Solar generated electrical energy may also be used to electrolyze water, and the end product, hydrogen, is stored as fuel for future use. Furthermore, it is possible to store solar energy in terms of hydraulic potential energy. Solar energy may also be stored as the bond energy of chemical compounds, and energy can be repeatedly stored in and released by the same materials through reversible chemical reactions. Kanzawa and Arai (1981) and Ragaini (1982), gave possible reactions suitable for thermo chemical energy storage. They include the following Ca (OH) 2 CaO + H 2 O (9) N 2 O 4 2NO 2 (10) Photochemical reactions such as 2NOCL + photons 2 NO + CL 2 (11) are other possibilities for solar energy storage (Hsieh, 1986). However, some of these possible means of solar energy storage need further research and development to make them technically and economically feasible. At the present time, the most common practice is the storage of solar- converted thermal energy in the form of sensible or latent heat of a liquid of solid medium. The choice of storage media depends to a large extent on the nature of the solar thermal process. The storage capacity required to ensure reasonable continuous operations of a solar system depends, among other factors, on the availability of solar radiation, the nature of the thermal process, and the economic assessment of solar versus auxiliary energy supplies. Meanwhile, the space and other requirements for a given storage capacity depend mostly on the physical and chemical properties of the storage medium employed.

37 Storage in Sensible Heat Materials Specific heat depends on the mass, or weight of material and its temperature. The amount of storage at any time is directly related to the storage temperature. In order to calculate the amount of usable energy in the storage mass, it is necessary to determine the lowest feasible operating temperature for storage. Amount of heat stored = specific heat x weight x t (12) Where t = difference between storage temperature and the lowest feasible temperature. With the specific heat storage method, there are two common types of storage media or materials, namely: water storage and pebble bed storage. The table 1 below lists the specific heats on both mass and volume basis for some common sensible heat storage materials. For air heating systems, sensible heat storage in pebble beds is often the best choice. Table 1: Properties of Sensible Heat Storage Materials Materials Specific heat, KJ/Kg.K Density, Kg/m 3 volumetric specific heat, KJ/m 3. K Thermal conductivity, W/m o C Adobe Aluminum Brick Concrete Fibre glass Batt insulation polyurethane board insulation Rock pebbles

38 38 Steel Stone (granite) water wood Source (Hsieh, 1986, Fisk and Anderson, 1982) PEBBLE BED HEAT STORAGE Pebble beds that are well designed have several characteristics that are desirable for solar energy applications. This includes low cost in addition to very good heat transfer between air and the solids of the bed. This tends to minimize temperature difference from air to solids on heating the bed and solids to air on cooling the bed. Insulation is usually minimal. Particle sizes should be uniform to obtain large void fraction and thus minimize pressure drop (Duffie and Beckman, 1974). Also, if the pebbles are too big, only the exterior of individual rocks may be heated and thus lowering the effectiveness of temperature stratification in the bed. Sizing of the pebble is by passage through a screen. The pebbles that pass through a 4cm screen but not through a 2cm screen are suitable. Uniform, spherical rock pebbles size (river rock) of 0.35m is selected. Norton et al. (1977) recommended a size of 3.5cm-5cm. Air flow direction in the pebble bed should be vertical. Vertical airflow generally results in better performance than horizontal flow (Fisk and Anderson, 1982; Kreider and Kreith, 1982). Heating of the pebble bed is usually by passing preheated air through the bed or exposure to the sun to be charged statically. The former has the advantage of uniform heating while the latter heats only the rocks directly in contact with incoming radiation. If the pre-heated air method is used, the efficiency of the storage bed is given as:

39 39 Energy stored is pebbles Storage efficiency x 100 (13) Energy stored by air It is also generally recommended that approximately 0.25m 3 (range ) of m pebbles be provided for every square metre of optimized airtype collector area of range m 2 (Beckman et al., 1977) and (Kohler, 1978) WATER HEAT STORAGE Water heat storage, like the pebbles can be achieved by heating stored water either by passing heated air through the water tanks, allowing water to pass through the collector, that is, water being the heat transfer medium, or heated statically directly by solar radiation. The method by which water extracts the heat from the collector and is stored it is the most common. In this method, the heat stored in the water is given by: Q s = mc p t (14) Where Q s = Amount of heat stored M = Weight C p = Specific heat t = Temperature difference of water Water has the advantage of weighing less than pebble, but pebbles have no danger of corrosion, leaking or freezing problems (Michels, 1979) PHASE-CHANGE HEAT STORAGE Phase-change heat storage by fusion is feasible for many solar applications. It is well known that heat is required to melt a solid substance and heat is given up when a fluid solidifies. Heat released by any quantity of solidifying material is

40 40 nearly as great as that required to raise its temperature from room temperature to boiling point. There are substances with considerably higher melting point which, after being melted and solidified again, release latent heat of fusion at temperatures well suited for space heating. To use the heat of fusion for thermal storage, the trick is to find compounds or mixtures which satisfy the following conditions: a) The phase transition must occur at a temperature compatible with the heating or cooling requirement. b) The process must be reversible over a large number of cycles without serious performance degradation, and c) The material must be inexpensive and can be used safely. If these conditions are fulfilled, phase change heat storage becomes a space saver. A few salt hydrates (salts bonded to water molecules) pass the desired qualities to serve as phase-change heat storage media. Table 2 below gives the properties of several salt hydrates that are suitable for solar energy storage. The cheapest salt is sodium sulphate decahydrate, commonly known as Glauber s salt. It is widely available in salt lakes. The transition point between the decahydrate and the anhydrous salt is approximately 32 o C, making it suitable for space heating storage needs. The decomposition process follows the relation: Na SO. 10 H O Energy Na SO ( solid ) 10 H O ( liquid ) (15) with decomposition energy of about 250KJ/kg. The total stored energy will depend on the temperature range over which the salt is heated to account for the sensible heat effects of the salt crystal and the solution (Hsieh, 1986). Energy is released when sodium sulphate (Na 2 SO 4 ) crystallizes from water to form sodium sulphate decahydrate (Na 2 SO 4.10H 2 O). Usually, the salt is filled into standard five gallon drums, which are hermitically sealed so that on heating, no gas or odour can be produced. During

41 41 hours of heat collection the salt within the drum melts thereby absorbing fusion heat, and on solidifying, the latent heat is returned to the surrounding air and conveyed by the circulation to where the heat is needed. Table2: Properties of Selected Salt Hydrates Name and Chemical symbol Sodium Sulphate decahydrate Na 2 SO 4.10H 2 O Disodium phosphate dodecahydrate Na 2 HPO 4.12H 2 O Sodium Thiosulphate pentahydrate Na 2 S 2 O 3.5H 2 O Applicable phase 32 o c 36 o c 50 o c change temperature or melting point. Heat of fusion 251kJ/kg 265kJ/kg 209kJ/kJ Volumetric heat of fusion 373,000kJ/m 3 402,000kJ/m 3 346,000kJ/m 3 Source (Hsieh, 1986) Salt have the advantage of storing 8.9 times the amount of heat the water would and 26 times the amount of heat in the same volume of rocks. However, there are problems encountered when salt hydrates. The major problems are super cooling and phase separation. Super cooling is a phenomenon when a material in the liquid state is cooled below its freezing point without crystallization. Another problem associated with use of salts is that after a long cycle, the salts can change and become chemicals with undesired characteristics. Also, there is the problem of finding suitable container for the salt in some cases plastic container becomes brittle from heat.

42 42 Energy Stored, Ea per unit mass of salt is given by Ea = C a (T m -T 1 ) + h a1 + C 1 (T 2 -T m ) (16) Where C a = Specific heat of solid phase, kj/kg. C 1 = specific heat of liquid phase, kj/kg.k H a1 = Latent heat of phase transition, kj/kg T m = Melting point, o C T 1 = Initial temperature of the solid, o C T 2 = Final temperature of the liquid, o C Chemical Storage Electrical energy generated by solar cells can be stored chemically in batteries. It is electrolysis of sulphuric acid (H 2 SO 4 ) using lead plates as electrolytes. This is accomplished by applying a 2-3 volt charging potential to the plates, using the power supply from solar cells. The anode (+) plate will oxidize to lead oxide and at the cathode (-) will remain lead with hydrogen gas, H 2 being formed. By reversing the charging direction many times, the roughening up of both electrodes can increase the surface to perhaps 100 times the geometric area of the plate and hence increase the energy storage capability of the cells (Norton et al, 1977). The chemical reaction for the operating battery is as follows. 2Pb SO 4 + H 2 O Ch arging + electrode - electrode discharging PbO 2 + 2H 2 SO 4 + Pb (17)

43 43 During charging, the reactions are as follows: At cathode Pb 2+ (aq) + 2 e- Pb(s) (18) SO 2-4 into solution At anode Pb 2+ (aq) + 2H 2 O(l) 2e - PbO 2 (s) + 4H + (aq) (19) During charging, the cathode grid acquires a filling of spongy lead and the anode grid one of lead (iv) oxide. Passage of ions into solution in the proportion of 2SO 2-4 to 4H + increases the concentration and density of the acid. At full charge, the emf is a little above 2 volts and the acid density cm -3. During discharging, the cell yields electrical energy by the following changes: At cathode Pb(s) Pb 2+ (aq) + 2 e- (20) 2- From solution, SO 4 PbSO 4 deposits At anode PbO 2 (s) + 4H + (aq) + 2e - Pb 2+ (aq) + 2H 2 O(l) (21) 2- From solution, SO 4 PbSO 4 deposits. During discharging, the reactions are such that the cathode becomes the anodic electrode in that it gives up electrons but in this case to the external circuit. These electrons perform the electrical work required and are absorbed at the anode. Absorption of ions (4H + : 2SO 2-4 ) from the electrolyte decreases the concentration and density of the acid. The emf fall to 2 volts soon after discharge begins and stays constant until it is almost complete, then falling to 1.8 volts. At

44 44 this point recharging is required. Chemical storage or the storage of energy by chemical reaction is perhaps the best potential method of heat storage. It is the only true long-term storage method because the energy is stored as long as the chemical components remain separated. All other heat storage methods depend on temperature, which means they are subject to heat losses. No insulation is necessary for chemical storage. Most chemical reactions used for chemical storage require very high temperatures, which would have to be provided by high efficiency concentrating solar collectors. Regulation of the pressure within the reaction vessel is sometimes very important. Some of the chemicals used in these reactions are very costly and some are toxic. Until these and other problems are overcome, chemical storage of heat will remain largely experimental (Fisk and Anderson, 1982). Some of the energy storing reactions under study are reported by Fisk and Anderson (1982), and they include: a) The distillation of water from sulphuric acid (H 2 SO 4 ) and the recombination of the concentrated acid and water (Huxtable and Poole, 1976). b) High-temperature dehydration of magnesium hydroxide (Mg(OH) 2 ) to magnesium oxide (MgO) and calcium hydroxide (Ca(OH) 2 ) to calcium oxide, lime (CaO), and their rehydration (Bauerle, 1976; Elvin, 1975) and c) The decomposition of metal hydrides (MH) to the metal plus-hydrogen gas on heating and their recombination (Gruen and Sheft, 1975)

45 Solar Energy Applications in Crop Drying Using the sun to dry crop and grain is one of the oldest and most widely used applications of solar energy. The simplest and least expensive technique is to allow crops to dry naturally on the field, or to spread grain and fruit out in the sun after harvesting. The disadvantage of these methods is that the crops and grain are subject to damage by birds, rodents, wind, and rain, and contamination by wind blown dust and dirt. More sophisticated solar dryers protect the product, reduce losses, dry faster and more uniformly, and produce a better quality product than open-air methods. The basic components of a solar dryer are an enclosure or shed, screened drying trays or racks, and a solar collector. In hot, arid climate the collector may not even be necessary. The southern side of the enclosure itself can be glazed to allow sunlight to dry the material. The collector can be as simple as a glazed box with a dark coloured interior to absorb the solar energy that heats the air. The air heated in the solar collector moves, either by natural convection or forced by a fan, up through the material being dried. The size of the collector and rate of airflow depends on the amount of material being dried, the moisture content material, the humidity in the air and the average amount of solar radiation available during the drying season Types of Solar Dryers Solar drying research has been going on in Nigeria mostly in universities, research institutes and polytechnics. Ofi (1982) constructed and evaluated a solar dryer using a flat plate collector. Although he did not dry any crop with it, he obtained a plenum chamber temperature that was about 50 o C above the ambient under maximum sunshine conditions. Awachie (1982) used a solar hot box to dry fish, coconut, and maize. He obtained air temperature range of o C in the chamber during the wet seasons

46 46 and above 50 o C during the dry seasons. He reported that 0.74Kg of coconut lost 0.143Kg of moisture within 24hours and that uncut fresh fish were fully dried in less than 72 hours, with about 50% of the moisture lost within the first 24hours. Arinze (1985) has successfully designed a commercial solar dryer for maize and other cereals that will be suitable for the northern part of Nigeria. Before 1980, some experimental solar dryers were tested at the universities of Nigeria, Nsukka and Ife. In 1980, the design and evaluation was reported for a solar dryer with a reflector/concentrator type of collector in which shelled maize and sliced plantain were dried (Igbeka, 1980). This dryer was also used to dry other crops. Later, a flatplate collector was used to heat air for drying maize on cob and sliced Okro (Igbeka, 1982) and this dryer was modified as a solar dryer/storage system by Araonye (1984) with very satisfactory results. Solar dryers fall into two broad categories, active and passive solar dryers Active Solar Dryer Active solar dryers require an external means, like fans or, pumps, for moving the solar energy in the form of heated air from the collector areas to the drying beds. These dryers can be built in almost any size. Either air or liquid collectors can be used to collect the sun s energy. The collector should face due south if you are in the northern hemisphere or due north if you are in the southern hemisphere. At or near the equator, they should also be adjusted east or west in the morning and afternoon, respectively. The collectors should be positioned at an appropriate angle to optimize solar energy collection. However, since it is more difficult to move air long distances, it is best to position the collectors as near the dryer as possible. The solar energy collected can be delivered as heat immediately to the dryer air stream, or it can be stored for later use. Storage systems are helpful in areas where the percentage of sunshine is low and a guaranteed energy source is

47 47 required or in carrying out round-the-clock drying. In active dryer, the solar heated air flows through the solar drying chamber in such a manner as to contact as much surface area of the product as possible Passive Solar Dryer Passive dryers can be further divided into direct and indirect models. A direct (passive) dryer is one in which the product is directly exposed to the sun s rays. In an indirect dryer, the sun s rays do not strike the product to be dried. Passive solar dryers use natural means (radiation and convection) to heat and move the air. The direct dryer consists of drying chamber that is covered by transparent cover made of glass or plastic. The drying chamber is a shallow, insulated box with holes in it to allow air to enter and leave the box. The product is placed on a perforated tray that allows the air to flow through it and the product. Solar radiation passes through the transparent cover and is converted to low-grade heat when it strikes an opaque wall. This low-grade heat is then trapped inside the box in what is known as the green house effect. Simply stated, the short wavelength solar radiation can penetrate the transparent cover. Once converted to low-grades heat, the energy radiates on a longwavelength that cannot pass back through the cover. The bottom and sides of the dryer should be insulated. Blackening the inside of the box will improve the dryer efficiency, but toxic material and lead-based paints should not be used. An indirect dryer is one in which the sun s rays do not strike the product to be dried. In this system, drying is achieved indirectly by using an air collector that channels hot air into a separate drying chamber. Within the chamber, the product is placed on mesh trays so that air flows through them. The solar collector can be of any size and should be tilted toward the sun to optimize collection. By increasing the collector size, more heat energy can be added to the air to improve overall efficiency. Larger collector areas are helpful in places with little solar energy, cool

48 48 or cold climates, and humid regions. Tilting the collectors is more effective than placing them horizontally, for two reasons. First, more solar energy can be collected when the collector surface is more nearly perpendicular to the sun s rays. Secondly, by tilting the collectors, the warmer, less dense air rises naturally into the drying chamber. The drying chamber should be placed on support legs, but it should not be raised so high above the ground that it becomes difficult to work with. The base of he collector should be vented to allow the entrance of air to be heated for drying. The vents should be evenly spaced across the full width of the base of the collector to prevent localized areas within the collector from overheating. The vents should also be adjusted so that the air flow can be matched with operating conditions and / or needs. Solar radiation, ambient air temperature, humidity level, drying chamber temperature, and moisture level of the product being dried must all be considered when regulating the air flow. The collector should be connected to the drying chamber through the bottom for easy flow of heated air. As the warm air flow through several layers of the product on trays, it becomes moister. This moist air is vented out through a chimney. This increases the amount of air flowing through the dryer by speeding up the flow of the exhaust air Advantages of Solar Energy Applications In Crop Drying Solar dryers have the principal advantage of using solar energy which is a free, available, and limitless energy source that is also non polluting. Drying most products in sunny areas should not be a problem. A solar dryer improves upon the traditional open-air system in the following ways: 1. It saves time. Products can be dried in a shorter time. Solar dryers enhance drying times in two ways. First, the translucent or transparent glazing over the collection area traps heat inside the dryer, raising the

49 49 temperature of the air. The capability of enlarging the solar collection area allows for the concentration of the sun s energy. 2. It is more efficient. Since products can be dried more quickly, less will be lost to spoilage immediately after harvest. This is especially true of produce that requires immediate drying. 3. Since the products are dried in a controlled environment, they are, less likely to be contaminated by pests, insect, marauding animals, and also can be stored with less likelihood of the growth of toxic fungi. 4. It is healthier. Drying foods at optimum temperatures and in a shorter amount of time enables them to retain more of their nutritional value especially vitamin C. An extra bonus is that foods will look and taste better which enhances their marketability. 5. It is cheaper. Using solar energy instead of conventional fuels to dry products, or using a cheap supplementary supply of solar heat in reducing conventional fuel demand, can result in a significant cost savings. Solar drying lowers the costs of drying, improves the quality of products, and reduces losses due to spoilage. Also in conventionally fueled dryers, which are the primary alternative to solar dryers, a fuel is burned to heat the drying air and in some cases, the gaseous products of combustion are mixed with the air to achieve the desired temperature. Although these drying systems are used around the world with no apparent problems, there is the possibility of a mechanical malfunction which might allow too much gas into the drying stream. If this occurs, the food in the dryer can become contaminated. Use of wood may contribute to problems of deforestation; coal may cause pollution and fossil fuels are becoming increasingly expensive and are not always available.

50 Limitations of Solar Energy Applications in Crop Drying Solar dryers do have short comings. They are subjects to daily, seasonal variations and other climatologically factors. Drying cannot be continuous throughout the day. Drying depends on the rate of solar radiation, time of the day, season and also the humidity of the environment. They are of little use during cloudy weather. They require heat storage systems that can store solar energy so that drying can continue during periods of low sun radiation. Also solar dryers have problem of low airflow through the dryers, because air is circulated by natural convection and electricity or other energy forms to increase airflow are not available in most rural areas.

51 51 CHAPTER 3 MATERIALS AND METHODS The design of the solar dryer was done and followed by the fabrication of the dryer using the following materials: mild steel, angle iron, aluminum absorber plate, plain transparent plastic cover, perforated trays, hinges, aluminum frame, metal handle, bolts and nuts, enamel black paint, pebbles and suction fan, powered with electricity. The machines / tools used in the fabrication work include: The welding, grinding/ cutting and drilling machines, vice, handfile, spanner, handsaw, hand drill, plier and other handtools. After the fabrication work, testing of the solar dryer was carried out considering the following parameters: Temperature at different locations in the collector and drying chamber, ambient temperatures, relative humidity, air flow rates, moisture contents of the products in their chip forms (about 3mm thick), actual capacity of the dryer, the drying rate, effect of the suction fan on the drying rate. Omega RS- 232 Data Logging thermometer was used for temperature measurements. The initial and final moisture content of the root crops in their chip forms were determined by oven method. Dry / Wet bulb hydrometer was used for measurement of relative humidity values. Solar radiation data on the test dates was obtained from the Meteorological unit of National Root Crops Research Institute (NRCRI), Umudike using Gunn- Bellani Pyranometer. Timer was used to obtain the drying period for each of the products. The performance of the dryer was evaluated with respect to its capacity, drying rate, and quality of the dried products using the following root crop products: cassava, yam, sweet potato, ginger, cocoyam and turmeric obtained from NRCRI research plot. The products were prepared in chip forms by using the chipping machine obtained from NRCRI food processing factory. The quantity of product the drying trays

52 52 can accommodate in thin layer spread was studied and this helps to determine the capacity of the dryer. Weight of moisture removed from the products against the drying period was used to determine the drying rate. Statistical analysis (ANOVA) in RCBD was also used to analyse the effects of the different drying methods / crop types on the drying rates. The quality of the dried chips was examined for colour and occurrence of mould by storing them for the period of 3 months. Some of the products like yam and tumeric need to be blanched for about 5 minutes before chipping to enhance drying and also quality of the dried products. The moisture content (%) wet basis of the samples was obtained. This was calculated using the procedure details of Henderson et al, MC cb = W i - W d x 100 (22) W i Where MC wb = moisture content wt basis, % W i = Initial weight of sample, kg W d = Dried weight of sample, kg The efficiency of the collector, was calculated from equation given by Hottel- Whiller as presented by Agbo, S.N. (2005). = Q u (23) A c I T Where Q u = Collector useful energy gain, W A c = Area of the collector, = 1.52m 2 I T = Solar radiation, W/m 2

53 53 CHAPTER 4 DESIGN AND DESCRIPTION OF ACTIVE SOLAR DRYER 4. DESIGN CONSIDERATIONS The main design consideration is the determination of the solar collector s area that will provide the desired rise in temperature of about 40 o C. Also the slope of 7 o which is an approximation of latitude of 6 o 52 N at Nsukka location was used. A covered plate collector consisting of glass transparent cover and aluminum absorber sheet (painted black) was chosen to maintain high incident radiation on the collector surface. Aluminum sheet was considered because of its high specific heat capacity of 0.896KJ/kg o K. The drying chamber of dimension 0.95m x 0.75m x 0.70m was chosen to accommodate three trays for spreading of products. Also, the dryer was covered with transparent material to allow direct solar radiation on the products. Entrance to the drying chamber was provided for easy loading and off loading of products. The dryer can be used to dry any agricultural product with initial moisture content range of about 80%w.b. The shape of the products was considered since it influences how fast it dries. The products should be prepared in chip forms of about 2 5mm thick. Also, the material used for the drying trays has openings to allow free air movement through the mass. The thermal properties of the root / tuber crop products used were also considered. These products have unique physicochemical properties mostly because of their amylose and amylopectin ratio due to the size of their starch granules (Sievert, et al, 1995). 4.1 DESIGN ASSUMPTIONS It was assumed that the products to be dried should be prepared in chip forms of about 2 5mm thick, in order to permit drying air to pass through easily. The ambient temperature was expected not to below 20 o C. Also, the ambient relative humidity should not be above 100%. It was also assumed that agricultural products

54 54 with initial moisture content of less than 90% w.b to be used. The loading density of the product should be within the range of 4 8kg/m 2 in thin layer spread of not more than 8mm thickness. Also, blanched or unblanched products can be used. 4.2 Description of the Solar Dryer The solar dryer consists of three main essential units, namely: The solar collector, Air outlet unit, Drying chamber and heat storage unit The Solar Collector The solar collector absorbs radiation from the sun and transfer the heat to the drying chamber. The solar collector as shown in fig.2 below is a flat plate collector of dimension 1.60m x 0.95m x 0.10m with glass transparent cover. The absorber of the collector was made of aluminum sheet that was painted black. The base and sides of the collector are insulated with glass fibre and cover made of mild steel sheet. A provision for air entrance into the collector was in front with the dimension 0.05m x 0.60m, and these air inlets can be closed during periods of low sun radiation. The collector was connected the floor of the drying chamber through the centre. The collector has slope of 7 o to the horizontal The Drying Chamber The drying chamber is the section where drying of product takes place. It has the dimension of 0.95m x 0.75m x 0.70m, and is covered with a transparent material to allow direct solar radiation on the products. It consists of 3 detachable perforated metal screens trays for spreading of the products in thin layer. Perforated screens were used to allow free passage of heated air through the drying products easily. The dimension of the perforated tray is 0.95m x 0.75m and each can hold up to 20kg of the product/batch. There is a provision of handling and opening of the chamber from one side to enhance monitoring/removal of the products during and after drying. Heated air from the collector passes through the heat storage unit to the drying chamber. The air outlet unit has the dimension of 0.41m x 0.41m. This unit enclosed the suction fan, having speed range of rpm. The suction fan was used to

55 55 increase the amount of air flowing through the dryer by speeding up the flow of the exhaust air. To obtain the air flow rate of the suction fan, the formular given below was used (William and Morey, 2002). Air flow rate m 3 /s = Area of air outlet opening x Where n is the fan speed in rpm. 2 x N 60 Area of the air outlet opening = 0.41m x 0.41m = m 2 Airflow rate of the suction fan = (24) Area of the air outlet opening x 2 60 x N 2 x = x x = m 3 /s Airflow rate = m 3 /s Heat storage unit This unit helps to store heat so that drying can continue during periods of low sun radiation. The dimension of the heat storage unit is 0.85m x 0.65 x 0.35m. Pebbles of average diameter of 0.035m in size were used. The pebble volume was calculated to be m 3. The sides and base of this unit are insulated with glass fibre to minimize heat loss from it. In the design, air flow is vertical with air ducts at the top and bottom. This is selected because vertical airflow generally results in better performance than horizontal flow (Fisk and Anderson 1982; Kreider and Kreith, 1982). They also recommend a pressure drop greater than 25pa (0.15in of water) but less than 75pa (0.30 in of water, and an airflow of 0.028m 3 to 0.085m 3 /min per square metre of collector.

56 56 To calculate the amount of heat stored in the pebble bed, the equation given below was used. Heat stored Q p = mcp t (25) Where m = weight of pebbles Cp = specific heat of pebble t = difference between storage temperature and the lowest feasible temperature. The specific heat and density of pebble according to Hsieh, 1986; Fisk and Anderson, 1982 are 0.88Kj/Kg o K and 1600kg/m 3 respectively. The dimensions of the pebble bed were 0.85m x0.65m x 0.35m, given a volume of m 3. Total weight of pebbles is kg. The amount of heat stored in the pebble bed is then calculated from equation (25) as Q p = mcp t Q p = 0.88 x x 25 = KJ Amount of heat stored by the pebble bed is therefore KJ

57 Fig.2: Design of Active Solar Dryer 57

58 Design Calculations Determination of the Collector Area To determine the collector area required for the expected rise in temperature of 40 o C, the total incident radiation on the tilted collector surface, Ht should be calculated first. The values of H t, H B and H BN were calculated from Hsieh (1986). H t = H R (26) Where H = the total radiation on the horizontal surface, R = the total radiation tilt factor. H = H B + H d (27) Where H B is the beam incident on a horizontal surface and H d, is the diffuse radiation. H B = H BN sinα (28) Where H BN = beam radiation at normal incidence a = the altitude angle. H BN = Ae -Bsinα (29) From Appendix 3 (Hsieh,1986), the values of A, B and C for the month of march are W/m2, and respectively. Where A = total radiation value

59 59 B and C = dimensionless constant To calculate the altitude angle, α α = arc sin (sin L sin δ + cos L cos δ cos h) (30) L, is the latitude value of 7 o and h, is the hour angle δ, is the solar declination. The solar declination δ, for any day can be calculated approximately by the equation Where δ = sin 360 (284 + n) (31) 365 n = day of the year. Hour angle, h = + 1/4 (number of minutes from local solar noon) (32) Where the + sign applies to afternoon hours and sign to morning hours. Also, diffused radiation H d = CH BN Fa (33) Where Fa, is the angle factor which has the value of 1.0 for horizontal surfaces and 0.5 for vertical surfaces. To calculate for the total radiation tilt factor, R R = H B R B + H d (1+cos S) + g (1- cos S) (34) H H 2 2 Where R B is the beam radiation tilt factor S = The surface tilt angle (7 o )

60 60 R B = cos (L-S) cos δ cos h + sin (L-S) sinδ Cos L cosδ cosh + sin L sinδ (35) g = 0.2 assuming ground reflectivity of 0.2 (Liu and Jordan, 1977). The total radiation on the tilted surface, H t is then calculated as: H t = H R Collector Heat Gain The area of collector, Ac is given by the Hottel Whiller equation (Agbo, S.N et al, 2005) as: Ac = Qu (36) Ht where Qu is the collector useful energy gain is average efficiency of flat plate collector and Ht is the total incident radiation on the tilted collector surface. The collector useful energy gain, Qu =mcp t (37) Where m, is the mass flow rate of air and Cp, is the specific heat capacity of air at 300 o K and t is the desired rise in temperature. According to Akani (1990), the mass flow rate of air per unit collector area is Kg/s. Also the value of Cp at 300 o K from Appendix (Hsieh, 1986) is KJ/Kg o.the average efficiency, of flat- plate collectors at Nsukka ranges between 73% to 81% (Ezeike, 1986), considering = 80% In the design, the collector internal dimensions are 0.95m width and 1.6m length, giving the area of 1.52m 2. This area is greater than the calculated area of 1.406m 2. It also agrees with the recommendation of Sayigh (1979) that the maximium area of a collector should be 2m 2.

61 61 CHAPTER FIVE 5.0 PERFORMANCE EVALUATION The performance of the solar dryer was evaluated by considering the capacity, drying rate, airflow rate, collector heat gain / efficiency and quality of the dried products. The following parameters were used to calculate the actual capacity of the dryer: Drying tray dimension = m 2 Number of trays = 3 Each tray can accommodate 5kg of product Full capacity = 5x3 = 15kg Analysis of variance (ANOVA) in RCBD was used to determine the effect of different drying methods and crop types on the drying rate in both seasons as presented from tables The drying rate, D r was calculated as: D r (kg/hr) = M w (38) t Where M w = Amount of moisture removed, kg t = Time, hr The airflow rate of the dryer was calculated using equation (24). Also, the collector heat gain was calculated from equation (37), while the efficiency was calculated using equation (23), and presented from table 5 13 and fig. 6 and 7. For quality assessment of the dried chips. The chips were examined for colour change and occurrence of mould by storing them for the period of 3 months. The chips dried with the solar dryer were of better quality than those dried with open sun drying method because they retained their colour and also free from mould after storage for period of 3 months.

62 EXPERIMENTAL PROCEDURES A test was carried out with the solar dryer on the 24 th, 25 th, 26 th, 27 th, 29 th and 30 th of March, Temperature measurements were taken at the drying chamber, heat storage unit and the collector area, while the relative humidity measurements were taken at the drying chamber for eleven hour period, starting from 7am to 5pm. Omega RS- 232 data logging thermometer was used for measurement of temperature both in the collector area, heat storage unit and the drying chamber. It was also used in measuring the ambient temperature. Dry and wet bulb hydrometer was used in measuring the ambient relative humidity and relative humidity of the drying chamber. While the solar radiation data on the test dates was obtained from the meteorological unit of National Root Crops Research Institute (NRCRI) Umudike. The data was collected using the Gunn- Bellani pyranometer. Test was conducted using the following root and tuber crops: yam, cassava, cocoyam, sweet potato, ginger and tumeric in their chip forms of about 3mm thick. The products were prepared in chip forms with the chipping machine obtained from NRCRI food processing unit. Moisture contents of the root and tuber crops were determined by oven method. The initial moisture content of the products was within the range of 65 78% w.b, and the final moisture level of 11 12% w.b. The root and tuber crops used were all obtained from NRCRI, Umudike. The testing was done with controls, solar dryer without suction fan and solar dryer with suction fan at three rates of air flow (22.89, and 27.29m 3 /s). Experimental test with control and solar dryer without suction fan were conducted on the 24 th to 26 th March, 2010, while test using solar dryer with suction at three different flow rates were conducted from 27 th to 30 th March, The repeated trial with control, solar dryer without suction fan and solar dryer with suction fan was conducted from 22 nd to 24 th June, Test with control involves traditional sun drying of the six samples in order to make provision for comparisons.

63 63 Fig.3: Omega RS- 232 Data Logging Thermometer 5.2 WEATHER CONDITIONS ON THE TEST DATES Hourly Solar Radiation The solar radiation data on the test dates was obtained from the meteorological unit of National Root Crops Research Institute (NRCRI) Umudike. The data was collected using Gunn- Bellani pyranometer. The total values for the test in March were 26.79, 27.06, 27.59, 28.17, and ml on the 24 th, 25 th, 26 th, 27 th, 29 th, and 30 th March, 2010 respectively. According to Okeke and Awachie (1988), 1 ml = 1.357MJ/m 2 day, therefore the solar radiation on the test dates

64 64 were calculated to be 36.35, 36.72, 37.44, 38.23, and MJ/m 2 respectively. Also, the total values for the repeated test in June were 23.09, and ml on 22 nd, 23 rd and 24 th June, 2010 respectively Temperature The maximum and minimum ambient temperature during the period of the test on 24 th, 25 th, 26 th, 27 th, 29 th and 30 th March 2010 are 25.2 o C and 36.7 o C, while that of the repeated test in June are 22.4 o C and 31.2 o C respectively. The maximum and minimum temperature of the collector and the drying chamber were (76 o C, 30 o C), (59 o C, 30 o C) and (62 o C, 26 o C), (49 o C, 25 o C) for the two seasons in March and June respectively Rainfall and Wind speed There was no rain during the period of the test in March, It was sunny and bright throughout, and the prevailing wind speed during the time of test averaged 1.06 m/s. For the repeated trial in June, 2010, the rainfall data were 2.7mm, 0.8mm and 33.9mm on 22 nd, 23 rd and 24 th June respectively.

65 ANALYSIS AND PRESENTATION OF RESULTS Analysis of variance (ANOVA) in RCBD was used to determine the effect of drying methods and crop types on the drying rates in both seasons. The data were presented in tables 28 and 30 while the ANOVA tables were presented in tables 29 and 31. Table 28: Data obtained for the drying rates (kg/hr) during the dry season test Drying Method Cassava Yam Sweet potato Cocoyam Ginger Turmeric Total Open Sun drying Solar Dryer Solar Dryer+ Suction fan Total Table 29: ANOVA Table for the drying rates during the dry season test Source S.S D.F M.S F - Cal Drying Methods ** Crop Products n.s Error Total ** = Highly significant at ( P<0.01) n.s = Non significant effect

66 66 The result of the ANOVA table above shows: -That drying methods used have highly significant effect on drying rates of the root / tuber crops at 1% level (P<0.01). -That the different root / tuber crop products used have no significant difference on their drying rates at 5% level (P<0.05) Table 30: Data obtained for the drying rates (kg/hr) during the wet season test Drying Methods Cassava Yam Sweet potato Total Open Sun Drying Solar Dryer Solar Dryer + Suction fan Total Table 31: ANOVA table for the drying rates during the wet season test Source S.S D.F M.S F - Cal Drying Methods Crop Products ** n.s Error Total ** = Highly significant at (P<0.01) n.s = Non significant effect

67 67 The result of the ANOVA table 31 shows: -That the drying methods have highly significant effect on the drying rates of the root / tuber crop products at 1% level (P<0.01). - That the different root / tuber crop products used have no significant difference on their drying rates at 5% level (P<0.05) Solar Collector Temperature variation in the collector for 11- hour period starting from 7am to 7pm on the test dates in March, 2010 is shown in table 3. The temperature in the collector ranged from minimium of 30 o C at 7am to a maximium of 76 o C at 1pm. The collector heat gain and efficiency from 24 th 30 th March were computed as shown in table 5 to 10. Figures 4 and 6 also show the graphs of maximium temperature of collector, and efficiency versus time respectively. The hourly total radiation values in ml for the test dates in March as obtained from meteorological unit of National Root Crops Research Institute (NRCRI) Umudike was shown in table 14. Also table 15 shows the hourly total radiation in MJ/m 2. The corresponding hourly total radiation values for the repeated test in June 2010 were presented in table 16 and 17 for values in ml and W/m 2 respectively. Also, fig. 8 and 9 show the hourly total radiation versus time for the test dates.

68 Collector/ Average Amb. Temp.(oC) Collector/ Average Amb. Temp.(oC) th 25th 26th 27th 29th 30th ACT AAT 7am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig. 4: Collector / Average Ambient Temperature versus Solar Time for Test in March, nd 23rd 24th ACT AAT 0 7am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig.5: Collector / Average Ambient Temperature versus Solar Time for Test in June, 2010

69 Efficiency of Collector (%) Efficiency of Collector (%) th 25th 26th 27th 29th 30th am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig.6: Efficiency of Collector versus Solar Time for March, nd 23rd 24th 7am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig. 7: Efficiency of Collector versus Solar Time for June, 2010

70 Hourly Total Radiation (W/m2) Hourly Total Radiation (W/m2) th 25th 26th 27th 29th 30th 0 7am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig. 8: Hourly Total Radiation versus Solar Time for March, nd 23rd 24th 0 7am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig.9: Hourly Total Radiation versus Solar Time for June, DRYING CHAMBER The temperature variation in the solar dryer starting from 7am to 5pm on the test dates in March, 2010 was shown in Table 18 and Fig. 10. The temperature in the dryer ranged from a minimium of 30 o C at 7am to a maximium of 59 o C at 2pm during the drying season period. For the rainy season period represented in Table 19

71 Temp. of Drying Chamb./Average Amb. Temp.(oC) 71 and Fig.11, the temperature ranged from a minimum of 25 o C to maximum of 48.6 o C. Fig. 10 and 11 show comparisons of the temperature of the drying chamber with the average ambient temperature values which were represented as (AATemp). The average relative humidity of the drying chamber as shown in Table 20 and Fig. 12 indicate minimum value of 28% at 3pm and maximum value of 90% at 7am for the test in March, But for the test in June, 2010 the average relative humidity of the drying chamber indicates minimum value of 52% at 2pm and maximum value of 92% at 7am as shown in table 21 and Fig. 13. Fig. 12 and 13 also show comparisons of the relative humidity of the drying chamber with the average ambient relative humidity values which were represented as (AARHum). It was also observed in Table 20, Fig. 12 and 13 that the three days (27 th -30 th ) March 2010 and also on 24 th June, 2010 during which the suction fan was used had lower values of relative humidity than the other days when the solar dryer was used alone am 8am 9am 10am 11am Solar Time (hr) 12noon 1pm 2pm 3pm 4pm 5pm 24th 25th 26th 27th 29th 30th AATemp Fig.10: Temperature of Drying Chamber / Average Ambient Temperature Versus Solar Time for March, 2010

72 Rel. Humidity of Drying Chamb./ Average Amb. Rel. Humidity (%) Temp. of Drying Chamb./ Average Amb. Temp.(oC) am 8am 9am 10am 11am Solar Time (hr) 12noon 1pm 2pm 3pm 4pm 5pm 22nd 23th 24th AATemp Fig.11: Temperature of the Drying Chamber / Average Ambient Temperature Versus Solar Time for June, th 25th 26th 50 27th th 30th AARHum 0 7am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig.12: Relative Humidity of the Drying Chamber / Average Ambient Relative Humidity versus Solar Time for March, 2010

73 Rel. Humidity of Drying Chamb./ Average Amb. Rel. Humidity(%) nd 23rd 24th AARHum am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig.13: Relative Humidity of the Drying Chamber / Average Ambient Relative Humidity versus Solar Time for June, OPEN SUN DRYING (CONTROL) As shown in table 22, for test with traditional sun drying method, it took maximium of 50 hours and minimium of 42 hours to dry the root / tuber crops: cassava, yam, sweet potato, cocoyam, ginger and turmeric to moisture content of 12% for the test in March, Also table 23 indicates that for the test in June, 2010, it took maximium of 58 hours and minimium of 52 hours to dry cassava, yam, sweet potato, cocoyam, ginger and turmeric. Table 22 and Fig. 14 also show that for test in March, using the solar dryer without the suction fan, the drying period was between (24 29) hours. This indicates that using solar dryer with suction fan at the three different flow rates gave the minimium drying period of between (8 11) hours. Results of the test in June, 2010 also indicate that the minimum drying period of (16 18) hours was obtained using the solar dryer and the suction fan at the suction rate of 27.29m 3 /s. while using the solar dryer without suction gave period range of (40 46) hours as shown in table 23 and Fig. 15. The corresponding range of ambient

74 74 temperature as shown in Table 24 was 25.2 o C to 36.7 o C. For the test in June, 2010 the ambient temperature range was between 22.4 o C to 31.2 o C as shown in Table 25. While the prevailing ambient relative humidity for the test dates in March, 2010 was a minimium of 41% at 3pm and maximium of 96% at 7pm as shown in Table 26, and also represented in Fig.18, when the solar drying relative humidity ranged from 90% to 26% as represented in Table 20 and Fig.12. For the test in June, 2010 the ambient relative humidity had minimium value of 71% at 4pm and maximium value of 97% at 7am as represented in Table 27 and Fig. 19, when the solar drying chamber relative humidity ranged between 92 to 50% as represented in Table 21 and Fig. 13. Bar chart representation of the drying periods against the methods: open air drying, solar dryer without suction fan and solar dryer with suction fan at three different flow rates 22.89, and 27.29m 3 /s was represented in Fig.14 as SD+SF1, SD+SF2 and SD+SF3 respectively. The results indicate that the three test dates (27 th 30 th ) March, 2010 as represented in Table 20 and also on the 24 th June, 2010 as shown in Table 21 during which the suction fan was used had lower relative humidity values within the drying chamber. Fig. 20 and 21 show the picture of the active solar dryer. Fig. 23 and 24 show the products dried with the solar dryer.

75 Drying Period (hr) Drying Period (hr) Control SD SD+SF1 SD+SF2 SD+SF3 0 Cassava Yam Sweet Pt Cocoyam Ginger Turmeric Drying Methods Fig.14: Bar Chart representation of Drying period versus Method Used for March, Control SD SD + SF Cassava Yam Sweet Potato Drying Methods Fig.15: Bar Chart representation of Drying period versus Methods used

76 Ambient Temperature oc Ambient Temperature oc th 25th 26th 27th 29th 30th 0 7am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig.16: Ambient Temperature versus Solar Time for March, nd 23rd 24th 0 7am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig.17: Ambient Temperature versus Solar Time for June, 2010

77 Ambient Relative Humidity (%) Average Relative Humidity (%) th 25th 26th 27th 29th 30th 0 7am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig.18: Ambient Relative Humidity versus Solar Time for March, nd 23rd 24th am 8am 9am 10am 11am 12noon 1pm 2pm 3pm 4pm 5pm Solar Time (hr) Fig.19: Ambient Relative Humidity versus Solar Time for June, 2010

78 Fig.20: The Active Solar Dryer (Side view) 78

79 Fig.21: Active Solar Dryer (Front view) 79

80 Fig.22: Testing of the Active Solar Dryer 80

81 81 Fig.23: Products dried with the Solar Dryer (Sweet Potato, Cocoyam and Turmeric) Fig.24: Products dried with the Solar Dryer (Yam, Cassava and Ginger)

82 82 Fig.25: Products dried with Open Air: Ginger, Yam and Cocoyam) Fig.26: Products dried with Open Air: Sweet Potato, Cassava and Turmeric