SOLAR DRYING Importance of solar drying • The world population is more than 6 billion and about 800-900 million people do not have enough food to eat. • There are three methods to solve hunger problem: – Increase food production – Reduce population growth – Reduce loss of food during and after harvesting • It has been estimated that world as a whole more than 20-30 percent food grains and 30-50 percent vegetables, fruits/fish etc. are lost before it reaches to the consumers. • Drying is a traditional method for preserving food. Solar drying is an effective method to preserve food. • Solar energy is diffuse in nature and thus suitable for crop drying, locally available and thus saves transportation, solar dryers can be made locally of any size and capacity and solar dryers are economical if cash crops are dried. Advantages in Favour of Crop Drying • It permits early harvesting and reduces the field losses of the products, • Reduces the risk of field losses caused by wild animals, • It permits better planning of harvesting season, • It reduces spoilage in storage drastically, • It permits the farmer to sell his product at better price during early period of harvesting season, • Quality of the product gets enhanced significantly and hence farmer gets more money for his product, and • Transportation is easy with dried product. OPEN SUN DRYING • It is the simplest method of drying used in most developing countries. • The food / crop is spread out in the open under the sun. Advantages (1) No technology involved (2) Very low cost (cost of labour only) Disadvantages (1) Contamination of the product due to dirt and insects. (2) Wastage by birds / mice. (3) Spoilage due to sudden and unpredicted rain. (4) There is no control of temperature over crop drying. (5) Overdrying may cause loss of germination power, nutritional changes, sometimes complete damage. PARAMETERS FOR SOLAR DRYING • The drying of product depends on external variables like temperature, humidity and velocity of air stream and internal variables which is a function of drying material and depends on parameters like surface characteristics (rough or smooth surface), Chemical composition (sugar, starch, etc.), physical structure (porosity, density, etc.). and size and shape of the product. The rate of moisture movement from the product inside to the air outside differ from one product to another and very much depends weather the material is hygroscopic or non-hygroscopic. Non- hygroscopic materials can be dried to zero moisture level while the hygroscopic materials like most of the food products will always have a residual moisture content. • The design of a solar dryer depends on : solar radiation, temperature of air, relative humidity of air, moisture content of the product, amount of product to be dried, time required for drying, availability of auxiliary energy, material of construction of dryer and the resource availability. PHYSICS OF DRYING • Heat by convection and radiation to Surface of product Goes to interior of product • Increase in temperature • Formation of water vapour Evaporation of moisture from Surface Drying can be accelerated by: • Increasing flow rate of air • Increasing temperature of drying air • Initial Drying - Surface drying, later on drying depends on type of materials. • Non hygroscopic- drying possible upto zero moisture content. • Hygroscopic - grains, fruit, food stuff have residual moisture. RATIONALE FOR CONTROLLED DRYING 1. Grain • Improves product quality, • Improves storage capability, • Reduces time and space requirement for drying, • Facilitates quick preparation of fields for next cropping, • Facilitates wet season harvesting and storage, • Improves drying hygene. 2. Timber • Improves product quality, • Reduces period capitoltied up in drying stock, • Improves low expertise, low capital, improved drying options, • Expands range of usable timber species, • Improves attainable drying level. 3. Fruits, Vegetables & Fish • Reduces product seasonability, • Improves marketing control of farmer, • Reduces spoilage, • Improves drying hygene, • Improves storage capability, • Reduces nutritional fluctuations. EARLY WORK ON SOLAR DRYING IN INDIA 1. In NPL, New Delhi in 1954 solar heated air was used for drying of coal fines. 2. In NPL, New Delhi in 1955, plane glass mirror concentrators with overall dimensions of 1.83m x 0.91 m were used for making jaggery from sugar cane and palm juice. 3. Khadi & Village Industries Commission (KVIC), Ahmedabad in 1968, using similar mirrors erected a pilot plant which dehydrates palm nira and turns it into gur or syrup. 4. Forest Research Institute (FRI), Dehradun, Developed a 7.1 cu.m, capacity timber seasoning kiln in 1972. 5. Central Arid Zone Research Institute (CAZRI), Jodhpur developed a solar cabinet dryer in 1972 and tested it for several years for drying fruits and vegetables. 6. Annamalai University, in the year 1978 developed one ton per day solar paddy dryer. 7. National Industrial Development Corporation (NIDC) of India developed several solar grain dryer in 1980 and put to use in few cities in India. REQUIREMENTS FOR SOLAR ENERGY AS AN ALTERNATIVE OF HIGH ENERGY DRYERS • Capacity of solar dryers must be equivalent to fossil fuel based dryer • The labour input for solar dryer operation should not increase • Solar dryer operation must be independent of weather conditions • With solar dryer the quality of dried product should not be lowered • The operating conditions should be reliable • Total drying cost should not increase Estimated Energy Requirements for drying of some crops Crops Initial moisture Final moisture Water removed Energy content (%) wb content (%) wb (kg/t of dried requirement product) (106 kJ/t) GRAIN Wheat, barley, rye, 20-25 14-16 50-147 0.30-0.88 oats, paddy Corn 25-45 12-14 147-600 0.80-3.60 VEGETABLES Peas, beans Potatoes 60-70 5-10 1250-2157 7.50-13.00 Onion, Garlic, 65-85 14 1458-4733 8.75-28.40 Carrot, beets, 70-80 5-10 2000-3750 12.00-22.50 Cabbage, Tamatoes 80-90 5-10 3500-8500 21.00-51.00 90-95 5-10 8000-3300 48.00-108.00 FRUITS Apples, 75-80 14-23 2080-3300 12.48-19.80 Apricots, peaches, prunces, grapes, figs, banana FORAGES Hay 40-60 10-14 433-1250 2.60-7.50 Grass, alfala 80-90 10-14 3300-8000 19.80-48.00 CLASSIFICATION OF SOLAR DRYERS • DIRECT TYPE DRYERS : In direct or natural convection type dryers, the agricultural product is placed in shallow layers in a blackened enclosure with a transparent cover. The solar radiations are directly absorbed by the product itself. The food product is heated up and the moisture from the product evaporates and goes out by the natural convection/circulation. • INDIRECT TYPE DRYERS : In these dryers the food product is placed in a drying chamber. The air is heated in solar air heaters and then blown through the drying chamber. In some of the designs, dryers receive direct solar radiations and also heated air from solar air heaters. In these dryers manipulation of temperature, humidity and drying rate is possible to some extent. • FORCED CIRCULATION TYPE DRYERS : In these dryers, hot air is continuously blown over the food product. The food product itself is loaded or unload continuously or periodically. These kind of dryers are comparatively thermodynamically efficient, faster and can be used for drying large agricultural product. These dryers can be of Cross-flow type, concurrent flow type or counter-flow type. (a) Direct type solar dryers (b) Indirect type solar dryers (c) Forced circulation type solar dryers TYPE OF SOLAR DRYERS Drying terminology • Percent moisture content, dry basis : The mass ratio of water to dry solid multiplied by 100. W d M (dry ) 100% d where, W is the wet mass and d is dry mass of the product. • Percent moisture content, wet basis: The mass fraction of water in the commercially dry solid multiplied by 100. W d M ( wet ) 100% W • Bound moisture : The amount of water in the material which exhibits a vapor pressure less than that of pure liquid at the same temperature. • Unbound moisture: The moisture contained by a material which exerts an equilibrium vapour pressure equal to that of pure liquid at the same temperature. • Equilibrium moisture : The amount of moisture in the material that is in the equilibrium with its vapour in the gas phase. For a given temperature and humidity conditions, the material cannot be dried below its corresponding equilibrium moisture content. • Hydrogroscopic material : A material that may contain bound moisture in small capillaries. • Non-hydroscopic material: The material which cannot hold moisture in the bound form. • Constant rate period: The part of the drying process during which the drying rate is constant and is controlled by external rather than internal conditions. • Falling rate period: The part of the drying during which the drying rate varies with time. Internal factors, i.e., physical and transport properties of the material, control the drying process. • External drying factors: The independent variables associated with the conditions and flow of gas phase. • Internal drying factors: The properties of the material that influence the transport of heat and mass within the phase. • Critical moisture content: The moisture content of a material at the end of constant rate period. The critical moisture content is not a unique moisture property of a material but is influenced by its physical shape as well as the conditions of the drying process. • Batch drying: The type of drying operation in which the material is fed to and discharged from drying chambers in batches at definite intervals of time. • Specific volume: The volume per unit weight of dry air. • Enthalpy: The total energy contents of the substrate. For moist air, it is equal to the heat content of the moist air per unit weight of dry air above a certain reference temperature. Basics of Solar Drying • Drying or dehydration of material means removal of moisture from the interior of the material to the surface and then to remove this moisture from the surface of the drying material. • The drying of product is a complex heat and mass transfer process which depends on external parameters such as temperature, humidity and velocity of the air stream; drying material properties like surface characteristics (rough or smooth surface), chemical composition (sugar, starches, etc) physical structure (porosity. density. etc.); size and shape of the product. • The rate of moisture movement from the product inside to the air outside differs from one product to another and very much depends on whether the material is hygroscopic or non-hygroscopic. Non-hygroscopic material can be dried to zero moisture level while the hygroscopic materials like most of the food products will always have a residual moisture content. This moisture in hygroscopic material may be a bound moisture (remains) in the material due to closed capillaries or due to surface forces) or unbound moisture which remains in the material due to surface tension of water. When the hygroscopic material is exposed to air, it will either absorb moisture or desorb moisture depending on the relative humidity of air. The equilibrium moisture content (EMC) will soon be reached when the vapour pressure of water in the material becomes equal to the partial pressure of water in the surrounding air. The equilibrium moisture content is, therefore, important in the drying since this is the minimum moisture to which the material can be dried under a given set of drying conditions. Basics of Solar Drying (contd.) • A series of drying characteristic curves can be plotted. The best is, if the moisture content M of the material is plotted versus time as shown in Fig. Another curve can be plotted between drying rate i.e. dM/dt versus time t as shown in Fig. But more information can be obtained if a curve is plotted between drying rate dM/dt versus moisture content M as shown in Fig. As is seen from this figure for both hygroscopic and non- hygroscopic materials, there is a constant drying rate terminating at the critical moisture content followed by falling drying rate. The constant drying rate for both non-hygroscopic and hygroscopic materials is the same while the period of falling rate is little different. For non-hygroscopic materials, in the period of falling rate, the drying rate goes on decreasing till the moisture content becomes zero. In the hygroscopic materials, the period of falling rate is similar until the unbound moisture is completely removed. then it further decreases and some bound moisture is removed; this continues till the vapour pressure of material becomes equal to the vapour pressure of drying air. When this equilibrium reaches then the drying rate becomes zero. Moisture in the drying material Rate of moisture loss 22 Drying rate with time curve Typical drying rate curve Basics of Solar Drying (contd.) Eight thermodynamic properties of moist air, generally used in drying are vapor pressure, relative humidity, humidity ratio, dry bulb temperature, dew point temperature, web bulb temperature, enthalpy, and specific volume. These parameters are correlated and the effect of one on another can be seen on psychometric chart. The drying process can be explained with the help of the psychometric chart of Fig. If the air is not saturated (say dry bulb temperature is 30C and wet bulb temperature is 20C) and is allowed to pass over the material and if no external heat is applied, then the sensible heat of air and material is exchanged for latent heat of vaporization of water. The path travelled on psychometric chart will be 20C wet bulb line shown by line AB. During this process the humidity ratio changes from 0.0140 to 0.0104 i.e. about 0.0036 kg of vapour per kg of dry air is absorbed. Now by using solar energy, the air is heated to 45C with a relative humidity of 17 per cent and is passed over the drying material. During the drying process, this air is cooled-adiabatically along the 24 C wet bulb line, then the final humidity ratio will be 0.0189. Thus the moisture evaporated with the. heated air will be 0.0075 kg of vapour per kg of dry air which is almost double the water evaporated compared to when air was not heated. Psychometric chart for pressure 101.35 kPa with drying process indicated Natural Convection or Direct type Solar Dryer • These dryers appear to be more attractive for use in developing countries since these do not use fan or blower to be operated by electrical energy. • These dryers are low in cost and easy to operate. • Some of the problems with these dryers are: slow drying, no control on temperature and humidity, small quantity can be dried, and some products change colour and flavour due to direct exposure to sun. • Several direct type dryers are fabricated, tested, and analysed in many countries. • The simplest direct type solar dryer is solar cabinet dryer. Solar Cabinet Dryer • The solar cabinet dryer in its simple form consists of a wooden (or of any other material) box of certain width and length (length is generally kept as three times its width), insulated at its base and preferably at the sides and covered with a transparent roof. • The inside surfaces of the box are coated with black paint and the product to be dried is kept in the trays made of wire mesh bottom. These trays loaded with product are kept through an openable door provided on the rear side of the drier. • Ventilation holes are made in the bottom through which fresh outside air is sucked automatically. Holes are also provided on the upper sides of the dryer through which moist warm air escapes. • This dryer has given encouraging results and reduced the drying time by one third compared to open sun drying. Details of solar cabinet dryer Photograph of Solar Cabinet Dryer Mixed Mode Type Solar Dryer • In the mixed mode type of solar dryers, the solar air heater with or without any electric fan along with a drying bin is used. • Such simple mixed mode type solar dryer was developed at AIT Bangkok for drying paddy and therefore named as rice dryer. • It consists of a solar air heater made of a frame of bamboo poles and wire covered with 0.15 mm thick transparent PVC sheet. The ground is covered with burnt rice husk which absorbs the solar radiation and heats the air in contact. • The hot air in this air heater rises to the drying chamber which either consists of transparent PVC sheets on bamboo frame absorbing directly the solar radiation or a bamboo frame covered from all the four side with some opaque material. • The drying material (rice etc.) is kept on the nylon net tray in thin layer through which hot air heated from air heaters enters its bottom and goes up into the chimney. Mixed Mode Type Solar Dryer (contd.) • The chimney is a long cylinder made of bamboo frame covered with black PVC to keep the inside air warm. There is a cap at the top of the chimney, leaving some space in between chimney top and cap to allow warm humid air to go out and protecting the product from rain and other foreign materials. • The height of the chimney and the hot air inside it creates a pressure difference between its top and bottom thereby creating forced movement of air through the rice bed to the top of the chimney. • The drying rate will depend on the depth of the bed, initial moisture content of the material, solar insolation, ambient temperature, and the design of the dryer. Cross section of chimney type solar dryer Photograph of Solar Rice Dryer Forced Circulation Type Solar Dryer • As the name implies, such dryers use some kind of one or several electric operated blower/exhaust fan to circulate air between air heater/storage bin/drying chamber. • Such dryers are more efficient, faster, reliable, preferred and can be used for drying large quantities of agricultural products. • These dryters can be used at low as well as at high temperatures and used for drying large quantities of product. • These dryers are of bin type, tunnel type, belt type, column type, or rotary type. • Some forced circulation type solar dryers use some kind of thermal storage unit, heat recovery wheel and auxiliary heating arrangement. • Auxiliary energy may be supplied either by electric heating or oil or gas burners and used only when solar air heaters or the heat from the thermal storage device is not sufficient to supply necessary energy for drying the product. Forced Circulation Type Solar Dryer (contd.) • Several storage systems are proposed but the most preferred one is the rock bed storage system which stores the heat in the form of sensible heat and performs the dual function of storing the heat and that of a heat exchanger. • A hybrid solar dryer (solar assisted) was developed at Fresno, California for drying large amount of fruits and vegetables. • It consists of several solar air heaters with a total area of 1350 m2, a thermal storage (rock bed type) of 350m3 volume, a rotary wheel type heat recovery whell and a tunnel dehydration in which 14 trucks loaded with prepared food move at a rate of 24 hours per truck in one direction and the heated air is sent from the other direction. • The system is designed for a fixed air flow rate of 9.5m3/s to the dehydrator 24 hours a day. The drying temperature varies from 60C in the beginning of June to 66C in August to September. • The solar contribution in this hybrid systems is 1582 MJ/hr which is about 60 per cent of the total heat requirement of drying. Photograph of Forced Circulation Type Solar Dryer Important Conclusions • Experience over the past four decades has shown that inspite of high potential of solar drying it has not taken off. Some of the reasons are; • Systematic work on solar dryer has been done only in few countries. • Solar dryer has not been developed as a system. • In industralized countries, there is great interest towards solar drying. However, neither the temperature nor the heat requirement can be achieved with solar collector. • Solar drying is considered more applicable to low temperature in- storage type drying in tropical and subtropical countries. • Pre-healing of drying air in batch dryers has been demonstrated to be techno-economically viable. • Solar drying should be disseminated for medium and low scale farmers for drying cash crops. • To popularise solar drying, pilot demonstration followed by training and workshop will have to be intensified.