CHAPTER 1

IMPORTANCE OF SOLAR ENERGY

1.1. INTRODUCTION:

In today's climate of growing energy needs and increasing environmental concern, alternatives to the use of non-renewable and polluting fossil fuels have to be investigated. One such alternative is solar energy. Solar energy is the name of energy which has been made using the power of the sun. It is given out by the sun in the form of heat and light and it fuels the solar system. Without heat and light this planet would not be as we know it. There would be no trees or plants as they need light to grow and therefore not enough oxygen for man to have evolved in the way that we have. The sun energy is very powerful and lately we have been trying to find ways of better using that energy. It has been known for years that it is worth using the suns energy and houses have been built to face the sun in order to aid with their heating and lighting, long before electricity was invented. Now technology has advanced, but with the environmental damage that fossil fuels are producing, solar power is one of the ways that people are trying to use a renewable resource.

There is likely to be an increase in demand for energy produced using renewable resources as consumer demand for it grows and governments are pressured into reducing their country's environmental impact. The demand has already increased a great deal and it is likely to continue to grow as people become more concerned for the environment. This is a great way of taking advantage of a free and natural resource and using it instead of something which is expensive, getting low in supply and polluting the atmosphere.

The sun is a large sphere of very hot gases, the heat being generated by various kinds of fusion reaction. Its diameter is 1.39 ×106 km, while that of the earth is 1.27×104 km. The mean distance between the two is 1.496×108 km. Although the sun is large, it subtends an angle of only 32 minutes at the earth’s surface. Thus, the beam radiation received from the sun on the earth is almost parallel. The brightness of the sun varies from its centre to its edge. [1]

1.2. SOLAR RADIATION:

Solar radiation is the radiant energy emitted by the Sun in the form of electromagnetic waves. The sun emits vast amount of radiant energy. The earth intercepts only a fraction of it. It is essential to drive directly or indirectly all biological and physical processes on the Earth. The earth is the only planet in the solar system, which receives an optimum amount of solar radiation that makes life sustainable on it. Solar spectrum resembles to that of a black body at approximately 5800K. About 98% of the total emitted energy lies in the spectrum ranges from 250nm to 3000nm. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum. Solar radiation having wavelength less than 0.286nm (called

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ultraviolet) is absorbed by ozone layer in stratosphere. The ultraviolet radiation not absorbed by the atmosphere is responsible for the change of colour in skin pigments. The solar radiation, that traverses the atmosphere further, is subjected to scattering, reflection and absorption by air molecules, aerosols and clouds. [2]

The radiation from the Sun travels in the space as electromagnetic wave. Above the earth’s atmosphere, sunlight carries 1367 watts of power per square meter. This is known as solar constant. We define solar constant as the amount of solar radiation received outside the earth’s atmosphere on a unit area perpendicular to the rays of the sun, at the mean distance of the earth from the sun. [2]

The Earth receives 1.8 x 1017 W of incoming solar radiation continuously at top of its atmosphere. But only half of it reaches the earths’ surface. Factors like absorption, scattering and reflection of light during its passage through the atmosphere are responsible for reduction of the amount of solar radiation available on the earth’s surface .It is particularly suitable in our country with dearth of economic resources. Our country has abundant sunlight throughout the year and electricity is not available in rural areas and solar energy can solve the problem. Figure 1.1 shows the solar radiation over India.

Fig 1.1: solar radiation in India [2]

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1.3. ADVANTAGES:

a) Availability -

About half the incoming solar energy reaches the Earth's surface. The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses.

b) Green Energy -

Solar energy is one of the cleanest energy resources in our planet, so when we use solar energy, we will also help our environment, the most effective solution for the current series environmental problems likes the Global warming.

c) Reduces reliance On Fossil Fuels.

d) In case of areas with no or unreliable connection to the grid solar panels can be used as: 1. Power supply for telecommunication.

2. Power supply for agricultural equipment like pumps.

3. Power supply for individual house-holds.

e) Contributes to energy security of region.

f) Minimizes the need for expensive and inefficient transmission and delivery systems.

1.4. APPLICATIONS:

1. Water heating

2. Space heating

3. Power generation

4. Space cooling and refrigeration

5. Distillation

6. Drying

7. Cooking

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CHAPTER 2

LITERATURE REVIEW

2.1. RESEARCH PAPER REVIEW:

V.K.Sharma et al [3] have worked on mathematical modelling and experimental evaluation of a natural convection type solar cabinet dryer. This paper gives the brief view of design methodology, performance studies and analytical solution of a cabinet type solar dryer. The analytical solution is based on the principle of simultaneous heat and mass-transfer.

H. P.Garg et al [4] have done the design and performance studies of a solar dryer suitable for rural applications. This research paper is aimed at investigating means for food preservation using processes suitable for implementation in rural areas, where energy resources are scarce. Bearing in mind the low cost of capital investment and utilization possibilities, mainly agricultural drying, they have fabricated different types of solar dryers. Air flow in the drying system is by natural convection. The performance of the various types of solar dryers, along with a preliminary heat transfer analysis, is presented.

O.V. Ekechukwu et al [5] have given the review of solar-energy drying systems II: an overview of solar drying technology. In this paper review of the various designs, details of construction and operational principles of the wide variety of practically-realised designs of solar-energy drying systems has been given. A systematic approach for the classification of solar-energy dryers has been evolved. Two generic groups of solar-energy dryers can be identified, which are passive or natural-circulation solar-energy dryers and active or forced-convection solar-energy dryers. Three sub-groups of these can also be identified, such as integral-type (direct mode), distributed-type (indirect mode) and the mixed-mode type. The appropriateness of each design type for application by rural farmers in developing countries is discussed.

A. Saleh et al [6] have discussed modelling and experimental studies on a domestic solar dryer. This paper gives the brief view of a domestic solar dryer with transparent external surfaces. This paper aims to propose a solar dryer with a uniform temperature profile that meets the requirements of the exponential model over a wide range of cases, thus, providing a simple and accurate design tool. The performance was tested under different operational conditions and the drying characteristics were experimentally investigated by conducting the experiments on two local herbs, Jew’s mallow and mint leaves.

P. Gbaha et al [7] have done experimental investigation of a solar dryer with natural convective heat flow. In this paper a direct type natural convection solar dryer was designed. It was constructed with local materials (wood, blades of glass, metals) and then different food products

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were tested. The study relates mainly kinetics and establishment of drying heat balances. The influence of significant parameters governing heat and mass transfers, such as solar incident radiation, drying air mass flow and effectiveness, is analysed in order to evaluate its thermal performances.

2.2. SOLAR FOOD DRYER:

1. Introduction:

Preservation of fruits and vegetables is essential for keeping them for a long time without deterioration in the quantity of the product. Various methods such as canning, refrigeration; chemical treatments, controlled atmospheres, dehydration and the use of sub atomic particles are available for their preservation. Dehydration is the modest approach in which the major energy source is solar energy which is abundant in India. As an economically viable energy device a solar energy seems to be the most promising way of preservation of various agricultural products, such as food grain, fruits, vegetables, medical plants etc.[4]

Although for commercial production of raisins (or any other agricultural product), the forced convection solar dryer provides better control of required drying air conditions, the natural convection solar dryer does not require any other energy during operation. Hence, the natural convection solar dryer may become a more suitable proposition for the rural sector and other areas in which electricity is scarce and in irregular supply. Various designs for small scale solar dryers have been proposed for their applications in developing countries in the past and still a good deal of work is going on in this direction. Still, solar drying units are not readily available, commercially, and those that are available are not functioning properly. The non-uniform temperature distribution, no control over drying temperature, non-availability of technical know-how, etc. are some of the major problems associated with the drying phenomenon. In a controlled cabinet type drying process, the external parameters that affect the drying process are carefully controlled to provide the desired drying rate, compatible with the attainment of the desired product quality. [3]

Solar food drying can be used in most areas but how quickly the food dries is affected by many variables, especially the amount of sunlight and relative humidity. Typical drying times in solar dryers range from 1 to 3 days depending on sun, air movement, humidity and the type of food to be dried.

2. Why solar drying?

Drying is necessary because, while a high moisture content of agricultural product is desirable during harvesting to minimize losses, the moisture content must be low during storage to prevent the product from being spoiled. High moisture level and warm temperatures promote mould growth, insect growth and increase the respiration rate of the product. The products with high moisture percentage at the time of storage are liable to attack by fungal growth and toxic materials which are harmful to human beings. The length of time for which any agricultural product can be stored varies with the moisture content and the type of product. Under the

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circumstances, the reduction of moisture content of the product is essential to improve its market value, or to store it for use at a later date. With the reduction of moisture content, growth of yeasts, mould and bacteria is inhibited and the product can be stored for relatively long periods of time without deterioration.

Thus, drying of agricultural product permits:

(1) Early harvest

(2) Planning of the early harvest season

(3) Long-term storage without deterioration

(4) Taking advantage of a higher price a few month after harvest

(5) Maintenance of the viability of seed

(6) Selling a better quality product

3. Systematic classification of drying systems [5]

All drying systems can be classified primarily according to their operating temperature ranges into two main groups of high temperature dryers and low temperature dryers. However, dryers are more commonly classified broadly according to their heating sources into fossil fuel dryers (more commonly known as conventional dryers) and solar-energy dryers. Strictly, all practically-realised designs of high temperature dryers are fossil fuel powered, while the low temperature dryers are either fossil fuel or solar-energy based systems.

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Fig 2.1: Classification of dryers and drying modes. [5]

3.1. High temperature dryers:

High temperature dryers are necessary when very fast drying is desired. They are usually employed when the products require a short exposure to the drying air. Their operating temperatures are such that, if the drying air remains in contact with the product until equilibrium moisture content is reached, serious over drying will occur. Thus, the products are only dried to the required moisture contents and later cooled. High temperature dryers are usually classified into batch dryers and continuous-flow dryers. In batch dryers, the products are dried in a bin and subsequently moved to storage. Thus, they are usually known as batch-in-bin dryers. Continuous-flow dryers are heated columns through which the product flows under gravity and is exposed to heated air while descending Because of the temperature ranges prevalent in high temperature dryers, most known designs are electricity or fossil-fuel powered. Only a very few practically-realised designs of high temperature drying systems are solar-energy heated.

3.2. Low temperature dryers:

In low temperature drying systems, the moisture content of the product is usually brought in equilibrium with the drying air by constant ventilation. Thus, they do tolerate intermittent or variable heat input. Low temperature drying enables crops to be dried in bulk and is most suited also for long term storage systems. Thus, they are usually known as bulk or storage dryers. Their ability to accommodate intermittent heat input makes low temperature drying most appropriate

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for solar-energy applications. Thus, some conventional dryers and most practically-realised designs of solar-energy dryers are of the low temperature type.[5]

2.2.3.3. Classification of solar-energy drying systems:

Fig.2.1 illustrates a systematic classification of drying systems, indicating the sub-classes and the group lineage of solar drying systems. Solar-energy drying systems are classified primarily according to their heating modes and the manner in which the solar heat is utilised.

In broad terms, they can be classified into two major groups, namely

I. Passive solar-energy drying systems:

Conventionally termed natural circulation solar drying system.

II. Active solar-energy drying systems:

Most types of which are often termed hybrid solar dryers.

Three distinct sub-classes of either the active or passive solar drying systems can be identified (which vary mainly in the design arrangement of system components and the mode of utilisation of the solar heat, namely

integral-type solar dryers

distributed-type solar dryers

Mixed-mode solar dryers

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Fig 2.2: Typical solar energy dryer design [5]

I. Passive solar drying systems [5]

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a) Open-to-sun drying:

There are two traditional approaches by which passive solar crop drying is undertaken in tropical countries, namely:

The plant bearing the crop is allowed to die, either in contact with the soil or is cut down but not removed, thus the crop is dried ``in situ''.

The crop is spread on the ground, mat, cemented floor or placed on either horizontal or vertical shelves exposed to solar radiation and to natural air currents. The crop is usually stirred occasionally in order to expose different parts of it to the sun and thereby encourage more rapid removal of the saturated air.

Despite the rudimentary nature of the processes involved, such techniques still remain in common use. Because the power requirements (i.e. from the solar radiation and the air's enthalpy) are readily available in the ambient environment, and as little or no capital cost is required and running costs low (often labour only), these are frequently the only commercially viable methods in which to dry agricultural produce in developing countries. Though utilised widely, natural open-to-sun drying techniques have inherent limitations: high crop losses ensue from inadequate drying, fungal and insect infestation, birds and rodent encroachment and weathering effects. The process is intermittent, being affected by cloudiness and unexpected rain. Output is low and can be of very poor quality. For tropical climates, sun drying poses serious practical problems during the wet season, as periodically but irregularly, the crop has to be removed to storage or protected from rain. The quality of the dried product is often degraded seriously, sometimes beyond edibility. Thus, at present, a large proportion of the world's supply of dried fruits and vegetables continue to be ``sun dried'' in the open under primitive conditions. Whilst more efficient solar drying methods are being developed, the traditional drying methods do have the following positive attributes:

Small capital cost.

Low running cost.

Independence from fuel supplies.

b) Natural-circulation solar-energy crop dryer:

Natural-circulation solar-energy dryers depend for their operation entirely on solar-energy. In such systems, solar-heated air is circulated through the crop by buoyancy forces or as a result of wind pressure, acting either singly or in combination. These dryers are often called ``passive'' in order to distinguish them from systems that employ fans to convey the air through the crop. The latter are termed ``active'' solar dryers. Natural-circulation solar-energy dryers appear the most attractive option for use in remote rural locations. They are superior operationally and competitive economically to natural open-to-sun drying. The advantages of

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natural-circulation solar-energy tropical dryers that enable them to compete economically with traditional drying techniques are:

They require a smaller area of land in order to dry similar quantities of crop that would have been dried traditionally over large land areas in the open.

They yield a relatively high quantity and quality of dry crops because fungi, insects and rodents are unlikely to infest the crop during drying.

The drying period is shortened compared with open air drying, thus attaining higher rates of product throughput.

Protection is afforded the crop from sudden down pours of rain.

Commercial viability, i.e. their relatively low capital and maintenance costs because of the use of readily available indigenous labour and materials for construction.

Three generic types of natural-circulation solar-energy dryers have evolved and both retain many of the advantages of traditional open-to-sun drying.

These are:

1. integral-type natural-circulation solar-energy dryers

2. distributed-type natural-circulation solar-energy dryers

3. Mixed-mode natural-circulation solar-energy dryers

1. Integral-type natural-circulation solar-energy dryers:

Integral-type natural-circulation solar-energy dryers (often termed direct solar dryers), the crop is placed in a drying chamber with transparent walls that allow the insulation necessary for the drying process to be transmitted. Thus, solar radiation impinges directly on the product. The heat extracts the moisture from the crop and concomitantly lowers the relative humidity of the resident air, thereby increasing its moisture carrying capability. In addition, it expands the air in the chamber, generating its circulation and the subsequent removal of moisture along with the warm air. The features of a typical integral passive solar dryer are illustrated in Fig 2.3

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Fig 2.3: Features of a typical integral-type (direct) natural-circulation solar-energy dryer [5]

Direct exposure to sunlight enhances the proper colour ripening of greenish fruits by allowing, during dehydration, the decomposition of the residual chlorophyll in the tissue .For certain varieties of grapes and dates, exposure to sunlight is considered essential for the development of the required colour in the dried product, and for Arabica coffee, a period of exposure to sunlight is considered inviolable for the development of full flavour in the roasted bean.

Passive solar cabinet dryer:

These are usually relatively small units used to preserve ``household'' quantities of fruit, vegetables, fish and meat. They are usually single or double-glazed insulated hot boxes with holes at the base and upper parts of the cabinet's walls. The solar radiation necessary for the drying process is transmitted through the cover and is absorbed on blackened interior surfaces as well as on the product. Air circulation is provided by the warm moist air leaving via the upper apertures under the action of buoyancy forces while replenishing fresh air is drawn from the base. The dryer consists of a container, insulated at both its base and sides and covered with a double-layered transparent roof. Drying temperatures in excess of about 80 0C were reported for the dryer.

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Fig 2.4: A typical natural-circulation solar-energy cabinet dryer [5]

2. Distributed-type natural-circulation solar-energy dryers:

These are often termed indirect passive solar dryers. Here, the crop is located in trays or Shelves inside an opaque drying chamber and heated by circulating air, warmed during its flow through a low pressure drop thermos phonic solar collector. Because solar radiation is not incident directly on the crop, caramelization and localised heat damage do not occur These dryers are also recommended generally for some perishables and fruits for which their vitamin content are reduced considerably by direct exposure to sunlight and for colour retention in some highly pigmented commodities that are also very adversely affected by direct exposure to the sun. Distributed passive solar dryers have higher operating temperatures than direct dryers or sun drying and can produce higher quality products. Thus, they are recommended for relatively deep layer drying. Their shortcomings, however, are the fluctuations in temperatures of the air leaving the air heaters, thereby making it difficult to maintain constant operating conditions within the drying chamber, and the operational difficulties of loading and unloading the trays and occasional stirring of the product.

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Fig 2.5: Features of a distributed-type (indirect) natural-circulation solar-energy dryer [5]

Distributed-type dryers, though, have an inherent tendency towards greater efficiency, as the component units can be designed for optimal efficiency of their respective functions. They are, however, relatively elaborate structures requiring more capital investment in equipment and incur larger running (i.e. maintenance) costs than the integral units. A typical distributed natural-circulation solar-energy dryer would be comprised of the following basic units:

An air-heating solar-energy collector;

Appropriately insulated ducting;

A drying chamber; and

A chimney.

3. Mixed-mode natural-circulation solar-energy dryer:

In these dryers a solar air heater with a drying bin but without a drying bin is used. A typical dryer consists of an air heater, drying chamber and a tall chimney (used to increase convection effect). This type of dryer is mainly used to dry paddy and similar products which entail slow and low temperature drying.

The ground is covered with rice husk which absorbs solar radiation and heats the air in contact. The hot air rises to the drying chamber which may absorb heat directly (PVC sheets) or may be covered (with bamboo). The material to be dried is kept in nylon net tray the hot air enters

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through its bottom and rises to the chimney. Chimney is covered with black PVC to keep the air warm. There is a cap at the top of the chimney which allows humid air to escape. Height of the chimney and hot air inside creates a pressure differential which causes air flow from bed to top of the chimney. Drying rate depends on depth of the bed, initial moisture content, ambient temperature in the bed etc. It is observed that the product in the bottom layer gets overheated while the top layer is under dried, hence stirring is necessary.

Fig 2.6: Mixed-mode natural-circulation solar-energy dryer [5]

II. Active solar drying systems [5]

Active solar drying systems depend only partly on solar-energy. They employ solar-energy and/or electrical or fossil-fuel based heating systems and motorised fans and/or pumps for air circulation. All active solar dryers are, thus, by their application, forced-convection dryers. A typical active solar dryer depends solely on solar-energy as the heat source but employs motorised fans and/or pumps for forced circulation of the drying air. Other major applications of active solar dryers are in large-scale commercial drying operations in which air heating solar-energy collectors supplement conventional fossil-fuel fired dehydrators, thus reducing the overall conventional energy consumption, while maintaining control of the drying conditions.

A variety of active solar-energy dryers exist which could be classified into either the integral-type, distributed-type or mixed-mode dryers.

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1. Integral-type active solar-energy drying system:

These are solar drying designs in which the solar-energy collection unit is an integral part of the entire system, thus, no special ducting to channel the drying air to a separate drying chamber is required. Three distinct designs of integral-type active solar dryers can be identified.

Direct absorption dryer:

In this design of active solar dryers, the product absorbs solar radiation directly, thus no separate solar collectors are required. Practically-realised designs include large-scale commercial forced-convection greenhouse dryers, illustrated in figure 2.7

Fig 2.7: Typical active solar-energy cabinet dryer [5]

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2. Distributed-type active solar-energy drying system:

A distributed-type active solar dryer is one in which the solar collector and drying chamber are separate units. A typical design would be comprised of four basic components,

Namely,

The drying chamber

The solar air heater, fan and/or pump

The ducting.

For conventional drying systems, drying efficiencies increase with temperature, thus encouraging drying at temperatures as high as the product can withstand. However, for distributed-type active solar dryers, the maximum allowable temperature may not yield an optimal dryer design, as the efficiencies of solar collectors decrease with higher outlet temperatures. Thus, a critical decision in the design of distributed active solar dryers would be either to choose high drying air temperatures and, consequently, accommodating lower air-flow rates (implying the use of smaller fans and requiring high levels of insulated ducting) or to employ low temperature drying, thus minimising the cost of insulation, since heat losses are low.

Fig 2.8: Interior-plastic-absorber greenhouse active solar dryer [5]

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However, the efficiency of high-temperature distributed active solar dryers is significantly improved by high air-flow rates, thus a balance has to be made between the size of fans used and the level of insulation for a cost effective design.

3. Mixed-mode active solar-energy dryer:

These are rather uncommon designs of active solar dryers. Mixed-mode designs combine some features of the integral and distributed-types. Typical designs would comprise the following components: a solar air heater, air ducting, a separate drying chamber and a fan and/or pump as in a distributed-type dryer. However, the drying chamber is glazed so that the product absorbs solar radiation directly as in direct absorption integral designs. Features of an active mixed-mode solar dryer are illustrated in Fig 2.9.

Fig 2.9: A continuous-flow active grain dryer [5]

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2.3. COMPARISON OF OPEN-SUN DRYING AND CABINET DRYING:

Drying is a traditional method used to preserve food for later use. The age old practice of drying used in India is spreading foods in open sun known as open sun drying. In this technique, the food is spread in thin layers on a hard platform and the product is turned once or twice a day.

This natural sun drying is simple and economic but suffers from many drawbacks such as:

There is no control over drying rate, crop may be over dried resulting in discoloration, loss of germination power, nutritional changes and sometimes complete damage.

Drying is non-uniform.

In case of slow drying, there may be deterioration of food due to fungi and bacteria.

The rain and dust storm can destroy the crop since in open drying, there is no protection.

There may be considerable damage due to birds, rodents, insects, etc.

Whereas there are several advantages of controlled drying such as

Best Product quality.

Better nutrient retention.

Reduce wastage, time, and space.

Better returns due to improved quality and improved transportability.

2.4. ADVANTAGE OF DRIED FOOD:

a) Dried foods are in more concentrated form.

b) Reduction in moisture content results in reduction in weight & volume hence it increases the ease of packing, handling, storage & transport.

c) Enhanced shelf life of product

d) Gives the product that has characteristics suitable for further processing.

e) Products have greater convenience in use.

f) Earlier dehydrated food products are particularly suitable for defence forces & now are being manufactured for common man’s use.

g) They are less costly than foods preserved by other ways due to low cost of labour.

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2.5. APPLICATIONS:

1. The solar dryers are useful for drying a variety of materials. Both food & non-food materials can be effectively dried without changing basic properties.

2. All types of cereal grains & their products (Flours, Maida, Wafers, vermicelli, Noodles etc.), legumes, green leafy vegetables, root vegetables, other vegetables, fruits & fruit products-bars, toffees, spices & condiments, herbs, flowers, gums, mushrooms, forest produce, meat, shrimps, fish, papads, chemicals etc are well dried in the solar dryer under clean conditions in a reasonably short time. The dryer ensures well-dried product irrespective of the season / climate / location.

3. Solar drying technology enables processing fruits & vegetables under clean & hygienic conditions meeting the international standards for quality.

4. Drying of herbal product for medical purpose.

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CHAPTER 3

CASE STUDY

The moisture content can be expressed on either a wet or dry basis. The amount of moisture in a product is designated on the basis of the weight of water and is usually expressed as a percentage. The moisture content on a wet basis is obtained by dividing the weight of water present in the material by the total weight of the material [4]:

Percentage moisture content (wet basis) ¿100(W −d )/W (1)

where,

W = mass of the wet sample

d = mass of the dry material in the sample.

The percentage moisture on a dry basis is determined by dividing the weight of water by the weight of dry matter. The moisture content on a dry basis is always larger than on the wet basis. A number of different methods, viz. oven methods, distillation methods, drying with desiccants, electrical resistance methods, dielectric methods, chemical methods, hygrometric methods and various other methods, are available for the measurement of moisture content.

In the laboratory, the oven method is convenient. The sample should be left in the oven until weight loss stops. It is practically impossible to remove all the moisture from a sample without deterioration of the product. If the sample is left in the oven too long, the organic materials will be reduced, and a loss of weight occurs which appears as moisture loss. For some air-oven determinations (at 100°C), it is specified that the sample be kept in the oven for a minimum of 72 hr. For accurate moisture content determinations, it is necessary to prevent the sample from absorbing moisture after the moisture has been removed from sample.

3.1 THE ENERGY BALANCE FOR DRYING:

Hygroscopic materials have bound moisture in the form of water trapped in closed capillaries, the water component of juices, or water absorbed by surface forces as well as unbound water held within the material by the surface tension of the water itself, The removal of water from a surface requires an amount of heat equal to the latent heat of vaporization of water, plus a current of air moving past the surface to carry away the water vapour produced, We have

mv . hfg=(T¿¿ f −T i) .ma .C p¿ (2)

where

mv= weight of water evaporated,

ma = weight of air circulated,

h fg = latent heat of vaporization

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C p = specific heat of air,

T f , T i = final and initial temperature.

The quantity of water is calculated from the initial and desired final moisture content with the help of the equation

mw=mi(M i−M f) /(100−M f ) (3)

where

mw = initial mass of sample

M i ,M f = initial and final moisture content,

The volume of air can be determined using the gas laws:

V a

mv

=ma

mv

.RTP

(4)

where

mv, the amount of water evaporated,

Can be read from a moisture ratio scale. Because the vapour pressure of bound water in hygroscopic material is less than saturation, the effect of bound water is also to be taken into account. Also, the latent heat value should be chosen to correspond to a very low vapour pressure,

3.2.THE QUANTITY OF AIR NEEDED FOR DRYING :

The quantity of air needed for drying green peas by means of the dehydration process has been estimated, both by using an energy-balance equation as well as by a psychrometric chart. The volumes of air needed for drying a specified quantity, under given conditions, as calculated by both methods, are found to be in satisfactory agreement. The calculation details are as given below.

The quantity of air needed for drying green peas

Use of a psychrometric chart.

Using the psychrometric chart I have calculated the amount of air needed to dry 1 kg of green peas (as a whole) from an initial moisture content of 70% (wet basis) to a final moisture content of 12%. The ambient air has a temperature of 30°C and r.h. = 50%. The final temperature of the air is assumed to be 60°C. The path AB, representing the heating process on the psychrometric chart, shows that the r.h. of the air is reduced to 15%. The path BC represents the change in state of the air as it passes through the drying material. The point D, with temperature 36°C and humidity ratio 0.0240, represents the end of the process, since here, the r.h. of 65% is in equilibrium with peas having the moisture content 11.2% On average, therefore, the humidity

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ratio of the drying air can be expected to rise by about 0.0105. The amount of water to be extracted from 1 kg of green peas, calculated from equation (3), is 0.662 kg.

The mass of air needed = 0.662/0.0105 = 63.06 kg (by definition of the humidity ratio). Using the expression Pv = m, RT, I have the required volume of air when P = 101.3 kPa, T = 309 K (36°C) and R = 0.291 kPa m3/kg K, which is about 55.97 m3.

From the energy-balance equation.

The calculation of the volume of air needed for drying from the energy-balance equation (2), in the above example, is as follows. Amount of water to be extracted--0.662 kg, specific latent heat of vaporization =2.8 MJ/kg and specific heat capacity = 1.02 kJ/kg°C; then, by using the same data, we get the mass of air needed: 64.90 kg. The corresponding volume of air, calculated as before, using the expression Pv = maRT, is 57.60 m3. So, in view of the approximate nature of the calculations, these values are in satisfactory agreement.

The area of absorber required

The area required to collect sufficient solar energy to dry a given quantity of green peas, potatoes and grapes can be determined [4], provided we 'know' the mass of water to be evaporated, the specific latent heat of vaporization of the water, the quantity of global solar radiation falling on the unit per day and the efficiency of the drying unit. An attempt has been made to elaborate this concept as below.

The calculation given earlier shows that the amount of water to be extracted = 6.621 kg, for 10 kg of green peas, and hence, the amount of heat required to evaporate the water in the drying process is 18.54 MJ. The daily global solar radiation ranges from 15 to 25 MJ/m2 per day. So 20 MJ/m2 per day may be taken as an average value. Also, the efficiency of the solar unit is assumed to be 40%. The amount of heat provided by the collector per unit area in 3 days is, therefore, estimated to be 24MJ/m2 on average. Since,

QR = amount of heat required to evaporate the water

¿(τα )e I Tt × A

Where

τ=¿ Transitivity = 0.8

α=¿ Absorptivity = 1

I Tt = Daily global solar radiation range = 20 MJ/m2 per day

So A = 0.91m2

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3.3. DRYING PERIODS:

In convection dryers, the heat is transferred from the warm gas to the surface of the drying material by convection. Heat from the surface penetrates into the material. The transfer of liquid inside the material may occur by several mechanisms, such as diffusion in continuous homogeneous solids, capillary flow in granular and porous solids, flow caused by shrinkage and pressure gradients etc. The factors governing the rates of the two processes, heat transfer and mass transfer, determine the drying rate. A drying curve can be obtained by plotting moisture content (% on a dry basis) vs time (h). The drying rate can be obtained by differentiating these curves graphically. The drying rate can then be plotted vs time and moisture content, separately, to determine the various periods of drying. On the other hand, the drying time, t, can also be calculated readily from the following equation [4], for a given final mean moisture content:

L W DM (M i−M f )=Cam(T i−T f ) t

Where

L = latent heat of vaporization,

M i = the initial moisture content of the product (dry basis),

M f = the final moisture content of the product (dry basis),

m = mass flow rate,

Ca= specific heat of drying air,

T i , T f = the initial and final dry bulb temperatures of the drying air,

t = drying time

W DM = weight of the dried product.

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CHAPTER NO 4

REFERENCES

1. S. P. Sukhatme, “SOLAR ENERGY principles of thermal collection and storage”, Tata McGraw-Hill publication, second edition.

2. Solar radiation hand book 2008, solar energy centre, MNRE Indian metrological department.

3. V.K.Sharma, S.Sharma, H.P.Garge, “Mathematical modelling and experimental evolution of a natural convection type solar cabinet dryer”, Indian Institute of Technology, Delhi -110016, Energy convers.Mgmt. Vol.26.No.31, pp 65 -73 – 119, 1991.

4. V.K.Sharma, S.Sharma, R.A.Ray, H.P.Garge, “Design and performance studies of a solar dryer suitable for rural application centre of energy studies”, Indian Institute of Technology, Delhi -110016, Energy convers.Mgmt. Vol.26.No.1, pp 111 – 119, 1986.

5. O.V.Ekechukwu, B.Norton, “Review of solar-energy drying systems II: an overview of solar drying technology”, Energy Conversion & Management 40 (1999) 615 – 655.

6. A. Saleh, I. Badran, “Modelling and experimental studies on a domestic solar dryer”, Philadelphia University, Amman -19392, Jordan, Renewable Energy 34 (2009) 2239–2245.

7. P.Gbaha, H.Yobouet Andoh, J.Kouassi Saraka, B.Kamenan Koua, S.Toure, “Experimental investigation of a solar dryer with natural convective heat flow”, Renewable Energy 32 (2007) 1817–1829.

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