4.10. Improved sun-drying and solar drying: basic considerations and selected applications

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- D. Adair

INTRODUCTION

The principles underlying improved sun-drying and solar drying are explained in simple terms and attention drawn to the basic questions which the decision-maker must ask when the introduction of these technologies is under considerations. Examples of methods of improving sundrying are described and several types of solar driers illustrated. It is suggested that carefully monitored demonstration projects be established as the first step to introduction of this technology into new areas.

Sun-drying has been used universally as a method of preserving agricultural produce, but it is not universally applicable to a common standard of efficiency and reliability. In many countries, therefore, it has been largely superseded by drying processes in which biomass or fossil fuels are used as sources of energy. The biomass fuel option may be regarded as an indirect method of solar drying - biomass concentrates the sun's energy and stores if for use at the processor's convenience, day or night.

In countries where there are serious constraints on fuel supplies, and where climatic conditions are not inherently favourable for sun-drying, developments in food preservation may depend on the introduction or improvements of the traditional sun drying process. The decision-maker must ask three important questions when considering specific proposals in this connection:

1. Has the proposed technology proved efficient and reliable?
2. Is it suitable for the intended users and for the produce available at the places in question?
3. Are its benefits proportionate to its costs?

The first of these questions concerns purely technological matters, and accordingly the decision-maker must be guided by appropriate specialist advice. Caution is necessary in this connection, however, as such advice generally derives from the findings of drying studies, or from established empirical practice elsewhere. These findings are always to some extent -and perhaps to a large extent- site-specific. There must be assurance that the findings have been related correctly to conditions at the proposed new sites of application - particularly to the solar irradiation levels and distribution patterns' and to the prevailing ambient temperatures and relative humidities. The state of the art is such at the present time that assurance of this kind will generally be obtained only by carrying out carefully monitored performance trials at the new site, with provision for adaptive changes of the technology.

THE DRYING PROCESS

The decision-maker will be better able to use specialist advice effectively if he possesses at roast a rudimentary understanding of the principles underlying the technology on offer, and these principles are not difficult to comprehend. They may be summarized as follows:

1. Sun-drying and solar drying would more correctly be called sun/air drying and solar/air drying, Their efficiency is largely determined by their provisions for moving air across the surfaces of the produce and for heating the air.
2. The produce being dried may receive energy by direct absorption of solar radiation, by transfer from the air surrounding it, and by transfer from the surface e on which it lies.
3. The energy received may raise the temperature of the produce as well as causing the evaporation of moisture from its surfaces.
4. For most produce the drying rate in the early stage of drying is determined by the rate of evaporation of moisture from its surfaces; this depends largely on the temperature and humidity of the surrounding air -which are interrelated- and on the speed of air movement. Raising the bulk temperature of the produce may be counter-productive at this stage.
5. In the later stage of drying, the drying rate is generally determined by the rate of movement of moisture from the interior of the produce to its surfaces; this depends largely on the temperature of the produce. Raising the bulk temperature of the produce is helpful at this stage, provided levels harmful to product quality are not attained.
6. Drying rate depends also on the shape and size of the produce's constituent unite and on the depth to which they are packed for example thin slices dry more quickly than thick slices' but for cassava optimum drying rates are attained by chipping rather than by slicing.

It is important to boar in mind that most communities are highly conservative with regard to their foods. A new drying technique, however efficient it may be, will not be accepted if it results in a product significantly different in character to that with which the consumer is familar.

THE STARTING POINT

In the simplest traditional sun-drying process, produce is spread in thin layers on the ground, turned occasionally and covered or moved to a shelter when rain falls. It exposes the produce for extended periods to risks of deterioration caused by dust and dust-borne organisms, and to the depredations of insects, rodents, birds and other animals. For fruit and vegetable produce the simple process may entail a loss of nutritive value e.g. through destruction of vitamin C or provitamin A and may lead to discolouration and the development of off-flavours. Where ambient humidity is high the process may be unreliable, even where solar irradiation levels are high, and extensive mould growth may take place in the produce under these conditions.

IMPROVED SUN-DRYING

Over the centuries, farming communities have recognised the limitations of the simple process and have improved upon it, using me, serial readily available to them. More recently, such improvements have been made the subject of systematic scientific research.

USE OF DRYING FLOORS AND PLATFORMS

Process hygiene is greatly improved if, instead of spreading produce on open ground, a clean firm, smooth surface is employed - such as plastic sheets, cement, concrete, wood or metal. Where land is available for the purpose, specially constructed drying floors are used, or platforms raised above ground level.

The improvement in hygiene may be accompanied by a minor improvement in drying efficiency arising from the fact that the materials used to make the floor or platform absorb solar radiation more efficiently than coos soil, and thus becomes hotter and transfer more energy to the produce. This effect is most evident when metal sheeting -such as the flat roof of a building is used. It partly explains also why many farmers use the surfaces of adjacent roadways as drying floors, despite the obvious disadvantages of this practice.

USE OF BLACKENED SURFACES

Matt black surfaces absorb solar radiation more efficiently than others, and so the improvements not d above can be enhanced by use of such surfaces. It has been demonstrated for example (Thanh 1978) that the time required to dry cassava chips on a concrete floor is reduced by about 15% if the floor is painted black. Where plastic sheeting is used as the surface supporting the bed of produce' black is preferred to transparent for this reason.

USE OF WOVEN MATTING

In many countries, produce is spread on woven matting for sundrying purposes. This practice probably evolved as a protection against contamination and for convenience of handling but it has been shown (Duff 1974) to speed up drying to a small extent by facilitating air movement around the produce.

USE OF MESH TRAYS

Movement of air around produce is further facilitated by drying on mesh trays rather than on solid platforms. This practice is adopted in Australia, where some 100.000 tonnes/annum of dried fruit -mainly grapes- are produced by sun-air drying. It is also used widely in Colombia both for coffee drying and for cassava drying. Work carried out in Colombia under a joint TDRI/CIAT project has led to an improvement in the eatablished tray drying process for cassava chips. The trays used in this are illustrated in Figure 1 (see Figure 1. Cassava Drying Trays (Colombia) - Sources Best (1979)). They are mde of plastic netting (35 holes/cm ) stretched on wooden frames and supported by chicken wire. The trays are mounted on bamboo supports at an inclination clove to the angle of repose for the chips (28 ), facing the direction of the prevailing wind. In practice the actual angle of inclination depends to some extent on the wind speed, and this also determines the quantity of chips which can be placed on the tray for drying without intermittent turning. For light winds (up to 1 m/s) loadings of 10 kg/m are possible; for steady winds (over 2 m/s) this can be increased to 16 kg/m . The time used for drying by this method is approximately the same as that for drying of chips on a blackened concrete surface. For the latter, however, loadings cannot be increased above approximately 6 kg/m . When all costs are taken into account' the tray drying method is found to be less expensive than drying on blackened concrete (Best 1979).

Drying on trays is essentially a wind assist d drying method. Where wind conditions are favourable, appreciable drying occurs overnight for cassava chipped and spread in the late afternoon. This effect does not occur to any significant extent for chips spread overnight on blackened concrete surfaces.

SOLAR DRYING

In sun drying, produce is exposed directly to solar radiation and -more or less effectively- to the wind. In solar drying, the produce is contained in an enclosed space, and the air in contact with it is heated by solar radiation. By heating the air' its humidity is reduced and thus its efficiency as a vehicle for removal of moisture is increased For this reason, solar drying affords a means of preservation particularly suitable for use in those places whore sundrying is unreliable because ambient humidities are too high.

In small driers, matched to the requirements of a single farmer' movement of air across the produce is generally induced by natural convection - the tendency of warm air to rise. In larger units -suitable for communal or co-operative use or for industrial use- air is moved by means of a fan; this is termed "forced convection". The latter units are inherently more efficient, but they require a supplementary source of energy to drive the fan. (It may be mentioned, however that TDRI and the Kenya Industrial Research and Development Institute are currently planning a collaborative study of the performance of driers using fans powered by photovoltaic cells).

In some solar driers the covers, or part of them, are transparent to allow exposure of the produce to screened solar radiation. This is not essential' however' and it must be avoided for those kinds of produce which deteriorate in sustained exposure to sunlight.

SMALL DRIERS

Numerous small driers have been described; these fall into three broad categories:

Cabinet driers
Tent driers
Driers with pre-heating chambers

The first of these is of sturdy construction - essentially a glass or plastic topped box with base and sides constructed from wood, plastered earth, brickwork, or other suitable material. The aides and base can be fabricated to give high insulation, and thus the cabinet dryer is particularly suitable for use whore high temperatures are required in the drying process. By restricting the ventilation of the cabinet, temperatures as high as 80 C have been obtained in work carried out in Barbados by the Brace Institute (Lawand 1966).

The second and third types of drier are generally constructed from plastic sheeting or film -black or transparent- held on a structure comprised of a suitable locally available material e.g. bamboo.

TDRI is not aware of any country in which driers of these three types are in routine use by the farming community. There is a considerable fund of knowledge of their performance, however, gained on a trial basis or through the efforts of enterprising farmers in many countries. Frequently' also, the ideas incorporated in their design have been discovered or adopted by farmers; for example many coffee producers in Colombia use plastic sheeting to protect sun-drying trays from rain in a manner which effectively converts the trays into a solar drier.

Figures 2-4 illustrate designs of drier which have bean used in TDRI work overseas.

Figure 2. Solar Cabinet Drier - Sources Curran and Trim (1982)

Figure 3. Solar Tent - Source: Curran and Trim (1982)

Figure 4. Solar Chimney Drier- Source: Curran and Trim (1982)

SOLAR DRIERS WITH FORCED CONVECTION

As noted above, these driers require motive power to drive a fan. They are suitable for use by groups of farmers under central management and in agro-industries. They offer considerable economies in their costs of construction. Two examples drawn from TDRI experience may be cited in this connection.

PEPPER DRYING IN THE REPUBLIC OF KOREA

Red peppers (Capsicum annuum L) are important in the dietaries of the Republic of Korea and? as the harvest extends only from August to October, the greater part of the crop is preserved by drying. Traditionally whole peppers are sun-dried, a process which can take up to three weeks when weather conditions are unfavourable. Substantial reductions in drying time can be effected by preslicing the peppers, but this leads to loss of pungency. In recent years medium and large scale producers have turned to artificial drying of whole peppers using fossil fuels. The Food Research Institute, Suweon, -with TDRI collaboration funded by FAO- has developed the use of solar drying, with forced convection as an alternative process.

The drier employed has been described in detail by Trim and Ko (1982) and is illustrated in Figure 5 (see Figure 5. Forced Convection Solar Drier - Source: Trim and Ko (1982)). Essentially it consists of: a frame made of bamboo; an outer covering of clear plastic and an inner covering of clear plastic and an inner covering of black plastic, separated by a distance of approximately 10 cm; and a fan/ducting system to draw air from one side of the wall cavity which distributes it within the dryer, and expels it through the roof. Peppers are placed on drying racks constructed from plastic mesh stretched on bamboo frames. The total rack area is 22 m² and a loading of approximately 14 kg/m² of whole peppers is attained.

In operation, temperature variation within the drier is negligible with a maximum of 5°C, when based on average daily temperatures at 6 selected widely-spaced points. Table 1 shows a typical set of figures for hourly variation of average drier temperature; in this instance a maximum rise of 33 C above ambient was attained.

Table 1 - Hourly variation in ambient and solar drier temperatures

Sources: Trim and Ko (1982

Table 2- Average drier temperatures attained on 11 days of varying insolation

* Collector Efficiency = (Energy Collected x 100) / (Solar Energy Received)
Source: Trim and Ko (1982)

Average drier temperatures attained on 11 days of widely varying insolation are shown in Table 2 alongside the corresponding average ambient temperatures.

Comparative trials have been made of solar drying and the traditional sun-drying process. The results of one of these are shown in Figure 6 (see Figure 6 - Comparison of efficiency of solar drying and traditional sun-drying of red pepper). These demonstrate that for whole peppers the solar drying process is more rapid' and capable of yielding a product of lower moisture content' than the sun-drying process. Organoleptic tests and laboratory assessments of colour and capsicin content (an index of pungency) have revealed no significant difference between the products.

This type of drier is widely adaptable for use with a variety of produce.

COFFEE DRYING IN KENYA

Action is currently being taken in Kenya to improve the drying techniques for parchment coffee in smallholder co-operative factories. TDRI is responsible for the design, installation and commissioning of three prototype drying installations based on a solar assisted drying process. The construction of the first of these was completed and commissioned at Rukera in November 1983.

The design of the drier is shown in Figure 7 (see Figure 7 - TDRI Parchment Coffee Drying System). The drying bins are contained in a building of which the roof forms a simple "bare plate" solar collector black painted corrugated iron with a hardboard ceiling suspended 30 cm beneath it. Air is drawn through the cavity between the roof and ceiling by means of a diesel-powered fan' the air being drawn also around the diesel engine block before entering the fan and being propelled into the drying bins. Temperature increases of up to 15 C have been obtained by passage of air at 8.5 m³/s across the roof cavity, with an additional 3-4°C increase derived from the waste heat of the engine.

INTRODUCING THE TECHNOLOGY INTO NEW AREAS

Most farmers appreciate the importance of efficient produce drying, but where this is perceived as an operation involving no direct expenditure, there may be resistance to the idea of purchasing materials to construct improved sun-drying facilities or solar driers. Such resistance may best be overcome through the establishment of well managed demonstration projects in which selected technologies are employed on sites readily accessible to the neighbouring farmers. Farmers will recognise the following benefits:

1. Increased output of dried products from a given area engaged in drying operations; this is important in areas, where there is a heavy constraint on land availability.
2. Substantial reduction in wastage where climatic conditions at harvest are unfavourable for simple sundrying.
3. Improved quality of a marketed product. This improvement is generally recognized on the market, where the product is valued accordingly.
4. Extension of the season in which drying can be undertaken with a resulting larger harvest, or additional crops to be grown and preserved.

In addition, there may be other advantages to the community which would be apparent in a full cost/benefit study, for example: improvement in public health resulting from increased retention of the vitamin content of produce' or simply from increase in the quantity of food available, reduction in cash out-flow resulting from decrease in the quantity of food purchased from outside the community; increased foreign exchange savings or earnings for the country.

The costs of purchase and maintenance of drying facilities have to be weighed against these benefits. Maintenance may be high in drying' unless farmers are careful in their handling of the relatively flimsy structures of solar driers and their relatively fragile plastic coverings. The driers are subject to damage in high winds and pouring rain' and their plastic covers are also to a certain degree subject to photodegradation. A realistic estimate of maintenance costs can be made only on the basis of on-site experience over several seasons of drier operations. This will also provide information on additional costs involved in crop-drying, for example the costs of provision for packaging or storage.

Communal or co-operative processing facilities are generally of more sturdy construction, even when they are constructed largely from plastic sheeting, and they are generally under closer supervision when in use. These advantages may be decisive where the choice between on-farm and centrally managed drying has to be made, particularly if the question of extension of credit to cover drying costs arises.

REFERENCES

Best R. (1979) Cassava Drying. Cali, Colombia: Centro International de Agricultura Tropical. p. 24

Curran C. L and Trim D.S. (1982) Comparative study of solar and sundrying of fish. Paper presented at IPFC Workshop on Dried Production and Storage.

Duff B., Nichols F.E., Campbell J. and Lee C.C. (1974) International Rice Research Institute Semi-Annual Progress Report 17-18 Las Bonos.

Lawand T.A. (1966) A solar cabinet dryer. Solar Energy 10 (4), 158-164.

Thanh N.C., Muttamara S. and Lohani B.N. (1978) Drying techniques for the improvement of tapioca chips in Thailand. Thai J. Agric Sci 11 (1), 45-55.

Trim D.S. and Ko H.Y. (1982) Development of a forced convection solar dryer for red peppers. Trop Agric (Trinidad) 59 (4 ), 319-323.

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