Solar Energy - Chapter 2

Solar dryers

CONTENTS

2.1 Introduction

2.2 Principles of drying with the sun's warmth

2.3 The principle of the flat-plate collector with cover

2.4 Different designs and constructions of solar driers

2.5 Practical tips

2.6 References

2.1 INTRODUCTION

In contrast to water heating and the generation of electricity, crop drying utilizes the sun's energy directly.
Using solar energy to dry crops is nothing new in the tropics. Many edible, and even cash crops such as cocoa and coffee beans, have for decades been dried on racks placed in the sun.

2.2 PRINCIPLES OF DRYING WITH THE SUN'S WARMTH

Imagine a closed heated space in which a damp agricultural crop has been stored.
Two things happen:

• the crops is warmed by the heat from the stove of fire
• air around the heat source is heated up - whereby it can take up a great deal of moisture - and, rising, is continually replaced.

As the crop is warmed up, including the air between the plant fibres, the water it contains quickly evaporates. Pretty soon the air within and surrounding the crop is saturated with water vapour. Fortunately the air moving alongside, warm and unsaturated, can take up this moisture and transport it away. A small fan will of course help this process, but it is not strictly necessary.

At a certain moment the air in the room has taken up so much moisture from the crop that the windows suddenly mist up (though this will depend on the outside temperature); the air against the cold windows has been cooled to below the 'dew point'.
In this way the water in the crop is transferred to the window panes, where it can be wiped off, or allowed to fall into a gutter which leads outside the room.
If in this account 'heat source' is replaced by 'sun', a solar drier has effectively been described. The 'cold window' (which works as a condensor) is sometimes encountered in indirect drying, where the warming of the air and the drying of the crop are separated, if the product has been stacked too high or too close together. See also paragraph 3.2.

Solar drying is a technique particularly suited to the warmer parts of the world, since:

• there is abundant sunlight.
• the air temperature is high and relatively constant over the whole year.

Figure 1. Annual mean global irradiance on a horizontal plane at the surface of the earth W/m averaged over 24 hours (Source: Budyko, 1958)

A high and stable air temperature is actually just as important as the sunshine itself, since it limits loss of generated warmth. It allows a simple solar drier to maintain the temperature of the drying crop during the day around 40°C.

Drying edible crops

The temperature within the solar drier is higher than that outside it. Consequently water on and in the product evaporates. The air takes up more and more of this moisture until a certain equilibrium is reached. Ventilation ensures that this saturated air is replaced with less saturated air, and so the product eventually dries out.
Drying is intended to evaporate and dispel the free water in a product, to make it unavailable to micro organisms. This water can also be bound, by adding salts (pickling) or sugar (preserving). Both techniques can also be used after drying.
Dried products attract moisture from the air, just as salt does. This moisture remains much freer - to micro organisms - than the moisture which was removed from the product; so even in conditions of relatively low humidity the product will rot.
This means that dried products must be given airtight packing unless the humidity is otherwise controlled, for example in a silo.
The level of dessication, i.e. the unavailability of free water, at which decay is stopped varies from product to product.

Table 1. Specifications for drying of agricultural products.
(Source: Herbert et al., 1984)

The warmth in the drier actually encourages rotting in products that are not yet completely dried. For this reason the speed at which the drying takes place is important. The fastest drying is brought about by strong ventilation with dry air.

Under such circumstances the difference between the internal and external temperature is less important than simply getting rid of the moisture as fast as possible. At a later stage the evaporation is less abundant, and much more temperature dependent. If the ventilation is now limited, the air in the drier will be warmed up, and the drying process improved further.

These considerations apart, the quality of the original product (its freshness and cleanliness) and of the drying air both exert a critical influence on the quality of the end product.

Forced drying using warm air circulation

Good ventilation is of crucial importance. It determines on the one hand the exchange of warmth from the absorbent surface to the air next to it and on the other hand the evaporation of the water on and in the product. A stronger ventilation leads to a lower average temperature but also to a more efficient overall transfer of warmth. This leads to a reduction in the relative humidity and improved drying.

Electric fans strongly increase the transfer of warmth to the drying air. This is especially true if the product is stacked close together, impeding the air circulation. It is important, therefore, to rack and shelve the products in such a way that the air circulation is impeded as little as possible. Forced air circulation is only worthwhile if sufficient solar energy can be taken in by the drier; this supposes a large enough (with regard to the mass to be dried) and efficient enough absorbent surface (for example, porous materials), and special glass for covering.

If these factors are not taken into account, the temperature within the drier will not be much higher than that outside it - which of course does not promote efficient drying, and certainly not at the last drying stage. Forced air circulation becomes economic in larger installations drying 50-100 kg per day or more. In non-forced air circulation, or natural ventilation a site is chosen which makes best use of prevailing winds, the air inlet and outlet being oriented accordingly, or a chimney is added to improve the draught.

2.3 THE PRINCIPLE OF THE FLAT-PLATE COLLECTOR WITH COVER

Physical description

The principle underlying the solar collector is that 'visible light' falling onto a dark object is converted into tangible warmth. The colour of the object does not in fact need to be black; it is rather the absorptive qualities of the material which determine the effect. A painted plate can be warmed, but so can a suitable fibrous material such as charred rice chaff.

The cover is of secondary importance, but still has a decisive influence on the total working efficiency; it prevents the created warmth from being blown away and also limits the warmed-up objects' heat loss through reradiation. Moreover it allows a controlled airstream over the warmed objects, which would not otherwise be possible.

To exploit the warmth in the heated objects or surface a medium (water, air) is directed alongside which takes up the warmth and takes it to wherever it is needed. When air is used, it can pass under the collector, above it, or through canals embedded within it. It can be a 'forced' or a 'natural' current. The various possibilities are examined in paragraph 3.2.

In drying, the relative and absolute humidity are of great importance. Air can take up moisture, but only up to a limit. This limit is the absolute (= maximum) humidity, and is temperature dependent.

In practice, however, the air is very rarely fully saturated with moisture. The degree of saturation at a given temperature is called the relative humidity and is expressed as a percentage of the absolute humidity at that temperature.

If air is passed over a moist substance it will take up moisture until it is virtually fully saturated, that is to say until absolute humidity has been reached.

However, the capacity of the air for taking up this moisture is dependent on its temperature. The higher the temperature, the higher the absolute humidity, and the larger the uptake of moisture.

If air is warmed the amount of moisture in it remains the same, but the relative humidity falls; and the air is therefore enabled to take up more moisture from its surroundings.

If fully-saturated air is warmed and then passed over the objects to be dried, the rise in absolute humidity (and the fall in relative humidity) allows still more water to be taken up.

Figure 2. Simple solar dryer

Basic technical details of the drier

Every solar drier is constructed using the same basic units, namely:

1. A transparent cover which admits sunlight and limits heat loss (glass or plastic)
2. An absorbent surface, made dark in colour, which takes up sunlight and converts it to warmth, then giving this warmth to the air within; this can also be the product that needs drying itself
3. An insulating layer underneath
4. An air intake and an outlet, by which means the damper air can be replaced with fresh drier air

These four elements can be modified if necessary, and/or other elements added, for example a fan or a chimney.

2.4 DIFFERENT DESIGNS AND CONSTRUCTIONS OF SOLAR DRIERS

Basic types and their applications

In choosing a certain type of drier account must be taken of the following six criteria:

• the use of locally available construction materials and skills
• the investment of the purchase price and maintenance costs
• drying capacity, holding capacity
• adaptability to different products
• drying times
• (fall in) quality of the end product

Figure 3. Solar drier directly employed

Solar driers can be constructed out of ordinary, locally available materials, making them well suited for domestic manufacture.

Solar driers can be divided into two categories:

1. driers in which the sunlight is directly employed; warmth absorption occurs here primarily by the product itself.
   These are further divisible into three sorts:
   • traditional drying racks in the open air
   • covered racks (protecting against dust and insects)
   • drying boxes provided with insulation and absorptive material
2. driers in which the sunlight is employed indirectly (see fig. 4). In this method, the drying air is warmed in a space other than that where the product is stacked. The products, then, are not exposed to direct sunlight. Various sorts of construction are possible; this design can also be provided with powered fans in order to optimise the air circulation.

Advantages and disadvantages of the various designs

Direct drying

Tradition open-rack drying enjoys four considerable advantages:

• it demands a minimum of financial investment
• low running costs
• it is not dependent on fuel
• for certain products the drying time is very short

On the other hand the products are exposed to unexpected rain, strong winds and the dust they carry, larvae, insects and infection by, amongst others, rodents.

Figure 4. Solar drier indirectly employed

Moreover, certain sensitive products can become overheated and eventually charred. Dried fruit so spoiled necessarily loses its sale value.

Commercially available driers often appear to be economically unfeasible. Specifically, not enough product can be dried fast enough to recoup the outlay. Larger (combined) installations are more cost-effective but call for sophisticated management if the input and output of products is to be held at a controlled, and high, level. They are also fitted with artificial heating (fires) and fans.

Indirect drying

The advantages if the indirect system are that:

• the product is exposed to less high temperatures, whereby the risk of charring is reduced
• the product is not exposed to ultraviolet radiation, which would otherwise reduce the chlorophyll levels and whiten the vegetables.

However, its use demands some care. Faulty stacking of the product to be dried can lead to condensation; rising hot air in the lowest layers becomes saturated, but cools so quickly as it rises that the water condenses out again in the upper layers: see also paragraph 2.1.

This problem can be overcome by

• stacking the product less high
• stacking it less close together
• a larger collector, higher working temperature, faster circulation of more air, or
• a deeper collector, the same working temperature and speed of circulation but more volume.

The higher cost and the complexity of the indirect method drier are also disadvantages.

Technical design

A drier which operates optimally is usually the result of a number of adjustments whose value is established by trial and error and simple drying tests.

It is therefore important that if a solar drier is bought or made, these adjustments can be made.

A summary of these adjustments is given below.

With regard to temperature regulation:

• the available sunlight is dependent on the season and the location and limited to 4-7 kW. hr/day/m . The absorbent area can be effectively increased by directing extra sunlight onto it with reflectors. The angle of the absorber is also specified by the latitude. Take care that the collector is facing the sun and that it is out of shadow as far as possible.
• it is a simple matter to insulate the drier better and thereby raise the degree of heat absorption (and air warmth uptake). The wall of a covered drier - which the sun cannot pass through - is better replaced by insulating material, which lines the box and is painted black.

The heat collector of an indirect drier can be improved by:

• enlarging the absorptive capacity
• reducing heat loss, by means of insulation and keeping hot-air-glass contact to a minimum.

This is usually only worthwhile if the airflow has been artificially increased.

In the absence of a forced ventilation, the chimney-effect is crucial. The difference in height between the air intake and outlet largely determines the draught and therefore the 'natural' ventilation. A chimney will help provided that:

• it is wide enough; if it is too small it will obstruct the draught.
• it is warm enough.

The air must not cool - this causes a reverse airflow! A wooden chimney is suitable.

A chimney less than 40 cm high will in this case suffice.

Despite the many experiments carried out in almost every tropical area, it still appears to be impossible to design the 'ideal' solar drier. Depending on the building materials used, the products that need drying, and the season in which the drying must take place, the 'ideal' dryer will take many forms.

Solar energy storage

Excess heat generated during the hottest hours of the day can be stored by passing the air through, for example, a container full of stones. This only works in forced circulation systems, as the stones cause considerable pressure loss in the airflow. Storing solar warmth in this way allows the excess heat generated by oversized collectors to be used again during the night for more drying.

Such an installation makes it possible to control the air temperature in the drying room, and thus to ensure that the different drying stages work well (for example, for sowing-seeds). In the first drying stage higher temperatures are allowable because the considerable free water still present in the product.

2.5 PRACTICAL TIPS

For the transparent cover, glass is the suggested material, but it is often difficult to obtain and rather expensive. Plastic offers a reasonable alternative. It is less radiation-efficient, but often enough more readily available. If plastic is stretched over the collector it will sag. Dust and rain can collect in the hollow. This can be remedied by fitting a supporting rib across the collector along its longest axis. If this is fixed slightly higher than the edges of the collector the plastic cover will slope down slightly on either side of the rib. Take care that there are no air leaks at the rib ends.

Dust on the cover reduces its efficiency, and should be removed as often as possible.

If the collector is strongly tilted, this favours the airflow and therefore promotes good heat transfer.

However, the further it is tilted below the sun the less sunlight it receives. For this reason the indirect dryers are often better in practice.

Watch out for excessive surrounding air humidity, for instance during misty early mornings! It is vital that the drier is only set into operation (by opening the air intake and outlet) after the mist has risen and the air humidity has fallen. Otherwise there is a risk that in the weak early morning sunshine the product, instead of being dried, attracts condensation.

In drying grain whose capacity for germination must remain, such as sowing-seeds, the maximum temperature is limited to around 40 C.

2.6 REFERENCES

• Brace Research Institute, A survey of solar agricultural dryers, Quebec, Canada, 1975.
• Arbeitsgemeinschaft für Entwicklungsplanung, Devices for food drying, GATE, Stuttgart, FR Germany.
• ILO, Solar drying: Practical methods of food preparation, Geneva, Switzerland, 1986.