[Science & Technology]

Drying of agriculture crops

SOLAR DRYER

to the rescue

Tahir Anwar

SOLAR POWER: Since ages man has been using energy from the Sun to dry agricultural produce. The material to be dried is simply spread out in the open, on the ground or a suitable platform, for proper exposure. This method has some inherent problems. For one, the method depends too much on the weather.

So, there is no protection in the event of rain. However, a simple system can be devised which utilises solar energy to dry crops, while protecting the same from the elements.

But before we discuss the nitty-gritty of such systems, let's discuss the potential of solar technologies in various parts of the world. In industrialised countries, solar technologies have to compete with efficient and reliable conventional technologies. At today's low energy prices, the use of solar energy is only economically viable in some niches, such as solar drying of selected crops.

Situation in the developing countries is comparatively better. Abundant supply of Sun's rays, decentralised use, low energy demand and high fossil fuel prices favour the use of solar energy in drying. This is why the introduction of low-cost, locally manufactured solar dryers offers a promising alternative that serves to reduce post-harvest losses.

This can have significant impact on the food supply situation. The possibility of producing high quality marketable products could also improve the economic situation of farmers.

Drying of agricultural crops in order to preserve perishable products is an accepted technique, which is still used on a large proportion of world's crops.

Traditional methods of drying directly in the Sun have been improved upon and drying methods and equipment have been developed which commercially dehydrate a wide variety of products.

Reliable estimates show that losses on account of poor drying practices may be more than 10 per cent and may even exceed 50 per cent in some developing countries.

Considerable losses of the produce may occur during drying due to various external influences, such as insects, birds, rainstorms and microorganisms.

The quality of produce may also be affected - by contamination from dust and bird droppings. The inability to adequately dry the produce will, at the very least, slow down processing and possibly lead to mould growth or discoloration. Any produce with even a trace of mould cannot be used for processing.

The sale value of mouldy chillies can be less than 50 per cent of the normal one. In extreme cases the whole crop can be lost. Therefore, modifications have to be made in conventional techniques to stamp out some of the existing problems.

With the exception of the northern areas of the country, plenty of sunshine on an acre on a sunny day in an hour is equal to heat energy produced by burning about 1,600kg of animal dung or 700kg of firewood. But the question remains if we may use solar energy in Pakistan for drying purposes at this point in our history.

Let me just say that there is ample scope to make improvements in our traditional drying techniques so that their demerits are avoided. As an example, take the drying of chillies in Sindh.

Drying of produce is simply the process of removing water from it. Biologically, the process is known as dehydration. The dehydration of grain crops, much like other produce, involves the movement of water from the interior of the grain to its outer surface and then evaporating from there.

The degree of evaporation depends on the difference in vapour pressures of the drying air and that of the grain. When both are equal, the vapour pressure in question is equilibrium vapour pressure and at that time water content of the grain remains constant and the process of drying stops.

However, if there is a pressure difference, the grain will either lose or gain moisture. If the temperature of drying air is then raised by only a few degrees, the relative humidity of even humid air is lowered enough for making it effective for drying.

Traditional Sun-drying is a slow process and losses may reach 30 to 40 per cent in tropical countries, the process requiring approximately 1ha of drying surface per 20ha of cropland. While conventional high-tech dryers have high efficiency and produce a uniformly high quality product, they are expensive and so are not affordable.

An alternative approach is to use a solar dryer that uses free solar energy available from the Sun's radiation. However, at least ten years' meteorological data is necessary to obtain an optimum performance of a solar dryer in different seasons. A comprehensive evaluation is necessary to determine the technical and financial viability before replicating a solar dryer in Pakistan.

Through analysis, we can choose an appropriate type of drying system with necessary modification and improvement, according to the climatic conditions of a specific site plus requirements of the user. Following are some of the common methods of crop drying:

Natural Sun-drying: Under conventional or natural Sun-drying, the crop to be dried is to be placed on the ground and overturned intermittently, to expose all the material to the Sun and air. In more advanced systems, the crop is placed on grills for an easy flow of air through it.

But in such a system environmental effects - such as rain, dew, dust and microorganisms - deteriorate the quality of the dried products. A solar dryer avoids these problems.

The simplest type is the cabinet solar dryer which can be constructed out of locally available materials like bamboo, coir fibre or nylon weave. However, construction costs are high and a full financial evaluation should be made to ensure that a higher income from better quality spices could justify the additional expense.

During the wet season or times of high humidity, which often coincides with the harvest of spices, a solar dryer cannot be used effectively. An artificial dryer which uses a cheap energy source is necessary. This may be a wood or husk burning dryer or a combined wood burning and solar dryer.

Indirect solar dryers: This method is more suitable for dehydration of fruits and vegetables because it avoids all contamination. In this process, pre-heated air enters from one side of an opaque box, circulates inside the box and exits through the opposite side. Circulation of the air could be natural or forced.

The products to be dried are placed on racks. We can call this a conventional dryer, since air is heated by solar energy instead of fossil fuel. A solar dryer where a chimney consists of a cylindrical polyethylene-clad vertical chamber supported by steel framework, and draped internally with a selectively absorbing surface, has been described by scientists and the performance of the chimney has been monitored extensively.

Glass roof dryer: This is the commonly available solar dryer. It is easy to build and simple to operate, maintain and control. It can be used to dry a wide variety of agricultural produce.

This dryer is a very useful and effective device for small-scale food preservation. Products to be dried are enclosed in a cabinet and the top is covered by glass sheets. Temperature of the drying air and products is raised to enhance the rate of the drying process.

To facilitate circulation of air, perforated side covers are used. An interesting form of this system is that of the glass roof dryer. This may be used for preservation of food on a large scale. It consists of two parallel rows of drying platforms with a central passage for the operator to move about.

A fixed glass roof above the drying platform allows the radiation of the Sun to penetrate the dryer and also prevents the ingress of rain or dew at night. In Pakistan, both green (fresh) and dried chillies are consumed. Chilli is not only an important part of the diet but also one of our main cash crops.

Considering the importance of chillies in Pakistan and the need to dry them when they are available abundantly, there is a need to study improved drying systems that are reliable and affordable by the farmers of Sindh.

Chilli is normally dried on open ground with no shelter. This results in poor quality which is often associated with huge financial losses to farmers. Studies have been carried out to estimate the loss of chillies from drying in this way and no alternative options have been investigated to improve the drying technique in Pakistan.

A cost-benefit analysis is based on the prevailing market prices for both the fresh and dried products. Even if the technical performance of a dryer is found to be satisfactory, its use in Pakistan needs to be tested from an economic point of view. Industrial drying offers quality drying, whereas its high cost limits its use.

Open Sun-drying suffers from quality considerations though it enjoys cost advantages. Choosing the right drying system is thus important in the process of chilli drying. In the regions where some crops have to be dried during rainy season, special care is required in choosing the drying system.

Studies comparing traditional Sun-drying and other solar drying techniques show that the use of solar dryer leads to a considerable reduction in drying time and to a significant improvement of the product quality.

The writer works as a senior scientific officer for the Pakistan Agricultural Research Council

Exploding stars

Shane Blok

SPACE: Halfway along the Milky Way a large group of stars bulges out above and below the band of stars. This is the centre of the Galaxy, our next destination.

Another gas cloud like the Orion nebula is passed. Blue stars can be seen stretching through space in a long curving tail behind the cloud. Following the trail of blue stars, they get older and older, the further from the gas cloud they are found.

Suddenly, a gigantic red star comes into view. It is where a blue star should be. So blue stars must also turn into large red stars as they are dying. This red star is much bigger than any seen so far and it is called a red supergiant.

Flying on, there are fewer and fewer blue stars around. They are all growing old and turning into red supergiants. Near the centre of a red supergiant, the star is built in layers, rather like an onion. Different chemicals are found within each of these layers.

Right at the centre of the star a core of iron is being made. It builds up in the same way as the ash at the bottom of a fire. Eventually, the iron core grows so large that it can no longer support its own weight. In a tiny fraction of a second it collapses into a tiny ball of matter.

As the iron core collapses, the outer layers of the star race downwards at speeds of up to 30,000 kilometres per second. When the gas collides with the collapsed core, an explosion begins that races out through the rest of the star, blowing it to pieces. Such an explosion is called a supernova. A supernova explosion happens about once a century in our Galaxy.

Dying stars

Flying out from the Orion nebula, stars appear again. Most lie in a beautiful band that stretches across space. This is called the Milky Way. A vast collection of stars like this is called a galaxy. The Milky Way is our Galaxy.

There are many kinds of stars throughout space, some are similar to the sun, others are very different. The colour of a star shows the temperature of the gas at its surface. A blue giant star is the hottest type of star and a red dwarf is the coolest. Yellow stars, like the sun, are somewhere in the middle. Counting the number of stars on the voyage shows that hot blue stars are quite rare but there are lots of yellow and red stars.

The hot blue stars blaze energy into space using up all their fuel in less than a billion years. Cooler yellow stars are smaller and shine more gently.

They can live for several billion years. Weakest of all are the tiny red dwarf stars which glow only dimly but do so for many billions of years. Blue stars are rare because they only live short lives but how exactly do any of the stars die ?

Flying further through space, it becomes obvious that not all the red stars are dwarfs. Some are much bigger.

These big ones are the dying stars. When a yellow or red dwarf star gets old it swells up to become a giant red star. It then splutters out gas and tiny particles into space.

As the star throws off its outer layers, they create delicate patterns of glowing gas.

These beautiful displays of colour mark the passing away of the star with a reminder of the glorious life it has had. Gradually, over several thousand years, the gas fades and floats away. The small, dead heart of the star is all that remains.

Known as a white dwarf, it is a tiny ball of matter that will drift through outer space for evermore.

References
Stars and planets
The big book of space
Space
Stars
Astronomy Encyclopedia