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CHAPTER ONE

INTRODUCTION

1.1 Drying

Drying is an excellent way to preserve food and solar dryers are appropriate food preservation technology for sustainable development. Drying was probably the first ever food preserving method used by man, even before cooking (Alamu et al., 2010). It involves the removal of moisture from agricultural produce so as to provide a product that can be safely stored for a longer period of time.

“Sun drying” is the earliest method of drying farm produce ever known to man and it involves simply laying the agricultural products in the sun on mats, roofs or drying floors. This has several disadvantages since the farm produce are laid in the open sky and there is greater risk of spoilage due to adverse climatic conditions like rain, wind, moist and dust, loss of produce to birds, insects and rodents (pests); totally dependent on good weather and very slow drying rate with the danger of mould growth thereby causing deterioration and decomposition of the produce. The process also requires large areas of land, takes time and highly labour intensive. In order to protect the products from above mentioned disadvantages and also to accelerate the time for drying the products, control the final moisture and reduce wastage through bacterial action, different types of solar dryer can be used (Exell 1980; Fohr and Figueredo 1987; Ghazanfari and Sokhansanj 2002; Janjaia et al., 2008; Khalil et al., 2007, Roa and Macedo 1976; Ting and Shore 1983; Yaldyz and Ertekyn, 2001). With cultural and industrial development, artificial mechanical drying came into practice, but this process is highly energy intensive and expensive which ultimately increases product cost. Recently, efforts to improve “sun drying” have led to “solar drying”.

Solar dryers are specialized devices that control the drying process and protect agricultural produce from damage by insect pests, dust and rain. In comparison to natural “sun drying”, solar dryers generate higher temperatures, lower relative humidity, lower product moisture content and reduced spoilage during the drying process. In addition, it takes up less space, takes less time and relatively inexpensive compared to artificial mechanical drying method. Thus, solar drying is a better alternative solution to all the drawbacks of natural drying and artificial mechanical drying.

The solar dryer can be seen as one of the solutions to the world’s food and energy crises.

With drying, most agricultural produce can be preserved and this can be achieved more efficiently through the use of solar dryers.

Solar dryers are a very useful device for:

Agricultural crop drying.
Food processing industries for dehydration of fruits and vegetables.
Fish and meat drying.
Dairy industries for production of milk powder.
Seasoning of wood and timber.
Textile industries for drying of textile materials, etc.

Thus, the solar dryer is one of the many ways of making use of solar energy efficiently in meeting man’s demand for energy and food supply, total system cost is a most important consideration in designing a solar dryer for agricultural uses. No matter how well a solar system operates, it will not gain widespread use unless it presents an economically feasible alternative to other available energy sources.

1.2 Some Background to the Drying Concept

The idea of using solar energy to produce high temperature dates back to ancient times.

The solar radiation has been used by man since the beginning of time for heating his domicile, for agricultural purposes and for personal comfort. Reports abound in literature in the 18th century works of Archimedes on concentrating the sun’s rays with flat mirrors;

Modern research on the use of solar energy started during the 20th century. Developments include the invention of a solar boiler, small powered steam engines and solar battery, but it is difficult to market them in competition with engines running on inexpensive gasoline. During the mid 1970’s shortages of oil and natural gas, increase in the cost of fossil fuels and the depletion of other resources stimulated efforts in the United States to develop solar energy into a practical power source. Thus, interest was rekindled in the harnessing of solar energy for heating and cooling, the generation of electricity and other purposes (Leon, et al., 2002).

1.3 Capturing Solar Energy

Solar radiation can be converted either into thermal energy (heat) or into electrical energy.

This can be done by making use of thermal collectors for conversion into heat energy or photovoltaic collectors for conversion into electrical energy. Two main collectors are used to capture solar energy and convert it to thermal energy, these are flat plate collectors and concentrating collectors. In the course of this project, emphasis is laid much on the flat plate collectors which are also known as non-focusing collectors.

1.4 Importance of Solar Dried Food

For centuries, people of various nations have been preserving fruits, other crops, meat and fish by drying. Drying is also beneficial for hay, copra, tea and other income producing non-food crops. With solar energy being available everywhere, the availability of all these farm produce can be greatly increased. It is worth noting that until around the end of the 18th century, when canning was developed, drying was virtually the only method of food preservation. (Bena et a.l, 2002)

Ikejiofor (1985) stated that the energy input for drying is less than what is needed to freeze or can, and the storage space is minimal compared with that needed for canning jars and freezer containers. It was further stated that the nutritional value of food is only minimally affected by drying.

Also, food scientists have found that by reducing the moisture content of food to 10 to 20%, bacteria, yeast, mold and enzymes are all prevented from spoiling it (Gallali, et al., 2000).

Microorganisms are effectively killed when the internal temperature of food reaches 145°F. The flavour and most of the nutritional value of dried food is preserved and concentrated. Dried foods do not require any special storage equipment and are easy to transport (Waewsak, et al., 2006).

Dehydration of vegetables and other food crop by traditional methods of open-air sun drying is not satisfactory, because the products deteriorate rapidly, studies showed that food items dried in a solar dryer were superior to those which are sun dried when evaluated in terms of taste, colour and mould counts (Gallali, et al., 2000).

Solar dried food are quality products that can be stored for extended periods, easily transported at less cost while still providing excellent nutritive value (Alamu, et al., 2010). This project work, therefore presents the design and construction of a domestic solar dryer.

1.5 Objectives

The objectives of this project are

To design and construct a solar dryer
To evaluate the solar dryer’s performance

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