Design And Manufacture A Natural Convection Solar Dryer Which Requires Little Or No Maintenance
This work is aimed at designing and fabricating a convectional solar dryer which can be used in drying spent grains after brewing. This will no doubt save considerable human labours and time wastage involved in conventional method of drying the spent grains by spreading on the ground. It also saves as a means of employments for our teaming population. This machine was designed after due consideration of its ease of production, maintenance and servicing. This study was therefore carried out on the construction of a convectional solar dryer. The dryer mainly consists of a solar collector panel, drying chamber, chimney and a charcoal stove. The solar collector is made up of 5 mm thickness single layer glass, 2 mm black painted aluminum absorber plate and 3 mm fiber glass insulation which is enclosed in a casing made from wood. The drying chamber is made from plywood with 2 cm thickness. Galvanized metal sheet of 1 mm thickness was rolled and welded to make the chimney. The total cost of the dryer was estimated to be N291,847.50 (US$ 765.00*). Different tests were carried out in order to evaluate the performance of the dryer. No load test, i.e. test without keeping any material to be dried, was performed and it indicated temperature could rise up to 53.3 oC in the dryer. Average collector temperature recorded was 56.4 oC. In the evening, the dryer temperature was kept above the ambient and collector temperature by burning charcoal using the backup stove. As a result, after three hours of heat supply the drying temperature reached 50.8 oC. The dryer performance was also evaluated using pineapple and mango. For the different tests carried out the performance parameters used for evaluation included moisture content, drying rate and drying efficiency.
The moisture content of pineapple and mango was reduced from 87 % and 85 % to 16 % and 13 %, respectively, within two to three days. When using only solar energy as a heat source, the drying rate for pineapple was found to be 23.7 g/h whereas for mango it was 15.5 g/h. These values were found to be 25.2 g/h and 18.4 g/h, for pineapple and mango, respectively, when solar drying was performed with the backup heater (heater used in the evening only). But a higher drying rate was obtained, 32.5 g/h for pineapple and 19.3 g/h for mango, when the backup heater was used with the solar energy during both the day time and in the evening. The collector efficiency was found to be 31.7 %. Drying efficiency was also found to be 9.7 %, 7.5 % and 8.7 % for solar drying, hybrid mode (backup heater used in the evening) and solar drying in hybrid mode (backup heater used during day time and evening), respectively.
1.1 Background To The Study
Drying is one of the oldest methods of food preservation. For many years, people have been preserving agricultural products by drying until caning was developed at the end of the 18th century (Ekechukwu and Norton, 1999). The importance of dried foods cannot be overestimated as the kitchens and food stores in any country will confirm the quantity and diversity of dried food in use. Drying is essentially a process of moisture removal due to simultaneous heat and mass transfer. It is a classical method of food preservation which provides longer shelf life,lighter weight for transportation and small space for storage. Drying could be of various methods such as open sun-drying,direct and indirect solar drying etc.
Open sun-drying which is the most commonly used method to preserve agricultural products like grains, fruits and vegetables in most developing countries. Such drying under hostile climatic conditions leads to severe losses in the quantity and quality of the dried product. This losses related to contamination by dirt, dust, bird droppings and infestation by insects, rodents and animals as well as the quality of the products being degraded due to uncontrolled heat in sundrying even up t the extent that the food products are inedible (Whitefield, 2000). Therefore, the introduction of solar dryers in developing countries can reduce crop losses and improve the quality of the dried product significantly when compared to the traditional methods.
Solar dryers could be either Natural convection of forced convection solar dryers. The Natural solar dryer is such that it controls the drying process and protects the agricultural produce from contamination by insects, dust, bird droppings, invasions by animals, and in comparison to Natural solar dryers, it generates higher temperatures, lower relative humidity, lower produce moisture content and reduce spoilage during drying process in addition, it takes less time and is relatively inexpensive when compared to artificial mechanical drying method (Ged-Gujur at Energy development Agency, 2003, www.geda.com). Dried foods using natural convection solar dryers are quality products that can be stored for extended nutritive values.
The major problem facing breweries in the country in how to utilize the spent grains after brewing. Though some breweries use these spent grains as aids by drying and burning them. We hence see this method of recognition as not being economical but wasteful.
There propelled us to designing a new method of drying these spent grains to the required moisture content through the use of a convectional solar dryer. When these spent grains are properly dried, it could be a major constituent in the production of poultry feeds.
With this convectional solar dryer, the unemployed masses could gain jobs by fabricating or buying this machine for drying of spent grains, and recycling it to a useful feed for poultry.
It is worthy to note that the production of a convectional solar dryer is designed in a way that even the unskilled workers could operate it, in fact, we are designing this project as our quota towards the eradication of unemployment and poverty, which this present government is wedging war against.
Moreover, technology could be bought, borrowed or stolen. Our project is a borrowed technology from the western world, which was primarily used for drying of clothes, but we have expanded this scope to deal favourably with drying of spent grains to the desired moisture content.
Agriculture in Nigeria is mainly carried out on a smallholder basis and it is mainly the traditional system of farming. About 90% of farm holdings are smaller than two hectares in size (MOFA, 2011). But there are some large farms and plantations, especially palm oil, rubber and coconut and to a lesser extent, maize, rice and pineapples. In Nigeria, cocoa, oil palm, coconut, cola and rubber are considered as the major industrial crops while cassava, cocoyam, yam, maize, rice, millet, sorghum and plantain are the main starchy and cereal staples in the country. The main agricultural produce under the category of fruits and vegetables are citrus, pineapple, banana, pawpaw, cashew, mango, tomato, okro, egg plant, pepper, asian vegetables and onion (MOFA, 2011).
MOFA (2011) stated that although agriculture is the largest sector of the economy in Nigeria, contributing about 39% of GDP, there are basic problems faced by this sector which include high post harvest losses as a result of poor postharvest management. For instance, Zakari (2012) has given an estimate showing that the average postharvest loss of mango is between 20 % and 50 %. The main reason for losses has been attributed to the fruit fly presence and a host of diseases as well as lack of cold chain facilities, and long transit time. Antwi (2007) also suggested that there would be loss of fresh produce during the harvest period because of excess production which could lead to unsold produce. This surplus produce should be stored so that it can be used later. But it might be unsafe to keep these produce over a long period due to high moisture content, physical damage, pathogens etc.
In order to reduce such postharvest losses to enable farmers increase the quality of their products, efficient and affordable drying methods are necessary. Locally manufactured low cost convectional solar dryer machines provide a means of reducing postharvest losses (Weiss and Buchinger, 2002).
1.2. Problem Statement
More than 80% of most fruits is water (GEPC, 2005). Micro-organisms can obtain nutrients and water for their growth from the fruit in which they grow. Hence, the fruit must be dried in order to stop the multiplication of micro-organisms and store it for longer period.
Traditional open sun drying is a common and widely used method for drying of agricultural produce including fruits, vegetables and cash crops. It is the simplest way of drying foods by direct exposure of the product to the sun. Even though sun drying is the cheapest method, the quality of the dried product is far below standards. This method has some disadvantages including contamination, damage by birds or insects and slow or intermittent drying. Dried product quality improvement and reduction of losses can be achieved by the introduction of suitable drying technologies such as solar drying.
However, most convectional solar dryer machines that are constructed use only solar energy as a heat source for drying. This makes the convectional solar dryer machine to be dependent on climatic conditions limiting its use in cloudy periods and at night. As a result, agricultural produce that are harvested in the rainy season are still subjected to spoilage.
Fruits that can be dried in Nigeria include pineapples, papaya, mango, banana and coconut. Dried fruit is mainly consumed as a snack and as an ingredient for breakfast cereals, healthy ready-to-eat snacks and desserts. Breakfast cereal mixtures and bakeries are one of the largest end users of dried fruit (GEPC, 2005).
In Nigeria, the international market has been the target market for dried fruit products. Dried fruit is not yet popular in terms of both consumption and exportation. But as awareness is created locally, it is expected that demand will eventually grow and attract more operators in the sector (Zakari, 2012).
In recent years, the use of solar energy has become more popular. Solar radiation is the main source of energy for solar drying. The use of solar energy in the agricultural sector to preserve grains, fruits, and vegetables is feasible, economical and ideal for farmers in many developing countries (Mustayen et al., 2014). But for most crops harvested during the rainy season, preservation by using only solar energy proves difficult (Barki et al., 2012). Hence, an additional means of heat supply must be incorporated into solar drying.
This makes the dryer to operate continuously at night and in cloudy days.
1.4. Research Objectives
The main objective of the research was to design, construct and evaluate the performance of a convectional solar dryer machine incorporating a charcoal stove which can be used as an additional heat source.
The specific objectives of the research were:
- To design and construct a convectional solar dryer machine with charcoal stove as a backup heat source.
- To evaluate the performance of the dryer using different parameters such temperature, moisture content of the produce, drying period, drying rate and efficiency.
- To compare the performance of the convectional solar dryer machine with and without the backup heater.
Conclusion And Recommendations
An indirect type convectional solar dryer machine with a backup heater was designed and constructed with materials readily available in the market. The dryer is easy to operate and handle. An additional system, backup heater consisting of a charcoal stove, was included in order to make drying continuous throughout the night and cloudy periods.
Under no-load condition, the average collector temperature reached 56.4 oC and that of the dryer reached 45.1 oC while the average ambient temperature was 34.6 oC. When only the backup heater was used in the evening by burning charcoal a temperature as high as 50.8 oC was recorded on the bottom tray. This indicated that the temperature in the dryer was raised above the ambient temperature creating a suitable condition for drying.
The performance of the dryer was evaluated using pineapple and mango in which the initial moisture contents were reduced from 87 % and 85 % to 16 % and 15.5 %, respectively, within two to three days. A better dryer performance in terms of drying rate was obtained when the dryer was operated in a hybrid mode, i.e. when heat was supplied by burning charcoal as a backup system. As a result, drying rate increased by 26.9 % (pineapple) and 19.8 % (mango) than the drying rate in convectional solar dryer machine.
The collector efficiency obtained from no load test was 31.5 %. This value is well in the range recommended by different literature for natural convection convectional solar dryer machines. The drying efficiencies were 9.7 %, 8.7 % and 7.5 % for solar drying, backup heater used throughout the drying period and backup heater used only in the evening.
It was found that the convectional solar dryer machine can dry high initial moisture content fruits such as pineapple and mango to the recommended value of moisture content for safe storage within two to three days. The convectional solar dryer machine can be used during any time and season as a result of the heat provided using the backup stove. Hence, it can provide a means of preserving agricultural produce that are harvested in the rainy season.
The performance of the dryer can further be enhanced by making modifications and following the recommendations given below:
- The glass cover of the collector should be insulated on the edge. In addition, the gap between the collector and drying chamber should be covered with permanent insulation that can withstand rain.
- The gap on the drying chamber where the backup heater is attached should be well covered using insulation material when the convectional solar dryer machine is used with only solar energy as a heat source.
- Insulating the drying chamber will help to attain a higher drying temperature, especially at night when the backup heater is the only source of heat supply.
- Design modifications are required to maintain the same amount of drying temperature in the dryer when the backup heater is used. One such suggestion would be to internally extend the metal tube to the adjacent sides of the drying chamber. This would help to minimize the non-uniformity of heat transfer on a tray.
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