The agricultural produces that were harvested, they initially have high water content in them which needs to be reduced in order to be stored for a longer period of time. The reduction of the water content in the produce to approximately 20% will prevent the development of mold, yeast, bacterial and enzymes that causes spoilage in the produce.
The drying process is one of the methods used to dry in order to reduce wastage of the produce. During drying the changes that occur in the produce are: the shape, size, texture, colour and shrinkage.
Solar drying technology is one of the better drying methods mostly used in developing countries as it is able to dry produces with better quality. It is different with the open sun drying by construction as it designed with components that enhance the collection of solar radiation in order to dry efficiently.
The advantage of solar dryers is that they are able to generate high temperature with lower relative humidity which are required to improve the drying rates and lower final water content in the produce. According to MONGI RJ (2013) the drying time can be reduced by 65% compared to open sun drying because inside the solar dryer it is warmer. In turn the spoilage becomes less and reduced microbiological infestation making the produce to have better product quality. However, the biggest disadvantage of solar drying technology is its dependency on weather for drying operation.
There are two drying mechanism involved simultaneously the first one is the transferring of heat to the product for evaporation and the second mechanism is the transfer of the mass of moisture out of the product to the air (Om and Anil, 2017).
As a result, the process of the reduction of moisture result in the reduction of water content in the product. This reduction of water content results in the prevention of development of biological degradations (i.e. growth of microorganism) and chemicals in the product. In the process of drying.
TP energy is required so that evaporation of water content present in the produce can occur, it can be either supplied directly or indirectly. Direct dryers are mostly used, whereby the hot air is blown on the surface of the product and the heat transferred into the product. The heat transferred into the product increases the temperature and thereby moisture is vaporized and this increases the vapour pressure of the product which makes it to be higher than of the surrounding air. As a result, this pressure difference causes evaporation of moisture from the product to the air. Pressure gradient is the driving force in the removal of moisture content from the product. The evaporation of moisture is continuous until the process reaches the equilibrium state.
Solar radiation or sun rays can be converted into either thermal energy or electrical energy. This is possible with the use of thermal conductors for converting to heat energy or photovoltaic collector for conversion into electrical energy. When the solar dryer has electrical components there will be two collectors to be used to capture solar energy and conversion to thermal energy.
Solar collectors are mostly flat which are of glazing material put at an angle depending on the direction of sun rays. Solar panel are used as photovoltaic system which will absorb and convert the solar radiation to electricity and supply it to the electrical components such as fans.
Solar dryers can be divided into two categories base on their air circulation which are:
A forced conversion solar dryer utilizes solar energy as the primary heat source and has ventilators for circulation of air in the drying chamber. Incorporating fans in the system improves air circulation and that in turn gives higher drying rates compared to open sun drying and natural conversion solar dryer. The better airflow is achieved by electrical components.
A forced conversion solar dryer consists of a solar collector, drying chamber and a fan or pump for air circulation; figure 3 depicts a solar collector with a fan to force air in. They are mainly used in larger commercial purposes and large-scale farming. This type of dryer is suitable for the drying of produce with higher moisture content. The Passive solar dryers With this type of a solar dryer the air is heated and circulated naturally due to wind pressure and buoyancy force. This solar dryer consists of a solar collector, a chimney and drying chamber.
Passive solar dryers are mostly used in the developing countries and tropical regions which practise small scale farming. They are preferred because they are inexpensive, easy construction and lastly their independence to the electricity grid thus there are no electrical components in the system. For a passive solar dryer shown at figure 3 its solar collector does not have a fan. Passive solar dryers are mostly for drying small batch of fruits and vegetables. The drying period is less with higher throughput compared to that of sun drying and this yields higher production of dried produce with better quality compared to sun drying.
Comparison between passive and active solar drying. According to Khuthadzo (2015), the efficiencies of passive and active solar drying are nearly the same but differ with the drying rates. Figure 3 shows the drying time of forced air circulation(active) and natural (passive) solar drying. However, the active solar drying is considered most of the time because of its better drying rates but the electrical components increases expenses. So, this project intends to develop a solar dryer based on operation and one that is economical based on budget.
Factors which are considered to influence the drying process:
According to the tests which were carried out in different regions it was found that the drying temperature average is 60 degrees Celsius for the drying of fruits and vegetables (Khuthadzo, 2015). The higher temperature than 80 degrees Celsius was considered to affect the colour and texture of the fruits. TP and vegetables. It was found that the final drying temperature to be most productive was 55 degrees Celsius as it is able to prevent the browning of the produce. During the drying process the temperature fluctuation of about 20 degrees Celsius is considered to stimulate the development of bacteria.
Shading in the solar collector caused by neighbouring building and (or) the drying chamber can make the solar collector not receive enough solar radiation and that will decrease the temperature of the absorber plate and make the drying less efficient. To eliminate shading in the solar collector, the solar collector should be placed where there is little or no shading that can affect it.
The solar collector needs to be tilted at an angle that it will be able to receive maximum solar radiation at the time of use. When it is tilted facing the solar radiation the absorber plate will be able to absorb the solar radiation and generate heat energy. The inclination will allow the run off of rain and enhance the circulation of air.
Sub-systems of the solar dryer.
Solar collector is the main part that converts the solar radiation to heat energy and it is able to raise temperature of about 10 – 30 degrees Celsius above the ambient temperature (MONGI RJ, 2013). The solar collector has an absorber plate inside which is of a material of high thermal conductivity, the recommended material is copper or aluminium and coated on the surface with a black colour because black colour is able to absorb more heat. The glazing material is the top cover of the solar collector; its function is to retain the heat inside the solar collector.
The materials which can be used for glazing are plastic and glass because they are able to transmit the solar radiation to the absorber plate. TP Solar collectors can be of any size and be tilted so that it can face the incoming solar radiations and also by tilting the solar collector the warmer less dense air will be able to rise naturally into the drying chamber. The solar collector size depends on the ambient temperature and the amount of solar radiation, when the size is big the more the dryer becomes efficient.
The drying chamber is an enclosed container where the produces are placed inside for drying and it is insulated and sealed to reduce the heat loss inside. The chamber has a number of trays for placing the produce evenly which allow the operator to be able to load and offload produces. The trays are made of material safe for food contact and handling by the operator. There are door(s) which are incorporated in the chamber usually placed at the back side of the dryer.
A drying chamber operates by adding hot air inflow at a wanted flow rate as the hot are flows it heats up the produce inside and then the moist air is released. The temperature range of drying chambers is generally between ambient temperature of 5 degree Celsius lower limit and around 300 degree Celsius as the upper limit, but there are already units which can go above that and enable temperature less than 0 degree Celsius depending on what it will be used for.
TP The sizes of drying chamber unit are different according to application. The smallest unit holds around 30 litres and the large one can hold several hundred litres.
As there is a high temperature generated and lower relative humidity than ambient conditions in the drying chamber, so to control the temperature inside an exhaust is needed to release the hot air inside. A chimney is often used to channel the hot air out that picked up moisture from the produce in the chamber. For air to flow through out, the air which is inside the drying chamber must have a higher temperature than the ambient air so that the density of air outside is greater than inside in that way there will be flow of air out of the system and the drying temperature will be maintained.
It was found that when a wind ventilation system is installed the solar dryer’s efficiency be improved from 31.2% to 46.7% (Khuthadzo, 2015). Most of the time forced air convectional solar dryer use a chimney and fans as the ventilation system whereas, natural convectional solar dryers use only chimney as the ventilation system.
The performance of the solar dryer is influenced by the design of the solar collector, it is the main component that has too much influence on the drying temperature. When there are major losses occurring in the solar collector the system becomes less efficient. The glazing and absorber plate materials play. TP an important role in the efficiency of the solar collector, so the materials need to absorb solar radiation as much as possible and retain it.
The materials which are used to design the glazing material are plastic and glass. They are considered because of the better absorption and transmission and reflection. Plastic is more efficient than glass because is able to transmit 0.4 of the short-wave and long-wave. However, the disadvantage of plastic is its limited life span. Glass is able to transmit the upcoming short-wave radiations and long-wave radiations of about 90% and has a better life span with that regard it will be considered.
Absorber plate needs to be made of a material that has a high thermal conductivity. The materials which are considered for high thermal conductivity are copper and aluminium and with their surface coated. A black paint on the surface is used as it has absorption rate of 0.92-0.98 (Khuthadzo, 2015). The surface that is considered to have as a good absorber has a high infrared emittance and good radiator of heat which is why black surfaced material is good.
Absorber plate is designed from material that absorbs heat and generate high temperatures. So, to minimise the heat losses at the back side of the absorber plate there should be an insulation to minimise them. The material should be able to withstand high temperature.