DESIGN AND PERFORMANCE STUDY OF SOLAR DRYERPAPER The major two categories of the dryers are natural convection solar dryers and forced convection solar dryers. In the natural convection solar dryers, the airflow is established by buoyancy, induced airflow while in forced convection solar dryers the airflow is provided by using fan operated by either electricity /solar module or fossil fuel. Now the solar dryer designed and developed for and used in tropics and subtropics are discussed under two headings.
2.1. Natural Convection Solar Drying: Natural convection solar drying has advantages over forced convection solar drying is that it requires lower investment though it is difficult to control drying temperature and the drying rate may be limited. Due to low cost and simple operation and maintenance, natural convection solar drier appears to be the obvious option and popular choice for drying of agricultural products. It is the oldest type of solar dryer and consists of a solar collector with a transparent cover on the top and a drying unit with an opaque cover on the top. These are connected in series. Natural convection solar dryers do not require power from the electrical grid or fossil fuels. Hence, the obvious option for drying would be the natural convection solar dryers. Several designs are available, and these are (i) cabinet type solar drier suitable for drying fruits and vegetables (Sharma et.al, 1995), (ii) indirect natural convection solar drier for paddy drying (Oosthuizen, 1995), and mixed-mode AIT drier for drying paddy (Excell, 1978). These dryers have been widely tested in tropical and subtropical countries.
2.1.1. Cabinet types solar dryer Cabinet type solar drier suitable for drying fruits and vegetables. Direct-solar cabinet-type driers are used to dry small quantities of food or vegetables and provide moderate drying temperatures (37-58°C) and airflow rates. Both the 1 m2 and 2 m2 model dryers have been tested for fish and banana drying. The 1 m2 model can dry about 15- 20 kg of fresh fish over a period of two or three days while the 2 m2 model can dry double this quantity. The dryer consists of four main parts: a base frame, drying chamber, drying trays, and loading door. The rectangular base frame has six supporting legs on which rest a drying chamber. A solar collector, supported by another three legs, is attached to the drying chamber.
2.1.2. Solar Box Dryer The solar box dryer has been designed to be suitable for household drying of agricultural products. The dryer can dry about 4-5 kg of fish, fruits, and vegetables in a single batch, at a drying air temperature of about 40-50°C. The dryer design was based on thermal performance and product quality optimization. The dryer consists of a rectangular inner box made of GI sheet, with an open top.
2.1.3. Mixed-mode solar dryer The mixed-mode solar dryer consists of a separate solar collector and a drying unit, both having a transparent cover on the top. Solar radiation is received in the collector as well as in the dryer box. The dryer is shown in Fig. 5 and the solar collector consists of a matt-black substance spread on the ground and provided with transparent top and side covers. The dryer was initially designed with a bed of burnt rice husk as the absorber, and clear UV stabilized polyethylene plastic sheet as a transparent cover. However, these materials could be substituted with locally available materials such as charcoal, black plastic or black-painted metal sheets, dark-colored pebbles, etc.
2.2. Force Convection solar dryer 2.2.1. Solar tunnel dryer: Solar tunnel dryer was developed at the University of Hohenheim, Germany in the early eighties for small-scale production of dried fruits, vegetables, spices, fish, etc [8]. This type of dryer has been widely tested and attained economic viability. A low-cost version of this drier has been designed at Bangladesh Agricultural University, Mymensingh, Bangladesh and the pictorial view of the dryer under construction The design of the solar tunnel dryer has been further improved and tested by Janjai (2004) at Silpakorn University at Nakhon Pathom in Thailand. The dryer still consists of two parts, namely the solar collector part and the drying part similar to the original version. Instead of using PE plastic sheet, the roof of the new design dryer is made of polycarbonate plates fixed with the sidewalls of the dryer [9]. The plate has an inclination angle of 5° for the drainage of rain. As the loading of products to be dried cannot be done from the top of the dryer, rectangular windows were made at the sidewall of the drying part for loading and unloading products. Back insulation was made of high-density foam sandwiched between two galvanized metal sheets. A 15 watt-solar cell module was used to power a dc fan for ventilating the dryer. The collector part and the drying part have the area of 1.2×4 m2 and 1.2×5 m2 , respectively. In the solar collector, the drying temperature is heated up to 60°C, which is the optimum drying temperature for most fruits.
2.2.2. Greenhouse solar dryer A PV-ventilated greenhouse solar dryer was developed at Silpakorn University (Janjai, 2004).The dryer essentially consists of a parabolic shape greenhouse with a black concrete floor with an area of 5.5×8.0 m2. The parabolic shape can withstand well the tropical rain and storm. The roof of the dryer is covered with polycarbonate plates. The floor of the dryer is made of concrete mixed with black powder paint to serve as a basement of the dryer as well as a solar radiation absorber. The loading capacity of the PV ventilated greenhouse solar dryer is 100-150 kg of fresh chilies. Drying in the PV ventilated greenhouse results in a considerable reduction in drying time (50%), and the quality of the dry products is high quality dried products regarding color and texture.
2.2.3. Roof-integrated solar dryer The roof-integrated solar dryer consists of a roof-integrated solar collector and a drying bin with an electric motor operated the fan to provide the required airflow. The bin is connected to the middle of the collector through a T- type air-duct connection. The roof-integrated collector consists of two arrays of collector: one facing the south and other facing the north with a total area of 108 m2. These arrays of these collectors also serve as the roof of the building. The roof-integrated collector is essentially an insulated black painted roof serving as an absorber that is covered with a polycarbonate cover.