In  open  air  Solar drying the heat is  supplied  by  direct absorption  of  solar radiation by material  being dried. The vapor  produced  is  carried  away by  air  moving  past  the material,  the  air  motion  being  due  either  to   natural convection resulting from contact with the heated material or to winds. In dehydration using fuel heat, the material being dried is placed  in  an  enclosure and heated air is  blown  past  the surface  of the surface of the material to remove moisture. Solar   dryer   make  use  of   solar   radiation,   ambient temperature, relative humidity. Heated air is passed naturally or  mechanically circulated to remove moisture from material placed in side the enclosure.

Many  variation  of solar dryers offer the processor  a  wide range  of opportunity to select various type of solar dryers depending  on degree of control over the drying  process  and handaling  of  the  material. The principal  types  of  solar dryers are described briefly below.

Solar Cabinet Dryer :
Solar Cabinet dryer mainly consist of a drying cabinet .  One side  of the cabinet is glazed to admit solar  radiation, which is converted in to low grade thermal heat thus  raising the temperature  of  the air, the drying  chamber,  and the produce.  Usually  the  sun  light  shines directly  on  the material  being dried. The moisture evaporated by solar heat is removed by air circulation. This is accomplished  either by designing  to  encourage natural convective air flow or  by forcing circulation with fans or blowers. The material to  be dried is placed in shallow layers on trays inside the drying cabinet.  Proper air vents are provided for  displacement  of hot air. Solar Cabinet drivers are ,

Solar green house dryers :
Solar   green  house  dryers  are  characterized  by   having extensive glazing on their south facing side while the other sides  are  well insulated. Inside the  enclosed  area,  some means  are provided to store the daytime excess  heat. Vents are strategically sized and positioned to control air flow. A well  designed  greenhouse dryer permits a greater degree  of control over the drying process than the solar cabinet dryers and should be used where relatively large quantity of product is to be dried.
 
Indirect Solar Dryer :
In this type of dryer, the produce is placed on trays  inside an  opaque  drying chamber to which is attached an  air type solar  collector.  The  sun does not shine  directly  on  the material  to  be  dried,  instead the  air  heated  in solar collector  is ducted to the drying chamber  for  dehydration. Air  circulation can be by natural convection, however it  is often  forced  by  blowers. These  dryers  result  in  higher temperature  than the cabinet dryers or sun drying,  and  can produce higher quality product.

In  many large scale commercial drying operations,  air  type solar   collectors  are  being combined  with   fuel fired dehydrators  in order to reduce the fuel consumption and  yet maintained  fine control of the drying conditions. The solar heated  air is used directly if it is hot enough,  otherwise, the  fossil fuel  system  boost the  air  temperature to  the required level Thus the effect of fluctuations in energy  out put  from the collector is less serious since the fuel fired system  is  automatically  controlled  to  provide   specific optimum temperature.

Design  and specifications of different type of  dryers  vary from case to case and it should be worked out individually.

Performance :
Performance   of  a solar dryer changes for different type  of dryer operated at different temperature. Basic requirement for installation of a solar dryer  is  the open space free from any obstacle to the solar radiation. For indoor drying/dehydration processes by solar heated air,  air heating  collectors  are  to be installed in  open  space on ground,  on  roof or on a terrace and ducted  to  the  drying chamber.

Operation and maintenance :
Operation of solar dryers is very simple and it is considered to  be  maintenance free. However for  efficient performance general  maintenance  of cleaning of the  glazing  ,  regular coating of absorbing surface and instruments and controls.