Solar  drying is in practice since the time  im-memorable  for preservation  of  food and agriculture crops. This  was  done particularly  by   open sun drying under open the  sky.  This   process has several disadvantages like spoilage of  product due to adverse climatic condition like rain, wind, moist, and dust,   loss   of  material due  to birds and animals, deterioration  of the material by decomposition, insects  and fungus growth. Also  the process is highly labor intensive,  time  consuming and   requires large area. With  cultural   and   industrial development artificial mechanical drying came in to practice. This  process is highly energy intensive and expensive  which ultimately  increases product cost.  Thus solar drying  is  the best  alternative  as  a solution of  all  the  drawbacks of natural drying and artificial mechanical drying. 

Solar  dryers used in agriculture for food and crop drying  ,for industrial drying process, dryers can be proved  to  be most  useful device  from energy conservation point of  view. It not only save energy but also save lot of time, occupying less area, improves quality of the product, make the  process more efficient and protects environment also. Solar dryers circumvent some of the major  disadvantages  of classical drying.  Solar drying can be used for the  entire drying process  or  for supplementing  artificial drying systems,  thus reducing  the total amount  of fuel  energy required.

Solar dryer is a very useful device for

•   Agriculture crop drying 
•   Food  processing  industries  for  dehydration  of  fruits, potatoes, onions and other                   vegetables, 
•   Dairy industries for production of milk powder, casein etc. 
•   Seasoning of wood and timber. 

Textile industries for drying of textile materials. Drying  implies  the  partial  removal  of  water  from   the material.  But  when  fuel fired equipment  is  used  in  the process then the term dehydration is often applied. Condition essential to both drying and dehydration include a supply  of heat  energy  to evaporate the water and a supply of  air  to carry away the water vapor produced.

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.

http://www.geda.org.in/solar/so_slr_dry.htm