• It's good to list all types of solar air collectors; e.g. different types of flat plate, CPC, solar chimney collector, ...
• Define each and state normal size and applications.
• You may go to the thermal performance of each.
• Cost and maintenance.
• Reliability.
• List Advantages and disadvantages.
• Then, comparison could be done.

If you want to compare on different solar air collectors, then first you can classify for what purpose you want to use so that you can use for regeneration of desiccant. therefore some parameters are selected which Influence your purpose like temperature, wind velocity etc.

Then as Elhadi Ibrahim Dekam told that you first choose the category of solar collector like FPC, CPC, parabolic collector, etc

2. What kind of material to use

3. design of system such as size, length, diameter single layer or double layer

4. find thermal performance

5. Reliability, Economics and cost analysis

6. life span, payback period

7. other factor such as single pass, multi pass air supply, absorbing surface material and its design.