Portable solar powered power bank

[Baxxxter]

Hi,
the reason I am posting this here is this not an assignment given to me, this is a project I chose to do as my degrees final year project.

Abstract:
I am going to design and build a portable solar powered device which is able to charge a USB device and drive a 4w fan. It also includes lithium ion batteries to store energy in case the direct illumination is not present.
Objectives:
*Portable ( foldable pv panel)
*Efficient ( combination of MPPT and PMW)
*keep the fan running in full speed and charge USB devices at the same time.(Ideally)
* Prevent the battery from getting over-charged and over-discharged.
*Satisfy USB devices charging specifications.(assuming be able to charge a mobile phones 3500mah battery within 1 hours in full illumination)
*relatively low cost.

I would appreciate it if you guys help me to find out where should I start in calculating . what steps should I take. what information's do I need. Components have to carefully calculated so u have to keep assumption minimal. In the end of the day this is a device that I eventually need to build and it has to work.
P.S: attached a diagram of the system.

[robermeyer]

You should provide some more requirements of the system.

battery voltage
Fan voltage
Fan current

This would decide buck or boost for 5volt usb. Buck is more efficient.
What size battery is in system?
Why MPPT to charge a battery and drive fan?  Excess solar is going where?
Normally you would take solar and generate 5v for USB.
Any excess is wasted.