Use of a fan will give minimal power without very tight aerodynamic coupling. Direct access to the fan's power source would be far more efficient.
Various small WT (wind turbines) are available commercially that claim to be able to charge a battery pack and power electronic portable devices. All I have ever seen are junk.
Available power from an air stream is
Power = 0.6 x A x V^3 x K Watts
V = air velocity in m/s
A = cross section area of WT rotor in m^2
0 <= K <= 1 = efficiency.
K = :
0.4+ is available in large (MW level) well designed commercial WTs.
0.2 - 0.3 DIY ascended masters and savy commercial startups.
0.15 - 0.2 achievable with very well designed and built DIY devices. Not at all easy.
~= 0.1 - creditable achievement for small DIY units.
From above - Power = 0.6 x A x V^3 x K Watts
At 1 m/s air velocity you get 60 mW per square meter.
At 2 m/s you get 480 mW / m^2- the cubed term in V helps
At 3 m/s you get 16 W/m^2.
A 100mm dia rotor has an area of 0.00785 m^2. Wow.
At (even) 3 m/s you get 16 x 0.00785 = 125 mW.
To charge an eg cellphone at 5V. 1A you need 5 Watts.
A 700mm dia WT rotor has area = 0.385 m^2.
Power at 3 m/s with K=0.1 = 6.2 Watts - about right for a 5W cellphone charger. About right to charge a 1400 mAh battery (iPhone etc).
3 m/s =~ 11 km/hour "breeze".
A 700mm dia = 2'-4" dia WT in a 3 m/s wind (across whole disk) is about entry level for your requirement using the above assumptions.
Typical pretty looking garbage:
Think about these specs - rotor size, power out and needed wind speed.
The claimed spec amounts to an achieved K of about 0.15 (at about 15 kph wind speed).
HYmini is a handheld hybrid mini ‘green’ power station. Measuring only 5.4 inches (134mm) X 3.4 inches (87.5mm), it uses three power sources: renewable wind power, solar power, and the conventional electric power outlet. The energy collected by the HYmini is stored in its internal battery, providing the user with a compact, portable device that can be used to recharge cell phones, MP3 players, iPods, PDAs, digital cameras or other 5V devices.
HYmini can harness wind power via its built-in micro wind power generator – a supplemental power source with a 65 mA capacity. It requires a minimum 9mph wind speed in order to start recharging.