I'm making a mini solar car from a capacitor, solar panel and motor but the specifications for the competition say that I can only charge the capacitor for 15mins. Is there any way I can make the capacitor charge quicker, by changingthe circuit without changing the solar cell, motor or capacitor? The circuit I'm currrently using I've attached.

I'd appreciate an answer quickly so I can purchase more solar panels if necessary.

 

It will only charge as fast as determined by the current supplied by the solar panel. You might be able to reduce the time by employing a switching converter to more efficiently charge the capacitor but it's likely easier just buy a bigger solar panel.

 

I think what Crutschow is thinking about is a Maximum Power Point controller. The problem of charging the capacitor as quick as possible from a solar panel is one of obtaining the most current from the panel. A Maximum Power Point controller would assume an effective input impedance that optimized the power from the panel, and output the power, in the case of charging a capacitor, as a current source, the current being the maximum obtainable from the input circuit.

The difference between Maximum Power Point Controllers that charge batteries and what you want is that you want the output to be maximum current until it capacitor is "full".

To wit:
http://www.youtube.com/watch?v=TENJcOcD5VU

http://electronicdesign.com/article/power/maximum-power-point-tracking-solar-battery-charger.aspx