Small Scale power generation

[cjkogan111]

Hello,
My purpose in writing this post is that I am looking for ideas to help with the design of a project that I am interested in working on. Rather than ask a specific question then, I will just overview the problem, and describe what I am considering for a solution.

I work with a research group that flies high altitude balloons. The longer the flight, the more batteries need to be included to power the system during flight. Thus, I was somewhat interested in looking at energy sources other than batteries. One major possibility is solar power. While this type seems more promising, I have decided to look into another source - namely extracting energy from balloon/payload oscillations.

The balloon/payload oscillations provide motion on a single axis that can be transferred to rotational motion by pushing a wheel against the main line. This can be turned into electricity by driving a DC motor with the oscillations (I'm pretty sure the output will be less than 200mA at 5V - however, who knows, it could be less than 1mA.) The output will be AC, because the motor will be turning back and forth. Thus, I will need a AC/DC power board to convert the voltage to DC. Then I can just store the charge in a capacitor to account for the changes in the magnitude of the oscillations - so that a constant current can be drawn.

Some problems I see:
I don't know how efficient DC motors are, or if there is a better way.
I don't know how heavy a AC/DC power board will be
I'm not sure how changes in the magnitude of the oscillations will effect the system

Any suggestions?
Thanks!
-cjkogan111