Lets say I have a critical device, which requires almost no ripple in voltage and draws a very high current. What are some ways, other than a simple capacitor which take out the ripple from the full-wave rectified ac voltage?
The low frequency of the ripple is the problem. Should the ripple frequency be 10kHz it's a lot more easily solved with components that are small.
I'd use an on-line switching power supply and forget about conventional transformers, bridge rectifiers and big capacitors. The output you get might have some higher frequency ripple and noise but this could be removed with low pass filtering from inductors and capacitors that are a fraction of the cost of filtering at 120Hz (full wave 60Hz).