q12023,
Why so abrasive (chuckle) with the good doctor? He has a point about consumption, as it's not as easy to replace an abrading solid as it is to pipe in fresh fuel. As for a practical friction energy source, you would a pre-existing and plentiful input like flowing water. I think any purpose-built input would require more energy than you would get out (that confounded prohibition of perpetual motion).
One elegant application of all this is regenerative braking, which is used in some of the prototype solar cars to reclaim some of the energy you waste slowing down. Is that the sort of system you allude to, q? If you know of other applications being attempted, I'm interested to hear about them.