well sollar energy as wind would be the way to be able to roi but how many time you think withoyt pay eletricity would take 1 year? more less with the current price of 230 dollars.... would be better to sell and buy it ?
Don't forget that with solar and wind (or any other electricity source you buy and generate your own) you will have to ROI on the investment for that device too, on top of the miner. Essentially, it only extends your ROI and makes your income higher after the break-even point, where the difficulty probably would have already risen.
Very true, but a miner that mines for free is never obsolete so theoretically you could keep mining forever. Technically you'll always make your ROI eventually, although as you have said because of difficulty rises and other factors like price that could take some time. I really only think green/free energy is the way to go for people that really want to get into mining but have really high power costs or people who don't care as much about the money as they do the fun of mining and other cool and interesting projects.