If you don't want to put a lot of money into mining equipment, then search for coins that are ASIC resistent like Monero, Vertcoin, etc.

Buy 1-2 rigs in the beginning and then with the profit you'll make, add more with time. Since you will have free electricity you make profit on most of the coins you'll mine, like ZombieWorm said. Reinvest your profit until you are pleased with how much money you earn per month selling what you mine.

This is the approach I would try if I had free electricity.