If you already got a harvest of turmeric rhizomes, you need to make sure that they are stored in a place, which is completely dry and without moisture. This storage area is best in sand, where your turmeric rhizomes will not have to bother about infection, fungus and moisture. That is why other rhizomes like ginger are also preserved in sand for a long while.
When you purchased turmeric for preservation at home, make sure that the rhizomes are fresh. They should give a soft odour which is a mixture of oranges as well as ginger.
You can always chop a little bit of these rhizomes, and use it instead of the powder, because you are waiting for the rhizomes to dry out in the sun, before you grind them into ground turmeric. Sun-dried turmeric is best kept in airtight containers in a dark and shady place. You may want to air them occasionally in the sun, so that any possibility of moisture and fungus dies at the very onset itself.