You will for sure have to collect your baggage in Delhi and clear Immigration and Customs..

Also, bad news, since you have a 15 hour layover, you can't simply recheck in your baggage after immigration and then walk away.. most airlines in India won't open the check in for the next flight till like 3 hours before the flight.. so you would have to take your luggage with you, wherever you go.. Not sure if Delhi airport has a luggage storage facility..