With the increasing changes in the climate, more and more people are hibernating their tortoises in fridges, because it's a place where you can control the temperature and monitor your tortoise. It's recommended to use a fridge that doesn't have a freezer compartment, and to use an alarmed thermometer so that if the temperatures goes above or below the recommended levels you are alerted. You don't need an air pump - must opening the fridge door every day for a few seconds ensures enough air exchange. Here's a good article on fridge hibernation:
http://www.tortoisetrust.org/articles/Refrigerator.htm

And here's an account by someone who uses fridge hibernation:
http://www.tortoisetrust.org/articles/Refrigerator_2.htm

Nina