Okay, I'm not completely sure how to do this, but I got an idea today while eating.
As I was sitting waiting for my soup and my tea to cool a little I notice that I was just letting some energy go. I mean I really wanted to cool my soup and my tea but I had not better way than let that energy warm the air. Yet I knew that later on I would want to warm something for dinner. So it would be great if I could take the heat from the soup and save it to use it to warm my dinner. I thought about a ball with a big heat capacity (high calorific coefficient) that I could save in a small bag that isolates temperature. So I could use this "heat trap" to save some of the heat of my soup for later.
I have some idea of how to make this, at least from a theoretical point of view, however I have to idea how to actually build it =/ I even wonder if the amount of energy you could save make it worth it (probably you spend more energy building it than you'll ever save with it). But here's an idea and maybe someone else has another idea and maybe if we join them we can build something better so feel free to comment!