So I kept thinking about the saving some heat for later idea
which was basically to save the heat from something you want to cool down (like a hot soup or coffee) to use it later to warm up something else (let's say your dinner). You just need something that stores heat (something with high calorific coefficient) and a thermally isolated bag (to save it). This seems easy to design, yet the amount of energy you save is probably not worth it (you might end up spending more energy in building the "heat trap" than what you can actually save with it)... or not?
After thinking a little more about this I realized the problem is:
1) Even if there are lots of things that waste heat (bulbs, hot food, computers, etc.), usually the energy you can save is small because we are talking about a lot of SMALL sources of heat
2) Even with an isolated bag, you're always losing some heat from the time you warm up your "heat trap" to the time you actually use that heat
How to overcome both of this?
Well let's say instead of trying to cool down your coffee, you work in an industry that needs to cool down something (I remember an internship I did in a glass manufacturing company, after "baking" the glass they have to cool it again and they did this all day long). And let's think the industry just next to you needs to warm up something (many industries need vapor so they have to warm up water).
Why not make some heat sharing pipes along and industrial park where you can "dump" any extra heat you don´t need for others to use it. I know many industries do this internally, but I'm guessing even with this some industries should have some extra heat (for example huge ventilation systems are used to extract hot air which has energy that might be usefull to someone else). Also, having this system at a bigger scale can have some benefits like getting higher amounts of heat that could be used to get cheap energy to light the common areas of the industrial park for example.
What I'm talking about is just a coordinated heat collecting system. This should be hard to design or expensive (again you just need something to conduct the heat without losing much of it in the way) and the amount of energy you save can be important if you do it in the proper scale!