I had never heard of these until I found this article:

http://en.wikipedia.org/wiki/Vacuum_flask_cooking

That sounds interesting. It would be an efficient way
to cook food with minimal heat input, especially
important during this hot weather.

However, I haven't used my crockpot in ages,
so I doubt I'll be buying a vacuum flask cooker
any time soon. It seems to me these things are
only useful for long slow cooking, and I can't
remember the last time I did that.