on ebay at the moment is an evacuated tube mini oven. It can apparently bake bread and looks like it probably holds about 20 fluid ounces, or a pint. You tube has a video of some-one using a double glass walled tea infuser filled with water and stainless steel wool to boil water in seconds using a fresnel lens with a steel beam, and the cleanrepublic video has 6 foot long lidless glass thermos flasks boiling water on a house wall. The inside of the inner glass wall has a coating that is dark on the outside and reflective on the inside. I would only use one or two of these tube-fuls of hot water in a day, and presumably with lids on the water would stay hot until it was needed. I hope you can buy these in Europe soon, without paying twice the item's worth in postage. It's a Chinese invention from centuries ago, improved on by modern scientists. I am hoping boiled rainwater is drinkable, with or without the addition of a uv filter, but I've read it has scary things growing in it that can't be got rid of so easily.
the 6ft one sounds like a standard vac tube. the likes of navitron sell the 1.5m flask for about £15 i believe. maybe put a cork homebrew bung (the type with a hole in it used for demijohns) and a s-bend homebrew trap (also used for demijohns) to keep the bugs out, and you should be able to heat water for your cooking.
the 58mm tubes (flasks) hold about 0.8L and are only a £1-£2 more.
the 47mm tubes should do about 40w on reasonably good summer's day and the 58mm about 60w, so somewhere around 1/40th as powerful as a kettle, but then, only half the water of an average kettle, so maybe 40 mins to an hour might get it near boiling (just a guess)?
now, if you're handy and can figure a way of mounting foil wrapped guttering behind it as a reflector, you'll get even better performance
read ivan's advice on the following link which should help avoid the flask cracking due to thermal shock