I have been using outdoor showers, on and off, for most of my adult life. They are great on boats, in secluded country gardens and even when you go camping ! And I am also passionate about reusing stuff, anything, and saving energy.
What I like about this 'free hot water maker' video by John Starnes, is that you can use the hot water for whatever you like. You could take it indoors and use it to wash up, or have a wash in your sink before bed, or even hand wash your woollies.
Of course, if you want to use it for an outdoor shower (or indoor too I suppose), you will need to build a suitable shower setup also - that will be for my next post !
John says in his video that he scavenged all of the materials, even the car screen reflector was left over from something else and only cost him $1 from a cheapo pound store kinda place.
So here is John's video about heating water for free - the set up isn't pretty and won't win an award from Jasper Conran, but it shows how you can make something incredibly useful, from junk you have around the place.