A solar infusion is simply covering the herb you wish to make an infusion from with water from the tap (or if you are lucky enough to have one nearby, spring). Keep it in either a glass jar with a lid (keeps out insects) or a glass cafetiere, then leave in the sun for several hours. The sun will gently warm the water so if you put it out there first thing by lunch time you will have a warm and well steeped brew, all without the use of gas or electricity!
A cold infusion can be made even without the presence of the sun. I usually make them over night. Again it is simply a matter of covering the relevent herb with water and leaving it to infuse. I tend to brew both nettle and elderflower with the cold infusion method, it somehow preserves the flavour of the herb better, and feels more kind than pouring boiling water over a freshly harvested sprig!
I also find that when I use the cold infusion method as the herb is not damaged by scolding water it can be used again, just add another small sprig to freshen it up, but it definately makes the herb go further. The good news about either the regular cold infusion or solar infusion method is that you can make your tea without the use of electricity or gas, so it is perfect for camping or even when you are hiking – just make sure you have a glass screw top jar with you, stick it in a side pocket of you back pack and by the time you break for lunch you will have a lovely warm herby cuppa at the ready 🙂