Hes off again !
Why pay for gas all year round ?
Build a box
Put 10m of bent up copper 10mm pipe in it .
My immediate reaction is that what's required is as much surface area as possible so your setup absorbs as much solar energy as possible. I'd find a used radiator, flush it out very well, paint it with matt black paint then put it in a case with a used double glazed panel in front of it.
Your 10m x 1cm setup has a total area of 0.1square metres.
A rough guide to solar energy is about 1Kw/m^2. The specific heat capacity of water is about 4,200Joules/degC.
So if you have 0.1m^2 of pipe that's 100% efficient, you'll be able to raise the temperature of one litre of water by 0.1 x 1000w / 4,200 = 2 hundredths of a degree centigrade every second. Pretty sure that maths is correct but I'm open to correction.
You need a bigger area. Even a tiny 60cm x 30cm radiator would be nearly double the area and would all be dead square on to the sunlight.
Vin