Existing Open Source Designs

OSE France's Design

Whats the deal with OSE France's main page, http://www.opensourceecologie.org/ ? As of 2/17/19 ~19:20 it appears to be down
The instructable for this

But this requires replacement of insides with a u- copper tube, which is a valiant effort. Tbis is because evacuated tubes have a closed heat exchanger mechanism - a closed copper tube with heat exchanger bulb.
Unless one can find low cost u-tube heat exchangers, the evacuated system appears to be too complicated compared to plain black tubes.
Does not appear highly relevant to saturated water storage.

$25 per 6 foot evacuated Tube, no inner tube - water directly in glass? - [1] - affordable.

Max T that a metal can get when in full sun in 90C - see nice explanation at [2] combined with Stefan Boltzmann constant - [3]
So we see that the max temp in K varies as the 4th root of power density hitting the object. For example, to double the temp in K, need 16x solar concentration.
90C = 363K. Double to 726K - max 450C.
At this point, conduction and convection play a good role.
We need T to be 1.3x this
We need 482K, or 1.33x this K
1.33^4 = 3.1 - so only 3x concentration is needed if there were no other losses
Convective loss calculator - [4]

Paper on Medium Temperature Concentrators - overview for tech up to 250C. We need 200C for saturated water. [5]