Thanks for the info. Some inter­est­ing info would now be, if the aver­age pow­er gen­er­a­tion (pho­to­volta­ic) is high­er or not then (maybe with some addi­tion­al (non-electrical) cool­ing improve­ments), and how much a fres­nel lens can cost from an eco­nom­ic point of view. But this high­ly depends on the instal­la­tion environment.

Regard­ing alter­na­tive pow­er gen­er­a­tion… I even have some links to DIY ther­mal col­lec­tors which look inter­est­ing, but with­out heavy changes in my house first (adding just the tubes from the heat­ing room to the roof means that I have to ren­o­vate the bath as I have to destroy a good part of it), I can not explore this inter­est­ing field.