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.