Interesting, thanks for the link.
The one thing that is invariably missing from all these cute "alternative energy" projects is any economic analysis.
He says he heated five gallons of water from 70F to 110F in "a couple of hours". I calculated that it takes about 415kJ of energy to do that. 415kJ is about 0.12 kHh or about 1 cent worth of electricity.
Just think, if he repeats this twice a day, in just about 250 sunny days from now he'll have broken even on his $5 investment. Not counting the cost ductape for maintenance, of course.