I see some logistical issues:
The lens would have to be HUGE to make a difference – Glass would be very heavy, and I am not sure plastic coule bear the heat.
Also, it seems this would need to be mounted well above ground to have the greatest likelihood of catching the largest amount of sun. How would this be built, and more to the point, how could it swivel and follow the sun’s path?
These people are doing something similar but are aiming the sun’s reflected rays instead of using lenses.