"The only waste product from silicon power stations would be large amounts of solid silicon-dioxide “ash” but this could be recycled back to the smelters to be reduced to silicon again."
So, if ash is a 'waste product' then it cannot be produced with 100% efficiency as the 'non-waste' is the goal of the process and not the 'waste' itself. Also, if you take that 'waste' and 'recycle it back to smelters', another round of inefficiency is introduced (as no physical process is 100% efficient).