Complex sentence: The three rows of heliostats furthest away from the tower are having their mirrors replaced by photovoltaic cells, and some of them are to follow the sun, and some not, so as to measure the difference in efficiency.
Split into simpler sentences: The three rows of heliostats furthest away from the tower are having their mirrors replaced by photovoltaic cells. Of these cells, some will follow the sun while others will not, so as to measure the difference in efficiency.