Complex sentence: On the other hand, certain requirements of photobioreactors: cooling, mixing, control of oxygen accumulation and biofouling, make these systems more expensive to build and operate than ponds.
Split into simpler sentences: On the other hand, photobioreactors have several limiting factors, including: cooling, mixing, control of oxygen accumulation and biofouling. As a result, these systems more expensive to build and operate than ponds.