In this paper, a comparative study of $CO_2$ and water-based brine secondary cooling systems for refrigeration applications is presented. $CO_2$ secondary systems are simple and could prove to be attractive for high temperature (HT) and medium temperature (MT) in industrial and commercial refrigeration applications regardless of climate. Calculations show that $CO_2$ secondary systems give a possibility of energy savings in the range of 10-20% for HT systems and more than 20-30% for MT systems. The design and particular challenges of $CO_2$ systems are discussed. Th analysis is illustrated by case studies. Further possibilities to optimized secondary $CO_2$ systems by modulating pressure or applying frequency converters are discussed. The use of secondary $CO_2$ systems in commercial refrigeration installations (supermarket systems) is briefly discussed.