Bluerise has developed a system that uses cold seawater to cool down a fresh water loop. This loop is used for fresh water production. The goal of this study is to improve the overall heat transfer coefficient of the working medium in this system by at least 5%, which will also reduce its size. The overall heat transfer can be improved by adding various additives to the water, although this new medium also requires more power to pump it through the system. The examined additives are: immiscible- and miscible fluids, and nanoparticles. After a theoretical analysis it became clear that only the nanoparticles improve the heat transfer. Experiments showed that a 0.5 and 1 %vol Al2O3 nanoparticles suspended in water gives respectively an enhancement of the heat transfer coefficient of between 0.8% and 22.3% compared to water, not taken into account agglomeration or pollution of the system. However, with a higher pressure-drop, the ratio of pump power needed per heat transfer deteriorated by a maximum of 80%, so although the initial goal of a 5% improvement of heat transfer coefficient is achieved, additional impacts need to be taken into account.