Thermodynamic assessment of nanofluids as secondary coolants in refrigeration applications: design guidelines for nanofluids developers.

In a previous work, the suitability of nanofluids as secondary coolants in refrigeration systems was assessed aiming at the external irreversibilities of the heat exchangers of the refrigeration cycle. The results suggested that, in the case where the heat exchangers are optimized to minimize the rate of entropy generation, water-based nanofluids do not perform as well as pure water. Nanofluids enthusiasts, however, might advocate that nanosuspensions would allow the refrigerator to operate with lower condensing and higher evaporating temperatures, thus increasing the system COP. The present paper is therefore aimed at assessing both the internal and external irreversibilities of the refrigeration cycle when nanofluids are used as secondary coolants. The analyses pointed out that although most of the nanofluid research and development has been focused on the transport properties (viscosity, conductivity), the thermodynamic properties related with the thermal capacity (specific heat, density) showed to be the most influential factors on the total system COP. In the case of most water-based nanofluids, the nanoparticles have twice the density but only a quarter of the specific heat of the base fluid, reason why it is unlikely that such nanosuspensions will perform better than pure water.