A thermodynamic study of water-based nanosuspensions as secondary heat transfer fluids in refrigeration systems

The suitability of nanosuspensions as secondary heat transfer fluids in refrigeration systems has been 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 have pointed out that despite most of the nanofluid research and development has been focused on the transfer properties (viscosity, conductivity), the transport 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 water-based nanofluids, the nanoparticles have twice the density but only a quarter of the specific heat of pure water, suggesting that such nanosuspensions are not likely to perform better than the base fluid in HVAC-R applications.