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Session: Poster session and Ouzo tasting in room Kallirhoe
Session starts: Monday 09 September, 17:20

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George Kosmadakis (National Center for Scientific Research “Demokritos”)
Panagiotis Neofytou (National Center for Scientific Research “Demokritos”)

Abstract:
The mixing of nanoparticles (NPs) with refrigerants introduces some favourable effects, with the main being the heat transfer enhancement during boiling and condensation. However, the resulting effective viscosity increases for higher NP loading, resulting to a larger pressure drop. Various nanorefrigerants have been examined in refrigeration and heat pump units, demonstrating the heat transfer coefficient increase. The present work provides some further insights of nanorefrigerants in an ORC supplied by a low-temperature heat source of 100 oC, employing R245fa and R1234ze(Z) as the base fluids. For this purpose, an existing thermodynamic model is extended to account for these particles, introducing their effect in the energy balance, and the heat transfer and pressure drop correlations. Two NP materials with different thermophysical properties are considered, already used in many refrigeration cases, namely Al2O3 and Cu. The numerical model has been applied to simulate the ORC with a variable NP mass fraction (from 0 up to 10% wt.), with the aim to identify the performance improvement. Focus is given on the heat transfer coefficients in the evaporator and the condenser, as well as the pressure drop in the different parts of the cycle. The results show that the thermal efficiency slightly increases for higher NP mass fraction and especially with the use of Al2O3. This is mostly attributed to the expansion process, in which the NPs act as internal heat sources to the vapour organic fluid during this process. The effect of the NPs on the heat transfer process in the two heat exchangers is minor, due to their low volume fraction in the vapour phase, resulting to small changes in the thermal conductivity. The increase in the pressure drop is minor even if the effective viscosity increases for higher NP mass fraction. Finally, a preliminary cost analysis of the nanorefrigerants in an ORC has been conducted, showing that their use results to a payback period of 12.8 years with Al2O3. The sensitivity analysis revealed that this period can be reduced to half, assuming that they can sustain a long-term operation under cycling conditions.