Nanorefrigerants are created by adding nanoparticles in suspension to a base refrigerant. They have a thermal conductivity that is 15% to 104% higher than that of the corresponding base refrigerants and contribute to improve the coefficient of performance (COP) of the refrigeration systems in which they are used. However, an increase in nanoparticle concentration also increases viscosity, which may be detrimental to the thermal performance of nanorefrigerants.
What is a nanorefrigerant ?
Nanorefrigerants are created by adding nanoparticles in suspension to a base refrigerant.
Nanoparticles are solid particles with a mean size of 1–100 nanometre (nm). A nanometre (nm) is one-billionth of a metre (10-9). Several types of nanoparticles can be used to create nanofluids: metal nanoparticles, metal oxide nanoparticles, silicon nanoparticles, silica nanoparticles, carbon nanotubes (CNT), etc.  CNTs are carbon nanofibres with graphene layers wrapped into perfect cylinders. CNTs can be either single-walled (SWCNT) with a one dimensional cylindrical shape or multi-walled (MWCNT) with multiple rolled layers in concentric tubes, as schematized on the image.
Thermophysical properties of nanorefrigerants
The increase in thermal conductivity resulting from the introduction of nanoparticles improves heat transfer performance. Several studies have shown that nanorefrigerants have a thermal conductivity that is 15% to 104% higher than the corresponding base refrigerants. , 
Factors affecting the thermal conductivity of nanorefrigerants include: nanoparticle type, nanoparticle size, nanoparticle concentration, temperature, type of base refrigerant and the thermophysical properties of both the base refrigerant and the nanoparticles.  For instance, higher nanoparticle concentration or smaller nanoparticle size have been reported to lead to an increased thermal conductivity.
In one experiment using carbon nanotubes (CNT), the thermal conductivity of the R113/CNT nanorefrigerant was 50-104% higher than that of the base R113 refrigerant, with a CNT volume fraction of 1.0 %. Several types of CNTs were tested, revealing that the diameter of carbon nanotubes has a significant effect on thermal conductivity enhancement: the smaller the diameter of the CNT, the higher the thermal conductivity of the R113/CNT nanorefrigerant. 
It should be noted that the mass fraction of nanoparticles continuously declines over time, due to alternating processes of condensation and evaporation. Eventually, the decrease in the mass fraction of suspended nanoparticles may result in a decline in the thermal conductivity of the nanorefrigerant. 
Viscosity, like thermal conductivity, is a property influencing heat transfer performance. Several experiments have found that the viscosity of nanorefrigerants increases with increasing nanoparticle concentration. However, it decreases with increasing temperature.  One experiment found the viscosity of the Al2O3/R134a nanorefrigerant to be 13.68% higher than the viscosity of R134a alone, over a temperature range of 283K to 308K.  Higher viscosity worsens heat exchange and increases the amount of energy required to circulate the nanorefrigerants, due to a higher friction factor. 
Performance of refrigeration systems using nanorefrigerants
Performance of vapour compression systems
Various experimental studies have reported a COP rise of about 5–15% when using nanorefrigerants. ,  One study reported a COP rise of 43.93% for Al2O3/R600a compared to base refrigerant R600a.  In practical applications involving high capacity chillers (about 1800–3500 kW), a COP increase of about 5% can be achieved, which saves a considerable amount of energy. 
Performance of absorption systems
Recent studies have shown that adding nanoparticles to the working fluid of absorption refrigeration systems can improve their COP. , ,  For instance, in an experiment with a diffusion absorption refrigeration system (DARS), the working fluid was an ammonia-water mixture with 2% mass fraction of Al2O3 particle concentration. Results showed a 5% increase in COP.  In an experiment testing a novel hybrid fuel cell-absorption refrigeration system using Ag-water as the working fluid, results showed an 18.5% increase in COP. 
Useful links for further information
For further information, the following documents are available for download on FRIDOC.
Articles from the International Journal of Refrigeration
Pourfayaz F., Imani M., Mehrpooya M., & Shirmohammadi R. (2019). Process development and exergy analysis of a novel hybrid fuel cell-absorption refrigeration system utilizing nanofluid as the absorbent liquid. International Journal of Refrigeration, 97, 31-41. https://iifiir.org/en/fridoc/process-development-and-exergy-analysis-of-a-novel-hybrid-fuel-141564Yıldız G., Ağbulut Ü., & Gürel A. E. (2021). A review of stability, thermophysical properties and impact of using nanofluids on the performance of refrigeration systems. International Journal of Refrigeration. https://iifiir.org/en/fridoc/a-review-of-stability-thermophysical-properties-and-impact-of-using-144038
Simulation of a solar cooling system using nanofluids and membrane-based components in the absorption chiller. (CYTEF 2020) https://iifiir.org/en/fridoc/simulation-of-a-solar-cooling-system-using-nanofluids-and-143075
 A. Bhattad, J. Sarkar, et P. Ghosh, « Improving the performance of refrigeration systems by using nanofluids: A comprehensive review », Renew. Sustain. Energy Rev., vol. 82, p. 3656‑3669, févr. 2018, doi: 10.1016/j.rser.2017.10.097.
 S. S. Sanukrishna, M. Murukan, et P. M. Jose, « An overview of experimental studies on nanorefrigerants: Recent research, development and applications », Int. J. Refrig., vol. 88, p. 552‑577, avr. 2018, doi: 10.1016/j.ijrefrig.2018.03.013.
 E. C. Okonkwo, I. Wole-Osho, I. W. Almanassra, Y. M. Abdullatif, et T. Al-Ansari, « An updated review of nanofluids in various heat transfer devices », J. Therm. Anal. Calorim., juin 2020, doi: 10.1007/s10973-020-09760-2.
 W. Jiang, G. Ding, et H. Peng, « Measurement and model on thermal conductivities of carbon nanotube nanorefrigerants », Int. J. Therm. Sci., vol. 48, no 6, p. 1108‑1115, juin 2009, doi: 10.1016/j.ijthermalsci.2008.11.012.
 S. Bobbo, B. Buonomo, O. Manca, S. Vigna, et L. Fedele, « Analysis of the Parameters Required to Properly Define Nanofluids for Heat Transfer Applications », Fluids, vol. 6, no 2, Art. no 2, févr. 2021, doi: 10.3390/fluids6020065.
 I. M. Mahbubul, A. Saadah, R. Saidur, M. A. Khairul, et A. Kamyar, « Thermal performance analysis of Al2O3/R-134a nanorefrigerant », Int. J. Heat Mass Transf., vol. 85, p. 1034‑1040, juin 2015, doi: 10.1016/j.ijheatmasstransfer.2015.02.038.
 M. Aktas, A. S. Dalkilic, A. Celen, A. Cebi, O. Mahian, et S. Wongwises, « A Theoretical Comparative Study on Nanorefrigerant Performance in a Single-Stage Vapor-Compression Refrigeration Cycle », Adv. Mech. Eng., vol. 7, no 1, p. 138725, janv. 2015, doi: 10.1155/2014/138725.
 V. Nair, P. R. Tailor, et A. D. Parekh, « Nanorefrigerants: A comprehensive review on its past, present and future », Int. J. Refrig., vol. 67, p. 290‑307, juill. 2016, doi: 10.1016/j.ijrefrig.2016.01.011.
 F. Pourfayaz, M. Imani, M. Mehrpooya, et R. Shirmohammadi, « Process development and exergy analysis of a novel hybrid fuel cell-absorption refrigeration system utilizing nanofluid as the absorbent liquid », Int. J. Refrig., vol. 97, p. 31‑41, janv. 2019, doi: 10.1016/j.ijrefrig.2018.09.011.
 A. Sözen, E. Özbaş, T. Menlik, M. T. Çakır, M. Gürü, et K. Boran, « Improving the thermal performance of diffusion absorption refrigeration system with alumina nanofluids: An experimental study », Int. J. Refrig., vol. 44, p. 73‑80, août 2014, doi: 10.1016/j.ijrefrig.2014.04.018.
(Image source: Anwar T, Kumam P, Khan I, Watthayu W. Heat Transfer Enhancement in Unsteady MHD Natural Convective Flow of CNTs Oldroyd-B Nanofluid under Ramped Wall Velocity and Ramped Wall Temperature. Entropy. 2020; 22(4):401. https://doi.org/10.3390/e22040401 )
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