Enhancing the coefficient of performance in vapour compression refrigeration systems with carbon nanotube lubricant additives: an experimental study

Abstract

The application of nanoparticles as an additive with the lubricant in refrigeration systems has been proved to bring efficient outcomes. Carbon nanotube nanoparticles (CNTNs) have been established as a viable solution to achieve efficient refrigeration and reduced power consumption at an affordable cost. POE oil has been extensively applied in Vapour Compression Refrigeration systems (VCRs) as it is compatible with most refrigerants and thermally stable. However, no study has been conducted so far to determine the optimal combination of CNTNs to be used with POE oil or its efficiency in achieving enhanced Coefficient Of Performance (COP), reduced power consumption and increased heat transfer capacity. In this background, this study investigates the impact of adding carbon nanotube nanoparticles in POE oil upon the performance of the compressor in Vapour Compression Refrigeration systems (VCRs). Further, it also intends to identify the optimal concentration of CNTNs to be added in POE oil for efficient performance. At first, the CNTNs were added with POE oil at varying concentrations such as 0.025 g L−1, 0.05 g L−1, 0.075 g L−1 and 0.1 g L−1 while characterisation techniques such as Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were applied to determine the morphological structure as well as the dispersion quality of the CNTNs. The results revealed optimal nanoparticle dispersion with diameters ranging from 10 to 30 nm. The stability of the nanoparticles was calculated using zeta potential value which confirmed that the CNTNs were within the acceptable stability limits (zeta value = 34 mV). At 0.075% concentration, a minimal specific wear rate was achieved compared to the base oil. The key performance measurement criteria considered for the CNTN included Coefficient of Friction (COF), viscosity and thermal conductivity. At 0.075% concentration, the COF value got reduced after which it increased, when the concentration increased up to 0.1% and 0.15%. The results confirmed that with the addition of nanoparticles, both viscosity as well as thermal conductivity (up to 12.54% compared to the base oil) increased though the former declined with an increase in the temperature. According to the study findings, the heat removal rate increased up to 23.31% while the compressor work got reduced up to 8.69%. Further, a 32% improvement in the coefficient of performance (COP) was observed at optimal concentration of the CNTN. These findings demonstrate that the CNTNs are highly effective additives to the lubricant and can significantly enhance the efficiency of the VCRs. Future research is recommended to develop a viscosity index for various oil types to better comprehend the effects of temperature on viscosity and stability. Further, machine learning and AI techniques can be involved through simulation models to identify the optimal criteria for the CNTN in terms of performance, efficiency, cost and availability so that both cost and time can be saved from the empirical investigations.