Dispersion and Stability Behaviour of Graphene and MWCNT Based Nano-Lubricant for Vapour Compression Refrigeration (VCR) System

Incorporating nanoparticles into oil compressors marks a significant advancement in scientific research. It has been widely demonstrated that the inclusion of nanoparticles can markedly improve the system's efficiency by reducing energy consumption, decreasing evaporator temperatures, and increasing condenser temperatures. Nanoparticles made of carbon, including those of metal, metal oxide, and metal hybrids, are particularly valued for their superior properties and are deemed to have considerable potential for achievements. This study focuses on examining the stability of graphene and multi-walled carbon nanotubes (MWCNT) in polyolester (POE) oil as a precursor to replacing conventional lubricating oil in refrigeration systems separately. Graphene nano-lubricants were tested at concentrations ranging from 0.1 to 0.3 g/L with the selected surfactant, cetyltrimethylammonium bromide (CTAB), in a 1:1 ratio, while MWCNT nano-lubricants were tested from 0.5 to 0.7 g/L. All samples were prepared using a two-step method that involved magnetic stirring at 1250 rpm for 45 minutes, followed by ultrasonication for 30 minutes. The samples underwent evaluation through three methods of observation. Initial visual observation was conducted immediately after production, then after a 24-hour settling period, and finally, after seven days, the three most optimal samples were identified. For MWCNT, an additional stability test was necessitated due to the high concentration levels, making it challenging to determine the three most stable concentrations. The UV-VIS spectrophotometer played a crucial role in analyzing the stability of the samples, providing data that enabled more precise results. The absorption peak at a wavelength of 294 nm reached its highest absorption value of 0.892 (a.u.) at a concentration of 0.2 g/L. In the case of MWCNT, after testing all samples, concentrations of 0.5, 0.55, and 0.65 g/L were identified as the best options based on peak absorption at different wavelengths, with their peaks at 4.87, 4.045, and 4.116 (a.u.), respectively. The Zeta Potential Analyzer confirmed the stability of these concentrations, indicating a moderate stability with values around -37.4 mV. This level of stability was also observed in MWCNT samples, with the three chosen concentrations showing excellent stability at 88.4, -84.9, and -137 mV, respectively. Among all tested nano-lubricant samples, those with concentrations of 0.2 g/L and 0.65 g/L for Graphene/CTAB/POE and MWCNT/POE, respectively, demonstrated the highest stability and optimum performance. This suggests that these concentrations have the potential to serve as replacements for POE oil in refrigeration systems.