*8.4. Limitations in Air Conditioning and Refrigeration Systems*

Based on the research conducted by Hu et al. [499], it was shown that some carbonbased nanomaterials need surfactant(s) to ensure long-term stability and avoid agglomeration when they are mixed with the oil–refrigerant in the AC&R system. Additionally, surfactant might help to increase the performance of the AC&R system because it enhances

the nucleate pool boiling heat transfer coefficient. However, this improvement is only attained under limited conditions because it is highly dependent on certain specifications (e.g., nanoparticle type, nanoparticle size, nanoparticle concentration, surfactant type, surfactant concentration, nanolubricant concentration, base fluid type, and heat flux). In addition, Cheng and Liu [500] recommended further investigation on nucleate pool boiling and flow boiling for refrigerant based-nanofluids. Therefore, further investigations must be conducted toward carbon-based nanofluids with refrigerant–oil–surfactants for AC&R applications. According to Bahiraei et al. [501] and Dalkılıç et al. [502], nanofluids helped to improve the heat transfer coefficient in heat exchangers (i.e., spiral-type and double-type heat exchangers). However, due to the increase in the nanofluid's viscosity, the pressure drop can be escalated, especially for low mass flow rates [503,504]. In AC&R systems, the pressure drop is an important factor that needs to be incorporated during the system design phase because any additional increase in the pressure drop in the heat exchangers during the operation of the AC&R system will result in significant degradation in the system COP performance as reported by Sunardi et al. [505] and Tashtoush et al. [506]. Yet, pressure drop due to a carbon-based nanofluids additive has not been studied in AC&R systems and needs to be further investigated. In fact, engineers need robust software tools to design AC&R systems with nanofluid additives. Some studies provided correlations for all thermophysical properties of the nanofluids (heat transfer coefficient, friction factor, thermal conductivity, viscosity, etc.) [507–510]. However, computational models integrating component models (as employed in Bahman et al. [511] and Loaiza et al. [512]) and the influence of carbon-based nanofluids in AC&R systems are still lacking. In addition, those kinds of models might have the potential to predict the overall system performance and ensure optimal operations. Furthermore, experimental investigations were limited to a specific concentration amount of nanofluids. The optimal concentration for maximum AC&R performance can be obtained numerically with the formerly mentioned computational models. Moreover, the determination of the optimal amount of concentration has not been proposed yet in the literature for carbon-based nanofluids in AC&R systems. In the AC&R system, nanofluid additives ultimately enhance the viscosity of the lubricating oil. Conversely, this might relatively increase the compressor power consumption. Therefore, it is vivid to find the relationship between nanofluid viscosity and compressor pumping power for AC&R application [513–515]. A limited number of research employed energy and exergy analysis on carbon-based nanofluids applied in AC&R systems. They have shown that there is a high potential for decreasing the irreversibility with carbon-based nanofluids due to their higher thermal conductivity compared to oxide materials [516]. Therefore, more compressive studies similar to Bahman and Groll [517] are required to identify the AC&R components with major irreversibility when employing nanofluids. Moreover, the literature is scarce in techno-economic analysis for nanofluids in AC&R applications. Although Bhattad et al. [518] showed that nanofluids could result in a higher payback period than the AC&R's components (i.e., heat exchangers), however, further studies need to be conducted on carbon-based nanofluids because by optimizing the number of nanoparticles with respect to operating condition and stability, it can be more cost-effective. Finally, AC&R systems combine several components, as nanofluid pass through these components, it would be compressed, expanded, or changed phases. All these processes may lead to the possibility of getting nanoparticles to be separated from the carrier fluid during long-term operation and probably degrade the system performance. Therefore, the long-term operating performance of the nanorefrigerant (and nanolubricant) must be investigated.
