*6.3. Air Conditioning and Refrigeration Systems*

Air conditioning (AC) is a process used to controls air's thermal and physical properties and then supply it with cooling or heating to an allocated area from its central plant or rooftop units. It also maintains and controls the temperature, humidity, air cleanliness, air movement, and pressure differential in a space within predefined limits so that conditioned space occupants or products enclosed satisfy comfort and health standards [391]. A typical AC or refrigeration system uses a vapor compression cycle to accomplish cooling or heating. The vapor compression cycle consists mainly of a compressor, an evaporator, a condenser, an expansion device, indoor and outdoor fans, and a working fluid. Additionally, secondary heating and cooling loops are implemented to accommodate more extensive systems, as shown in Figure 22. The AC system is potentially used for providing a clean, healthy, and comfortable indoor environment, and saving energy by developing high-efficiency equipment in residential and industrial sectors. However, none of these uses come without associated challenges. The AC&R systems can be operating with a very high temperature lift (different between heat source and heat sink temperatures). For instance, the AC system operates in hot and dry climate countries needs to maintain indoor temperature as low as 18.3 ◦C (65◦ F) [392], whereas the refrigeration system needs to run continuously for long hours to sustain freezing chamber temperatures [393]. As a result, the AC&R systems generate a tremendous amount of heat loss to the environment during the compression process, which increases the pressure ratio across the compressor and degrades its efficiency; it increases the compressor discharge temperature and jeopardizes its reliability. Simultaneously, the cooling demand is compromised and the AC or the refrigeration system strives to provide enough cooling in the unit's evaporator (or reject heating in the unit's condenser) and therefore degrades the overall system coefficient of performance (COP).

loops.

**Figure 22.** Schematic for a typical vapor compression system with secondary heating and cooling **Figure 22.** Schematic for a typical vapor compression system with secondary heating and cooling loops.

One of the methods to improve the COP of AC&R systems is to reduce the power consumption in the compressor. Many researchers have already shown that adding nanoparticles to the compressor oil (nanolubricant) reduced its energy consumption because it enhanced the lubricating oil's tribological and thermal properties, which helped improve the compression process, and therefore increased the system COP [394–402]. Lee et al. [403,404] studied the effects of adding nanoparticle to mineral oil. Their results showed that the improvement in the lubricating properties of the mineral oil increases with the addition of the nanoparticle. The authors found that adding the nanoparticle to the compressor oil decreased its friction coefficient by 90%, and thus causing improvement in the compression process and reducing the energy losses in the compressor. Jia et al. [405] investigated the effects of using mineral-based nano-oils in a domestic refrigerator compressor with two different refrigerants, namely, R-134a and R-600. They concluded that the One of the methods to improve the COP of AC&R systems is to reduce the power consumption in the compressor. Many researchers have already shown that adding nanoparticles to the compressor oil (nanolubricant) reduced its energy consumption because it enhanced the lubricating oil's tribological and thermal properties, which helped improve the compression process, and therefore increased the system COP [394–402]. Lee et al. [403,404] studied the effects of adding nanoparticle to mineral oil. Their results showed that the improvement in the lubricating properties of the mineral oil increases with the addition of the nanoparticle. The authors found that adding the nanoparticle to the compressor oil decreased its friction coefficient by 90%, and thus causing improvement in the compression process and reducing the energy losses in the compressor. Jia et al. [405] investigated the effects of using mineral-based nano-oils in a domestic refrigerator compressor with two different refrigerants, namely, R-134a and R-600. They concluded that the COP values increased by 5.33% when the nano-oil was utilized in the compressor with R-600, whereas no effects were noticed when the same nano-oil was used with R-134a.

COP values increased by 5.33% when the nano-oil was utilized in the compressor with R-600, whereas no effects were noticed when the same nano-oil was used with R-134a. Another method to improve the cooling COP is to increase the heat transfer coefficient in the heat exchangers of the AC&R system. Many studies have already shown that mixing nanoparticles with the refrigerant enhanced the heat transfer coefficient of the refrigerant (nanorefrigerant) in the condenser and the evaporator because of the additional nucleate boiling and the higher thermal conductivity of the nanoparticles that enhanced the heat transfer rate, and thus increasing the system COP [286,395,401,406–411]. Since carbon-based nanofluids (i.e., ND, graphene, CNTs, etc.) have better performance due to their superior features compared to other known nanomaterials [17,406,412–418], they could result in significant system performance improvement. Therefore, researchers have further investigated carbon-based nanoparticles for various AC&R applications [419], such as the ones demonstrated in Figure 23. The following sections present a literature review on studies investigating the effect of carbon-based nanoparticles on the thermo-Another method to improve the cooling COP is to increase the heat transfer coefficient in the heat exchangers of the AC&R system. Many studies have already shown that mixing nanoparticles with the refrigerant enhanced the heat transfer coefficient of the refrigerant (nanorefrigerant) in the condenser and the evaporator because of the additional nucleate boiling and the higher thermal conductivity of the nanoparticles that enhanced the heat transfer rate, and thus increasing the system COP [286,395,401,406–411]. Since carbonbased nanofluids (i.e., ND, graphene, CNTs, etc.) have better performance due to their superior features compared to other known nanomaterials [17,406,412–418], they could result in significant system performance improvement. Therefore, researchers have further investigated carbon-based nanoparticles for various AC&R applications [419], such as the ones demonstrated in Figure 23. The following sections present a literature review on studies investigating the effect of carbon-based nanoparticles on the thermophysical properties of AC&R refrigerants, followed by a literature review on studies investigating the effect of carbon-based nanoparticle on the AC&R system's performance.

physical properties of AC&R refrigerants, followed by a literature review on studies investigating the effect of carbon-based nanoparticle on the AC&R system's performance.

**Figure 23.** Nanofluids employment in AC&R applications, namely; air conditioning units, air handling units, industrial refrigerators, and domestic refrigerators. **Figure 23.** Nanofluids employment in AC&R applications, namely; air conditioning units, air handling units, industrial refrigerators, and domestic refrigerators.

6.3.1. Influence of Carbon-Based Nanoparticles on the Thermophysical Properties of Working Fluid in AC&R Systems 6.3.1. Influence of Carbon-Based Nanoparticles on the Thermophysical Properties of Working Fluid in AC&R Systems

It is evident from the literature that there are many researchers investigated the performance properties (i.e., heat transfer coefficient and viscosity) of nanoparticles applied to the refrigerants in AC&R systems including copper (Cu), aluminum (Al), nickel (Ni), copper oxide (CuO), zinc oxide (ZnO), aluminum oxide (Al2O3), titanium oxide (TiO2), and other metal nanoparticles [325,419–434]. However, only a limited number of research work is available for ND, graphene, and CNTs, which can be summarized in Table 10. Park and Jung [435] investigated the possible contribution of CNT on the nucleate boiling heat transfer coefficients of R-123 and R-134a. They reported an enhancement up to 36.6% in nucleate boiling heat transfer coefficients of the nanorefrigerant at low heat flux compared to the baseline refrigerant. However, as the heat flux increases the enhancement decreased due to robust bubble generation that prevented the CNT from penetrating the thermal boundary layer and touch the surface. The flow boiling heat transfer characteristics and pressure drop were also investigated experimentally by Zhang et al. [436], using MWCNT dispersed in the R-123 refrigerant with SDBS surfactant flowing in a horizontal circular tube heat exchanger. Their results showed that the nanorefrigerant heat transfer coefficient and frictional pressure drop increased with the increase of nanoparticle concentration, mass flux, and vapor quality. Similar conclusions were observed by Sun et al. [437] when they investigated MWCNT with R-141b. Jiang et al. [438] studied the influence of CNT diameters and aspect ratios on CNT–R-113 nanorefrigerant. The study involved four different groups of CNTs with different physical dimensions (diameters, length, and aspect ratio). Their experimental results showed that the thermal conductivities of CNT nanorefrigerant increased proportionally with the increase of CNT's volume fraction and aspect ratio and with the decrease of CNT's diameter. The maximum increase in the thermal conductivity was about 104% for a volume fraction of 1.0 vol. %. Peng et al. [439] studied the influence of CNT physical dimensions such as diameters, length, and aspect ratios for the CNT–R-113–oil mixture. They used the same four different groups of CNTs with different physical dimensions as Jiang et al. [438] and VG68 ester lubricating oil. An enhancement of up to 61% was obtained in the nucleate pool boiling heat transfer coeffi-It is evident from the literature that there are many researchers investigated the performance properties (i.e., heat transfer coefficient and viscosity) of nanoparticles applied to the refrigerants in AC&R systems including copper (Cu), aluminum (Al), nickel (Ni), copper oxide (CuO), zinc oxide (ZnO), aluminum oxide (Al2O3), titanium oxide (TiO2), and other metal nanoparticles [325,419–434]. However, only a limited number of research work is available for ND, graphene, and CNTs, which can be summarized in Table 10. Park and Jung [435] investigated the possible contribution of CNT on the nucleate boiling heat transfer coefficients of R-123 and R-134a. They reported an enhancement up to 36.6% in nucleate boiling heat transfer coefficients of the nanorefrigerant at low heat flux compared to the baseline refrigerant. However, as the heat flux increases the enhancement decreased due to robust bubble generation that prevented the CNT from penetrating the thermal boundary layer and touch the surface. The flow boiling heat transfer characteristics and pressure drop were also investigated experimentally by Zhang et al. [436], using MWCNT dispersed in the R-123 refrigerant with SDBS surfactant flowing in a horizontal circular tube heat exchanger. Their results showed that the nanorefrigerant heat transfer coefficient and frictional pressure drop increased with the increase of nanoparticle concentration, mass flux, and vapor quality. Similar conclusions were observed by Sun et al. [437] when they investigated MWCNT with R-141b. Jiang et al. [438] studied the influence of CNT diameters and aspect ratios on CNT–R-113 nanorefrigerant. The study involved four different groups of CNTs with different physical dimensions (diameters, length, and aspect ratio). Their experimental results showed that the thermal conductivities of CNT nanorefrigerant increased proportionally with the increase of CNT's volume fraction and aspect ratio and with the decrease of CNT's diameter. The maximum increase in the thermal conductivity was about 104% for a volume fraction of 1.0 vol. %. Peng et al. [439] studied the influence of CNT physical dimensions such as diameters, length, and aspect ratios for the CNT–R-113–oil mixture. They used the same four different groups of CNTs with different physical dimensions as Jiang et al. [438] and VG68 ester lubricating oil. An enhancement of up to 61% was obtained in the nucleate pool boiling heat transfer coefficient compared to R-311–oil mixture without CNTs. They also showed that the improvement of the nucleate pool boiling heat transfer coefficient increased as the CNTs length increases

and as CNTs outside diameter decreases. The heat transfer performance of MWCNT–oil– R-600A nano-refrigerant in horizontal counter-flow double-pipe heat micro-fin tube heat exchanger, was studied by Ahmadpour et al. [440]. Their experiments covered a wide range of parameters, including mass velocity, vapor quality, and condensation pressure. Their results showed that an increase up to 74.8% in the heat transfer coefficient could be achieved with 0.3% nanoparticles concentration at 90 kg/m<sup>2</sup> .s mass velocity compared to the pure refrigerant. Kumaresan et al. [441] conducted an experimental study on the convective heat transfer characteristics of secondary refrigerant nanofluids in a tubular heat exchanger. The objective of the secondary refrigerant loop is to reduce the primary refrigerant charge in vapor compression refrigeration systems. The nanofluid used in the study consists of MWCNT dispersed in a water-EG mixture. Their results showed that the maximum enhancement in convective heat transfer coefficient was 160% for the nanofluid containing 0.45 vol. % of MWCNT compared to the base fluid. However, the friction factor was also increased by 8.3 times, which might increase the pumping power and reduce the advantage of the increase in the heat transfer coefficient of the nanofluid [442]. Similar findings were attained by Baskar et al. [443] and Wang et al. [444] when they experimentally tested MWCNT–IPA and graphene–EG in a secondary refrigeration loop, respectively.

The dispersion stability of MWCNT in the R-141b refrigerant with the addition of surfactant was investigated by Lin et al. [445]. Three different types of surfactants, including SDBS, hexadecyl trimethyl ammonium bromide, and nonylphenoxpoly ethanol (NP-10), were tested to prevent the aggregation and sedimentation of MWCNTs during the long-term operation. SDBS was found to have an excellent adsorption ability on the MWCNT surface. It was also shown that the relative concentration increased with decreasing MWCNT length or outer diameter and increasing ultrasonication time. The optimal SDBS concentration for the highest dispersion stability increased proportionally with the increase of the initial MWCNT concentration. However, the SDBS might reduce the nanorefrigerant's thermal conductivity at higher operating temperatures. The thermophysical properties and heat transfer performance of SWCNTs dispersed in the R-134a refrigerant was also investigated by Alawi and Sidik [446]. They found that up to a 43% increase in thermal conductivity can be reached when 5 vol. % of nanoparticle concentration is used in the MWCNT–R-134A nanorefrigerant compared to the pure R-134A refrigerant. Similar to other nanofluids, the thermal conductivity increases with the increase of nanoparticle volume concentrations and with the increase in the temperature of the nano-refrigerant. Moreover, the increase of volume fractions at a constant temperature led to a significant increase in the viscosity and density of the nanorefrigerant.

Dalkilic et al. [447] investigated the stability and viscosity of MWCNTs–polyolester (POE) oil nanolubricants. The study involved using four different refrigeration compressor oil with different values of viscosity (i.e., 32 mm2/s, 68 mm2/s, 100 mm2/s, and 220 mm2/s) tested at a maximum temperature of 50 ◦C and a concentration of MWCNTs up to 1 wt %. They reported a substantial augmentation in viscosity up to 90% compared to the viscosity of the base oil. This could reduce the refrigeration efficiency due to the possible increase in the compressor pumping power. Most of the review studies [325,419,420,431,448] have shown that adding nanoparticles always enhances the heat transfer coefficient of the nanofluid mixture due to the higher thermal conductivity of nanorefrigerant and due to the reduction of the thermal boundary layer thickness caused by the presence of nanoparticles. Additionally, nanoparticles increased the viscosity of the nano-refrigerant causing an increase in the frictional pressure drop and therefore might reduce the AC&R system performance. The review studies of references [325,419,420,431,448] covered only CNTs nanomaterial from the carbon family, and therefore further investigations on other types of carbon-based nanoparticles, such as diamonds and graphene, needs to be conducted.


**Table 10.** List of studies related to carbon-based nanoparticles effect with working fluid in AC&R systems.

6.3.2. Influence of Carbon-Based Nanofluids on the COP and Overall Cooling Performance of AC&R Systems

A limited number of studies are available on how ND, graphene, and CNTs improve system COP and cooling capacity, which can be summarized in Table 11.

Abbas et al. [449] examined CNT mixed with POE oil in an R-134a refrigeration unit. They found that the system COP increased by 4.2% with nanoparticle concentration of 0.1 wt %. The experiment was infeasible beyond this concentration because the main challenge was with the agglomeration due to the strong Van der Waals interactions during the preparation phase. Jalili et al. [450] mixed various concentrations of MWCNT with water to assess the cooling performance of the secondary fluid in the evaporator of the refrigeration system. The transient analysis results showed that the evaporator's inlet temperature increased by 6.5% while the outlet temperature decreased by 14.5% when the water contains 2000 ppm of MWCNT. The significant enhancement in evaporator outlet temperature confirmed the tremendous increase in heat transfer coefficient with MWCNT. According to Kruse and Schroeder [451] and Cremaschi [452], the existence of oil lubricant in heat exchangers acted as insulation and resulted in heat transfer coefficient reduction. However, if the addition of nanoparticles enhanced the oil lubricant, the heat transfer coefficient would be compensated in the heat exchangers due to the improved overall thermophysical properties. Vasconcelos et al. [453] examined MWCNT–water as a secondary fluid in a 4–9 kW refrigeration unit with R-22 as a refrigerant. Due to the high thermal conductivity of the nanofluid, the cooling capacity increased up to 22.2% at the coolant's inlet temperature range of 30–40 ◦C. Vasconcelos et al. [453] found no significant reduction in the total power consumption. However, the increase in cooling load helped the compressor power consumption to relatively reduced because of the relative increase in evaporation pressure, and therefore the COP increased up to 33.3%. Kamaraj and Manoj Babu [454] replaced the POE oil with POE–mineral oil nano lubricants containing CNT

particles with the amount of 0.1 and 0.2 g/L in a R-134a refrigerator. Besides the reduction in cooling time using the new nanofluids by approximately 40%, the COP was improved by 16.7% for 0.2 g/L of CNT using mineral oil. This is mainly due to the enhancement of the heat transfer coefficient in the evaporator without any significant reduction in the compressor power. Yang et al. [455] analyzed graphene nanosheets blended with SUNISO 3GS refrigerant oil in a R-600a refrigerator/freezer. The authors found that the cooling rate freezing rate improved by approximately 5.6% and 4.7%, respectively. The energy analysis yielded that the three concentrations nanolubricants (10 mg/L, 20 mg/L, and 30 mg/L) helped in reducing the compressor discharge temperature by 2.5%, 3.8%, and 4.6%, and dropping the energy consumption by 14.8%, 18.5%, and 20.4%, respectively. Hence, the energy saving was estimated to be up to 20% compared to using pure refrigerant oil. Indeed, the addition of graphene nanosheets with lubricant oil helped to decrease compressor friction losses. However, using graphene nanosheets as nanolubricant required additional surfactants (dispersants), which might increase the compression power because surfactants increase the viscosity and reduce the thermal conductivity at higher operating temperatures. Pico et al. [456] investigated two mass concentrations of ND–POE in a 17.6 kW vapor compression refrigeration system that used variable-speed compressor and refrigerant R-410A. The results showed that the compressor power consumption remained the same due to the type of compressor (i.e., hermetic scroll). On the other hand, the discharge compressor temperature reduced by approximately 3 ◦C and 4 ◦C, while the cooling capacity increased by 4.2% and 7%. Therefore, the overall system COP increased by 4% and 8% at 0.1% and 0.5% mass concentrations, respectively. Furthermore, Pico et al. [457] experimentally investigated the same ND–POE nanolubricant with R-32 as a substitute for R-410A. The results showed that for 0.5% mass concentration of diamond nanoparticle added to POE lubricant, the cooling capacity increased by 2.4% and the discharge compressor temperature decreased by approximately 2 ◦C, and hence the COP enhanced by 3.2%. The reduction of ND–POE performance with R-32 compared to R-410A can be justified with the low mass flow rate, which affected the oil circulation rate of the system operating with R-32.


**Table 11.** List of studies related to carbon-based nanoparticles effect on AC&R systems performance.
