Parametric investigation of a chilled water district cooling unit using mono and hybrid nanofluids

This study presents a novel parametric investigation into the performance of a district cooling system using mono (Al2O3 and TiO2) and hybrid (Al2O3–TiO2) nanoparticles in the base fluids of water and ethylene–glycol water (EG-water) at a 20:80 ratio. The study analyses the effect of variables such as secondary fluid flow rate, evaporator and inlet temperatures, nanoparticle concentration, and air flowrate on the COP, total electrical energy consumption, and design of the district cooling unit. The analysis is performed with a thermal model developed and validated using operations data obtained from the McQuay chilled water HVAC unit operating in one of the facility plants at the Education City campus. The results of the study show that the use of nanofluids increased the overall heat transfer coefficient in the system by 6.6% when using Al2O3–TiO2/water nanofluids. The use of nanofluids in the evaporator also led to an average reduction of 23.3% in the total work input to the system and improved the COP of the system by 21.8%, 20.8% and 21.6% for Al2O3–TiO2/water, Al2O3/water, and TiO2/water nanofluids, respectively. Finally, an enhancement of 21.6% in COP was recorded for Al2O3–TiO2/EG-water nanofluids at a 5% nanoparticle volume concentration.


List of symbols
www.nature.com/scientificreports/ as an intercooler. The nanofluids were used in the shell side and the highest increment of 49.32% in COP was observed for the nanofluids at 0.75% concentration and a flow rate of 2 L/min, while a 12.2% reduction in power consumption was also recorded. The effectiveness and energy savings potential of methanol and silver/methanol nanofluids used in an HVAC unit were compared 29 . The results show that the nanofluid had an energy savings of about 8.1-31.5% and 18-100% when the supplied air stream in the HVAC unit is reheated. Alawi et al. 30 after a review of nanofluid in refrigeration systems concluded that nano-refrigerants have the potential to decrease the energy consumption of the system, while the use of nano-lubricants has the potential to improve the rate of heat transfer with the system. Finally, they stated that the use of nanoparticles can help improve the COP and rate of cooling of the refrigeration cycle. Some other studies on the use of nanofluids in both the vapour compression and absorption systems are listed in Table 1.
The use of nanofluids in HVAC has gained research attention over the past years. However, information on the performance of a new type of nanofluid that combines the properties of two or more nanoparticles known as hybrid nanofluids on HVAC systems is very scarce. Hybrid nanofluids have enhanced thermal and rheological properties due to the new chemical and physical bond formed within the fluid, these bonds result in better heat transfer characteristics for the resultant nanofluids [31][32][33][34][35] . Also, many studies on the HAVC system utilizes pure water-based nanofluids as a secondary fluid in the evaporator of the chiller. This, however, presents a technical restriction to their application, as water due to its freezing point is rarely used in chillers because of the possibilities of corrosion and icing in the flow channel. The commonly used fluids in these loops are inhibitor glycols (anti-freeze coolants) due to their much lower freezing points and anti-corrosion properties 1 . Studies using glycol-based nanofluids in this system for cooling are also rear in literature. On this basis, this study intends to fill these gaps and present a novel parametric investigation into the use of mono (Al 2 O 3 and TiO 2 ) and hybrid (Al 2 O 3 -TiO 2 ) nanoparticles in the base fluids of water and ethylene glycol water (EG/water) at 20:80 ratio for use in an HVAC unit. The study considers the effect of variables such as the secondary fluid flow rate, evaporator and inlet temperatures, nanoparticle concentration, and air flowrate on the COP, total energy consumption, and design of the HVAC unit. The analysis is performed with a thermal model developed on the Engineering Equation Solver (EES). For the simulations of the nanofluids, their thermal properties and Nusselt number correlations are determined using equations from literature.

System description and working fluids
System description. Figure 1 shows a schematic representation of the air-cooled chilled water HVAC unit being studied. The unit consists of 3 loops namely, the chiller (first loop), the secondary fluid loop, and the air handling unit loop. The first loop is the typical vapour compression refrigeration system operating with R134a as a refrigerant. At state 1, saturated vapour undergoes isentropic compression at the compressor to give high temperature, high pressure (same as condenser pressure) superheated vapour. At state 2, the vapour enters the condenser at a temperature higher than that of the surrounding where with the help of a fan (air-cooled), heat is rejected to the surrounding at constant pressure. The refrigerant leaves the condenser at state 3 as saturated vapour before it is throttled down with an expansion valve to the evaporator pressure. The throttling process drops the temperature of the refrigerant to below 0 C levels, which are well below the temperature of the working fluid in the secondary loop. As the R134a refrigerant goes through the tube side of the shell and tube evaporator, it absorbs heat from the working fluid in the secondary loop (in this case water, EG-water, or nanofluids) flowing in the shell side of the evaporator, until it becomes saturates vapour at state 1 and the whole process repeats. The flow rate of the chilled working fluid (water, EG-water, nanofluids) is maintained by a pump and it transfers its energy to the air distribution system in the building which consists of a series of fans and coils connected by air ducts. The distribution system (air handling units (AHUs)) circulates filtered air from the ventilated space and outside air through the fan coil with the help of a blower into the ventilated space.
Working fluids. This study evaluates the performance of eight different working fluids. Two different base fluids (water and EG-water at a ratio of 20:80 representing the percentage by volume of both fluids) are used and www.nature.com/scientificreports/ a comparison of their performance with Al 2 O 3 and TiO 2 nanoparticles in the base fluids are also investigated. The properties of the base fluids are obtained from the EES library, while that of the nanoparticles can be seen in Table 2.
Theoretical properties of the nanofluids. To calculate the properties of the nanofluids, the properties of the nanocomposite (Al 2 O 3 + TiO 2 ) must first be determined. The hybridization effects of the nanoparticles were studied at a ratio of 1:1 and each nanoparticle are represented by ϕ 1 for Al 2 O 3 and ϕ 2 for TiO 2 . The total concentration of the hybrid nanoparticle is obtained using Eq. (1) 31 .
To obtain the density of the equivalent hybrid nanoparticle ρ np the Eq. (2) is used The specific heat capacity of the nanocomposite can be obtained using Eq. (3).
The thermal conductivity of the nanocomposite is given as: where the subscripts np-1 and np-2 represents the equivalent properties of Al 2 O 3 and TiO 2 nanoparticles, respectively. The theoretical model for obtaining the density of the nanofluids which accounts for the variation in concentration at a constant temperature is given in Eq. (5) 43 .
where the subscripts nf, bf, and np represent nanofluid, base fluid, and nanoparticles, respectively, while ϕ represents the volume concentration of the nanoparticle.
The Eq. (6) below is used to theoretically obtain the specific heat capacity values of the nanofluid 43 .
The thermal conductivity of nanofluids is obtained using the Bruggeman model 44 . This equation is valid for spherical particles and considers the interaction between the particles. The Brinkman model is used to calculate the dynamic viscosity of the nanofluids 45 .
The equation for viscosity is a general equation that is used to investigate all nanofluids. It should also be noted that Eqs. (5)-(8) account only for the variation in the base fluid properties and do not account for nanoparticle size or temperature. However, these limitations are the same for all examined nanofluids and are proper for the comparison of both mono and hybrid nanofluids.

Theoretical modeling and simulation
The thermal modelling of the configuration in Fig. 1 is presented in this section. The system is modelled using the laws of conservation of energy, mass, and momentum. The dimensions and configurations of the key parameters are presented in Table 3. The system was modelled on the Engineering Equation Solver (EES) and the following assumptions are made.
• The system operates under steady-state conditions. • Potential and kinetic energy changes in the fluid streams are negligible.
• Heat transfer in the evaporator heat exchanger is adiabatic.
• Both the flow of the refrigerant and secondary loop working fluids are fully developed.
• Uniform heat transfer occurs along the tubes of the evaporator heat exchanger.
The general mass balance equation for each element is described as follows The general form of the energy balance equation used for any component can be written as follows. www.nature.com/scientificreports/ where E is the change in energy of a system, and this value is zero for any system operating under steady-state conditions. Q and Ẇ represents the heat transfer and work rates within a system. The mass flow rate and specific enthalpy at each state point is represented by ṁ and ḣ respectively.
Evaporator heat exchanger equations. Since the focus of the study is to investigate, the effect of using various nanofluids as a coolant in the shell and tube evaporator, detailed modelling of this component is needed to account for all possible thermal interaction within the unit. The refrigerant R134a enters and exits from the tube side of the heat exchanger, while the secondary loop working fluid (water, EG-water (20:80), or nanofluids) flows through the shell side of the heat exchanger. An expression for the rate of heat transfer in the evaporator is given by where F is the correction factor, which depends on the geometry of the heat exchanger and the inlet and outlet temperatures of the hot and cold fluid streams. Due to the condensation of the R134a refrigerant in the tube side of the heat exchanger, the value of F is 1. In this study, the hot fluid (T h ) is the nanofluids and the cold fluid (T c ) is the R134a refrigerant. Also, T 1 = T hi − T co and T 2 = T ho − T ci . The overall heat transfer coefficient (U) and the total heat transfer area (A o ) are determined by the following equations 46 .
The heat transfer relationship between the two working fluids in the evaporator heat exchanger can be expressed as www.nature.com/scientificreports/  where the area of the flow-through window A w = 1.04A s and the friction factor is given as 49 where C f is 0.2873 for Re h ≤ 10 4 and C f = 0.129for Re h ≥ 10 449 The Nusselt number (Nu) correlation for the Al 2 O 3 or TiO 2 nanofluids are calculated using the Pak and Cho correlation 50 The Nusselt number in the shell side of the heat exchanger with a 25% baffle cut is calculated using Kern's correlation 51 .
The correlation is valid for 10 < Re < 10 6 and provided the best agreement with the experimental results compared to other theoretical correlations.
The mass flow rate defines the rate at which the fluids passes through the shell or tube and it is expressed as: where v is the velocity of the fluid flowing inside the tube.
Another parameter used in this study to expresses the flow rate is the volume flow rate (L/min) and it is expressed as: Cooling coil. The cooling energy from the shell and tube evaporator, ventilated space cooling capacity from the fan coil unit (FCU) is obtained using Eqs. (30)- (35).
where h condensate is obtained at the enthalpy of vaporization of the secondary working fluid at T 8 , which equals zero in this study.
The nanofluid local heat transfer coefficient is calculated using the following equation:  www.nature.com/scientificreports/ The Nusselt number for turbulent flow valid for Reynold numbers between 2300 to 5 × 10 6 and Prandtl number between 0.5 to 2000 is obtained from the following equation 52 : The friction factor is presented in Eq. (30) 52 .
The pressure drop in the cooling coil tube is calculated using Eq. (35) 52 .
Work input. The work done by the system is the sum of the work done by the pump ( Ẇ p ) and the compressor ( Ẇ c ), these work equations are presented in Eqs. (36)- (38).
The pump work in the chilled water loop is expressed as; The total work in the system is expressed as; The ensemble coefficient of performance of the HVAC system is evaluated using.

Validation
The HVAC system was designed based on the design specifications provided by the McQuay centrifugal compressor water chillers. The manual of the McQuay system provided specific design parameters for the chiller with an evaporator pressure of 124.1 psi (855.7 kPa) and condenser pressure of 35 psi (241.3 kPa) 53 . For the given pressures, the temperatures at both evaporator and condenser can be calculated. The design parameter of the chiller was used as input values for the present model. The manufacturer specified that for the stated pressures, the flow rate of the refrigerant controls the cooling output of the chiller. The results of varying refrigerant flow rates on the cooling outputs for both the plant and the model are presented in Fig. 2. The model results show a good agreement with the operational results with a percentage error of less than 0.6%. www.nature.com/scientificreports/

Results and discussion
This section evaluates the effects of the base fluids, along with the mono and hybrid nanofluids on the performance of the HVAC unit. The effect of the parameters of nanoparticle volumetric concentration, volume flow rate, and operating temperatures of the evaporator and secondary loop on the system's performance is analytically investigated. These parameters are outlined in Table 4, along with the operating range considered in this study. The overall heat transfer coefficient, cooling energy, total work input, and COP of the HVAC are amongst the key performance outputs studied. Finally, an optimization of the key parameters listed in Table 4 is performed to obtain the optimum conditions of these parameters to achieve the maximum COP for each working fluid. In this study, the nanoparticle volume concentration is limited to 5%, as there are limited experimental studies on the behaviour of nanofluids beyond the volume concentration of 5% and it has been ascertained that higher concentrations of nanoparticles affect the stability of the nanofluids 14,42 . Also, the accuracy of Eqs. Effect of studied parameters on the overall heat transfer coefficient in the HVAC evaporator. The overall heat transfer coefficient in the evaporator is an important factor that determines the cooling effect obtained from the HVAC unit. This factor accounts for the individual heat transfer coefficient in the tube and shell sides of the evaporator. As seen in Eqs. (14)- (21), this parameter is a function of the properties of the working fluids on both sides of the evaporator heat exchanger, the mass flow rate of the working fluid, and the dimensions of the heat transfer surface in the evaporator. As the working fluid (R134a) flowing in the tube side of the evaporator is the same for all secondary fluids considered, the value of h i in the tube side will be the same for all considered working fluids in the shell side. The effect of using nanoparticles in water and EG-water solution is seen in Fig. 3. The presence of nanoparticles enhances the properties of the base fluids hence the resultant nanofluids have a much higher overall heat transfer coefficient than the base fluids (Fig. 3A). The water-based nanofluids perform better than the EG-water nanofluids because they possess much higher thermal conductivity and lower viscosity values. The highest enhancement of 7.8% in the overall heat transfer coefficient is seen with the use of Al 2 O 3 -TiO 2 / EG-water based nanofluids, while the Al 2 O 3 /EG-water and TiO 2 /EG-water nanofluids had 6.5% and 3.3% respectively. The enhancement recorded for the water-based nanofluids were 6.6%, 5.5%, and 2.7% for the Al 2 O 3 -TiO 2 / water, Al 2 O 3 /water, and TiO 2 /water nanofluids, respectively. All values are obtained at a flow rate of 200 L/min and nanoparticle concentrations of 5%.
The mass flow rate of the working fluid tends to favour the overall heat transfer coefficient in the evaporator as seen in Fig. 3B. The increase in the volume flow rate increases the Reynolds number of the flow stream and this results in a higher rate of heat transfer to the outer walls of the evaporator. However, the logarithmic nature of Fig. 3B shows that there exists an optimum flow rate after which further increase in flow rate will result in no significant enhancement in the overall heat transfer coefficient. Figure 3B also shows that the nanofluids performed relatively better than the corresponding base fluids although the effects of the nanoparticles are seen to decrease as the mass flow rate increases. This is due to the increased effect of turbulent flow as a result of the fluid flow rate exceeding the benefits of the fluid property from the use of the nanofluids. The Al 2 O 3 -TiO 2 hybrid nanofluids are seen to show the best performance ahead of Al 2 O 3 and TiO 2 nanofluids, respectively. Figure 4 shows the effect of the evaporator temperature (Fig. 4A) and the secondary fluid inlet temperature (Fig. 4B) on the overall heat transfer coefficient in the evaporator. The evaporator temperature represents the temperature of the refrigerant (in this case R134a) in the tube side of the evaporator heat exchanger. This temperature is fixed in most chillers but varies depending on the working fluid (refrigerant) used in the chiller as different refrigerants have different evaporative temperatures and can range anywhere between − 40 to 0 C. From Fig. 4A, it is observed that the value of the overall heat transfer coefficient decreases with an increase in the evaporator temperature. This is because the logarithmic temperature difference is greater at lower evaporator temperatures and this decreases as the temperature of the evaporator increases. Although the overall heat transfer coefficient in the evaporator decreases, the enhanced performance of the nanofluids is obvious from the figure. However, an inverse effect is seen when the inlet temperature of the secondary working fluid is increased (Fig. 4B). The overall heat transfer coefficient is generally improved for all working fluids because the properties of the working fluids are temperature-dependent and these fluids become less viscous at higher temperatures, resulting in higher values of Reynolds number (Eq. 18) and hence better heat transfer results. www.nature.com/scientificreports/ Effect of studied parameters on the cooling output of the HVAC unit. The effects of the studied parameters on the characteristic performance of the HVAC unit is analysed as a function of the cooling effect. The cooling effect reflects the quantity of energy used in cooling the ventilated space. In this section, this energy is represented in kilowatt (kW) or tons of refrigeration (R Ton), where 1 R Ton = 3.516 kW. Figure 5 shows the impact of nanoparticle loading on the cooling effect obtained from the HVAC unit. The cooling effect obtained from the nanofluids is seen to marginally increase, with the use of Al 2 O 3 -TiO 2 hybrid nanofluids showing the best performance for both the water and EG-water base nanofluids, respectively. The cooling output is increased due to the Brownian motion of particles in the nanofluids as the concentration of nanoparticles is increased. This random motion of particles aid in the thermal transport mechanism within each nanofluid. The more thermally enhanced hybrid nanofluids perform better due to the enhanced thermal conductivity properties of the nanoparticles as compared to Al 2 O 3 and TiO 2 nanofluids, respectively. Also, the higher thermal properties of water as compared to EG-water mean that the resulting water base nanofluids have better thermal properties than that of the EG-water nanofluids. The enhancement in cooling effect as a result of using nanofluids accounts for 1.5%, 1.2%, and 0.6% for Al 2 O 3 -TiO 2 , Al 2 O 3 , and TiO 2 water-based nanofluids respectively, while 1.8%, 1.5% and 0.8% were witnessed for the Al 2 O 3 -TiO 2 , Al 2 O 3 , and TiO 2 EG-water based nanofluids respectively at 5% nanoparticle concentration. The effect of the mass flow rate on both the working fluids in the secondary loop and the air flow rate across the fan coil unit is seen in Fig. 6. Similar to the trend observed in Fig. 3B, the effect of increasing the flow rate of the secondary fluid results in an increase in the cooling effect from the HVAC (Fig. 6A). The less viscous water and its nanofluids are seen to perform better than the EG-water solution. The enhanced properties of the nanofluids result in a better performance than their respective base fluids at the same flow rate. The air flow rate in the fan coil unit (Fig. 6B) shows, a similar trend to Fig. 6A. In both cases (FCU and evaporator), the convection coefficient is seen to increase with an increase in the flow rate across both heat exchangers. This increase would result in a higher cooling return from the HVAC unit. As shown in Fig. 6B, the cooling capacity is increased from 3.4 R Ton to over 19 R Ton as the airflow rate is adjusted from 0.2 to 6.2 m 3 /s. An average airflow rate of 0.16-0.18 m 3 /s is needed for 1 R Ton of cooling.  www.nature.com/scientificreports/ Effect of studied parameters on the work input to the HVAC unit. Figure 7A shows that the work done by the HVAC is decreased with nanoparticle loading. The major contributing factor to the work in the HVAC comes from the pump used to maintain the flowrate in the secondary loop. Figure 7A shows that the EG-water based nanofluids require more pump power than the water-based nanofluids, this is because of the much higher viscosity of EG-water as compared to pure water. At a constant mass flow rate, the use of nanofluids benefits the system, as less work is required by the pumps. An average of 23.29% reduction in the total work of the HVAC unit is achieved when using the nanoparticles at a volume concentration of 5%. Figure 7B shows the effect of the mass flow rate on the total work input in the HVAC. The pressure losses in the secondary loop are strongly correlated to the required pump work into the system. As the pressure losses increase due to an increase in flow rate, the amount of work required by the pump also increases. This influences the overall performance of the HVAC as seen with Eqs. (34) and (35). The water-based nanofluids as explained earlier are significantly less viscous than the EG-water nanofluids and hence witness much fewer pressure losses and pump work.
The effect of the evaporator temperature and inlet temperature of the working fluids on the pump work of the HVAC is seen in Fig. 8. The total work required by the HVAC is decreased with an increase in the evaporator temperature (Fig. 8A). It should be noted that this reduction in the work input into the system is largely due to a reduction in the compression work as the temperature of the evaporator increases. This reduction is due to the decrease in the temperature and pressure difference between states 1 and state 2. As can be seen from Fig. 8A, the pump work remains the same for all working fluids even as the evaporator temperature changes. The effect of the inlet temperature change on the pump work of the secondary loop is seen in Fig. 8B. The pump power required to maintain flow is seen to decrease as the inlet temperature increases owing to the reduction in the viscosity of the various working fluid with rising temperatures. This shows that the effect of fluid friction is minimized, and the pressure losses are decreased leading to a decrease in the pump requirements of the system. of the nanoparticle volume concentration on the COP of the system is seen in Fig. 9A. It is observed that an increase in nanoparticle loading increases the COP of the HVAC. The most beneficial concentration of nanoparticle for the flow rate considered is 5%, and the COP is seen to improve for both sets of the nanofluids studied. The enhancement in the water-based nanofluids are higher than those witnessed in the EG-water nanofluids, the Al 2 O 3 -TiO 2 /water nanofluid showed the best performance with a 21.9% enhancement while Al 2 O 3 /water and TiO 2 /water nanofluids had 20.8% and 21.6% enhancement, respectively. For the EG-water based nanofluids, the Al 2 O 3 -TiO 2 /EG-water nanofluid showed the best performance with a 21.6% enhancement while Al 2 O 3 /EGwater and TiO 2 /EG-water nanofluids witnessed a 20.5% and 21.2% enhancement, respectively. The COP performance for changing flowrate is seen in Fig. 9B. The decrease in COP is attributed to an increase in the power requirement of the HVAC system. This parameter affects all the working fluids studied, however, the nanofluids are still seen to perform better than the base fluids. Figure 10 shows the impact of the evaporator and inlet temperatures on the COP of the HVAC system. From Fig. 10A, the COP is increased as the evaporator temperature increases. A rule of thumb is that the COP improves by 2 to 4% for each C the evaporating temperature is raised. This is because an increase in the evaporator temperature increases the evaporator pressure and therefore less work is required by the compressor. Also, in Fig. 10B, the pump work decreases with an increase in the inlet temperature of the secondary fluids, and this favour an increase in the COP of the system. The use of nanofluids has the potential to impact the design of parts of the HVAC system. This is supported by Eq. (12), which proves that at a constant rate of heat transfer, the use of nanofluids can lead to a reduction in the required heat transfer surface of the heat exchanger. This outcome can result in improvements in the design of the size and volume of the evaporator used in the chiller.

Conclusion
This study presents a novel parametric investigation into the use of mono and hybrid nanoparticles in the chilled water loop of a district cooling unit. The study analysed the effect of several variables on the COP, total energy consumption and design of the HVAC district cooling unit. The analysis is performed using a thermal model developed on the Engineering Equation Solver (EES) and validated with operations data obtained from a McQuay chilled water unit operating in one of the facility plants at the Education City campus of Qatar Foundation. From the findings, the following conclusions are made: • The use of nanofluids was found to increase the rate of heat transfer in the HVAC system. The rate of cooling was significantly increased by adding the nanoparticles into the base fluids. • Within the range of nanoparticle concentration used in this study, it was discovered that increasing the volume concentration of nanoparticles in all the base fluid studied improves the performance of the district cooling system. • At a nanoparticle volume concentration of 5%, the use of nanofluids led to a 23.3% average reduction in the pump work of the district cooling system. The highest reduction in additional pump work recorded was 24.4% for TiO 2 /water nanofluid while the least observed was 22.18%, which was, recorded when Al 2 O 3 /EG-water nanofluid is used. • The overall heat transfer coefficient in the district cooling unit was increased by using the nanofluids. At a 5% volume concentration, a 6.6% enhancement in this parameter was recorded for Al 2 O 3 -TiO 2 /water nanofluid while 5.5% and 2.7% were obtained when using Al 2 O 3 /water and TiO 2 /water nanofluids, respectively. An enhancement of 7.8%, 6.5% and 3.3% was recorded when using Al 2 O 3 -TiO 2 , Al 2 O 3 , and TiO 2 EG-water nanofluids, respectively.   www.nature.com/scientificreports/ • The use of nanofluids in the evaporator improved the coefficient of performance of the district cooling system. Al 2 O 3 -TiO 2 /water nanofluid had the best performance with a 21.8% enhancement in COP while an increase of 20.8% and 21.6% were achieved with Al 2 O 3 /water and TiO 2 /water nanofluids, respectively. • The use of the hybrid nanofluids also recorded the highest enhancement in COP for the EG-water based nanofluids, as an enhancement of 21.6%, 20.5%, and 21.1% were recorded for Al 2 O 3 -TiO 2 /EG-water, Al 2 O 3 / EG-water, and TiO 2 /EG-water nanofluids, respectively. • The enhancement in cooling effect as a result of using nanofluids accounts for 1.47%, 1.24% and 0.63% for Al 2 O 3 -TiO 2 , Al 2 O 3 , and TiO 2 water-based nanofluids respectively, while 1.84%, 1.54%, and 0.81% were witnessed for the Al 2 O 3 -TiO 2 , Al 2 O 3 , and TiO 2 /EG-water nanofluids respectively. • The application of nanoparticles in the secondary loop improves heat transfer within the evaporator of the district cooling unit and can lead to a reduction in the required heat transfer surface of the heat exchanger. This outcome can help designers and engineers improve the size and volume of the evaporator used in the chiller.
This study shows that nanofluids can have a significant impact when used in the HVAC systems, a reduction in pump and compressor work, enhanced cooling output and improved COP are all benefits of using these fluids. As earlier stated this can lead to improved system design as these systems can now become more compact thereby lead to reduced cost, and the energy savings obtained as a result of its use can lead to a reduction in the carbon emission associated with electricity production. Future works on this should involve a pilot-scale study as well as observing the wider effect of nanoparticles on a district cooling network. We expect that this would reveal some other physical quantities and benefits not accounted for in this theoretical study.    Figure 10. Effect of (A) evaporator temperature and (B) secondary fluid inlet temperature on the COP of the HVAC.