Transient Thermal Analysis of a Solar-Assisted AHU by Focusing on Heat Recovery and Nanoparticles: Jeddah Climate Zone

In the Jeddah climate region, a lot of energy is assigned to the air handling unit (AHU) sector, which should be reduced by using energy-efficient solutions. As the air passes through the cooling coil, a lot of energy is consumed to reduce the temperature along with humidity so that if the air is precooled in the previous stages, energy consumption in this energy-intensive section will be diminished. Using the coldness of the return air in the heat recovery unit (HRU), the incoming air is precooled. Based on the thermodynamic calculations, in June, July, and August, the cooling coil power demand reduces by 11.6, 13.3, and 12%, respectively. In summer, owing to using HRU, an energy-saving by 76.08 MWh is achieved (12.34% reduction in energy demand). By the incorporation of the solar collectors in the AHU, heating coil demand diminishes by 1,206, 1,399, and 1,367 kWh in June, July, and August, respectively. To improve the solar-assisted AHU effectiveness, the MWCNT nanoparticles are injected into the collectors, and it is found that the saving-energy capability improves by 17.7% using MWCNT-water at 0.1 vol.%.


INTRODUCTION
Residential buildings, along with commercial buildings, are heavily involved in CO 2 emission and energy demand. Buildings consume 40% of energy consumption (EIA, 2019) while accounting for about 35% of CO 2 emission. To reduce the energy demand along with CO 2 emission, it is recommended to focus on the intensive-energy sector (Jokar et al., 2017;Ahmadi et al., 2019;Attia et al., 2020;Hashemi-Tilehnoee et al., 2020;Jamei et al., 2021;Thalib et al., 2021), one of which is the air handling unit (AHU). To reduce the energy demand, many researchers focused on recovery-based approaches (Noussan et al., 2017;Eades, 2018;Pacak et al., 2019;Dodoo, 2020;Liu et al., 2020;ShahsavarGoldanlou et al., 2020). Almitani et al. (2021) added an energy recovery unit (ERU) to the AHU to reduce the use of energy. In their attractive design, the exhaust air energy is used to preheat the fresh air. Fresh air passes through ERU, absorbs the sensible/latent energy of the exhaust air, and therefore, its energy rises. When the outside air is very cold, there is a risk of frostbite, so it is necessary to preheat the fresh air by installing an auxiliary heater. Hence, first, the fresh air is preheated and then enters ERU. Through recovery energy, energy demand in various devices reduces. Due to recovery of sensible/latent energies in ERU (owing to exchanging temperature and humidity), the heater and humidifier energy demand reduce. Due to the temperature growth in ERU, the auxiliary heater was able to raise the air temperature by consuming less energy (up to 37.4%). Humidifier also increases humidity by consuming less steam (up to 61.2%). Because, the fresh air humidity is increased by passing through ERU by passing through ERU. Reducing energy demand in these sectors reduced the total AHU power load by 31.8% and increased efficiency (η AHU ) by 46.6%. Also, the authors considered the exergy destruction in AHU and revealed that installing ERU decreased irreversibility from 82.54 to 55.33 kW. Therefore, another positive point of ERU can be considered as a reduction in exergy losses by 33%. Kalbasi et al. (2020a), through developing a program in EES software, examined the sensitivity of ERU to the extent of humidity in ambient. They added an ERU to the AHU and considered different scenarios. In the first scenario, they increased ϕ Amb from 30 to 70% (T Amb cte) and found that energy recovery augmented by 234%. They also considered the variations of the cooling coil energy-consuming (PC cc ). Taking into account the reduction in PC cc by 13.63% at ϕ Amb 30% and by 27.7% at ϕ Amb 70%, it is concluded that ERU effectiveness is more in a humid region. They also examined the effects of T Amb . At ϕ Amb 50%, as T Amb rises from 30 to 40°C, PC cc intensifies by 122 kW which is equivalent to a 38.18% intensification in PC cc . The variation in PC cc effects η AHU and decreases it by 23.56%. Note that the sensitivity of PC cc to T Amb depends on the ambient relative humidity (ϕ Amb ). As ϕ Amb rises, PC cc and η AHU become more sensitive to T Amb . The authors examined the total energy recovery in ERU. At 30°C and 30%, ERU could recover the energy by 42.6 kW while at 40°C and 70%, this figure was 300 kW. This indicates that ERU in humid and hot climates has more efficacy.
In another study (Kalbasi et al., 2020b) the authors studied the exergy performance of an AHU + ERU. The major irreversibility corresponded to the heating coil (11.47 kW) while the minimum irreversibility (1.067 kW) was through the mixing box. Owing to using ERU, irreversibility diminished by 2.87 kWh. This is very acceptable because an 8.7-percent reduction in irreversibility improves the second efficiency by 4.6%.
In a numerical/experimental study, Ribé et al. (2019) examined the effects of relative humidity on the performance of ERU. Since in the heat recovery unit (HRU), only the sensible heat is exchanged; therefore, the humidity content does not affect the HRU effectiveness. The latent heat transfer potential of the ERU unit depends on ϕ Amb . Although in dry climates, they showed that there was not much difference between ERU and HRU, but for wet climates, the amount of recovered energy in ERU is higher. Taking into account the Spanish climatic zone, they recommended using HRU in the dry climate (owing to lower cost) and ERU in the wet climate.
In a similar study, Yari et al. (2019) examined the effects of ϕ Amb on the usefulness of adding ERU in AHU. In dry climate ϕ Amb 10%, PC cc reduced only by 0.161 kW (0.9%) while in the wet climate, PC cc diminished by 10 kW (27%). Moreover, the efficiency was influenced by ϕ Amb so that at ϕ Amb 10%, installing ERU improved η AHU by 1% while at ϕ Amb 70% this figure was 36.8%. Therefore in humid climates the usefulness of ERU becomes more evident. Zheng et al. (2020) added two ERUs in an AHU and focused on PC AHU . Under the Kuwait city climate, the thermodynamic analysis showed that PC AHU for AHU without the ERU unit is 438.74 kW while after incorporating two ERUs in AHU, PC AHU reduces by 163.63 kW. This means that PC AHU diminished owing to installing ERUs by 37.3%.
Controllers with the apparent commands can reduce PC AHU . In a study by Homod (2014), it was revealed that PC AHU experiences a 32-percent reduction if control scheduling is optimized. The results of other studies in the field of energy consumption reduction are reported.
In this study, two strategies are used to reduce AHU energy consumption (PC AHU ). In the first strategy, by reducing the fresh air temperature through using HRU, PC CC diminishes. In the second strategy, a solar collector is used to reduce PC HC . This collector, which is filled with MWCNT nanoparticles, can provide a part of the required energy for the heating coil and thus improve the overall performance of AHU. Finally, the amount of energy-saving in each scenario is compared.

DESCRIPTION OF THE MODIFIED AHU
The description of the solar system integrated with AHU is shown in Figure 1. In general, AHUs have two main purposes: 1-providing the suitable temperature (T room ) along with the apparent relative humidity (ϕ room ), 2-providing fresh air ( _ m fresh ) to satisfy comfort conditions. As shown in Figure 1, the ambient fresh air enters the AHU with the thermodynamic properties of T Amb and ϕ Amb (Figure 2). The average of T Amb in June is 31.5°C while in July and August it is 33.5°C. The average of ϕ Amb is 56.6, 51.2, and 58.3% in June, July, and August, respectively.
If the task of the AHU is only to supply _ m fresh , it can be said that the AHU needs a fan and duct to transfer fresh air, and therefore PC AHU will be insignificant. There is always a big difference between T Amb and T room along with ϕ Amb and ϕ room and hence the energy content of the indoor (E room ) and outdoor (E Amb ). Taking into account E Amb > E room in summer, the AHU should dissipate the effects of Q s and Q s through the cooling and heating coils. For meeting the ventilation requirements, _ m fresh and supply air mass flow rate ( _ m supply ) should be justified as follows: where N is the number of people, 0.006 m 3 /s is the fresh air mass flow rate per person, ϑ is the specific volume, n 6 is the number of changes of air per hour, and ∀ denotes the room volume. First, air enters HRU. The absolute temperature and humidity of the air at the outlet are determined according to the following equation: where ε HRU is the effectiveness of the HRU unit. Note that in this study a transient analysis is performed for an AHU. In the transient analysis, there are changes in energy within the system so that its value depends on zT/zt. However, if the time step is selected small enough, the temperature changes within each control volume can be considered negligible ((zT/zt) → 0). In this study, the same technique (assuming a small time step) is used. After that, the precooled air enters the mixing box. The outlet properties are (Kalbasi et al., 2020c): Because through passing CC, both humidity and temperature decrease, so PC CC is obtained from the following equation: The conditions for the supply air depend on Q s sensible and Q l latent and obtained from the following equations: Equations 9 and 10 reveal that the supply air properties are influenced by Q s and Q l . In air conditioner calculations, the parameter of SHF Q s /(Q s + Q l ) is usually used. The variations in Q s along with SHF are shown in Figure 3.
In this study, using a solar hot water supply (SHWS), the heating coil power demand (PC HC ) is supplied. The solar system consists of three main parts: collectors, storage tank, and auxiliary heater. In the storage tank, the water must be set at a certain temperature (T set ). For this purpose, if the power of the collectors is not enough to regulate the temperature, the electric heater is turned on to adjust the temperature. The power of the collectors is obtained from the following equation: where _ m wf is the working fluid mass flow rate. The parameters of η c and I c are collector efficiency and solar intensity. The variations in η c and I c are illustrated in Figure 4 and Figure 5, respectively.

RESULTS
In this study, it is tried to reduce PC AHU in two scenarios. In the first scenario, the main focus is on reducing PC CC , while in the second scenario, the parameter of PC CH is targeted.

FIRST SCENARIO
In summer, outdoor air energy (E Amb ) is at a higher level than the indoor one (E room ). To reduce E Amb − E room , a cooling coil should  be used. Finally, according to the ambient conditions shown in Figure 2, the ambient dew point is more than 6°C and therefore the air humidity along with temperature reduces through passing the cooling coil. By installing HRU, the temperature and humidity at the inlet of the cooling coil experience a reduction, and this means a reduction in PC CC . As shown in Figure 6, the temperature of the air intake for AHU + HRU is lower than that in AHU. Figure 7 illustrates the cooling coil power changes for AHU and AHU + HRU. It is clear that in June, the values of PC CC are lower for AHU + HRU, and this trend is also true in July and August.
A lower PC CC for AHU + HRU means energy-saving. The total amount of energy-saving in June, July, and August is 21.78, 27.2, and 27.1 MWh, respectively (Figure 8). In other words, cooling coil energy consumption reduces by 11.6, 13.3, and 12% in June, July, and August, respectively. Throughout the summer, PC cc decreases by 12.34%, which resulted in a 76.09 MWh energy-saving.

SECOND SCENARIO
In the second scenario, the focus is on the heating coil. Many studies showed the effectiveness of using solar energy (Ahmadi et al., 2017a;Ahmadi et al., 2017b;Ahmadi et al., 2018;Dabiri et al., 2018;Gholipour et al., 2020). In the heating coil, the temperature rises during a process in which the humidity ratio remains constant. The inlet temperature in the heating coil is preset according to the cooling coil outlet. On the other hand, the outlet temperature of the heating coil is also determined according to the room conditions using Eq. 9. Therefore, its input and output are predetermined. Generally, in AHUs, the heating coil supplies its energy through either a boiler or an electric heater. In this study, the heating coil power demand is supplied from a solar system. As mentioned in the previous section, the solar system consists of several collectors and a storage tank equipped with an electric heater. If solar energy cannot heat the water in the tank, the auxiliary heater must heat the water. In this study, in addition to the auxiliary heater, the tank is equipped with two sensors. The first sensor is located exactly at the top of the tank and the second sensor is located in the middle of the tank. The sensors function in such a way that they can keep the temperature in the whole tank in the range of 35-40°C. Since the power of the heating coil is obtained from PC HC _ m hotwater × 4180(ΔT hotwater ), so the parameters of _ m hotwater and ΔT hotwater are both known as independent variables. In air conditioning calculations, the selection of ΔT hotwater and _ m hotwater should be done in such a way that the difference between the inlet and outlet temperatures (ΔT hotwater ) is equal to 20°F (ASHRAE American Society of Heating and Engineers, 2016). Therefore, taking into account ΔT hotwater 20℉, the required hot water mass flow rate follows Figure 9. It is clear that the parameter of _ m hotwater varies within the range of 0.4 − 0.8 kg/s.
Owing to the temperature range of 35 − 40 ℃ inside the storage tank, the heater power will change according to Figure 10. In this figure, the useful heat gain through the collector is also shown. This figure clearly shows that the heat gain in the collector is not enough to completely cover PC HC .
For comparison, it is assumed that AHU provides heating power in the heating coil through an electric heater. Thermodynamic calculations affirm that accumulated heating power (PC HC ) in June, July,and August is 20.462,20.303,and 21.142 MWh, respectively. However, by integrating AHU with a solar system, PC HC can be reduced. Figure 11 illustrates the monthly analysis of PC HC and shows that it is less for the AHU + solar system.
Owing to integrating a solar system with AHU, PC HC reduces by 0.85, 0.97, and 0.95 MWh in June, July, and August, respectively.   It seems that by increasing the number of collectors in parallel arrangement (increasing the collector area), it is possible to strengthen the ability of energy-saving. As the area increases (as shown in Figure 12), the saving energy intensifies. In June, the saving energy increases by 41.48% (from 850 kWh to 1,206 kWh) as the area rises from 5 to 10 m 2 . This figure for July and August is 44.22 and 43.9%. This implies that there is no linear relationship between the collector area and energy-saving content. Note that this figure in summer is 43.4% (with increasing area from 5 to 10 m 2 ).
Nanoparticles are another technique for improving the collectors' efficiency (Qu et al., 2019;Mahmoudi et al., 2020;Alawi et al., 2021;Sadeghi et al., 2021). These materials have widely been examined by many researchers (Osman et al., 2019;Aghakhani et al., 2020;Eshgarf et al., 2021;Mahdavi et al., 2020;Nguyen et al., 2020;. Nanoparticles can improve heat transfer within the collector (Muhammad et al., 2016;Tong et al., 2019;Elcioglu et al., 2020;Gholipour et al., 2021;Mustafa et al., 2021). As shown in Figure 5, the efficiency for the MWCNTbased collector is higher. Therefore, it is expected that MWCNTs will increase the energy-saving content. In Figure 13, it can be seen that for a collector filled with water, the energy-saving is 3,972 kWh. This figure for collector filled with MWCNTs (at 0.01 vol.%) is 4,170 kWh. This indicates that using MWCNT is recommended owing to a 5% enhancement in energy-saving.
The positive effects of MWCNT increase as the nanoparticle concentration intensifies. Adding further MWCNTs leads to more enhancement in energy-saving. At 0.05 and 0.1 vol.%, the energy-saving is 4,520 and 4,675 kWh which is equivalent to 13.8 and 17.7% enhancement.

CONCLUSION
Considering the 40 and 35% contributions of the building sector in energy consumption and greenhouse gas emissions, two techniques were used to reduce both the parameters. In the first technique, a unit heat recovery (HRU) was used to reduce energy demand in the cooling coil, while in the second technique, the energy consumption in the heating coil was reduced by focusing on solar energy. By developing energy and continuity equations in individual sections, transient thermal analysis of the AHU was performed. The main results were: ❖ First technique (installing HRU): 1. By installing HRU, the hotness from the fresh air was transferred to the return air and consequently, the fresh air was precooled. Owing to entering with a lower temperature into the cooling coil, the power demand reduced by 76.09 MWh in summer which is equivalent to a 12.34% reduction.   ❖ Second technique (using solar collectors + stratified storage tank): 1. In summer, the task of the heating coil is to adjust the thermodynamic conditions of the supply air. With the installation of the solar system, energy consumption was reduced by 3,972 kWh in June + July + August which is equivalent to a 6.41-percent reduction. 2. To boost the solar collector effectiveness, MWCNT nanoparticles were loaded into the water and it was found that the energy-saving potential increased by 17.7% (from 3,972 to 4,675 kWh).

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.