**1. Introduction**

Decreasing the energy consumption of heating, ventilation and air conditioning (HVAC) systems is a very important issue, due to their high environmental and energy costs, together with a significant actual increase in their demand from the market. Several studies have described various technologies and techniques that can be used to reduce HVAC energy consumption, one of which is nanofluids [1,2].

Nanofluids are engineered heat transfer fluids which can be used to improve heat transfer, thus increasing energy efficiency in a variety of applications based on chillers, heat pumps and other hydronic HVAC systems. They are suspensions of nanoparticles dispersed in a liquid that are formulated to achieve higher heat transfer performance than their basefluids. Numerous studies have demonstrated that nanofluid thermal conductivity can be improved based on some variables, such as nanoparticle volume concentration, size, morphology, etc.

In early experiments on nanofluids, Lee et al. [3] measured thermal conductivity with the transient hot-wire method, demonstrating that a small amount of nanoparticles was enough to increase the thermal conductivity of the base fluid.

Beck et al. [4] presented data for the thermal conductivity enhancement in seven nanofluids containing 8–282 nm diameter alumina nanoparticles in water or ethylene glycol.

**Citation:** Milanese, M.; Micali, F.; Colangelo, G.; de Risi, A. Experimental Evaluation of a Full-Scale HVAC System Working with Nanofluid. *Energies* **2022**, *15*, 2902. https://doi.org/10.3390/ en15082902

Academic Editor: Chi-Ming Lai

Received: 17 March 2022 Accepted: 12 April 2022 Published: 15 April 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

They found that the thermal conductivity enhancement in these nanofluids decreases as the particle size decreases below about 50 nm. This finding could be attributed to phonon scattering at the solid–liquid interface.

To confirm this result, Colangelo et al. [5,6] designed, built and tested a new experimental setup to investigate the physical phenomena involved in the thermal conductivity enhancement of nanofluids.

In recent years, experimental investigations on the effects of nanofluids on convective heat transfer coefficients in laminar and turbulent conditions were developed, demonstrating a significant improvement with respect to conventional heat transfer fluids [7].

Balla et al. [8] studied several suspensions of Cu and Zn nanoparticles with a size of 50 nm in water base fluid. They found that the heat transfer coefficient of nanofluids was higher than its base fluid. Similar results were found by Kai et al. [9] studying nanofluid heat transfer in a mini-tube using SiO2 nanoparticles.

Recently, several numerical and experimental studies on nanofluids and their applications have been developed, such as solar thermal systems. Lee et al. [10] and Alsalame et al. [11] studied photovoltaic thermal systems based on nanofluids. Colangelo et al. [12,13] experimentally investigated the use of nanofluids in flat solar thermal collectors: tests on traditional solar flat panels revealed some technical issues, due to the nanoparticles' sedimentation. Therefore, the modification of the panel shape allowed this problem to be fixed. Flat plate solar collectors based on nanofluids were also studied by Chaji et al. [14]. Furthermore, the application of nanofluids on different solar thermal energy conversion systems was investigated in [15–18].

Different studies have been carried out to increase the performance of internal combustion engines. Zhang et al. [19] improved the heat-transfer performance of a diesel-engine cylinder head by means of a nanofluid coolant.

Micali et al. [20] developed an experimental campaign related to the use of CuO nanofluid as the coolant in a biodiesel four-strokes engine. They measured reductions in temperature up to 13.6% on the exhaust valve seat and up to 4.1% on the exhaust valve spindle.

Further studies related to the use of nanofluid within electronic devices [21], geothermal heat exchangers [22,23], and a cooling system for wind turbines [24], demonstrated a significant increase in heat transfer performance versus traditional fluids.

Considering these thermal performance improvements, the use of fluids containing suspended solid particles in HVAC systems is expected to show significant enhancements of their efficiency.

Devdatta et al. [25] observed that the use of nanofluids inside the heating system of the building is a suitable solution to reduce the size of the heat transfer system, and, in particular, the size of the heat exchangers, heat pumps and other components as well. This will reduce energy consumption and will, thus, indirectly reduce environmental pollution.

Ahmed and Ahmed Khan [26] used nanofluids in the external cooling jacket around the condenser of an air conditioner. In particular, they studied the benefits of two types of nanofluids, made of copper and aluminum oxide, respectively, on the performance of an air/water conditioner. Their experimental results showed a significant enhancement in the coefficient of performance (COP), up to 22.1% with Al2O3 nanofluid and 29.4% with CuO nanofluid.

Hatami et al. [27] experimentally tested three types of nanoparticles (SiO2, TiO2 and Carbon Nanotubes), dispersed in water inside HVAC systems. They found the best result, in terms of energy consumption reduction, with SiO2-based nanofluid.

In order to use nanofluids as a heat transfer fluid within full-scale HVAC systems, different problems have to be solved, such as nanoparticle stability in suspension [28] and increment of viscosity [29]. Regarding the first issue and according to Awais et al. [29], sedimentation and agglomeration of nanoparticles within nanofluids can produce fouling on heat transfer surfaces and, therefore, higher pressure drops and damages in ducts, pumps, etc. On the other hand, the use of nanoparticles, having an optimal shape, size and

volume fraction in the base-fluid coupled with the addition of surfactants can improve the suspension stability, avoiding the above problems [30].

The electrical potential at the shear slippage plane is called the zeta potential (ZP), and its value aids in evaluating nanoparticles' (NPs) stability in suspension [31,32], according to Bogdan [33] and Lee [34]: indeed, particles in colloidal suspension tend to develop a surface charge by the adsorption of ions from the base fluid. This superficial charge is double-layer-structured, with a sliding surface located beyond the first layer. In the nanofluid formulation used in this investigation, an anionic surfactant was used in order to improve the stability. The stability of the aluminum oxide suspension depends on the dispersant to modify the ZP and the surface repulsion between particles. In cases with anionic dispersant, a ZP value higher than 25 mV (absolute value) is necessary to achieve enough repulsion forces.

In the case of a sample at rest, the settling occurs over a long time, with 50% of particles settled in 6 months.

Regarding the second issue (viscosity), it is important to remark that the variation in nanofluid viscosity is directly proportional to the particles' concentration in suspension. In the nanofluid formulation, a volume of only 2% nanoparticles has been added to achieve a very limited viscosity increment. Furthermore, the test campaign was carried out in a large HVAC system, where the relevant diameter of the pipes and relevant size of the heat exchangers limited the impact of viscosity increment on the pressure drop in the system. Pantzali et al. [35] studied nanofluid use in industrial applications, mainly focusing on the pressure drop increment related to the viscosity of the nanofluid. They concluded that in the case of industrial heat exchangers and large pipes with turbulent flow, usually developed inside, the substitution of conventional fluids by nanofluids had no relevant incidence on pressure drops in the system.

In order to overcome the above discussed problems, this work was based on a nanofluid composed of water–glycol and aluminum oxide (Al2O3) nanoparticles, having a controlled size distribution (Dv90 = 617 nm) and good stability, that deliver efficient, reliable, and consistent performance over a wide temperature range, with little effect on viscosity, and, therefore, on system fluid pumping energy. Particularly, in [36] Colangelo et al. developed dynamic simulations in order to compare the efficiency of two full-scale HVAC systems (installed at the educational building "Corpo O" in the Campus of University of Salento, Lecce, Italy), working with a traditional water–glycol mixture and with Al2O3-nanofluid, they found a numerical increment in efficiency of about 10%. As a followup to that study, the objective of this work was to carry out an extended experimental campaign on the same building in order to quantify, over a long period of time and under real operating conditions, the increase in performance of the HVAC system due to the use of nanofluids. These results will also allow the validation of the numerical results previously found, verifying their congruence with the experimental measurements.

## **2. Test Conditions and Experimental Apparatus**

The experimental campaign was carried out on an educational building, named "Corpo O" (Figure 1), at the Campus of University of Salento, which is in Lecce, Italy at latitude 40◦21 and longitude 18◦10 .

The building consists of two symmetrical wings (left and right wings), each of which has its own HVAC system. Each wing is composed of three floors: ground floor, first floor and second floor, with a total area of 2400 m2 and a total volume of 13,163 m3. The HVAC systems are used for air conditioning of offices and labs inside the building.

**Figure 1.** Building "Corpo O" at Campus Ecotekne of the University of Salento in Lecce, Italy.

The experimental test campaign was focused on data acquisition during winter (heating mode) and summer (cooling mode).
