*2.2. Nanofluid Characteristics*

In order to evaluate the increase in performance due to the use of a nanofluid as a heat transfer fluid, in this study an aluminum oxide-based nanofluid has been loaded inside the HVAC-1 system. This nanofluid has been chosen taking into account its high stability and its low viscosity, which are comparable to the base fluid ones. Table 2 summarizes the main specifications of the nanofluid.

**Table 2.** Specifications of the nanofluid.


The nanofluid was made of aluminum oxide nanoparticles at 2% in volume concentration and size distribution with Dv90 = 617 nm, with a density of 1079 g/L. Density is strictly related to the particle concentration, therefore sample density has been measured over the test campaign to monitor sedimentation phenomena inside the system. According to regulation and restrictions, the use of propylene glycol in the composition of the nanofluid for the test campaign comes from the necessity to avoid a toxic grade of glycol, as ethylene glycol is.

The remaining 2% in weight are dispersants, anti-corrosion inhibitors and aluminum oxide nanoparticles.

#### *2.3. Test Instrumentation and Data Acquisition System*

The coefficient of performance (COP) of each HVAC unit was calculated as the ratio between thermal (Eth) and electrical (Eel) energy:

$$\text{COP} = \text{E}\_{\text{th}} / \text{E}\_{\text{el}} \tag{1}$$

Therefore, the energy monitoring system required the installation of several instrumentations, such as electricity meters, temperature sensors and mass flow rate meters:


**Figure 2.** Acquisition system for the electrical energy measurements IME—NEMO D4.

**Figure 3.** Drawing of the acquisition system for thermal energy Caleffi—Conteca Easy—Ultra.
