*3.3. February–March 2021 Results*

In order to confirm the results acquired during winter 2020, the previous heat transfer fluid (water–glycol 30% vol) was reloaded within the HVAC-1 system and a long data set was acquired, in order to compare the performance of the two systems over a long time period (from 27 November 2020 to 5 February 2021). Therefore, on 8 February, the nanofluid was reloaded again within the HVAC-1 system and the performance was monitored until 9 March 2021. Figure 10 shows the hourly COP comparison between HVAC-1 and HVAC-2, from 1 January 2021 to 9 March 2021.

**Figure 10.** Hourly COP comparison between HVAC-1 and HVAC-2 (1 January 2021–9 March 2021).

It can be seen that the hourly COP values related to the HVAC-2 (orange dots), until 5 February are mainly higher than the HVAC-1 values. After loading the nanofluid (8 February) the trend reverses, with blue dots over orange ones.

This effect is particularly visible in Figure 11, where a comparison of the instantaneous COP is shown, following the loading of the nanofluid around noon. A significant increase in HVAC-1 performance is evident, due to the heat transfer fluid change.

**Figure 11.** Instantaneous COP comparison between HVAC-1 and HVAC-2. Data acquired on 8 February 2021.

Finally, Figure 12 shows the average daily COP comparison and the daily COP ratio between HVAC-1 and HVAC-2 from 27 November 2020 to 9 March 2021. Clearly, it can be observed that during the entire experimental period, in which the two plants operated with the same fluid (from 27 November to 5 February), the performance of HVAC-1 was worse than HVAC-2, with an average COP1/COP2 ratio of 0.970. After the nanofluid loading, for the period from 9 February to 9 March 2021, the COP1/COP2 ratio was 1.078, with a peak of 1.121 and an average increase of 10.5%.

All the results shown in the above graphs demonstrate that the increased performance of the HVAC-2 system is not due to favorable environmental conditions, but only to the positive action of the nanofluid. The above discussed experimental results essentially agree with the numerical results found by Colangelo et al. [36].

In order to better understand the results described above, the performances of the HVAC-1 working with the base fluid and nanofluid have been investigated in depth and compared. In particular, the data related to the period January–March 2021 have been collected as a function of outside air temperature and fluid temperature at the inlet of HVAC-1. These parameters have been chosen since they directly affect the heat pump COP. Figure 13 shows the results.

As it can be seen, the COP was strongly influenced by the operating conditions of the heat pump. Nevertheless, the performance growth between the base fluid (data until 5 February) and nanofluid (data related to the following days) seems quite constant during the whole period of experimentation and equal to 13% on average.

**Figure 12.** Average daily COP comparison and daily COP ratio between HVAC-1 and HVAC-2 (27 November 2020–9 March 2021).

**Figure 13.** Average COP of HVAC-1 working with base fluid and nanofluid (January 2021–March 2021).

## *3.4. Practical Significance/Usefulness*

The results of this work suggest that the use of nanofluids within hydronic HVAC systems can have a big impact from an environmental, energetic and economic perspective. In fact, taking into account the annual energy consumption of the building "Corpo O" [36], it was possible to calculate annual energy savings equal to 50.2 MWh. According to the Italian CO2 emission factor [38], it was possible to preliminarily evaluate an annual avoided CO2 emission equal to 21.8 tons related to the use of the nanofluid.

Finally, it is important to remark that the replacement of the traditional heat transfer fluid with a high performance nanofluid does not require important modification to the HVAC plant, resulting in an easy and effective retrofitting of old systems.
