3.2.3. Solar Energy Technology

After the analysis of monthly and annual solar radiation on each rooftop (Figure 10), taking into account 570 heated, pitched buildings and roof surface with annual solar radiation higher than 1200 kWh/m<sup>2</sup>/year, the ST collectors and PV modules were dimensioned according to domestic hot water consumption and electrical consumption of residential and nonresidential users.

According to the Italian Decree 28/2011, 50% of domestic hot water consumption of residential sector is covered by ST collectors. The percentage reaches 100% in June, while in the winter months (December and January) ST production is able to cover about 7% of the residential consumption (Figure 11a). In addition, there is a GHG emission reduction of 1958 ton/CO2/year. The requirements indicate that 50% of consumption must be covered; this dimensioning is appropriate, given that in July hot water can only be used to cover domestic hot water.

**Figure 10.** Solar energy technology assessment: (**a**) Annual solar radiation, solar roofs are identified with a black outline; (**b**) identification of areas (in red) with annual solar radiation <1200 kWh/m<sup>2</sup> (not suitable for solar energy production).

The PV panels can be dimensioned in two ways: By covering 100% of consumption in the month of maximum irradiation or by reaching 100% of annual self-consumption, taking into account that the overproduction in the summer months that is fed into the grid will be consumed in winter. In this work, the PV panels were dimensioned according to the National Decree 28/2011 using the footprint area of the buildings (*A*), where the installed power is equal to *A* divided by a *K* coefficient = 50 (Figure 11b). According to the installed power and the annual utilization hours of use (in the Piedmont region are 1130 h), the electricity produced from PV panels was compared to the electrical consumption. Therefore, knowing that a typical Turin family needs about 2000 kWhel/year for electricity supply and, in the district analyzed, the number of families is equal to 10,638 (ISTAT data, 2011), the 13% of the annual residential and nonresidential electrical consumption has been covered with a GHG emission reduction of 1853 ton/CO2/year. Considering only the residential sector, PV production covers the 18% of electricity consumption. In the summer months it covers 38% and in the winter months, 2–3%. Table 6 shows the total roof area, the quota well exposed with no disturbances (15–35%), the quota used for the ST collectors to satisfy the domestic hot water (DHW) consumptions, and the quota for the PV panels, as requested by the standards (1/50 kW/m2). Using the maximum energy potential that can be produced from PV panels with the left available roof area, it is possible to cover 82% of residential electrical consumption; with the reverse procedure, an optimal value of *K* of about 11 m<sup>2</sup>/kW was calculated.



From this analysis, it emerged that to reach the 50% coverage of domestic hot water, heating, and cooling consumption, it is necessary to use not only solar technologies but also other renewable energy technologies, such as the energy taken from the cold source with heat pumps for heating or PV panels for cooling. In fact, in the city, there are few renewable energy sources available, but there are sources that can be exploited in public spaces, such as the PV panels on shelters, micro-power plants (of which in Turin city there are three) and mini-wind on commercial buildings, considering the acoustic impact.

**Figure 11.** Solar energy technology assessment (for the 2014 year): (**a**) Comparison between domestic hot water (DHW) consumption of residential sector and solar thermal (ST) production considering four collector typologies (collectors' annual average efficiency: ST1 = 0.59, ST2 = 0.77, ST3 = 0.80, and ST = 0.79); (**b**) comparison between electrical consumption, photovoltaic (PV) production with coefficient *K* = 50 m<sup>2</sup>/kW (according to the Decree 28/2011), and PV max producible.

#### *3.3. Energy Savings: Heating and Cooling*

Green and high-reflectance roofs (cool roofs) have a significant effect in reducing energy consumption during cooling and heating seasons. In accordance with literature review [38,63–67], from this work it emerged that cool roofs are more effective in reducing heat gain in the cooling (C) season from 15 April to 14 October, than heat loss in the heating (H) season from 15 October to 14 April (Figure 12).

**Figure 12.** Comparison of heat fluxes (Wh/m2) between common roof, insulated common roof, insulated high-reflectance roof, and insulated green roof: (**a**) Cooling season; (**b**) heating season.

This analysis was carried out using weather data measurement recorded by Politecnico weather station (WS) for the period from 2011 to 2016. The thermal performance of a refurbished roof was compared to the typical common roof. In particular, three roof solutions were taken into account: (1) Insulated common roof, (2) insulated high-reflectance roof, and (iii) insulated green roof. The heat flux (*Q*) in the roof was quantified according to Equation (3). Table 7 describes the characteristics of roof solutions and the main energy efficiency results. GHG emissions were quantified using 0.210 tonCO2/MWh for natural gas and 0.46 tonCO2/MWh for electricity [68].


**Table 7.** Characteristics of roof solutions and energy efficiency results.

Figures 13 and 14 show the results for three consecutive hot days (21–23 July 2015) and cold days (15–17 January 2012). From the comparison of hourly heat fluxes (W/m2) between common roof, insulated common roof, insulated high-reflectance roof, and insulated green roof, it emerged that using an insulated green roof there was less heat gain during the summer season and with an insulated roof there was less heat loss in the winter season.

**Figure 13.** Hourly values of global solar radiation (*Ii)*, solar radiation entering in the system (*In*), and the external air temperature (*Tae*) for three consecutive days: (**a**) 21–23 July 2015; (**b**) 15–17 January 2012.

**Figure 14.** Comparison of hourly heat fluxes (W/m2) between common roof, insulated common roof, insulated high-reflectance roof, and insulated green roof for three consecutive days: (**a**) 21–23 July 2015; (**b**) 15–17 January 2012.

This methodology, used to evaluate the effect of green and cool roofs, will be implemented in future work, adding the effect of evapotranspiration on energy performance of a building.
