Solar Energy Technology

Solar thermal (ST) collectors and photovoltaic (PV) panels that exploit renewable energy sources (RES) provide environmental and economic benefits [57–59].

In this work, the solar technologies were assessed considering the potential roofs' area with better solar exposition. In Italy, the most used types of low-temperature ST collectors are *flat glass collectors* with high e fficiency and low cost and *vacuum tubes,* which have greater e fficiency compared to flat glass collectors, due to the lower dispersions by thermal convection inside the vacuum tubes, but higher cost [60]. On average, a solar thermal system in Italy has a monthly e fficiency of 40–85% with flat collectors and 70–86% with vacuum tubes; a collection area of 0.7–1.2 m<sup>2</sup>/person for flat collectors and 0.5–0.8 m<sup>2</sup>/person for vacuum tubes (considering the production of domestic hot water), with the month of maximum solar radiation being considered for the dimensioning; and a cost of 1000 euro/m<sup>2</sup> for flat collectors and 1200 euro/m<sup>2</sup> for vacuum tubes.

Regarding PV modules, the e fficiency of converting solar energy into electricity varies mainly according to the type of technology chosen. The average e fficiency values vary from 22% (high e fficiency *monocrystalline silicon*) to 4% (*amorphous silicon*). The cost of a PV solar system depends on the installed power, which is around 2000 euro/kWp where kWp is the peak power). The capturing surface depends on the e fficiency of the module and ranges from 5.5 m<sup>2</sup>/kWp for high e fficiency monocrystalline silicon to 11 m<sup>2</sup>/kWp for amorphous silicon.

In assessing the e fficiency of converting solar energy, it is also necessary to consider the energy losses of all system components, in addition to solar panels; it is estimated to be around 20–25%.

The GIS tool 'Area solar radiation' was used to quantify how much solar radiation each rooftop in the district receives throughout the year. The sun and sky models were elaborated, considering the monthly data of atmosphere transparency (τ) and ratio of di ffuse radiation to global radiation (ω) identified from the 'Photovoltaic Geographical Information System PVGIS' of Joint Research Centre (JRC). In particular, considering the period 2013–14, τ was taken to equal 48%, 62%, and 72% in winter, midseason, and summer periods, respectively, and ω was taken to equal to 48%, 45%, and 35%, similarly.

According to European Standard (EN) 12975-2:2006 and Italian Standard (UNI) 11300-4:2016, the two typologies of ST collectors have, on average, respectively, zero-loss e fficiencies η*0* of 0.94 and 0.88, linear heat loss coe fficients *a1* of 3.34 and 1.57 <sup>W</sup>/m<sup>2</sup>/K, quadratic heat loss coe fficients *a2* of 0.02 and 0.01 <sup>W</sup>/m<sup>2</sup>/K2, and, for the whole system, a performance ratio of 75%. Then, their monthly efficiencies vary from 0.37 to 0.87 for the flat glass collectors and from 0.69 to 0.87 for the vacuum tubes.

The PV modules have an e fficiency of 15% (standard e fficiency polycrystalline silicon module), and both PV and ST have a system performance around 75%. The hypothesized ST areas were dimensioned in order to not have an overproduction of hot water during summertime.

The monthly energy consumption was simulated for a district of Turin as follow:

