*3.1. Life Cycle Assessment*

As shown in Figure 3, the system's main environmental hotspots are the solar collector in nine of the 18 impact categories (contributions between 35% in OFPh and 73% in FETP), the PCM in five categories (between 26% in GWP and 65% in WDP), the PCM tank in three categories (between 35% in MEP and 49% in ODP), and the heat exchanger is the main contributor to HTPc (30% of the total). Altogether, these parts contribute over 83% in all the 18 impact categories evaluated.

For solar collectors the evacuated tube collector is the leading environmental hotspot, representing between 67% (water consumption, WDP) and 90% (land use, ALOP) of solar collector-related impacts in all categories evaluated. These impacts are mainly associated with the raw materials (particularly copper and borosilicate glass) in the evacuated tube collector and the production processes. For example, over 90% of the impacts of the evacuated tube collector in the TEP, FEP, and HTPnc categories are associated with its copper content. The borosilicate glass is the primary environmental hotspot of the evaluated tube collector in the GWP, IRP, OFPh, ALOP, and FDP impact categories, with contributions over

35%. In GWP, the impact is mainly associated with the electricity and heat consumption in the borosilicate glass production plant. Electricity consumption is also responsible for 82% of the solar collector-related IRP impact. In the OFPh category, NOx emissions generated in the glass tube production plant are responsible for 54% of the impact of the evacuated tube collector. Heat and electricity used in the production contribute, respectively, 40% and 35% of the FDP impact of the evacuated tube collector.

**Figure 3.** Environmental contribution analysis of a solar domestic system with latent heat thermal energy storage technology with phase change material (S-LHTES-PCM). (All impacts are expressed per kWh of heat produced. GWP: global warming potential; ODP: ozone depletion potential; IRP: ionising radiation potential; OFPh: ozone formation potential, human health; PMP: fine particulate matter formation potential; OFPt: ozone formation potential, terrestrial ecosystems; TAP: terrestrial acidification potential; FEP: freshwater eutrophication potential; MEP: marine eutrophication potential; TEP: terrestrial ecotoxicity potential; FETP: freshwater ecotoxicity potential; HTPc: human carcinogenic toxicity potential; HTPnc: human non-carcinogenic toxicity potential; ALOP: agricultural land occupation potential; MDP: mineral depletion potential; FDP: fossil depletion potential; WDP: water depletion potential; CED: Cumulative energy demand; SAT: Sodium acetate trihydrate).

> With regards to the PCM subsystem, SAT contributes to the highest environmental burden, accounting for 68% (MEP and TEP) and 96% (IRP) of the impact of the PCM. As illustrated in Figure 3, SAT is the leading environmental contributor in the whole S-LHTES-PCM system in the GWP (23% of the total), IRP (62%), FDP (35%), WDP (48%), and CED (37%) categories. The main environmental hotspots in the SAT life cycle are associated with

the electricity used in the production of the components (sodium hydroxide and acetic acid), and to a lesser extent, in the production process of sodium acetate trihydrate. As for the PCM tank, steel and polyurethane (foam) represent the major environmental impacts. These parts of the PCM tank represent 68% (ALOP) and 100% (ODP) of the impact of the PCM system. Polyurethane is one of the main environmental hotspots of the PCM tank in the categories of ODP (93% of the contribution of the whole PCM system), MEP (87%), FDP (57%), WDP (71%), and CED (52%). These impacts are mainly due to the use of methylene diphenyl diisocyanate (MDI) in polyurethane production. Nitrate emissions to water in MDI production are responsible for 80% of the MEP impact of the PCM system. The other categories mentioned above (ODP, FDP, WDP, and CED) are strongly influenced by the use of aniline in the MDI production process. If we focus on the PCM tank, steel is the main environmental hotspot in 13 out of the 18 impact categories analysed. In the GWP category, heat production from hard coal industrial furnaces and electricity production are the steel main environmental hotspots. Emissions of particulate matter < 2.5 μm in the ferrochromium production process is the main environmental hotspot in the PMP category, accounting for 47% of the impact of steel on the PMP category. In the TEP category, air emissions of copper in ferronickel production are responsible for 60% of the steel impact. Air emissions of chromium VI associated with ferrochromium production are responsible for 48% of the HTPc impact, while nickel ore consumption in ferronickel production is responsible for 85% of the impact in the MDP category.

End-of-life stages generate both environmental benefits and impacts. Steel recycling generates higher environmental benefits in eight impact categories (GWP, OFPh, OFPt, MEP, ALOP, MDP, FDP, and CED), while copper does so in ten (ODP, IRP, PMP, TAP, FEP, TEP, FETP, HTPc, HTPcn, and WDP). Overall, recycling reduces the total environmental impacts of the solar energy and S-LHTES-PCM system by up to 28% (FETP).
