*3.2. Sensitivity Analysis: Lifetime Extension*

A sensitivity analysis was performed based on the lifetime of the S-LHTES-PCM system (see Section 2.3). As shown in Figure 4, the increase of life expectancy of the S-LHTES-PCM system to 25 years significantly reduces the environmental impacts in all the evaluated categories. However, when the lifetime increases to 30 years, the environmental impacts of five categories (FETP, TEP, HTPnc, FEP, and TAP) increase over the impacts of the original system (20 years lifetime). This is because the S-LHTES-PCM system requires the replacement of the solar collector, which has reached its lifetime at 25 years. The solar collector is the leading environmental hotspot of the S-LHTES-PCM system in the five categories mentioned above. Because of this, and given that the system will have a lifetime of 30 years, the new solar collector will only be used for five years, and, therefore, not being able to amortize its environmental burden over its potential lifetime, which increases the impacts of the system.

As illustrated in Figure 4, when the lifetime increases from 30 to 40 years, most of the impacts decrease because the environmental burden of the leading environmental hotspots of the system (PCM and solar collector) are distributed throughout their lifetime. FETP is the only impact category that did not decrease, but had a negligible increase (0.6%). The TEP and HTPnc impact categories showed minor decreases when the lifetime increased to 40 years (3% and 4%, respectively). These small variations can be a result of the inherent uncertainty associated with the data. However, these minor variations can be justified because the solar collector is the primary environmental hotspot in these categories (FETP, TEP, and HTPnc). The solar collector initially has a lifetime of 20 years and, although it increases its lifetime to 25 years, two solar collectors are used when the useful life of the S-LHTES-PCM system increases to 40 years, which does not affect these categories. However, these categories are affected to a lesser extent by the PCM tank that doubles its lifetime, which generates the environmental benefit observed in the TEP and HTPnc categories. On the other hand, the HTPc, ODP, and GWP categories showed the most significant decreases with 53%, 40%, and 36% reduction, respectively. HTPc presents the highest reduction in all

the environmental impacts, because its main environmental hotspots are PCM tank (28%) and heat exchanger (30%). Both parts of the S-LHTES-PCM system increase from 20 years to 40 years, which influences the environmental benefits of the HTPc category. For ODP, the main environmental hotspot is the PCM tank (49%), and, therefore, increasing its lifetime decreases the impact of this category. In GWP, the PCM tank (26%) and the heat exchanger (10%) are two of the main environment hotspots, and increasing their lifetime decreases the global warming impact. This will imply a reduction from 0.29 kg CO2 eq./kWh to 0.19 kg CO2 eq./kWh when the lifetime of the S-LHTES-PCM system increases to 40 years. However, it should be noted that increasing the lifetime of the system could potentially increase the cost of repairs and maintenance. Therefore, the economic impact of increasing the lifetime of the system should be analyzed in future studies.

**Figure 4.** Percentual variation of the environmental impacts for different lifetimes of a solar domestic system with latent heat thermal energy storage technology with phase change material (S-LHTES-PCM). For impact nomenclature, see Figure 3.
