*2.3. Sensitivity Analysis*

A sensitivity analysis was performed to assess the effect on expanding the life expectancy of the S-LHTES-PCM system. The lifetime of each part of the system is highly uncertain as it depends on variable aspects such as the level of maintenance. S-LHTES-PCM systems are a new and under development technology, and there is the potential to increase the life expectancy of the different parts of the system with appropriate maintenance, in line with the circular economy strategy of slowing [42–44]. For this reason, it has been estimated that the heat exchanger and inner tank can increase their useful life up to 40 years (from the initial 20 years; see Table 1), while the solar collector, expansion vessels, and PCM useful life can be increased to 25, 20, and 15 years, respectively (from the initial

20, 15, and 10 years; see Table 1). For pipes, valves, and water pumps, the useful life was not changed. Considering these premises, the sensitivity analysis considers different levels of lifetime extension (25, 30, 35, and 40 years of operation) and compares them with the current situation (20 years).
