**4. Conclusions**

This study analyses, for the first time, 18 environmental impact categories for an S-LHTES-PCM system. The environmental performance of the system is compared against traditional heat sources in the UK (biomass, heat pumps, and natural gas). Moreover, a sensitivity analysis was performed based on the lifetime of the S-LHTES-PCM system.

Thanks to the implementation of the life cycle assessment methodology, it was possible to identify the main environmental hotspots of the S-LHTES-PCM system: the solar collector, PCM, PCM tank, and heat exchanger. Altogether, these parts contribute over 83% in all the 18 impact categories evaluated. The environmental impacts are mainly associated with the system's raw materials and the energy consumption in the production processes. For this reason, extending the lifetime of the systems according to circular economy principles improves the system's environmental performance. In this sense, thanks to a sensitivity analysis, it has been demonstrated for the first time that when the S-LHTES-PCM system lifetime is increased to 40 years (from the initial 20 years considered), the environmental performance improves and can be a competitive option from an environmental perspective if compared with other traditional household energy sources, like natural gas, biomass, and heat pumps.

The results of this study will work as a baseline to identify where LHTES systems need to improve before being implemented commercially in the UK. Even more, the general conclusions and hotspots identified can be applied to the implementation of similar systems in other countries. Future research should investigate the social and economic aspects to have a holistic vision that considers the three pillars of sustainable development. The results obtained could be of special interest to stakeholders of the construction and energy sectors and policymakers interested in potential solutions to achieve a more sustainable delivery of space and water heat in households.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/su132011265/s1, Table S1. Main features of the piping in the phase change material (PCM), collector and water tank circuits. Adapted from Englmair et al. (2020).

**Author Contributions:** Conceptualization, D.C.B., E.M., G.M., A.S. and A.G.-S.; Data curation, D.C.B. and E.M.; Formal analysis, A.G.-S.; Funding acquisition, A.S., H.A. and A.G.-S.; Investigation, D.C.B., E.M. and A.G.-S.; Methodology, A.G.-S.; Project administration, A.S., H.A. and A.G.-S.; Resources, G.M. and A.G.-S.; Software, D.C.B. and E.M.; Supervision, A.S. and A.G.-S.; Validation, A.G.-S.; Writing—original draft, D.C.B., E.M. and A.G.-S.; Writing—review & editing, D.C.B., A.S., H.A. and A.G.-S. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to acknowledge the financial support received by the EPSRC Supergen Energy Network (EP/S00078X/1) through the Flex Funding of the project 'Whole system analysis impact of thermal energy storage technologies in future UK energy networks'.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The images in Figure 2 were obtained from Wikimedia Commons. The authors would like to thank Xi Zhang for his contribution to the article with data regarding the main sources of heat in UK households.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **Acronyms**


