Introduction Global decarbonization efforts have accelerated the deployment of renewable infrastructures, particularly photovoltaic (PV) systems, due to their scalability and cost-efficiency. However, this transition increases demand for Critical Raw Materials (CRMs), which often involve energy-intensive extraction and refining processes with high greenhouse gas emissions [1]. While traditional assessments focus on material demand, supply risks, and economic security, they frequently overlook the physical quality of resources and the thermodynamic irreversibility of their transformation. Specifically, high-purity refining entails significant exergy losses, indicating material degradation. This study explores existing methodological frameworks to discuss how a thermodynamic perspective could offer a physics-based, complementary metric to socio-economic and environmental analyses, better characterising the long-term sustainability of material-intensive technologies.

Methods This study integrates a quantitative material demand assessment for PV technologies with an exploratory analysis of thermodynamic and thermoeconomic methods addressing resource quality. Using the Italian National Energy and Climate Plan (PNIEC) as a 2030 reference scenario, we estimate cumulative requirements for PV systems, specifically silicon, silver, copper, aluminium, steel, and concrete, and we use their potential environmental impacts are associated using Carbon Footprint (CF) ranges from the JRC report [2]. In this context, it can be useful to complement these quantities with an overview of exergy-based and thermoeconomic approaches and metrics [3, 4] to explicitly address resource quality. The analysis focuses on the conceptual relevance of these physical indicators and their capacity to inform sustainability assessments for material-intensive technologies.

Results and Discussions PNIEC-based assessments confirm that large-scale PV deployment requires substantial bulk and highly refined materials. Projections on future energy demand can be used to calculate the future CO_2eq emissions linked to PV technologies (or their CF). While this metric clarifies climate impacts, it offers a primarily environmental perspective. Integrating thermodynamic frameworks can enhance resource characterization by addressing physical aspects that emission-based indicators overlook, such as resource quality and degradation. Frameworks such as cumulative exergy demand, exergy replacement costs, exergy footprint, and thermodynamic rarity explicitly account for irreversibility and resource quality degradation, e.g., the thermodynamic rarity (ex_TR) accounts for the “amount of exergy resources needed to obtain a mineral commodity from an accessible common rock, using the best prevailing technology” with respect to a reference state, in which the minerals are too dispersed in the crust. This quantity includes a physical cost, meaning the exergy resources needed to convert a mineral into a commodity (embodied exergy, or exergy cost), and a non-visible cost, which accounts for the “natural” cost (exergy replacement cost) of the actual concentrations of ores in mines instead of being dispersed in the crust. It takes into account both the scarcity in the crust together with ore degradation, but also the technology improvements that can be introduced. As concerns the case study, PNIEC PV-related cumulative 2020-2030 demand has been evaluated as: 147-240 kt of PV-grade silicon, 450 kt of aluminium, 276 kt for copper, and 600 t of silver. CF values of PV systems range from 10.8 to 44.0 g_CO2eq/kWh, driven by silicon manufacturing electricity mixes, material intensity, and structural components. Technology lifetime further influences emission intensity per unit of energy. Meanwhile, the thermodynamic rarity values, ex_TR [GJ kg^-1], of the main elements used in PV technologies can vary a lot: Silicon (77.0), Aluminium (681.7), Copper (348.7), Silver (8937.6). This allows to obtain a complementary thermodynamic quantity, related to the second law and to the degradation of resources, that can be further used to assess the resources' sustainability. Consequently, these approaches can offer a thermodynamic foundation for assessing the long-term sustainability of material-intensive technologies, although such methodologies are rarely applied in CRM-oriented policy analyses or systemic evaluations of green technologies.

Conclusions While material demand and CF indicators are essential, they overlook the physical implications of resource transformation. Thermodynamic approaches complement these metrics by addressing resource quality and irreversibility, fostering a systemic sustainability evaluation of material-intensive technologies.

References

[1] International Energy Agency. Global Critical Minerals Outlook 2024. Paris: IEA; 2024.

[2] European Commission, JRC. Harmonised rules for the calculation of the CF of PV modules in the context of the EU Ecodesign Directive. Luxembourg: Publications Office of the European Union; 2025.

[3] Szargut J. Exergy Method: Technical and Ecological Applications. Southampton: WIT Press; 2005.

[4] Sciubba, E. A Thermodynamic Measure of Sustainability. Frontiers in Sustainability 2021, 2, 739395. DOI: 10.3389/frsus.2021.739395

[5] Iglesias-Émbil et al. Raw material use in a battery electric car - a thermodynamic rarity assessment. Resources, Conservation & Recycling 2020, 158, 104820. DOI: 10.1016/j.resconrec.2020.104820