Defining Embodied Emissions for Concentrated Solar System, in the framework of the PYSOLO project

The PYSOLO project—Pyrolysis of biomass by concentrated solar power, funded through Horizon Europe—explores the use of solar energy to drive biomass pyrolysis in a sustainable way. This study focuses on the environmental performance of the solar subsystem, which consists of a heliostat field and a rotary kiln solar receiver. A distinctive feature of the system is its use of particle heat carriers (PHCs), including silica sand, bauxite, and olivine, which circulate inside the rotary kiln. The kiln simultaneously acts as both the solar receiver and the heat transfer medium in a central tower setup. Once heated, the particles are directed to a downstream reactor where pyrolysis occurs. The solar subsystem is designed to deliver 15 kW of thermal power, using a configuration of five heliostats to ensure reliable performance under realistic solar conditions. Three operational scenarios are evaluated, each employing a different PHC material. The life cycle assessment (LCA) defines the functional unit (FU) as 1 kWh of useful thermal energy delivered to the particle bed in the kiln. A Net Functional Unit (NFU) of 98,550 kWh is used to represent a laboratory baseline equivalent to two hours of daily operation over 15 years. Since PHCs are continuously reused, only replacement material needed to account for wear and loss is considered. This allows consistent and fair comparison of the three PHC scenarios. As the system is still at a development stage, the analysis relies on numerical simulations, engineering assumptions, and scaled estimates rather than measured data. The LCA was performed with OpenLCA, in line with ISO 14040/44 standards, using the Ecoinvent v3.10 database complemented with custom data. The study covers the entire life cycle of the solar subsystem—from construction and operation to end-of-life—and evaluates six environmental impact categories using the ReCiPe 2016 Midpoint (H) method. Particular attention is paid to greenhouse gas emissions, non-renewable energy use, land use, and water consumption. The results show that heliostat construction is the dominant contributor to environmental impacts across categories. In the climate change category, for example, heliostat construction accounts for 57% of emissions, followed by heliostat operation (25%), rotary kiln construction (12%), and end-of-life treatment (4% for heliostats and 1% for the kiln). Rotary kiln operation contributes only 1%. Human toxicity is also mainly linked to heliostat construction (77%), while resource use in the bauxite case is largely driven by heliostat (48%) and kiln (32%) construction. For land use (sand case), heliostat construction again dominates (62%), followed by heliostat operation (25%). The choice of PHC material leads to only minor variations, with the overall impact profile consistently shaped by heliostat construction. Despite its reliance on modeled data, this study provides the first comprehensive environmental assessment of the PYSOLO solar-biomass system. It identifies key environmental hotspots, highlights the influence of design choices, and offers guidance for improving the sustainability of future PYSOLO deployments.