Multi-objective optimisation of renewable energy integration in circular brine valorisation/treatment: The case of the CARMEn chain

Circular brine-valorisation chains offer a promising route to address water scarcity and critical raw-material recovery, but their large electrical and thermal energy demands can offset environmental benefits if powered by fossil sources. This work develops a fully integrated, hourly resolved simulation and multi-objective optimisation framework for the CARMEn treatment chain − combining nanofiltration, Mg(OH)2 precipitation, softening, electrodialysis with bipolar membranes, membrane distillation and reverse electrodialysis – coupled with a hybrid solar energy system comprising photovoltaic panels, lithium-ion battery storage, flat-plate solar-thermal collectors and a hot-water storage tank. Three representative brine feeds are analysed at a saltwork site in Trapani (Italy): reverse-osmosis brine, nanofiltration retentate and saltwork bittern, each targeting 50 t/y of Mg(OH)2 production. A deterministic Python model computes 8760 h energy balances, system dispatch, discounted cash flows and operational CO2 emissions. A multi-objective genetic optimisation algorithm explores the trade-offs between net present value, electrical and thermal coverage, and capital expenditure. Results show that all scenarios are economically viable at baseline market conditions (net present value of 224–277 k€, internal rate of return of 11–11.5 %, payback of about 8 years), with nanofiltration retentate achieving the best economic performance. Bittern attains comparable renewable shares (electrical coverage of 54 %, thermal coverage of 74 %) at the lowest capital expenditure (514 k€). Sensitivity analysis identifies the electricity export tariff as the primary economic driver, while battery capital cost governs storage sizing, with an economic break-even threshold around 150 €/kWh. Avoided CO2 emissions range from 37.8 t/y for bittern to 85.4 t/y for reverse-osmosis brine, with the thermal contribution dominating in heat-intensive scenarios. The proposed framework provides a systematic tool for the co-design of solar-powered circular brine-valorisation systems, with the specific quantitative results reflecting the solar resource and regulatory context of the Trapani case study considered here.