Energy recovery from wastewater in deep-sea mining: Feasibility study on an energy supply solution with cold wastewater

Deep-sea mining has been regarded as one of the most promising technologies to meet the increasing demands for minerals and metals. The exploration of renewable energy supply solutions should be prioritized because the current solutions are highly dependent on fossil fuels. In this study, recognizing that the wastewater transported with minerals to the production support vessel is a potential cold source, the wastewater energy recovery is proposed for the first time. Focusing on exploring an efficient and cost-effective solution for wastewater energy recovery, the organic Rankine cycle (ORC) using six working fluids and Kalina cycle are investigated, and a multi-objective optimization for the levelized cost of electricity (LCOE) and exergy efficiency is presented. Based on the design criteria and measured data of the Solwara 1 Project of Nautilus Minerals, optimization results show that R601 with the highest critical temperature used in ORC performs best in terms of LCOE (0.102 $/kWh) and exergy efficiency (40.2%), compared with 0.125 $/kWh and 42.5% of ammonia-water mixer (0.966/0.034) used in Kalina cycle. In addition, the discounted payback periods of ORC and Kalina cycle are 12.71 years and 18.95 years respectively, indicating that ORC is economically more feasible than Kalina cycle. Moreover, the feasibility of the proposed system is validated by its economic benefits, good performance compared with ocean thermal energy conversion systems, and high flexibility and compatibility. The wastewater energy recovery is expected to provide a sustainable and low-carbon energy supply idea for deep-sea mining.