Optimal planning and operation for a grid-connected solar–wind–hydro energy system in wastewater treatment plants

This study proposes a multi-objective optimization model for a grid-connected wind–solar–hydro system in wastewater treatment plants, addressing trade-offs among electricity utilization cost, self-sufficiency, complementary effect, and carbon emission. Two modes are explored: one allowing energy export and another leading to curtailment. A case study from a plant with design scale of 50,000 m/day in China found that a higher complementary rate can slightly increase costs, lower self-sufficiency, and increase carbon emissions. Compared to the initial scheme without the solar–wind–hydro system, the design modes of export and curtail reduce electricity costs by 2.15 and 0.87 million CNY, carbon emissions by 2760 and 1439 tons annually, and increase self-sufficiency to 52% and 42%, respectively. The levelized cost of energy is 25% higher and total installed capacity decreases by 54% when surplus cannot be exported. A 6.7% curtailment minimizes the electricity utilization cost. Complementarity rates show seasonal fluctuations, peaking in August, with the highest yearly rate of 0.1528 at a solar-to-wind capacity ratio of 1.747. Output ratios remain stable across different treatment scales under curtailment mode: imports about 60%, wind and solar about 20% each, and hydropower 2%. Price changes and potential for zero-carbon operation are further discussed.