Technical and economic analysis of pump-as-turbine systems versus commercial turbines for harnessing energy from small water potentials

Greenhouse gas emissions from human activity pose a significant threat to the sustainability of modern society. Utilizing renewable energy sources is a key strategy to mitigate these emissions. Hydroelectricity, the prevailing renewable energy source, effectively fulfills approximately 17 % of the global requirements for electrical power. One potential strategy to reduce the cost of electromechanical equipment in hydropower plants is the utilization of pumps as turbines (PAT). Harvesting energy from dams not designed for pumped storage systems and with capacities below 5 MW poses technical and economic challenges due to seasonal variations in head and flow. This study presents two key novelties. First, the selection and arrangement of PATs for a dam with variable head and flow are economically optimized using the particle swarm optimization (PSO) algorithm. The impact of inverters on economic performance is also considered in the optimization process. Second, it presents a technical and economic comparison between hydroelectric power plants utilizing Francis turbines and those employing PATs. A comprehensive database of approximately 100 pumps was assembled, and the optimization process was carried out monthly. The research objective function includes net present value (NPV) and a penalty function to align the operating flow rate with the dam average flow rate. The case study of this research is a dam in Iran. The findings reveal that variable-speed PATs offer the greatest operational flexibility by closely aligning the operating flow rate with the dam average flow rate compared to PATs without inverters. However, using inverters increases both capital costs and the payback period. The results also demonstrate that the Francis turbine generates approximately 10 % more energy than the PAT power plant; however, its capital cost is nearly four times higher. Consequently, the payback period is extended by a similar factor and the NPV is decreased by approximately 4 %, despite the improvement in energy efficiency. In conclusion, if minimizing cost and achieving a reasonable return are most important, the PAT power plant with inverter is the preferred option. But if maximizing energy output and achieving optimal water management are the priorities, The utilization of the Francis turbine is the better choice.