Economic benefit analysis of retrofitting a fixed-speed pumped storage hydropower plant with an adjustable-speed machine

For pumped storage hydropower plants with fixed-speed electrical machines, turbines typically operate between 70 and 100 percent of rated power whereas pumps usually run at constant load. If the fixed-speed machine is replaced with an adjustable-speed machine, the operating range of the pump increases, thereby increasing flexibility and enabling the provision of ancillary services in pump mode. This paper estimates the economic benefits of converting an existing fixed-speed pumped storage hydropower plant to adjustable speed. We consider the test case of the Grimsel 2 pumped storage hydropower plant, located in Switzerland. We implement a mixed-integer linear program to optimize operations at Grimsel 2 considering the structure of Swiss energy and ancillary service markets. Our results show that upgrading a fixed-speed plant to adjustable speed could increase total revenue up to 58 percent.