Transforming conventional hydropower plants into pumped storage by turbine replacement

Abstract

While the global community has started to realize the impact of the climate changes, the global temperatures are rising, extreme weather occurs more frequently and certain species are struggling to adapt. To slow down the worrying development, a fast transition into a renewable and sustainable society is critical. Recent years, the development of wind and solar power in Europe has been formidable, and the growth will most likely continue in the years to come. As the share of non-regulative energy sources at the continent continue to grow, the grid has become less stable. Periods with too much excess energy has occasionally caused problems for power companies, who has been either forced to halt the production or forced to produce at negative electricity prices to handle regulatory demands and safety. The opposite situation has caused pressed supply conditions, where the industry and private households has been encouraged to lower their consumption to cover the demand. It is no doubt that this situation will persist in the years to come, as even more non-regulative energy sources are deployed. Preparing the grid for the future has caused a need for regulatory effects in the power system. Pumped storage is excellent for this purpose, as electricity can be produced in periods with shortage and consumed in periods with excess energy by pumping the water back to the upper reservoir for storage. However, there are few suited locations for new pumped storage project left. An option is to transform conventional hydropower plants into pumped storage, but that also brings forward many uncertainties and challenges. The focus of this research has been to investigate how an existing Francis turbine can be redesigned into a reversible pump-turbine to reduce the extent of construction work needed for a transformation. A 50 MW Francis turbine at Roskrepp hydropower plant has been used as a test object for this purpose. A booster pump, located in series with the pump-turbine, is proposed as a solution to handle some of the pumping challenges and cavitation issues. The research is not revolving much around the booster pump technology itself. Three reversible pump-turbines have been designed with wrap angles 90◦, 110◦ and 130◦. The primary objective has been to investigate the wrap angles’ effect on stability and performance. The results conclude that the wrap angle has an important effect on the distribution of relative velocities within the runner channel. Controlling the wrap angle can improve the slip, but also reduce relative circulation between the runner vanes, which is especially important working with a limited pump-turbine design scope. The slip effect is most severe for the pump-turbine with a 130◦ wrap, while the pump-turbine with a 90◦ wrap is most exposed to re-circulation along the blade pressure side. The pump-design with a 110◦ wrap seem to provide the most stable results, as both the slip effect and re-circulation is reduced. However, having a varying wrap distribution along the leading edge in pump mode will most likely improve the performance further.