Design method and optimization of Deriaz pump turbine for hydraulic energy storage

POLITECNICO DI MILANO Abstract Master’s Degree in Energy Engineering Design Method and Optimization of Deriaz Pump-Turbine for hydraulic energy storage by Alessandro Morabito Hydro-power energy is available in abundance in form such as falls, canals, rivers and dams. Hence, there are various types of sites with different parameters like flow rate and heads. Depending upon the sites, water turbines are designed and manufactured to avail hydro power energy. To cope with possible variations in load, more careful design choices are required. Although pumps and turbines are designed for a particular flow rate, in practise, they operate across a certain range of flows rates. Indeed, it is totally realistic that a machine might work temporary in off-design especially in hydraulic plants with remarkable changes in operating conditions. At off-design conditions of flow rates, the flow pattern deviates from the ideal. The flow separates at the impeller tips, in the impeller and in the diffuser or volute. Recirculation may appear at the inlet, flow returning from the impeller eye into the inlet pipe. Recirculation may also occur at the impeller exit, fluid flowing back into impeller from the diffuser of volute. Not only it deteriorates the efficiency at off-design condition, but cavitation can appear, which can destroy the impeller. Excessive vibrations appear which reduces bearing life and reliability, and even the entire pump system can enter into a resonant oscillation. Catastrophic failures are unusual, but off-design point operation for a longer period of time seriously reduced pump life. In selecting a pump, much care must be given in estimating margin of excess capacity that might be required by the system, supplied by the pump or required at the turbine. Manufactures usually provide guidelines for the safe range of their turbo-machines. A too large margin normally causes the pump to operate far from off-design but at bad efficiency value: accordingly they work at conditions that reduce its reliability and life. It becomes an economic issue whether low efficiencyat nominal operating conditions (and corresponding operating cost) will balance avoiding losses from lack of excess capacity at unusual occasions. This project aims to draw the state of the art of a not well-known pump-turbine type Deriaz, showing cases and analysis of its designed model. The Deriaz pump turbine promises high efficiencies in a wide range of partial load, thanks to its efficient regulation by means of adjustable runner blades and guide vanes. Since the actual cultural trend shows wisely an increasing interest in a smarter use of energy, the philosophy of the complementary use of renewable energies and storage energy system is strengthened. One of these technologies is the Pump Hydroelectric Storage (PHS). It can be combined excellently with a reversible turbo-machine able to adapt quickly to fluctuations of demands of electrical power. Hence this study wants to analyse the design steps and the theory which are needed to optimizing this type of reversible pump. These tasks are supported by the check of flow behaviour in computational fluid dynamic (CFD) simulations. The study is so carried out for design of part-load conditions at different operating points in turbine mode and in pump mode in order to confirm its versatile behaviour and its feasibility.