Modelling the hydraulic performance of an array of centrifugal pumps operating under a variable pressure in a hydro-pneumatic energy storage system

Abstract The large pressure variation in hydro-pneumatic energy storage systems presents a major challenge for maintaining a high operating efficiency of the turbomachinery used in the hydro-electric energy conversion processes. This paper applies numerical modelling to compare the hydraulic performance of two pumping unit variants used for charging an offshore hydro-pneumatic energy storage (HPES) accumulator. The first unit consists of a single large centrifugal pump operated at variable speed while the second pumping unit is made up of an array of smaller, dynamically similar fixed-speed pumps. A number of series/parallel configurations for the latter option were derived in order to optimise the operational efficiency at various states of charge of a gas-charged accumulator, specifically between 80 and 200 bar. A comparative analysis of the operational hydraulic efficiencies for both pumping units shows that using an array of small fixed-speed pumps arranged in series/parallel configurations is more energy efficient than using a single large pump operated at variable speed.