Variable Speed Regulation for Pump as Turbine in a Micro Pumped Hydro Energy Storage Application

The need for storage technologies is recently emerging and more discussed in micro scale. Pumped hydro energy storage (PHES), the most used technology in energy storage, represents a valid option for its maturity, proven long life-span and rather high efficiency. Using a centrifugal or diagonal Pump As Turbine (PAT) in the hydroelectric generation is a valid trade-off between capital cost and performance. However, commercial pumps are not designed to run in reverse mode. Hydraulic efficiency and working range are so limited. Variable speed regulation allows the PAT to maintain a nearly constant high efficiency all the time regardless of the head availability or the instant surplus of energy and enlarge the operability range in pumping and generating modes. This paper presents an overview of an existing micro-PHES installation of about 17 kWh capacity. This facility is equipped with a PAT coupled with a Variable Frequency Driver (VFD), which allows the system to deal with variable load keeping high efficiency. The characteristic curves of the pump in reverse mode are analysed defining the exploitable PAT working range. The use of variable speed pumping in PHES integrated into a smart grid aims to give flexibility in storing energy. The extraordinary adaptability recorded on the site in partial load until 30% clearly endorses the coupling of the PAT and the variable speed driver.