Pumps as turbines for pumped hydro energy storage systems - A small-size case study

Pumped Hydro Energy Storage (PHES) technology has been used since early 1890s and is, nowadays, a consolidated and commercially mature technology. PHES systems allow energy to be stored by pumping water from a lower-to a higher-level reservoir. Subsequently, this energy can be released through a turbine placed in a penstock, which connects the two reservoirs, to produce energy. Although these plants have historically been employed at large power scales (in the order of hundreds of MW), in recent years, micro- and small-scale plants are becoming more interesting, due to their possibility of being integrated with renewable energy systems (RES) used in autonomous island grids. Capital costs associated with hydraulic machines used in PHES systems represent the most critical economic factor, which can be mitigated by using commercial centrifugal pumps in reverse mode (Pumps as Turbines, PATs) in place of small hydro turbines. These expected economic benefits must be weighed in each specific case study, with some drawbacks related to the use of PATs, mainly associated to a lower round-trip efficiency with respect to specifically designed pumps and turbines. In this work, a small-scale PHES plant has been studied coupled to an existent photovoltaic system for the integration in the electric grid of a small island in Southern Italy. Two different PHES outlines have been compared based on techno-economic considerations. The former is a typical PHES system composed of both pumps and a turbine, while the latter uses only an array of parallel pumps which work also in reverse mode. The analysis demonstrates the feasibility of integrating a photovoltaic and PHES plant, which results in a lower cost of electricity production, while PHES performance in the PAT-based outline results penalized by the lower efficiency of PATs with respect to the hydraulic turbine.