Optimized design procedure for small-scale Pumped Hydro Energy Storage systems based on the use of pumps as turbines

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Abstract Pumped Hydro Energy Storage (PHES) is a well-established technology offering large storage capacities. In recent years, small-scale PHES plants are becoming more interesting, due to their possibility of being integrated with renewable-based microgrids. However, the high capital costs represent the most critical economic factor for such storage systems. The cost of the hydraulic machines can be mitigated by using commercial centrifugal pumps in reverse mode (the so called Pump as Turbine, PAT, technology) in place of small hydro-turbines. One of the main issues in PHES systems that operate with a single machine is the procedure followed to select the optimal PAT for a given case study. In this study, an automatic selection method was developed, utilizing commercial pump data and optimization algorithms in MATLAB to identify the optimal PAT configuration based on site-specific characteristics and user load profiles. To test the effectiveness of the proposed procedure, the case study of a small-scale PHES located in Sardinia (Italy) coupled with a photovoltaic plant is considered. Results demonstrated that while the choice of the PAT closest to the theoretical best efficiency point exhibit higher nominal efficiencies, their actual operational efficiencies showed considerable variation. This underscores the importance of both developing an algorithm capable of comparing different PATs and defining a method to determine an adequate group of PAT candidates. Additionally, the number of PATs installed significantly impacts the performance of PHES systems. Increasing the number of PATs generally enhances flexibility and efficiency in energy usage during the charging phase, leading to greater energy output during discharge phases. However, operating an excessive number of PATs far from their nominal conditions can result in efficiency losses.