###### Abstract  
Hydro generation is the simplest and oldest method of electricity generation, with a century of successful operation. Using a pump as a turbine (FAT) is an optimal solution for minimizing the cost, particularly in low-head and small-scale hydro plants. Commercially available centrifugal pumps have become a popular solution for small-scale hydro and pumped-hydro facilities owing to their simple geometry, ease of operation, maintenance, and abundant availability in local markets. Variations in the water flow in hydro facilities, such as pumped-hydro and small-scale hydro facilities, are common; however, a PAT is unable to respond to variable flows because it is a fixed-speed device. To overcome this problem, different techniques have been suggested by researchers: (a) a system of parallel PATs; (b) geometrical modifications in the impeller of the PAT; and (c) power electronics-based variable frequency drives. All the aforementioned techniques have limitations, such as low output, high cost, complexity, transportation, and operation and maintenance. In this study, a simple and economical technique is proposed to smooth the output of a PAT on variable/decreasing water flow profiles. In the proposed technique, water columns connected in parallel (PWCs) are used to produce a pressurized water flow, as they have a convergent nozzle at the outlet. The PWC creates more space for water, and this additional mass of water boosts the water flow at the outlet. In this manner, the PWC technique maintains the flow at the inlet of the turbine. A serial integration of five PWCs with the same dimensions was conducted to inject the additional flow into the existing PAT pentstock, governing a 37-kW generator. The design flow was maintained at 192.1 L/s without any additional power usage at the inlet of the PAT, and the output was smoothed even at the minimum water flow/head. Pump design and computational fluid dynamics simulations were performed using ANSYS software, whereas generator simulations were performed using MATLAB/Simulink software. energy resources; hydropower generation; small hydro; pump as turbine; PWC technique; smooth output power +
Footnote †: journal: _Sci. In pumped hydro storage (PHS) and small-scale hydro plants, using pumps in reverse (PAT) instead of conventional turbines is preferable to minimize the capital cost [10; 11; 12]. However, commercially available pumps are not designed to function in reverse mode, and if this mode is exercised, the pumps prove to be low-efficiency devices. Despite their low efficiency, PATs are used in small-scale hydro-generation and PHS facilities [13]. The use of PATs is abundant for numerous reasons, such as their low cost, bulk availability in local markets, and ease of installation. Furthermore, no specialty is required for operation and maintenance [14]. However, small-scale PHS and hydro-generation plants encounter flow fluctuations for several reasons, including intermittent upstream inflows and the decrement of the head in the upper reservoir of the PHS facility during generation. A PAT is unable to respond to these flow fluctuations owing to its simple geometry, and the output of the plant decreases. The design flow, which is obtained by the design head, determines the maximum efficiency of a PAT. Variations in the flow from its original design flow, for any reason, have an adverse effect on the output [15]. Researchers have adopted various methods to mitigate the adverse effects of flow fluctuations on the output, such as parallel PATs of different capacities, geometrical modifications of a PAT, and power electronics-based variable frequency drives (VFDs). In parallel PATs, when the flow is reduced and a PAT with a large capacity cannot deliver its rated output, it is either manually or automatically turned off. A PAT with a lower capacity is operated to minimize the adverse effects of flow fluctuations.