**1. Introduction**

Environmental effects, fear of the extinction of conventional energy sources (such as oil, gas, nuclear, and coal), and the high running costs of these costs have compelled planners toward renewable energy sources [1–4]. Among these resources, hydro energy contributes to nearly 16% of the global energy mix [5,6]. However, extensive capital costs, danger to aquatic life, significantly long construction times, and limited locations are the drawbacks of hydro-generation technology. To overcome these drawbacks, new arrangements for hydro facility construction, such as run-of-river, small hydro, and mini, micro, and picohydro plants, have been studied [7–9]. These new arrangements of hydro technology have reduced the initial cost, construction time, and environmental effects while increasing the location spectrum for the construction of hydro plants.

**Citation:** Hussain, S.; Humza, M.; Yazdan, T.; Abbas, G.; Cho, H.-W. Parallel Water Column Technique for Obtaining a Smooth Output Power of the Pump as a Turbine at a Variable Water Flow Rate. *Appl. Sci.* **2023**, *13*, 3232. https://doi.org/10.3390/ app13053232

Academic Editor: Luis Hernández-Callejo

Received: 31 January 2023 Revised: 27 February 2023 Accepted: 1 March 2023 Published: 2 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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 [10–12]. 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. However, the output is reduced by the newly available water flow, and the cost increases owing to paralleling, thereby making the design more complex [13,16]. Regarding the geometrical modifications of PATs, different researchers have suggested certain design modifications of the impeller [13,17–22], which better manage the variable flows. However, these modifications are unable to manage all the flow variations, and the cost is increased. Power electronics-based VFDs can provide any speed profile of variable flow inputs [23,24]. When the flow decreases, which affects the output of the plant, the VFD changes the speed of the generator, and the output of the plant becomes smooth. However, this method is significantly costly because devices with high ratings are required, causing installation, operational, maintenance, and transportation problems.

In this study, a simple technique is proposed to mitigate the adverse effects of flow variations on the output of a plant. This study is novel because a parallel water column (PWC) technique is used to smooth the output of a PAT system while maintaining the water flow at the inlet of the PAT by acting as an auxiliary penstock. With this technique, water columns connected in parallel are used with the existing penstock of a plant to inject additional flow at the inlet of the PAT in the event of a decreasing flow. This additional flow will maintain the smooth rated output of the plant, even at the minimum head. The effectiveness of the PWC technique was confirmed by performing a simulation using ANSYS software for the case study of a variable flow profile. Subsequently, based on the output obtained from the PAT, the electrical output of the generator was analyzed using MATLAB/Simulink software.
