**1. Introduction**

Use of renewable energy sources is one of the best solutions to reduce society's dependence on fossil fuels so as to reduce emission of both conventional pollutants and greenhouse gases. Amongst various renewable energy sources, solar photovoltaic (PV) panels are the most commercially viable due to the ease in installation, cost and scalability, especially in countries close to the equator [1]. Wide adoption of renewable energy as a power generation source is only possible if the power can be delivered on and as per the demand of the consumer. In case of solar or wind energy, there is uncertainty with the amount of energy available at any given time as the exploitable energy depends on a number of factors that include location of the PV panel or wind turbine, time of the day, and seasonal and weather conditions [1,2]. Energy storage systems coupled with these energy sources can reduce the impact of their natural fluctuations and can provide power needed by the consumer. An energy storage system used for solar PV applications requires specific properties to aid this integration while maintaining the good performance and long life of the system. High cycle life, high capacity appreciation at slow rate of discharge, good reliability under cyclic discharge conditions, low equalizing and boost charging requirements, high watt-hour (round-trip energy) efficiency and ampere-hour (coulombic) efficiency at different states of charge (SOC) levels, low self-discharge, wide operating temperature range, robust design and low maintenance and cost effectiveness are some of the necessary characteristics anticipated from a battery for storing solar energy [3,4]. Energy storage integrated with a renewable energy source can serve as a stand-alone power generation system for a variety of applications [5,6].

**Citation:** Parmeshwarappa, P.; Gundlapalli, R.; Jayanti, S. Power and Energy Rating Considerations in Integration of Flow Battery with Solar PV and Residential Load. *Batteries* **2021**, *7*, 62. https://doi.org/10.3390/ batteries7030062

Academic Editor: Kai Peter Birke

Received: 30 July 2021 Accepted: 2 September 2021 Published: 8 September 2021

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**Copyright:** © 2021 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/).

Lead acid or lithium ion type batteries have been conventionally used as battery storage devices in integration studies. These batteries are best operated in their safe operating range; in the context of solar PV applications, the life of these batteries degrades considerably due to varying input from solar energy and due to their limited range of operating depth of discharge (DOD) and operational conditions. Lead acid battery is economical, sustainable and has good operational safety and quick response time, but it suffers from low efficiency, low energy density (30–50 Wh/kg) and has limitations on low power rating and DOD for assured life. Lithium ion battery has higher energy density and higher efficiency than lead acid battery but is expensive and prone to thermal runaway and can thus be a fire hazard [7]. In addition, solid state batteries have a fixed power to energy (P/E) ratio due to fixed volume of electrolyte.

A number of studies have been reported recently on improving the characteristics and performance of integrated renewable energy source-energy storage systems. Angenendt et al. [8] studied stringent control strategies based on forecast for a PV-battery system to improve the system performance and battery life. PV self-consumption improvement using frequency restoration reserves was analyzed by Litjens et al. [9] for both residential and commercial applications. Hybridization is another way of improving the system robustness and enhancing its performance with improved life. Single energy source with multiple storage systems (for example, PV with lead acid battery and supercapacitor for improved battery life) has been studied by Jing et al. [10]. Multiple energy sources, for example, solar PV, wind turbine, diesel generator coupled with single or multiple storage systems such as lead acid batteries, lithium ion batteries, flow batteries, reversible fuel cells, etc. have been investigated extensively and various algorithms and optimization models have been employed for better energy management and reduced system cost [11–18].

The present work focuses on redox flow batteries as energy storage system. A redox flow battery (RFB) is an electrochemical energy storage device that has several attractive features especially for large-scale stationary storage, such as independent scalability in energy and power levels, large number of life cycles and absence of fire hazard [19–23]. The energy storage capacity of a typical redox flow battery is determined by the volume of the electrolyte taken, while the power at which the energy can be delivered or absorbed is controlled by combination of active area and number of cells in a stack [20–22]. The positive (catholyte) and negative (anolyte) electrolyte species are stored in separate reservoirs. Each electrolyte is circulated through the respective electrodes of a stack for either charging or discharging of the battery. There is broad consensus that RFBs can be highly costcompetitive when used in large scale power applications such as microgrids, power islands, peak shaving and renewable energy applications [24]. The integration of solar cell and redox flow battery offers a unique advantage, namely, the liquid electrolytes of redox flow battery system can also be used as a coolant for the photovoltaic panels and the battery stacks so as to have integrated thermal management capabilities. Vanadium based redox flow batteries have gained significance and market penetration compared to other flow battery systems in view of its same chemical species on positive and negative redox couples and ease of recyclability [3,20]. Although the VRFB has considerable capacity fade induced by crossover of vanadium species through membrane, either the cross-over can be reduced using operating protocols or the active state can be reversed back by remixing schedules [25–27]. Compared to a lithium-ion battery, it suffers from relatively low efficiency, low energy density and high electrolyte cost. It is considered is to be cost-competitive for GWhscale energy storage applications [24] and several studies have recently reported on its integration with renewable energy sources. Garcia-Quismondo et al. [23] reported on a nine-month performance analysis of a 5 kW/ 5kWh VRFB system coupled to a PV system. Bhattacharjee and Saha [28] designed an electrical equivalent model of 1 kW/6 kWh VRFB system, validated it in MATLAB/SIMULINK and later integrated with solar PV for residential application. Zhang et al. [29] studied an integrated solar PV- VRFB system for residential applications using MATLAB and brought out the importance of battery sizing considering cost and battery efficiency. Sarkar et al. [30] designed an integrated solar

PV, wind turbine, biomass and VRFB system and studied its performance using Homer software. A virtual power plant was designed by Behi et al. [31] using solar PV (810 kW) with VRFB (700 kWh, 350 kW) to cater to 67 dwelling power requirements.

Several design/ simulation studies [12–17,29] have highlighted the importance of optimal sizing of battery energy storage system to ensure uninterrupted energy availability, improved life span of battery, less maintenance and cost, etc. The present work on simulation of the integrated system is primarily experimental and is focused on dealing with natural fluctuations that arise both from supply side (solar PV) and demand side (residential load) in a solar PV-VRFB integrated system. By running different scenarios with a VRFB stack over a seven-day solar PV-load profile, the study brings out how the power and energy characteristics influence the sizing of VRFB and the PV systems in terms of power and energy ratings.
