Optimal power flow management for distributed energy resources with batteries q Henerica Tazvinga ⇑ , Bing Zhu, Xiaohua Xia Center of New Energy Systems, Department of Electrical, Electronic and Computer Engineering, University of Pretoria, Pretoria 0002, South Africa article info Article history: Received 29 September 2014 Accepted 7 January 2015 Available online 29 January 2015 Keywords: Conflicting Variable nature Energy management Hybrid energy system Dispatch strategy abstract This paper presents an optimal energy management model of a solar photovoltaic-diesel-battery hybrid power supply system for off-grid applications. The aim is to meet the load demand completely while sat- isfying the system constraints. The proposed model minimizes fuel and battery wear costs and finds the optimal power flow, taking into account photovoltaic power availability, battery bank state of charge and load power demand. The optimal solutions are compared for cases when the objectives are weighted equally and when a larger weight is assigned to battery wear. A considerable increase in system opera- tional cost is observed in the latter case owing to the increased usage of the diesel generator. The results are important for decision makers, as they depict the optimal decisions considered in the presence of trade-offs between conflicting objectives. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction A hybrid renewable energy (RE) system comprising solar photo- voltaic (PV) generators, diesel generators (DGs) and battery storage in a hybrid system can solve single source power supply problems. RE systems incorporating DGs and batteries have been studied by various authors, such as Muselli et al. [1], Dufo-Lopez and Ber- nal-Augustin [2], Jenkins et al. [3], Adaramola et al. [4], but battery wear has not been evaluated in the analyses. The lifetime charac- teristics of battery energy storage systems have therefore not been fully considered in many RE based hybrid energy management optimization studies. The variable nature of RE sources means that the battery banks in PV applications experience a wide range of operational conditions, including varying rates of charge and dis- charge, depth of discharge (DoD), temperature fluctuations and charging strategies [5,6]. These operating conditions vary signifi- cantly in different locations and applications. Battery lifetime is thus determined by operating conditions, which are a function of the system sizing and the dispatch strategy. In most RE based hybrid systems, battery banks constitute a major part of the invest- ment costs and are often the most expensive component when considering the lifetime costs, as their lifetime is considerably shorter than that of any of the other hybrid components [7]. Talaq and EI-Hawary [5] investigate the performance and expected life- times of different sized batteries, using a previously developed lead acid battery model. The results, based on the lifetime algorithm assumptions used, show that the lifetime of a battery should increase linearly with battery size. Kaiser [8] developed a battery management system that considers the various characteristics of the individual battery strings and decides how the strings are trea- ted considering the load profile. A grid-tied microgeneration and storage model has been developed for quantifying the performance of energy storage options and the challenges of relying on micro- generation for autonomy are highlighted [9]. Riffonneau et al. [10] also propose a grid-tied system with a peak shaving service as a way of increasing the penetration of PV production in the grid and consider battery ageing, but the PV generation is not opti- mized. An optimal hybrid scheme of a micro-grid with combined heat and power that consists of a gas-engine, wind generator, and PV generator, with the objective of minimizing fuel consump- tion, is proposed by Hernandez-Aramburo et al. [11]. The bone of contention is that in most optimisation work battery wear cost is neglected, yet battery lifetime in RE based applications poses great uncertainty for investors owing to the replacement cost during the hybrid system’s lifetime. This paper minimizes the operational cost of a PV-diesel-bat- tery (PDB) hybrid system in which lead-acid batteries are used. The main contribution is the consideration of battery wear cost, as battery wear has a great impact on battery life and this has not been considered in the optimization of RE based distributed http://dx.doi.org/10.1016/j.enconman.2015.01.015 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved. q This article is based on a four-page proceedings paper in Energy Procedia Volume 61 (2015). It has been substantially modified and extended, and has been subject to the normal peer review and revision process of the journal. ⇑ Corresponding author. Tel.: +27 12 420 2068. E-mail address: henerica.tazvinga@up.ac.za (H. Tazvinga). Energy Conversion and Management 102 (2015) 104–110 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman