Optimisation of a photovoltaic battery ultracapacitor hybrid energy storage system

Autonomous photovoltaic panels are intermittent sustainable energy sources which require energy storage to balance generation and demand, as photovoltaic generation is time and weather dependent. Traditionally batteries are the most common storage technology for photovoltaic systems. Photovoltaic batteries can encounter extended periods of low State of Charge (SOC), resulting in sulphation and stratification, reducing battery lifetime. Standalone photovoltaic systems are often used in remote areas away from the national grid for water irrigation system, requiring dc motor starting resulting in high inrush current, cathodic protection systems for oil and gas pipelines, emergency phones, warning signs, and telecommunication repeater stations, resulting in pulse discharging of the battery. A combination of depleted battery SOC and high burst current can result in premature loss of load due to stringent battery Low Voltage Disconnect (LVD) limits implemented by the battery management system. A combination of Valve Regulated Lead Acid (VRLA) batteries and ultracapacitors in a Hybrid Energy Storage System (HESS), which increases the power density of the overall system, is examined. Operating the ultracapacitor bank under high power conditions reduces the strain of large current extraction from the battery bank. The addition of the ultracapacitor bank presents the need for a methodology to optimise the photovoltaic system to prevent excess battery storage. This paper outlines the methodology utilised to optimise the combination of photovoltaic panels, batteries, and ultracapacitors for a given solar radiation and load profile employing Matlab software. 2012 Elsevier Ltd. All rights reserved.