A Risk Assessment Methodology to Prioritise Risks in Large, Stationary Li-Ion Battery Systems

Abstract

Performing a risk analysis is the basis in all engineered products. No specific methodology exists for stationary batteries. Indications on the needed risk assessment may be derived from standards that cover large stationary Li-ion battery storage systems in their scope [1]. However, they hardly go further than referring to the general risk assessment techniques with as examples Failure Mode and Effect Analysis (FMEA) and Fault Tree Analysis (FTA). Despite this lack of specific guidance, a risk assessment for large battery systems has their uniqueness due to their structure. There are many levels between battery cells and the complete system. Moreover, a huge number of cells are used that must be correctly dealt with in the assessment. In the scientific literature only a few papers are dedicated to risk assessment on batteries. This contribution sets up a complete framework of methodologies and protocols for safety testing of stationary Li-ion batteries for large-scale grid-connected applications. The risk assessment is based on the well-known Failure Mode Effects Analysis (FMEA) method. In a first step, a generic hierarchical system diagram of a stationary battery is developed to be used as the system under study in the FMEA. To improve the completeness and the objectivity of the analysis, improvements to the FMEA methodology are proposed and used. The main goal of these improvements is to limit the dependency on expert judgement and inspiration and to improve the prioritization of risks without adding unnecessary complexity. A core issue is that the method of calculating the Risk Priority Number (RPN) is not sufficient fo large battery systems. This problem was also detected by [2]. The RPN does not take suitable care of the numbers of cells, modules, etc. and has no aggregation per level. Risks that are mitigated on several system levels have to be aggregated in an advanced RPN methodology. This leads to a thorough risk assessment, in which it is easy and meaningful to select the most prominent project risks. This contribution is an outcome of the European FP7 project Stallion. The project has set up safety tests beyond state of the art for stationary Li-ion battery systems. It also resulted in a handbook on battery system safety for the broad audience [3]. [1] Survey on standards for batteries and system integration with them. VITO, 2015. [Online] batterystandards.vito.be.[2] Analysis of Battery Safety and Hazards’ Risk Mitigation. Ashtiani, C.N. 2008, ECS Transactions, Vol. 11, pp. 1-11 .[3] STALLION Handbook on safety assessments for large-scale, stationary, grid-connected Li-ion energy storage systems, DNV GL, Arnhem 2015. [Online] stallion-project.eu Figure 1