Techno-Economic Assessment of Free Cooling for Liquid-Cooled BESS

The thermal management of Battery Energy Storage Systems (BESS) plays a critical role in ensuring both safety and optimal performance. In stationary storage applications, liquid cooling is gradually replacing air cooling; however, the use of outdoor air temperature for direct cooling of the BESS heat transfer fluid, through free cooling technology, has not been extensively explored in the literature. Despite its potential for improving system efficiency and ease of implementation, this approach remains under-evaluated. This thesis presents the development of an extension to the BESS simulation tool SimSES, enabling the modelling of liquid cooling in combination with free cooling. Based on literature and industrial data, the extension incorporates a hybrid free cooling-vapor compression system, based on a BBR Energie design, as well as a generic vapor-compression system for comparison purposes. Subsequently, the impact of free cooling on the overall efficiency of BESS is assessed through simulations of a 4 MWh/2 MW BESS participating in Frequency Containment Reserve (FCR) operations. The study considers three distinct climatic environments in Europe: Stockholm (Sweden), St. Etienne (France), and Madrid (Spain). The results show an efficiency improvement between 3.2% and 3.49%, reaching a maximum value of 87.64% in the Stockholm simulation. Notably, the efficiency difference between the locations remained below 0.1% when free cooling was employed, indicating a significant potential for widespread application of this technology across Europe. Furthermore, the economic benefits derived from the efficiency gains over the BESS lifetime are evaluated. In this use case, when the BESS State of Charge (SOC) limits were reached, electricity was traded on the Day-Ahead Market to meet FCR requirements. The estimated financial savings at the end of the system lifetime, resulting from reduced electricity purchases on the Day-Ahead Market, amounted to approximately €387,300, yielding net profits of around €357,000. This work contributes to the development of open-source BESS simulation tools, specifically facilitating the modelling of liquid and free cooling systems. The case-study illustrates a potential application of this tool extension and highlights the importance of long-term BESS simulations to account for battery ageing impact on HCS solicitation.