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Lithium ion batteries are currently the main technology for storing energy in electric vehicles thanks to their high power density and energy density. Among the many challenges, safety related aspects must be faced. The thermal runaway caused by overcharge is one of the main weak points to be addressed to ensure a proper level of safety, necessary for the diffusion of this technology.Nowadays, many cells have integrated protection devices that reduce the risk of thermal runaway triggered by overcharging. In this study, the efficiency of devices used to prevent the thermal runaway, caused by overcharging cylindrical 18650 lithium-ion cells with graphite anode, is evaluated. Experimental tests were carried out at different ambient temperatures and overcharge currents.In all the cases the intervention of the Current Interrupt Device avoids the thermal runaway occurrence, preventing the intervention of the others protection devices such as the Positive Temperature Coefficient device and the safety vent. more...

Lithium titanate oxide cells are gaining attention in electric vehicle applications due to their ability to support high-current charging and their enhanced thermal stability. However, despite these advantages, safety concerns, particularly thermal runaway, pose significant challenges during abuse conditions such as overcharging. In this study, we investigated the effectiveness of various dielectric fluids in mitigating thermal runaway during overcharge abuse tests of cylindrical LTO cells with a capacity of 10 Ah. The experimental campaign focused on overcharging fully charged cells (starting at 100% State of Charge) at a current of 40A (4C). The tests were conducted under two conditions: the first benchmark test involved a cell exposed to air, while the subsequent tests involved cells submerged in different dielectric fluids. These fluids included two perfluoropolyether fluorinated fluids (PFPEs) with boiling points of 170 °C and 270 °C, respectively, a synthetic ester, and a silicone oil. The results were analyzed to determine the fluids’ ability to delay possible thermal runaway and prevent catastrophic failures. The findings demonstrate that some dielectric fluids can delay thermal runaway, with one fluid showing superior performance with respect to the others in preventing fire during thermal runaway. The top-performing fluid was further evaluated in a simulated battery pack environment, confirming its ability to mitigate thermal runaway risks. These results provide important insights for improving the safety of battery systems in electric vehicles. more...

Lithium-ion batteries currently represent the most suitable technology for energy storage in various applications, such as hybrid and electric vehicles (HEVs and BEVs), portable electronics and energy storage systems. Their wide adoption in recent years is due to their characteristics of high energy density, high power density and long life cycle. On the other hand, they still face challenges from a safety point of view for the possible faults that could generate several problems, ranging from simple malfunctioning to a dangerous thermal runaway. Overcharge is one of the most critical types of faults, and, depending on the level of abuse, it may trigger a thermal runaway. To prevent high levels of overcharge abuse, some cells include integrated protection devices that cut off the circuit when a critical condition is met. In this paper, the performance of these protection devices is evaluated to assess their effectiveness. The cells were tested at different ambient temperatures and current levels. In the worst-case scenarios, the maximum cell temperature slightly exceeded 70 °C and the State of Charge (SOC) reached a peak of 127% when the Current Interruption Device (CID) was activated. These conditions were not critical, so serious events such as thermal runaway were not triggered. These outcomes confirm the effectiveness of the CID, which always intervenes in maintaining a safe state. However, since it never intervened in the overcharge abuse tests, a specific set up was also used to investigate the operation of the other protection device, the Positive Temperature Coefficient. more...

Phase change materials (PCMs) are an interesting solution to increase the efficiency of thermal energy storage (TES) systems. The present work explores, with an experimental and computational study, the behavior of a paraffin wax encapsulated in a spherical containment system during the melting and solidification phases. The experimental tests were conducted by immersing the spherical capsule in a thermostatic water bath at different temperatures and measuring the temperatures at four different points inside the capsule. A two-dimensional CFD model of the sphere was then applied to investigate the effect of the insertion of a solid foam into the sphere to increase the system’s responsiveness under demanding conditions. In addition, an analysis of the solidification process considering two different wall materials (HDPE and aluminum) with different thermal conductivity was performed. The results suggest that embedded foams can represent a useful tool to increase the efficiency of a PCM-based TES, but, at the same time, they also highlight that a considerable increase in thermal conductivity is required to achieve significant advantages with respect to pure PCM systems. more...

Li-ion batteries are being widely used as power sources in a continuously increasing number of applications (from portable devices to electric vehicles and even more complex systems). Nonetheless these components are still characterized by serious concerns connected with their safety and stability, which often hinder their more widespread use. In particular, their operation is strictly dependent on their temperature which derives from the balance between the heat internally produced during operation and that dissipated towards the external environment. Beyond certain temperatures a thermal runaway can occur with possible dangerous events, such as fires and explosions. In the present paper, 3D simulations have been carried out to investigate the cooling efficiency of an air flow, under different operating conditions, on a cylindrical Li-ion cell located in a whole battery pack. Under the investigated configurations, it was found that, beyond a minimum value of the passing air velocity, it is possible to keep the cell within safe conditions, thus preventing a thermal runaway. more...

Fuel cell electric vehicles represent a possible solution to meet the objectives of the energy transition currently underway, which sees the replacement of combustion vehicles with low environmental impact vehicles. For this reason, this market is expected to markedly grow in the coming years. Currently, the most suitable fuel cell technology for both light and heavy transport applications is the Proton Exchange Membrane fuel cell. This review provides a comprehensive description of the state of the art of fuel cell electric vehicles at different levels: vehicle configuration, fuel cell stack, and all the necessary operation systems. The current advantages and limits of the mentioned technology are highlighted, referring to recent studies aimed at optimizing the efficiency of the system and providing future perspectives. more...

Due to their attractive properties, such as high energy and power density, Lithium-ion batteries are currently the most suitable energy storage system for powering portable electronic equipment, electric vehicles, etc. However, they are still affected by safety and stability problems that need to be solved to allow a wider range of applications, especially for critical areas such as power networks and aeronautics. In this paper, the issue of overdischarge abuse has been addressed on Lithium-ion cells with different anode materials: a graphite-based anode and a Lithium Titanate Oxide (LTO)-based anode model. Tests were carried out at different depths of discharge (DOD%) in order to determine the effect of DOD% on cell performance and the critical conditions that often make the cell fail irreversibly. Tests on graphite anode cells have shown that at DOD% higher than 110% the cell is damaged irreversibly; while at DOD% lower than 110% electrolyte deposits form on the anodic surface and structural damage affects the cathode during cycling after the overdischarge. Furthermore, at any DOD%, copper deposits are found on the anode. In contrast with the graphite anode, it was always possible to recharge the LTO-based anode cells and restore their operation, though in the case of DOD% of 140% a drastic reduction in the recovered capacity was observed. In no case was there any venting of the cell, or any explosive event. more...

This work deals with the risk related to the flammability and toxicity of low Global Warming Potential (GWP) refrigerants used in heat pumps for residential applications. Some new generation refrigerants were analyzed assuming to make a drop-in for a typical 50 kW heat pump, suited for small multi-family buildings (4 ÷ 6 dwellings). The theoretical maximum Coefficient of Performance (COP) was calculated for the selected fluids, identifying the best performing one from an energy point of view. Subsequently, an analysis of some of the potentially more dangerous accident scenarios was performed, considering the outdoor/indoor release of gases. More in detail, two accident scenarios were analyzed, assuming a refrigerant leak from a hole in the pipeline downstream of the heat pump compressor: in one case the gas is released in an open environment with an ignition near the release point (jet fire), in the other case the release happens within a confined environment. In both cases, the conditions in which it is possible to operate safely were determined. more...

Lithium-ion batteries are now a widespread technology in automotive applications. Together with the life of the batteries and their performance, safety plays a fundamental role in ensuring the spread of electromobility in our society. Overcharge is one of the most severe safety problems for the large-scale application of lithium-ion batteries. In this work the results of the overcharge tests performed on Lithium Ion cells with different anode materials are presented: a comparison was made between graphite-based anode Li-ion batteries and Lithium Titanate Oxide (LTO)-based anode Li-ion batteries. Experimental tests were performed with different current intensities: it was thus possible to analyze the effects of an overcharge as the current supplied varies. The graphite-based anode Li-Ion batteries are equipped with protection devices which act by blocking the passage of current in the cell and avoiding venting and/or explosion phenomena; on the other hand, LTO-based anode Li-Ion Batteries, although considered intrinsically safer batteries, experienced thermal runaway during the overcharge tests. Increasing the overcharge current, the effects of the electrical abuse are more destructive. more...

Lithium ion batteries represent a well established technology in a range of applications (laptops, mobile phones, etc.) but they are becoming key factors in many other areas were reliability and safety are of paramount importance (e.g. the space and automobile industries). However, a number of drawbacks still raise concerns about their wider use and hamper a more structured introduction in these additional applications. In particular, the management of heat effects remains a challenge, as an excessive temperature rise can cause reduction of cycle life, battery failures and, above all, may lead to thermal runaway of individual cells or of an entire battery pack, with associated damages to the surrounding people or environment. In the present paper, a simplified model capable of predicting the thermal behaviour of a battery pack refrigerated with a cooling fluid, is presented. It allows to quickly estimating the efficiency of a given cooling system under specific working conditions, and thus identify the range of operation within which a given energy storage system can safely operate. more...

Due to their characteristic of providing high energy density and long cycle life, Li-ion batteries are being widely used as power sources for a range of applications, including electric vehicles. The temperature of a Li-ion cell is important for its performance, capacity, efficiency and safety, and it is strongly influenced by the charging and discharging process modalities. Especially during electric vehicle operation, considerable heat is generated in the battery pack that needs to be removed. In the present paper a simplified model for predicting the temperature trend within a battery module with cylindrical cells, is presented. This allows to estimate the requirements for a given cooling system under specific discharge conditions. Three cooling fluids have also been experimentally compared: air, a dielectric oil and a perfluorinated polyether. The best performance was shown by a commercial perfluorinated polyether, by which it was possible to work in safe conditions with a very low pumping power: at 0.02 W, a maximum temperature of 48°C is reached at the end of the discharge using Galden HT135, while 55°C is reached with the Midel ICE. The results also showed that, under the assumed conditions, an air-cooling system needs between 100 and 1700 times more energy than the other methods to keep the same average temperature. more...