Comparison of the heat transfer efficiency of nanofluids

The continuously increasing power involved in many applications, coupled with the very small size of a number of component devices, is pushing the technical community to look for more efficient heat transfer systems, to remove the heat generated and keep the system under controlled operating conditions. In particular, significant interest has been devoted to the use of the so-called nanofluids, obtained by suspending nano-sized particles in conventional heat transfer liquids. According to some literature, these suspensions present enhanced heat transfer capabilities, without the inconveniencies of particles settlement and clogging of the channels encountered using larger particles. However, other results show that the actual improvement in the heat transfer efficiency may depend on the adopted working conditions and on the reference parameters (fluid velocity, Reynolds number, pressure drop, etc.) assumed to compare the performances of the nanoparticles suspensions with those of the clear thermal fluid. In the present work heat transfer experiments were carried out on a number of nanofluids systems, varying the type and the concentration of the nanoparticles, and the fluid dynamic regime. The investigated suspensions gave rise to heat transfer coefficients different from those of their respective clear thermal fluid, the thermal efficiency being higher or lower, depending on the fluid dynamic parameter used as a base for comparing the systems. Generally speaking, in most cases nanofluids may give an advantage from the heat transfer point of view only when the conditions are unfavorable for the traditional thermal fluid

Temperature Control of Lithium-ion Battery Packs under High-Current Abuse Conditions

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

Experimental evaluation of heat transfer coefficient for nanofluids

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The paper reports the results of heat transfer experimental tests on nanofluids. Measurements were performed in a two-loop test rig for immediate comparison of the thermal performances of the nanofluid with the base-fluid. The convective heat transfer was evaluated in a circular pipe heated with uniform heat flux and with flow regimes from laminar to turbulent. Tests have been performed to compare the heat transfer capability of nanofluids and water at the same velocity or Reynolds number , and they have been compared with values calculated from widely used correlations. In particular ten different nanofluids and three base fluids (in addition to the water) have been used. The analysis of the experimental data shows a different behavior depending on the parameter used in the comparison, and, as a consequence, the addition of nanoparticles to the heat transfer fluid can result advantageous or not, depending on the specific point of view. Furthermore some classical correlations have been used to estimate the heat transfer coefficients, and the analysis shows that they are able to provide good agreement with the experimental data both for the nanofluid and water

Experimental Investigation of Overdischarge Effects on Commercial Li-Ion Cells

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

Renewable energy and safety concerns: the case of secondary batteries

In the framework of the continuous effort in reducing the emissions of greenhouse gases and increasing the amount of renewable energy sources, secondary batteries are playing a more and more significant key role. In fact, on one hand, they allow to make the use of the energy derived from these sources more continuous, in contrast with the intermittent availability which often characterizes them (e.g. wind energy); on the other hand, they allow its adoption in a much wider range of application, such as in the automotive industry, where the number of electric and hybrid electric vehicles is constantly increasing. Consequently, a strong interest is present in the availability of efficient and reliable rechargeable energy storage systems both to be inserted in the main electric network (large stationary systems and off-grid solar PV power systems), and to be installed on mobile electric vehicles. From this point of view, secondary Lithium-ion batteries represent the most promising technology available at the moment. However, despite the wide adoption of these batteries in a number of commonly used technologies (mobile cellular phones, laptops, etc.), a number of past accidents have raised concern about their introduction in the above mentioned larger systems where even much higher powers and energy densities are required (e.g. in the aeronautical and aerospace technologies). In the present paper, an analysis of the causes and of the final consequences of as many as possible of the failures reported in the literature will be carried out. In addition, based on the main characteristics of the energy storage systems and of the specific life cycle under consideration, an efficient risk analysis methodology framework will be suggested, with specific reference to the Li-ion battery technology. This would allow a safer use of this important technology in a wider range of practical applications, thus leading to a more efficient use of renewable energy sources and, at the same time, reducing the risk to the possibly exposed people (either workers or consumers) and to the environment

Hazardous scenarios identification for Li-ion secondary batteries

Lithium ion rechargeable batteries represent an energy storage technology already commonly used in a number of applications (mobile cellular phones, laptops, etc.), and will play an even increasingly important role in the next future. However, a number of past accidents have raised concern about their reliability and safety, and thus delayed their introduction in larger and more strategic systems like the main electrical network or large photovoltaic power systems. With the aim of identifying the largest number of dangerous scenarios associated with the use of these systems, and based on the available information on this technology, Failure Modes and Effects Analysis (FMEA) has been selected for the hazard identification process and applied to a number of common system configurations. The main focus of the analysis has been on the possible negative interactions between the battery system and its surrounding environment (powered system, location of installation, but also modality of use, and so on). The resulting tables collect data from a wide range of sources of information, thus allowing to identify the most important predictable dangerous scenarios, and to suggest adequate mitigation actions to be implemented in any phase of the battery's life cycle (installation, operation, etc.). This would allow a safer use of this technology in a wider range of practical applications, enabling more reliable systems operation and reducing the risk to the possibly exposed people and to the environment

Experimental evaluation of heat transfer coefficient for nanofluids

The paper reports the results of heat transfer experimental tests on nanofluids. Measurements were performed in a two-loop test rig for immediate comparison of the thermal performances of the nanofluid with the base-fluid. The convective heat transfer was evaluated in a circular pipe heated with uniform heat flux and with flow regimes from laminar to turbulent. Tests have been performed to compare the heat transfer capability of nanofluids and water at the same velocity or Reynolds number , and they have been compared with values calculated from widely used correlations. In particular ten different nanofluids and three base fluids (in addition to the water) have been used. The analysis of the experimental data shows a different behavior depending on the parameter used in the comparison, and, as a consequence, the addition of nanoparticles to the heat transfer fluid can result advantageous or not, depending on the specific point of view. Furthermore some classical correlations have been used to estimate the heat transfer coefficients, and the analysis shows that they are able to provide good agreement with the experimental data both for the nanofluid and water

Thermal analysis of Lithium-ion batteries: an experimental investigation

The performance and stability of secondary batteries depend on the working temperature of the cells. This paper describes a set of experimental tests carried out to better understand the thermal behavior of Lithium-ion batteries under load. Different types of batteries have been analyzed to check the influence of a number of parameters that characterize the cells. The generation of hot spots has been registered, their presence being independent of the cell geometry and size; instead, the battery’s history and age, appear the main factors in determining the onset of hot spots on the surface of the cell, with increases of more than 20-30 degrees with respect to the average surface temperature. Nickel Manganese Cobalt-oxide cells presented major problems from the thermal point of view, while new LiFePO4 cells could withstand charge/discharge rates well beyond the maximum allowed values with no signs of excessive heating

Thermal management of lithium-ion batteries: an experimental investigation

This paper describes a set of experimental tests carried out to better understand the thermal behavior of Lithium-ion batteries under load and the capability of various cooling fluids in maintaining the working conditions within a safe range for the cells. Despite several theoretical models are available in the literature, very few experimental data are reported. Different types of cells have been analyzed. The generation of hot spots has sometimes been registered, their occurrence being independent of cell geometry and size; instead, the battery's history and age, appeared the main factors in determining the onset of hot spots on the surface of the cell. Two experimental rigs have been set up to test the capability of different cooling fluids in removing the surplus heat generated in a Li-ion battery module, where the cells of interest have been replaced with electrically heated elements with the same thermal characteristics of the cells. It was thus possible to safely investigate “extreme” operating conditions, where the occurrence of a thermal runaway is possible. Among the tested fluids, air was unable to adequately limit the surface temperature increase, while a perfluorinated polyether, allowed to work within the optimal temperature range, even under severe operating conditions

Analysis of Passive Temperature Control Systems Using Phase Change Materials for Application to Secondary Batteries Cooling

Temperature control is one of the most significant factors to improve the performance and extend the cycle life of a battery. It is, therefore, important to design and implement an effective battery thermal management system (TMS). Phase change materials (PCMs) can be used as a cooling means for batteries. In the present paper, a preliminary analysis of the thermal behavior of PCMs used to cool down a heated metal surface was carried out. Tests have shown that pure PCMs are able to limit the temperature increase, but only for relatively low-heat fluxes. At higher values of the heat produced, the thermal conductivity of the PCM was increased by using solid foams characterized by higher thermal conductivity; it was, thus, possible to keep the surface temperature within safe limits for longer times. A computational fluid dynamics (CFD) model of the composite material (PCMþsolid foam) was also developed, which allowed to predict the temperature trend within the system under different boundary conditions. However, the average thermal conductivity of the composite system that best fitted the experimental results was found to be much lower than that theoretically predicted by using common semiempirical correlations