The thermal management of batteries holds an important place in technological developments and require innovative improvement solutions. Nanotechnology shows the potential for the development of better battery cooling systems. Nanofluids and nanostructured fins are suggested as promising solutions to battery cooling.
Energy storage technologies have become one of the most important research areas with increasing energy demand for technological devices. One of the main concerns with the development of battery technology is effective thermal management. Uneven or high temperatures affect energy storage, life cycle, durability, and efficiency of the battery. Overheating or uncontrolled exothermic reactions in the battery can lead to thermal runaway, which causes the device to burst into flames and combust, resulting in serious safety issues. Hence, a proper battery thermal management system is crucial for further improvement of energy storage. The operating temperature range which is commonly reported as between 0 and 30°C is especially important for Li-ion batteries. Both low temperatures and high temperatures that are outside of this region will lead to degradation of performance and irreversible damages, such as lithium plating and thermal runaway. Similarly, temperature rise in photovoltaic (PV) systems leads to considerable efficiency drop. Studies show that the electrical efficiency of the PV module decreases by 0.5% with every unit degree increment in the temperature of the module above 25°C.
To this day, several different cooling strategies have been suggested for Li-ion batteries, electric vehicle batteries, or PV systems. Some of the cooling strategies include cooling duct geometries, cooling channel, cooling plate, and corrugated channel on the battery. Air, liquids, and phase change materials (PCM) are all utilized in different scenarios as cooling agents. Forced or natural air draft is one of the most commonly used means of battery cooling due to ease of use and low price of the system. However, it is one of the least effective systems at the same time. Liquids such as water or deionized water are also considered suitable candidates. But, the thermal conductivity of most liquids is found to be insufficient. To their advantage, PCMs provide effective thermal cooling without the use of moving parts within the thermal management system, a relatively simple design, and lower cost compared to active cooling systems. Although phase change materials are considered to be better suited to the job, they are only capable of absorbing heat equal to the latent heat of the PCM, limiting its use to moderately energy dense cells. Hence, the need for an effective system remains. Therefore, researchers have turned to nanotechnology for satisfying answers. Nanofluids, aerogels, and nanostructure designs are amongst the most promising options for battery thermal management systems.
Battery Cooling with Nanofluids
One of the suggested methods for the improvement of thermal management of batteries is improving the thermal properties of heat exchange materials. It is clear that the thermal properties of coolants in use today as heat transfer fluids exhibit rather poor thermal conductivity when compared to solid metals. The heat transfer capacity of these coolants can be improved by incorporating solid nanoparticles with high thermal conductivity. Such liquid-solid dispersions are named nanofluids. Unlike micro solid particles, nanoparticles do not cause clogging, sedimentation, or erosion. Several different nanofluids are considered promising coolants in battery systems. Water, deionized water, ethylene glycol, synthetic oils, or composites of these materials and PCMs are used as the base material in nanofluids for battery cooling. Different nanoparticles are incorporated into nanofluids in the hopes of improving the thermal properties. The most promising nanomaterials include Carbon Nanotubes (CNTs), Al2O3, Cu, CuO, SiC, ZnO, TiO2, and SiO2. Carbon nanotubes have especially attracted attention due to their extremely high thermal conductivity of 3500 W/mK. Studies report that the addition of carbon nanotubes to synthetic oil results in a 150% enhancement in the thermal properties of the base fluid. Studies show that Cu nanoparticles dispersed in ethylene glycol can increase the thermal conductivity of the base fluid up to 40%. The thermal conductivity of water increases 10-25% with the inclusion of alumina nanoparticles. Similarly, it is possible to obtain a 20-80% increase in thermal efficiencies with the above-mentioned nanoparticles. It is important to note that the thermal properties of nanofluids are considerably affected by different parameters. This is why understanding and designing such parameters is important for the development of better battery thermal management systems.
What are the Parameters Affecting the Battery Cooling with Nanofluids?
The thermal properties and the performance of battery cooling with nanofluids are affected by several different factors. Nanofluid properties can change depending on the base fluid type, nanoparticle concentration, nanoparticle size, nanoparticle type, and nanofluid flow rate. On the other hand, battery cooling performance is commonly affected by the circulation method and channel geometry.
The choice of nanoparticle type has an important effect on the quality and thermal properties of nanofluid. Metallic nanoparticles are favored over metalloid nanoparticles since metalloids increase the viscosity and decrease the specific heat capacity of the nanofluid. Every metallic nanomaterial inherently has different thermal and chemical properties. Hence, the selection of nanomaterials must be done carefully. Researches have shown that SiC/water nanofluids show the most promising battery cooling performances. However, it is still too soon to disregard different options and further investigation is required on the subject. Similar to nanoparticles type, the choice of base fluid also affects the thermal properties of nanofluid. Thermal properties of the base fluid and its interaction with the nanomaterial must be taken into consideration to obtain an efficient nanofluid.
Another important parameter for battery cooling with nanofluids is the nanoparticle concentration. Although this parameter affects the thermal properties of the nanofluid greatly, the results on the effect of concentration are contradictory. It seems that the optimum concentration of nanomaterial in base fluids must be determined for each different case.
Nanoparticles have excellent heat transfer rates due to their high surface area. The size of nanoparticles is reported to have a direct relationship with the heat transfer of the nanofluid. However, further research is required to conclusively narrate the effects of nanoparticle size on the cooling performance of nanofluids.
Similar to any heat exchanging system, the flow rate, flow regime, circulation method, and channel geometry significantly affects the battery cooling with nanofluids. These parameters should be adjusted based on heat exchange principles to obtain optimum conditions.
Battery Cooling with Nanostructures
Nanotechnology can also be utilized to design effective nanostructures for battery cooling. The high surface area of nanostructures offers great advantages for heat transfer between two different media. An increased surface area dramatically improves the heat exchange rate between the coolant and battery surface. Furthermore, utilizing nanostructures inhibits the undesired pressure drop across the heat exchange system. The nanostructures are incorporated in the microchannel coolers which have been suggested as a promising cooling solution. However, the extreme pressure drop associated with microchannel flow prevents it to be employed in a fluid loop with a micro-pump due to the pumping power limit. To eliminate this drawback, improved cooling designs must be developed. For this purpose, carbon nanotubes are utilized to replace silicon fins in microchannel coolers or to decorate the entire heat exchange surface with fins. Carbon nanotubes especially attract attention due to their high thermal conductivity of 3500 W/mK. Thus, utilizing nanostructures such as carbon nanotube fins improve the performance of battery cooling systems.
Electrolyte Lithium Hexafluorophosphate (LiPF6)
Electrolyte Lithium Hexafluorophosphate is a crystalline powder white in color. Electrolyte possesses an important role in lithium-ion batteries (LIB) as the medium to carry lithium ions between the two electrodes of the battery. High purity and battery-grade electrolyte solutions are thus crucial for lithium-ion battery performance. The most common LIB electrolytes are derived from solutions of the salts of lithium-ion, such as LiPF6 in non-liquid solvents, such as solvent blend or alkyl carbonates with low prices and high purity.
The electrolyte solution is one mole of salt in a liter of solvent in a ratio of 4:3:3 by volume of EC + DMC + DEC. The maximum voltage is 4.5 V. The electrolyte is sealed in a container of stainless steel.
Lithium Cobalt Oxide (LiCoO2)
Lithium Cobalt Oxide (LiCoO2) is a crystalline powder, black in appearance with a specific area of 0.2~0.5m2/g and tap density over 2.8 g/cm3. The nature of Lithium Cobalt Oxide (LiCoO2) is basic with pH ranging from 9 to 11. The cell coin capacity of a cell of voltage level ranging from 3.0 V to 4.3 V is more than 151mAh/g with moisture level less than 0.1 %.
Lithium Iron Phosphate (LiFePO4)
Lithium Iron Phosphate is a powder form material used to synthesize cathodes for acid-based and Lithium-ion battery. It has a tap density of 1.132 g/cm3 and a resistance of 114.9 Ohms.cm. The materials contain 1.29 % of carbon with slightly basic nature at a pH of 8.92. The electrodes made from Lithium Iron Phosphate has a discharge efficiency of more than 97 % at the first capacity of 155.5 mAh/g.
Lithium Manganese Oxide (LiMn2O4)
This is also a powder form material for the synthesis of the cathode for batteries consisting of different metals which include Nickel, Iron, Sodium, Copper and metal impurity of less than 25 ppb with moisture content of less than 0.2 %. Lithium Manganese Oxide has a melting point of 400o C, the specific area from 0.4 ~ 1.0 m2/g, tap density ranging from 4 – 5 at standard room temperature and is insoluble in water.
Lithium Nickel Manganese Cobalt Oxide (LiNiCoMnO2)
As the name indicates, this physically powdered material consists of many metals in a particular proportion. These metals include Nickel, Manganese, Cobalt, Lithium, Sodium, Iron, Copper and a part of moisture or water. It has the first discharge capacity of 154.1 ~ 154.8 mAh/g at an efficiency of 87 %.
Apparently, N-Methyl-2-Pyrrolidone is a colorless transparent liquid, with a molar mass of 99.13 g/mol. It possesses a melting point of -24 oC and boils at 202 oC with 1013 hPa. N-Methyl-2-Pyrrolidone has a flashpoint of 91 oC, the ignition temperature of 245 oC and vapor pressure of 0.32 hPa at standard room temperature. It is soluble in water, ether, alcohol, ester, aromatic hydrocarbons, ketone, and halogenated hydrocarbons.
Nickel foam is an exceptional anti-corrosive material with a porosity of 95 % consisting of 80 – 110 pores per inch. It is extensive on both sides of the length and width to some extent. These are used to make cathode for long term use and less maintenance required.
PVDF Binder for battery’s cathodes, which are mostly deployed in the manufacturing of lithium-ion batteries to maintain the active materials and their particles together in place and in contact with the existing collectors. It brings many benefits to the industry of Li-ion battery when it is used as a binder in the electrode formulation as well as in the separator design.
TIMCAL SUPER C45 and C65 Conductive Carbon Black
These two are the two forms of conductive black carbon with a difference in the ratio of the contents present in conductive carbon black. Due to difference in the ratio of the contents of the materials, it causes some differences in the properties and the resulting products as well. they have different values for volatile content, toluene extract, ash content, grit content for different micron size, moisture, density, Sulphur content, and metal elements.
TIMCAL Super P Conductive Carbon Black
It is the most graded level of conductive carbon black, ‘Super P’. it is used mainly as a stabilizer against UV, rubber reinforcement, black pigment, etc. It has a high density of 160 kg/m3.
The role of energy storage technologies in the modern world has become more and more important with each technological development. An aspect that falls short to keep up with developing technology is the effective thermal management of energy storage systems. The exothermal reactions taking place in batteries can cause uneven and high temperatures which result in poor energy storage, life cycle, and durability. Nanotechnology is employed to overcome this problem. Nanofluids and nanostructures offer promising solutions in battery cooling technologies. Nanofluids show better heat transfer properties compared to traditional fluid coolants. Nanoparticles included in the base fluids improve the thermal conductivity and heat exchange properties of the nanofluid. As a result, a better performing cooling agent is obtained to circulate in heat exchange systems. On the other hand, nanostructures are utilized to improve the heat transfer performance of heat exchanging systems due to their high surface area and thermal conductivity. Carbon nanotubes are one of the most preferred nanostructures for this purpose. Researches clearly show that nanotechnology holds a high potential for improving battery cooling systems. However, further investigation is required to obtain optimum applications.