Batteries can overheat when pushed to their limits. The lack of space and sufficient ventilation area in electric vehicles is directing the design of thermal management towards new solutions so that the batteries do not overheat and cause extreme damage to the entire system. Kulr Technology Group is developing, manufacturing, and licensing carbon fiber thermal technologies for batteries and other electronic devices for space to keep them cool. Its thermal management solution used on the Mars mission will enable the next electric luxury sports cars.
Lithium battery fire incidents involving hoverboards, smartphones, and electric vehicles are a serious public safety concern for electronics and battery manufacturers, creating a surge in demand for improved battery safety technologies and thermal management solutions (figure 1). In an interview with EE Times, Michael Mo, chief executive officer at Kulr Technology Group, highlighted how the company’s carbon-fiber technology designed with NASA to regulate the extreme temperatures of sensitive components in space for the Perseverance mission, will be employed by Drako Motors for a new electric supercar. With a 1,200-horsepower architecture designed, Drako GTE is an EV platform that highlights the difficulties of thermal management and thus the importance of new solutions that can improve performance.
We’re seeing changes in the world of power electronics with the automotive industry moving to vehicle electrification and the advent of 5G communications technologies that will accelerate the growth of cloud computing. These applications require more power and also thermal management solutions or cooling technologies for batteries and other powertrain systems.
The electrons, in their passage through the conductors and semiconductors, produce a lot of heat, and they negatively affect the final performance of the circuit. In recent decades, the power density in electronic devices has increased significantly. The tendency to reduce the size of the devices has increased thermal problems within electronic circuits. Therefore, temperature management in power devices remains an extremely critical factor.
Several interfaces between high-power components generate heat and the heat sinks themselves. Thermal management issues can arise from the minimal contact areas that are created between the two surfaces (interface) due to micro-scale surface roughness. This reduces heat conduction across the interface since air voids have low thermal conductivity. Surface irregularity is the main cause of thermal contact resistance. “The goal of Kulr’s solutions is to increase the contact between the two surfaces and thus decrease the thermal resistance of the interface,” said Mo.
The temperature changes the reliability and durability of the electrical and electronic components. A device malfunction is almost always caused by a thermal problem. High temperatures not only make the system work unstable but also reduce the average life of the components, thus leading to their deterioration.
The first precaution to take is to adopt and implement a strategy to disperse the heat of the electrical and electronic circuits. The heat transfer efficiency of the heat sinks is linked to the thermal resistance between the heat sink and the ambient space. It measures the ability of a material to dissipate heat. An ideal heat sink material must have high thermal conductivity, low thermal expansion coefficient, low density, and low cost.
The quantity of heat depends on the power and on circuit design. The optimal arrangement of the electronic component, on a circuit, should provide for excellent air circulation and intelligent placement of the parts, taking into consideration the specifications of the electrical circuit.
Thermal interface materials
Kulr is using phase change material with vertically aligned carbon fibre (carbon fiber thermal interface, or FTI) for electronics and lithium-ion batteries to serve the world of electric transportation, energy storage, battery safety, 5G infrastructure, cloud computing, and aerospace and defense applications (figure 2).
Carbon fiber can dissipate heat while reducing size, weight, and manufacturing complexity. Kulr has developed a proprietary manufacturing technology that organizes 5 to 10-micron carbon fiber strands onto a base material in a way that looks and feels like black velvet.
With the ability to produce 1,800 continuous amps and 2,200 peak amps, the battery in Drako GTE is designed to offer megawatt-order power output and cooling capacity to withstand track-level sports performance on various world circuits.
Mo pointed out that producing an electric supercar needs to involve extreme power, and by maintaining limited space for heat sinks, the thermal interface has its importance. Bringing the technology used in space environment with high temperatures, the electric transport could support more power thus ensuring proper dissipation and avoiding overheating. “But we have some challenges to solve: the biggest thing that the consumer world is looking for is the price to be very cost-effective, and high thermal conductivity performance,” said Mo.
Mo explained how the FTI solutions family specifically includes Alcor and Mizar FTI materials. “The ALCOR has a density of < 0.7 g/cm^3, and very low contact pressure to achieve low thermal impedance. The MIZAR FTI increases power densities of a board layout and relieves mechanical stress — resulting in an overall increase in thermal stability and reliability,” added Michael Mo.
ARA is another solution from Kulr aimed at solving thermal management problems in the aerospace and defense industry, as it has a thermal capability that has experimentally proven effective over a small temperature range. It finds use in systems with a large amount of computing power in short time intervals. Michael Mo said they developed a proprietary high thermal conductivity fiber core material to deliver a good performance required by space.
HYDRA is another solution that acts as a heat sink for lithium-ion batteries and prevents the thermal runaway propagation (TRP): an important parameter in electric vehicles (figure 3). A short circuit in a battery pack can cause thermal runaway and, therefore, the ignition of fire and combustion of materials such as to raise the temperature of neighboring cells. Rising temperatures increase the possibility of shorting adjacent cells. “Hydra aims to prevent the temperatures of neighboring cells from rising above 100 °C and thus prevent thermal runaway,” said Mo.
Typically, a thermal runaway is caused by excessive current or a high ambient temperature and develops through several stages: starting at a temperature of about 90-100 °C, the heat generated causes organic solvents to rupture, resulting in the release of gas, increasing the pressure inside the cells. Despite this, the gas does not ignite due to the lack of oxygen. If, however, the temperature continues to rise, exceeding 135 °C, the separator melts and causes a short circuit between the anode and cathode, leading to the rupture of the metal oxide cathode at 200 °C and releasing oxygen. This allows the electrolyte and hydrogen gas to burn.
As part of battery testing, Kulr developed the LYRA internal short circuit (ISC) trigger cell to identify a cell’s failure conditions to study the failure modes and safety issues that could arise within battery packs.
Interest in battery electric vehicles is steadily increasing. The real challenge is the availability of fast-charging stations, thus reducing the battery charging time. This will lead to a significant thermal increase in powertrain systems resulting in controlled thermal management to optimize heat flow.
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