Week 10 Thermal Management

AIM:

To summarize a report for the thermal management techniques for electric car battery pack

Procedure:

# Thermal management should be explained briefly.

# Types of thermal management must be explained.

# Cooling techniques used for cooling the battery pack of the electric car.

BTMS- BATTERY THERMAL MANAGEMENT SYSTEM

# It plays a vital role in the thermal control of battery thermal behavior.

# These systems are analyzed through a trade-off between performance, weight, size, cost, reliability, safety, and energy consumption.

# For the safe functioning of the battery pack, the BTMS should be installed with the right functions such as cooling, heating, insulation, ventilation.

# Thermal management systems are an important aspect of an electric vehicle. as with the usage of motors, batteries, and inverter which has to be cooled for optimal performance, as it has seen through various simulations that the battery functions best when operated near its optimal temperature range.

# A battery pack is one of the most important and expensive components in the EV, HEV & PHEV. One of the most important factors that affect the performance of the battery is operating temperature. All batteries depend on their action on an electrochemical process whether charging or discharging and we know that these chemical reactions are in some way dependent on temperature.

# Nominal battery performance is usually specified for working temperatures somewhere in the + 20°C to +30°C range however the actual performance can deviate substantially from this if the battery is operated at higher or lower temperatures.

# There are several designs used for cooling of batteries, primary air, oil, and water-glycol if the battery is very cold then the discharge curve will be very short and the battery will discharge very quickly and if the battery gets heated above 50 degrees celsius then there will be a thermal runaway which means the rate of charge of heat will increase drastically and may even cause explosions.

# Lithium-ion cells degrade quickly if operated above 45 degrees celsius.

WHY DO BATTERIES PRODUCE HEAT?

# In a pack, the number of cells is connected in series and parallel connections by different methods.

# These connections at the cell terminals cause more concentration of heat which leads to a rise in temperature at charge-discharge cycles.

# These images will give a brief idea about how the battery pack looks like, how the cell will transmit the current to the load, how cell terminals get heated up, how it transmits the energy to the atmosphere from the cell.

Cooling techniques for EV batteries

Air cooling – air cooling is the simplest technique among other cooling techniques. Air removes heat from a battery pack using the convection technique. As air runs over the surface, it will carry away heat. Air can be taken from the atmosphere or from cabin. Air colling is less sophisticated and cheap but it is not efficient for high-performance applications due to the inability to cope up with a wide range of ambient temperatures. In hot ambient temperature, thermal runaway becomes real. One of the earliest electric cars Nissan leaf’s battery pack installed with an air cooling system.

Liquid cooling – besides air-liquid is also used for cooling. It is better than air cooling because they have a higher heat capacity and heat conductivity than air. The liquid cooling technique can be implemented with two types - direct contact with cells and indirect contact with cells. At present no one using direct cooling but some developers are trying to use non-conducting coolants. Indirect cooling is the most common technique and it uses a mixture of glycol and water mostly. This technique is used by EV manufacturers like Tesla, Jaguar, BMW, others.

The liquid cooling system can be installed easily but it has its own safety and leakage concerns like glycol could be dangerous and there are disposal concerns also.

Phase change material – as the name shows in this technique material changes the phase from solid to liquid after absorbing heat from the battery pack. PCM stores absorbed heat as latent heat up to its maximum limit. The thermally conductive structure holding the PCM provides a fast heat transfer path through the battery pack, minimizing temperature difference between the cells This technique can meet the cooling requirement but volume change during phase change limits its application and it gathers heat not transfers it away, so it is not useful for the electrical vehicle.

Thermoelectric cooling – in recent years, the thermoelectric effect in a semiconductor has been widely investigated because it can produce a temperature gradient when an electric current is launched and thermoelectric generator cooling can promptly create a relatively lower temperature than the surrounding temperature. The degree of cooling can be adjusted according to the current size. This system can be combined with an air cooling system to get the further lower temperature.

Heat pipe – cooling through a heat pipe is another way to reduce the temperature of the battery pack. The heat pipe can be installed with an air cooling system to get enhanced cooling. Heat pipe poses high thermal conductivity, compact and flexible structure, easy maintenance, and long life. Heat pipe works on phase change technique means mostly water is used as working fluid and it goes under phase change (liquid to vapor) by absorbing heat from the battery pack. The use of heat pipe is more reliable because it involves no moving parts and we know moving part consumes energy and they make the system unstable.

We have discussed some cooling techniques, but which one should be used for EV applications. We saw every method has its unique perks and cons, but to get the efficient cooling performance it can be used in combination like – liquid cooling with direct refrigerator cooling, combined liquid system with PCM, heat pipe, or thermoelectric material with air cooling.

# Due to enthalpy changes, electrochemical reactions a large amount of energy is generated inside the cell.

# Resistive heating is also another factor for the battery because of the cell resistance inside the cell.

Immersion cooled:

# Immersion cooling involves submerging the battery cells in a liquid coolant, in doing so superior thermal contact and homogeneity can be achieved. Additionally, the flame retardant nature of the fluids acts as a safety feature, suppressing thermal runaway events before they propagate between cells. A key consideration here is obviously the choice of coolant fluid used, through primary research.

# These fluids are all dielectrics but vary in their properties with factors such as weight, thermal conductivity, environmental consideration, and the cost are extremely important. The expected outcome is to keep the cell temperature within the optimum range with a method that's both easier/cheaper and thus increase available power output/charging power and extend cell longevity.

Benefits of Immersion cooling:

# Eliminates all conductive fluids in the battery

# Increased thermal transfer through direct contact of the coolant with the entire casting and tabs of the battery (as opposed to just tab cooling with cold plates).

# Simplification of the internal battery structure as all the heat sinks, thermal transfer materials, complex support matrices, and cold plates are eliminated and replaces with a simple container that surrounds the batteries and is filled with a minimum amount of fluid.

# Reduced weight through the elimination of the excess materials and complex systems.

# Icreased reliability through greater thermal transfer and elimination of the failure scenarios.

# Better vehicle range and faster charging due to better thermal management.

# The EV battery is large with good heat dissipation possibilities by convection and conduction and subject to a low-temperature rise due to its high thermal capacity. On the other hand, the HEV battery with fewer cells, but each carrying higher currents, must handle the same power as the EV battery in less than one-tenth of the size. With a lower thermal capacity and lower heat dissipation properties, this means that the HEV battery will be subject to a much higher temperature rise.

Battery cooling system in Tesla Model S:

The Tesla Model S battery cooling system consists of a patented serpentine cooling pipe that winds through the battery back and carries a flow of water-glycol coolant, thermal contact with the cells is through their sides by thermal transfer material. Again this will remove heat from the side of the cells rather than from the tabs, and overheating a Tesla battery pack under hard driving is easy to do.

Conclusion:

Battery Thermal Management System is very important in any electrical appliances especially in an electric vehicle because the whole system depends on the battery pack, if the battery temperature is imbalanced there are severe damages take place. These are some of the Battery thermal management techniques used in electrical vehicles. So, all the different management systems need to be work together in sync to ensure the battery pack gives its best performance.