Week 10 Thermal Management
Battery model with thermal effects
The inner cell Temperature is assumed to be uniform, and taken as the average temperature inside the cell.The cell temperature can be computed by solving the heat equation of a homogenous body exchanging heat with the environment.
`C_T((dT)/dt)= P_s - ((T-T_a)/R_T)`
`C_T` = heat capacity
T = inner cell temperature
`T_a` = ambient temperature
`R_T` = convection resistance
`P_S` = Power dissipated inside the cell
Thermal management systems are an important aspect of an electric vehicle, as with the usage of motors, batteries and motor inverter which has to be cooled for optimal performance, as it has been seen through various simulations that the battery functions best when operated near its optimal temperature range, there are several designs used for cooling of the motors, primarily these are air, oil and water-glycol, there are several designs used for cooling of the motors, primarily these are air, oil and water-glycol if the battery is very cold then the discharge curve will be very short and will lose its discharge very quickly, and if the battery gets heated above 50 degree celsius then there will be a thermal runaway which means the rate of change of heat will increase drastically and may even cause explosions. Lithium cells can degrade quickly id operated above 45 degree celsius
Thermal management has many advantages as it ensures the safety of the battery pack from thermal runaway or explosions due to heat, it ensures extended battery life with thermal management systems ie. more life cycles compared to no thermal management system in a battery pack
Three common types of thermal management systems:
- convection to air either passively or forced
- using dielectric to flood the battery and pass on the heat to a heat exchanger
- circulation using water-based cooling or glycol as coolant
types of thermal management systems:
1) Air Cooling and Heating
Air systems use air as the thermal medium. The intake air could be direct either from
the atmosphere or from the cabin and could also be conditioned air after a heater or
evaporator of an air conditioner. The former is called a passive air system and the latter
is an active air system. Active systems can offer additional cooling or heating power. A
passive system can offer hundreds of watts cooling or heating power.
Because in both cases the air is supplied by a blower, they are also called forced air
systems.
2) Liquid-cooled
a flooded dielectric cooling system will be primarily cooling by extracting heat from the surfaces of the cells. Tab cooling is best achieved by a water-based coolant or an organic refrigerant circulated through a cold plate system built into the battery pack by a pump. The coolant can be used to remove heat from the pack and to also provide heating of the pack for fast charging at low temperatures.
The Tesla models S battery cooling system consists of a patented serpentine cooling pipe that winds through the battery pack 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.
3) 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 considerations, and cost, being 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.
Advantages of immersion cooling:
- Elimination of all conductive fluids in the battery
- Increased thermal transfer through direct contact of the coolant with the entire casing 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 replaced with a simple container that surrounds the batteries and is filled with a minimum amount of fluid.
- Reduced cost through the elimination of the complex systems
- Reduced weight through the elimination of all the excess materials and complex systems.
- Increased reliability through greater thermal transfer and elimination of the failure scenarios.
- Better vehicle range and faster charging due to better thermal management.