Week 10 Thermal Management

Air Cooling
Air systems use air as the thermal medium. The intake air could be direct either from atmosphere or from the cabin and could also be conditioned air after a heater or evaporator of an air conditioner. The former is called passive air system and the latter is active air system. Active systems can offer additional cooling or heating power. A passive system can offer some hundreds of watts cooling or heating power and an active system power is limited to 1 kW. 
Because in both cases the air is supplied by a blower, they are also called forced air systems. 

Liquid Cooling
There are generally two groups of liquids applied for thermal management systems. One is dielectric liquid (direct-contact liquid) which can contact the battery cells directly, such as mineral oil. The other is conducting liquid (indirect-contact liquid) which can only contact the battery cells indirectly, such as a mixture of ethylene glycol and water. Depending on the different liquids, different layouts are designed. For direct-contact liquid, the normal layout is to submerge modules in mineral oil. For indirect-contact liquid, a possible layout can be either a jacket around the battery module, discrete tubing around each module, placing the battery modules on cooling/heating plate or combining the battery module with cooling/heating fins and plates. Between these two groups, indirect contact systems are preferred in order to achieve better isolation between battery module and surroundings and thus better safety performance.
By different heat-sinks for cooling, liquid systems can also be categorized into either passive systems or active systems. In passive liquid system, the heat-sink for cooling is a radiator. This system has no ability to heat. Heat transfer fluid is circulated by the pump within a closed system. The circulating fluid absorbs heat from battery pack and releases heat via a radiator. The cooling power depends strongly on the temperature between ambient air and battery. Fans behind the radiator can improve the cooling performance, but if ambient air is higher than the battery temperature or the difference between them is too small, the passive liquid system becomes ineffective.

There are two loops. The upper is called the primary loop and the lower the secondary loop. The primary loop is similar to the loop in a passive liquid system, where the heat transfer fluid is circulated by pump. The secondary loop is actually an air conditioning loop (A/C loop). The upper heat exchanger instead of being a radiator works as an evaporator (EVAP) for cooling operation and connects both loops. During heating operation, the 4 way valve will be switched, and the upper heat exchanger works as a condenser (COND) and the lower heat exchanger works as an evaporator. The heating operation loop is also called heat pump loop.

Direct Refrigerant Cooling

A direct refrigerant system (DRS) consists of an A/C loop, but DRS uses refrigerant directly as heat transfer fluid circulating through battery pack.

PCM
During melting, heat is absorbed by PCM and is stored as latent heat until the latent heat is up to the maximum. The temperature is kept at melting point for a period and the temperature increase is delayed. Therefore, PCM is used as conductor and buffer in battery thermal management systems. Also, a PCM is always combined with air cooling system or liquid cooling system to manage the battery temperature.

Thermo-electric Module
Thermo-electric module can convert electric voltage to temperature difference and viceversa. Here the former effect is adopted. That means it transfers heat through the module by consuming electricity directly. Two fans are installed to improve heat transfer by forced convection. To combine a passive air system with thermo-electric module, the combined system is able to cool down the battery even lower than the intake air temperature, but the power is still
limited to around some hundreds of watts and less than one kW. It’s easy to switch between cooling and heating operation. To achieve that, the poles of electrodes need to be reversed.

Heat Pipe

The flat copper envelope of the heat pipe was under partial vacuum. The capillary structure is made of sintered
copper powder. The heat pipe uses water as the working fluid. Water on the evaporator side will absorb heat and become vapour lower 100°C due to low pressure inside. Water on the condenser will dissipate heat to the surrounding and become liquid again. This cycle repeats again and again.

PTC Heater
PTC thermistors have many self-heated applications by utilizing their own voltagecurrent or current-time characteristics. One of the applications is as a self-regulating heater known as a PTC heater. The temperature of a PTC heater can be kept at a fixed point through adjusting the resistance of PTC heater automatically.

Tube Cooling

A wave formed aluminum extrusion with 8 microchannels passes by all the cells. The coolant flowing through the microchannels is a mixture of ethylene-glycol with water (50/50% by volume). The cells are
held in place with two polycarbonate clam shells (Two plates on each side with extruded rims to hold each cell in its specific place). The distance between two adjacent cells is 1.5mm (Minimum possible distance between cells provided by Northvolt). The interface between the cells and the aluminum extrusion is filled with TIM (Thermal Interface Material) also known as gap pad or thermally conductive pad. TIM is a solid material (often wax or silicon-based) that aid heat conduction between the heat sink and the material that is being cooled. They are used to prevent air gaps on imperfect flat surfaces on a thermal contact interface. This type of cooling is inspired by Tesla battery modules used in Model S and X cars.

Bottom cold plate
Bottom cold plate follows the same principle as tube cooling. The cells are placed more compact relative to Tube cooling since there is no material between the cells. The advantage of this cooling method is that it takes advantage of higher axial thermal conductivity of the cells. In this model the cells are inserted 10mm into a thermally conductive polymer, thus the cold plate acts as a clamshell on the bottom of the cells as well. There are a series of channels inside, the plate to ensure that the coolant can flow along the plate and cool it down uniformly. The coolant used is Ethylene-glycol and water mixture like the tube cooling.