Week 10 Thermal Management

Aim:

To study the thermal management techniques for electric car battery pack.

Theory:

Why we need Battery Thermal Management System (BTMS):

The battery temperature should be controlled within temperature limits to avoid the thermal issues and improve the performance. The temperature range affects battery power and battery cycle life.

Higher operating temperature increases the capacity of battery pack and at the same time decreases it's life span. Temperature higher than the specified range can cause thermal runaway.

The decrease of the operating temperature will reduce reaction rate and the capacity of carrying current during charging or discharging.

Different techniques used for BTMS:

1. Air cooling and heating:

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. (Valeo, 2010)
Because in both cases the air is supplied by a blower, they are also called forced air systems. The following figure shows a schematic description of systems.

2. Liquid cooling and heating:

Besides air, liquid is another heat transfer fluid to transfer heat. 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. (Pesaran, 2001) 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 a passive liquid system, the heat-sink for cooling is a radiator. This system has no ability to heat. Figure below presents the systematic scheme of a passive liquid system. 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.

Figure below shows the systematic scheme of an active liquid system. 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.

3. Direct refrigerant cooling and heating:

Similar to active liquid systems, a direct refrigerant system (DRS) consists of an A/C loop, but DRS uses refrigerant directly as heat transfer fluid circulating through battery pack. Below fig. shows the layout of Direct refrigerant cooling and heating.