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Modeling Refrigerant Cooled Batteries in KULI

One concept used frequently in average to high performance BEVs and PHEVs is combining a coolant cooled battery (in a dedicated battery cooling circuit) with a chiller that couples this cooling circuit to the air conditioning system. This allows battery cooling (with a battery target temperature usually between 30°C and 40°C) even in hot ambient conditions (higher than 40°C).

One valid question is why the battery should not be integrated into the refrigerant circuit more directly. For example, having battery cooling plates with integrated refrigerant channels would completely eliminate the need for the additional battery cooling circuit, including its coolant pump, the chiller and possibly an additional radiator.

 

 

Figure 1: Circuit schematics for direct and indirect refrigerant battery cooling

 

Due to evaporation of the refrigerant in the cooling plates, the heat removed from the battery is latent heat, the refrigerant temperature remains unchanged, and thus very homogeneous temperatures can be achieved.

Disadvantages include long necessary refrigerant lines (because the AC system is usually in the front of the car and the battery pack is usually in the back) and generally strongly increased refrigerant volume. Nevertheless, the concept of direct refrigerant cooling is definitely worth looking in to.

In the given example, the battery pack consists of 4 modules which are bottom-cooled by a refrigerant plate. This plate has serpentine-shaped internal channels and is modeled by point masses in our simulation model. The point masses are connected to the 4 battery modules by heat transfer components. In addition, heat spreaders between the modules are modeled by further heat conduction components connecting to the module sides.

 

Figure 2: KULI model for refrigerant cooling plate and battery modules

 

On the refrigerant circuit side, the cooling plate is integrated parallel to the AC system evaporator, with both components controlled by dedicated thermal expansion valves (required to guarantee sufficient superheating in front of the compressor).

Simulating this circuit shows the following results:

 

Figure 3: Vapor Quantity in the cooling plate and related ph-diagram

 

The thermal expansion valve controls the refrigerant flow so that superheating is only reached just before leaving the cooling plate. This leads to a constant refrigerant temperature of 3.14°C, and to very homogeneous plate, module and cell temperatures accordingly. One disadvantage of the displayed bottom cooling example is a fairly high temperature gradient in z-direction (depending on the battery load case). However, this is independent from whether the battery is refrigerant- or coolant-cooled.

In summary, we can say that direct refrigerant cooling definitely offers some interesting benefits. Whether it is actually feasible strongly depends on the specific application and integration situation.