When a medium changes phase (e.g. from solid to fluid) due to some heat input, the result is that the applied heat does not increase the medium temperature (latent heat). The energy is used for the phase transition and only when the complete medium is melted, the medium temperature continues to rise (sensible heat).
This process can be modeled in KULI by using the PCM subsystems provided on our homepage. The general functionality can be explained best by the following simple example:
We take an ice-cube with an initial temperature of -10°C and throw it into a water tank constantly preconditioned to 80°C. Let us assume that the water from the ice-block and the surrounding water do not mix (e.g. the ice is contained in a box) and we have heat transfer according to the temperature gradient. We further simplify the model by assuming one average temperature for the complete ice-block (point mass). Simulation now yields the following results (Figure 1):
Figure 1: Transient warm-up of a melting ice cube
Initially the ice-block warms up until it reaches the melting point at 0°C (sensible heat). After this there is constant heat flow to the ice-block, but this heat is used to melt the ice at a constant temperature of 0°C (latent heat). When all the ice is melted, the resulting water warms up further with the temperature gradient between the 80°C water tank and the melted ice cube decreasing (sensible heat).
We now want to use this approach to model battery pack cooling with a (for example wax-based) phase change material (PCM) element. We take a battery pack with 297 cells (distributed to 9 modules with 33 cells each) which is cooled by a low temperature cooling circuit (Figure 2).
Figure 2: KULI battery model
This battery pack will now be subjected to fast-charging at 120kW electrical power. This means that due to the high charging currents we will see significant thermal losses in the battery which will cause potentially dangerous warm-up of the individual cells.
In order to prevent this, we want to distribute phase change material between the 8 modules which should provide additional cooling (by absorbing latent heat) when temperature levels become critical. These PCM elements will not interfere with the primary fluid cooling and provide additional thermal buffer.
Figure 3: Battery cooling layout with additional PCM elements
Simulation shows that this approach actually improves the temperature levels during fast-charging.
Figure 4: PCM elements help to decrease cell temperatures
In Figure 4 we see a comparison between temperature levels for a battery pack without PCM elements (bold lines) and the same battery pack with additional PCM cooling (dotted lines). The red line shows the warm-up of the hottest module whereas the green line shows the warm-up of the coldest module. Where hot temperature levels reach 40°C without the use of PCM elements, in this example 8kg of phase changing material can reduce the maximum temperature level by more than 4K. Additionally the temperature span between different modules can be slightly decreased.
In summary we see the following effects:
While being completely uncritical during a standard driving cycle, battery temperatures can still reach dangerous levels during high power charging
PCM elements can be used to buffer this effect and reduce both the maximum cell temperature and the temperature spread in the pack.
Different PCM layouts can of course yield different temperature improvements.
Subsystems for modeling PCM elements are now available for download from the KULI Homepage.