2.2.2. Heat Pipes

Heat pipes (HP) are passive capillary-driven two-phases systems that represent another solution to manage the temperature of electric vehicles battery packs. The two-phase system means that the heat transfer occurs due to a phase change and, in this case, due to the liquid–vapor one. A heat-pipe based cooling system is made of three main parts: the evaporator (heat source), the adiabatic section (which links the first and the third parts and along which the heat transport happens), and the condenser (heat sink). Evaporation and condensation rule the thermodynamic cycle: the coolant within the pipes (usually made of copper) absorbs the heat of the battery cells, which causes the evaporation of the cooling liquid itself, then the fluid moves along the pipes, towards the condenser, with an efficient heat transfer thanks to the latent heat of vaporization. Once the vapor reaches the condenser, another phase change occurs, and it turns to liquid again and heat is dissipated. Simulations [35] have shown that BTM systems based on heat pipes provide energy savings in electric vehicles, keeping the maximum temperature of the battery under 50 ◦C, the temperature difference under 5 ◦C, and a good temperature distribution. The possibility to curve and bend the pipes makes them suitable in almost any battery design, realizing unique and efficient cooling systems. Figure 2 shows a scheme of a heat-pipe-based BTMS, equipped with a further U-pipe system (remote heat transfer heat pipe system, RHE-HP) which helps in transporting heat away.

**Figure 2.** Schene of a heat pipe-based BTMS assembly equipped with a RHE-HP system, as studied in [52].

The interface plate connects the heat pipe cooling plates system (HPCP) and the RHE-HP one. Several types of research have been carried on studying the influence of many factors on the efficiency of HP systems. For their experiments, Putra et al. [37] use different coolants and modify the heat flux load studying the maximum temperature and the best system performance. The coolants chosen are distilled water, alcohol 96%, and acetone 95% with a filling ratio of 60%. Looking at the evaporator temperatures during the transient case, it is shown that they increase quickly followed by a drop. This is due to super-heating, which occurs when the boiling point is reached. As the heat flux load increases, the duration of the transient decreases. Another factor that influences the transient is the capacitance of

the system, which is a function of the mass of the system itself. For all the evaporator and condenser temperatures, the shortest transients are obtained with acetone and alcohol as coolants. During the steady-state case, the same temperature distribution can be seen for distilled water and alcohol, except when the highest heat flux load is reached: in that case, alcohol and acetone have an equal temperature distribution. The evaporator temperature is always under 50 ◦C for all the heat flux loads and almost all the coolants. The lowest temperature difference between the evaporator and the condenser is obtained with acetone in all the experiments carried out. Another consideration is that as the heat flux load increases, at the beginning, it is possible to see the alcohol evaporator temperature close to the distilled water one and then closer to the acetone one. This is because a rise in heat flux load is followed by a pressure increase, which is the cause of higher temperatures. Depending on the coolant, the evaporator temperature will be different. It has been found that, for the highest heat flux load, the acetone condenser temperature is very different from the other coolant ones: this is due to super-heating in the evaporator and for the vapor infiltration with a consequent increasing of the liquid temperature and much more heat transported towards the condenser. However, the lowest temperature difference is obtained with acetone for the most moderate heat flux load. The best performance is reached using acetone as a coolant and with the highest heat flux load. Wang et al. [53] studied cylindrical cells' battery performances, taking into account the effect of several structural parameters such as the spacing, the thickness of the conductive elements, and the angle between the battery and the conductive elements. Simulations show that the height of the conductive element is the parameter that most influences the temperature distribution. Moreover, the angle is a second influencing factor, while the thickness of the conductive element and the spacing have a minimum effect.
