When the knowledge in materials and technologies for thermal energy management, conversion and storage of the Thermal Energy Solutions (TES) area of CIC energiGUNE is combined with those of the Electrochemical Energy Storage (EES) area, the result is the emergence of disruptive innovations in thermal management focused on batteries.

The BTMS´s (Battery Thermal Management System) objective is to prevent accelerated battery deterioration by managing the heat generated by its components so that it operates continuously under optimum temperature conditions.

Although existing commercially available cells can operate safely between -40 and 60 ºC, the operating range preferred by manufacturers to maximize their performance is indeed between 15 and 35 ºC. In this sense, it is also recommended that within the battery-pack there should not be a difference of more than 5 ºC between the cells.

It should be noted that exposing the battery to extreme conditions can have fatal consequences. For example, its operation at very high temperatures (> 80 ºC) can cause the well-known thermal runaway, resulting in fire and, in the worst scenario, the explosion of the battery with the consequent personal safety implications.

The BTMS is the battery-pack component responsible for ensuring that the cells operate under the optimum temperature conditions specified by the manufacturer.

Thermal management technologies

When selecting a BTMS for a battery-pack, there is no one single alternative. The following figure shows an outline of the leading thermal management technologies that are commercially available or are being investigated by the scientific community:

The first major classification of BTMS corresponds to those systems in which there is fluid in motion and those in which there is not. The first ones are known as active BTMS and the second ones as passive BTMS.

Active BTMS

Nowadays, active BTMS based on forced air or coolant are the most commonly used in electric vehicles. For example, both Toyota and Lexus use fans that circulate cold air through the battery cells. On the other hand, Tesla or Audi use channels in direct contact with the cells through which a cooling fluid (commonly, a mixture of water and ethylene glycol) circulates.

When liquid coolants are used, they can be in direct contact with the cells (immersed in the fluid) or circulate inside pipes and act indirectly. The above examples of liquid cooling are all indirect systems.

One of the main disadvantages of indirect systems compared to direct systems is the loss of heat transfer efficiency, mainly due to the resistance to heat transfer at the interface between the pipe containing the refrigerant and the cell itself.

However, since there is no direct contact between the fluid and the electrical components of the battery, indirect systems allow the use of conventional coolants already used in combustion vehicles. For this reason, and because of its low cost, this is the preferred alternative today by manufacturers who implement liquid cooling.

In recent years, the immersion of the cells in cooling fluids has acquired a great interest at both scientific and industrial levels. The main advantage of this configuration is the direct contact between the cooling fluid and the cells, which allows a more effective heat transfer. Studies indicate that the transfer can be increased by up to four times compared to indirect systems.

However, there are significant challenges that hinder the implementation of this solution in electric vehicles today. The main one is the need for further research into dielectric fluids that guarantee the correct operation of the cells, that are not incompatible with any of the battery-pack components (cells, current collectors, electronics...), that have a reasonable cost and that guarantee the safety of the vehicle in the event of an impact.

A more extreme case of this alternative is the use of fluids with a boiling point in the desired temperature range for the cells, in order to benefit from the liquid-vapor phase change.

There are scientific studies on these fluids, with which it is estimated that heat transfer can be increased by up to 10 times compared to the use of fluids without phase change. However, these fluids are at a very low TRL (Technology Readiness Level) and are not expected to be implemented in vehicles in the short term.

In general, the advantages and disadvantages of active BTMS can be summarized as follows:

• Advantages:
  • Relatively simple design of forced-air based ones.
  • High efficiency in maintaining the battery-pack in the desired temperature range in the case of liquid-based ones.
• Disadvantages:
  • High operating costs in forced-air based plants due to the need to implement large airflows.
  • Low efficiency in achieving temperature homogeneity between cells.
  • Leakage problems may arise in liquid-based systems.
  • Occupied volume and complexity of liquid-based systems.

Passive BTMS

Passive systems are an alternative to active BTMS that overcome their disadvantages. Although these types of systems are not currently implemented in electric vehicles, they have recently become very important due to their operational advantages.

Two large families stand out among the different passive solutions: phase change materials (PCMs) and heat pipes (HPs).

PCMs -especially those with a solid-liquid phase change- have been extensively studied for their application in BTMS. The interest of these materials lies in the possibility of exploring the high energies associated with phase changes (typically >150 J/g) that occur at a nearly constant temperature. These two characteristics make them attractive when maintaining a homogeneous temperature throughout the battery-pack, close to the phase change temperature of the implemented PCM.

The most studied compounds for these applications are paraffins, fatty acids or hydrated salts. Generally, these compounds/mixtures have melting points in the range of 30-50 ºC, making them ideal for battery thermal management.

However, in general, the aforementioned families of PCMs have a relatively low thermal conductivity, a feature that limits heat transfer from the cells to the PCM itself and from the PCM to the battery-pack exterior.

To address this limitation, numerous works in the literature propose embedding the PCM in porous structures (generally metallic), doping the PCM with nanoparticles, fibers or expanded graphite, among others.

Despite their good performance in achieving good thermal homogeneity in the battery pack, PCMs have certain limitations that make them not the preferred option today. These include the following:

• Low thermal conductivity.
• When PCM is doped, it loses energy density.
• Limited thermal storage capacity.
• Increases the weight of the battery-pack.

A second alternative to active systems are heat pipes. These are fluid-filled vacuum tubes (usually with water) that operate by using the vapor-liquid phase change of the fluid.

In general, a heat pipe is composed of three sections: an evaporator (area in contact with the hot source/cell), an adiabatic section through which the vapor circulates, and a condenser (area in contact with the cold source/outside of the battery pack). And, although they are not currently used in battery-packs, their use in the cooling of electronic components is very widespread.

The main characteristics that make them of great interest for implementation in BTMS are their flexible geometry, high thermal conductivity (nearly twice that of solid conductors) and virtually zero maintenance. On the other hand, the main limitations of this technology are its complexity and the cost of the complete solution.

Hybrid BTMS

Finally, in order to take advantage of the benefits of active and passive systems, hybrid systems have emerged, combining two or more of the alternatives described above.

The most studied combinations include the use of PCMs with forced air, PCMs with liquid cooling or PCMs with heat pipes. In the first case, the objective is to achieve a good temperature distribution in the battery-pack and the use of forced air or liquid cooling to evacuate the heat generated to the outside.

In the case of PCMs with heat pipes, the aim is to improve the heat transfer from the PCM to the outside of the cells, so that the cells can be cooled by natural convection.

Although these BTMS systems show a much more effective performance than pure passive or active systems in thermally managing the battery pack, their complexity and cost are a limiting factor for implementation in electric vehicles.

A sector with high expectations

Whichever alternative finally dominates the market in the coming years, what does seem certain is the significance that this sector will gain in the short and medium-term. Not only because of the gradual implementation of electric vehicles, but also because of its usefulness and application in other uses and sectors where the optimum operation and temperature of cells and batteries is critical for their correct operation.

Not surprisingly, if we focus only on the expected market prospects for the BTMS industry associated with electric vehicles, we note that this activity is estimated to reach a value of 12-13 billion euros only until 2024, with a CAGR of almost 40% in the coming years (considerably above the average of other industries).

This only reaffirms the big bets that companies such as Samsung, CATL or LG Chem have started to make on these technologies, demonstrating the future and potential that these new solutions are expected to have in the coming years.

In this sense, at CIC energiGUNE we are also working on disruptive thermal management alternatives in collaboration with cell manufacturers, whose information will be expanded in future articles.