An Introduction to Thermal Management in Electric Propulsion Systems

Reduction in CO2 emissions is driving the development of different electric, turbo-electric and hybrid electric propulsion systems for various applications and industries including space, aviation, automotive and marine. Electric propulsion (EP) is not a new concept, having been studied in parallel with chemical propulsion for many years. EP is a generic name encompassing all the ways of accelerating a propellant using electric power by different possible electric and/or magnetic means. The simplest way to achieve electric propulsion is to replace the heat generated by combustion in conventional chemical engines with electrical heating.

Electric propulsion systems offer several advantages compared to other conventional propulsion systems. It not only helps reduce the environmental emissions but also helps reduce fuel consumption and increases safety levels. Electric propulsion has become a cost effective and sound engineering solutions for many applications. Electric propulsion engines are also more efficient than others. It is proven to be one of the most energy saving technologies as we can use more renewable sources of energy (due to the versatility of electricity generation) instead of non-renewable sources of energy like gasoline. The major limitation of electric propulsion, when compared with conventional propulsion is limited by the available electric power capacity on board, this may be the reason, it is not the default propulsion system.

Electric Propulsion Architectures
Figure 1. Electric Propulsion Architectures. SOURCE: [4]
Generally, electric propulsion architectures vary depending on the application. Figure 1, above, shows the EP architectures for an aviation application. These architectures rely on different electric technologies (batteries, motors, generators, and so on). Typical aircrafts use gas turbine engines as the source of propulsion power, but all electric aircraft systems use batteries as the only source of propulsion power as shown in Figure 1 on the right. The hybrid systems use gas turbine engines for propulsion and to charge batteries which also provide energy for propulsion and accessories during one or more phases of flight as shown in Figure 1 on the left.

Electric propulsion systems require the development of high-power electric machines with integrated power electronics. Here, cooling remains an issue because of the dense packaging and close integration. The high-power density associated with these propulsion systems requires an advanced thermal management system (TMS). These thermal management systems allow controlling the temperature of a system by means of technology relying on thermodynamics and heat transfer. The TMS should be able to handle high heat flux on a large scale, have a high coefficient of performance and low weight. Efficient TMS in electric propulsion contribute significantly to improving the efficiency, health, and extending the overall lifespan of electric components such as the motor, generator, battery etc. Improving these efficiencies (motor/generator, battery etc.) increases the overall propulsion system efficiency. These electric components generate large amount of heat during operations, which should be managed by TMS, making them a crucial part of any electric propulsion system. The battery performance depends on both the temperature and the operating voltage of battery cells. Due to inefficiency, battery cells will not only generate electricity but also heat. This heat should be removed from the battery pack when battery temperature reaches the optimum temperature or even in advance to avoid thermal issues and improve performance and longevity. Thus, a cooling function is required in TMS of batteries.

There are several cooling technologies used in thermal management system batteries including:

  1. Air-cooling system
  2. Liquid-cooling system
  3. Direct refrigerant cooling system
  4. Phase change material cooling system,
  5. Thermoelectric cooling system

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The thermal management system in a battery (battery cooling system) is shown in Figure 2. This system uses coolant as the thermal medium. The coolant enters from one side of the cooling tube and removes some of the heat generated from the cells before leaving the cooling tube as shown in Figure 2. Due to heat transfer, the cells at the end of the cooling tube will have significantly higher temperature than cells at the beginning of the cooling passage.

Thermal Management System in Battery
Figure 2. Thermal Management System in Battery. SOURCE: [5]
Similarly, an electric motor also generates relatively large amount of heat due to their operating and driving cycles. To efficiently remove this heat and maintain the motors temperature within the prescribed range while minimizing energy consumption constitutes the cooling system’s main challenge. Thus, an efficient cooling system is required in thermal management of motors.

There are several cooling system strategies used in thermal management of electric motors such as:

  1. Surface air-cooling with a fan coupled to the shaft as shown in Figure 3(a) below
  2. Liquid cooling with coolant jacket as shown in Figure 3(b) below
  3. Heat pipe cooling with attached fins and a centrifugal fan as shown in Figure 3(c) below
  4. Hybrid cooling with heat pipes and liquid as shown in Figure 3(d) below

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Thermal Management System in Electric Motor
Figure 3. Thermal Management System in Electric Motor. SOURCE:[7]
These thermal management systems of electric motors includes air, liquid, heat pipes and hybrid cooling. Air cooling is straightforward and offers a simple structure but the cooling performance may not be sufficient. Moreover, a cooling fan is usually linked to the motor shaft which consumes energy and cannot be directly controlled. The cooling air from the fan passes through the motor surface in the axial direction and helps remove motor heat as shown in Figure 3(a). In Figure 3(b), coolant enters from the inlet of coolant jacket and the motor heat is removed by the flowing coolant before exiting through the outlet. Liquid cooling is effective; however, it consumes energy to run the coolant pump and radiator fan. Additionally, liquid cooling adds more weight and complexity due to the cooling lines.

In Figure 3(c) the presented heat pipe has two sections: an evaporator (hot end) and a condenser (cold end). Heat pipe evaporator was inserted into the motor housing to absorb heat and heat supplied to the evaporator end. The heat pipe is filled with a working liquid vaporizes as a result of the input heat. The vapor is then pushed towards the condenser and condenses back to liquid by rejecting heat. Finally, the liquid from the condenser end returns back to the evaporator due to capillary forces. Fins and a centrifugal fan are added to enhance the heat dissipation rate. However, this system has heat transfer limitations due to the capillary limit, fluid properties, operating temperatures, etc.

Figure 3(d) shows a hybrid cooling system, which features two parallel heat transfer pathways – heat pipes and liquid cooling systems. By adding heat pipes, the workload of the liquid heat transfer pathway will be reduced therefore providing more possibilities to minimize the cooling system energy consumption. These electric motor cooling methods should be selected based on the machine’s classification, power level and working environment.

Such cooling systems whether in a battery or electric motor make it possible to remove the heat generated during various operations and help to control the thermal behaviors of each. This makes an efficient thermal management system in both batteries and electric motors. These types of cooling flow systems can be modelled and analyzed using AxSTREAM NET™ by creating a cooling flow passage and analyzing it. One way to model cooling through convection and conduction between a hot and a cold flow in AxSTREAM NET™ is shown in Figure 4.

Convection Modelling in AxSTREAM NET
Figure 4. Convection Modelling in AxSTREAM NET™

Figure 4 represents a modelling example of convective heat transfer between a fluid flow and a solid pipe using thermal-fluid network approach. Here, both the hot and cold fluid flows are simulated through the pipe. The surface and thermal elements are added and connected to the fluid network of both hot and cold pipes to simulate convective heat transfer between the fluid flow and the solid wall of the pipes. In this way convection heat transfer can be modelled and analyzed using AxSTREAM NET™. The AxSTREAM NET™ software is a thermal-fluid system and analysis tool. It provides a flexible method to represent fluid path and solid structure as a set of 1D elements, which can be connected to each other to form a network. For each fluid path section, it calculates fluid flow parameters for inlet and outlet cross-sections, like velocity, pressure, temperature, etc.  By simulating convective heat transfer between fluid flow and solid structure we can estimate cooling or heating system effectiveness, based on values of heat flow rates, temperatures of fluid flow and solid bodies as shown in Figure 4.

To design or analyze fans, like the one coupled to the electric motor of Figure 3, SoftInWay offers a complete turbomachinery design and analysis tool called AxSTREAM® which can generate optimized designs with less time and effort starting from the specifications. Using this tool, designers are provided the opportunity to create thousands of designs from scratch with minimal data available. The flexibility of this software enables a person with basic knowledge of fan design and use of design tools to perform complex fan design and analysis tasks.

Examples of fans designed using AxSTREAM
Figure 5. Examples of fans designed using AxSTREAM®

In AxSTREAM® the design process starts with the preliminary design, then progresses to the streamline analysis and then to the profiling and blade design. It finally concludes with a 3D analysis. Examples of an Axial and a Centrifugal fan designed using AxSTREAM® are shown in Figure 5 (a) and 5 (b), respectively.

Are you interested in learning about how AxSTREAM NET™  and AxSTREAM® can help you to design or analyze a cooling flow path and axial or centrifugal fan? Reach out to us at Info@Softinway.com to schedule a demo!

References:

[1]https://arc.aiaa.org/doi/abs/10.2514/6.2018-4695

[2]Sabbadini. Marco, M. Buoso, and G. Saccoccia. Electric Propulsion: Technology Programmes. No. ESA-BR-187.2002

[3]Ian J.E. Jordan. Electric Propulsion. V. Pisacane as part of requirement for 744 Space Systems I course at JHU. Whiting School of Engineering

[4]https://www.nap.edu/read/23490/chapter/7

[5]https://insideevs.com/news/338711/tesla-model-3-battery-cooling-much-improved-track-mode/

[6]http://publications.lib.chalmers.se/records/fulltext/200046/200046.pdf

[7]https://www.researchgate.net/publication/331417484_A_Hybrid_Electric_Vehicle_Motor_Cooling_System-_Design_Model_and_Control