Thermal Management in Aerospace Electric Propulsion Systems

The growing interest towards electric propulsion system for various applications in aerospace industry is driven first by the ambitious carbon emissions and external noise reduction targets. An electric propulsion (EP) system not only helps reduce the carbon emissions and external noise, but also helps reduce operating cost, fuel consumption and increases safety levels, performance and efficiency of the overall propulsion system. However, the introduction of electric propulsion system leads engineers to account for certain key challenges such as electric energy storage capabilities, electric system weight, heat generated by the electric components, safety, and reliability, etc. The available electric power capacity on board may be one of the major limitations of EP, when compared with a conventional propulsion system. This may be the reason electric propulsion is not the default propulsion system. Now, let’s consider how electric propulsion is used in the aerospace industry. Following the hybridization or complete electrification strategy of the electric drive pursued on terrestrial vehicles, the aerospace industry is giving great attention to the application of electrical technology and power electronics for aircrafts.

Figure 1 Aircraft Electric Propulsion Architectures
Figure 1. Aircraft Electric Propulsion Architectures. SOURCE: [1]
Electric Propulsion in aircrafts may be able to reduce carbon emissions, but only if new technologies attain the specific power, weight, and reliability required for a successful flight. Six different aircraft electric propulsion architectures are shown in Figure 1, above, one is all-electric, three are hybrid electric, and two are turbo-electric.  These architectures, rely on different electric technologies (batteries, motors, generators, etc.).

Electric Propulsion in aircrafts may be able to reduce carbon emissions, but only if new technologies attain the specific power, weight, and reliability required for a successful flight. Six different aircraft electric propulsion architectures are shown in Figure 1, above, one is all-electric, three are hybrid electric, and two are turbo-electric.  These architectures, rely on different electric technologies (batteries, motors, generators, etc.). A typical aircraft uses a gas turbine engine as the source of propulsion power, but all electric aircraft systems use batteries as the only source of propulsion power on the aircrafts as shown in Figure 1(d). A hybrid system uses a gas turbine engine for propulsion and to charge batteries which also provide energy for propulsion and accessories during one or more phases of flight. With a parallel hybrid system, as shown in Figure 1(b), a battery-powered motor and a turbine engine are both mounted on a shaft that drives a fan, so that either or both can provide propulsion at any given time. With a series hybrid system, as shown in Figure 1(a), only the electric motors are mechanically connected to the fans; the gas turbine is used to drive an electrical generator, the output of which drives the motors and/or charges the batteries. Series hybrid systems are compatible with distributed propulsion concepts, which use multiple relatively small motors and fans.

The series/parallel partial hybrid system has one or more fans that can be driven directly by a gas turbine as well as other fans that are driven exclusively by electrical motors as shown in Figure 1(c), these motors can be powered by a battery or by a turbine-driven generator.

Full and partial turbo-electric configurations do not rely on batteries for propulsion energy during any phase of flight, as shown in Figure 1(e) and 1(f) respectively. Rather, they use gas turbines to drive electric generators, which power inverters and eventually individual direct current (DC) motors that drive the individual distributed electric fans. A partial turboelectric system is a variant of the full turboelectric system which uses electric propulsion to provide part of the propulsive power; the rest is provided by a turbofan driven by a gas turbine as shown in Figure 1(f). As a result, the electrical components for a partial turboelectric system can be developed with smaller advances beyond the state of the art than those required for a full turboelectric system. Because it is relatively easy to transmit power electrically to multiple widely spaced motors, turboelectric and other electric propulsion concepts are well-suited to distributed propulsion for higher bypass ratios (BPR). EP can provide aircraft design options for maximizing the benefits of boundary layer ingestion (BLI) in the fans.

Current all-electric technology aircraft have been operating as technology demonstrators and are starting to enter production. Some examples of these aircraft, which represent the state-of-the-art as of 2015 [1], are shown in Figure 2. They use relatively small electric motor systems, on the order of 60-80 KW.

Figure 2 Examples of all-electric-powered general aviation aircraft
Figure 2. Examples of all-electric-powered general aviation aircraft. SOURCE: Courtesy of Pipistrel (www.pipistrel.si), (top left); Jean-Marie Urlacher (http://www.urlachair.com) (top right); Wikimedia Commons user Adambro, “Boeing Fuel Cell Demonstrator_AB1,” https://commons.wikimedia.org/wiki/File: Boeing_Fuel_Cell_Demonstrator_AB1.JPG, Creative Commons Attribution-Share Alike 3.0 Unported (bottom left); Airbus, “Airbus-E-Fan-close-up,” https://www.cnn.com/2014/06/11/tech/airbus-electric-aircraft/index.html © Airbus Group (bottom right).

The SUGAR Volt concept from Boeing SUGAR (Subsonic Ultra Green Aircraft Research) is a family of aircraft which is based on a parallel hybrid electric system is shown in Figure 3. This SUGAR Volt concept features a twin-engine aircraft (see Figure 3) and relies upon projected advances in battery technology to enable a parallel hybrid electric propulsion system [1]. The aircraft’s concept engines were provided by General Electric and used a large electric motor attached to the low-pressure shaft of a gas turbine. This allowed the turbofan to run conventionally, burning aviation fuel, or it could use the electric motor to augment the power supplied to the low-pressure shaft (and hence the fan). The motor could also provide exclusive power to the fan when the core of the gas turbine is shut down during portions of the cruise mission, thereby significantly reducing emissions.

Figure 3 Boeing SUGAR Volt concept
Figure 3. Boeing SUGAR Volt concept. SOURCE: NASA, “Slimmed Down Aircraft Wing Expected to Reduce Fuel and Emissions by 50%,” April 4, 2016, http://www.nasa.gov/image-feature/ames/slimmed-down-aircraft-wing-expected-to-reducefuel-and-emissions-by-50.

Figure 4, shows the Distributed Open Rotor Aircraft electric propulsion concept which is based on a turbo-electric system with distributed propulsion [1]. This concept uses multiple propulsors distributed across a significant portion of the wing as shown in Figure 4. These propulsors could be either electrically powered by turbo-generators mounted on the wings or shaft driven. Regardless of the power distribution mechanism used for this concept, the slipstream of the propulsors significantly increases the wing lift coefficient at take-off, allowing the wing area to be reduced and enabling the wing to be designed with a very high aspect ratio, optimized for highly efficient cruise performance. For an electric propulsion system, the use of many fans lowers the power required of each motor. This potentially makes the aircraft practical sooner, as smaller motors can be used instead of waiting for the development of larger motors [1].

Figure 4 Rolls Royce
Figure 4. Rolls-Royce Distributed Open Rotor Aircraft concept for regional aircraft. SOURCE: © 2016 Rolls-Royce, plc.

Partial turboelectric systems are also being studied. The NASA STARC-ABL (single-aisle turboelectric aircraft with an aft boundary layer propulsor) is an example of a partial turboelectric system as shown in Figure 5 [3]. This conceptual design of STARC-ABL adds a tail cone propulsor to a typical tube-and-wing single-aisle configuration with downsized, podded turbofans (Figure 5). Unlike many proposed turboelectric architectures, the STARC-ABL does not include a dedicated turbo-generator; the power for the electric propulsor is generated from the turbofans [4].

STARC-ABL Turbo-Electric Concept
Figure 5. STARC-ABL Turbo-Electric Concept (NASA Image) [3]
Turbo-electric propulsion is one of the four high-priority approach for developing advanced propulsion and energy system technologies for larger aircrafts to reduce CO2 emissions that could be introduced into service during the next 10 to 30 years [1].

Hybrid-electric and all-electric systems are not recommended as a high-priority approach because the batteries with the power capacity and specific power required for a larger aircraft is limited.[1]. All-electric battery-powered airplane configurations will be limited to small aircraft (general aviation and commuter aircraft), which are not a significant source of CO2 emissions compared to larger aircrafts [1]. For large aircraft, it is likely that fuel cell applications will be limited to secondary systems such as auxiliary power units and starter systems, etc. [1]. Considerable improvements in the specific power of batteries and fuel cells will have to be attained before these power sources would be considered for large aircrafts [1].

Turbo-electric propulsion concepts are heavily dependent on advances in aircraft electrical power system technologies. These technologies include generator systems for electrical power generation; power electronics for power conversion, conditioning, and distribution; high power aircraft distribution that includes circuit protection; motors; and energy storage. These electric power system technologies present a number of challenges related to thermal management system (TMS). These electrical technologies also generate large amount of heat during operations, which should be managed by TMS, making them a crucial part of turbo-electric propulsion system. The thermal management system is an important aspect because it may not only affect the performance of the electric propulsion system, but also affect the aircraft performance, too. TMS allows controlling the temperature of a system by means of technology relying on thermodynamics and heat transfer.  As a result, more structurally and aerodynamically efficient configurations can help to address these challenges.

In a turbo-electric propulsion system, an electric motor generates relatively large amount of heat due to their operating and driving cycles. The main challenge of the cooling system is to efficiently remove this heat and maintain the motors temperature within the prescribed range while minimizing energy consumption. Thus, an efficient cooling system is required for the TMS of motors.

There are several cooling system strategies used in TMS of electric motors such as air-cooling, liquid cooling, heat pipes cooling, hybrid cooling with heat pipes and liquid, etc. An example of liquid cooling with a coolant jacket is shown in Figure 6.

Figure 6 Thermal Management System in Electric Motor
Figure 6. Thermal Management System in Electric Motor. SOURCE:[2]
In Figure 6, above, a coolant enters from the inlet of the 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. Electric motor cooling methods should be selected based on the machine’s classification, power level and working environment.

Such types of cooling systems in an electric motor make it possible to remove the heat generated during various operations and help to control the thermal behaviors of the motor. This makes an efficient thermal management system in electric motors. Such 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 7.

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

Figure 7 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 analysed using AxSTREAM NET™. In AxSTREAM NET™, 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 7.

To design or analyze fans, like as shown in Figure 1’s various aircraft electric propulsion architectures, 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 can generate thousands of designs from scratch with minimal data available. The flexibility of this software allows a person with basic knowledge of fan design and use of design tools to perform complex fan design and analysis tasks. An example of a fan designed using AxSTREAM® is shown in Figure 8.

Figure 8 - Fan Designed Using AxSTREAM
Figure 8. Fan Designed Using AxSTREAM®

AxSTREAM® can also assist with the development of new concept electric propulsion systems. The six different aircraft electric propulsion architectures, Figure 1, are selected based on the different aircraft applications. There is still a lot of space to push the electric propulsion limits, and this is what both industry and academics are focusing on. For turbo-electric concept to be applied in larger aircrafts, the aircraft electrical technologies such as motors, generators, power electronics and power distribution needs to be advanced further. For anyone who needs to develop a new concept in turbomachinery, SoftInWay can be a strong partner. With many years of experience in aerospace industry and with the aid of our AxSTREAM® platform, we can ensure the optimized electric propulsion systems for various applications.

Are you interested in learning about how SoftInWay can help you to develop a new concept in turbomachinery design? or/ to design or analyse a cooling flow path and fan using AxSTREAM NET™  and AxSTREAM® platform? Reach out to us at Info@Softinway.com to schedule a demo!

References:   

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

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

[3] J. Welstead, J. L. Felder, Conceptual Design of a Single-Aisle Turboelectric Commercial Transport with Fuselage Boundary Layer Ingestion, in: 54th AIAA Aerospace Sciences Meeting, San Diego, CA, 2016. doi:10.2514/6.2016-1027.

[4] B. J. Brelje and J. R. R. A. Martins. Electric, Hybrid, and Turboelectric Fixed-Wing Aircraft: A Review of Concepts, Models, and Design Approaches, Progress in Aerospace Sciences, 104:1– 19, January 2019. doi:10.1016/j.paerosci.2018.06.004

[5] Clemet Pornet. Electric Drives for Propulsion System of Transport Aircraft, Bauhaus Luftfahrt, Munich, Germany.