Power Density as the Key Enabler for Electrified Mobility
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Electrification for mobile systems is a long-term trend that touches every mode of transport we currently use. As these systems proliferate, and increase in power levels, there is a strong desire to increase the power density of the electrified systems on board. A key barrier to increased electrical power density is the associated thermal management that is necessary to prevent failures. This article provides an overview of a long-term effort aimed at integrating the research and education of combined electrical and thermal systems for the sake of increased power density. An overall motivation for this effort is provided in order to gain context. Then, specific examples are provided of how research and education can be performed within graduate programs across the United States (U.S.) and the world.
KeywordsMobility Transportation Electrification Power density
1 Electrified Mobility: Background and Motivation
We live in an increasingly electrified world. For stationary applications such as industry and manufacturing, this statement has been obvious since the start of the twentieth century as steam and belt drives in factories gradually gave way to electric motors for machining, conveyor lines, and all manner of other industrial applications (Atkeson & Kehoe, 2001). For domestic stationary applications, modern conveniences blossomed as electrification grew starting in the middle of the twentieth century (Gordon, 2016). Lighting, air-conditioning, cooking, and cleaning, as well as many types of in-home entertainment were fueled by growing abilities to provide relatively cheap electrical power over long distances.
Whereas much of the auxiliary componentry on aircraft used to be performed by mechanical means, including pneumatic or hydraulic power, increasingly the auxiliary systems are becoming electrified. Bleed air systems to run air cycle machines are being replaced by high-speed electric motors. Hydraulic actuators for flight surface control are being replaced by electric drives, or at least electro-hydrostatic actuators. The insatiable consumer appetite for communications and infotainment are leading to increased loads for networking within the aircraft as well as communicating to ground or satellite stations. Some of the benefits of electrified auxiliaries are increased efficiency, reduced emissions, and potentially increased long-term reliability (Sarlioglu & Morris, 2015). All this led to the more electric Boeing 787, the most advanced commercial aircraft, which boasts a megawatt of power coursing through its infrastructure.
While aircraft have a long and steady growth rate, there is an event horizon that is upcoming. Manufacturers and government are now looking past auxiliaries and subsystems as the sole domain of electrification and are focusing on the prospect of electric propulsors taking the place of the venerable gas turbine turbo-fan engine (Henke et al., 2018). Efforts under investigation include both battery-driven systems without any combustion engine as well as hybridized power trains where the engine provides electrical power to run the propulsor fans. These will take the electrical power levels well above 1 MegaWatt for commercial transport.
The results of the passenger vehicle and aerospace sector are not unique. Similar trends can be seen in delivery trucks, busses, off-highway equipment, and ships. Many of the same drivers apply here as well. For example, government regulations limit or prohibit idling of engines in delivery trucks while stopped to perform loading and unloading (D.C. Anti Idling Law, 2018). This is a natural incentive to electrify. An examination of Figs. 1, 2, and 3 makes at least two things clear. First, increased electrification in mobile transport is a long-term trend that shows no signs of reversing at any point in the future. In fact, it will likely accelerate as economies of scale drive unit prices down. Second, the level of electric power being used in mobility platforms is monotonically increasing, potentially with significant jumps as discussed with regard to electric propulsion.
There are clear and irreversible drivers governing these trends. A key question raised in this articles is “What are the challenges and barriers to progress faced by these systems?” In Section 2 of this article, we make the case that one of the main barriers to progress is the challenge associated with increasing power density. Power density is arguably the most important metric when discussing mobile systems. Yet, as we will see, there are limits to increasing electrical power density. Section 3 advocates for a systems-level solution to these challenges and provides specific examples of how this systems-level approach can bring tangible technological benefits. Since technology without people is just an abstract idea, Section 4 describes one approach to developing the right types of technologists to address these systems-level problems. Section 5 concludes the paper with some thoughts for our increasingly electrified future.
2 Power Density Challenges
When considering mobile systems, the power to weight ratio is the key metric that drives all other performance indicators. Increased power to weight allows for a given propulsion level to be accomplished in a smaller or lighter form factor. This results in greater cargo carrying capacity with similar acceleration performance or, conversely, greater acceleration with the same cargo carrying. For electrified systems, part of the cargo to be carried are the batteries that store energy in addition to the people and goods to be transported. Therefore, increased power density provides greater range, more passengers, and lower cost per passenger mile.
Power density can be measured either gravimetrically, as in kW/kg, or volumetrically, as in kW/l or kW/m3. Both have their relative merits depending on the application. However, increasing both result in processing larger amounts of electrical power in smaller volumes since weight is often tied closely with size. As electricity passes through a given volume, there is resistance which leads to resistive, or Joule heating. One way to minimize this is to utilize high temperature superconductors but this is outside the scope of consumer-ready transport. Joule heating leads to a buildup of thermal energy. Unfortunately, as a consequence of physics, it is far easier to put thermal energy into a confined volume than it is to take it out. A relatively thin wire embedded in a solid can create a significant amount of heat. It is simple to find heaters that can provide 100’s of watts of heat in cubic centimeter packaging. Moving the heat away from its source in the solid requires either conduction or convection via fluid mass transport. Convection requires relatively sizeable passages to get the fluid mass close to the source which results in increased bulk and reduced power density. Conduction results in a relatively slow transport of heat away from the source with a speed governed by the thermal conductivity of material. The choices of big (convection) or slow (conduction) result in key design barriers to the increase of electrical power density with significant impact.
Another thermal issue with electronics results from their construction. Power electronics are comprised of multiple heterogeneous systems including metal conductors, semi-conductors, capacitors, inductors, and substrates. These all have different coefficients of thermal expansion (CTE). When two or more materials, with dissimilar CTE’s, are bonded or joined, the heating and cooling of them causes stress due to a mismatch in displacement between the different materials. As these heterogeneous systems heat and cool due to the operation of the power systems, there is a mechanical stress that is cycled at the interface. Over time, and if sufficiently large, this stress leads to failure just like any other mechanical stress cycling phenomenon (Khasaka et al., 2015) (Durand et al., 2016). This reliability impact is one of the most important aspects of systems that process electrical power.
The thermal limits for electrified systems act as a barrier or wall to increased power density. As a result, modern systems have been limited in power or energy density. Motors have been limited to approximately 5 kW/kg, excluding the cooling system. Power electronics have been limited to approximately 2–5 kW/kg for inverters depending on the level of refinement of sinusoidal signals being sought. State of the art lithium-ion batteries range between 150 and 200 W*h/kg; much of this limitation is electrochemistry but there are also significant thermal limits associated with charge and discharge rates for batteries.
What is needed is a systems-level approach to increasing power density. This means a co-design of the electrical and thermal elements from the beginning. It also means a multi-disciplinary approach spanning electrical engineering, mechanical engineering, and materials science among other disciplines. Recently, a new effort was started to specifically address the systems-aspect to what is termed electro-thermal systems. The U.S. National Science Foundation has sponsored an Engineering Research Center entitled Power Optimization of Electro-Thermal Systems (POETS). This 10-year federal investment, starting in 2015, encompasses 4 U.S. universities, 2 international universities, many commercial partners, as well as multiple government partners. The U.S. universities are The University of Illinois at Urbana-Champaign as the lead, The University of Arkansas, Howard University, and Stanford University. The international universities are the University of Sao Paolo in Brazil and The Royal Institute of Technology (KTH) in Stockholm, Sweden. The goal is to develop a different approach to developing electro-thermal technology and the engineers who will carry this forward into the future.
3 A Systems Approach to Electro-Thermal Design and Operation
Mobile power systems involve generation, conversion, transmission, and storage. It should be understood that there is no single technology that will improve power density across the entire chain of subsystems. Moreover, the introduction of improved components may simply expose a weak link elsewhere in the chain. Consider a relatively new semiconductor silicon carbide (SiC) that is being advanced in the application of transistors and switching devices which are at the heart of power electronics (Ozpineci & Tolbert, 2011). Included in many of its superior attributes to its replacement, silicon, is its ability to operate at high temperatures. While silicon can operate up to 150 °C, SiC can safely operate to nearly twice that level. However, even if the active switching devices are made to operate at higher temperatures, there are still many other components within a power device (capacitors, control circuits, and drivers) that may not be able to withstand higher temperatures and may fail. Therefore, advancing the device development needs to go hand in hand with the overall component development and this component development needs to coordinate with the system development. The following presents two examples to illustrate this point.
Having a large area over which active devices operate has potential for good thermal transport. However the large area distribution poses an interesting challenge for efficient thermal management. The active devices need to couple their thermal power they are creating with the heat sink to take that power away and prevent overheating of the active device. Typical designs would have a thermal interface material (TIM) that interfaced between the active devices and a heat sink collecting the thermal energy given off. Slight mismatches in manufacturing alignment over the entire area of the heat sink mean each active device may be at a slightly different height with respect to the heat sink. This leads to problems in getting good contact across all active devices with the TIM and between the TIM and the heat sink. Many devices may have good contact but there may be some where the TIM is not in good contact and these may lead to heat buildup and failures.
The POETS approach taken is counter-intuitive from an electrical design perspective where power density focuses on the board and module design. The electrical designer would wish to pack the active devices as close as possible together, thereby increasing nominal electrical power density but giving up a lot in thermal management and reliability. The modular approach moved the active devices further away from each other to insert the modular heat sinks. This led to a larger electrical footprint than the non-modular approach. However, the modularity bought significant improvements in the thermal management which led to better and more uniform device cooling and a more efficient and reliable overall system. Overall, the power density of this system is 5–10× higher than comparable state of the art systems and maintains both active and passive device temperatures well within operational designs. Current designs are focused on optimizing the flow paths, reducing costs by having one fan instead of multiple distributed ones, and also the benefits to liquid modular cooling as an improvement over air. It is an excellent example of electro-thermal systems where considering both electrical and thermal functionality at the start results in an overall module improvement.
Examining the two examples in this section, from a component and systems level, it can be seen that consideration of the electrical and thermal interaction leads to better design of physical components as well as better interaction in operation for the various subsystems.
4 A New Type of Engineering and Engineer Needed
The previous section indicated a new type of multi-disciplinary engineering approach to increasing power density for electrified mobile systems. In addition to the two examples given, there are several more that could be used to illustrate the integration of electrical engineering with materials science and mechanical engineering. To accomplish this type of integration takes teams of engineers who are ready and willing to work across boundaries. It is important that academia actively prepares engineers for this setting.
In addition to the scientific and technical research being performed within POETS, there is also examination of how best to train students to become the ideal shown in Fig. 12. One active way of doing this is to create courses that are intentionally multi-disciplinary and require teams of students from different disciplines to address a complex project. This is currently being done across three different departments with cohorts of 20 graduate students per session. At a recent offering, the design project goal was to create the most power dense motors possible without the need for cryogenic cooling. The course was taught by 4 different faculty, each with a separate technical background. Early feedback from both students and faculty involved support the idea that this is a challenging, but very valuable pedagogical endeavor.
Another approach is to provide students with specific training in non-technical skills necessary to bridge disciplines. Short courses have been created within POETS targeted at training students in creating a business plan or structured brainstorming and ideation. These workshops are brief and focused but are recorded and archived to allow for asynchronous learning. Student feedback is positive. Even more positive is the feedback from industrial partners who are looking to have these students move into jobs at their companies after graduation. There is a significant appreciation for the students’ well-roundedness in some of the softer skills necessary to work on large complex projects in addition to their technical depth.
5 Future Outlook and Conclusions
In industrialized society, the use of coal power pioneered our mechanized mobility needs in the form of ships and trains. The age of coal gave way to the age of oil which pushed modern society further, faster, and higher than ever before with automobiles and airplanes. Now, with the advent of renewable energy and the need to meet climate change challenges, there is an inexorable shift away from oil and towards electrified transport. History has taught us that these transitions happen slowly, over generations. However, the signs are there that this is a direction we will not be turning back from. The drivers for this change differ by transport mode and by individual nation, but they are real.
The question posed, in summary, is “Who will benefit and who will be left out?” This is the key question to be asked when understanding these technology transitions. This article argues that those who will benefit the most are those who are able to work across disciplines to increase the power and energy density of an electrified mobile infrastructure. In particular, the ability to incorporate both electrical and thermal aspects of power and co-design tightly integrated systems will afford significant benefits. These individuals and organizations will become the leaders in this emerging technological landscape. Those who do not will, unfortunately, be left behind.
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