Category Archives: Analysis

Electric Car Batteries Are Topic of Presentation by ATS CEO Dr. Kaveh Azar

Electric Car Batteries

ATS CEO Dr. Kaveh Azar will deliver a presentation on the thermal management of electric vehicle batteries on Thursday, Sept. 22. (Photo courtesy of Wikimedia Commons)

On Thursday, Sept. 22, Advanced Thermal Solutions, Inc. (ATS), a leading-edge engineering and manufacturing company focused on the thermal management of electronics, will host the New England Section of Society of Automotive Engineers International (SAE NE) for a tour of its Norwood campus and a presentation by ATS founder, President, and CEO Dr. Kaveh Azar.

Dr. Azar’s discussion is entitled, “Battery Thermal Management – The Gateway to the Successful Operation of Electric Vehicles.” He will review the role of temperature in the longevity and performance of nickel metal hydride (NiMH) and lithium-ion electric vehicle batteries; drawing analogies between battery temperature and the junction temperature of modern electronics. As Dr. Azar notes, “Both play an identical role in successful operation of their respective systems.”

There will be a discussion of the analytical methods and design criterion for predicting battery temperature and establishing safe temperature limits. Dr. Azar will present high-level possibilities for thermal management in the electric vehicle sphere as well as cooling options that are deployed for battery thermal management. Current cooling designs can be active or passive. There are forced air, liquid cooling, natural convection and conduction systems used by manufacturers.

Several thermal solutions that engineers have incorporated include increasing the thermal density of the battery, using phase-change material to store transient heat loads and graphite-impregnated paraffin waxes as gap fillers. It is also important for the designs to control temperature distribution across the battery to avoid degradation of cells.

Thermal management is crucial in the design of electric vehicle batteries because temperature has a direct correlation on battery life and performance. It will affect the battery’s ability to store and deliver a charge, weaken polymer- or fiber-based cell dividers, and could potentially lead to thermal runaway.

“The engineers who will design the next hybrid vehicle battery packs will need to be cognizant of the growing need for thermal management,” read a recent article on coolingZONE. “The increased need for thermal protection, due to safety considerations; the reduced thermal capacity, due to lesser mass; and the reduced workable volume are among the challenges to be faced. The hybrid vehicle we may soon drive must have reliable and intelligent cooling systems to cool down their high-density battery packs.”

Why is this topic of particular relevance now?

Electric vehicle sales worldwide have jumped 57 percent from 2015 to 2016, according to data reported by Bloomberg New Energy Finance. The article referenced a Bloomberg report stating that electric vehicle sales could be as much as 47 percent of the automotive market by 2040 (dependent on factors such as oil prices). In the U.S., manufacturers have been urged by President Barack Obama’s EV Everywhere challenge to make electric cars as affordable and convenient as gas-powered vehicles by 2022.

Like cell phone technology in the past two decades, electric vehicles have the potential for widespread usage and to wide-ranging effects inside and outside of the automotive industry. The “digitization of the transport system” will effect, among others, oil companies, car dealerships, maintenance services, and utility suppliers.

“If it is hard to predict when phase change in complex systems begins, it is even harder to predict where it ends,” said Michael Leibreich and Angus McCrone, the authors of the Bloomberg article. “No list of potential impacts of the ‘Transformation of Transportation’ can be complete. However, one thing is for sure: if our predictions for the uptake of electric vehicles are anything like correct, there is no part of the global economy which will not, in some way, be affected.”

Currently, electric vehicles cost an average of $30,000 and travel 100 miles or less on a single charge. Tesla (Model 3) and Chevrolet (Bolt EV) have both promised electric vehicles that will travel 200 miles on a charge within the year. Other car makers, such as Volkswagen and BMW, have announced plans to turn a large portion of their production to electric vehicles in the next few years as well.

While the changes in infrastructure and the length of time that most car owners keep a vehicle (11 years on average) have limited electric vehicle sales to this point, according to Christopher Mims of the Wall Street Journal, the next vehicle that most consumers purchase is likely to be electric.

Mims explained, “It is the nature of disruptive technological shifts that it seems like nothing is changing—until it seems as if everything is changing at once. Electric vehicles have been a long time coming, but they now represent such a clear and present threat to the gasoline engine that Mr. Fox, of the service-station association, now recommends that members signing long-term contracts for fuel include an option to renegotiate if more than 10 percent of a state’s fleet goes electric.”

Electric vehicles offer a smooth drive with better acceleration, less moving parts requiring less maintenance, better air quality, and a better platform for autonomous driving, said Bloomberg. Electric vehicles are the future and that means designing better, longer-lasting, higher-performing batteries will be the future as well.

Cooling those batteries will be critical. As Dr. Azar will explain, without proper thermal management the electric vehicle battery will be inefficient and unable to provide the performance that consumers demand.

The Sept. 22 event is free for SAE NE members and $5 for non-members. It runs from 6-9:15 p.m. with tours of the ATS campus from 7-8:00 p.m. and Dr. Azar’s presentation at 8:00. Register online at http://www.sae.org/servlets/sectionEvent?PAGE=getSectionEvents&OBJECT_TYPE=SectionEventAdmin&HEIR_CODE=MS045#249128&saetkn=w1aFMMls8Y or contact SAE member Jeff Mobed at jeffrey.mobed@gmail.com or 508-367-6565.

Integral Modeling Is First Step for ATS Engineers

Integral Modeling

ATS engineers utilize integral or analytical modeling as a first step to solving thermal management issues in a design. [Advanced Thermal Solutions, Inc.]

In July, Future Facilities, a CAD software company, released the results of a survey it conducted of more than 350 electrical engineers (the link to the story is below) on how thermal management relates to reliability in electronics design. The survey coincided with the release of the company’s newest version of its thermal simulation software 6SigmaET.

Forty percent of the surveyed engineers believed thermal simulations for their projects to be too time-consuming or complex. Sixty-two percent of the engineers said that they would rather over-design a project than optimize thermal performance in the design process. In fact, 33 percent of the engineers called thermal issues an “irritation” and would prefer to not deal with them.

Tom Gregory, Product Manager at 6SigmaET, concluded, “It’s clear that a lot of engineers still don’t feel comfortable creating thermal simulations of their designs, a fact which is not being helped by the complex nature of most thermal simulation tools currently on the market.”

The engineers at Advanced Thermal Solutions, Inc. (ATS), a leading-edge engineering and manufacturing company focused on the thermal management of electronics, have long demonstrated that thermal solutions are a critical component to electronics design and that incorporating thermal management early in the design process will lead to a more cost-effective and reliable product.

By incorporating thermal management into the design process engineers optimize time between failure for individual components as well as the overall system. They actually reduce the cost of the system by limiting the need to overdesign it. Well thought out thermal solutions increase the likelihood that the final design will succeed and meet the specifications that were set out at the beginning of the project.

The survey results pointed to CFD analysis as the jumping off point for thermal solutions. But an easier and more efficient way to start the process is with an integral or analytical model, using pencil and paper or a spreadsheet.  In its 3-Core Design Process, ATS has utilized integral modeling as its first step to quickly and easily provide first order solutions and determine whether a design will succeed in meeting its thermal requirements.

Integral modeling, as Dr. Kaveh Azar, founder, President, and CEO of ATS, explained in a webinar (the link is below), utilizes standard equations based on the basic laws that govern thermal engineering: Conservation of Mass, Conservation of Momentum, Conservation of Energy, and Equation of State (i.e. the Ideal Gas Law).

Determining pressure, temperature, and air velocity differentials throughout a system and plugging those numbers into equations that most engineers will remember from undergraduate and graduate training will define the problem that will be faced in designing the system.

Dr. Azar said, “When I focus on integral modeling as I go through the process, you’ll see how easy it is and how broad-spectrumed the applications of these are and this is going to form the first foundation for any kind of analysis that we do in electronics cooling.”

Integral modeling is applicable to any domain and will give a substantiated, independent model to ensure the system is built within the proper parameters. Taking this early step saves time and money that may have been wasted on designing a system that ultimately would not work. Integral modeling also establishes parameters under which the system can be built to save costs after deployment.

Dr. Azar explained, “If we design it for the worst case scenario, we always have the adequate margins and as a result have lesser cost of deployment.”

It is a competitive market. Integral modeling is a quick first step to ensure thermal solutions are part of a design to save on component and system costs. A few quick calculations will have a major impact on the project’s bottom line.

The survey results from Future Facilities can be found at http://www.thermalnews.com/main/news/40-percent-of-electronics-engineers-find-thermal-simulation-too-complex-and-time-consuming.

For more information about the importance of integral modeling and practical applications, watch the webinar with Dr. Kaveh Azar of Advanced Thermal Solutions, Inc. below:

Does the process of thermal design for your next project seem daunting?  Contact us.  ATS offers a  free four-hour consultation in its lab.  Email ATS at ats-hq@qats.com.

ATS maxiGRIP and superGRIP Heat Sink Attachments

Advanced Thermal Solutions John O’Day and Len Alter showcase the patented heat sink attachments maxiGRIP and superGRIP. With its patented and discrete design, these heat sink attachments are well worth it for being your only choice for a cost-effective, high performing thermal solution.

Thermal Resistance and Component Temperature

To maintain operation, the heat must flow out of a semiconductor as such a rate as to ensure acceptable junction temperatures. This heat flow encounters resistance as it moves from the junction throughout the device package, much like electrons face resistance when flowing through a wire. In thermodynamic terms, this resistance is known as conduction resistance and consists of several parts. From the junction, heat can flow toward the case of the component, where a heat sink may be located. This is referred to as ÎJC, or junction to case thermal resistance. Heat can also flow away from the top surface of the component and into the board. This is known as junction to board resistance, or ΘJB.

Source: JESD51-2, Integrated Circuits Thermal Test Method – Natural Convection, JEDEC, March 1999.

ΘJB is defined as the temperature difference between the junction and the board divided by the power when the heat path is from junction to board only. To measure ΘJB, the top of the device is insulated and a cold plate is attached to the board edge (Figure 1). This is the true thermal resistance, which is the characteristic of the device. The only problem is that, in a real application one does not know how much power is being transmitted from different paths.

Due to the multiple heat transfer paths within a component, a single resistance cannot be used to accurately calculate the junction temperature. The thermal resistance from junction to ambient must be broken down further into a network of resistances to improve the accuracy of junction temperature prediction. A simplified resistor network is shown in Figure 2.

As board layouts become denser, there is a need to design optimized thermal solutions that use the least amount of space possible. Simply put, there is no margin to allow for over-designed heat sinks with tight component spacing. Accounting for the effect of board coupling is an important part of this optimization. The possibility for using an oversized heat sink exists only if the junction to case heat transfer path is considered.

To ensure a 105°C junction temperature at 55°C ambient a typical component (see Table 1) needs a heat sink resistance of 2.05°C/W (if we ignore board conduction). When board conduction is taken into account, the actual junction temperature could be as low as 74°C, assuming the board temperature is the same as the air temperature. This indicates a heat sink that is larger than necessary.

From this example, it is clear that all heat transfer paths from the component junction must be considered. Using just the ΘJC and ΘCA values can lead to a larger than optimal heat sink and may not accurately predict operating junction temperatures. Using the proposed correlation can also predict junction temperature when the board temperature is known from experimentation, as shown in Figure 3.

 

 

The Importance of Heat Flux Sensors

Heat flux sensors are practical measurement tools which are useful for determining the amount of thermal energy passed through a specific area per unit of time. Measuring heat flux can be useful, for example, in determining the amount of heat passed through a wall or through a human body, or the amount of transferred solar or laser radiant energy to a given area.

Affixing a thin heat flux sensor to the top of a component will yield two separate values which are useful in determining the convection heat transfer coefficient. If the heat flux can be measured from the top of the component to the ambient airstream and if the temperature at the top of the component and of the ambient airstream is measured, then the convection coefficient can be calculated.

Where:

q = Heat flux, or transferred heat per unit area

h = Convection coefficient

TS = Temperature at the surface of the solid/fluid boundary

TA= Ambient airstream temperature

Using a heat flux sensor can be useful for lower powered systems under natural convection scenarios. Under forced convection, the heat lost to convection off the top of a component can often be significantly higher than the heat lost to the board, particularly if the board is densely populated and the temperature of the board reaches close to the temperature of the device. Under natural convection situations, often the balance of heat lost to convection and heat lost through the board becomes more even and it therefore is of even greater interest to the designer to understand the quantity of heat dispersed through convection.

Experiments done at Bell Labs alluded to the effect of board density on the heat transfer coefficient. In these experiments, thin film heat flux sensors are affixed to DIP devices which populate a board. The total heat generation of the board is kept constant, so the removal of components from a densely populated board only increases the heat generation per component. The results of this particular experiment highlight an increase in the ratio of heat lost through convection from the surface of the component as board density increases and individual device power decreases.

 Surface Heat Flow vs. Board Density

Qs/Qt = the ratio of total heat flow through device surface to total heat generation

σ = the ratio of total device surface area to total board area

If a board was to be sparsely populated, a greater percentage of heat can be transferred to the board due to larger thermal gradients; however since the overall surface area of the sum of devices decreases, to some extent the heat transfer coefficient must increase to reflect a balance. As the number of components decreases, the power generation increases per component, and the larger resulting temperature gradients in the region around the component yield more convective flow and thus an increase in the heat transfer coefficient. On the other hand, if the board becomes more densely populated, the proportion of heat transferred through the surface, as compared to through the board increases, and the overall increase in heat transferred through the surface yields increased flow and heat transfer at an individual component surface.

The use of a heat flux gauge is an important tool for the electronics designer. In particular, by using a heat flux gauge, it is possible to experimentally determine the heat transfer coefficient at a certain location on the electronics board where it would have had to be simply predicted or estimated previously. Due to the complexity of many electrical systems as well as the irregular nature of many boards, often analytical or CFD methods are not accurate and the best approach is empirical techniques. The use of the heat flux sensor can give results which would be difficult to calculate using analytical or numerical simulations. However, like most other instruments, it is important to use the sensor correctly and carefully to decrease the errors within a system and increase the reliability.