Week 10 Thermal Management

AIM:- To prepare a report summarizing thermal management techniques for an electric car battery pack.

INTRODUCTION:-

All EVs contain large, complex, rechargeable batteries, sometimes called traction batteries, to provide all or a portion of the vehicle’s propelling power.

Battery technology is developing rapidly. However, lithium-ion (Li-ion) batteries are by far the most commonly used in electric cars and other alternative mobility options today. They may seem safe but the potential is there for catastrophe if they aren’t kept cool.

While advancements have been made in electric vehicle batteries that allow them to deliver more power and require less frequent charges, one of the biggest challenges that remain for battery safety is the ability to design an effective cooling system.

In EV batteries, current flow, both charge, and discharge generate heat inside the cells and in their interconnection systems. This heat is proportional to the square of the flowing current multiplied by the internal resistance of the cells (`I^2R` losses) and the interconnect systems. The more rapidly you charge or discharge a battery, the higher the current flow, and the more heat is generated.

Because batteries are only manufactured to work between certain temperature extremes, they will stop working if there is no cooling system to keep them in a working range. Cooling systems need to be able to keep the battery pack in the temperature range of about 20-40 degrees Celsius, as well as keep the temperature difference within the battery pack to a minimum. Also if there is a large internal temperature difference, it can lead to different charge and discharge rates for each cell and deteriorates the battery pack performance.

Why cooling systems in battery-powered vehicles are necessary:-

Thermal runaway can happen in EV batteries which are much, much larger. Typically, it’s due to a collision that pierces the Li-ion Battery pack or while it’s charging. Many automobiles are seen catching up fires. The energy contained in an EVs battery pack is immense. A cooling system prevents the battery from overheating, keeping the user safe while the user drives around in it.

Also, potential thermal stability issues, such as capacity degradation, and fire explosion, could occur if the battery overheats or if there is non-uniform temperature distribution in the battery pack. In the face of life-threatening safety issues, innovation is continually happening in the electric vehicle industry to improve the battery cooling system. 

A battery thermal management system (BTMS) is necessary to prevent temperature extremes, ensure proper battery performance, and achieve the expected life cycle. An effective BTMS keeps cell temperatures within their allowed operating range.

As defined by engineers at the U.S. Department of Energy’s NREL (National Renewable Energy Laboratory), EV battery pack thermal management is needed for three basic reasons:

1. To ensure the pack operates in the desired temperature range for optimum performance and working life. A typical temperature range is 15-35°C.
2. To reduce uneven temperature distribution in the cells. Temperature differences should be less than 3-4C°.
3. To eliminate potential hazards related to uncontrolled temperature, e.g. thermal runaway.

                                                                                   Figure:- EV's Battery Pack Arrangement

                                                                       Figure:- Colling system inside Battery Pack

THERMAL MANAGEMENT TECHNIQNIQUES in EVs:- 

Thermal management techniques in EVs for Battery Pack and their system applications are described below. Some may seem very similar to cooling systems in traditional petrol-engine vehicles while others are unique. All have the same purpose: to prevent battery failure.

1) AIR COOLING:-

The lowest cost method for EV battery cooling is with air. A passive air-cooling system uses outside air and the movement of the vehicle to cool the battery. Active air-cooling systems enhance this natural air with fans and blowers. The premise is all about passing as much air over the battery pack as possible. Heat transfers from the battery into the less-dense air and then carried off into the atmosphere.

Air cooling eliminates the need for cooling loops and any concerns about liquids leaking into the electronics. The added weight from using liquids, pumps, and tubing is also avoided.

                                          Figure:- The Nissan Leaf’s battery pack is cooled by air.

The trade-off is that air cooling, even with high-powered blowers, does not transport the same level of heat as a liquid system can. This has led to problems for EVs in hot climates where the ambient temperature is higher than the desired operating range, including more temperature variation in battery pack cells i.e safety concerns like thermal runaway become much more real. Blower noise can also be an issue.

                                                 Figure:- Air cooler battery thermal management system used in Toyota’s Prius.

 

2) LIQUID COOLING:-

A) Indirect Cooling:-

Piped liquid cooling systems provide better battery thermal management because they are better at conducting heat away from batteries than air-cooling systems. 

The most common way to cool EVs currently is with indirect liquid cooling systems. In this design, a series of pipes are routed through and/or around the battery pack much like a cooling system on an ICE vehicle. The fluid, typically glycol, is excellent at storing heat that is transferred from the warmer battery pack and circulating it to a heat exchanger like a radiator.

Liquid coolants have higher heat conductivity and heat capacity (ability to store heat in the form of energy in its bonds) than air, and therefore performs very effectively and own advantages like compact structure and ease of arrangement. Out of these options, liquid coolants will deliver the best performance for maintaining a battery pack in the correct temperature range and uniformity. 

This is the style used in EVs manufactured by Tesla, BMW, Jaguar, and Chevrolet, plus others.

Although the indirect cooling system is most common, it isn’t without its own issues. Fluid leaks inside the battery pack, for example, could be dangerous and there are environmental concerns regarding glycol disposal. For now, it remains the most desirable solution. Also, one downside is the limited supply of liquid in the system compared with the essentially limitless amount of air that can flow through a battery.

 

Tesla’s thermal management system (as well as GM’s) uses liquid glycol as a coolant. Both the GM and Tesla systems transfer heat via a refrigeration cycle. Glycol coolant is distributed throughout the battery pack to cool the cells. Considering that Tesla has 7,000 cells to cool.

                          

                                          Figure:- Tesla uses a metallic cooling tube that snakes through the EV battery pack.

The Tesla Model S battery cooling system consists of a patented serpentine cooling pipe that winds through the battery pack and carries a flow of water-glycol coolant; thermal contact with the cells is through their sides by thermal transfer material.

                                     Figure:- GM’s Chevrolet Volt uses cold plates interwoven with battery cells as the liquid cooling system.

General Motor’s Chevrolet Volt features a liquid cooling system to manage battery heat. Each rectangular battery cell is about the size of a children’s book. Sandwiched between the cells is an aluminum cooling plate. There are five individual coolant paths passing thru the plate in parallel, not in series as the Tesla system does. Each battery pouch (cell) is housed in a plastic frame. The frames with coolant plates are then stacked longitudinally to make the entire pack.

Thermodynamic engineers at Porsche develop and optimize each vehicle’s entire cooling system. This includes the battery, of course, and one example is the liquid-filled cooling plate from the traction battery in the Boxster E.

                                                         Figure:- Thermal model of a battery cooling plate in the Porsche Boxster.

Based on the results of the analysis in the thermal model described above, the cooling plate was designed geometrically and optimized using computational fluid dynamics (CFD). The result is a highly efficient and lightweight heat exchanger, optimally tailored and adapted to the battery pack, with low pressure losses, high cooling performance and a very even distribution of temperature.

 

B) Direct Cooling:-

Also known as Liquid Immersion.

The optimum cooling performance occurs when the coolant is in direct contact with the battery’s cells. A direct liquid cooling system would be able to absorb heat most efficiently, regulating the battery’s temperature precisely. However, the coolant would be required to be non-conducting so there would not be an electrical hazard.

Currently, no EVs use direct cooling systems but that could change soon. Developers such as XING Mobility and M&I Materials are leading the charge to get these non-conducting coolants into use in the auto industry.

Instead of snaking coolant through lines and chambers within a battery pack’s case, XING Mobility takes a different approach by immersing its cells in a non-conductive fluid with a high boiling point. The coolant is 3M Novec 7200 Engineered Fluid, a non-conductive fluid designed for heat transfer applications, fire suppression, and supercomputer cooling.

                                            Figure:- The XING Battery has 4,200 individual lithium-ion cells encased in liquid-cooled module packs.

XING’s batteries take the form of 42 lithium-ion-cell modules that can be put together to build larger battery solutions. The complete XING battery houses 4,200 individual 18,650 lithium-ion cells encased in liquid-cooled module packs.

 

Comparison of cooling Techniques:- 

 

CONCLUSION:-

                To prevent thermal runaway in EV batteries, cooling systems are needed. Many EV manufacturing companies such as Tesla, BMW, and LG Chem are using their own indirect cooling systems rather than other cooling systems