Challenge

Week 10 Thermal Management

BBlesswin Alexander9 Aug 2020

Battery Thermal Management System

Aim:

* The Need and Importance of a Thermal management system in in an Electric Vehicle battery pack is discussed and analysed.

* The different Type of Thermal Management techniques for an Electric vehicle's battery pack is Discussed.

Effect of High Temperature in Battery Pack:

* Battery temperature is the primary factor affecting the performance and life of the battery pack. Ideally, Li-ion batteries should operate between 25 Degree C and 40 Degree C for optimum performance and life.

* The energy storage and cycle life of the cell can be reduced significantly when the cell is operated at temperatures above 40 Degree C or below 0 Degree C.

* High temperatures promote growth of the solid electrolyte interface layer and increase the internal resistance, which would cause significant reduction in the power delivery.

* Under extreme conditions, the separator will melt, causing an internal short circuit which can potentially lead to thermal runaway of the cell.

* In the other hand, the capacity will be decreased if the battery is operated at temperatures below 0 Degree C and lithium plating during charging will occur.

* While higher temperatures can be tolerated temporarily, if the temperature is greater than 60 Degree C for a prolonged period with the battery fully charged, there is a risk that the batteries may rupture, explode or catch fire due to thermal runaway.

* Battery temperature and uniformity have a strong influence on the availability of the charging and discharging power, cell balancing and charge acceptance during regenerative braking.

* A large variation of temperature in a battery pack can lead to different cells charging and discharging at different rates and lead to electrically unbalanced cells and reduce the performance of the battery pack.

* Variation of temperature within a battery cell should be kept between 5 Degree C and 10 Degree C while the variation of temperature across the battery pack should be kept within 3 Degree C to 5 Degree C.

* A temperature difference of 5 Degree C can lead to about 10% degradation of power capability, and an increment of 25% of thermal aging kinetics.

* Furthermore, the self-discharge rate will also be affected over a long period of time and can lead to a reduction in the effective operational state of charge.

* Thermal and cyclic aging defines the calendar life of the battery. It reduces the capacity and increases the internal resistance of the cell. The rate is dependent on the cell temperature and SOC.

Need for an Thermal management in a Battery Pack:

]* The above mentioned points further emphasizes the need for a good understanding of the thermal issues of the Li-ion battery and a superior thermal management system for a battery pack.

* Electric Vehicles are projected as the most sustainable solutions for future transportation and they have many advantages over conventional internal combustion engines including energy efficiency, environmental friendliness, noiselessness and less dependence on fossil fuels.

* However, there are also many challenges which are mainly related to the battery pack, such as battery cost, driving range, reliability, safety, battery capacity, cycle life, and recharge time.

* The performance of EVs is greatly dependent on the battery pack.

* Temperatures of the cells in a battery pack need to be maintained within its optimum operating temperature range in order to achieve maximum performance, safety and reliability under various operating conditions.

* Poor thermal management will affect the charging and discharging power, cycle life, cell balancing, capacity and fast charging capability of the battery pack.

* Hence, a thermal management system is needed in order to enhance the performance and to extend the life cycle of the battery pack.

Thermal Management Techniques for Electric vehicle's Battery pack:

* Similar to conventional ICE vehicles, a battery pack in EVs or HEVs also needs a cooling system. An EV battery pack comprises cell modules, module interconnects, power management system and thermal management system.

* Different types of cooling systems will influence the performance and cost of the battery pack thermal management system.

* The heat transfer medium could be,

(i) Air Cooling

(ii) Liquid Cooling

(iii) Phase change material (PCM), heat pipe or a combination of them.

* The selection of the cooling system is dependent on the constraints of the vehicle, installation costs and the external environment.

* In the extreme environment and working under heavy duty cycles, an active cooling system is preferred in order to offer more effective thermal management.

(i) Air Cooling:

* Air systems use air as the thermal medium. The intake air could be direct either from atmosphere or from the cabin and could also be conditioned air after a heater or evaporator of an air conditioner. The former is called passive air system and the latter is active air system.

* Active systems can offer additional cooling or heating power. A passive system can offer some hundreds of watts cooling or heating power and an active system power is limited to 1 kW.

* Because in both cases the air is supplied by a blower, they are also called forced air systems. The following figure shows a schematic description of systems.

* Using air as a heat transfer medium is the simplest and cheapest approach but not as effective as liquid cooling.

* The disadvantages include limiting flow rate of cooling air to 100 m3/h – 250 m3/h, dependence on vehicle cabin air temperature, low cooling performance, noise, inhomogeneous temperature distribution within batteries.

* There is a risk of fouling and potential safety concerns due to emission of toxic gaseous from the battery pack.

(ii) Liquid Cooling:

* Liquid cooling systems are more complex than air cooling systems. In a battery thermal management system utilizing liquid cooling, the heat transfer between battery and liquid coolant is achieved by installing discrete tubing around the battery cells.

* It is done by jacket around the battery cells which places the heated liquid or cooled plate to the battery cell surface as shown above.

* If the liquid cooling system uses a non-direct contact method, water, ethylene glycol or refrigerants can be used as the cooling medium.

* On the other hand, in the direct contact method, we are submerging the cells in a dielectric fluid such as silicone-based or mineral oils to avoid any short circuit.

* The advantages of direct liquid cooling include greater compactness, greater temperature uniformity among the cells and higher cooling rate compared to an air-cooled system.

* The disadvantages of direct liquid cooling are that the cooling system can only operate when the power supply is available, and heating (which is required for ambient temperatures below 0 Degree C) cannot be integrated into the thermal management system.

* The indirect-contact liquid cooling method offers some advantages such as higher average energy efficiency, higher cooling performance compared with air cooling, allowing integration of heating and ease of maintenance.

* The disadvantages include requirements of larger space and increase of vehicle total weight, higher cost due to the

necessity of auxiliary components such as chiller and heat sink, and high inertia due to high thermal mass.

(iii) Phase change material (PCM):

* A battery cell is surrounded by the PCM, and as it heats up, the waxy material softens, absorbing the heat. When the cell cools off, the PCM hardens, releasing the heat into the atmosphere.

* The above figure shows the battery pack which has PCM Layers inbetween each cells.

* The key to using it in packs with a large number of individual cells is the material’s thermal conductivity. If you have an individual cell that has a problem, such as an internal short circuit or nail penetration, that cell will create more heat than others.

* When you have a big thermal runaway, the problem starts with a single cell, and then spreads quickly through the entire pack. This material can quickly move the heat away from the problematic cell and absorb it to prevent a chain reaction.

* During melting, heat is absorbed by PCM and is stored as latent heat until the latent heat is up to the maximum.

* The temperature is kept at melting point for a period and the temperature increase is delayed.

* Therefore, PCM is used as conductor and buffer in battery thermal management systems.

* Above figure shows the working mechanism of PCM on battery cells. Also, a PCM is always combined with air cooling system or liquid cooling system to manage the battery temperature.

* These are some of the cooling methods used to cool the electric vehicle's battery to prevent a Thermal Runaway, from happening inside the battery pack.

* The battery Thermal management system uses Liquid cooling method and Phase change materials methods more as they are very effective in saving the electric vehicle's battery pack from thermal Runaway.

Week 10 Thermal Management

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Ship work like this
in your own course.