Thermal Management 127 conductivity and phase change, in order to transfer heat. The heat pipe operates by sealing a liquid into a series of metal pipes. As the pipes are heated, the liquid converts into steam and returns to the other end of the pipe where it again returns to a liquid form as it cools. Heat pipes have found significant market penetration in the portable computer market but have yet to find a home in automotive or large energy storage systems. The biggest downfall of the heat pipe for automotive applications is that above a certain temperature (depending on the liquid) all of the liquid is converted to steam and the system thereby stops transferring heat at all.
Another thermal management device that has been evaluated by some designers is the Peltier device. The Peltier device is a nonmechanical device that is made up of two different types of materials. When current is applied to one side of the device, heat is transferred from one side to the other where a heat sync can be used to move the heat out of the pack. This can be used to create what I refer to as a hybrid thermal management system. A hybrid thermal manage- ment system is one that may include various combinations of liquid, air, and other cooling mediums combined into a single battery pack system.
One example may be a system design that seals the pack or module with an internal fan that circulates air within the pack; this would be combined with a heat exchange device such as an aluminum manifold or a Peltier device that the air is passed across. A cooled liquid that is entirely outside of the pack could then be run through the heat exchange device thereby cooling the air that is being circulated within the pack. One more thing that must be considered in your thermal management system design is the pressure drop.
The pressure drop is the difference between incoming and outgoing pressure of the fluid, which can be either liquid or air, and is basically a measure of the amount of pressure that the system must overcome in order to flow through the cooling system. In a liquid-cooled system, the pressure drop will determine the size and flow rate of the pump that is needed to move the fluid through the cooling system. In an air-cooled system, it will help to determine the size and flow rate of the fans that are needed to move the air through the system.
Passive Thermal Management Systems Passive thermal management is the process of managing the temperature of the cells and the pack without forcing air, liquid, or other cooling medium into the pack. This can be accom- plished through several methods. Most frequently, this is achieved through the use of alumi- num or metal housings transferring the heat of the cells to an aluminum or metal pack enclosure. This allows the heat to be dispersed throughout all of the metal of the pack and enables it to radiate into the environment. This is generally referred to as a heat sync and is most effective on packs with low discharge rates as they generate less heat.
Another method of passive cooling is to design the pack enclosure with fins such that as the vehicle is moving, air is forced over these fins thereby cooling the pack.