Most cars are equipped with air-conditioning systems to stabilize inside temperatures for comfort, particularly on a hot summer's day. However, providing sufficient cooling requires a lot of energy. Use of new phase change material (PCM) with unique thermo-regulating properties can lead to substantially enhanced thermal comfort inside the car as well as energy savings.
A phase change material ( PCM- solid at room temperature ) is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa. Thus PCMs are classified as latent heat storage (LHS) units. Initially, the solid-liquid PCMs behave like sensible heat storage (SHS) materials; their temperature rises as they absorb heat. However, when PCMs reach the temperature at which they change phase (their melting temperature) they absorb large amounts of heat at an almost constant temperature. The PCM continues to absorb heat without a significant raise in temperature until all the material is transformed to the liquid phase. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its stored latent heat. The temperature of the PCM and its surroundings remains nearly constant throughout the entire process. Phase change material (PCM) is a highly productive thermal storage mean. Phase change material (PCM) possesses the ability to change its physical state within a certain temperature range. In the reverse cooling process, the latent heat stored in the PCM is released into the environment in a certain temperature range and a reverse phase change from the liquid state to the solid state takes place. During this crystallization process, the temperature of the PCM and its surroundings remain nearly constant. When the phase change is complete, a continued heating/cooling process leads to a further temperature increase/decrease. The absorption or release of high amounts of latent heat without a temperature change is responsible for the appeal of the PCM as a suitable heat storage mean.

In addition to ice (water), over 500 natural and synthetic PCMs are known (paraffin, salt hydrates). These materials differ from one another in their phase change temperature ranges and their latent heat storage capacities. A study of the temperature development in different locations of the passenger compartment resulted in selection of the headliner, instrument panel and seats for PCM application:
In a closed car, hot air that builds up mainly by exposure to sunlight through the windows, moves to the top, heating up the headliner. A non-combustible salt hydrate-PCM is used via embedding in a one millimeter thick polymeric film. A textile composite in which the PCM-film is arranged between the fabric at the bottom of the headliner and its intermediate foam layer has been created. The PCM starts to absorb latent heat when its temperature rises above 30°C. The PCM integrated in the headliner possesses a latent heat storage capacity of about 240 kJ. This amount of latent heat absorption of the new PCM treated headliner is equivalent to the heat absorption of a common headliner whose temperature rises by about 100°C. If PCM is applied to the headliner, it will absorb the heat without a further rise in its temperature until the PCM's melting point is reached. Based on the latent heat absorption by the PCM, the normal rise in the headliner's temperature is delayed significantly. The heat stored in the PCM might be released through the roof into the environment during driving periods of the car or as a result of the overnight cooling.

Applied to the instrument panel, the PCM absorbs heat that penetrates mainly through the windshield into the passenger compartment. During the latent heat absorption of the PCM, the temperature of the instrument panel remains nearly constant at a comparatively low level, releasing less heat into the passenger compartment. This leads to improved thermal comfort, reduces the cooling requirements for the passenger compartment, and, therefore, results in energy savings. In its instrument panel application, the PCM is embedded in a polymeric film which is attached to the back side of the PVC cover material. The non-combustible salt hydrate-PCM absorbs latent heat when its temperature rises above 35°C. Due to the high demand for the latent heat absorption, a much larger amount of PCM is used in the instrument panel in comparison to the headliner. The latent heat storage capacity of the PCM applied to the instrument panel totals about 600 kJ.
Energy savings of about 15% are estimated for the PCM application in the instrument panel. By using PCM in the headliner and the instrument panel, overall energy savings of up to 40% can be obtained.

By applying PCM to a car seat, the thermal seating comfort is improved significantly, especially on hot summer days. The PCM absorbs surplus heat stored in the seat cover and heat released from the driver's body as soon as the driver occupies the seat. The heat transfer away from the seat's surface and the heat absorption by the PCM arranged inside the seat leads to an instant drop in the microclimate temperature until a comfortable level is reached and is maintained. In the seat application, the PCM is embedded in polymeric film patches that are arranged on top of the seat's foam cushion. The applied PCM absorbs latent heat when its temperature rises above 30°C. It possesses a latent heat storage capacity of about 100 kJ. The heat transfer to the PCM results in a substantial drop in the microclimate temperature. The microclimate temperature remains then at a comfortable level of about 33°C.