13-03-2012, 04:09 PM
COOLING OF HELMET WITH PHASE CHANGE MATERIAL
Motorcycle helmets greatly reduce injuries and fatalities in motorcycle accidents. Thus, many countries have laws requiring standard helmets to be worn by motorcycle riders. Safety is a primary feature of a motorcycle helmet. Modern helmets are fabricated from plastics, often reinforced with Kevlar and carbon fiber. The helmet has two principal protective components: a thin, outer shell made of acrylonitrile butadiene styrene(ABS) plastic, fiberglass or Kevlar and a soft, thick, inner liner about one-inch thickness usually made of expanded polystyrene foam or expanded polypropylene foam. The foam liner is very similar to that used in refrigerators as thermal insulation. The hard outer shell prevents puncture of the helmet by sharp pointed object and provides the framework to hold the inner liner. The inner foam liner is to absorb the crush during an impact. The one-inch thickness of insulation liner lining the interior of the helmet restricts and virtually eliminates the heat exchange with the outside wall of the most efficient part of the body. This creates an uncomfortable and dangerous hot environment to the head of the wearer. The interior of the helmet can quickly rise to the temperature between 37˚C and 38 ˚C.
2. Phase Change Materials (PCM)
A helmet cooling system eliminating the requirement of the power supply uses phase change material (PCM) to absorb all the heat generated from the head at a relatively constant temperature to provide cooling of the head. Thus, the interior is maintained at a certain cooled temperature which is close to the melting temperature of the PCM and creates a thermally comfort environment to the head without requiring external electrical power supply. The PCM is enclosed in a pouch and placed between the head and the helmet. When the head skin temperature is above the melting temperature of the PCM, the PCM begins to melt as it absorbs the heat from the head. A PCM-cooled system can be implemented in any kind of safety helmet.
4. Conceptual design of PCM-cooled helmet
The basic components of the PCM-cooled helmet are illustrated in Fig. 1. The key component of the helmet cooling system is the PCM pouch. The PCM is enclosed in the pouch. A flexible heat collector made of copper provides a good thermal path for conducting heat transfer from the wearer head to the PCM pouch. The heat collector is attached to a vinyl cushion which is filled with a water based solution (gel).
The PCM-cooled helmet is designed to maintain the head skin temperature at about Tskin = 30 °C. It is designed to be capable of cooling the head continuously up to 2 hours which is referred to as the loading time Δtload = 2h . After the loading time, the stored heat in the PCM pouch needs to be dissipated to the ambient which is referred to as the discharging time, Δtdiscaharge. The helmet is designed to allow quick and easy retrieval of the PCM pouch from the helmet for discharging. The PCM pouch can be left on a table for heat dissipation to the ambient by natural convection, or the pouch can be immersed in water at room temperature to enhance heat dissipation to shorten the discharging time. If the PCM pouch stays inside the helmet, the discharging time will be longer.