G Love: Design
The following picture is a design of our gloves using a Ni:Cr 80:20 wire located on the backhand side of the glove. The power source is a flat Li ion polymer battery also located on the back of the glove. A temperature is in the heating circuit at a temperature-sensitive location on the hand, to ensure that the glove does not overheat beyond a comfortable temperature, approximately 32 C. The outer shell is made of an insulating fabric, 100% polyester fleece, with a low thermal conductivity to lessen the heat transfer from the heating element away from the hands. The inner lining of the glove is a thin polyester fabric to easily allow heat flow from the heating element to the hand.
Phase change materials (PCMs) were also studied for use as a heating element. Two PCMs with melting temperatures near the temperature of the human body (37 C) were incorporated into the gloves using various embedding techniques. The PCM design is described further here.
The Materials Selection page provides detailed information about the materials selected for these glove components. The Background page also provides information about these compenents.
The heat supplied by the glove was determined by calculating the amount of heat supply needed to keep the hands comfortable while walking across the Harvard Bridge in 15mi/hr winds on a day when the temperature is -10°C. Click to see this heat loss calculation.
It was then possible to calculate both the required battery capacity and resistance of the heating element:
Battery Capacity:
The battery should ideally last at least 6 hours before it needs to be recharged in order to allow the user to participate in an all day outdoor activity such as skiing without running out of power. This means that the battery would need 1.7 Watt*6 hours = 10.2 Watt*hours of power. The voltage of a lithium ion polymer battery is 3.74V. Since current=power/voltage,
Battery Capacity = 10.2/3.74 = 2.7 Amp*hours
Since the battery selected actually has a lower capacity, it will only last about three hours. This should not be a problem because it is highly unlikely that the battery will run continuously over the entire period the gloves are worn (it will turn off periodically when the temperature inside gets too warm).
The wire is 100cm long. The wire is Ni:Cr 80:20 with a diameter of 0.40mm, and a measured resistance of 2.46 ohms/ft. Using the conversion factor 1 inch = 2.54cm, resistance of this 100cm wire = 8.04 ohms. Using R = 8.04 ohms and V = 3.74V in Equation 1 below, the heating element of this design will give off 1.73 Watts.
(1)
Although 2.7 Watts are needed to keep the hand at room temperature (according to the heat transfer model), 1.7 Watts is still enough to provide significant warmth to the hand. From Ohm's Law:
V = IR (2)
for V = 3.74V and R = 8.04 ohms, the current in the wires (I) = 0.47 Amps.
A rough comparison of using octadecane PCM as the heating element in place of this wire:
According to the DSC results on octadecane, it gives off 283.5 J/g of heat when it freezes. 5 grams of octadecane in the glove will give off the same amount of heat as the wire gives off in ~14 minutes. After the 5 grams have completely released all their stored heat, the gloves will rely on the fabric to contain the heat. The PCM may then be re-melted because of heat from the hand, leading to a repetitive cycle of melting and recrystallization that will continue to keep the hand warm.
The glove consists of two layers of fabric with the polymer battery and heating element sandwiched in the middle. The layer closest to the skin is a thin, soft 100% polyester fabric that allows heat from the heating element to come through. The outermost layer is a more insulating 100% polyester fleece that minimizes heat loss, but still allows the hand to breathe.
PCMs that best fit the desired thermal properties of the gloves are octadecane and polyethylene glycol (PEG) (see materials page). The challenge with using these materials was how to incorporate them into the glove, which was the determining factor in revising the design to include the PCMs as the heating element.
There were several design attempts for incorporating octadecane into the gloves. One was to fabricate the PCM into small particles or microspheres and embed it into a flexible base material. Another, which was used in the final product, was to crush the octadecane into larger particles (similar to sand) and mix it into polydimethyl siloxane (PDMS) resin.
Thin pieces of this material were sewn into pockets of the glove, similar to the method for attaching the lithium polymer battery. Pieces were placed along the fingers and on top of the hand as shown in the diagram below:
The second method, which was attempted with PEG, was to directly incorporate the PCM into a material that could potentially be processed into a very thin sheet or fabric and then sew that into a layer of the glove. This has been accomplished using polethylene glycol (PEG) in polypropylene (PP) in order to create dessicants (Mathiowitz et al, 2001), however a similar procedure could work in the gloves. The hydrophilic PEG and the hydrophobic polypropylene phase separate, creating either a network of channels as shown below, or encapsulated particles of PEG in polypropylene. The final structure of the material will depend on the ratio of PEG to polypropylene initially put into the mixture.
This can then be hot pressed to make a thin film. The film could then be sewn into the glove. This method would allow PCM to be evenly distributed all over the hand as shown below:
Due to the lack of proper facilities, the PEG/PP could not be processed. Instead, low density polyethylene (LDPE) and PEG were melted together but gave relatively poor results. This was due to the very different thermal properties of the two polymers. It was finally decided to heat seal the PEG in LDPE bags to create pockets of PCM that could be sewn into the glove.
References:
http://hyperphysics.phy-astr.gsu.edu/hbase/heacon.html
D.R. Poirier, G.H.Geiger, Transport Phenomena in Materials Engineering. The Minerals, Metals & Materials Society: Warrendale, Pennsylvania, 1994.
Mathiowitz, Edith et al. "Novel Dessicants Based on Designed Polymeric Blends." Journal of Applied Polymer Science, Vol. 80, pp. 317-327, 2001.