Tuesday, January 13, 2015

A Wearable Furnace: Keeping Toasty Warm With Nanowire Fabric

Runners using aluminum blankets to keep warm. Credit: Adapted from Ian Hunter | flickr
If you've ever stood exposed and shivering at the end of a run, you'll know just how quickly the body loses heat without protective clothing. To prevent this, aluminized plastic blankets are a common sight at the end of races; they insulate from cold air and reflect back the body's heat. But these blankets are impractical and uncomfortable for daily use, trapping in moisture as well as heat. 
Using some basic principles of radiation reflection and a coating of silver nanowires, physicists have now developed a new type fabric which can keep you toasty warm and comfortable. So warm in fact that the authors think their insulating fabric could be a solution to the large amounts greenhouse gases created by wintertime indoor heating.

Losing heat by convection.
Credit: Gary Settles
Humans lose heat by four methods, listed in order of decreasing importance:
  • infrared radiation: the body radiates infrared light, causing it to glow in infrared or night vision cameras. 
  • convection: conduction (see below) combined with moving air whisks heat away from the skin. This is why wind chill is a very real effect.
  • conduction: the transfer of heat from warm skin to cooler air (or water or that cold metal bench). The denser the surrounding medium, the faster the heat loss.
  • evaporation: sweat or water on the surface of the skin takes heat away from the body while transitioning from a liquid to a gas. Evaporation also occurs during breathing.
Normal clothing keeps us warm by trapping air against the skin, which heats up and helps reduce conduction and convection. But clothes do little to prevent infrared radiation, which is the main method of human body heat loss (see chapter 6 of The Physics of the Human Body by Irving Herman).

Silver-Encrusted Cloth

Now a Stanford University research team led by Yi Cui have reported in this month's Nano Letters that their new nanowire fabric can do it all: insulate against conduction and convection, prevent loss of infrared radiation, allow moisture to escape, and even actively heat up with the help of a battery.
This works because of the size of their fabric mesh. The researchers wanted a metallic layer in order to reflect infrared radiation back towards the body. But unlike the shiny blankets handed out at races, they also wanted to allow moisture to escape so that the fabric would be comfortable to wear all day. 
Reprinted with permission from Hsu et al. 2014. DOI: 10.1021/nl5036572. Copyright 2014 American Chemical Society.
The simple solution was to create a mesh with holes smaller than the typical wavelength of human body radiation (about 9 micrometers or 9000 nanometers) and yet still porous enough to allow sub-nanometer-sized water molecules to pass through.

By dipping a cloth in a solution of silver nanowires, the team created a metallic mesh layer with a spacing of about 300 nanometers and a thickness small enough to keep base fabric flexible. Experimentally this fabric provides 21 percent more thermal insulation than normal cloth.

A metallic fabric has the added bonus of being able to heat up like an electric blanket when a voltage is applied. Cui and his colleagues found that less than a volt was required in order to heat the nanowire fabric up to 100 degrees Fahrenheit (skin temperature is typically about 91 degrees). 

Energy Savings

With the combination of passive insulation and active heating, the researchers estimate that about 350 W per person could be saved by wearing clothes made out of their nanowire fabric, instead of heating an entire room on a cold winter day. 
Assuming winter lasts for four months, this is an energy savings of 1000 kWh per year per person! According to the World Energy Council, the average residential energy usage per person in the United States is about 4500 kWh, so about a quarter of the usage could be cut by wearing nanowire clothes.
"If one wanted to offset this power consumption via solar cells, then a 2-m2-size solar panel would be required per person," said Cui and his colleagues in their paper.
Recent studies have shown that 47 percent of energy worldwide is used on indoor heating and that this heating contributes up to 33.5 percent on greenhouse gas emission (see references within Cui's paper).
Cui and his colleagues believe that their nanowire cloth can change the way in which we keep warm, making the switch from heating an entire room to targeted heating and insulation via nanotechnology clothes. In doing so, they hope to "lessen our dependence on fossil fuel" and "provide an important part of the solution for global energy and climate issue".

By Tamela Maciel, also known as "pendulum"

1 comment:

  1. A wearable furnace sounds like something I need to wear during the winter. I have poor circulation in my legs, so I am usually a little colder than everyone else. It is so cool to see technology change and become better and better.