Yi CUI, et al.
Silver Nanowire Insulation
http://phys.org/news/2015-01-super-insulated-indoor.html
Jan 08, 2015
Super-insulated clothing could eliminate need for indoor heating
by Lisa Zyga
(a) Illustration showing that body heat passes through normal cloth but is reflected by AgNW cloth. (b, c) Photos of AgNW cloth and CNT cloth showing their flexibility. (d, e) SEM images of AgNW cloth and CNT cloth. Credit: Hsu, et al. ©2014 American Chemical Society
By wearing clothes that have been dip-coated in a silver nanowire (AgNW) solution that is highly radiation-insulating, a person may stay so warm in the winter that they can greatly reduce or even eliminate their need for heating their home. Considering that 47% of global energy is spent on indoor heating, and 42% of that specifically for residential heating, such highly insulating clothing could potentially have huge cost savings.
A team of researchers led by Professor Yi Cui, along with PhD student Po-Chun Hsu and others at Stanford University, have published a paper on the AgNW-coated textiles in a recent issue of Nano Letters.
As the researchers explain, most strategies to reduce indoor heating focus on improving the insulation of the buildings, such as by using high R-value insulation and low-emissivity windows. However, a large portion of the energy is still wasted on heating empty space and inanimate objects
To avoid this waste, the researchers have used a new strategy called "personal thermal management," which focuses on heating people. They've demonstrated that clothing dipped in a solution of metallic nanowires, such as AgNWs, achieves this goal by both providing passive insulation and allowing for active heating when connected to an external power source.
The main advantage of the AgNW-coated clothing is that it reflects over 90% of an individual's body heat (i.e., infrared radiation) back to the individual. This reflectance is much higher than even the warmest wool sweater, as the average clothing material reflects back only about 20% of body heat.
This increase in reflectance is due to differences in the materials' emissivity, which is a measure of heat radiation. Low-emissivity materials like silver, which has an emissivity of 0.02, emit less radiation and so provide much better insulation than high-emissivity materials like common textiles, which have an emissivity of about 0.8.
Of course, wearing clothing made completely of silver would be impractical and uncomfortable, not to mention expensive. A main reason for this discomfort is that silver, like all metals, is not breathable. For example, Mylar blankets, which are made of aluminum and plastic, are extremely warm but are not vapor-permeable, causing moisture to accumulate on a person's skin.
The new AgNW-coated clothing, on the other hand, is breathable due to the nanowires' porous structure. The large spacing between nanowires of about 300 nm offers plenty of room for water vapor molecules, which are about 0.2 nm, to pass through. The 300-nm spacing is still much too small to allow body heat to pass through, since human body radiation has a wavelength of about 9 µm and so interacts with the nanowire cloth as if it were a continuous metal film, and is reflected.
Clothing coated in AgNWs would feel virtually identical to normal clothing because such a small amount of AgNW solution is required to achieve high reflectivity. Dip-coating cotton cloth into the AgNW solution adds a mass of just 0.1 g/m2, which would be less than 1 gram for an entire outfit. Only a small fraction of this mass is silver, so the cost would be relatively inexpensive. Using other metals such as copper, nickel, or aluminum, which have similar properties as silver, could further reduce costs.
Besides providing high levels of passive insulation, AgNW-coated clothing can also provide Joule heating if connected to an electricity source, such as a battery. The researchers demonstrated that as little as 0.9 V can safely raise clothing temperature to 38 °C, which is 1 °C higher than the human body temperature of 37 °C.
Variables such as outdoor temperature, length of the winter season, and home size make it difficult to calculate exactly how much energy a person would save by wearing AgNW-coated clothing. However, the researchers have calculated a rough savings estimate of 8.5 kWh of heating energy per person per day, or 1,000 kWh per year assuming that the heating system operates for four months per year. This estimate is based on the average person requiring 367 W of heating power, compared with 12 W required by the AgNW-coating clothing when actively operating.
The researchers note that a 1,000 kWh savings in power consumption is equivalent to the power generated by a 2-square-meter solar panel. Plus, fabrication, installation, and maintenance of the solar panel would likely cost much more than the AgNW-coated clothing.
The researchers note that a 1,000 kWh savings in power consumption is equivalent to the power generated by a 2-square-meter solar panel. Plus, fabrication, installation, and maintenance of the solar panel would likely cost much more than the AgNW-coated clothing.
The researchers also fabricated and tested clothing coated in a carbon nanotube solution. However, although carbon nanotubes are conductive and therefore suitable for Joule heating, their high emissivity of 0.98 does not enable them to reflect body heat nearly as well as the AgNW coating.
http://pubs.acs.org/doi/abs/10.1021/nl5036572
Nano Letters.
DOI: 10.1021/nl5036572
November 30, 2014
Personal Thermal Management by Metallic Nanowire-Coated Textile
Po-Chun Hsu, et al.
Heating consumes large amount of energy and is a primary source of greenhouse gas emission. Although energy-efficient buildings are developing quickly based on improving insulation and design, a large portion of energy continues to be wasted on heating empty space and nonhuman objects. Here, we demonstrate a system of personal thermal management using metallic nanowire-embedded cloth that can reduce this waste. The metallic nanowires form a conductive network that not only is highly thermal insulating because it reflects human body infrared radiation but also allows Joule heating to complement the passive insulation. The breathability and durability of the original cloth is not sacrificed because of the nanowires’ porous structure. This nanowire cloth can efficiently warm human bodies and save hundreds of watts per person as compared to traditional indoor heaters.
http://phys.org/news/2015-01-nanowire-people-warmwithout.html#inlRlv
Jan 07, 2015
Nanowire clothing could keep people warm — without heating everything else
To stay warm when temperatures drop outside, we heat our indoor spaces—even when no one is in them. But scientists have now developed a novel nanowire coating for clothes that can both generate heat and trap the heat from our bodies better than regular clothes. They report on their technology, which could help us reduce our reliance on conventional energy sources, in the ACS journal Nano Letters.
Yi Cui and colleagues note that nearly half of global energy consumption goes toward heating buildings and homes. But this comfort comes with a considerable environmental cost - it's responsible for up to a third of the world's total greenhouse gas emissions. Scientists and policymakers have tried to reduce the impact of indoor heating by improving insulation and construction materials to keep fuel-generated warmth inside. Cui's team wanted to take a different approach and focus on people rather than spaces.
The researchers developed lightweight, breathable mesh materials that are flexible enough to coat normal clothes. When compared to regular clothing material, the special nanowire cloth trapped body heat far more effectively. Because the coatings are made out of conductive materials, they can also be actively warmed with an electricity source to further crank up the heat. The researchers calculated that their thermal textiles could save about 1,000 kilowatt hours per person every year — that's about how much electricity an average U.S. home consumes in one month.
http://espacenet.com ( European Patent Office )
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