[LINK] Harvesting heat

[stephen at melbpc.org.au]

Nanocrystal-coated fibers show promise for harvesting waste heat

By Dario Borghino, April 25, 2012 http://www.gizmag.com/thermoelectric-
energy-harvesting-nanocrystal-coated-fibers/22239/


Researchers at Purdue University in the U.S. have developed a new method 
of harvesting energy from waste heat. 

Using glass fibers dipped in a solution containing nanocrystals of lead 
telluride, the team led by Dr. Yue Wu is engineering a highly flexible 
thermoelectric system that generates electricity by gathering heat from 
water pipes and engine components.

The new Purdue system seeks to avoid many of the limitations that has 
hindered similar attempts in the past: it is less bulky, uses less toxic 
material, and could potentially be applied on a mass scale.

Using glass fibers coated by nanocrystals, Wu's team has engineered a 
promising new thermoelectric system that is best suited for use with 
power plants and car exhausts. It is scalable, eco-friendly, has 
comparatively low production costs and lends itself well to being mass-
produced. When heated on one side, the electrons in a thermoelectric 
material migrate to the opposite, cooler side. This generates an 
electrical current which can then be channeled into the main grid.

Although researchers have already created high-performance materials of 
this kind, these tend to be quite brittle and unable to conform to the 
bends along the pipes with which they are intended to operate. They are 
also manufactured using large quantities of materials which are either 
toxic or hard to come by.

Wu's solution was to dip glass fibers in a solution containing 
nanocrystals of lead telluride and then expose the fibers to heat, fusing 
the crystals together. The flexible fibers can easily follow the bends of 
water pipes and, by minimizing the use of expensive tellurium to a mere 
300-nanometer-thick coating, the production costs fall dramatically.

The concept was demonstrated using a system containing tubes of differing 
diameters nested inside a larger tube. Hot water flows through a central 
tube and cooler water flows through an outer tube, with a layer of 
thermoelectric material between the two.

The researchers believe the system has applications in power plants, car 
engines and residential heating. These are areas where energy waste 
through heat is a big issue - for example research from the Lawrence 
Livermore National Lab shows that between 2008 and 2010, 57.3 to 57.8 
percent of the total energy generated in the United States was rejected 
into the environment, the majority of which was dispersed as heat, with 
the main culprits being power plants and the automotive industry.

"If we could get just 10 percent back that would allow us to reduce 
energy consumption and power plant emissions considerably," Dr. Wu 
commented.

So how much energy can be harvested using the Purdue system?

"For those familiar with the thermodynamics of heat engines, the 
efficiency of a thermoelectric device made from our nanocrystal-coated 
fibers would be ~11 percent of the Carnot efficiency," Dr. Wu told 
Gizmag. "Considering some recent tests on a Chevy Suburban, a vehicle 
equipped with a thermoelectric device that uses our fibers would see a 
~3.6 percent improvement in mpg due to the thermoelectric generator 
supplying 250-440 W of power. We have not looked into precise numbers 
regarding co-generation of electricity during residential heating."

There are still degradation issues that need to be ironed out 
however. "We suspect the cracking of the coated film on the glass fibers 
upon bending is the cause of the degradation, and we are working on the 
issue," says Dr. Wu. "The future steps will include improving the coating 
quality, exploring better coating 'ink', developing power generation 
modules based on flexible fibers and testing their efficiency."

And the future might even bring some quite unexpected applications. 

Besides generating electricity when exposed to heat, in fact, the 
material can also work in reverse, absorbing heat whenever an electrical 
current is applied. This means that the fibers might one day be woven 
into clothing, or used in solid-state cooling devices.

The findings were detailed in a research paper appearing last month in 
the journal Nano Letters. The work is being funded by the National 
Science Foundation and the U.S. Department of Energy.

--

Cheers,
Stephen