In many science fiction series we often have waste heat seemingly ignored by the author, but it may not be as ignored as many think.
Think about how well lit a Galaxy Class is, and the glowy bits on the outside of the hull.http://physicsworld.com/cws/article/news/2012/mar/08/led-converts-heat-into-light wrote:LED converts heat into light
Mar 8, 2012 8 comments
A light-emitting diode (LED) that emits more light energy than it consumes in electrical energy has been unveiled by researchers in the US. The device – which has a conventional efficiency of greater than 200% – behaves as a kind of optical heat pump that converts lattice vibrations into infrared photons, cooling its surroundings in the process. The possibility of such a device was first predicted in 1957, but a practical version had proved impossible to create until now. Potential applications of the phenomenon include energy-efficient lighting and cryogenic refrigeration.
The energy of photons emitted by an LED is dictated by the band gap of the semiconductor used – the energy required to make an electron–hole pair. When an electron and hole recombine in a radiative process, a photon carries away the extra energy. The voltage across the LED creates the electron–hole pairs but its value does not affect the photon energy, since the semiconductor's band gap is a permanent feature of the material.
However, it is possible for the individual emitted photons to have energies that are different to the band gap. The vast majority of electron–hole recombinations actually result in the production of heat, which is absorbed by the semiconductor in the form of quantized lattice vibrations called phonons. These vibrations create a heat reservoir that can then boost the energy of photons produced by radiative recombination. In 1957 Jan Tauc at the Institute of Technical Physics in Prague pointed out that, since this provided a mechanism for radiation to remove heat from a semiconductor lattice, there was no barrier in principle to an LED being more than 100% efficient, in which case it would actually cool its surroundings.
Obeys the second law
At first glance this conversion of waste heat to useful photons could appear to violate fundamental laws of thermodynamics, but lead researcher Parthiban Santhanam of the Massachusetts Institute of Technology explains that the process is perfectly consistent with the second law of thermodynamics. "The most counterintuitive aspect of this result is that we don't typically think of light as being a form of heat. Usually we ignore the entropy and think of light as work," he explains. "If the photons didn't have entropy (i.e. if they were a form of work, rather than heat), this would break the second law. Instead, the entropy shows up in the outgoing photons, so the second law is satisfied."
Despite the soundness of the physics, over the past five decades nobody had managed to demonstrate an LED actually cooling its surroundings. One way researchers tried to maximize the number of photons produced was to increase the bias voltage across the LED, but this also increases the heat produced through non-radiative recombinations.
So, Santhanam and colleagues did the exact opposite and reduced the bias voltage to just 70 µV. They also heated the LED to 135 °C to provide more lattice heat. In this regime, less than 0.1% of the electrons passing through the LED produced a photon. However, when the researchers measured the minute power of the infrared radiation produced by the LED, they measured 70 pW of power being emitted by the LED while only 30 pW was being consumed, an efficiency of more than 200%. This happens because as the voltage approaches zero, both light output and power dissipation also vanish. However, the power dissipated is proportional to the square of current, whereas light output is proportional to the current – halving the bias voltage therefore doubles the efficiency.
One possible application of the effect is a refrigeration device that removes heat in the form of light. As an expert in this field, Jukka Tulkki of Aalto University in Finland, told physicsworld.com, "I think this is a historically important breakthrough…that could eventually lead to more useful and technologically relevant applications." However, he cautions that the cooling power of this particular device is extremely low and not great enough for any practical applications.
Santhanam, meanwhile, believes the principle may find applications in fields other than refrigeration. "My personal opinion is that it's more likely to be useful as a light source," he says. "Refrigerators are mostly useful when they are high power. Light sources, however, are used in all kinds of ways. In particular, light sources used for spectroscopy and communication don't necessarily need to be very bright. They just need to be bright enough to be clearly distinguishable from some background noise."
The research is published in Physical Review Letters.
About the author
Tim Wogan is a science writer based in the UK
http://phys.org/news/2014-04-tin-selenide-efficiently-electrical-energy.html wrote: Researchers find tin selenide shows promise for efficiently converting waste heat into electrical energy
Apr 17, 2014 by Bob Yirka
(Phys.org) —A team of researchers working at Northwestern University has found that tin selenide (SnSe) has the highest Carnot efficiency for a thermoelectric cycle ever found, making it potentially a possible material for use in generating electricity from waste heat. In their paper published in the journal Nature, the team describes work they've conducted on SnSe and how their discovery might lead to even more efficient materials. Joseph Heremans gives a short history of thermoelectric research in a News & Views companion piece and offers some insights into why SnSe might be so efficient and how it might lead the way to the discovery of even better materials.
As the planet continues to experience the impact of global warming, scientists around the world frantically pursue alternate ways to produce electricity—one such possibility is to convert waste heat from industrial process into electricity. To make that happen, a thermoelectric generator must be constructed and used. Such generators operate by taking advantage of differences in temperature experienced by a single material. Two thermoelectric semiconductors are exposed to a temperature gradient and are connected together by conducting plates. Thus far, however, the process has not proved to be efficient enough to warrant the expense of building and using such generators, despite doubling in efficiency over just the past fifteen years—from zT 1 to 2.
The increase in efficiency has been due mostly to research work involving nanotechnology, and the materials used have generally been based on lead telluride. The difficulty in finding better materials has been stymied by the dual properties required: low thermal conductivity and high electrical conduction. SnSe has been used by scientists for a variety of purposes, but due to its stiff bonds and distorted lattice was not really considered as a possibility. But that was because others had not taken into account the compound's low anharmonicity. When the team at Northwestern tested it as a possible material for use in a thermoelectric generator they found it had the highest zT ever found, 2.6.
http://physicsworld.com/cws/article/news/2012/mar/08/graphene-in-new-battery-breakthrough wrote: Graphene in new ‘battery’ breakthrough?
Mar 8, 2012 58 comments
Researchers at Hong Kong Polytechnic University claim to have invented a new kind of graphene-based "battery" that runs solely on ambient heat. The device is said to capture the thermal energy of ions in a solution and convert it into electricity. The results are in the process of being peer reviewed, but if confirmed, such a device might find use in a range of applications, including powering artificial organs from body heat, generating renewable energy and powering electronics.
Ions in aqueous solution move at speeds of hundreds of metres per second at room temperature and pressure. The thermal energy of these ions can thus reach several kilojoules per kilogram per degree. However, until now, little work had been done on finding out how to tap into this energy and produce power from it.
Zihan Xu and colleagues made their battery by attaching silver and gold electrodes to a strip of graphene – which is a film of carbon just one atom thick. In their experiments, the researchers showed that six of these devices in series placed in a solution of copper-chloride ions could produce a voltage of more than 2 V. This is enough to drive a commercial red light-emitting diode.
The technology is quite different to conventional lithium-ion batteries, for example, which convert chemical energy into electricity. "The output of our device is also continuous and it works solely by harvesting the thermal energy of the surrounding copper-chloride ions, which, in theory, is limitless," says Xu.
According to the researchers, the battery works rather like a solar cell. The copper ions (Cu2+) continually collide with the graphene strip in the battery. This collision is energetic enough to displace an electron from the graphene. This electron can then either combine with the copper ion or travel through the graphene strip and into the circuit.
Since electrons move through graphene at extremely high speeds (thanks to the fact that they behave like relativistic particles with no rest mass), they travel much faster in the carbon-based material than in the ionic solution. The released electron therefore naturally prefers to travel through the graphene circuit rather than through the solution. This is how the voltage is produced by the device, explains Xu.
Boosting voltage output
The researchers also found that the voltage produced by the device could be increased by heating the ionic solution and accelerating the Cu2+ ions with ultrasound. Both of these methods work because they increase the kinetic energy of the ions. The voltage also increases if the copper-chloride solution is more concentrated with Cu2+ ions, because the density of Cu2+ on the graphene is then greater. Other cationic solutions can be employed too, such as Na+, K+, Co2+ and Ni2+, although these produce lower voltage outputs.
The unique atomic-layer nature of graphene is crucial for this battery, say the researchers, who also experimented with graphite and carbon-nanotube thin films. They discovered that these materials only produced low voltages of around microvolts, which could be regarded as noise.
Bor Jang of Nanotek Instruments in Dayton, Ohio, who has worked on making supercapacitors from graphene, says that the concept described looks "very interesting" but that "more work will be needed to assess whether the approach could provide sufficient energy or power density for practical uses".
For its part, the Hong Kong team now plans to improve the power output of its graphene-based device and further investigate how it works.
The work is described in a preprint on arXiv.
About the author
Belle Dumé is a contributing editor to nanotechweb.org