Using a process known as a 3D modulation-doping strategy, the researchers are finding the gains by re-designing materials that scientists have been working with for years.

3D Modulation Doping Boosts Thermo Electric Performance. Click image for more info.

Silicon Germanium is valued for its performance in high-temperature thermoelectric applications.  The team found by altering the design of bulk SiGe with 3D modulation doping, a process borrowed from the thin-film semiconductor industry, helped produce a more than 50 percent increase in electrical conductivity.

Boston College Professor of Physics Zhifeng Ren and graduate researcher Bo Yu, and MIT Professors Gang Chen and Mildred S. Dresselhause and post-doctoral researcher Mona Zebarjadi paper has been published in the journal Nano Letters.

The 3D modulation-doping strategy succeeded in creating a solid-state device that achieved a simultaneous reduction in the thermal conductivity, which combined with conductivity gains to provide a high figure of merit value of ~1.3 at 900 °C.

Modulation doping is widely used in producing microelectronics and photonic devices such as computer chips and camera and scanner sensors. The purpose is to reduce impurity scattering and enhanced mobility of the doping material.

The team’s innovation to apply the doping to the SiGe enhanced the power factor significantly by using a thirty percent volume fraction of boron doped silicon nanoparticles in the base silicon germanium.  The improvement to the power factor signifies a new strategy to improve the electron performance in bulk materials.

The team has developed a simple model based on mixture rules to interpret the experimental data. Without any fitment parameters, the team is able to explain the experimental data within a maximum uncertainty of ± 20%.  The team expects that similar modulation-doping strategies can be applied to other thermoelectric materials following the developed general guidelines.

Bo Yu explains the research paradox, “To improve a material’s figure of merit is extremely challenging because all the internal parameters are closely related to each other. Once you change one factor, the others may most likely change, leading to no net improvement. As a result, a more popular trend in this field of study is to look into new opportunities, or new material systems. Our study proved that opportunities are still there for the existing materials, if one could work smartly enough to find some alternative material designs.”

There’s a bonus in the research from a cost standpoint.  Professor Ren pointed out that the performance gains the team reported compete with the state-of-the-art n-type SiGe alloy materials, with a crucial difference – the team’s design requires using 30 percent less Germanium.  “Using 30 percent less Germanium is a significant advantage to cut down the fabrication costs. We want all the materials we are studying in the group to help remove cost barriers. This is one of our goals for everyday research,” said Ren.

The calculation of the lost energy to escaped heat in the U.S. is about 60%.  Or not quite double of what you bought is simply lost.  From huge industrial and power generation stations down to the hot water pipes in homes, a stunning loss takes place every time fuels are used.  For transportation the numbers are even worse.

How the Boston College and MIT team’s work will translate into products is yet to be seen.  But a certainty is the thermoelectric performance is going to improve and costs will start to fall as mass production scales up.

When thermoelectric materials are heated on one side electrons flow to the cooler side, generating an electrical current.  The new Purdue material also could be used to create a solid-state cooling technology that does not require compressors and chemical refrigerants. The fibers might be woven into a fabric to make cooling garments.

Thermoelectric Coated Glass Fibers. Click image for more info.

Yue said, “The ugly truth is that 58 percent of the energy generated in the United States is wasted as heat.  If we could get just 10 percent back that would allow us to reduce energy consumption and power plant emissions considerably.”  That might be a generous number to start with.

The team uses glass fibers that are dipped in a solution containing nanocrystals of lead telluride and then exposed to heat in a process called annealing to fuse the crystals together.  Then the fibers could be wrapped around industrial pipes in factories and power plants, as well as on car engines and automotive exhaust systems, to recapture much of the wasted energy. The “energy harvesting” technology might dramatically reduce how much heat is lost, Wu said.

That would be a much more practical than today’s high-performance thermoelectric materials that are brittle and formed into large discs or blocks.  “This sort of manufacturing method requires using a lot of material,” Wu said.

The new flexible devices would conform to the irregular shapes of engines and exhaust pipes while using a small fraction of the material required for conventional thermoelectric devices.  Wu said, “This approach yields the same level of performance as conventional thermoelectric materials but it requires the use of much less material, which leads to lower cost and is practical for mass production.”

The coated fiber concept promises a method that can be scaled up to industrial processes, making mass production feasible.

The development is explained in a research paper appearing last month in the journal Nano Letters. The paper was written by Daxin Liang, a former Purdue exchange student from Jilin University in China; Purdue graduate students Scott Finefrock and Haoran Yang; and Wu.

Scott Finefrock starts the description; “We’ve demonstrated a material composed mostly of glass with only a 300-nanometer-thick coating of lead telluride. So while today’s thermoelectric devices require large amounts of the expensive element tellurium, our material contains only 5 percent tellurium. We envision mass production manufacturing for coating the fibers quickly in a reel-to-reel process.”

Plus, the fiber material also can be operated in a reverse manner: Applying an electrical current causes it to absorb heat, offering a possible solid-state air-conditioning method. Such fibers might one day be woven into cooling garments or used in other cooling technologies.

The team is showing the material has a promising thermoelectric efficiency, which is gauged using a formula to determine a measurement unit called ZT. One part of the formula is the “Seebeck coefficient,” named for 19th century German physicist Thomas Seebeck, who discovered the thermoelectric effect.  The others parts are having a low thermal and electrical conductivity results.  Having a low thermal conductivity, a high Seebeck coefficient and electrical conductivity results in a high ZT number.

Wu said, “It’s hard to optimize all of these three parameters simultaneously because if you increase electrical conductivity, and thermal conductivity goes up, the Seebeck coefficient drops.”

The Purdue researchers have used the ZT number to calculate the maximum efficiency that is theoretically possible with a material. “We analyze the material abundance, the cost, toxicity and performance, and we established a single parameter called the efficiency ratio,” Wu said.
While high-performance thermoelectric materials have been developed, the materials are not practical for widespread industrial applications.

Wu explains, “Today’s higher performance ones have a complicated composition, making them expensive and hard to manufacture. Also, they contain toxic materials, like antimony, which restricts thermoelectric research.”

Purdue’s nanocrystal lead telluride material is a critical ingredient, in part because the interfaces between the tiny crystals serve to suppress the vibration of the crystal lattice structure, reducing thermal conductivity. The materials could be exhibiting “quantum confinement,” in which the structures are so tiny they behave nearly like individual atoms.

“This means that, as electrons carry heat through the structures, the average voltage of those heat-carrying electrons is higher than it would be in larger structures,” Finefrock said. “Since you have higher-voltage electrons, you can generate more power.”

The Purdue team is also exploring other materials instead of lead and tellurium, which are toxic, and preliminary findings suggest other new materials are capable of a high ZT values, too.

Future work could focus on higher temperature annealing to improve efficiency, and the researchers also are exploring a different method to eliminate annealing altogether, which might make it possible to coat polymer fibers instead of glass.

The team may also work toward coating the glass fibers with a polymer to improve the resilience of the thermoelectric material, which tends to develop small cracks when the fibers are bent at sharp angles.

Wu adds, “Of course, the fact that our process uses such a small quantity of material – a layer only 300 nanometers thick, it minimizes the toxicity issue.  However, we also are concentrating on materials that are non-toxic and abundant.”

This work may finally get thermoelectric materials into practical use.  While lead and telluride are not inviting, the Purdue team has both the alternative material route and a coating method to solve the toxicity issue.

It looks like mass thermoelectric could get underway soon to capture a massive resource already at hand.

The team’s research appears in the American Chemical Society journal ACS Nano.  Lab results show the new oil additive could raise the efficiency of such transfer mediums by as much as 80 percent using an environmentally friendly base material.

The Rice Team's h-BN Comparative Results Graph. See the paper linked above for more details. Click image for the largest view.

While we may be interested in moving heat efficiently, the Rice team focused on electrical transformers.  Electrical transformers step voltage up or down and in doing so the wire resistance within causes heat to build up.  Currently most transformers are filled with fluids that cool and insulate the core and windings inside, as well as components that must remain separated from each other to keep voltage from leaking or shorting out.

The team has discovered that a very tiny amount of hexagonal boron nitride (h-BN) particles, which are two-dimensional cousins to carbon-based graphene, suspended in standard transformer mineral oils are highly efficient at removing heat from a system.

Narayanan said, “We don’t need a large amount of h-BN.  We found that 0.1 weight percentage of h-BN in transformer oil enhances it by nearly 80 percent.”  This news will light up a lot of heat management folks worldwide.

Taha-Tijerina adds to the depth of the discovery’s value with, “And at 0.01 weight percentage, the enhancement was around 9 percent. Even with a very low amount of material, we can enhance the fluids without compromising the electrical insulating properties.”

The background focuses on Taha-Tijerina, who was employed by an electrical transformer manufacturer in Mexico before coming to Rice, explains others working on similar compounds are experimenting with particles of aluminum, copper oxide and titanium oxide, but none of the compounds has the combination of qualities exhibited by h-BN.

Narayanan explains the technical details of h-BN particles that are about 600 nanometers wide and up to five atomic layers thick, disperse well in oil and, unlike highly electrically conductive graphene, are highly resistant to electricity. With help from Pasquali the team determined that the important quality of the oil’s viscosity is minimally affected by the presence of the new h-BN nanoparticle fillers.

Professor Ajayan who is Rice’s Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science and of Chemistry said, “Our research shows that with new materials and innovative approaches, we can add enormous value to applications that exist today in industry. Thermal management is a big issue in industry, but the right choice of materials is important; for transformer cooling, one needs dispersants in oils that take heat away, yet remain electrically insulating. Moreover, the two-dimensional nature of the fillers keeps them stable in oils without settling down for long periods of time.”

The Rice press release winds up adding Guanhui Gao, a visiting scholar in Ajayan’s lab; senior Matthew Rohde; and graduate student Dmitri Tsentalovich as team members.

Obviously the team has worked on the project in view of the perspectives of the team members and the interests of the supportive funding.  But the huge improvement of heat transfer is going to affect a much wider range of interests than electricity alone.

Moving heat, or conducting it, is an engineering challenge of getting as much as possible from the source to the work.  The less that’s lost the better.  The Rice University team’s discovery of a particle adding so much efficiency will stimulate a lot more research into other fluids and solids.  The result should be less fuel to drive the same amount of work.

The innovation that earns the applause is connecting the aspects of graphene to the h-BN molecule and particles finding a new result.  It’s a clue for more discoveries born of close observation and creative thought.

In today’s circumstances the main player is big temperature differentials where steam can be made to drive a Rankine thermodynamic cycle – commonly thought of as a turbine connected to a generator.  There are other ways using such circulation fluids like ammonia and freon types that will work as well.  These are often binary systems where two steps are used to get to flowing and working heat.

All the Rankine based ideas rely on a fluid heating up, expanding and vaporizing to drive a turbine or Stirling engine that makes mechanical motion to generate electricity. The vapor is then cooled, condensing it back into a fluid that is recycled back through to repeat the process.

Ian Marnoch Shows His MTP Engine. Click image for more info.

Marnoch’s heat engine works using the principle a little differently. There is no phase change in the vaporization of the working fluids.  Marnoch’s system relies on dry pre-pressurized air that expands as it’s heated and contracts as it’s cooled.  That change in volume and pressure causes pistons to move that can generate electricity.

Marnoch isn’t the first to grasp this, but Marnoch has configured his machine such to get an edge over other technologies. He says his engine configuration can process heat much faster and at bigger volumes than Rankine machines.

“It can process about three times as much heat by value as an Organic Rankine machine of the same size,” says Marnoch, adding that his heat engine can be designed to be much smaller and, therefore, less expensive.  All good.

But the new advantage is it can tap into lower temperatures that aren’t viable with other technologies.  This technology doesn’t need the boiling ammonia up to boiling water and beyond levels of temperatures.

Here’s the key – all Marnoch’ cycle needs is the right temperature differential, the spread between the heat source and the heat sink.  That could be cool air, the water in well, a deep mine shaft or the temperature at the bottom of an old oil or natural gas well.

Lots of folks are going to be realizing the opportunities are huge in the natural environment.

The news is all Marnoch needs is a 20º C (38º F) or higher temperature spread and there’s potential to generate electricity. The system becomes more economical the wider the gap.  That’s a lot of territory and the geothermal value can be heat or a heat sink.

It’s quite an idea for mechanical energy from low temperature heat spreads.

Marnoch and a team of PhD students and professors at the University of Ontario Institute of Technology (UOIT) have been working to perfect his patented heat engine.  Funding from the Canadian and Ontario governments have supported development of the machine for the past five years with early seed money from The Ontario Power Authority and Ontario Centres of Excellence.  The latest prototype of the machine is at UOIT’s new Clean Energy Research Laboratory.

Marnoch is understandably eager to get the machine out in the field and tested in a real-world situation. Companies are lining up.  Canada’s St. Marys Cement is exploring using the Marnoch engine to generate electricity from the waste heat of its Bowmanville cement plant.  Martin Vroegh, environmental manager at St Marys said, “It is in very early discussions but we are very enthusiastic about the potential and what this can mean for industries with large volumes of low-grade waste heat.”

Marnoch knows the current situation saying, “We just need to get out there and prove it works.”

With essentially any temp range where better than 20º C or 38º F can be had on the cheap money can be made or savings held or costs can be recovered.  Some folks are going to sit up and notice and Marnoch’s machine will get market legs.  Next up after field demonstrations is going to be working to more models and mass market pricing.

Go Marnoch!

Løvvik said in an interview in the UK’s Engineer, the key to the problem is that a good thermoelectric material ought to have high thermal resistance but low electrical resistance. Therefore, perhaps counter-intuitively, it is important to prevent heat dissipation through the material.

Løvvik in His Lab. Click image for more info.

The Norwegian team achieved this by introducing nanoscale barriers into various common semiconducting materials, which reflect waves of vibrating ‘hot’ energetic particles of certain frequencies.

Løvvik explains, “It looks easy on the outside, but on the inside the electrons are doing all the work. It’s essentially a heat engine but the working fluid is the electronic gas, because the electrons are free to move all around. It’s possible to choose your frequencies with care and then you can maintain the electronic conductivity while dramatically changing the heat dissipation – that’s what we aim for.

The fabrication method involves cooling down blocks of semiconducing materials to -196°C with liquid nitrogen to make them more brittle and less sticky, then grinding them down into nanoscale particles using a ‘mill’. Then particles are essentially compressed back together in a controlled fashion, leaving the essential nanoscale barriers within.

Here is Løvvik’s high moment – “We use the same kind of mill they use to make paint, it’s a well-established technique, it can be upscaled and it’s cheap, so that’s important,” he said.

Add in the calculations suggesting it could recover around 15 per cent of all energy losses in a variety of scenarios – and commercial production potential looks probable. The team is already in talks with a major automotive manufacturer (GM in the U.S.) with a view to placing the material in the exhausts of cars.

Løvvik grasps the significance, “This is just the starting point for using this technique to exploit the vast amount of waste heat that is available almost everywhere in society.”

Over half of all energy in the world is lost in useless waste heat. A car engine for example, only utilizes about 30 percent of the energy; the rest is lost as heat.  Recovering 15% of the 70% earns back 10.5% for a total efficiency of 40.5%, a 33% improvement.  Replacing the alternator in vehicles with a thermoelectric collector would also cut the load, saving even more.  It would be like $3 gasoline getting $4 of work done.

The thermoelectric effect was discovered in 1821 and essentially describes the generation of a voltage arising from a temperature difference across a material, generally made up of two different metals.  Here the Norwegians have managed to get worthwhile recovery efficiency with what seems to be a low cost monolithic material.

Cost and practicality have stopped thermoelectric devices from marketing successfully.  It looks like Løvvik’s team has made it over the barrier.

The progress could be very significant if the production of material is low cost enough and the applications simple and robust.  An enormous amount of heat is lost by energy intensive industry, business and in homes and transport.  15% doesn’t sound like a lot, but widely applied the results would be huge.

Keep in mind the Norwegian team is only at the first step. But the step looks to be commercial and marketable.

Today’s energy process as the U.S. Energy Information Administration explains, 92% of all the energy we use involves converting heat into mechanical energy and then often into electricity. A common path is coal or natural gas to steam to a turbine to a generator.

But today’s mechanical systems have drawbacks: their reliability is subject to moving parts breaking; their efficiency is relatively low; and they haven’t been successfully scaled down to the small sizes needed to match so many of today’s power-consuming devices, from sensors to smart phones to medical monitors.

Ivan Celanovic ScD ’06, research engineer in the Institute for Soldier Nanotechnologies (ISN) said, “Being able to convert heat from various sources into electricity without moving parts would bring huge benefits, especially if we could do it efficiently, relatively inexpensively, and on a small scale – a few millimeters, centimeters, or meters.”

Half a century ago, researchers developed thermophotovoltaics (TPV), an approach that couples a PV diode (the active part of a solar cell) with any source of heat. In a TPV system, a burning hydrocarbon, for example, heats up a solid piece of material called the thermal emitter; the thermal emitter radiates heat and light onto the PV diode; and the PV diode generates electricity. Because the thermal emitter is not as hot as the sun is, its radiation includes far more infrared wavelengths than occur in the solar spectrum. “Low band-gap” PV materials invented less than a decade ago can absorb more of that infrared radiation than standard silicon PVs can; but much of the heat is still wasted, so efficiencies remain relatively low.

Starting with TPV the MIT researchers have fabricated a button-sized power generator that’s fueled by butane, can run three times longer than a lithium-ion battery of the same weight, and can be recharged instantly by snapping in a tiny cartridge of fresh fuel. Another device, powered by a radioisotope, should generate electricity for 30 years without refueling or servicing—an ideal source of electricity for spacecraft that head out of our solar system.

Celanovic explains their solution is design a thermal emitter that radiates only the wavelengths that the PV diode can absorb and convert into electricity – and suppresses emission of all other wavelengths. “If you have perfect spectral matching between your heat source and your PV diode, you’ll get optimal efficiency for the overall system,” he said.

Marin Soljačić, associate professor of physics and ISN researcher takes the explanation further, “But how do we find a material that has this magical property of emitting only at the wavelengths that we want?” The answer: make a photonic crystal. Take a sample of material and on its surface create some nanoscale features – say, a regularly repeating pattern of holes or ridges. Light will now propagate through the sample in a dramatically different way than it did when the material was in its natural form.

This powerful approach – co-developed by John D. Joannopoulos, the Francis Wright Davis Professor of Physics and ISN director, and others – has been widely used to improve lasers, light- emitting diodes, and even optical fibers. But to the MIT team, this new type of material was exactly what they needed to engineer the spectrum of their thermal radiation to what their PV diode could use.

To start, they needed a material that could be heated to extremely high temperatures and then glow with an intensely bright light. The obvious choice was tungsten, which for 100 years has served as the filament in incandescent light bulbs. To make a slab of tungsten into a photonic crystal, they created an array of tiny pits – cylindrical cavities – on the surface. When the slab heats up, it generates a bright light but now with an altered emission spectrum.

MIT ThermoPhotoVoltaic Cell Sample Showing Geometry of Structure. Click image for more info.

Then each pit acts as a resonator, capable of giving off radiative heat at predetermined wavelengths. That provides wavelength selectivity. Celanovic offers as an analogy an acoustic resonator. “It’s kind of like when you put a seashell next to your ear and you hear a humming noise. You hear the noise amplified at the resonant frequencies of the seashell cavity. It’s the same principle, the same physics, but rather than acoustic resonance, this is electromagnetic resonance,” he said.

To implement their “designer material,” the researchers needed to find a practical means of fabricating a nanoscale structure in tungsten. After much work, they developed a method based on lithography and reactive ion etching, processing techniques used to make small features, for example, on micro-processors. In the MIT work, interference between two overlapping laser beams creates an etch mask with identical tiny holes, which are then transferred to a tungsten substrate by reactive ion etching. Using that approach, the MIT team has fabricated tungsten photonic crystals that are 1 cm in diameter with surfaces that contain billions of tiny holes, equally spaced from one another and nearly uniform in diameter and depth.
By controlling the geometry of the tiny cylinders bored in the tungsten the emission spectra can be distinctly set.

MITs New ThermoPhotoVoltaic Cell, Click image fior more info.

Heat raises the temperature of the tungsten photonic crystal – the thermal emitter – that in turn radiates heat at selected wavelengths toward the PV diode, and the diode converts it into electric power. Even with careful tailoring, the tungsten emitter delivers some heat at wavelengths that the PV diode cannot convert into electricity. To prevent that waste, the researchers mount on the face of the diode another photonic crystal, this one fabricated with a series of alternating layers of silicon and silicon dioxide – a nanostructure that can be tailored to transmit certain wavelengths and reflect others. Here, it reflects any radiation at unacceptable wavelengths back to the tungsten emitter, where it is reabsorbed and subsequently reemitted to provide more heat at wavelengths that the PV diode can use.

The MIT team is working with collaborators at MIT and elsewhere to create several novel electricity-generating devices.

One example is the micro-TPV power generator, a button-sized solid-state device that uses as its heat source hydrocarbon fuels such as butane or propane. At the center of the device is a “micro-reactor” designed by Klavs Jensen, the Warren K. Lewis Professor of Chemical Engineering, and fabricated in the Microsystems Technology Laboratories. This tiny reactor is a silicon chip with an interior channel where injected fuel undergoes catalytic reaction, generating heat. Photonic crystals are deposited on both the top and bottom surfaces of the micro-reactor, and low band-gap PV diodes are placed above and below the reactor separated by tiny gaps. Heated by the micro-reactor, the TPV system generates electricity, which passes into an electronic circuit that was specially designed by Robert Pilawa, graduate student, and David Perreault, associate professor, both of electrical engineering and computer science. The circuit dynamically adjusts the voltages and currents to suit a smart phone, sensor, or other device while extracting the maximum amount of power from the TPV system.

Celanovic said prototypes of their micro-TPV power generator are “pretty exciting.” The devices achieve a fuel-to-electricity conversion efficiency of about 3% – a ratio that may not sound impressive, but at that efficiency their energy output is three times greater than that of a lithium ion battery of the same size and weight. The TPV power generator can thus run three times longer without recharging, and then recharging is instantaneous: just snap in a new cartridge of butane. With further work on packaging and system design, Celanovic is confident that they can triple their current energy density. “At that point, our TPV generator could power your smart phone for a whole week without being recharged,” he says.

The MIT team has other ideas based on the technology starting in the lab, including radioisotopes that offer much promise.  But for most of us now the conversion of solar energy is in the forefront.  In that the scientists are looking at ways to use their photonic crystal to improve the conversion of solar energy into electricity. For example, optical concentrators such as parabolic mirrors could focus solar radiation onto a photonic crystal absorber and emitter, which would reshape the solar spectrum to better match the properties of a PV cell.

There’s a lot of potential in this field. While the seemingly low efficiencies are getting addressed, the applications look to be racing ahead.  The power by weight and size is very attractive as noted in the comparison with lithium batteries.

The MIT article is a long and worthwhile read.  Even this summary post is long.  As devices and tools get more compact and efficient the MIT TPV may have a much larger application field than thought.  Wherever there is a battery, there could be a TPV instead.

The experiments conducted at nanoGUNE show that infrared light can be transported and nanofocused with miniature transmission lines, consisting of two closely spaced metal nanowires. While lenses and mirrors manipulate visible light in its form of a free-space propagating wave, transmission lines guide the infrared light in the form of a tightly bound surface wave.

The nanoGUNE team adapted the concept of a classic transmission line to the infrared frequency range. Transmission lines are specialized cables such as the familiar ‘coax’ to carry for example TV and radio frequency signals. An old and simple form consists of two metal wires running closely in parallel, also called ladder line that you might recall as flat 300-Ohm antenna cable used to connect the rooftop antennae to a TV. 300-Ohm line was widely used from the early days of broadcast television until cable TV demanded higher capacity of frequencies.  Applied at MHz frequencies, where typical wavelengths are in the range of centimeters to several meters, flat 300-Ohm antenna cable is a prime example illustrating transport of energy in waveguides of strongly subwavelength-scale diameter.

Infrared Wire Topography and Image. Click Image for more info.

The researchers developed experiments demonstrating that infrared light can be transported in the same way, by scaling down the size of the transmission lines to below 1 micrometer. To that end, they fabricated two metal nanowires connected to an infrared antenna. The antenna captures infrared light and converts it into a propagating surface wave for traveling along the transmission line.

By gradually reducing the width of the transmission line to a tapering shape, the researchers demonstrate that the infrared surface wave is compressed to a tiny spot at the taper apex with a diameter of only 60 nm. This tiny spot is 150 times smaller than the free-space wavelength, emphasizing the extreme subwavelength-scale focus achieved in the experiments. The researchers applied their recently introduced near-field microscopy technique (Schnell et al., Nano Lett. 10 3524 (2010)) to map the different electrical field components of the infrared focus with nanoscale resolution.

Connecting a transmission line to the antenna, the infrared light captured by the nanoantenna can be transported over significant distances and nanofocused in a remote place.  While we’re not primarily interested in focusing the infrared energy here, the team’s target is nanofocusing of infrared light with transmission lines because it has important implications in spectroscopy and sensing applications. Rainer Hillenbrand, leader of the Nanooptics Group at the nanoscience institute nanoGUNE said, “This opens new pathways for the development of infrared nanocircuits.”  Martin Schnell who performed the experiments offers, “It is amazing that the classical radiofrequency concepts still work at infrared frequencies. That is 30 THz!”

But the door is opened now- the prospect of transmitting the infrared spectrum by wire is an intensely interesting prospect.  Any thermal process yielding less than 50% efficiency is spending more on waste than the energy used, an appalling situation.  This is a new research and development field.

The applications that jump to mind are the Stratosolar idea of solar harvesting in the stratosphere, geothermal where one could go to hot locations and simply send the heat up by wire and the waste heat recovery market.  The stunning advantage is the heat isn’t hot until you start the conversion process to most likely electricity for example.  One could also dump into a large number of storage solutions as well. You’re sure to think of more.

The engineering potential is stunning. While the folks at nanoGUNE are hot on a worthwhile goal, the discovery offers lots of other connections for innovative engineering and product and process development.  Hats off to the Spaniards on this one!

Getting from heat to electric power has a loss of the energy i.e. steam to turbine to generator with each step having energy losses of significance that the UM discovery may well skip in a working model.  The game is still about efficiency, but one alloy is found that can offer much further research, insight and innovation.  Step one is usually the hardest – getting from heat to electricity is now a now one step reality.  There’s a lot to learn, and now we have a place to start.

UM Aerospace Engineering and Mechanics Professor Richard James, who led the research team said, “This research is very promising because it presents an entirely new method for energy conversion that’s never been done before. It’s also the ultimate ‘green’ way to create electricity because it uses waste heat to create electricity with no carbon dioxide.”

Should development come quickly waste heat might be gathered taking efficiencies further.  Such devices could replace cooling towers, hot effluents, – all the range of cooling that spirits heat off into the atmosphere.

Multiferroic Energy Conversion Device. Click image for more info.

The UM team made the new alloy by combining elements at the atomic level to create a new multiferroic alloy, Ni45Co5Mn40Sn10. Multiferroic materials combine unusual elastic, magnetic and electric properties. The alloy Ni45Co5Mn40Sn10 achieves multiferroism by undergoing a highly reversible phase transformation where one solid turns into another solid. During this phase transformation the alloy undergoes changes in its magnetic properties that are exploited in the energy conversion device.

During a small-scale demonstration, the new material begins as a non-magnetic material, then suddenly becomes strongly magnetic when the temperature is raised a small amount. When this happens, the material absorbs heat and spontaneously produces electricity in a surrounding coil.

Some of this heat energy is lost in a process called hysteresis. A critical discovery of the team is a systematic way to minimize hysteresis in the phase transformations.  The team’s research has been published in the first issue of the new scientific journal Advanced Energy Materials.

A pdf file of the full paper is downloadable here.

Here again is a cross-disciplinary team with an astonishing result.  Professor James said, “This research crosses all boundaries of science and engineering. It includes engineering, physics, materials, chemistry, mathematics and more. It has required all of us within the university’s College of Science and Engineering to work together to think in new ways.”

Multiferroic Energy Conversion Team Members. Click image for more info.

With James the other members of the research team include University of Minnesota aerospace engineering and mechanics post-doctoral researchers Vijay Srivastava and Kanwal Bhatti, and Ph.D. student Yintao Song. The team is also working with University of Minnesota chemical engineering and materials science professor Christopher Leighton to create a thin film of the material that could be used, for example, to convert some of the waste heat from computers into electricity.

In the lab, University of Minnesota researchers show how a new multiferroic material they created begins as a non-magnetic material then suddenly becomes strongly magnetic as the piece of copper below it is heated a small amount. When this happens, it jumps over to a permanent magnet. This demonstration represents the direct conversion of heat to kinetic energy.

The new multiferroic alloy is a major step to the complete use of heat.  While no efficiency factor is available yet, cost estimate, or engineering designs for working models, the news deserves some acclaim.  Which begs the question, who can top this with what?

A great job, and a giant step. Congratulations.

Naomi Halas, Rice’s Stanley C. Moore Professor in Electrical and Computer Engineering, the paper’s lead researcher explains,  “We’re merging the optics of nanoscale antennas with the electronics of semiconductors. There’s no practical way to directly detect infrared light with silicon, but we’ve shown that it is possible if you marry the semiconductor to a nanoantenna. We expect this technique will be used in new scientific instruments for infrared-light detection and for higher-efficiency solar cells.”

Infrared Absorbing Antennas Embedded in Silicon.

Keep in mind that more than a third of the solar energy arriving on Earth comes in the spectrum of infrared light.  This energy is what warms the earth, feels good when basking in the sun and is in part re radiated back into space each night.  Is a new take on thermal energy gathering.

Silicon based solar cells convert mostly the visible spectrum of sunlight into electricity in the vast majority of today’s solar panels, but the technology doesn’t capture infrared light energy.

The semiconductor class of materials has spectrum gaps where light below a certain frequency passes directly through the material and is unable to generate an electrical current.  This is from the spectrum at the infrared being longer or simply bigger than semiconductors can react to.  This is where the antennae idea comes in.

The Rice team is showing they can extend the frequency range for electricity generation into the infrared by attaching a metal nanoantenna specially tuned to interact with infrared light to the silicon.

When infrared light passes into the antenna, it creates a “plasmon,” a wave of energy that sloshes through the antenna’s ocean of free electrons where the wave can create a current flow.

The study of plasmons is one of Professor Halas’ specialties, and the new paper resulted from basic research into the physics of plasmons that began in her lab years ago.

It’s been known that plasmons decay and give up their energy in two ways; they either emit a photon of light or they convert the light energy into heat. The heating process begins when the plasmon transfers its energy to a single electron, also known as a ‘hot’ electron.

Rice graduate student Mark Knight who is lead author on the paper, together with Rice theoretical physicist Peter Nordlander, his graduate student Heidar Sobhani, and Halas set out to design an experiment to directly detect the hot electrons resulting from plasmon decay.

By patterning a metallic nanoantenna directly onto a semiconductor to create a “Schottky barrier,” Knight showed that the infrared light striking the antenna would result in a hot electron that could jump the barrier, which creates an electrical current. This works for infrared light at frequencies that would otherwise pass directly through the device.

Here’s the payoff –according to Knight, “The nanoantenna-diodes we created to detect plasmon-generated hot electrons are already pretty good at harvesting infrared light and turning it directly into electricity. We are eager to see whether this expansion of light-harvesting to infrared frequencies will directly result in higher-efficiency solar cells.”

This opens up some big questions.  Foremost is how good is “pretty good”?  An estimated percentage or proportion would be worth knowing.  Next up would be the costs to add the antennas to a silicon photovoltaic cell.  That in turn asks if one even needs sunlight or would ant hot radiating source do?  Would it be efficient to just forego the photovoltaic costs all together?

It’s easy to see this could a lot of different directions.  When more is known, such as the characteristics of the current generated and the efficiency another look will be worthwhile.

One does wonder if there is a chart about the energy density of the points along the spectrum arriving on Earth.  (If you know leave a comment with a a link, please.)  At high efficiency and high density this technology could be very important.

A Micro Heat Pump Assembly Shown by Jan Kåre Bording. Click image for the largest view.

The new heat pump consists of many miniature heat pumps as small as one cubic millimeter. Heating a house requires several thousand of them. The miniature pumps assembled together into larger units that can be tall and thin or short and wide.

USNs Miniature Heat Pump Beside a Norwegian Coin. Click image for the largest view.

Doctor of Physics Jan Kåre Bording who is Chief Engineer at the USN says, “The most important advantages of the new heat pump is that you can regulate its size and form and that it is more durable than heat pumps are today. It is also more environmentally friendly.”

With colleague Professor of Materials Science Vidar Hansen, the team is developing a new heat pump that is thermo-electric. They have investigated its disadvantages and advantages compared with the heat pumps we use today.  They believe the heat pump will be fully developed and ready to be launched on the market in five to ten years.

The science is thermo-electric technology for heat pumps by making use of materials that produce electricity when they are subjected to differences in temperature.  With no moving parts the new heat pump has a dramatically longer life than today’s heat pumps.

Bording says,  “The heat pumps we use today consist of several movable parts. After some time different parts break down and will have to be changed. The new heat pump consists of several miniature heat pumps and these have a very simple design. In opposite to today’s heat pumps, these miniature heat pumps consist of only one part.

Because they consist of only one metal part it’s easier to avoid wear and tear. You can compare the heat pump to a golden ring. A golden ring won’t be broken. The miniature pumps will just continue to pump. We stick fans on them, and they must be replaced, but the heat pump itself will stay and be equally effective after 10,000 years.”

How’s that for a “Wow” moment?

The small heat pumps can be assembled together to form larger units. The USN team also envisages that it may be possible to place several thousand of the small heat pumps at different places in the house.

Hansen says, “We don’t want a large wood-burning stove in the middle of the house as in the old days. It’s (efficiency is) better with more, smaller heat sources.”

But to start with the team will create units that can be placed at one or two locations in the house. The new heat pumps offer great flexibility as to where in the house you want them. It would be an advantage to have them in places where it is especially cold.

“For example, it may be a good idea to put them under the floor, so that the floor will heat the room. When the heat pump has a large surface, it produces more heat,” Bording says.

The new heat pumps will be more environmentally friendly than those in use today. One problem with today’s heat pumps is that they can leak cooling gas. Cooling gas is usually freon gas, which destroys the ozone layer. There is no risk of losing cooling gas in thermoelectric heat pumps, since gas has been replaced with clean electricity.

Today’s common air to air heat pumps begin to deteriorate after one year. Then they need to be inspected annually incurring a professional expense.  After 10 to 20 years, larger parts of the pump such as the compressor have a higher likelihood of failure.  And they still, even as efficient as they are require an outside source of grid quality power.

The USN press release notes Bording saying thermal electric technology has long been used to generate electricity, as the phenomenon of thermo-electricity has been known for more than a hundred years.

Only now the Norwegians are busily trying to find out how we can use this phenomenon to pump heat into the house.  According to the researchers the new heat pump will be fully developed and ready to be launched on the market in five to ten years.

Time will tell if the thousands of micro heat pumps needed can be produced at scale at attractive prices.  If the home heating and some or all of the electrical demand can be met by this idea then a great share of the world energy load could be simply shed off cutting back in the need for power generation.

It might be too good to be true – but the lab units seem to be working . . .

We do have a few questions though, does the pump allow a straight electric output and can they reverse to provide cooling?  Plus how is the temperature difference employed?
It’s early in the technology – but the promise is strongly motivating.