Thermophotovoltaic devices without mirrors to concentrate sunlight to 1000 degrees celsius and 37% overall system efficiency

Researchers at MIT have found a way to use thermophotovoltaic devices without mirrors to concentrate the sunlight, potentially making the system much simpler and less expensive.

The key is to prevent the heat from escaping the thermoelectric material, something the MIT team achieved by using a photonic crystal: essentially, an array of precisely spaced microscopic holes in a top layer of the material.

If you put an ordinary, dark-colored, light- and heat-absorbing material in direct sunlight, “it can’t get much hotter than boiling water,” because the object will reradiate heat almost as fast as it absorbs it. But to generate power efficiently, you need much higher temperatures than that. By concentrating sunlight with parabolic mirrors or a large array of flat mirrors, it’s possible to get much higher temperatures — but at the expense of a much larger and more complex system.\

Diagram of angle-selective solar thermophotovoltaic system. Bermel et al. Nanoscale Research Letters 2011 6:549 doi:10.1186/1556-276X-6-549

Nanoscale research letters - Tailoring photonic metamaterial resonances for thermal radiation

Selective solar absorbers generally have limited effectiveness in unconcentrated sunlight, because of reradiation losses over a broad range of wavelengths and angles. However, metamaterials offer the potential to limit radiation exchange to a proscribed range of angles and wavelengths, which has the potential to dramatically boost performance. After globally optimizing one particular class of such designs, we find thermal transfer efficiencies of 78% at temperatures over 1,000°C, with overall system energy conversion efficiencies of 37%, exceeding the Shockley-Quiesser efficiency limit of 31% for photovoltaic conversion under unconcentrated sunlight. This represents a 250% increase in efficiency and 94% decrease in selective emitter area compared to a standard, angular-insensitive selective absorber.

More Efficient Sun-free photovoltaics

Using new nanofabrication techniques, MIT researchers made these samples of tungsten with billions of regularly spaced, uniform nanoscale holes on their surfaces. In their TVP system, this type of photonic crystal serves as a thermal emitter, absorbing heat and then—because of its surface structure—radiating to the PV diode only those wavelengths that the diode can convert into electricity. The inset shows a digital photo of the full 1 cm-diameter sample, illuminated by white light. The color suggests the diffraction of white light into green as a result of the surface pattern.

A new photovoltaic energy-conversion system developed at MIT can be powered solely by heat, generating electricity with no sunlight at all. While the principle involved is not new, a novel way of engineering the surface of a material to convert heat into precisely tuned wavelengths of light — selected to match the wavelengths that photovoltaic cells can best convert to electricity — makes the new system much more efficient than previous versions.

They used a slab of tungsten, engineering billions of tiny pits on its surface. When the slab heats up, it generates bright light with an altered emission spectrum because each pit acts as a resonator, capable of giving off radiation at only certain wavelengths.

In this novel MIT design, input heat from an energy source raises the temperature of the tungsten photonic crystal, which transmits radiative heat at selected wavelengths to the PV diode. A second photonic crystal—mounted on the face of the PV diode—lets through heat at wave- lengths that the diode can convert into electricity and reflects the rest back to the tungsten photonic crystal, where it is reabsorbed and reemitted. Electricity from the PV diode passes to an electronic circuit that adjusts its voltage to match the external device being powered.

Prototypes of their micro-TPV power generator are "pretty exciting," says Celanovic. 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.

Metamaterials for emitted blackbody radiation with efficiency beyond natural limits

A designer metamaterial has shown it can engineer emitted "blackbody" radiation with an efficiency beyond the natural limits imposed by the material’s temperature, a team of researchers report in Physical Review Letters. Illustration shows design of the infrared metamaterial absorber. (a) Top view of a single band metamaterial absorber unit cell. (b) Schematic of a dual-band metamaterial absorber. (c and d) Perspective view for single and dual-band metamaterial absorbers. Credit: Physical Review Letters

A designer metamaterial has shown it can engineer emitted "blackbody" radiation with an efficiency beyond the natural limits imposed by the material's temperature, a team of researchers led by Boston College physicist Willie Padilla report in the current edition of Physical Review Letters.

A "blackbody" object represents a theorized ideal of performance for a material that perfectly absorbs all radiation to strike it and also emits energy based on the material's temperature. According to this blackbody law, the energy absorbed is equal to the energy emitted in equilibrium.

The breakthrough reported by Padilla and colleagues from Duke University and SensorMetrix, Inc., could lead to innovative technologies used to cull energy from waste heat produced by numerous industrial processes. Furthermore, the man-made metamaterial offers the ability to control emissivity, which could further enhance energy conversion efficiency.

Physical Review Letters - Taming the Blackbody with Infrared Metamaterials as Selective Thermal Emitters

High-performance flat-panel solar thermoelectric generators with high thermal concentration

Nature Materials - High-performance flat-panel solar thermoelectric generators with high thermal concentration

The conversion of sunlight into electricity has been dominated by photovoltaic and solar thermal power generation. Photovoltaic cells are deployed widely, mostly as flat panels, whereas solar thermal electricity generation relying on optical concentrators and mechanical heat engines is only seen in large-scale power plants. Here we demonstrate a promising flat-panel solar thermal to electric power conversion technology based on the Seebeck effect and high thermal concentration, thus enabling wider applications. The developed solar thermoelectric generators (STEGs) achieved a peak efficiency of 4.6% under AM1.5G (1 kW m−2) conditions. The efficiency is 7–8 times higher than the previously reported best value for a flat-panel STEG, and is enabled by the use of high-performance nanostructured thermoelectric materials and spectrally-selective solar absorbers in an innovative design that exploits high thermal concentration in an evacuated environment. Our work opens up a promising new approach which has the potential to achieve cost-effective conversion of solar energy into electricity.

Micron-gap ThermalPhotoVoltaics update

A single 250-500 watt module

Micron-gap TPV (MTPV) provides a significant breakthrough in power source technology by transferring more power between the emitter and receiver by reducing the size of the gap between them. By employing a submicron gap, the achievable power density for MTPV can be increased by approximately an order of magnitude as compared to conventional TPV. Equivalently, for a given active area and power density, the temperature on the hot-side of an MTPV device can be reduced by about 45%. which allows for new applications for on chip power, waste heat power generation, and converter power.

“We will be entering the market at 1 Watt per square centimeter, and we believe we are about two years away from perfecting our second-generation technology capable of more than 40-50 Watts per square centimeter,” Mather said.

The company “is about 12 months away from the start of selling commercial products.” The “quad” module (pictured above) could be inserted into the gas flow of an industrial plant or the methane burn-off at a coal mine. It will generate about 300 Watts, about one-fourth the power needed to power a small home. Smaller modules are also in development.

MPTV has developed the concepts and now has small teams in Austin, Boston, and Wuhan, China. The company has developed modules which deliver 10-50 times more power than earlier thermal photovoltaics.

Stanford Has a Process to Make Solar Power Production More Than Twice as Efficient to as High as 60% efficient

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Caption: A small PETE device made with cesium-coated gallium nitride glows while being tested inside an ultra-high vacuum chamber. The tests proved that the process simultaneously converted light and heat energy into electrical current.Credit: Photo courtesy of Nick Melosh, Stanford University

Stanford engineers have figured out how to simultaneously use the light and heat of the sun to generate electricity in a way that could make solar power production more than twice as efficient as existing methods and potentially cheap enough to compete with oil. The new process excels at higher temperatures. Called 'photon enhanced thermionic emission,' or PETE, the process promises to surpass the efficiency of existing photovoltaic and thermal conversion technologies.

This heat from unused sunlight and inefficiencies in the cells themselves account for a loss of more than 50 percent of the initial solar energy reaching the cell.

Until now, no one had come up with a way to wed thermal and solar cell conversion technologies.

IMEC Introduces Software Radio Architecture that can Scale to 1 Gigabit per second and IMEC Makes Germanium Thermophotovoltaics Cheaper

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1. Imec introduces a cognitive baseband radio (COBRA) architecture targeting 4G requirements at up to 1Gbit/s throughput and multiple asynchronous concurrent streams (for instance simultaneous digital broadcasting reception and high-speed internet access). The low-cost, flexible architecture answers a new trend in wireless communication where terminals give their users ubiquitous broadband access to a multitude of services.

Heat Transfer Can Be 1000 Times Greater than Plancks Law at the Nanoscale

Courtesy / Sheng Shen A diagram of the setup, including a cantilever from an atomic force microscope, used to measure the heat transfer between objects separated by nanoscale distances

A well-established physical law, Planck's law, describes the transfer of heat between two objects, but some physicists have long predicted that the law should break down when the objects are very close together. MIT researchers have determined that heat transfer can be 1,000 times greater than the law predicts.

The new findings could lead to significant new applications, including better design of the recording heads of the hard disks used for computer data storage, and new kinds of devices for harvesting energy from heat that would otherwise be wasted.

By using the glass (silica) beads, they were able to get separations as small as 10 nanometers (10 billionths of a meter, or one-hundredth the distance achieved before), and are now working on getting even closer spacings.

The new findings could also help in the development of new photovoltaic energy conversion devices to harness photons emitted by a heat source, called thermophovoltaic, Chen says. "The high photon flux can potentially enable higher efficiency and energy density thermophovoltaic energy converters, and new energy conversion devices," he says.

Micron gap thermal photovoltaics were described at nextbigfuture.




Nanometer gap thermal photovoltaics could be super-efficient for energy conversion

Micron Gap Thermal Photovoltaics

MTPV, a startup based in Boston that has raised $10 million, says that it has developed prototype micron-gap thermal photovoltaics that are large enough for practical applications.

Thermal photovoltaics use solar cells to convert the light that radiates from a hot surface into electricity. The first applications will be generating electricity from waste heat, eventually the technology could be used to generate electricity from sunlight far more efficiently than solar panels do. In such a system, sunlight is concentrated on a material to heat it up, and the light it emits is then converted into electricity by a solar cell.

In a thermal photovoltaic system, light is concentrated onto a material to heat it up. The material is selected so that when it gets hot, it emits light at wavelengths that a solar cell can convert efficiently. As a result, the theoretical maximum efficiency of a thermal photovoltaic system is 85 percent.

In practice, engineering challenges will make this hard to attain, but DiMatteo says that the company's computer models suggest that efficiencies over 50 percent should be possible. The prototypes aren't this efficient: they convert about 10 to 15 percent of the heat that they absorb from the glass-factory exhaust into electricity, which DiMatteo says is enough to make the devices economical.

Thermal Photovoltaics could be superior to both regular photovoltaics and to thermal electrics in terms of efficiency of converting heat or sunlight to electricity.

In a conventional TPV system, most of the photons generated in the heated material are reflected back into the material when they reach its surface; it's the same phenomenon that traps light in fiber-optic cables. When the solar cell and the heated material are brought close together, so that the gap between the two is shorter than the wavelength of the light being emitted, the surface no longer reflects light back. The photons travel from one material to the other as if there were no gap between them. The close spacing also allows electrons on one side of the gap to transfer energy to electrons on the other side. (A vacuum between the heated material and the solar cell maintains a temperature difference between the two that is required to achieve high efficiencies.) Since the heated material emits more photons, the solar cell can generate 10 times as much electricity for a given area, compared with a solar cell in a conventional TPV.

That makes it possible to use one-tenth as much solar-cell material, which cuts costs significantly. Alternatively, it makes it possible to generate more power at lower temperatures, which Peter Peumans, a professor of electrical engineering at Stanford University, says is one of the key advantages of the approach. Conventional thermal photovoltaics can require temperatures of 1,500 °C, he says. The first prototypes from MTPV work well at less than 1,000 °C, and DiMatteo says that, in theory, the technology could economically generate electricity at temperatures as low as 100 °C. This large temperature range could make the technology attractive for generating electricity from heat from a variety of sources, including automobile exhaust, that would otherwise be wasted.

But Peumans says that the technology has a trade-off: because the heated material and solar cell are placed so close together, it's not possible to put a filter between them to help tune the wavelengths of light that reach the solar cell. This could limit the ultimate efficiencies that the system can reach.

Technology Newroundup, Next Gen Wifi, Laser Fusion, Infrared solar power

1. Several startups take Wifi to the next level:

Amimon WHDI - Wireless Home Digital Interface provides a high-quality, uncompressed wireless link which can support delivery of equivalent video data rates of up to 3Gbps (including uncompressed 1080p) in a 40MHz channel in the 5GHz unlicensed band.

The Quantenna High Speed (QHS) family of chipsets pioneers a new level of ultra reliability for delivering high-definition (HD) multimedia content over wireless networks. With its advanced architecture – which includes vector mesh routing, two or four concurrent bands and throughput link rates in excess of 1 Gbps.

PC World has some more information about Quentanna's merging a mesh of wifi nodes around a house to get full wifi coverage.

Celano said its switched MIMO approach uses up to eight antennas and beam forming to carry up to four high definition video streams across 50 meters and penetrate multiple walls




2. The European research project, called HiPER (High Power laser for Energy Research), has been kicked off. The 'proof of principle' of laser fusion is anticipated in the next few years based on two large-scale lasers currently nearing completion in the USA and France.

3. CIP Technologies, University of Oxford (Oxford, England) and Wafer Technology Ltd. (Bucks, England), and with partial funding from the UK Technology Strategy Board and EPSRC, have successfully completed a three-year collaborative research project. It has delivered first generation single-junction cells with energy conversion efficiencies up to 12% for thermo-photovoltaic (TPV) cells. This compares to 9% from existing, commercially available devices.

Thermo-photovoltaics are similar to solar cells, but operate at infrared rather than visible wavelengths, generating electricity directly from heat. They have applications in waste-heat recovery from industrial plants such as blast furnaces, combined heat and power (CHP) generation, and domestic boilers, as well as silent mobile power generation.

The consortium is now working on a second-generation cell design with a more complex, multi-layer construction that will improve infrared capture even further. This is expected to extend energy conversion efficiencies to over 15%, significantly widening the range of viable applications for the technology.