Wind Turbines by Jacqueline McBride at LLNL

Lawrence Livermore National Laboratory scientist Sonia Wharton and colleague Julie Lundquist of the University of Colorado at Boulder and the National Renewable Energy Laboratory are looking at that issue calling it “stability”.  Their paper appeared in the Jan. 12 edition of the journal Environmental Research Letters, where Wharton and Lundquist examined turbine-generated power data, segregated out the atmospheric stability, to determine the power performance at a West Coast wind farm.

The pair has found by looking at the stability of the atmosphere, wind farm operators could gain greater insight into the amount of power generated at any given time.  The result is surprising; while it seems obvious that power generated at a set wind speed is higher under stable conditions and lower under strongly unsteady conditions, the average wind power output difference is as high as 15% less wind power generation when the atmosphere is unstable.

Wharton said, “The dependence of power on stability is clear, regardless of whether time periods are segregated by three-dimensional turbulence, turbulence intensity or wind shear.”

The pair rolled in time to the research, while turbulence is a relatively well-known term in assessing turbine efficiency, wind shear – which is a difference in wind speed and direction over a relatively short distance in the atmosphere also plays an important role when assessing how much power a turbine generates over certain time scales.

The study offers up a challenge to improve performance.  Wharton and Lundquist show that wind farm operators could better estimate how much power is generated if the wind forecasts included atmospheric stability impact measurements.

The research follows on earlier research that looked at atmospheric stability effects on power output.  But few studies have analyzed power output from modern turbines with hub heights of more than 60 meters, a small unit by today’s standards.

For the new research, Wharton and Lundquist gathered a year of power data from upwind modern turbines (80 meters high) at a multi-megawatt wind farm on the West Coast. They considered turbine power information as well as meteorological data from an 80-meter tall tower and a Sonic Detection and Ranging (SODAR), which provided wind profiles up to 200 meters above the surface, to look at turbulence and wind shear. Looking at upwind turbines removed any influence that turbine wakes may have on power performance.

Mean Seasonal Normalized Power and 80m Nacelle Wind Speed

They also found that wind speed and power production varied by season as well as from night to day. Wind speeds were higher at night (more power) than during the day (less power) and higher during the warm season (more power) than in the cool season (less power). For example, average power production was 43 percent of maximum generation capacity on summer days and peaked at 67 percent on summer nights.

The research at the West Coast location also offered new operational data.  Wharton said, “We found that wind turbines experienced stable, near-neutral and unstable conditions during the spring and summer. But daytime hours were almost always unstable or neutral while nights were strongly stable.”

Lundquist winds up the press release saying, “This work highlights the benefit of observing complete profiles of wind speed and turbulence across the turbine rotor disk, often available only with remote sensing technology like SODAR or LIDAR (Laser Detection and Ranging,).  Wind energy resource assessment and power forecasting would profit from this increased accuracy.”

The ladies have made an important point.  While wind might seem free, leaving 15% of the efficiency out of the performance is a major opportunity for production and profit.  Without a fuel cost the operation expenses and amortization cost can always use reductions and speeding up.  Income per operating hour would be an operator’s watchword so adding 15% to income by increasing output could add market share and a chance to drop rates to consumers.

Thanks to Wharton and Lundquist for a more, better and cheaper potential from wind power.

For comparison external quantum efficiency for photocurrent is usually expressed as a percentage where the number of electrons flowing per second in the external circuit of a solar cell is divided by the number of photons per second of a specific energy (or wavelength) that enter the solar cell.  So far no other solar cells show external photocurrent quantum efficiencies above 100% at any wavelength from the solar spectrum.

The NREL team has reached an external quantum efficiency peak value of 114%.  At these efficiencies both solar electricity and solar fuels may be competitive with, or perhaps less costly than, energy from fossil or nuclear fuels.

The paper at Science Magazine entitled “Peak External Photocurrent Quantum Efficiency Exceeding 100 percent via MEG in a Quantum Dot Solar Cell,” is co-authored by NREL scientists Octavi E. Semonin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Arthur J. Nozikand Matthew C. Beard.

The mechanism for producing quantum efficiency above 100% with solar photons is based on a process called Multiple Exciton Generation (MEG).  In a MEG a single absorbed photon of appropriately high energy can produce more than one electron-hole pair in the solar cell.

The idea that would be so came from NREL scientist Arthur J. Nozik who first predicted in a 2001 publication that a MEG would be more efficient in semiconductor quantum dots than in bulk semiconductors.

On the technical side quantum dots are tiny crystals of semiconductor, with sizes in the nanometer (nm) range from 1-20 nm.  One nm equals one-billionth of a meter. At these dimensions semiconductors exhibit dramatic activity due to the quantum physics.

Most importantly quantum physics applied to the nano semiconductors forms correlated electron-hole pairs (called excitons) at room temperature, which enhances coupling of electronic particles (electrons and positive holes) through Coulombic forces.  As the quantum dots are reduced in size a rapidly increasing bandgap occurs.

The physics set up the quantum dots to confine the charges and harvest excess energy.

Quantum dots are still semiconductor crystals with tiny volumes.  When an electrical charge is confined in such a space the energy stays electric instead of becoming heat.  That’s how the high efficiency starts. Spilling out more than one electron per photon strike pushes the result even higher.

The NREL team achieved the 114% external quantum efficiency with a layered cell consisting of antireflection-coated glass with a thin layer of a transparent conductor, a nanostructured zinc oxide layer, a quantum dot layer of lead selenide treated with ethanedithol and hydrazine, and a thin layer of gold for the top electrode.  Other than the gold, the raw materials are low cost.  It is looking like fabrication of quantum dot solar cells may apply to inexpensive, high-throughput roll-to-roll manufacturing.

At this stage one has to ask is plus 100% even believable.  MEG was first demonstrated experimentally in colloidal solutions of quantum dots in 2004 by Richard Schaller and Victor Klimov of the DOE’s Los Alamos National Laboratory.  By 2006 NREL scientists Mark Hanna and Arthur J. Nozik showed that ideal MEG in solar cells based on quantum dots could increase the theoretical thermodynamic power conversion efficiency of solar cells by about 35 percent compared to the cells of the day.

Quantum Efficiency of Photon Energy Using NREL Quantum Dot Solar Collector. Click image for more info.

Meanwhile many researchers around the world, including teams at NREL, have confirmed MEG in many different semiconductor quantum dot designs.  But nearly all those experiments used ultrafast time-resolved spectroscopic measurements of isolated quantum dots dispersed as particles in liquid colloidal solutions.  No power out was measured.

The new NREL team result is a MEG built with an external photocurrent quantum yield greater than 100 percent.  The reporting on the study points out the cells showed significant power conversion efficiencies (defined as the total power generated divided by the input power) as high as 4.5 percent with simulated sunlight.

Still these new solar cells are un-optimized and thus exhibit relatively low power conversion efficiency, which is a product of the photocurrent and photovoltage.

This is still stage one.  The MEG demonstration has important implications because it opens new and unexplored approaches to improve solar cell efficiencies.

As well as being a milestone the new results confirm the previous time-resolved spectroscopic measurements of MEG and hence validate those earlier MEG results.  The confirmation improves when the external quantum efficiency is corrected for the number of photons that are actually absorbed in the photoactive regions of the cell.

In actual absorption the determined quantum yield is called the internal quantum efficiency. The internal quantum efficiency is greater than the external quantum efficiency because a significant fraction of the incoming photons are lost through reflection and absorption in non-photocurrent producing regions of the cell. A peak internal quantum yield of 130% was found taking these reflection and absorption losses into account.

That brings us to the questions.  If every incoming photon was to drive an exciton pair the potential efficiency would seem to be 200%, leaving a lot on the cell, so to speak.  Then the matter of a photon driving even more than just a pair comes to mind.  Follow this with the reality, the NREL team has made the measurement from lab built first successful experimental materials leaving a full range of innovation to come – where those levels could get to isn’t offered, yet.

Lastly, the baseline and materials are geared towards the high-energy end of the solar radiation arriving at the surface.  What can be done to improve the range of the useful spectrum is another intriguing question.

The best perspective is today’s solar cell at 10 to 20% efficiency and marketed with some success now could have a 5 fold increase in the future if the science milestone of this week can scale to commercial production.  And the prospects can only get better with more research and engineering.

On a clear bright day you’ll notice the shadows are a dark area with little light.  But on a day with some overcast the shadows are not so dark and as the overcast intensifies, the shadows can seem to disappear.

One might think, the light is down so much the light and the shadows are equalized, but with some thought – that can’t be.  Instead the high clouds do cut back on the total light, but importantly, the light is scattered sending some in indirectly to light up the shadowed area.  The folks at MIT had an “AHA!” moment in a very big way.

The typical solar panel today, actually all of them, are as flat as pancakes and respond best to directly incoming light. Mounted up on a roof they are a paving of dark glass.  Users can increase the efficiency by pouring in an investment of moving parts called solar tracking that aim the flat sheets to match the sun and can get a very good result. It’s a very costly motorized computer controlled mechanism with two axes to handle to keep things lined up over a day and through the year. It’s a maintenance task of considerable magnitude and expense.

So the MIT team came up with a three-dimensional solar panel design called 3DPV (3 Dimensional Photo Voltaic).  It’s such a good idea the peaks in power generation for the 3DPV designs are better with some overcast light scattering.  The sum of the effects yields an increase in the daily energy generation of 3DPV in cloudy weather to more than a flat panel in clear weather.

The Marco Bernardi team at MIT simulated the performance of various shapes and tested several of these on the roof of a building at MIT. Their results indicate that the 3DPV panel structures can increase the amount of energy that can be generated in a given area by as much as 20 times. The structures can also double the number of useful peak hours of generation and reduce the seasonal variation as well.

There is likely a bunch of pending orders being postponed today.

The new mounting structure is simply a cube, open at the top and covered inside and out with photovoltaic cells, like a box with a panel on both sides of the four sides and one in the bottom. That arrangement can generate as much 3.8 times the power of a flat panel with the same footprint. By comparison, a solar tracking mount, control system and activating machinery produces an increase of only up to 1.8 times.

For buyers it’s the money.  While a cube has 9 times the surface area for panels, the panels are cheap.  It’s the installation, energy connections and current conversion that are still the big dollars.  For most buyers the area to work with is the limitation.  Now the calculation has changed – nearly 4 times the energy for the same area with nearly the same cost for the installation, energy connections and conversion.

This whole thing has to be rethought from the engineering of panels to maximize output from the 3D perspective and drive down the watt-hour cost at the panel output on to the buyers rethinking the investment and the payback with a much-accelerated amortization.

Bernardi suggests the 3DPV panel structures could be shipped as flat packages that easily “pop up” into 3D structures when assembled.

MIT has made the entire pdf file of the published report available to download. Its fair to say every panel builder worldwide will be studying the MIT result intensely today. There’s enough information there to replicate and take the engineering concept much further.  New product choices can’t be far off.

With silicon and printed panels getting very cheap, with a small if any increase of investment for the installation and power management the industry could get a big boost this year.

Please note the Bernardi group has done the math to also show there is an increased range for photovoltaic cells as well.

Also this is just round one.  The idea is a revolutionary start on a new photovoltaic future.  Others are going to take this further and the consumer choices are going to be much more attractive.  The calculations may have a given area, a given level of investment, a required payback period or other baseline thresholds for starting the process to adopt the technology.

What the MIT team has done is shift those thresholds much closer to many more people.

The paper is well worth the time for reading and consideration.  More energy by area, beating out sun tracking, increasing by double the number of useful peak hours and reducing the seasonal and latitude variation of solar energy generation, with even higher productivity in case of cloudy weather all adds up a Major Breakthrough.

Zhu and his team have discovered that it’s possible to double the number of electrons harvested from one photon of sunlight using an organic plastic semiconductor material.  The team published their discovery Dec. 16 in Science.

Zhu explains saying, “Plastic semiconductor solar cell production has great advantages, one of which is low cost. Combined with the vast capabilities for molecular design and synthesis, our discovery opens the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies.”

As the technology sits today the maximum theoretical efficiency of the silicon solar cell in use today is holding at approximately 31 percent.  The rest of the sun’s energy hitting the cell is too high to be turned into usable electricity. That energy, in the form of “hot electrons,” is instead lost as heat. Harvesting the hot electrons instead of radiating of the heat could potentially increase the efficiency of solar-to-electric power conversion to as high as 66 percent.

About a year and a half ago Zhu and the team first came into the high efficiency through semiconductor nanocrystals, or quantum dots that are suggested for the cooling of hot electrons and can slow down the electrons. In a 2008 paper in Science, a research group from the University of Chicago showed this to be true unambiguously for colloidal semiconductor nanocrystals.

This Illustration of Solar Driving Quantum Dot Energy From the 2010 Technology.

Zhu’s group discovered that hot electrons could be transferred from photo-excited lead selenide nanocrystals to an electron conductor made of widely used titanium dioxide.

Back in 2010 Zhu knew, “The demonstration of this hot electron transfer establishes that a highly efficient hot carrier solar cell is not just a theoretical concept, but an experimental possibility,” and that their methods will work for quantum dots made of other materials, too.

Getting to 66% isn’t a done deal as Zhu explains, “For one thing, that 66 % efficiency can only be achieved when highly focused sunlight is used, not just the raw sunlight that typically hits a solar panel. This creates problems when considering engineering a new material or device.”

To get to the theoretical goal, Zhu and his team have found an alternative. They discovered that a photon produces a dark quantum “shadow state” from which two electrons can then be efficiently captured to generate more energy in the semiconductor pentacene.

Working out the numbers on the pentacene mechanism shows an increase of solar cell efficiency to 44 percent without the need for focusing a solar beam, which would encourage more widespread use of solar technology.

That takes the efficiency more than a third of the way to 66%.

What’s known is a photon striking a pentacene semiconductor creates an excited electron-hole pair called an exciton.  The exciton is coupled quantum mechanically to a dark “shadow state” called a multiexciton.  This dark shadow state can be the most efficient source of two electrons via transfer to an electron acceptor material, such as fullerene, which Zhu’s group used in the study.  Exploiting the dark shadow state to produce double the electrons should increase solar cell efficiency to 44 percent.

Zhu’s team was led by Wai-lun Chan, a postdoctoral fellow in Zhu’s group, with the help of postdoctoral fellows Manuel Ligges, Askat Jailaubekov, Loren Kaake and Luis Miaja-Avila.

The research shows there is still a huge jump to come in solar photovoltaic.  Just how it will come to market isn’t clear, but the physics and chemistry are on their way to new defining potentials.  Sixty six percent was a nutty idea not long ago, and how long it stands is open for more innovation and research.

At Washington University in St. Louis’s Photosynthetic Antenna Research Center (PARC) one scientific team has just succeeded in making a crucial photosystem component – a light-harvesting antenna – from scratch. The new antenna is modeled on the chlorosome found in green bacteria.

We may not be stuck with just one means to harvest solar energy.  The solar cell is only 70 years old and came from a new understanding of semiconductors, materials that can use light energy to create mobile electrons and an electrical current. Comparatively they are quite efficient, yet they have almost nothing to do with the biological photosynthesis in plants that use light energy to push electrons across a membrane and ultimately create sugars and other organic molecules.

Since 1941 no one had the depth of understanding of those complex assemblages of proteins and pigments well enough to exploit their secrets for the design of solar cells.

That’s over now.

At (PARC) scientists are exploring native biological photosystems, building hybrids that combine natural and synthetic parts, and building fully synthetic analogs of natural systems.  Now they have a light-harvesting antenna described in a recent issue of New Journal of Chemistry.

Chlorosomes are giant assemblies of pigment molecules. Perhaps Nature’s most spectacular light-harvesting antennae, they allow green bacteria to photosynthesize even in the dim light in ocean deeps.

Dewey Holten, PhD, professor of chemistry in Arts & Sciences, and collaborator Christine Kirmaier, PhD, research professor of chemistry are part of a team that is trying to make synthetic chlorosomes. Holten and Kirmaier use ultra-fast laser spectroscopy and other analytic techniques to follow the rapid-fire energy transfers in photosynthesis.

The PARC article explains chlorosomes as biological systems that capture the energy in sunlight and convert it to the energy of chemical bonds.  While they come in many varieties, but they all have two basic parts: the light harvesting complexes, or antennae, and the reaction center complexes.

Photosystem Harvesting Light. Click image for more info.

The activity of an antenna consists of many pigment molecules that absorb photons and pass the excitation energy to the reaction centers.

In the reaction centers, the excitation energy sets off a chain of reactions that create ATP, a molecule often called the energy currency of the cell because the energy stored ATP powers most cellular work. Cellular organelles selectively break those bonds in ATP molecules when they need energy hits for cellular work.

The PARC folks are intuitive enough to look at green bacteria, which live in the lower layers of ponds, lakes and marine environments, and in the surface layers of sediments, and have evolved large and efficient light-harvesting antennae very different from those found in plants bathing in sunlight on Earth’s surface.

Green bacteria offers a super antennae consisting of highly organized three-dimensional systems of as many as 250,000 pigment molecules that absorb light and funnel the light energy through a pigment/protein complex called a baseplate to a reaction center, where it triggers chemical reactions that ultimately produce the desired ATP.

In plants and algae (and in the baseplate in the green bacteria) photo pigments are bound to protein scaffolds, which space and orient the pigment molecules in such a way that energy is efficiently transferred between them.  But chlorosomes don’t have a protein scaffold – instead the pigment molecules self -assemble into a structure that supports the rapid migration of excitation energy.

That’s intriguing because it suggests chlorosome mimics might be easier to incorporate in the design of solar devices than biomimetics that are made of proteins as well as pigments.

The PARC team’s goal was to see whether synthesized pigment molecules could be induced to self-assemble – even though the process by which the pigments align and bond is not well understood.

Holten explains, “The structure of the pigment assemblies in chlorosomes is the subject of intense debate, and there are several competing models for it.”  To design a pigment for a photosynthetic organism a chemist first builds one of three molecular frameworks. All three are macrocycles, or giant rings: porphyrin, chlorin and bacteriochlorin.  “One of the members of our team, Jon Lindsey can synthesize analogs of all three pigment types from scratch,” said Holten.

With that in mind the team wanted to study many variations of a pigment molecule to see what favored and what blocked assembly and Lindsey had also developed the means to synthesize chlorins, the basis for the pigments found in the chlorosomes of green bacteria. The chlorins push the absorption to the red end of the visible spectrum, an area of the spectrum scientists would like to be able to harvest for energy.

Doctoral student Olga Mass and coworkers in Lindsey’s lab synthesized 30 different chlorins, systematically adding or removing chemical groups thought to be important for self-assembly but also attaching peripheral chemical groups that take up space and might make it harder for the molecules to stack or that shift around the distributions of electrons so that the molecules might stack more easily.

The powdered pigments were shipped Holten’s lab at WUSTL and to David Bocian’s lab at the University of California at Riverside.

The two labs made up green-tinctured solutions of each of the 30 molecules in small test tubes and then poked and prodded the solutions by means of analytical techniques to see whether the pigment had aggregated and, if so, how much had formed the assemblies. Holten’s lab studied their absorption of light and their fluorescence (which indicated the presence of monomers, since assemblies don’t normally fluoresce) and Bocian’s lab studied their vibrational properties, which are determined by the network of bonds in the molecule or pigment aggregate as a whole.

In one crucial test Joseph Springer at Holten’s lab, compared the absorption spectrum of a pigment in a polar solvent that would prevent it from self-assembling to the spectrum of the pigment in a nonpolar solvent that would allow the molecules to interact with one another and form assemblies.

“You can see them aggregate. A pigment that is totally in solution is clear, but colored a brilliant green. When it aggregates, the solution becomes a duller green and you can see tiny flecks in the liquid,” Springer said.

The absorption spectra indicated that some pigments formed extensive assemblies and that the steric and electronic properties of the molecules predicted the degree to which they would assemble.

The PARC scientists have already taken the next step toward a practical solar device.  Along with Pratim Biswas, PhD, the Lucy and Stanley Lopata Professor and chair of the Department of Energy, Environmental & Chemical Engineering the team has demonstrated getting the pigments to self-assemble on surfaces, which is the next step in using them to design solar devices, explained Holten.

Holten cautions, “We’re not trying to make a more efficient solar cell in the next six months. Our goal instead is to develop fundamental understanding so that we can enable the next generation of more efficient solar powered devices.”

There is a very long way to go, and modeling from biology isn’t always a path to success.  But knowledge and understanding of biology is growing exponentially.  The research here offers a depth of understanding that will have an impact on further engineering efforts.

Man’s ability to grasp and understand vastly speeds up the time compared to what nature has needed to build complex organisms and systems.  This research is another example of basic research opening new doors for technology change.

We have a prior work to consider that harvests the infrared from nearly four years ago by Steve Novak at the Idaho National Lab.  Then this year saw a group in Spain focus infrared light so that much more light could strike a collector.

Now scientists in Israel, professors Koby Scheuer, Yael Hanin and Amir Boag have achieved a much higher conversion rate from light into useable energy combined with a lower material cost, which could mean a cost-effective way to harvest a larger part of the solar radiation.

The professor’s technology was recently presented at Photonics West in San Francisco and will be published in the conference proceedings.

Professor Scheuer explained that both radio and optical waves are electromagnetic energy.  When these waves are harvested, electrons are generated that can be converted into electric current. Traditionally, detectors based on semiconducting materials like silicon are used to interface with light, while radio waves are captured by antenna.

For optimal absorption, the antenna dimensions must correspond to the light’s very short wavelength — a challenge in optical frequencies that’s plagued engineers in the past, but now the Israeli team is able to fabricate antennas less than a micron in length.

To test the efficacy of the antennas, Prof. Scheuer and his colleagues measured their ability to absorb and remit energy. “In order to function, an antenna must form a circuit, receiving and transmitting,” said Prof. Scheuer, who points to the example of a cell phone, whose small, hidden antenna both receives and transmits radio waves in order to complete a call or send a message.

By illuminating the antennas, the researchers were able to measure the antennas’ ability to re-emit radiation efficiently, and determine how much power is lost in the circuit — a simple matter of measuring the wattage going in and coming back out. Initial tests indicate that 95 percent of the wattage going into the antenna comes out, meaning that only five percent is lost or wasted.

These are efficiencies undreamed of by the photovoltaic folks.

According to Prof. Scheuer, these “old school” nano sized antennas also have greater potential for solar energy because they can collect wavelengths across a much broader spectrum of light. The solar spectrum is very broad, he explained, with UV or infrared rays ranging from ten microns to less than two hundred nanometers. No semiconductor can handle this broad a spectrum, and they absorb only a fraction of the available energy. A group of antennas, however, can be manufactured in different lengths with the same materials and process, exploiting the entire available spectrum of light.

The team’s plan is a finished nanoantenna solar panel will be a large sheet of plastic which, with the use of a nano-imprinting lithography machine, will be imprinted with varying lengths and shapes of metallic antennas.

The researchers have already constructed a model of a possible solar panel. The next step, said Professor Scheuer, is to focus on the conversion process — how electromagnetic energy becomes electric current, and how the process can be improved.

That also is the most likely engineering issue faced by the earlier research.  The folks at the Idaho National Lab found the energy harvested coming in at frequencies never before considered.  What progress has been made isn’t known yet.

Everyone’s goal is not only to improve the efficiency of solar panels, but also to make the technology a viable option in terms of cost. Silicon is a relatively inexpensive semiconductor, but in order to obtain sufficient power from antennas, you need a very large panel — which makes them expensive.

Professor Scheuer pints out the obvious to most of us by noting energy sources need to be evaluated not only by what they can contribute environmentally, but also the return on every dollar invested.   “Our antenna is based on metal — aluminum and gold — in very small quantities. It has the potential to be more efficient and less expensive,” he said.

It seems to this writer that nanoantennas are the leading candidate to make solar to electric power a fully economically viable path to harvesting energy.  It comes with a great sense of surprise that the Israeli team has a working model losing only 5% of the incoming energy.

But after all this, antennas deliver current. So far the current they make isn’t very useful as the frequencies are so very, very high.

Now though, the incentive is getting clearer for investing the effort to harvest the antenna’s output and make free solar energy an affordable capital cost.  Novak has figured out that his design could work into the night gathering the latent infrared energy.  Just how the multiple sized antennas printed on plastic might perform will be an intensely interesting test.

John Hagopian, who is leading the effort involving 10 Goddard technologists announced the team of engineers working at NASA’s Goddard Space Flight Center in Greenbelt, Md., reported their findings recently at the SPIE Optics and Photonics conference, the largest interdisciplinary technical meeting in this discipline. The team has since reconfirmed the material’s absorption capabilities in additional testing.

Hagopian said in the press release, “The reflectance tests showed that our team had extended by 50 times the range of the material’s absorption capabilities. Though other researchers are reporting near-perfect absorption levels mainly in the ultraviolet and visible, our material is darn near perfect across multiple wavelength bands, from the ultraviolet to the far infrared. No one else has achieved this milestone yet.”

For solar thermal this is big news.

The nanotech-based coating is a thin layer of multi-walled carbon nanotubes, tiny hollow tubes made of pure carbon about 10,000 times thinner than a strand of human hair. They are positioned vertically on various substrate materials much like a shag rug. The team has grown the nanotubes on silicon, silicon nitride, titanium, and stainless steel, materials commonly used in space-based scientific instruments.

Carbon Nanotube Light Absorber Close Up. Click image for more info.

To grow the carbon nanotubes, Goddard technologist Stephanie Getty applies a catalyst layer of iron to an underlayer on silicon, titanium, and other materials. She then heats the material in an oven to about 1,382 degrees Fahrenheit. While heating, the material is bathed in carbon-containing feedstock gas.  Sounds simple and probably isn’t.

The tests indicate that the nanotube material is especially useful for a variety of spaceflight applications where observing in multiple wavelength bands is important to scientific discovery. One such application is stray-light suppression. The tiny gaps between the tubes collect and trap background light to prevent it from reflecting off surfaces and interfering with the light that scientists actually want to measure. Because only a small fraction of light reflects off the coating, the human eye and sensitive detectors see the material as black.

Of particular interest, the team found that the material absorbs 99.5 percent of the light in the ultraviolet and visible, dipping to 98 percent in the longer or far-infrared bands. “The advantage over other materials is that our material is from 10 to 100 times more absorbent, depending on the specific wavelength band,” Hagopian said.

We’re not going to complain about 98% infrared absorption.

Goddard engineer Manuel Quijada, who co-authored the SPIE paper and carried out the reflectance tests said, “We were a little surprised by the results. We knew it was absorbent. We just didn’t think it would be this absorbent from the ultraviolet to the far infrared.”  Let’s take surprised up to thrilled, these are impressive results.

Obviously NASA has its agenda.  If used in detectors and other instrument components, the technology would allow scientists to gather hard-to-obtain measurements of objects so distant in the universe that astronomers no longer can see them in visible light or those in high-contrast areas, including planets in orbit around other stars, Hagopian said. Earth scientists studying the oceans and atmosphere also would benefit. More than 90 percent of the light Earth-monitoring instruments gather comes from the atmosphere, overwhelming the faint signal they are trying to retrieve.

Currently, instrument developers apply black paint to baffles and other components to help prevent stray light from ricocheting off surfaces. However, black paints absorb only 90 percent of the light that strikes it. The effect of multiple bounces on paint makes the nano tube coating’s overall advantage even larger, potentially resulting in hundreds of times less stray light.

Additionally black paints do not remain black when exposed to the cold cryogenic temperatures in space. They take on a shiny, slightly silver quality, said Goddard scientist Ed Wollack, who is evaluating the carbon-nanotube material for use as a calibrator on far-infrared-sensing instruments that must operate in super-cold conditions to gather faint far-infrared signals emanating from objects in the very distant universe. If these instruments are not cold, thermal heat generated by the instrument and observatory, will swamp the faint infrared they are designed to collect.

Another NASA agenda point that’s worth noticing is black materials also serve another important function on spacecraft instruments, particularly infrared-sensing instruments, added Goddard engineer Jim Tuttle. The blacker the material, the more heat it radiates away. In other words, super-black materials, like the carbon nanotube coating, can be used on devices that remove heat from instruments and radiate it away to deep space. This cools the instruments to lower temperatures, where they are more sensitive to faint signals.  We earthlings are thinking about heat exchangers and heat sinks now.

Ed Wollack said, “This is a very promising material, it’s robust, lightweight, and extremely black. It is better than black paint by a long shot.”

While NASA is hot on the outer space applications and should be, this is another technology that has powerful transfer potential to the private industrial sector.  Thermal solar power is already well on its way to being a leading alternative energy source and a new technology of nearly complete solar radiation absorption is strong new building block for even more development.

It will be interesting to re report on this if and when the technology is released to manufacturers and what the gain might be for a thermal collector with the new nanotubes harvesting essentially, the whole of the available energy.  For now its exciting news, no costs are available, nor details of production. But the interest is going to be high if the technology can be used in concentrated collector designs.  If the nanotubes can stand it we might be getting process dry steam from the sun – and that would be something.

Prof. Kribus has developed a new technology that combines the use of conventional fuel with the lower pressures and temperatures of steam produced by solar thermal power, allowing plants to be hybrid, replacing 25 to 50 percent of their fuel use with green energy.

The professor explains most power plants create power by using fuel.  Solar thermal power plants use high temperatures and pressures generated by sunlight to produce turbine movement are currently the industry’s environmentally friendly alternative. But it’s an expensive option, especially when it comes to equipment made from expensive metals and the solar high-accuracy concentrator technology used to harvest solar energy.

In a solar thermal power plant, sunlight is harvested to create hot high-pressure steam, approximately 400 to 500 degrees centigrade. This solar-produced steam is then used to rotate the turbines that generate electricity.

Prof. Kribus cautions that it is somewhat unrealistic economically for the power industry. “It’s complex solar technology,” he explains. The materials alone, which include pipes made from expensive metals designed to handle high pressures and temperatures, as well as fields of large mirrors needed to harvest and concentrate enough light, make the venture too costly to be widely implemented.

A Turbine Inspection

With his graduate student Maya Livshits, Prof. Kribus is developing an alternative technology, called a steam-injection gas turbine.  “We combine a gas turbine, which works on hot air and not steam, and inject the solar-produced steam into the process,” he explains. “We still need to burn fuel to heat the air, but we add steam from low-temperature solar energy, approximately 200 degrees centigrade.” This hybrid cycle is not only highly efficient in terms of energy production, but the lowered pressure and heat requirements allow the solar part of the technology to use more cost-effective materials, such as common metals and low-cost solar collectors.

Kribus and Livshits’ method presents a potentially cost-effective and realistic way to integrate solar technology into today’s power plants is to be published in a future issue of the Solar Energy Journal.

Its not a totally alternative solution, but substituting a quarter to half of the fuel with solar is no small thing. A hybrid plant does offer a more realistic option for the coming decades.

Krivbus points out electricity from solar thermal power plants currently costs about twice as much as electricity from traditional power plants.  If this doesn’t change, solar technology may never be widely adopted.  Kribus and Livshits’ hope that a hybrid plant will have a comparable cost to a fuel-based power plant, making the option of replacing a large fraction of fuel with solar energy more competitive and practical.

The news story offered at the university site doesn’t address the peak load issues, nor the impact on a plant’s productivity after sundown.  But from the point of view in the Middle East where air conditioning is a major part of power use, the harvest of solar during the day’s heating period should make great sense.

Kribus and Livshits are starting a collaboration with a university in India to develop the design in more detail, and are looking for corporate partnerships that are willing to put hybrid technology into trial use.  Kribus knows it’s a stepping-stone that will help introduce solar energy into the industry in an accessible and affordable way.

This is quite an innovative approach.  We’ll be keeping an eye out for how the application of Indian engineering and more minds up close can improve the basic concept.  The first impression on the physics seem good, now if they can just keep the power level up into night . . .

The paper published in Nature Communications entitled, ‘Broadband Polarization-Independent Resonant Light Absorption Using Ultrathin Plasmonic Super Absorbers’ says in the abstract, “Our super absorber yields broadband and polarization-independent resonant light absorption over the entire visible spectrum (400–700 nm) with an average measured absorption of 0.71 and simulated absorption of 0.85.”

That’s right – the claim is the entire visible spectrum leaving some infrared and ultraviolet yet to be gathered.

Aydin explains, “The solar spectrum is not like a laser – it’s very broadband, starting with UV and going up to near-infrared. To capture this light most efficiently, a solar cell needs to have a broadband response. This design allows us to achieve that.”

Solar cells are only as efficient as the amount of sunlight they collect.  Collect a small percent and the efficiency is a small percent.  Collect a larger percent and the efficiency rockets up.

Light Absorption Grate. Click image for more info.

Aydin and his team used two unconventional materials, metal and silicon oxide – to create thin but complex, trapezoid-shaped metal gratings on the nanoscale that can trap a wider range of visible light. The use of these materials is unusual because on their own, they do not absorb light; however, they worked together on the nanoscale to achieve very high absorption rates

The uniquely shaped grating captured a wide range of wavelengths due to the local optical resonances, causing light to spend more time inside the material until it gets absorbed. This composite metamaterial was also able to collect light from many different angles – a useful quality when dealing with sunlight, which hits solar cells at different angles as sun moves from east to west throughout the day.

The catch is as Aydin explains, is the research is not directly applicable to solar cell technology because metal and silicon oxide cannot convert light to electricity. In fact, the photons are converted to heat and might allow novel ways to control the heat flow at the nanoscale. However, the innovative trapezoid shape could be replicated in semiconducting materials that could be used in solar cells.

But, if the geometry and technique can be applied to semiconducting materials, the technology could lead to thinner, lower-cost, and more efficient solar cells.  Aydin thinks it can be done.

Aydin has taken light physics and materials chemistry together to control light absorption and refraction at the nanoscale with an impressive result.  Collecting all the visible light is a significant achievement with materials that seem to be likely to make the transition to the solar cell research area.

Solar electrical generation is in a low spot with a certain widely publicized bankruptcy dragging down a business that is intermittent in performance and very difficult to make completive with things a simple as thermal solar.  The industry needs a shot of innovation and the Aydin’s team at Northwestern might have the answer for the industry and the energy independence minded consumer.

For the global warming crowd it’s a promising technology that simultaneously reduces atmospheric carbon dioxide and for everyone else the carbon dioxide can be directed to produce fuel.  Whichever suits, humanity would be a much more active player in a complete planetary atmospheric carbon cycle.

The team paper has been published in the journal Science.

Clearly defined, artificial photosynthesis is the process of converting carbon dioxide gas into useful carbon-based chemicals, most notably fuel or other compounds.  In plants, photosynthesis uses solar energy to convert carbon dioxide (CO2) and water to sugars and other hydrocarbons. Then mankind steps in and fuels are built from sugars extracted from crops such as sugarcane and corn.

In artificial photosynthesis, an electrochemical cell uses energy from a solar collector or a wind turbine to convert CO2 to simple carbon fuels such as formic acid or methanol, which are further refined to make ethanol and other fuels.

Dioxide Materials founded by retired chemical engineering professor Richard Masel who is the firm’s CEO and a co-principal investigator of the paper explains,  “The key advantage is that there is no competition with the food supply and it is a lot cheaper to transmit electricity than it is to ship biomass to a refinery.”  There is a lot of power involved, this big hurdle and the technology needed has kept artificial photosynthesis from vaulting into the mainstream.

Solar CO2 to Fuel Reactor Compared to Biomass. Click image for more info.

The first step to making fuel, turning carbon dioxide into carbon monoxide, consumes too much energy. It requires so much electricity to drive the carbon dioxide recovery reaction that more energy is used to produce the fuel than can be stored in the fuel.

The Illinois group’s innovative development is to use a novel approach involving an ionic liquid to catalyze the reaction, greatly reducing the energy required to drive the process. The ionic liquids stabilize the intermediates in the reaction so that less electricity is needed to complete the conversion.

The researchers used an electrochemical cell as a flow reactor, separating the gaseous CO2 input and oxygen output from the liquid electrolyte catalyst with gas-diffusion electrodes. The cell design allowed the researchers to fine-tune the composition of the electrolyte stream to improve reaction kinetics, including adding ionic liquids as a co-catalyst.

Kenis, who is also a professor of mechanical science and engineering and affiliated with the Beckman Institute for Advanced Science and Technology comments on the results said,  “It lowers the overpotential for CO2 reduction tremendously. Therefore, a much lower potential has to be applied. Applying a much lower potential corresponds to consuming less energy to drive the process.”

As the headline says, this is a step for an incomplete process. The Illinois team hopes to tackle the problem of throughput. To make their technology useful for commercial applications, they need to speed up the reaction and maximize conversion.  “More work is needed, but this research brings us a significant step closer to reducing our dependence on fossil fuels while simultaneously reducing CO2 emissions that are linked to unwanted climate change,” Kenis said.

It would be a dream come true when a solar collector can take CO2 from the air and produce a liquid or gaseous fuel.  The technology is not commercial or scalable yet, but the day a collector on the roof can provide an energy dense fuel useful in the home and car isn’t so far off.

Congratulations are in order for the Illinois and Dioxide Materials team.  Stay with it.