Rice University’s solar-powered approach for purifying salt water with sunlight and nanoparticles is even more efficient than its creators first believed.
Researchers in Rice’s Laboratory for Nanophotonics (LANP) this week showed they could boost the efficiency of their solar-powered desalination system by more than 50% simply by adding inexpensive plastic lenses to concentrate sunlight into “hot spots.” The results are available online in the Proceedings of the National Academy of Sciences.
“The typical way to boost performance in solar-driven systems is to add solar concentrators and bring in more light,” said Pratiksha Dongare, a graduate student in applied physics at Rice’s Brown School of Engineering and co-lead author of the paper. “The big difference here is that we’re using the same amount of light. We’ve shown it’s possible to inexpensively redistribute that power and dramatically increase the rate of purified water production.”
In conventional membrane distillation, hot, salty water is flowed across one side of a sheetlike membrane while cool, filtered water flows across the other. The temperature difference creates a difference in vapor pressure that drives water vapor from the heated side through the membrane toward the cooler, lower-pressure side. Scaling up the technology is difficult because the temperature difference across the membrane—and the resulting output of clean water—decreases as the size of the membrane increases. Rice’s “nanophotonics-enabled solar membrane distillation” (NESMD) technology addresses this by using light-absorbing nanoparticles to turn the membrane itself into a solar-driven heating element.
Dongare and colleagues, including study co-lead author Alessandro Alabastri, coat the top layer of their membranes with low-cost, commercially available nanoparticles that are designed to convert more than 80% of sunlight energy into heat. The solar-driven nanoparticle heating reduces production costs, and Rice engineers are working to scale up the technology for applications in remote areas that have no access to electricity.
The concept and particles used in NESMD were first demonstrated in 2012 by LANP director Naomi Halas and research scientist Oara Neumann, who are both co-authors on the new study. In this week’s study, Halas, Dongare, Alabastri, Neumann and LANP physicist Peter Nordlander found they could exploit an inherent and previously unrecognized nonlinear relationship between incident light intensity and vapor pressure.
Alabastri, a physicist and Texas Instruments Research Assistant Professor in Rice’s Department of Electrical and Computer Engineering, used a simple mathematical example to describe the difference between a linear and nonlinear relationship. “If you take any two numbers that equal 10—seven and three, five and five, six and four—you will always get 10 if you add them together. But if the process is nonlinear, you might square them or even cube them before adding. So if we have nine and one, that would be nine squared, or 81, plus one squared, which equals 82. That is far better than 10, which is the best you can do with a linear relationship.”
In the case of NESMD, the nonlinear improvement comes from concentrating sunlight into tiny spots, much like a child might with a magnifying glass on a sunny day. Concentrating the light on a tiny spot on the membrane results in a linear increase in heat, but the heating, in turn, produces a nonlinear increase in vapor pressure. And the increased pressure forces more purified steam through the membrane in less time.
“We showed that it’s always better to have more photons in a smaller area than to have a homogeneous distribution of photons across the entire membrane,” Alabastri said.
Halas, a chemist and engineer who’s spent more than 25 years pioneering the use of light-activated nanomaterials, said, “The efficiencies provided by this nonlinear optical process are important because water scarcity is a daily reality for about half of the world’s people, and efficient solar distillation could change that.
“Beyond water purification, this nonlinear optical effect also could improve technologies that use solar heating to drive chemical processes like photocatalysis,” Halas said.
For example, LANP is developing a copper-based nanoparticle for converting ammonia into hydrogen fuel at ambient pressure.
Halas is the Stanley C. Moore Professor of Electrical and Computer Engineering, director of Rice’s Smalley-Curl Institute and a professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering.
NESMD is in development at the Rice-based Center for Nanotechnology Enabled Water Treatment (NEWT) and won research and development funding from the Department of Energy’s Solar Desalination program in 2018.
What gives? If solar panels and wind turbines became so much cheaper, why did the price of electricity rise instead of decline?
Electricity prices increased by 51 percent in Germany during its expansion of solar and wind energy. EP
One hypothesis might be that while electricity from solar and wind became cheaper, other energy sources like coal, nuclear, and natural gas became more expensive, eliminating any savings, and raising the overall price of electricity.
The price of nuclear and coal in those place during the same period was mostly flat.
Electricity prices increased 24 percent in California during its solar energy build-out from 2011 to 2017. EP
Another hypothesis might be that the closure of nuclear plants resulted in higher energy prices.
Evidence for this hypothesis comes from the fact that nuclear energy leaders Illinois, France, Sweden and South Korea enjoy some of the cheapest electricity in the world.
Since 2010, California closed one nuclear plant (2,140 MW installed capacity) while Germany closed 5 nuclear plants and 4 other reactors at currently-operating plants (10,980 MW in total).
Electricity in Illinois is 42 percent cheaper than electricity in California while electricity in France is 45 percent cheaper than electricity in Germany.
But this hypothesis is undermined by the fact that the price of the main replacement fuels, natural gas and coal, remained low, despite increased demand for those two fuels in California and Germany.
That leaves us with solar and wind as the key suspects behind higher electricity prices. But why would cheaper solar panels and wind turbines make electricity more expensive?
The main reason appears to have been predicted by a young German economist in 2013.
In a paper for Energy Policy, Leon Hirth estimated that the economic value of wind and solar would decline significantly as they become a larger part of electricity supply.
The reason? Their fundamentally unreliable nature. Both solar and wind produce too much energy when societies don’t need it, and not enough when they do.
Solar and wind thus require that natural gas plants, hydro-electric dams, batteries or some other form of reliable power be ready at a moment’s notice to start churning out electricity when the wind stops blowing and the sun stops shining.
And unreliability requires solar- and/or wind-heavy places like Germany, California and Denmark to pay neighboring nations or states to take their solar and wind energy when they are producing too much of it.
Hirth predicted that the economic value of wind on the European grid would decline 40 percent once it becomes 30 percent of electricity while the value of solar would drop by 50 percent when it got to just 15 percent.
Hirth predicted that the economic value of wind would decline 40% once it reached 30% of electricity, and that the value of solar would drop by 50% when it reached 15% of electricity. EP
In 2017, the share of electricity coming from wind and solar was 53 percent in Denmark, 26 percent in Germany, and 23 percent in California. Denmark and Germany have the first and second most expensive electricity in Europe.
By reporting on the declining costs of solar panels and wind turbines but not on how they increase electricity prices, journalists are — intentionally or unintentionally — misleading policymakers and the public about those two technologies.
The Los Angeles Times last year reported that California’s electricity prices were rising, but failed to connect the price rise to renewables, provoking a sharp rebuttal from UC Berkeley economist James Bushnell.
“The story of how California’s electric system got to its current state is a long and gory one,” Bushnell wrote, but “the dominant policy driver in the electricity sector has unquestionably been a focus on developing renewable sources of electricity generation.”
Part of the problem is that many reporters don’t understand electricity. They think of electricity as a commodity when it is, in fact, a service — like eating at a restaurant.
“The price we pay for the luxury of eating out isn’t just the cost of the ingredients most of which which, like solar panels and wind turbines, have declined for decades.
Rather, the price of services like eating out and electricity reflect the cost not only of a few ingredients but also their preparation and delivery.
This is a problem of bias, not just energy illiteracy. Normally skeptical journalists routinely give renewables a pass.
The reason isn’t because they don’t know how to report critically on energy — they do regularly when it comes to non-renewable energy sources — but rather because they don’t want to.”
That could — and should — change. Reporters have an obligation to report accurately and fairly on all issues they cover, especially ones as important as energy and the environment.
A good start would be for them to investigate why, if solar and wind are so cheap, they are making electricity so expensive.
New MIT research shows that, unless steady, continuous carbon-free sources of electricity are included in the mix, costs of decarbonizing the electrical system could be prohibitive and end up derailing attempts to mitigate the most severe effects of global climate change. Image: Chelsea Turner
To curb greenhouse gas emissions, nations, states, and cities should aim for a mix of fuel-saving, flexible, and highly reliable sources.
In major legislation passed at the end of August, California committed to creating a 100 percent carbon-free electricity grid — once again leading other nations, states, and cities in setting aggressive policies for slashing greenhouse gas emissions. Now, a study by MIT researchers provides guidelines for cost-effective and reliable ways to build such a zero-carbon electricity system.
The best way to tackle emissions from electricity, the study finds, is to use the most inclusive mix of low-carbon electricity sources.
Costs have declined rapidly for wind power, solar power, and energy storage batteries in recent years, leading some researchers, politicians, and advocates to suggest that these sources alone can power a carbon-free grid. But the new study finds that across a wide range of scenarios and locations, pairing these sources with steady carbon-free resources that can be counted on to meet demand in all seasons and over long periods — such as nuclear, geothermal, bioenergy, and natural gas with carbon capture — is a less costly and lower-risk route to a carbon-free grid.
The new findings are described in a paper published today in the journal Joule, by MIT doctoral student Nestor Sepulveda, Jesse Jenkins PhD ’18, Fernando de Sisternes PhD ’14, and professor of nuclear science and engineering and Associate Provost Richard Lester.
The need for cost effectiveness
“In this paper, we’re looking for robust strategies to get us to a zero-carbon electricity supply, which is the linchpin in overall efforts to mitigate climate change risk across the economy,” Jenkins says. To achieve that, “we need not only to get to zero emissions in the electricity sector, but we also have to do so at a low enough cost that electricity is an attractive substitute for oil, natural gas, and coal in the transportation, heat, and industrial sectors, where decarbonization is typically even more challenging than in electricity. ”
Sepulveda also emphasizes the importance of cost-effective paths to carbon-free electricity, adding that in today’s world, “we have so many problems, and climate change is a very complex and important one, but not the only one. So every extra dollar we spend addressing climate change is also another dollar we can’t use to tackle other pressing societal problems, such as eliminating poverty or disease.” Thus, it’s important for research not only to identify technically achievable options to decarbonize electricity, but also to find ways to achieve carbon reductions at the most reasonable possible cost.
To evaluate the costs of different strategies for deep decarbonization of electricity generation, the team looked at nearly 1,000 different scenarios involving different assumptions about the availability and cost of low-carbon technologies, geographical variations in the availability of renewable resources, and different policies on their use.
Regarding the policies, the team compared two different approaches. The “restrictive” approach permitted only the use of solar and wind generation plus battery storage, augmented by measures to reduce and shift the timing of demand for electricity, as well as long-distance transmission lines to help smooth out local and regional variations. The “inclusive” approach used all of those technologies but also permitted the option of using continual carbon-free sources, such as nuclear power, bioenergy, and natural gas with a system for capturing and storing carbon emissions. Under every case the team studied, the broader mix of sources was found to be more affordable.
The cost savings of the more inclusive approach relative to the more restricted case were substantial. Including continual, or “firm,” low-carbon resources in a zero-carbon resource mix lowered costs anywhere from 10 percent to as much as 62 percent, across the many scenarios analyzed. That’s important to know, the authors stress, because in many cases existing and proposed regulations and economic incentives favor, or even mandate, a more restricted range of energy resources.
“The results of this research challenge what has become conventional wisdom on both sides of the climate change debate,” Lester says. “Contrary to fears that effective climate mitigation efforts will be cripplingly expensive, our work shows that even deep decarbonization of the electric power sector is achievable at relatively modest additional cost. But contrary to beliefs that carbon-free electricity can be generated easily and cheaply with wind, solar energy, and storage batteries alone, our analysis makes clear that the societal cost of achieving deep decarbonization that way will likely be far more expensive than is necessary.”
A new taxonomy for electricity sources
In looking at options for new power generation in different scenarios, the team found that the traditional way of describing different types of power sources in the electrical industry — “baseload,” “load following,” and “peaking” resources — is outdated and no longer useful, given the way new resources are being used.
Rather, they suggest, it’s more appropriate to think of power sources in three new categories: “fuel-saving” resources, which include solar, wind and run-of-the-river (that is, without dams) hydropower; “fast-burst” resources, providing rapid but short-duration responses to fluctuations in electricity demand and supply, including battery storage and technologies and pricing strategies to enhance the responsiveness of demand; and “firm” resources, such as nuclear, hydro with large reservoirs, biogas, and geothermal.
“Because we can’t know with certainty the future cost and availability of many of these resources,” Sepulveda notes, “the cases studied covered a wide range of possibilities, in order to make the overall conclusions of the study robust across that range of uncertainties.”
Range of scenarios
The group used a range of projections, made by agencies such as the National Renewable Energy Laboratory, as to the expected costs of different power sources over the coming decades, including costs similar to today’s and anticipated cost reductions as new or improved systems are developed and brought online. For each technology, the researchers chose a projected mid-range cost, along with a low-end and high-end cost estimate, and then studied many combinations of these possible future costs.
Under every scenario, cases that were restricted to using fuel-saving and fast-burst technologies had a higher overall cost of electricity than cases using firm low-carbon sources as well, “even with the most optimistic set of assumptions about future cost reductions,” Sepulveda says.
That’s true, Jenkins adds, “even when we assume, for example, that nuclear remains as expensive as it is today, and wind and solar and batteries get much cheaper.”
The authors also found that across all of the wind-solar-batteries-only cases, the cost of electricity rises rapidly as systems move toward zero emissions, but when firm power sources are also available, electricity costs increase much more gradually as emissions decline to zero.
“If we decide to pursue decarbonization primarily with wind, solar, and batteries,” Jenkins says, “we are effectively ‘going all in’ and betting the planet on achieving very low costs for all of these resources,” as well as the ability to build out continental-scale high-voltage transmission lines and to induce much more flexible electricity demand.
In contrast, “an electricity system that uses firm low-carbon resources together with solar, wind, and storage can achieve zero emissions with only modest increases in cost even under pessimistic assumptions about how cheap these carbon-free resources become or our ability to unlock flexible demand or expand the grid,” says Jenkins. This shows how the addition of firm low-carbon resources “is an effective hedging strategy that reduces both the cost and risk” for fully decarbonizing power systems, he says.
Even though a fully carbon-free electricity supply is years away in most regions, it is important to do this analysis today, Sepulveda says, because decisions made now about power plant construction, research investments, or climate policies have impacts that can last for decades.
“If we don’t start now” in developing and deploying the widest range of carbon-free alternatives, he says, “that could substantially reduce the likelihood of getting to zero emissions.”
David Victor, a professor of international relations at the University of California at San Diego, who was not involved in this study, says, “After decades of ignoring the problem of climate change, finally policymakers are grappling with how they might make deep cuts in emissions. This new paper in Joule shows that deep decarbonization must include a big role for reliable, firm sources of electric power. The study, one of the few rigorous numerical analyses of how the grid might actually operate with low-emission technologies, offers some sobering news for policymakers who think they can decarbonize the economy with wind and solar alone.”
The research received support from the MIT Energy Initiative, the Martin Family Trust, and the Chilean Navy.
Solar energy is clean and abundant, but when the sun isn’t shining, you must store the energy in batteries or through a process called photocatalysis. In photocatalytic water splitting, sunlight separates water into hydrogen and oxygen, which can then be recombined in a fuel cell to release energy. Now, a new class of materials — halide double perovskites — may have just the right properties to split water, according to research in Applied Physics Letters. In this image: Novel, lead-free double perovskites as potential photocatalysts for solar water splitting. Credit: George Volonakis
Now, a new class of materials — halide double perovskites — may have just the right properties to split water, according to a newly published paper in Applied Physics Letters, from AIP Publishing.
“If we can come up with a material that can be useful as a water-splitting photocatalyst, then it would be an enormous breakthrough,” said Feliciano Giustino, a co-author on the paper.
Researchers have experimented with many photocatalytic materials before, such as titanium dioxide (TiO2). While TiO2 can harness sunlight to split water, it’s inefficient because it doesn’t absorb visible light well. So far, no photocatalytic material for general water splitting has become commercially available.
Using supercomputers to calculate the quantum energy states of four halide double perovskites, George Volonakis and Giustino, both of the University of Oxford, found that Cs2BiAgCl6 and Cs2BiAgBr6 are promising photocatalytic materials because they absorb visible light much better than TiO2. They also generate electrons and holes (the positively charges absence of electrons) that have sufficient energy (or nearly ideal energies) to split water into hydrogen and oxygen.
Very few other materials have all these features at once, Giustino said. “We can’t say this will work for sure, but these compounds seem to have all the right properties.”
Giustino and his team originally discovered this type of perovskite while looking for materials to make solar cells. Over the last several years, perovskites have garnered interest as materials to boost the efficiency of silicon-based solar cells through tandem designs that integrate a perovskite cell directly onto a high-efficiency silicon cell, but they contain a small amount of lead. If they were used for energy harvesting in a solar farm, the lead could pose a potential environmental hazard.
In 2016, using computer simulations to identify alternative materials, the researchers found a new type of lead-free perovskite with potential for high-efficiency solar cells. The present paper shows these new materials may also split water. “These new double perovskites are not only promising as a complementary material for tandem solar cells, but they can also be promising in areas like photocatalysis,” Volonakis said.
Still, the new analysis is theoretical, assuming the compounds form perfect crystals. The next step, the authors said, is for experimentalists to see if the material works in the real world as well as predicted. In the meantime, the researchers are using their computational techniques to explore whether these double perovskites have properties useful for other applications like light detectors.
For quantum dot (QD) materials to perform well in devices such as solar cells, the nanoscale crystals in them need to pack together tightly so that electrons can hop easily from one dot to the next and flow out as current. MIT researchers have now made QD films in which the dots vary by just one atom in diameter and are organized into solid lattices with unprecedented order. Subsequent processing pulls the QDs in the film closer together, further easing the electrons’ pathway. Tests using an ultrafast laser confirm that the energy levels of vacancies in adjacent QDs are so similar that hopping electrons don’t get stuck in low-energy dots along the way.
Taken together, the results suggest a new direction for ongoing efforts to develop these promising materials for high performance in electronic and optical devices.
In recent decades, much research attention has focused on electronic materials made of quantum dots, which are tiny crystals of semiconducting materials a few nanometers in diameter. After three decades of research, QDs are now being used in TV displays, where they emit bright light in vivid colors that can be fine-tuned by changing the sizes of the nanoparticles. But many opportunities remain for taking advantage of these remarkable materials.
“QDs are a really promising underlying materials technology for energy applications,” says William Tisdale, the ARCO Career Development Professor in Energy Studies and an associate professor of chemical engineering.
QD materials pique his interest for several reasons. QDs are easily synthesized in a solvent at low temperatures using standard procedures. The QD-bearing solvent can then be deposited on a surface—small or large, rigid or flexible—and as it dries, the QDs are left behind as a solid. Best of all, the electronic and optical properties of that solid can be controlled by tuning the QDs.
“With QDs, you have all these degrees of freedom,” says Tisdale. “You can change their composition, size, shape, and surface chemistry to fabricate a material that’s tailored for your application.”
The ability to adjust electron behavior to suit specific devices is of particular interest. For example, in solar photovoltaics (PVs), electrons should pick up energy from sunlight and then move rapidly through the material and out as current before they lose their excess energy. In light-emitting diodes (LEDs), high-energy “excited” electrons should relax on cue, emitting their extra energy as light.
With thermoelectric (TE) devices, QD materials could be a game-changer. When TE materials are hotter on one side than the other, they generate electricity. So TE devices could turn waste heat in car engines, industrial equipment, and other sources into power—without combustion or moving parts. The TE effect has been known for a century, but devices using TE materials have remained inefficient. The problem: While those materials conduct electricity well, they also conduct heat well, so the temperatures of the two ends of a device quickly equalize. In most materials, measures to decrease heat flow also decrease electron flow.
“With QDs, we can control those two properties separately,” says Tisdale. “So we can simultaneously engineer our material so it’s good at transferring electrical charge but bad at transporting heat.”
Making good arrays
One challenge in working with QDs has been to make particles that are all the same size and shape. During QD synthesis, quadrillions of nanocrystals are deposited onto a surface, where they self-assemble in an orderly fashion as they dry. If the individual QDs aren’t all exactly the same, they can’t pack together tightly, and electrons won’t move easily from one nanocrystal to the next.
Three years ago, a team in Tisdale’s lab led by Mark Weidman Ph.D. ’16 demonstrated a way to reduce that structural disorder. In a series of experiments with lead-sulfide QDs, team members found that carefully selecting the ratio between the lead and sulfur in the starting materials would produce QDs of uniform size.
“As those nanocrystals dry, they self-assemble into a beautifully ordered arrangement we call a superlattice,” Tisdale says.
Scattering electron microscope images of those superlattices taken from several angles show lined-up, 5-nanometer-diameter nanocrystals throughout the samples and confirm the long-range ordering of the QDs.
For a closer examination of their materials, Weidman performed a series of X-ray scattering experiments at the National Synchrotron Light Source at Brookhaven National Laboratory. Data from those experiments showed both how the QDs are positioned relative to one another and how they’re oriented, that is, whether they’re all facing the same way. The results confirmed that QDs in the superlattices are well ordered and essentially all the same.
“On average, the difference in diameter between one nanocrystal and another was less than the size of one more atom added to the surface,” says Tisdale. “So these QDs have unprecedented monodispersity, and they exhibit structural behavior that we hadn’t seen previously because no one could make QDs this monodisperse.”
Controlling electron hopping
The researchers next focused on how to tailor their monodisperse QD materials for efficient transfer of electrical current. “In a PV or TE device made of QDs, the electrons need to be able to hop effortlessly from one dot to the next and then do that many thousands of times as they make their way to the metal electrode,” Tisdale explains.
One way to influence hopping is by controlling the spacing from one QD to the next. A single QD consists of a core of semiconducting material—in this work, lead sulfide—with chemically bound arms, or ligands, made of organic (carbon-containing) molecules radiating outward. The ligands play a critical role—without them, as the QDs form in solution, they’d stick together and drop out as a solid clump. Once the QD layer is dry, the ligands end up as solid spacers that determine how far apart the nanocrystals are.
A standard ligand material used in QD synthesis is oleic acid. Given the length of an oleic acid ligand, the QDs in the dry superlattice end up about 2.6 nanometers apart—and that’s a problem.
“That may sound like a small distance, but it’s not,” says Tisdale. “It’s way too big for a hopping electron to get across.”
Using shorter ligands in the starting solution would reduce that distance, but they wouldn’t keep the QDs from sticking together when they’re in solution. “So we needed to swap out the long oleic acid ligands in our solid materials for something shorter” after the film formed, Tisdale says.
To achieve that replacement, the researchers use a process called ligand exchange. First, they prepare a mixture of a shorter ligand and an organic solvent that will dissolve oleic acid but not the lead sulfide QDs. They then submerge the QD film in that mixture for 24 hours. During that time, the oleic acid ligands dissolve, and the new, shorter ligands take their place, pulling the QDs closer together. The solvent and oleic acid are then rinsed off.
Tests with various ligands confirmed their impact on interparticle spacing. Depending on the length of the selected ligand, the researchers could reduce that spacing from the original 2.6 nanometers with oleic acid all the way down to 0.4 nanometers. However, while the resulting films have beautifully ordered regions—perfect for fundamental studies—inserting the shorter ligands tends to generate cracks as the overall volume of the QD sample shrinks.
Energetic alignment of nanocrystals
One result of that work came as a surprise: Ligands known to yield high performance in lead-sulfide-based solar cells didn’t produce the shortest interparticle spacing in their tests.
“Reducing that spacing to get good conductivity is necessary,” says Tisdale. “But there may be other aspects of our QD material that we need to optimize to facilitate electron transfer.”
One possibility is a mismatch between the energy levels of the electrons in adjacent QDs. In any material, electrons exist at only two energy levels—a low ground state and a high excited state. If an electron in a QD film receives extra energy—say, from incoming sunlight—it can jump up to its excited state and move through the material until it finds a low-energy opening left behind by another traveling electron. It then drops down to its ground state, releasing its excess energy as heat or light.
In solid crystals, those two energy levels are a fixed characteristic of the material itself. But in QDs, they vary with particle size. Make a QD smaller and the energy level of its excited electrons increases. Again, variability in QD size can create problems. Once excited, a high-energy electron in a small QD will hop from dot to dot—until it comes to a large, low-energy QD.
“Excited electrons like going downhill more than they like going uphill, so they tend to hang out on the low-energy dots,” says Tisdale. “If there’s then a high-energy dot in the way, it takes them a long time to get past that bottleneck.”
So the greater mismatch between energy levels—called energetic disorder—the worse the electron mobility. To measure the impact of energetic disorder on electron flow in their samples, Rachel Gilmore Ph.D. ’17 and her collaborators used a technique called pump-probe spectroscopy—as far as they know, the first time this method has been used to study electron hopping in QDs.
QDs in an excited state absorb light differently than do those in the ground state, so shining light through a material and taking an absorption spectrum provides a measure of the electronic states in it. But in QD materials, electron hopping events can occur within picoseconds—10-12 of a second—which is faster than any electrical detector can measure.
The researchers therefore set up a special experiment using an ultrafast laser, whose beam is made up of quick pulses occurring at 100,000 per second. Their setup subdivides the laser beam such that a single pulse is split into a pump pulse that excites a sample and—after a delay measured in femtoseconds (10-15 seconds)—a corresponding probe pulse that measures the sample’s energy state after the delay. By gradually increasing the delay between the pump and probe pulses, they gather absorption spectra that show how much electron transfer has occurred and how quickly the excited electrons drop back to their ground state.
Using this technique, they measured electron energy in a QD sample with standard dot-to-dot variability and in one of the monodisperse samples. In the sample with standard variability, the excited electrons lose much of their excess energy within 3 nanoseconds. In the monodisperse sample, little energy is lost in the same time period—an indication that the energy levels of the QDs are all about the same.
By combining their spectroscopy results with computer simulations of the electron transport process, the researchers extracted electron hopping times ranging from 80 picoseconds for their smallest quantum dots to over 1 nanosecond for the largest ones. And they concluded that their QD materials are at the theoretical limit of how little energetic disorder is possible. Indeed, any difference in energy between neighboring QDs isn’t a problem. At room temperature, energy levels are always vibrating a bit, and those fluctuations are larger than the small differences from one QD to the next.
“So at some instant, random kicks in energy from the environment will cause the energy levels of the QDs to line up, and the electron will do a quick hop,” says Tisdale.
The way forward
With energetic disorder no longer a concern, Tisdale concludes that further progress in making commercially viable QD materials will require better ways of dealing with structural disorder. He and his team tested several methods of performing ligand exchange in solid samples, and none produced films with consistent QD size and spacing over large areas without cracks. As a result, he now believes that efforts to optimize that process “may not take us where we need to go.”
What’s needed instead is a way to put short ligands on the QDs when they’re in solution and then let them self-assemble into the desired structure.
“There are some emerging strategies for solution-phase ligand exchange,” he says. “If they’re successfully developed and combined with monodisperse QDs, we should be able to produce beautifully ordered, large-area structures well suited for devices such as solar cells, LEDs, and thermoelectric systems.”
Panasonic is far from being satisfied with only a battery supplier role. The Japanese company has greater ambitions and intends to offer its scalable “ePowertrain” platform for small EVs.
The main target for the ePowertrain are EV bikes and micro EVs. These should now be easier to develop and produce using Panasonic’s power unit (with an on-board charger, junction box, inverter and DC-to-DC converter) and a motor unit. Of course, batteries are available too.
“Panasonic Corporation announced today that it has developed a scalable “ePowertrain” platform, a solution for the effective development of small electric vehicles (EVs). The platform is a systematized application of devices used in the EVs of major global carmakers, and is intended to contribute to the advancement of the coming mobility society.
Global demand for EVs is expected to expand rapidly, along with a wide variety of new mobility. These include not only conventional passenger vehicles but also new types of EVs, such as EV bikes and micro EVs, which suit various lifestyles and uses in each region.
The platform Panasonic has developed for EV bikes and micro EVs is an energy-efficient, safe powertrain that features integrated compactness, high efficiency, and flexible scalability. It consists of basic units, including a power unit (with an on-board charger, junction box, inverter and DC-to-DC converter) and a motor unit. The platform will help reduce costs and lead time for vehicle development by scaling up or down the combination of basic units in accordance with vehicle specifications such as size, speed and torque.
Panasonic has developed and delivered a wide range of components – including batteries, on-board chargers, film capacitors, DC-to-DC converters and relays – specifically for EVs, plug-in hybrids, and hybrid EVs. Panasonic will continue to contribute to the global growth in EVs through system development that makes use of the strengths of our devices.”
In the case of full-size cars, Panasonic is most known for its battery cells supplied to Tesla. The partnership was recently expanded to include solar cells.
Panasonic feels pretty independent from Tesla, stressing that it has its own battery factory “inside” the Tesla Gigafactory, however the cells were “jointly designed and engineered”.
Annual production of 35 GWh is expected in 2019.
Production of New Battery Cells for Tesla’s “Model 3”
Panasonic’s lithium-ion battery factory within Tesla’s Gigafactory handles production of 2170-size*1 cylindrical battery cells for Tesla’s energy storage system and its new “Model 3” sedan, which began production in July 2017. The high performance cylindrical “2170 cell” was jointly designed and engineered by Tesla and Panasonic to offer the best performance at the lowest production cost in an optimal form factor for both electric vehicles (EVs) and energy products. Panasonic and Tesla are conducting phased investment in the Gigafactory, which will have 35 GWh*/year production capacity of lithium-ion battery cells, more than was produced worldwide in 2013. Panasonic is estimating that global production volume for electric vehicles in fiscal 2026 will see an approximately six-fold increase from fiscal 2017 to over 3 million units. The Company will contribute to the realization of a sustainable energy society through the provision of electric vehicle batteries.
In regards to solar cells, Panasonic expects 1 GW output at the Tesla Gigafactory 2 in Buffalo, New York in 2019.
The solar cells are used both in conventional modules, as well as in Tesla Solar Roof tiles.
Strengthening Collaboration with Tesla
In addition to the collaboration with Tesla in the lithium-ion battery business (for details, refer to pages 5-6), Panasonic also collaborates with the company in the solar cell business and will begin production of solar cells this summer at its Buffalo, New York, factory. Solar cells produced at this factory are supplied to Tesla. In addition, the solar cells are used in roof tiles sold by Tesla, a product that integrates solar cells with roofing materials.Panasonic will continue its investment in the factory going forward and plans to raise solar cell production capacity to 1 GW by 2019.
Over the past half-century, scientists have shaved silicon films down to just a wisp of atoms in pursuit of smaller, faster electronics. For the next set of breakthroughs, though, they’ll need novel ways to build even tinier and more powerful devices.
A study led by UChicago researchers, published Sept. 20 in Nature, describes an innovative method to make stacks of semiconductors just a few atoms thick. The technique offers scientists and engineers a simple, cost-effective method to make thin, uniform layers of these materials, which could expand capabilities for devices from solar cells to cell phones.
Stacking thin layers of materials offers a range of possibilities for making electronic devices with unique properties. But manufacturing such films is a delicate process, with little room for error.
“The scale of the problem we’re looking at is, imagine trying to lay down a flat sheet of plastic wrap the size of Chicago without getting any air bubbles in it,” said Jiwoong Park, a UChicago professor with the Department of Chemistry, the Institute for Molecular Engineering and the James Franck Institute, who led the study. “When the material itself is just atoms thick, every little stray atom is a problem.”
Today, these layers are “grown” instead of stacking them on top of one another. But that means the bottom layers have to be subjected to harsh growth conditions such as high temperatures while the new ones are added—a process that limits the materials with which to make them.
Park’s team instead made the films individually. Then they put them into a vacuum, peeled them off and stuck them to one another, like Post-It notes. This allowed the scientists to make films that were connected with weak bonds instead of stronger covalent bonds—interfering less with the perfect surfaces between the layers.
“The films, vertically controlled at the atomic-level, are exceptionally high-quality over entire wafers,” said Kibum Kang, a postdoctoral associate who was the first author of the study.
Kan-Heng Lee, a graduate student and co-first author of the study, then tested the films’ electrical properties by making them into devices and showed that their functions can be designed on the atomic scale, which could allow them to serve as the essential ingredient for future computer chips.
The method opens up a myriad of possibilities for such films. They can be made on top of water or plastics; they can be made to detach by dipping them into water; and they can be carved or patterned with an ion beam. Researchers are exploring the full range of what can be done with the method, which they said is simple and cost-effective.
“We expect this new method to accelerate the discovery of novel materials, as well as enabling large-scale manufacturing,” Park said.
A medium-sized commercial weed grow with around 50 lights stands to save about $13,500 in electricity costs a year with the use of two Tesla Batteries. Those will also protect the plants in case of power outages while making the operation less visible to law enforcement. Elon Musk just made growing weed easier.
Unveiled last night, the Tesla Battery gives home owners and businesses an easy, slick, affordable way to store electricity at home. The 10kWh battery costs just $3,500 and can be “stacked” in sets of up to nine units. Larger capacity batteries of infinitely-scaleable capacity will be available to large businesses and governments. There’s three general use cases for the battery:
Storing electricity purchased during cheaper, Off-peak hours for use during high-demand periods;
Storing electricity generated by solar power or other renewable sources for use around the clock; and
As a backup power source for when the grid goes down.
Know who uses an awful lot of electricity? Weed growers. We just called one and put him on the phone with a commercial energy use management expert to figure out how the Tesla Battery will benefit his home operation and others like it.
Our friend’s operation is small, but profitable. With eight to ten grow lights running 16-20 hours a day in his garage, as well as air-conditioning during hotter parts of the year, his monthly electricity bill is around $2,100, including his home use.
As a domestic consumer of electricity, he’s currently purchasing flat-rate power. In that current arrangement, the Tesla Battery would not save him money day-to-day. Where it would help would be during a power outage, where it would enable him to keep at least some of his lights on, part of the time. In total, those lights alone are using up to 250kWh of power a day, so even two 10kWh batteries could only keep some of the lights on part time.
But, that could be enough to prevent a large financial loss. “The plants start to get angry after about 72 hours without power,” the grower explains. “They won’t die, but the plants in veg will think it’s time to flower and switch over.”
In the lifecycle of a marijuana plant, the vegetative state is where the plants are growing. Depending on the individual plants and the method with which they’re being grown, this stage can last from two weeks to two months. Premature flowering would lead to smaller plants producing fewer, smaller buds and therefore a smaller crop.
The point in the plant lifecycle at which a power outage occurs, its duration and the amount of marijuana being grown will combine to determine the financial loss, but it’s safe to say that the Tesla Battery could throw growers a lifeline during extreme weather or natural disasters.
We’ve all heard stories about growers being outed by the energy intensive nature of their work. Roofs over grow rooms free of snow during winters or insanely high electricity bills have all, in those stories at least, tipped off the cops.
“It doesn’t work that way,” the grower explains. “The cops have to present a warrant to the electricity company to get your bill and, for that, they need probable cause. No, the electricity companies don’t always demand that warrant, but generally, this isn’t how it works. They’re not going through every power bill, looking for suspiciously high ones.”
One of the other touted benefits of the Battery is its ability to facilitate off-grid living. By hooking it up to solar panels, the Battery can store energy during the day, then keep your house powered throughout the night. Or your off-grid grow, maybe?
“I haven’t seen any solar-powered indoor grows yet,” says our guy. “I suspect the costs of the panels are still way too high.”
He’s right. The most powerful solar panel kit currently available at Home Depot costs $12,388 and produces only 3,800 to 8,900kWh a year. Best case scenario, that yearly total is only enough to power our buddy’s 8-10 lights for a little over a month. Look at it from a cost perspective and 10 times the price of his monthly electricity bill (lights only) nets him about 1/10th the power. And that’s before buying any batteries, Tesla or otherwise.
At this point, the real savings possible with the Tesla Battery come with scale. But not that much more.
Our commercial energy consumption management expert sat down and ran the numbers assuming a medium-sized, 50-light commercial operation running its A/C during the day. These numbers are based on commercial electricity rates here in California, where the company is paying a premium during high-demand hours.
With two 10kWh Tesla Batteries giving this commercial grow the ability to shift some of its load to off-peak hours, savings in demand charges alone would total $8,000 a year, while use charges would lower by $5,500, for a total savings of $13,500.
Of course, even just at 50 lights, we’re talking about a multi-million dollar operation, making this sound like relative chump change. Worthwhile — the batteries would be paid for in just over 6 months of savings — but hardly revolutionary.
“Where these batteries might start to make sense for small growers is when LEDs are optimized for herb,” says our grower. He’s skeptical of the light quality produced by current LED grow lights, but sees that technology being optimized for marijuana in the near future. When it is, it could drastically lower the energy consumption of growing, reducing electricity used by the lights alone by 60 percent or more. Lower outright energy consumption will reduce the cost of growing, of course, but it also shifts the amount of consumption into a range that could be more easily handled by Tesla Batteries.
Given the current pace of marijuana legalization, the need for clandestine home grows may largely be eliminated by the time dipping energy consumption and increasing battery capacity meet in a home solar power sweet zone, but as a massive electricity consumer, it does look like the marjiuana industry is going to profit from the same Tesla Battery benefits everyone else will — reduced peak demand and increased stability during outages.
Are there ‘soon to be coming to market – more energy dense batteries’ available?
Prof. Karen Gleason has come up with a low-cost, environmentally friendly way to make solar cells on ordinary tracing paper. Photo: Len RubensteinSpring 2012
Solar Cells on Paper
Chemical engineer Karen K. Gleason would like to paper the world with solar cells. Glued to laptops, tacked onto roof tiles, tucked into pockets, lining window shades, she envisions ultrathin, ultra-flexible solar cells going where no solar cells have gone before.
Silvery blue solar cells seem to magically generate electricity from sunlight the way Rumpelstiltskin spun straw into gold but in their present form, they’re more akin to gold than straw.
Karen Gleason develops a low-cost, environmentally friendly way to make solar cells on tracing paper, which one day might charge a cell phone.
The cost of manufacturing crystalline and thin-film solar cells with silicon, glass, and rare earth materials like tellurium and indium is high.
Gleason, the Alexander and I. Michael Kasser Professor of Chemical Engineering, has collaborated with Vladimir Bulovic, professor of electrical engineering; former chemical engineering graduate student Miles C. Barr; and others to come up with a low-cost, environmentally friendly way to make practically indestructible solar cells on ordinary tracing paper.
One day, a paper solar cell might help us charge a cell phone. “A paper substrate is a thousand times cheaper than silicon and glass. What’s more, these solar cells can be scrunched up, folded a thousand times, and weatherproofed,” she says.
Using abundant, inexpensive organic elements like carbon, oxygen, and copper — “nothing exotic,” she says — in a vacuum chamber, layers are “printed” through a process called vapor deposition, similar to frost forming on a window. At less than 120 degrees Celsius, the method is gentler and cooler than that normally used to manufacture photovoltaic materials, allowing it to be used on delicate paper, cloth, or plastic. “We repeat that five times and you end up with a solar cell,” she says; tweaking the composition of the five layers of materials determines the cells’ energy output.
The research is funded by MITEI founding member Eni SpA, Italy’s biggest energy company, which is pursuing new advances in biofuels, solar, and other forms of alternative energy.
“The challenge of the project appealed to me,” she says. “I also thought it would be fun.” Her students display a prototype solar cell (a sheet of paper embossed with a pinstripe and chain-link design) folded into a paper airplane as a power source for an LCD clock.
Gleason would like to see the first commercial solar paper devices hit the market in five years, but first the cells’ efficiency has to be ramped up from nearly 4 percent to at least 10 percent. (Commercial solar cells have an efficiency of around 15 percent.) MIT engineers believe this is doable.
Then, the sky’s the limit — solar cells could power iPads, generate lighting inside tents, keep ski clothing toasty.
“The paper cells’ portability could have a big impact in developing countries, where the cost of transporting solar cells has been prohibitive.
“Rather than confining solar power to rooftops or solar farms, paper photovoltaics can be used virtually anywhere, making energy ubiquitous,” Gleason says.
Using sunlight to drive chemical reactions, such as artificial photosynthesis, could soon become much more efficient thanks to nanomaterials.
This is the conclusion of a study published today led by researchers in the Department of Physics at Imperial College London, which could ultimately help improve solar energy technologies and be used for new applications, such as using sunlight to break down harmful chemicals.
Sunlight is used to drive many chemical processes that would not otherwise occur. For example, carbon dioxide and water do not ordinarily react, but in the process of photosynthesis, plants take these two chemicals and, using sunlight, produce oxygen and sugar.
The efficiency of this reaction is very high, meaning much of the energy from sunlight is transferred to the chemical reaction, but so far scientists have been unable to mimic this process in manmade artificial devices.
One reason is that many molecules that can undergo chemical reactions with light do not efficiently absorb the light themselves. They rely on photocatalysts – materials that absorb light efficiently and then pass the energy on to the molecules to drive reactions.
In the new study, researchers have investigated an artificial photocatalyst material using nanoparticles and found out how to make it more efficient.
This could lead to better solar panels, as the energy from the Sun could be more efficiently harvested. The photocatalyst could also be used to destroy liquid or gas pollutants, such as pesticides in water, by harnessing sunlight to drive reactions that break down the chemicals into less harmful forms.
Lead author Dr Emiliano Cortés from the Department of Physics at Imperial, said: “This finding opens new opportunities for increasing the efficiency of using and storing sunlight in various technologies.
“By using these materials we can revolutionize our current capabilities for storing and using sunlight with important implications in energy conversion, as well as new uses such as destroying pollutant molecules or gases and water cleaning, among others.”
The material that the team investigated is made of metal nanoparticles – particles only billionths of a metre in diameter. Their results are published today in the Journal Nature Communications.
The team, which included researchers from the Chemistry Department at University of Duisburg-Essen in Germany led by Professor Sebastian Schlücker and theoreticians from the Rensselaer Polytechnic Institute and Harvard University at the US, showed that light-induced chemical reactions occur in certain regions over the surface of these nanomaterials.
They identified which areas of the nanomaterial would be most suitable for transferring energy to chemical reactions, by tracking the locations of very small gold nanoparticles (used as a markers) on the surface of the silver nanocatalytic material.
Now that they know which regions are responsible for the process of harvesting light and transferring it to chemical reactions, the team hope to be able to engineer the nanomaterial to increase these areas and make it more efficient.
Lead researcher Professor Stefan Maier said: “This is a powerful demonstration of how metallic nanostructures, which we have investigated in my group at Imperial for the last 10 years, continue to surprise us in their abilities to control light on the nanoscale.
“The new finding uncovered by Dr Cortés and his collaborators in Germany and the US opens up new possibilities for this field in the areas photocatalysis and nanochemistry.”