Most of the world’s electricity-producing power plants — whether powered by coal, natural gas, or nuclear fission — make electricity by generating steam that turns a turbine. That steam then is condensed back to water, and the cycle begins again. But the condensers that collect the steam are quite inefficient, and improving them could make a big difference in overall power plant efficiency. Now, a team of researchers at MIT has developed a way of coating these condenser surfaces with a layer of graphene, just one atom thick, and found that this can improve the rate of heat transfer by a factor of four — and potentially even more than that, with further work. And unlike polymer coatings, the graphene coatings have proven to be highly durable in laboratory tests. 
An uncoated copper condenser tube (top left) is shown next to a similar tube coated with graphene (top right). When exposed to water vapor at 100 degrees Celsius, the uncoated tube produces an inefficient water film (bottom left), while the coated shows the more desirable dropwise condensation (bottom right). (Image courtesy of the researchers) The findings are reported in the journal ("Scalable Graphene Coatings for Enhanced Condensation Heat Transfer") by MIT graduate student Daniel Preston, professors Evelyn Wang and Jing Kong, and two others. The improvement in condenser heat transfer, which is just one step in the power-production cycle, could lead to an overall improvement in power plant efficiency of 2 to 3 percent based on figures from the Electric Power Research Institute, Preston says — enough to make a significant dent in global carbon emissions, since such plants represent the vast majority of the world’s electricity generation. “That translates into millions of dollars per power plant per year,” he explains. There are two basic ways in which the condensers — which may take the form of coiled metal tubes, often made of copper — interact with the flow of steam. In some cases, the steam condenses to form a thin sheet of water that coats the surface; in others it forms water droplets that are pulled from the surface by gravity. When the steam forms a film, Preston explains, that impedes heat transfer — and thus reduces the efficiency — of condensation. So the goal of much research has been to enhance droplet formation on these surfaces by making them water-repelling. Often this has been accomplished using polymer coatings, but these tend to degrade rapidly in the high heat and humidity of a power plant. And when the coatings are made thicker to reduce that degradation, the coatings themselves impede heat transfer. “We thought graphene could be useful,” Preston says, “since we know it is hydrophobic by nature.” So he and his colleagues decided to test both graphene’s ability to shed water, and its durability, under typical power plant conditions — an environment of pure water vapor at 100 degrees Celsius. They found that the single-atom-thick coating of graphene did indeed improve heat transfer fourfold compared with surfaces where the condensate forms sheets of water, such as bare metals. Further calculations showed that optimizing temperature differences could boost this improvement to 5 to 7 times. The researchers also showed that after two full weeks under such conditions, there was no measurable degradation in the graphene’s performance. By comparison, similar tests using a common water-repelling coating showed that the coating began to degrade within just three hours, Preston says, and failed completely within 12 hours. Because the process used to coat the graphene on the copper surface — called chemical vapor deposition — has been tested extensively, the new method could be ready for testing under real-world conditions “in as little as a year,” Preston says. And the process should be easily scalable to power plant-sized condenser coils. “This work is extremely significant because, to my knowledge, it is the first report of durable dropwise condensation with a single-layer surface coating,” says Jonathan Boreyko, an assistant professor of biomedical engineering and mechanics at Virginia Tech who has studied condensation on superhydrophobic surface. “These findings are somewhat surprising and very exciting.” Boreyko, who was not involved in the research, adds that this method, if proven through further testing, “could significantly improve the efficiency of power plants and other systems that utilize condensers.”
read more "Graphene coating on condensers could make power plants more efficient"
An uncoated copper condenser tube (top left) is shown next to a similar tube coated with graphene (top right). When exposed to water vapor at 100 degrees Celsius, the uncoated tube produces an inefficient water film (bottom left), while the coated shows the more desirable dropwise condensation (bottom right). (Image courtesy of the researchers) The findings are reported in the journal ("Scalable Graphene Coatings for Enhanced Condensation Heat Transfer") by MIT graduate student Daniel Preston, professors Evelyn Wang and Jing Kong, and two others. The improvement in condenser heat transfer, which is just one step in the power-production cycle, could lead to an overall improvement in power plant efficiency of 2 to 3 percent based on figures from the Electric Power Research Institute, Preston says — enough to make a significant dent in global carbon emissions, since such plants represent the vast majority of the world’s electricity generation. “That translates into millions of dollars per power plant per year,” he explains. There are two basic ways in which the condensers — which may take the form of coiled metal tubes, often made of copper — interact with the flow of steam. In some cases, the steam condenses to form a thin sheet of water that coats the surface; in others it forms water droplets that are pulled from the surface by gravity. When the steam forms a film, Preston explains, that impedes heat transfer — and thus reduces the efficiency — of condensation. So the goal of much research has been to enhance droplet formation on these surfaces by making them water-repelling. Often this has been accomplished using polymer coatings, but these tend to degrade rapidly in the high heat and humidity of a power plant. And when the coatings are made thicker to reduce that degradation, the coatings themselves impede heat transfer. “We thought graphene could be useful,” Preston says, “since we know it is hydrophobic by nature.” So he and his colleagues decided to test both graphene’s ability to shed water, and its durability, under typical power plant conditions — an environment of pure water vapor at 100 degrees Celsius. They found that the single-atom-thick coating of graphene did indeed improve heat transfer fourfold compared with surfaces where the condensate forms sheets of water, such as bare metals. Further calculations showed that optimizing temperature differences could boost this improvement to 5 to 7 times. The researchers also showed that after two full weeks under such conditions, there was no measurable degradation in the graphene’s performance. By comparison, similar tests using a common water-repelling coating showed that the coating began to degrade within just three hours, Preston says, and failed completely within 12 hours. Because the process used to coat the graphene on the copper surface — called chemical vapor deposition — has been tested extensively, the new method could be ready for testing under real-world conditions “in as little as a year,” Preston says. And the process should be easily scalable to power plant-sized condenser coils. “This work is extremely significant because, to my knowledge, it is the first report of durable dropwise condensation with a single-layer surface coating,” says Jonathan Boreyko, an assistant professor of biomedical engineering and mechanics at Virginia Tech who has studied condensation on superhydrophobic surface. “These findings are somewhat surprising and very exciting.” Boreyko, who was not involved in the research, adds that this method, if proven through further testing, “could significantly improve the efficiency of power plants and other systems that utilize condensers.”
Garment-based printable electrodes developed in the lab of Joseph Wang, distinguished professor of nanoengineering at UC San Diego, and lead principal investigator of ATTACH. (Image: Jacobs School of Engineering/UC San Diego) The smart fabric will be designed to regulate the temperature of the wearer’s skin—keeping it at 93° F—by adapting to temperature changes in the room. When the room gets cooler, the fabric will become thicker. When the room gets hotter, the fabric will become thinner. To accomplish this feat, the researchers will insert polymers that expand in the cold and shrink in the heat inside the smart fabric. “Regardless if the surrounding temperature increases or decreases, the user will still feel the same without having to adjust the thermostat,” said Wang. “93° F is the average comfortable skin temperature for most people,” added Renkun Chen, assistant professor of mechanical and aerospace engineering at UC San Diego, and one of the collaborators on this project. Chen’s contribution to ATTACH is to develop supplemental heating and cooling devices, called thermoelectrics, that are printable and will be incorporated into specific spots of the smart fabric. The thermoelectrics will regulate the temperature on “hot spots”—such as areas on the back and underneath the feet—that tend to get hotter than other parts of the body when a person is active. “This is like a personalized air-conditioner and heater,” said Chen. Saving energy “With the smart fabric, you won’t need to heat the room as much in the winter, and you won’t need to cool the room down as much in the summer. That means less energy is consumed. Plus, you will still feel comfortable within a wider temperature range,” said Chen. The researchers are also designing the smart fabric to power itself. The fabric will include rechargeable batteries, which will power the thermoelectrics, as well as biofuel cells that can harvest electrical power from human sweat. Plus, all of these parts—batteries, thermoelectrics and biofuel cells—will be printed using the technology developed in Wang’s lab to make printable wearable devices. These parts will also be thin, stretchable and flexible to ensure that the smart fabric is not bulky or heavy. “We are aiming to make the smart clothing look and feel as much like the clothes that people regularly wear. It will be washable, stretchable, bendable and lightweight. We also hope to make it look attractive and fashionable to wear,” said Wang. In terms of price, the team has not yet concluded how much the smart clothing will cost. This will depend on the scale of production, but the printing technology in Wang’s lab will offer a low-cost method to produce the parts. Keeping the costs down is a major goal, the researchers said. The research team Wang, the lead principal investigator of ATTACH, has pioneered the development of wearable printable devices, such as electrochemical sensors and temporary tattoo-based biofuel cells. He is the chair of the nanoengineering department and the director for the Center for Wearable Sensors at UC San Diego. His extensive expertise in printable, stretchable and wearable devices will be used here to make the proposed flexible biofuel cells, batteries and thermoelectrics. Chen specializes in heat transfer and thermoelectrics. His research group works on physics, materials and devices related to thermal energy transport, conversion and management. His specialty in these areas will be used to develop the thermal models and the thermoelectric devices. Meng’s research focuses on energy storage and conversion, particularly on battery cell design and testing. At UC San Diego, she established the Laboratory for Energy Storage and Conversion and is the inaugural director for the Sustainable Power and Energy Center. Meng will develop the rechargeable batteries and will work on power integration throughout the smart fabric system. Jin specializes in functional materials for applications in nanotechnology, magnetism, energy and biomedicine. He will design the self-responsive polymers that change in thickness based on changes in the surrounding temperature. Windmiller, former Ph.D. student and postdoc in Wang’s nanoengineering lab, is an expert in printed biosensors, bioelectronics and biofuel cells. He co-founded Electrozyme LLC, a startup devoted to the development of novel biosensors for application in the personal wellness and healthcare domains. Electrozyme will serve as the industrial partner for ATTACH and will lead the efforts to test the smart fabric prototype and bring the technology into the market.
Reconstruction of the focused X-ray wave. The lens achieves the spot size of 8 nanometres at the smallest waist. (Image: Saša Bajt/DESY) "X-rays are used to study the nano world, as they are able to show much finer details than visible light and their penetrating power allows you to see inside objects,” explains Bajt. The size of the smallest details that can be resolved depends on the wavelength of the radiation used. X-rays have very short wavelengths of only about 1 to 0.01 nanometres (nm), compared to 400 to 800 nm for visible light. A nanometre is a millionth of a millimetre. The high penetration of X-rays is favoured for three-dimensional tomographic imaging of objects such as biological cells, computer chips, and the nanomaterials involved in energy conversion or storage. But this also means that the X-rays pass straight through conventional lenses without being bent or focussed. One possible method to focus X-rays is to merely graze them from the surface of a mirror to nudge them towards a new direction. But such X-ray mirrors are limited in their convergence power and must be mechanically polished to high precision, making them extremely expensive. An alternative means to bend X-rays is to use crystals. A crystal lattice diffracts X-rays, as the German physicist Max von Laue discovered a century ago. Today, artificial crystals can be tailor-made to sharply focus X-rays by depositing different materials layer by layer. From this building block comes the multilayer Laue lens or MLL, made by coating a substrate with thin layers of the chosen substances. “However, conventional Laue lenses are limited in their converging power for geometric reasons,” explains Bajt. “To gain the maximum power, the layers of a MLL need to be slightly tilted against each other.” As theoretical calculations have shown, all layers of such a “wedged” MLL must lie perpendicular on a circle with a radius of twice the focal length. This rather specific condition could not be fabricated — until now. Bajt’s team invented a new production process, where a mask partially shields the substrate from the depositing material. In the half-shade of the mask a wedged structure builds up, and the tilt of the layers is controlled simply by adjusting the spacing of the mask to the substrate. The wedged MLL is then cut from the penumbra region. "Before us, no one came close to building such a wedged lens", says Bajt. 
Scanning electron microscope image of the novel X-ray lens. The bright area is built from 5500 alternating layers of tungsten and silicon carbide. The dark area is the substrate. The width of the lens is 40 micrometres. (Image: Saša Bajt/DESY) The researchers manufactured a wedged lens from 5500 alternating layers of silicon carbide (SiC) and tungsten (W), varying in thickness. The final lens cut from these deposits was 40 micrometres (millionths of a metre) wide, 17.5 micrometres thick and 6.5 micrometres deep. The team tested their novel lens at DESY's ultra brilliant X-ray source PETRA III. The test at the experimental station P11 showed that the lens produced a focus just 8 nm wide, which is close to the design value of 6 nm. The tests also showed that the intensity profile across the lens is very uniform, a prerequisite for high quality images. The lens design allows to transmit up to 60 per cent of the incoming X-rays to the sample. The scientists focussed the X-ray beam in just one direction, resulting in a thin line. Focussing in two dimensions to obtain a small spot can be done by simply using two lenses in line, one focusing horizontally and the other vertically. “Our results prove out our fabrication technique to achieve lenses of high focussing power. We believe we have the requisite control to achieve even higher power lenses,” elaborates Bajt. “It appears that the long-sought goal of focusing X-rays to a nanometre is in reach.” This will put X-ray imaging on par with the quality achieved with scanning electron microscopes, that typically have a resolution of 4 nm. The advantage is that X-ray imaging is not limited to viewing surfaces or extremely thin samples only, but can penetrate a sample. “Our novel lens concept will help scientists to peer deeper into the nanocosm and make previously inaccessible details visible,” says Bajt.