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### Topics - Md. Khalid Hasan

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1
##### Electrical Machine / Starting Of Slip-Ring Motors
« on: April 10, 2019, 04:08:10 PM »
Slip-ring motors are started with full line voltage, as external resistance can be easily added in the rotor circuit with the help of slip-rings. A star connected rheostat is connected in series with the rotor via slip-rings as shown in the fig. Introducing resistance in rotor current will decrease the starting current in rotor (and, hence, in stator). Also, it improves power factor and the torque is increased. The connected rheostat may be hand-operated or automatic.
As, introduction of additional resistance in rotor improves the starting torque, slip-ring motors can be started on load.
The external resistance introduced is only for starting purposes, and is gradually cut out as the motor gathers the speed.

2
##### Electrical Machine / Starting Of Squirrel Cage Motors
« on: April 10, 2019, 04:07:40 PM »
Starting in-rush current in squirrel cage motors is controlled by applying reduced voltage to the stator. These methods are sometimes called as reduced voltage methods for starting of squirrel cage induction motors. For this purpose, following methods are used:
By using primary resistors
Autotransformer
Star-delta switches
1. Using Primary Resistors:
primary resistors starting of induction motor
Obviously, the purpose of primary resistors is to drop some voltage and apply a reduced voltage to the stator. Consider, the starting voltage is reduced by 50%. Then according to the Ohm's law (V=I/Z), the starting current will also be reduced by the same percentage. From the torque equation of a three phase induction motor, the starting torque is approximately proportional to the square of the applied voltage. That means, if the applied voltage is 50% of the rated value, the starting torque will be only 25% of its normal voltage value. This method is generally used for a smooth starting of small induction motors. It is not recommended to use primary resistors type of starting method for motors with high starting torque requirements.
Resistors are generally selected so that 70% of the rated voltage can be applied to the motor. At the time of starting, full resistance is connected in the series with the stator winding and it is gradually decreased as the motor speeds up. When the motor reaches an appropriate speed, the resistances are disconnected from the circuit and the stator phases are directly connected to the supply lines.
2. Auto-Transformers:
auto transformer starting of induction motors
Auto-transformers are also known as auto-starters. They can be used for both star connected or delta connected squirrel cage motors. It is basically a three phase step down transformer with different taps provided that permit the user to start the motor at, say, 50%, 65% or 80% of line voltage. With auto-transformer starting, the current drawn from supply line is always less than the motor current by an amount equal to the transformation ratio. For example, when a motor is started on a 65% tap, the applied voltage to the motor will be 65% of the line voltage and the applied current will be 65% of the line voltage starting value, while the line current will be 65% of 65% (i.e. 42%) of the line voltage starting value. This difference between the line current and the motor current is due to transformer action. The internal connections of an auto-starter are as shown in the figure. At starting, switch is at "start" position, and a reduced voltage (which is selected using a tap) is applied across the stator. When the motor gathers an appropriate speed, say upto 80% of its rated speed, the auto-transformer automatically gets disconnected from the circuit as the switch goes to "run" position.
The switch changing the connection from start to run position may be air-break (small motors) or oil-immersed (large motors) type. There are also provisions for no-voltage and overload, with time delay circuits on an autostarter.

3
##### Electrical Machine / Direct-On-Line (DOL) Starters for Three Phase Induction Motors
« on: April 10, 2019, 04:06:45 PM »
Small three phase induction motors can be started direct-on-line, which means that the rated supply is directly applied to the motor. But, as mentioned above, here, the starting current would be very large, usually 5 to 7 times the rated current. The starting torque is likely to be 1.5 to 2.5 times the full load torque. Induction motors can be started directly on-line using a DOL starter which generally consists of a contactor and a motor protection equipment such as a circuit breaker. A DOL starter consists of a coil operated contactor which can be controlled by start and stop push buttons. When the start push button is pressed, the contactor gets energized and it closes all the three phases of the motor to the supply phases at a time. The stop push button de-energizes the contactor and disconnects all the three phases to stop the motor.
In order to avoid excessive voltage drop in the supply line due to large starting current, a DOL starter is generally used for motors that are rated below 5kW.

4
##### Electrical Machine / Working Principle And Types Of An Induction Motor
« on: April 10, 2019, 04:01:45 PM »
Induction Motors are the most commonly used motors in many applications. These are also called as Asynchronous Motors, because an induction motor always runs at a speed lower than synchronous speed. Synchronous speed means the speed of the rotating magnetic field in the stator.

There basically 2 types of induction motor depending upon the type of input supply - (i) Single phase induction motor and (ii) Three phase induction motor.

Or they can be divided according to type of rotor - (i) Squirrel cage motor and (ii) Slip ring motor or wound type

5
##### Electrical Machine / Difference Between Synchronous Motor And Induction Motor
« on: April 10, 2019, 04:01:04 PM »
Difference In Working
Synchronous motor: Stator poles rotate at the synchronous speed (Ns) when fed with a three phase supply. The rotor is fed with a DC supply. The rotor needs to be rotated at a speed near to the synchronous speed during starting. If done so, the rotor poles get magnetically coupled with the rotating stator poles, and thus the rotor starts rotating at the synchronous speed.
Synchronous motor always runs at a speed equal to its synchronous speed.
i.e. Actual speed = Synchronous speed
or  N = Ns = 120f/P
Induction motor: When the stator is fed with two or three phase AC supply, a Rotating Magnetic Field (RMF) is produced. The relative speed between stator's rotating magnetic field and the rotor will cause an induced current in the rotor conductors. The rotor current gives rise to the rotor flux. According to Lenz's law, the direction of this induced current is such that it will tend to oppose the cause of its production, i.e. relative speed between stator's RMF and the rotor. Thus, the rotor will try to catch up with the RMF and reduce the relative speed.
Induction motor always runs at a speed which is less than the synchronous speed.
i.e. N < Ns

6
##### Review and Opinion / বৃষ্টির কারণে ঢাকায় বাতাসের মানোয়ন্নন
« on: April 10, 2019, 03:04:47 PM »
ঝড়-বৃষ্টির কারণে রাজধানী ঢাকায় বাতাসের মান উন্নত হয়েছে। এয়ার কোয়ালিটি ইনডেক্সে (একিউআই) মঙ্গলবার সন্ধ্যা ৭টা ২ মিনিটে ২২তম অবস্থানে উঠে এসেছে ঢাকা। আগে এর স্কোর ছিল ৮২, যা নির্দেশ করে বাতাসের গুণগত মান মাঝারি মানের।

মঙ্গলবার সন্ধ্যায় রাজধানীতে কালবৈশাখী ঝড় আঘাত হানে। এ সময় বাতাসের তীব্রতা ছিল ঘণ্টায় ৮৫ কিলোমিটার। সন্ধ্যা ৬টা থেকে পরবর্তী এক ঘণ্টায় ১১ মিলিমিটার বৃষ্টিপাত রেকর্ড করেছে আবহাওয়া অধিদপ্তর।

এদিকে আজ সকাল ৮টায় এয়ার কোয়ালিটি ইনডেক্সে (একিউআই) ঢাকার স্কোর ৮০ তে অবস্থান করলেও ৯টায় তা ৮৩তে দাঁড়ায়।

অন্যদিকে ১৫৩ স্কোর পেয়ে দূষিত বাতাসের শহরগুলোর তালিকায় প্রথমে রয়েছে দিল্লি। এর পরে আছে ব্রাসেলস ও চিয়াঙ মাই।

উল্লেখ্য, প্রতিদিনের বাতাসের মান নিয়ে তৈরি করা একিউআই সূচক একটি নির্দিষ্ট শহরের বাতাস কতটুকু নির্মল বা দূষিত সে সম্পর্কে মানুষকে তথ্য দেয়। এছাড়া বাতাস কোন ধরনের স্বাস্থ্য ঝুঁকি তৈরি হতে পারে কিনা তা জানায়।

ঢাকা দীর্ঘদিন ধরেই দূষিত বাতাস নিয়ে হিমশিম খাচ্ছে। গ্রীষ্মকালে শহরটিতে বায়ু দূষণ চরমে উঠলেও বর্ষায় অবস্থার উন্নতি দেখা যায়।

7
##### Review and Opinion / ২০০ উপজেলায় স্বয়ংক্রিয় আবহাওয়া স্টেশন স্থাপন করবে সরকার
« on: April 10, 2019, 02:58:02 PM »
আবহাওয়া সম্পর্কে নির্ভুল পূর্বাভাস পেতে ২০০ উপজেলায় স্বয়ংক্রিয় আবহাওয়া স্টেশন স্থাপনে স্থানীয় সরকার বিভাগের সঙ্গে চুক্তি করেছে প্রতিরক্ষা মন্ত্রণালয়।

গতকাল মঙ্গলবার সচিবালয়ে স্থানীয় সরকার বিভাগের সম্মেলনকক্ষে এই চুক্তি স্বাক্ষর হয়।

উপজেলা পরিষদের ভূমি ব্যবহার করে কৃষিভিত্তিক স্বয়ংক্রিয় আবহাওয়া পর্যবেক্ষণাগার স্থাপনের জন্য প্রয়োজন হবে মাত্র ২২০ বর্গফুট জায়গা।

স্থানীয় সরকার বিভাগের সিনিয়র সচিব এস এম গোলাম ফারুক এবং প্রতিরক্ষা মন্ত্রণালয়ের সচিব আখতার হোসেন ভূঁইয়া চুক্তিতে স্বাক্ষর করেন।

অনুষ্ঠানের প্রধান অতিথি স্থানীয় সরকার, পল্লী উন্নয়ন ও সমবায়মন্ত্রী মো. তাজুল ইসলাম বলেন, অঞ্চলভিত্তিক আবহাওয়ার পূর্বাভাস প্রাপ্তি অর্থনৈতিক কর্মকাণ্ডকে বেগবান করবে। বিশেষত কৃষিক্ষেত্রে আবহাওয়ার পূর্বাভাস কাজে লাগিয়ে লাভবান হওয়া যাবে।

অনুষ্ঠানে আরও উপস্থিত ছিলেন স্থানীয় সরকার প্রতিমন্ত্রী স্বপন ভট্টাচার্যসহ সংশ্লিষ্ট মন্ত্রণালয় ও অধিদপ্তরের ঊর্ধ্বতন কর্মকর্তারা।

8
##### Review and Opinion / Japanese energy policy needs clarity, says business group
« on: April 10, 2019, 02:53:30 PM »
In a report released today entitled Rebuild The Power System That Supports Japan, Keidanren underlined the importance of energy policy to the resource-poor country.

Energy is "the economic backbone of the nation" and so discussions on the country's energy future should be held in public, based on long-term perspectives, scientific information and modelling, it said.

Japan is in a "crisis situation" of low investment and in the electricity business it has stagnated, except in certain areas, such as nuclear safety measures, it said.

"Japan stands at a crossroads as to whether it can maintain the foundations of technology and industrial competitiveness built up as a small resource country and a technological nation," it said, adding that investment in R&D, generating reserve and new capacity construction had come to a halt.

In the current situation, it is challenging to maintain the pre-requisites of energy policy -  secure supply, affordability and environmental acceptability, it said. Meanwhile Japan is expecting to evolve into what Keidanren called Society 5.0, whereby social problems can be solved using advanced technology, Japan's population will age and decline, while in common with other places it expects decentralised energy sources supported by digitalisation to become very significant in regional and national supply. A highly decentralised future could see great variety in Japan, given its geography, distribution of renewable sources and dense clusters of population and industry. If it is to be successful, such a system needs planning.

For these reasons the report calls for the government to provide clarity for energy policy after 2030, so that business can set "a timely and proportionate" investment strategy.

9
##### Power System and Renewable Energy / Effects of renewable energy sources on the power system
« on: April 10, 2019, 02:49:53 PM »
Recently, growing electrical energy market and increasing integration of renewable energy sources (RES) in power systems have lead to new challenges on network planning step and operation, thus it is required to investigate and analyze properly the impacts of integrated RES on the power system. In this paper, the electricity transmission network with wind farms (WF) is modeled. For the grid model, a part of Izmir region is chosen due to the amount of installed generation plants based RES especially wind energy in this region. The comparison between unexpected variations to voltage profile of the power system before and after RES integration to the power system is demonstrated. In the modeling of the electricity transmission network with RES, Digsilent/Power Factory is used as software. The different case studies in integration of different amounts of RES are implemented on the developed grid model. As a result of the conducted case studies, effects of RES on existing power system are evaluated and graphics obtained from the simulation are presented. Especially, the voltage profile of power system is examined.

10
##### Power System and Renewable Energy / Renewable Energy in Power Systems
« on: April 10, 2019, 02:48:40 PM »
Renewable Energy (RE) sources differ from conventional sources in that, generally they cannot be scheduled, they are much smaller than conventional power stations and are often connected to the electricity distribution system rather than the transmission system.  The integration of such time variable ‘distributed’ or ‘embedded’ sources into electricity networks requires special consideration.
This new book addresses these special issues and covers the following:

The characteristics of conventional and RE generators with particular reference to the variable nature of RE from wind, solar, small hydro and marine sources over time scales ranging from seconds to months
The power balance and frequency stability in a network with increasing inputs from variable sources and the technical and economic implications of increased penetration from such sources  with special reference to demand side management

11
##### Electrical Machine / Estimation of Additional PV Converter Losses Operating Under PF ≠ 1 Based on Man
« on: March 12, 2019, 02:56:49 PM »
The increasing penetration of photovoltaic (PV) systems in distribution networks often causes overvoltage problems. One solution to address this issue is the provision of reactive power (RP) by the PV converters. This can cause increased power losses on the PV converters leading to additional operational costs. However, the manufacturers of commercially available PV converters provide data regarding the converter losses only under unity power factor (PF). The data are also limited regarding the technical details of the PV converters. This paper presents a methodology to estimate analytically the power losses in two-stage PV converters under an RP provision based on the efficiency curves at PF = 1 for different PV voltages and the limited information given in the PV converter datasheet. The losses are separately estimated for the dc–dc converter and the dc–ac inverter, because the losses on the former are not affected by the RP, while the losses on the latter are. The method is validated with field measurements of PV converter losses under RP provision and with detailed simulations.

12
##### EEE / Fibers that can hear and sing
« on: May 10, 2018, 01:10:48 PM »
For centuries, "man-made fibers" meant the raw stuff of clothes and ropes; in the information age, it's come to mean the filaments of glass that carry data in communications networks. But to Yoel Fink, an associate professor of materials science and principal investigator at MIT's Research Lab of Electronics, the threads used in textiles and even optical fibers are much too passive. For the past decade, his lab has been working to develop fibers with ever more sophisticated properties, to  enable fabrics that can interact with their environment.

In the August issue of Nature Materials, Fink and his collaborators announce a new milestone on the path to functional fibers: fibers that can detect and produce sound. Applications could include clothes that are themselves sensitive microphones, for capturing speech or monitoring bodily functions, and tiny filaments that could measure blood flow in capillaries or pressure in the brain. The paper, whose authors also include Shunji Egusa, a former postdoc in Fink's lab, and current lab members Noémie Chocat and Zheng Wang, appeared on Nature Materials' website on July 11, and the work it describes was supported by MIT's Institute for Soldier Nanotechnologies, the National Science Foundation and the U.S. Defense Department's Defense Advanced Research Projects Agency.

Ordinary optical fibers are made from a "preform," a large cylinder of a single material that is heated up, drawn out, and then cooled. The fibers developed in Fink's lab, by contrast, derive their functionality from the elaborate geometrical arrangement of several different materials, which must survive the heating and drawing process intact.

The right stuff

The heart of the new acoustic fibers is a plastic commonly used in microphones. By playing with the plastic's fluorine content, the researchers were able to ensure that its molecules remain lopsided — with fluorine atoms lined up on one side and hydrogen atoms on the other — even during heating and drawing. The asymmetry of the molecules is what makes the plastic "piezoelectric," meaning that it changes shape when an electric field is applied to it.

In a conventional piezoelectric microphone, the electric field is generated by metal electrodes. But in a fiber microphone, the drawing process would cause metal electrodes to lose their shape. So the researchers instead used a conducting plastic that contains graphite, the material found in pencil lead. When heated, the conducting plastic maintains a higher viscosity — it yields a thicker fluid — than a metal would.

Not only did this prevent the mixing of materials, but, crucially, it also made for fibers with a regular thickness. After the fiber has been drawn, the researchers need to align all the piezoelectric molecules in the same direction. That requires the application of a powerful electric field — 20 times as powerful as the fields that cause lightning during a thunderstorm. Anywhere the fiber is too narrow, the field would generate a tiny lightning bolt, which could destroy the material around it.

Sound results

Despite the delicate balance required by the manufacturing process, the researchers were able to build functioning fibers in the lab. "You can actually hear them, these fibers," says Chocat, a graduate student in the materials science department. "If you connected them to a power supply and applied a sinusoidal current" — an alternating current whose period is very regular — "then it would vibrate. And if you make it vibrate at audible frequencies and put it close to your ear, you could actually hear different notes or sounds coming out of it." For their Nature Materials paper, however, the researchers measured the fiber's acoustic properties more rigorously. Since water conducts sound better than air, they placed it in a water tank opposite a standard acoustic transducer, a device that could alternately emit sound waves detected by the fiber and detect sound waves emitted by the fiber.

In addition to wearable microphones and biological sensors, applications of the fibers could include loose nets that monitor the flow of water in the ocean and large-area sonar imaging systems with much higher resolutions: A fabric woven from acoustic fibers would provide the equivalent of millions of tiny acoustic sensors.

Zheng, a research scientist in Fink's lab, also points out that the same mechanism that allows piezoelectric devices to translate electricity into motion can work in reverse. "Imagine a thread that can generate electricity when stretched," he says.

Ultimately, however, the researchers hope to combine the properties of their experimental fibers in a single fiber. Strong vibrations, for instance, could vary the optical properties of a reflecting fiber, enabling fabrics to communicate optically.

Max Shtein, an assistant professor in the University of Michigan's materials science department, points out that other labs have built piezoelectric fibers by first drawing out a strand of a single material and then adding other materials to it, much the way manufacturers currently wrap insulating plastic around copper wire. "Yoel has the advantage of being able to extrude kilometers of this stuff at one shot," Shtein says. "It's a very scalable technique." But for applications that require relatively short strands of fiber, such as sensors inserted into capillaries, Shtein say, "scalability is not that relevant."

But whether or not the Fink lab's technique proves, in all cases, the most practical way to make acoustic fibers, "I'm impressed by the complexity of the structures they can make," Shtein says. "They're incredibly virtuosic at that technique."

13
##### EEE / Tuning in to a new hearing mechanism
« on: May 10, 2018, 01:10:15 PM »
More than 30 million Americans suffer from hearing loss, and about 6 million wear hearing aids. While those devices can boost the intensity of sounds coming into the ear, they are often ineffective in loud environments such as restaurants, where you need to pick out the voice of your dining companion from background noise.

To do that, you need to be able to distinguish sounds with subtle differences. The human ear is exquisitely adapted for that task, but the underlying mechanism responsible for this selectivity has remained unclear. Now, new findings from MIT researchers reveal an entirely new mechanism by which the human ear sorts sounds, a discovery that could lead to improved, next-generation assistive hearing devices.

“We’ve incorporated into hearing aids everything we know about how sounds are sorted, but they’re still not very effective in problematic environments such as restaurants, or anywhere there are competing speakers,” says Dennis Freeman, MIT professor of electrical engineering, who is leading the research team. “If we knew how the ear sorts sounds, we could build an apparatus that sorts them the same way.”

In a 2007 Proceedings of the National Academy of Sciences paper, Freeman and his associates A.J. Aranyosi and lead author Roozbeh Ghaffari showed that the tiny, gel-like tectorial membrane, located in the inner ear, coordinates with the basilar membrane to fine-tune the ear’s ability to distinguish sounds. Last month, they reported in Nature Communications that a mutation in one of the proteins of the tectorial membrane interferes with that process.

Sound waves

It has been known for more than 50 years that sound waves entering the ear travel along the spiral-shaped, fluid-filled cochlea in the inner ear. Hair cells lining the ribbon-like basilar membrane in the cochlea translate those sound waves into electrical impulses that are sent to the brain. As sound waves travel along the basilar membrane, they “break” at different points, much as ocean waves break on the beach. The break location helps the ear to sort sounds of different frequencies.

Until recently, the role of the tectorial membrane in this process was not well understood.

In their 2007 paper, Freeman and Ghaffari showed that the tectorial membrane carries waves that move from side to side, while up-and-down waves travel along the basilar membrane. Together, the two membranes can work to activate enough hair cells so that individual sounds are detected, but not so many that sounds can’t be distinguished from each other.

Made of a special gel-like material not found elsewhere in the body, the entire tectorial membrane could fit inside a one-inch segment of human hair. The tectorial membrane consists of three specialized proteins, making them the ideal targets of genetic studies of hearing.

One of those proteins is called beta-tectorin (encoded by the TectB gene), which was the focus of Ghaffari, Aranyosi and Freeman’s recent Nature Communications paper. The researchers collaborated with biologist Guy Richardson of the University of Sussex and found that in mice with the TectB gene missing, sound waves did not travel as fast or as far along the tectorial membrane as waves in normal tectorial membranes. When the tectorial membrane is not functioning properly in these mice, sounds stimulate a smaller number of hair cells, making the ear less sensitive and overly selective.

Until the recent MIT studies on the tectorial membrane, researchers trying to come up with a model to explain the membrane’s role didn’t have a good way to test their theories, says Karl Grosh, professor of mechanical and biomedical engineering at the University of Michigan. “This is a very nice piece of work that starts to bring together the modeling and experimental results in a way that is very satisfying,” he says.

Mammalian hearing systems are extremely similar across species, which leads the MIT researchers to believe that their findings in mice are applicable to human hearing as well.

New designs

Most hearing aids consist of a microphone that receives sound waves from the environment, and a loudspeaker that amplifies them and sends them into the middle and inner ear. Over the decades, refinements have been made to the basic design, but no one has been able to overcome a fundamental problem: Instead of selectively amplifying one person’s voice, all sounds are amplified, including background noise.

Freeman believes that by incorporating the interactions between the tectorial membrane and basilar membrane traveling waves, this new model could improve our understanding of hearing mechanisms and lead to hearing aids with enhanced signal processing. Such a device could help tune in to a specific range of frequencies, for example, those of the person’s voice that you want to listen to. Only those sounds would be amplified.

Freeman, who has hearing loss from working in a noisy factory as a teenager and side effects of a medicine he was given for rheumatic fever, worked on hearing-aid designs 25 years ago. However, he was discouraged by the fact that most new ideas for hearing-aid design did not offer significant improvements. He decided to conduct basic research in this area, hoping that understanding the ear better would naturally lead to new approaches to hearing-aid design.

“We’re really trying to figure out the algorithm by which sounds are sorted, because if we could figure that out, we could put it into a machine,” says Freeman, who is a member of MIT’s Research Laboratory of Electronics and the Harvard-MIT Division of Health Sciences and Technology. His group’s recent tectorial membrane research was funded by the National Institutes of Health.

14
##### EEE / New way to grow microwires
« on: May 10, 2018, 01:09:32 PM »
Microwires made of silicon — tiny wires with a thickness comparable to a human hair — have a wide range of possible uses, including the production of solar cells that can harvest much more sunlight for a given amount of material than a conventional solar cell made from a thin wafer of silicon crystal. Now researchers from MIT and Penn State have found a way of producing such wires in quantity in a highly controlled way that could be scaled up to an industrial-scale process, potentially leading to practical commercial applications.

Other ways of making such wires are already known, and prototypes of solar cells made from them have been produced by several researchers. But these methods have serious limitations, says Tonio Buonassisi, MIT professor of mechanical engineering and a co-author of a paper on the new work that was recently published online in the journal Small, and will soon appear in the print edition. Most require several extra manufacturing steps, provide little control over the exact sizes and spacing of the wires, and only work on flat surfaces. By contrast, the new process is simple yet allows precise control over the wire dimensions and spacing, and could theoretically be done on any kind of curved, 3-D surface.

Microwires are thought to be capable of reaching efficiencies close to those of conventional solar cells in converting sunlight to electricity, but because the wires are so tiny they would do so using only a small fraction of the amount of expensive silicon needed for the conventional cells, thus potentially achieving major reductions in cost.

In addition to microwires’ potential use in solar cells, other researchers have proposed ways such microscopic wires could be used to build new kinds of transistors and integrated circuits, as well as electrodes for advanced batteries and certain kinds of environmental monitoring devices. For any of these ideas to be practical, however, there must be an efficient, scalable manufacturing method.

The new method involves heating and intentionally contaminating the surface of a silicon wafer with copper, which diffuses into the silicon. Then, when the silicon slowly cools, the copper diffuses out to form droplets on the surface. Then, when it is placed in an atmosphere of silicon tetrachloride gas, silicon microwires begin to grow outward wherever there is a copper droplet on the surface. Silicon in the gas dissolves into these copper droplets, and then after reaching a sufficient concentration begins to precipitate out at the bottom of the droplet, onto the silicon surface below. This buildup of silicon gradually elongates to form microwires each only about 10 to 20 micrometers (millionths of a meter) across, growing up from the surface. The whole process can be carried out repeatedly on an industrial manufacturing scale, Buonassisi says, or even could potentially be adapted to a continuous process.

The spacing of the wires is controlled by textures created on the surface — tiny dimples can form centers for the copper droplets — but the size of the wires is controlled by the temperatures used for the diffusion stage of the process. Thus, unlike in other production methods, the size and spacing of the wires can be controlled independently of each other, Buonassisi says.

The work done so far is just a proof of principle, he says, and more work remains to be done to find the best combinations of temperature profiles, copper concentrations and surface patterning for various applications, since the process allows for orders-of-magnitude differences in the size of the wires. For example, it remains to be determined what thickness and spacing of wires produces the most efficient solar cells. But this work demonstrates a potential for a kind of solar cell based on such wires that could significantly lower costs, both by allowing the use of lower grades of silicon (that is, less-highly refined), since the process of wire growth helps to purify the material, and by using much smaller amounts of it, since the tiny wires are made up of just a tiny fraction of the amount needed for conventional silicon crystal wafers. “This is still in a very early stage,” Buonassisi says, because in deciding on a configuration for such a solar cell “there are so many things to optimize.”

Michael Kelzenberg, a postdoctoral scholar at the California Institute of Technology who has spent the last five years doing research on silicon microwires, says that while others have used the copper-droplet technique for growing microwires, “What's really new here is the method of producing those liquid metal droplets.” While others have had to place the droplets of molten copper on the silicon plate, requiring extra processing steps, “Buonassisi and his colleagues have shown that metal can be diffused into the growth substrate beforehand, and through careful heating and cooling, the metal droplets will actually form on their own — with the correct position and size.”

Kelzenberg adds that his research group has recently demonstrated that silicon microwire solar cells can equal the efficiency of today’s typical commercial solar cells. “I think the greatest challenge remaining is to show that this technique is more cost-effective or otherwise beneficial than other catalyst metal production methods,” he says. But overall, he says, some version of silicon microwire technology “has the potential to enable dramatic cost reductions” of solar panels.

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##### EEE / Turning windows into powerplants
« on: May 10, 2018, 01:07:44 PM »
If a new development from labs at MIT pans out as expected, someday the entire surface area of a building’s windows could be used to generate electricity — without interfering with the ability to see through them.

The key technology is a photovoltaic cell based on organic molecules, which harnesses the energy of infrared light while allowing visible light to pass through. Coated onto a pane of standard window glass, it could provide power for lights and other devices, and would lower installation costs by taking advantage of existing window structures.

These days, anywhere from half to two-thirds of the cost of a traditional, thin-film solar-power system comes from those installation costs, and up to half of the cost of the panels themselves is for the glass and structural parts, said Vladimir Bulović, professor of electrical engineering in the Department of Electrical Engineering and Computer Science. But the transparent photovoltaic system he developed with Richard Lunt, a postdoctoral researcher in the Research Laboratory of Electronics, could eliminate many of those associated costs, they say.

A paper by Bulović and Lunt describing their new system has been published online in the journal Applied Physics Letters, and will appear in a forthcoming issue of the print edition.

Previous attempts to create transparent solar cells have either had extremely low efficiency (less than 1 percent of incoming solar radiation is converted to electricity), or have blocked too much light to be practical for use in windows. But the MIT researchers were able to find a specific chemical formulation for their cells that, when combined with partially infrared-reflective coatings, gives both high visible-light transparency and much better efficiency than earlier versions — comparable to that of non-transparent organic photovoltaic cells.

In a new building, or one where windows are being replaced anyway, adding the transparent solar cell material to the glass would be a relatively small incremental cost, since the cost of the glass, frames and installation would all be the same with or without the solar component, the researchers say, although it is too early in the process to be able to estimate actual costs. And with modern double-pane windows, the photovoltaic material could be coated on one of the inner surfaces, where it would be completely protected from weather or window washing. Only wiring connections to the window and a voltage controller would be needed to complete the system in a home.

In addition, much of the cost of existing solar panels comes from the glass substrate that the cells are placed on, and from the handling of that glass in the factory. Again, much of that cost would not apply if the process were made part of an existing window-manufacturing operation. Overall, Bulović says, “a large fraction of the cost could be eliminated” compared to today’s solar installations.

This will not be the ultimate solution to all the nation’s energy needs, Bulović says, but rather it is part of “a family of solutions” for producing power without greenhouse-gas emissions. “It’s attractive, because it can be added to things already being deployed,” rather than requiring land and infrastructure for a whole new system.

Fine-tuning the cells

The work is still at a very early stage, Bulović cautions. So far, they have achieved an efficiency of 1.7 percent in the prototype solar cells, but they expect that with further development they should be able to reach 12 percent, making it comparable to existing commercial solar panels. “It will be a challenge to get there,” Lunt says, “but it’s a question of excitonic engineering,” requiring optimization of the composition and configuration of the photovoltaic materials.

The researchers expect that after further development in the lab followed by work on manufacturability, the technology could become a practical commercial product within a decade. In addition to being suitable for coating directly on glass in the manufacture of new windows, the material might also be coated onto flexible material that could then be rolled onto existing windows, Lunt says.

Using the window surfaces of existing buildings could provide much more surface area for solar power than traditional solar panels, Bulović says. In mornings and evenings, with the sun low in the sky, the sides of big-city buildings are brightly illuminated, he says, and that vertical “footprint” of potential light-harvesting area could produce a significant amount of power.

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