EEWeb Issue 87
February 26, 2013
Richard Barry Founder, FreeRTOS
TECHNICAL ARTICLE
Solar Power Breakthroughs SPECIAL FEATURE
Cree, Inc. The SiC Advantage and the Future of Power Semiconductors
Electrical Engineering Community
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TABLE OF CONTENTS
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Richard Barry FOUNDER OF FREERTOS An interview with the founder of the completely free real-time operating system, FreeRTOS.
Featured Products
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The SiC Advantage and the Future of Power Semiconductor Devices
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Why Cree Inc., a market-leading semiconductor company, is using silicon carbide technology in their power and RF devices and how it sets them apart from the rest.
Solar Power Breakthroughs Vs. Exponential Energy Concerns
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BY ALEX TOOMBS With annual global energy consumption now over 15 TW, solar power is becoming an ever more economical option for providing the electrical energy demanded world over.
RTZ - Return to Zero Comic
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Richa
Barry
FreeRTOS is a completely free, real time operating system that supports 33 architectures and boasts 103,000 downloads a year. We spoke with Richard Barry, the founder of FreeRTOS, about how his extensive software background led him to create his own RTOS, the benefits of using his product, and his reasons for giving away the product for free.
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RTOS
free
ard
INTERVIEW
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EEWeb PULSE How did you get into electrical engineering? I received my degree at a local university here in the UK, which was vocational in nature. It was a dedicated real-time computing course, and my training is exactly what I ended up doing. A lot of embedded engineers are primarily educated in electronic engineering and get into software because they have to program or test what they’re designing. I actually came from an education in software engineering, and in the embedded space, rather than web-programming or applications-programming, there are not that many people who are primarily trained in software. My education helped prepare me for my career because that skill is often the missing piece of a team. When I’m in a team, there are lots of people that know a lot more about electronics than I do, but I generally know more about software design. They’re complementary skills. While at university, I got a firstclass honors degree, but turned down the opportunities the degree presented to further my education with a Master’s degree. Back then, I wanted to do something outside of academia, but I would be interested in an academic career now that I’ve done a lot of other things. I was always quite entrepreneurial; even when I was in secondary school I worked for myself washing cars, and earned about three times what my friends were earning with more usual teenage jobs, like working in supermarkets. I actually set up my first proper incorporated company in my last year of university. What did your first company do? We developed operator interface terminals for industrial equipment, or at least that was the idea. We
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created a couple of models. There was a lot of electronic, software, and mechanical design. Looking back on it now it seems very naïve, thinking you can really break into a global competitive market like that when you are so young and have no work experience at all. My business partner did have 10 years of experience and had already built up a lot of contacts, so maybe it’s not quite as strange as it sounds. He was the person with experience and I was the person with the youthful enthusiasm. We kept that company going for a few years. In fact, the company still exists, but I resigned as a director long ago, and the company doing something completely different these days. That experience taught me an awful lot very quickly. If I had gone into a big company like Hewlett Packard or ST Micro (both big employers in Bristol who typically take people with my skills), I would have been at the bottom of the pile. I wouldn’t have gained any of the commercial or customer exposure. When you’re in your own small company you’re doing the development, the marketing, the financing, everything -- I might not have been very good at it at that age, but having to do all that stuff means you definitely learn and gain confidence very quickly. Starting a company taught me a lot about the unpredictability of business as well. We set out to manufacture interface terminals for industrial equipment and supported as many different manufacturers as we could. That meant we had to code up lots of different communication bus protocols, and eventually devise ways of passing data between incompatible domains. This was in the days before field buses; everything was RS422,
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or 4-20mA loop, or something like that, so it was not just the software layers of the protocols that were incompatible. Writing the protocols was a necessity to produce the operator terminals, but the terminals didn’t do that well and we actually ended up selling dedicated protocol converters instead. That was the niche we found ourselves in accidentally, and since then I have found that pattern repeated in subsequent ventures – you head off in one direction with good intentions to keep a steady course, but end up turning down unanticipated paths as niches present themselves along the way. Those were the early days. What did you do after that? After that I did a few freelance jobs. In fact, that was nothing new; the first business was partly funded by part time freelance work. Half the week we would be doing contract work to raise cash, and the other half we would be spending the money we made developing products. After that I stayed contracting for a while with the contacts I had already made and a few others I found. People employed me to do things like optimization, where a product had been designed and was functioning, sort of, but was too slow on the target hardware. I also had to do board bring-up, which is the bit between hardware design and readying the development environment for the application writers. I was always acting like a troubleshooter with other people’s code and hardware. My deep embedded skills meant I could use the debugger on the real hardware more efficiently than the apps guys, as well as profile code, and working out how to create interfaces and speed things up. There was always the communication protocol work too -- again it’s a bit of a niche that
INTERVIEW I had happened to learn a lot about. High-level application engineers often knew how to interface to a communications protocol through an API, but less about how to actually implement, test, and debug the protocol itself. These were all very short term tasks. After that I hooked up with WITTENSTEIN, who will crop up again later no doubt. At that time, about 13 years ago, WITTENSTEIN were already a successful and rapidly growing high precision motor and gearbox company. They were starting a company in the UK
office - it was that kind of experience. Although WITTENSTEIN was a moderately large organization, the part I worked in had a small business feel to it because we were remote from the headquarters. It was like starting a business from scratch, but this time with very secure backers. It suited me well. The managing director in the office was an aeronautical engineer called David Cowling. He is very well-respected in his field, with a specialization in active stick technology - or haptic controls if you like. Our first successful project was
detents. The best feature was what we called software end stops, end stops that made it feel like you were hitting the stick against something mechanical. All the characteristics of the stick feel were in reality completely controlled by software. A force sensor measured the pilot force and two high precision motors were controlled in real time to move the stick around, giving the illusion that the pilot was moving the stick himself. The precision of the WITTENSTEIN hardware was what made everything so realistic. The flight-worthy project was where
“I passionately wanted to make FreeRTOS easy to use and fast to learn so I didn’t just create source code but complete prepackaged projects that were ready to compile.” to get their cutting edge actuation technology into the aerospace market. They entered the medical market with miniature implantable actuators around the same time, so they were getting into critical development in a big way. Software was not their core product, but their aerospace and medical products required software, and for those markets the software had to be provably well engineered. I was lucky enough to be their first employee in the UK office, and things literally started out by finding office space and then equipping the
what turned out to be the world’s first production flight worthy active side stick. Most people know of the passive side stick controllers found in Airbus aircraft. The pilot maneuvers the plane by using the side stick like a joystick, but in passive systems the pilot doesn’t get feedback from the stick. An active side stick provides lots of feedback. There are simple things like stick shakers, but there are much more complex feedback features like giving the pilot the feeling that he is pulling or pushing against a spring, or adding in notches and
I learned all about software quality standards, knowledge that was later applied to the SafeRTOS product. I think that is a real differentiator for SafeRTOS. Some RTOS vendors sub out their safety work, or have their customers do certification. WITTENSTEIN has that knowledge in house and they do it themselves. Their work is of course validated externally by organizations like TÜV SÜD. That’s necessary to ensure the mandated independence level is maintained. WITTENSTEIN even manufacture physical aerospace and medical products themselves. Visit www.eeweb.com
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When did you start FreeRTOS? I started FreeRTOS, coming up on 10 years ago now. After a few years with WITTENSTEIN, I left and was employed on a couple of consulting jobs. In one consulting job I was asked to find a replacement for an operating system that was costing the client too much in royalty payments. After researching their requirements, I found they didn’t actually need that much and that their existing system was already overkill. I then spent probably too long looking at open source alternatives for them, but found the open source offerings at that time largely unusable for a variety of reasons. For a start, they assumed you were a Linux user, but more than that they were presented in a very unfriendly way, largely undocumented and of dubious quality. I thought, “Well, there’s no way I can use these,”
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so I recommended a different commercial system that didn’t have any royalties, and that worked out well. Later on I used this experience to ensure all the objections I had to using those open source systems did not apply to FreeRTOS; it was time well spent. When the job was finished I thought again about the open source options. I thought to myself “you know, I’ve spent days and days looking at all these open source offerings, without finding any I thought were usable, especially in a Windows environment. How many thousands of other people in the world have done the same thing?” Then, because I’m a geek and enjoy that sort of thing, I thought, “okay, I’ll write one myself, but make it how I would want to find it, easy to use, especially for Windows developers”. So over a period of
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time, I created what evolved into FreeRTOS. I passionately wanted to make it easy to use and fast to learn so I didn’t just create source code but complete pre-packaged projects that were ready to compile. I also created a lot of documentation, which is not very glamorous or fun I know, but obviously necessary. So I just put it all out on the web and thought that would be the end of it. I have to admit there was no planning or any sort of business model being contemplated, it had just been a bit of fun for a geek and hopefully useful to other people. Anyway, my hunch turned out to be right, there was evidently a massive market for it -- I use the word “market” in the loosest sense, I really mean “user base,” as nothing was being sold. The whole thing just took off and I was bombarded with requests to support more devices.
INTERVIEW I was surprised myself, and from that starting point it has continued to accelerate with a life of its own. It’s just ridiculous how many engineers are out there. FreeRTOS has now polled as the leading choice of engineers for comparable systems for three years. Last year unique visitors to the website still rose by 26%, and it was downloaded in 103,000 times during 2012 alone. Of course now a lot more thought goes into planning and maintenance. What started as the “FreeRTOS project” is now professionally managed by an incorporated company Real Time Engineers Ltd. As we have gone on over time, people have come up with all the normal reasons about why they can’t use open source software – and every time someone comes up with a reason, we shoot it down, basically. When the reason was, “oh, it’s not supported so we can’t possibly use it,” we started supporting it properly and for free, which may be a bit crazy, but I’m told the support is actually better than that received from commercial support contracts. The next objection raised was the risk of accidental IP infringement. People, say, “we don’t know where this code comes from -- we might put it in a product and sell a million parts,” and then someone will come in to say, “sorry that code has infringed my intellectual property.” We came up with the concept of moderated open source—when you download FreeRTOS the code is truly open source to you but any code contributed back the other way is kept separate from the official code base. It’s still made available, but if you stick to the official code base then we know where all the source code came from.
though they could get and use the code for free. When you dig into the rationale for this you realize what was really lacking was warranty and legal guarantees, indemnification if you like. I was not willing to offer that myself because it would have required all sorts of insurance and that sort of admin activity, but I then got back together with the WITTENSTEIN UK office, who at that time were looking to market their safety critical software skills to other markets. I licensed FreeRTOS to WITTENSTEIN to allow them to certify it as a proof of concept. They then set about certifying the code base to the IEC61508 standard to prove their safety critical development processes were just as applicable to industrial markets. They went through the entire safety lifecycle from HAZOPS to integration testing, including MC/DC testing, which is the most stringent code test coverage criteria used in aerospace projects. The
end result was SafeRTOS as a standalone re-engineered product. WITTENSTEIN were experts in the code base and had spent a long time picking it apart, adding safety features, and testing it after all, so they were also able to support and indemnify FreeRTOS commercially. They do this, again under license, using the OpenRTOS name. FreeRTOS and OpenRTOS are actually the same product, only the licenses are different. Real Time Engineers Ltd. still owns, maintain, develop and promote FreeRTOS. WITTENSTEIN adds a commercial and legal wrapper and add value with a wide range of top quality USB, file system, and networking components. It was a good deal for both of us. In my small company, I don’t want to be dealing with lots of separate customers, or the bureaucracy that goes with insurance and export controls, so as far as selling licenses
FreeRTOS has now polled as the leading choice of engineers for comparable systems for three years, last year unique visitors to the website still rose by 26%, and it was downloaded 103,000 times during 2012 alone.
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EEWeb PULSE go, I just license the code to WITTENSTIEN, which has all the admin and necessary systems covered already. That leaves Real Time Engineers free to concentrate on working with a few larger partners. This requires a much lower admin overhead. What kind of devices do you support? FreeRTOS started off with very small 8- and16-bit microcontrollers, and more by luck than judgment was able to ride on the back of the rapid growth in 32-bit devices, especially the ARM7 at that time. One of the very early ports I did was to an NXP ARM7 device. At that time there were also devices like the ColdFire, and a little later the PIC32 – entry level 32-bit stuff. I always see that as my core market, but people use FreeRTOS all over the place. Right now the Cortex-A9 gets requested a lot. I never imagined it would ever get into that class of processor, but what people like Xilinx are doing on their dual core A9 device is running Linux on one core to benefit from the entire infrastructure that comes with that,
and FreeRTOS on the other core to benefit from the true real time capabilities that go with that. Right now FreeRTOS has official support for 33 different architectures and something like 18 different compilers. In that, I’m really talking about architectures, so classing something like Cortex-M3 as one architecture even though that covers hundreds of parts from lots of manufacturers. FreeRTOS will run on any Cortex-M3 part, or any M4F part for that matter. Do you have any new features for the product? One of the latest features is what I call tickless idling. That allows you to enter really deep sleep low power modes because you can turn the periodic tick interrupt off without losing functionality. I have also released a fully reentrant and thread-aware UDP/IP stack. It’s not full TCP, but it does have a sockets interface, a zero copy sockets interface, DNS, DHCP, and all that sort of thing. It’s ideal for peer to peer communication in embedded control networks. Unless you want to run a webserver, lots of
embedded stuff is UDP because it’s faster, deterministic, and much smaller than TCP. We have a novel web server solution coming soon, but for now the UDP/IP stack, with everything included, takes less than 6K of code space. I’m quite pleased with that. Are you happy with the decision to make your product free of charge? Absolutely – you’re not going to become a millionaire giving stuff away for free, but I think if you can you can make a very good living, and really enjoy what you’re doing, then that’s great.
For more information about FreeRTOS, visit:
www.freertos.org
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The SiC Advantage
and the Future of Powe Semiconductor Device C
is a market-leading manufacturer of silicon carbide (SiC) based semiconductors. Paulree Kierstead
Founded in 1987, Director of Marketing forCree the was formed around silicon carbide and gallium nitride wide bandgap material technologies, which have proven their ability to sustain high power Cree Power Business Unit densities in smaller physical devices. From its earliest days, Cree’s materials technology has been the core of the company and remains that way today. Although Cree has seen considerable growth on the lighting and LED side of their business, they have also been working on power and RF devices since those early days. The first commercial power device that went into production was a silicon carbide Schottky diode in 2002 – some fifteen years after the company was founded. We spoke with Paul Kierstead, Director of Marketing for the Cree Power Business Unit, about his role in developing the SiC power semiconductors, the advantages of using silicon carbide in these devices, and Cree’s roadmap for the future of SiC power devices in various applications.
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SPECIAL FEATURE
e
er es
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SiC: Separating from the Pack What makes Cree’s power semiconductors different from the rest? According to Kierstead, while Cree’s basic devices are “not too different from silicon products,” the company’s silicon carbide technology enables them to “build power electronics devices with a significantly smaller die size for a given power handling capability than would be possible using silicon technology.” Another advantage comes from SiC’s inherent high thermal conductivity. “You can get a lot of power into a small device,” Kierstead says, “but, in contrast to silicon, with SiC you can get the heat generated out of the die quickly, efficiently, and easily.” When using silicon for high blocking voltages, the die size has to increase along with that voltage. Conversely, SiC’s smaller die size contributes to low capacitive switching losses. Further, the high power density allows use of SiC unipolar devices (Schottky diodes and MOSFETs), whereas silicon is required to utilize bipolar (PN diode and IGBT) parts. Smaller, unipolar SiC power semiconductors enable significant reductions in system switching losses versus their silicon counterparts. For systems requiring 600 volts, 1200 volts, or higher, Kierstead’s team can replace silicon PN-diodes and IGBTs with silicon carbide Schottky diodes and MOSFETs to build faster and more energy-efficient power systems. SiC Schottky diodes eliminate the reverse recovery losses common on PN diodes and SiC MOSFETs eliminate the tail currents associated with IGBTs, both of which contribute to switching inefficiency. According to Kierstead, building your high power system with all-SiC power devices results in minimal system losses, which in turn contributes to maximizing system performance in many ways.
designers a big switching advantage by reducing switching losses – a consequence that has three major benefits. The first is pure energy efficiency; simply replacing the silicon devices in a system with silicon carbide devices will allow users to immediately realize one to three percentage points of higher energy efficiency, which can reduce overall energy consumption. This benefit it particularly useful for applications like driving a large motor or powering a data center, as the few percentage points of higher energy efficiency gained by simply replacing a siliconbased system with a silicon carbide-based system will be magnified. The second benefit is that any time a system isn’t using energy efficiently it is generating excess heat, the dissipation of which requires costly and complicated thermal designs. In contrast, SiC devices’ inherent energy efficiency enables users to simplify and reduce the cost of their end systems and increase system reliability by lowering the overall temperature, allowing for the employment of smaller, simpler, and more cost-effective heatsinks, fans, and other thermal management components. The final and potentially most important advantage is that, since SiC devices’ switching efficiencies are much higher than those of silicon devices, users can increase their system switching frequency while maintaining high efficiency. In power systems that have a lot of filtering transformers and capacitors, higher frequency can be used to shrink and reduce the cost of those filter components, as they
Switching Advantages of Using SiC Another inherent advantage of using silicon carbide power devices from Cree is that they give system
Cree’s SiC Module
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SPECIAL FEATURE
“There was a quote from Walter Schottky in the 1950’s where he basically identified silicon carbide as the ideal material that you can build semiconductors on, but nobody could build one on the crystal quality that allowed them to do that, so the world went silicon. We’re kind of coming all the way back to where the materials advantage has simply allowed us to take advantage of some physics that has been there for a long time.” - Paul Kierstead their portfolios each have many times more versions of different amperages, blocking voltages, and switch types, multiplied by different package styles – a direct reflection of the larger installed base of silicon power electronics applications. Consequently, we asked Paul Kierstead about Cree’s roadmap for the future. Kierstead Cree’s SiC MOSFET Wafer explained that Cree is really focused on building out are sized inversely to the switching frequency. In other their initial set of devices. “We intend to increase our words, if a user can increase their system’s switching voltage ratings to beyond 1700 volts and are already working through the processes frequency, they can also shrink necessary to do so. For instance, its magnetics components, Walter Schottky we have demonstrated SiC which are all big iron and (1886 - 1976) was a MOSFETs in the lab that have copper devices, as well as German physicist up to 10 kV blocking voltages, its capacitors. “This provides whose research which is basically unheard of in several advantages to the system in semiconductorthe silicon world. Hence, there’s designer,” Kierstead adds. “First, device theory led a lot of headroom to move up in the magnetic components are to the development voltage ratings,” said Kierstead. large, heavy, and expensive, so of Schottky Diodes. “The availability of more power reducing their rating will save These diodes are ratings for existing devices will significant size, weight, and known for their low forward voltage allow deeper penetration in areas cost. Additionally, electrolytic drop and very fast switching action. such as solar inverters, server, capacitors are not only These inherent characteristics allow communications, industrial, and expensive, but they’re a point for better overall system efficiency, uninterruptible power supplies. of failure; typically, they’re one especially when constructed from Higher voltage devices will of the biggest reliability issues silicon carbide. Source: Wikipedia eventually open applications in in a system. Consequently, by systems like trains and windmills making them smaller, you can buy a better grade, likely for a similar or maybe even and in heavy industrial power conversion..” For lower cost. Overall, higher switching frequency allows Kierstead, there are three dimensions to Cree’s future system designers to make smaller, lighter, and less expansion with regards to their product line: “We will offer higher voltages, provide more amperage rating costly end systems.” options, and add more packaging technologies.” ■
The Future of Power Systems
If you look at Cree’s portfolio, you will see that there are nearly 50 power devices available that range from 600 volt ratings up to 1700 volts for Schottky diodes and MOSFETs in various amperage ratings. However, if you look at a typical manufacturer of silicon power devices, such as Infineon or Fairchild, you will see that
For more information about Cree’s products, visit:
www.cree.com
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EE Making Wireless Truly Wireless: Need For Universal Wireless Power Solution
Dave Baarman Director Of Advanced Technologies
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Solar P Brea E Vs. E Co Alex Toombs Electrical Engineering Student
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TECH ARTICLE
Power akthroughs Exponential Energy oncerns A
mong the more important contributions that electrical engineers can make to society is the pursuit of attaining global energy sustainability. Current projections estimate that, if we are not already at it, that peak oil production will come about within the next 50 years. That implies that we are producing the maximum amount of oil possibly extracted from the ground. Additionally, public concern over nuclear safety and proliferation have hindered efforts to develop and install safe, modern nuclear plants. With annual global energy consumption now over 15 TW, solar power is becoming an ever more economical option for providing the electrical energy demanded world over. As an energy source, solar is democratic, plentiful, clean, and fully renewable; the only issue is that the technology for effectively capturing light and storing electrical energy is not yet entirely there. However, recent technological innovations have made more progress toward that goal.
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EEWeb PULSE Solar panels have become much cheaper to fabricate over the past decade, largely due to improvements in manufacturing processes, especially for generation one solar cells. Generation one solar cells are those that consist of single semiconductor PN junctions, like a typical silicon diode. These cells, largely created from silicon, comprise the majority of those that are used in home installations, campground solar chargers, and other simple devices. They are subject to a fundamental limit known as the Shockley-Queisser limit, confining the maximum power extraction efficiency from a singlejunction solar cell to around 34%. This limit is due mostly to losses accounting to the recombination of holes and electrons in the cell, emission of energy by the cell due to blackbody radiation, and from the ability to absorb only a certain fraction of the solar spectrum of light incident upon the cell. With the cost of these solar cells coming down, more people are considering home installations of cells to reduce their monthly electricity bills. Often, many do not consider the fixed costs required for a home installation of solar cells. Among these are the costs paid to install the cells, the power electronics required to convert generated electricity to AC, and the costs associated with financing a solar installation. Installation of the cells is a cost that will always come into consideration, but can vary depending upon roof size and power demands. For example, some homeowners install solar trackers, which are mechanized devices that move the solar panel throughout the course of the day as a way of maximizing the light incident on the solar cell. Typically, these higher cost items only make sense for large scale and high efficiency applications. Power electronics, such as inverters, are required to convert the power generated by a solar cell, which is DC, and generate three-form AC power that can be used around the house or sold back to a utility company. Inverters and AC power generators are among the most expensive of costs associated with solar installations. These cost around as much as the cost of the solar cell— or even more. Fortunately, research in the area of power electronics has been lowering costs. High speed switching devices, such as gigahertz and terahertz transistors, need to become more economical in order for home installations of solar cells to become more viable. Solar installations for home use can range in the tens of thousands of dollars, which may require a loan from a bank in order to purchase. Despite government tax breaks, it is simply not economical for some to purchase solar cells. Assuming a fixed cost per kilowatt-
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hour from the power company over the lifetime of the solar cell, the cost of a solar installation, in addition to tax breaks and eventual writeoffs, it may simply make more sense for all of a household’s electricity to come from the utilities. At the moment, solar power is still more expensive in the long run. In addition to decreasing costs of power electronics, there are two different approaches, both with different applications, dedicated to solving the economic issues associated with solar power. The first possible solution lies with generation two solar cells, also known as organic solar cells. Unlike generation one solar cells, which are comprised of semiconductors like silicon, organic solar cells are made up of other materials that accept light and donate electrons, converting sunlight to electricity much in the same way. There are several formulations currently used, much as there are multiple semiconductors used in solar today. The main difference is that organic solar cells are cheap and available by design. An example cell is shown in Figure 1 below:
Electrode 1 (ITO, Metal)
Electron Donor Electron Acceptor Electrode 2 (Al, Ma, Ca)
Figure 1: Organic Solar Cell Diagram
As opposed to semiconductor solar cells, which require expensive fabrication labs, highly pure materials, and complicated assembly lines, the goal of organic solar is that the “cells” may be mixed anywhere, making a cheap and available solar cell. Organic solar cells are usually amorphous or liquid in addition to being low cost, meaning that any area exposed to sunlight can be coated and turned into a solar cell. Generation one cells are generally rigid and fixed in size, which can be a disadvantage in some installations. However,
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generation two cells are much less efficient than other comparable cells, generally limited to under ten percent conversion efficiency. Additionally, they still require inverters in order to utilize the electricity generated in any household connected to the electrical grid. To overcome the Shockley-Queisser, solar researchers have developed a variety of techniques. Generation three solar is the umbrella term for those cells that are designed in order to defeat that fundamental limit of power extraction. The foremost of these are multi-layer, or tandem, solar cells. Multi-layer cells are designed to defeat one aspect of the Shockley-Quiesser limit, that of the solar spectrum divisions. Each semiconductor has a band gap, measured in electron-volts, that corresponds to the amount of energy required to move an electron from the valence band to the conduction band— effectively freeing that electron to be used as electricity. These band gaps are inversely proportional to the maximum wavelength of light that may be absorbed by the cell, meaning that each cell can only absorb one part of the solar spectrum. To ensure that the rest of the solar spectrum may be absorbed, multiple solar cells are layered. The largest band gap cells come first, absorbing whatever light it can. The top cell is transparent to the remaining wavelengths of light, allowing it to pass through and get absorbed by the next layer, or the one thereafter. A typical application is shown in Figure 2. These multi-junction cells are not without their problems, however. Even solar cell stacks are subject to Ohm’s law, meaning that each cell in series is limited to the current of the lowest current-producing cell in the stack; this is known as the current-matching problem. Additionally, each cell is much more expensive than a similar generation one cell, meaning that it is much more difficult to pay to cover a roof with them. As research continues toward increasing the efficiencies associated with multi-junction cells, costs are also driven down with better manufacturing techniques. The problem of lattice matching remains a compelling one. Each semiconductor has a crystal lattice size that determines the spacing between atoms in a crystal. Growing different semiconductors on top of each other is difficult because each atom does not line up nicely. As of last year, the best lab-grown multi-junction cells were around 42% efficient. Despite the difficulties associated with it, solar technology offers the best chance at providing the electricity the world demands. Other challenges remain, such as
TECH ARTICLE Eg1 > Eg2 > Eg3
Cell 1 (Eg1)
Cell 2 (Eg2)
Cell 3 (Eg3) Figure 2: Generation Three Multi-Layer Solar Cell
storing generated electricity for use during hours when the sun has set. But considering current year over year increases in electricity consumption, current sources of electricity will not last, and solar power hurdles will need to be overcome. Oil and natural gas are fossil resources that will soon run out, and nuclear is currently tied up in safety, security, and health concerns. As long as the sun shines, solar power will make sense as a peak supply electricity source.
About the Author Alex Toombs is a senior electrical engineering at the University of Notre Dame, concentrating in semiconductor devices and nanotechnology. His academic, professional and research experiences have exposed him to a wide variety of fields; from financial analysis to semiconductor device design; from quantum mechanics to Android application development; and from low-cost biology tool design to audio technology. Following his graduation in May 2013, he will be joining the cloud startup Apcera as a Software Engineer. Visit www.eeweb.com
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