IEEE ELECTRIFICATION Dec 2013

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News, analysis and insights on electric vehicles, electric ships, electric trains and electric planes

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VOL. 1, NO. 2 DECEMBER 2013 ISSN 2325-5987

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IEEE Electrification Magazine (ISSN 2325-5987) (IEMECM) is published quarterly by the Institute of Electrical and Electronics Engineers, Inc. Headquarters: 3 Park Avenue, 17th Floor, New York, NY 10016-5997 USA. Responsibility for the contents rests upon the authors and not upon the IEEE, the Society, or its members. IEEE Operations Center (for orders, subscriptions, address changes): 445 Hoes Lane, Piscataway, NJ 08854 USA. Telephone: +1 732 981 0060, +1 800 678 4333. Individual copies: IEEEmembers US$20.00 (first copy only), nonmembers US$123.00 per copy. Subscription Rates: Society members included with membership dues. Subscription rates available upon request. Copyright and reprint permissions: Abstracting is permitted with credit to the source. Libraries are permitted to photocopy beyond the limits of U.S. Copyright law for the private use of patrons 1) those post-1977 articles that carry a code at the bottom of the first page, provided the per-copy fee indicated in the code is paid through the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923 USA; 2) pre-1978 articles without fee. For other copying, reprint, or republication permission, write Copyrights and Permissions Department, IEEE Operations Center, 445 Hoes Lane, Piscataway, NJ 08854 USA. Copyright 2013 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Periodicals postage pending at New York, NY, and at additional mailing offices. Postmaster: Send address changes to IEEE Electrificaton Magazine, IEEE Operations Center, 445 Hoes Lane, Piscataway, NJ 08854 USA. Canadian GST #125634188 PRINTED IN U.S.A.

MISSION STATEMENT: IEEE Electrification Magazine is dedicated to disseminating infor-mation on all matters related to microgrids onboard electric vehicles, ships, trains, planes, and off-grid applications. Microgrids refer to an electric network in a car, a ship, a plane or an electric train, which has a limited number of sources and multiple loads. Off-grid applica-tions include small scale electricity supply in areas away from high voltage power networks. Feature articles focus on advanced concepts, technologies, and practices associated with all aspects of electrification in the transportation and off-grid sectors from a technical perspec-tive in synergy with nontechnical areas such as business, environmental, and social concerns.

Sensible Transportation Electrification Get rid of inefficient powertrain designs.

6 New Horizons in DC Shipboard Power SystemsNew fault protection strategies are essential to the adoption of dc power systems.

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Transportation Electrification Conductive charging of electrified vehicles.

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2 ABOUT THIS ISSUE4 TECHNOLOGY LEADERS

74 DATES AHEAD75 NEWSFEED80 VIEWPOINT

The Role of Energy Storage in a Microgrid Concept Examining the opportunities and promise of microgrids.

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Reducing Fuel Consumption at a Remote Military Base Introducing an energy management system.

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Flywheels Store to Save Improving railway efficiency with energy storage.

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Lee Stogner. Page 80

Wide-Bandgap Semiconductor TechnologyIts impact on the electrification of the transportation industry.

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Cryogenic Power Conversion SystemsThe next step in the evolution of power electronics technology.

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Cryogenic power conversion systems. Page 64

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IEEE Electr i f icat ion Magazine / DECEMBER 20132

T IS MY PRIVILEGE TO PRESENT TO YOU THE SECONDissue of IEEE Electrification Magazine. After the successful launch of the print version of the inaugural issue in the

fall of 2013, we made the digital version open access so that anyone can read the first issue from anywhere in the world. If you have not done so already, I hope you will visit http://electricvehicle.ieee.org/ieee-electrification-magazine/ to take advantage of the free access to our information-filled first issue.

The first issue had six articles covering various areas of new electrification topics, and this second issue has eight articles by authors from Asia, Europe, and North Ameri-ca. In the first article, the authors argue where and how electrification makes sense for effi-cient transportation. The following two arti-cles address storage examples for railway transportation as well as in microgrids. The next tells us how electrification and energy management systems can help reduce fuel consumption in remote military bases.

Another article details the new concept of dc shipboard power systems. This is followed by an article on the conductive charging of electric vehicles. As electric vehicles gain popularity, the issue of different charging methods is attracting a lot of attention. Power electronics play a major role in the electrification of transporta-tion. The next article is about how wide-bandgap semiconductor technologies show their impact on the electrification of the trans-portation industry. This issue is wrapped up with an article that reviews cryogenic power conversion systems for the electrification of transportation. The titles of the articles are:

1) Sensible Transportation Electrification2) Flywheels Store to Save3) The Role of Energy Storage in a Microgrid Concept4) Reducing Fuel Consumption at a Remote Military Base

By Saifur Rahman

A Global View

Digital Object Identifier 10.1109/MELE.2013.2295002Date of publication: 26 February 2014

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EDITORIAL BOARDSaifur RahmanEditor-in-ChiefVirginia TechVirginia, [email protected]

Iqbal HusainEditor, Electric VehiclesNorth Carolina State UniversityNorth Carolina, [email protected]

Eduard MuljadiCoeditor, Electric VehiclesNREL: Wind ResearchColorado, [email protected]

Herb GinnEditor, Electric ShipsUniversitiy of South CarolinaSouth Carolina, [email protected]

Robert CuznerCoeditor, Electric ShipsDRS Power and Control TechnologiesWisconsin, [email protected]

Eduardo Pilo de la FuenteEditor, Electric TrainsEPRail Researchand [email protected]

Jose Conrado Martine Coeditor, Electric TrainsDirectcion de Estrategia y Desarrollo [email protected]

Bulent SarliogluEditor, Electric PlanesUniversity of Wisconsin-MadisonMadison, [email protected]

Christine RossCoeditor, Electric PlanesRolls-Royce CorpIndiana, [email protected]

MohammadShahidehpourEditor, Off-GridIllinois Instituteof TechnologyChicago, [email protected]

Steve PullinsCoeditor, Off-GridHorizon Energy GroupTennessee, USAspullins@horizonenergy group.com

IEEE PERIODICALS MAGAZINES DEPARTMENT445 Hoes Lane, Piscataway, NJ 08854 USA

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(continued on page 5)

Saifur Rahman.

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ABOUT THE MAGAZINEIEEE Electrication Magazine is a quarterly magazine dedicated to disseminating information on all matters related to microgrids onboard electric vehicles, ships, trains, planes, and off-grid applications. Microgrids refer to an electric network in a car, a ship, a plane or an electric train, which has a limited number of sources and multiple loads. Off-grid applications include small scale electricity supply in areas away from high voltage power networks.

The launch of IEEE Electrication Magazine has created an opportunity for professionals within the industry and academic community to submit articles for publication.We welcome your expertise in the areas of:

U Electric VehiclesU Electric TrainsU Electric PlanesU Electric ShipsU Off-Grid Electricity

Feature articles focus on advanced concepts, technologies, and practices associated with allaspects of electrication in the transportationand off-grid sectors from a technical perspective in synergy with nontechnical areas such as business, environmental, and social concerns.

There is no IEEE magazine which has a global view of electrication in both transportation and off-grid electrication applications. The IEEE Electrication Magazine will ll the need felt by engineers inindustry as well as policy makers who require information on the technology, use cases and eld experience of electrication.

Articles should be between 6 & 8 pages (approximately 5,000 words with graphics) and may be submitted via e-mail to [email protected] with a copy to [email protected]. The submittal should includethe authors name, complete mailing address, phone, fax and e-mail.

CALL FOR AUTHORS!Articles wanted for the new IEEE Electrication Magazine

For more info, please visit ieee-pes.org/publications/electrication-magazineDigital Object Identifier 10.1109/MELE.2014.2303505

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T E C H N O L O G Y L E A D E R S

By Blake Lloyd

S PRESIDENT OF THE IEEEIndustry Applications Soci-ety, one of the three IEEE

sponsoring Societies of this magazine, it is my great pleasure to welcome you to this second issue of IEEE Elec-trification Magazine. This magazine is aimed at bringing together informa-tion related to the electrification of all means of transportation, from bicy-cles and cars to trains, ships, and air-planes, and, by extension, the electrifi-cation of small stationary systems and microgrids. Many professionals are living in a technically multidisci-plinary world, with technical and business challenges specific to indus-tries. We hope this publication will provide a sufficient focus to be rele-vant to you, yet be broad enough to bring you new or different perspec-tives on your daily challenges.

This magazine is brought to you by three IEEE Societies: the Power & Ener-gy Society, the Industry Applications Society, and the Power Electronics Society. This collaboration is no acci-dent; it is made necessary by advanc-es in technology that blur the lines between the various scopes of each Society. At the same time, we look for-ward to such cooperation because it can only provide more breadth and depth to the presentation of the vari-ous subjects at hand.

Electrification has been a trendy topic popularized by the publicity around hybrid and electric vehicles (EVs) and current debates about the necessary action against global warm-ing. Of course, as professionals in this field, we know that electrification has a much longer history, from the automotive starter motor in 1911 to electric trains in the 1930s.

Many other needs also underlie this progression, beyond energy savings and the environment, including mechanical simplicity in trains,

ships, and mining equipment; reli-ability and diagnostics in aircraft sys-tems; and better performance (driv-ing a fast EV looks kind of cool!). Just the same, it is good for engineers to be associated with a problem and an industry of which the general nonen-gineering public is aware and may even have strong feelings about. Ihope this magazine will someday find its place in dentists waiting rooms, but in the near future, it can help decision makers, or those inter-ested in helping decision makers, get the broader and deeper technical out-look that is sometimes missing from the public debates.

IEEE Electrification Magazine is meant to be a complement to existing offerings: within IEEE, many magazines and journals are available from a num-ber of Societies, but they are more dis-cipline based or focused on a specific industry rather than across the board

on the subject of electrification. Various conferences, some focused on electrifica-tion, others on electric machines, power elec-tronics, grids, and system issues, are available from IEEESocieties across the globe. Of course, out-side of the IEEE, a number of trade and other journals are also

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The Electrification Progression

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IEEE Electr i f icat ion Magazine / DECEMBER 2013 5

available. However, since IEEE Electrifica-tion Magazine has the backing of the IEEE and is produced by IEEE volunteers and staff, you know you can count on the finest technical quality to help you in your professional development. For more information, the new IEEEElectrification Web portal (http://electricvehicle.ieee.org/) is a great one-stop information window to any IEEE offering in this field.

I extend the warmest thanks to the editorial staff, the volunteers from the industry, and the IEEE staff for the long hours spent developing and launching this new product. Sin-cere thanks to the authors who are sharing their deep subject matter expertise with the community. How-ever, as a reader, you are the most important stakeholder of this maga-zine: your feedback is critical to the

success and the direction of this young product. Please share the magazine with your colleagues and send us your opinion, suggest topics of interest, or even offer to author an article on a subject of interest. Please feel free to contact [email protected] with any questions or ideas.

About This Issue (continued from page 2)5) New Horizons in DC Shipboard

Power Systems6) Transportation Electrification7) Wide-bandgap Semiconductor

Technology8) Cryogenic Power Conversion

Systems.While the September 2013 and

this issue of the magazine covered

multiple topics addressing various transportation and off-grid concerns, beginning in 2014, each issue will focus on a particular topic. For exam-ple, the March 2014 issue will focus on microgrids. The June 2014 issue will cover electric vehicles, followed by electric trains in the September issue. The fourth and final issue of

2014 will be on electric planes. For issues in 2015 and beyond, we welcome guest editors to propose articles on particular themes covering our topics of interest. Please contact me or any of the editors/coeditors if you are interested in proposing a topic for an issue of this magazine.

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2325-5987/13/$31.002013IEEEIEEE Electr i f icat ion Magazine / DECEMBER 201366


Get rid of inefficient powertrain designs.

By Randy Reisinger and Ali Emadi

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HE AVERAGE INTERNAL COMBUSTION ENGINE (ICE)-PROPELLED AUTOMO-bile is roughly 1020% efficient on average at converting the energy in gaso-line into forward motion. The remainder of the energy is dissipated into heat or ejected and not fully burned. This means that 8090% of the fuel is wasted. If you consider an analogy where a person filling the gas tank pumped 12 gal

into the tank, then pumped 89 gal onto the ground, you begin to understand just how much fuel your automobile can waste. This may startle those who might think of their vehicle as clean burning and energy efficient; but, as we will see, the numbers actually get much worse.

If the vehicles mission is to transport a payload from point to point, then the mission effi-ciency depends on the ratio of the weight of the pay-load to the weight of the vehicle. Frequently, the driver is the only payload in the vehicle. The typical drivers weight is roughly 5% of the combined weight, yielding a mission efficiency of less than 1% (1020% combined thermodynamic and system effi-ciency multiplied by 5% payload efficiency = 0.51% mission efficiency). So based on our earlier analogy, now consider the example that 12 gal are in the tank and 89 gal on the ground but realize that 95% of the energy from the 12 gal that made it into the tank will be required to move the weight of the vehi-cle, while only 5% of the energy will actually move the person to their destination. Often, when people hear these numbers for the first time, they are astonished that automobiles could be so inefficient. Yet, engineers who understand the laws of thermo-dynamics will readily verify them. The next time you fill your gas tank, look at the total cost of the fuel and realize that less than 1% of the fuel will move you from place to placethe rest is essentially wasted in terms of your mobility.

So the next time you hop in your car and buckle up to drive a short distance, for example, to the grocery store to get a few items, you may want to think about the fact that you just strapped on 4,000 lb or so of steel to move your far-lighter body a mile or two to get a small bag of groceries. The energy you consume is probably around 200500 Wh/mi instead of about 1015 Wh/mi on a bicycle. Driving a typical conventional car consumes about 2050 times more energy than riding a bicycle (which has additional health benefits).

Energy Wasted in Transportation on a Global ScaleNow, let us consider how our single-car example measures up on a global scale. There are roughly 1 billion vehicles globally, consuming about 70% of oil production, or about 60 million barrels of oil per day, for transportation. Personal transportation, i.e., cars, sport-utility vehicles, minivans, and light trucks, consume about 36 million barrels of petroleum per day at less than 1% mission efficiency on average. They emit nearly 17 billion t of carbon and roughly 114 trillion BTUs of heat every day. The vehicle base is projected to double in 1520 years. At current effi-ciency levels, those 2 billion vehicles would consume roughly 120 million barrels of petroleum per day, a rate that raises concerns about oil production capabilities to supply fuel. The refining process yields about 33 gal of transportation fuel in the form of gasoline, diesel, and jet A fuels for aircraft (along with other products). So roughly 4 billion gal of transportation fuel are pro-jected to be burned, daily, in 1520 years. These numbers have significant air quality, health, and economic consequences.

At roughly 20 lb of carbon dioxide (CO2) per gallon of fuel consumed, the daily quantity of CO2 generated by transportation would be about 80 billion lb/day or 29 trillion lb/year of CO2. In such quantities, CO2, plus the other emissions generated in the process of burning fuel each day, raises serious concerns about global climate change and the breathability of air, especially in high-density urban and suburban areas where more than 50% of people globally will live.

Transportation is the second-highest contributor to carbon emissions, behind coal-fired power plants. The above data suggest that increasing transportation efficiency can

Superlightweight designs improve energy consumption, battery size or range, cost of the vehicle, and acceleration performance.

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dramatically reduce CO2 emissions without reducing transportation benefits. Simply eliminating much of the wasted energy can be very beneficial in improving both pressure on global climate change and air quality. As we will discuss later, this can be accomplished by improving ICE efficiency by increased electrification of the vehicles powertrain and by reducing the weight of the vehicle.

Air quality issues are not new to the auto industry. Severe smog was reported in Los Angeles (LA) in 1943, but the cause was not discovered until five years later. In 1947, the Air Pollution Control District was set up in LA County by then-Governor Earl Warren. In 1948, Arie Haagen-Smit, a scientist from the California Institute of Technology, linked the smog to ozone caused by the automobiles. LA residents own autos and factories were causing the smog that would plague them with stinging eyes and respiratory problems for decades.

To the auto companies, this was of little concern and largely ignored. It was not recognized as an indica-tion of things to come in other areas. In the 1950s, the notion that auto and factory emissions could cause respiratory problems was a novel idea. To Californians living in LA, first smog from their automobiles and then traffic jams on the freeways during ever-lengthening rush hours began to define their lives. Many people planned trips to the desert or mountains on weekends to get some fresh air. Usually, they drove. Ulti-mately, that smog would spark the process of regulating emissions, but no one at the time foresaw the potential magnitude of the problem or the possibility that air quality would one day become a global problem and the subject of global treaties and initiatives to solve.

Perhaps less noticed is the amount of heat generated by transportation. Roughly 8 gal of transportation fuel equal 1 million BTUs of heat. The heat generated from burning 4 billion gal/day is roughly 500 trillion BTUs/day.

For many people, these numbers are beyond compre-hension. For now, let us say that if our current fuel con-sumption levels have scientists and politicians worried about the effects on our planet, then the anticipated growth in auto usage to two times our current rates without improving fuel efficiency would be much more threatening.

Changing an Industry and ElectrificationThe above-mentioned concerns have influenced lawmak-ers to institute higher fuel-efficiency standards for auto-makers54.5 mi/gal by 2025 Corporate Average Fuel Economy (CAFE) Standards in the United States, for instance. As a result, many newer automotive propulsion systems use some degree of electrification, increase

efficiency, and/or substitute some of the petroleum fuel with electricity to offer consumers a more efficient choice for personal transportation.

As mentioned previously, massive amounts of petro-leum fuel are being burned in very inefficient convention-al vehicles, so most of the fuel burned for transportation is wasted because of the current inefficient powertrain designs used to propel excessively heavy vehicles. The CAFE standards increase is intended to require improve-ments in transportation efficiency so transportation can continue to grow without burning astronomical amounts of fossil fuels unnecessarily. Several strategies are being pursued by automakers to achieve greater efficiency, including more efficient ICEs, various designs for electri-fied propulsioneither as the only propulsion mecha-

nism, as in a fully electric vehicle (EV), or blended to some degree with an ICE, as in a hybrid electric vehicle (HEV)or other methods of generat-ing the electricity to power electric propulsion motors. These approaches are intended to improve powertrain efficiency. Another approach, reducing the weight of the vehicle, reduces the power required to accelerate the vehi-cle and, therefore, the amount of fuel required to move the vehicle and its payload down the road.

As engine makers strive to meet the new CAFE standards, new com-bustion techniques are being used to extract more of the energy available in fuel to gain engine efficiency and reduce unburned fuel for lower emis-sions. One approach, reducing the

compression ratio in diesel engines, has the added bene-fit of enabling lighter aluminum engine blocks to replace heavier steel, reducing the vehicles weight. In addition, advanced engines increasingly benefit from more sophis-ticated electronic control units. Improvements in ICEs can be amplified significantly when used in conjunction with electric motors in the powertrain. Examples include dif-ferent electrification levels from the engine startstop technology and the integration of the battery starter-gen-erators to various hybrid electric powertrain configura-tions with different hybridization factors.

Electrically Propelled VehiclesElectrically propelled vehicles can be on the order of 8090% efficient at converting electrical energy into forward motion, but storing enough electricity for long-distance travel is expensive. Consumers not yet familiar with EVs might believe recharging times should be no longer than refueling a petroleum-powered vehicle with fuel. However, a survey of EV owners/drivers indicates most charging takes place at home overnight, and drivers quickly adopt

Plug-in hybrid and range-extended EV designs that draw much of their energy from the grid are a great way to use energy from the grid, particularly if their battery packs are charged overnight.

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new behavior patterns consistent with the characteristics of the new technologies.

Traditional autos meet the needs of both short com-mutes and longer-distance travel with one vehicle and have thus set that expectation for most of the 1 billion auto owners around the world. While an electrically powered vehicle is far more efficient than an ICE-powered vehicle, storing enough electricity for long-distance travel demands several new strategies for future transportation vehicles.

An HEV, such as the popular Toyota Prius, uses an ICE in combination with two more-efficient electric motors to propel the vehicle. This strategy demonstrates an effective way to increase fuel efficiency without compromising long-distance travel. This blending of efficient electric motors with a smaller ICE that can operate at a more efficient engine speed uses the advantages of both technologies and overcomes the obstacle of the very large battery packs required of electric-only vehicles for long-range travel. When the Prius was released in North America, its average fuel economy was about 44 mi/gal, roughly double that of the fleet aver-age (22 mi/gal) at the time. Its rise in popularity placed a renewed emphasis on fuel economy in North America and, along with emboldened policy makers and consumer demands, spurred other automakers to improve their fuel economy in response.

All-Purpose Vehicles Versus Specialty VehiclesThe current ICE vehicles are designed to accommodate local commutes and distant travel with one vehicle. If con-sumers require one vehicle to meet all of their driving needs, then a vehicle designed to travel locally on electric-ity and use petroleum fuel in a range-extended EV pow-ertrain for longer trips is a logical solution. A current effective example of this type of vehicle is the Chevy Volt.

Another strategy for meeting different driving needs might be to commute locally with an all-EV but fly or rent a hybrid electric car or range-extended EV suitable for longer trips when long-distance travel is required. This notion arises because EVs are more suited to local use, where they can easily return to a place for charging, which is likely to require some time to complete. That said, the question is whether to choose different vehicles for different missions or a single vehicle designed to meet most drive cycles or missions. Both vehicle designers and consumers will likely face this ques-tion for another decade or so.

Similarly, a strategy for a family with varying needs might be to own a vehicle for the daily local commute, such as an all-EV, as well as a second car suitable for longer trips. Some forward-thinking companies are offering transporta-tion services that combine the availability of several types of vehicles for use by customers, depending on their

current needs, often priced as a monthly service fee. This concept offers driving flexibility without the cost of owner-ship of multiple vehicles. One example is the Smart2go pro-gram, a car-sharing program in Europe from Daimler AG.

Vehicle Weight and Energy ConsumptionEnergy consumption is determined by a combination of the efficiency of the powertrain and the weight of the vehicle. While powertrain design and optimization are topics beyond the scope of this article, a typical electric powertrain might be on the order of 7590% efficient. The vehicle mass or weight also has a significant impact on the overall energy consumption. A reasonable (perhaps overly simplified) rule of thumb is that every 10 lb (4.5 kg) of vehicle weight adds 1 Wh/mi of energy consumption; so a 3,000-lb vehicle is likely to have an energy consump-tion on the order of 300 Wh/mi, while a 1,000-lb vehicle is likely to consume around 100 Wh/mi. The actual con-sumption will depend on the driving conditions, speed,

aerodynamics, rolling resistance, etc., but assuming constant values for those variables, the vehicle weight is clearly the most important variable in determining the energy consumption of the vehicle at lower speeds. At highway speeds, aerodynamic drag also becomes important. A vehicle that consumes less energy can achieve a given range with a smaller battery, reducing the battery cost and the overall cost of the vehicle. There-fore, reducing the vehicle weight reduces energy consumption and, thus, the battery capacity require-

ment for a given range and, in turn, the cost of the vehicle.If lower costs and fuel savings are not enough of an

incentive, this lighter is better relationship also extends to performance. The lighter the vehicle, the better the acceleration with a given power. The conventional approaches toward lightweighting, as a means of improv-ing fuel economy, target a reduction in the vehicle weight of 100300 lb. In the example above, we considered a weight reduction of 2,000 lb, with spectacular results. Therefore, to eliminate any confusion between minor reductions in the weight (a few hundred pounds) of a 3,0004,000-lb vehicle and vehicles designed to perform at a weight of 1,0001,200 lb, perhaps we should refer to our 1,000-lb vehicle concept as a superlight vehicle. Based on acceleration models, with all other variables being equal, a 3,000-lb vehicle with the power to accelerate from 0 to 60 mi /h in 9 s would accomplish the same 060-mi/h accel-eration in roughly 3 s if the vehicle weighed 1,000 lb. Con-versely, if such high performance is not necessary, one could achieve the same 9-s acceleration as the 3,000-lb vehicle with one-third the power applied to the 1,000-lb vehicle. To summarize, superlightweight designs improve

Energy consumption is determined bya combination ofthe efficiency ofthe powertrain and the weight of the vehicle.

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the energy consumption, battery size or range, cost of the vehicle, and acceleration performance. With so many dimensions of improvement, it is difficult to argue against making vehicles lighter, but perceptions can be obstacles.

At the time of this writing, reports of the Volkswagen XL1 research vehicle point to the possibilities for efficient vehi-cles. The XL1 has reported 261 mi/gal (0.8 L/100 km) from a lightweight vehicle made largely of carbon fibers. It uses a diesel-electric plug-in hybrid powertrain and demonstrates efficiencies thought to be impossible only a decade or so ago.

Perception and Buyer BehaviorConsumers purchase cars based pri-marily on emotional attachments and perception. Choices are often made based on the image the pur-chaser desires to project or an image that appeals to him/her. The math in the above examples clearly indicates that lighter vehicles are better in many ways, for both the driver and others. However, consumers often associate the social status conferred by a car with its size, plush interior, or styling, for instance, just as a vehi-cles performance has often been judged by the roaring sound of its engine. To the extent that our 1,000-lb vehicle is large, with a plush inte-rior and great styling, the customer might become emotionally attached, but if the same vehicle is small, lacks a luxury interior, has less than stellar styling, and makes no roaring sounds, then the lack of traditional status symbolism may not engage the customers emotions. Thus, engi-neering a very efficient vehicle may be much easier than marketing one.

To be successful with light vehicles, consumers must make emotional connections with the vehicles. Tradi-tional dimensions such as performance and luxury have already been marketed heavily, but more subtle vectors such as energy consumption, informational displays, connectivity, safety, or other attributes specific to electri-fied vehicles or high technology require significant re-education and effort. Developing the right mix of elements to appeal to the senses and emotions of cus-tomers is as important as engineering the right blend of power during acceleration.

Energy Consumption and Battery CapacityIf you have not previously thought much about energy consumption in cars, our efficiency scenarios may not mean much, so let us further explore how energy con-sumption relates to a cars battery capacity, typically measured in kilowatt-hours (kWh). A 1,000-lb vehicle

consuming 100 Wh/mi will have roughly a 10-mi range with a 1-kWh battery pack (1,000 Wh/100 Wh/mi = 10 mi), whereas a 3,000-lb vehicle will consume roughly three times the energy and, thus, require three times the bat-tery capacity to provide the same range. To achieve a 60-mi all-electric range, our 1,000-lb vehicle would require a 6-kWh battery pack. To achieve the same 60-mi range, a 3,000-lb vehicle would require an 18-kWh battery pack.

Charging StrategiesIn the above examples, charging a 6-kWh battery pack for

the light vehicle could be accom-plished very quickly with a fast char-ger or in fewer than 1 h with a cost-effective 240-V Level 2 charger. Alternatively, a standard 120 Vac outlet (current similar to that of a small win-dow air conditioner) could be used for overnight charging. There are also sev-eral options for charging the 18-kWh pack of the heavier 3,000-lb vehicle. It could be done in fewer than 30 min with a dc fast charger or about 3 h with a 240-V, 30-A charger.

Consumers inexperienced with EVs surveyed by auto manufacturers often say they want their EV to charge as fast as they can currently fill their gas tank, with their thought process being that when they are traveling and their battery gets low, they want the convenience of pulling into a charging station and getting a

quick charge. However, when you ask the same question of someone who has used an EV for more than a few months, you get an entirely different answer. Experienced users expect to mostly charge overnight at their home. They only expect to charge at a public facility when they plan a trip outside of their normal commute range. In fact, many experienced EV users are skeptical of fast charging, concerned that it might reduce their battery life or battery capacity. One experienced EV user indicated that it takes about 30 s to charge the battery15 s to plug it in before bedtime and 15 s to unplug it in the morning.

As electrified vehicles proliferate, the time and intensi-ty of battery recharging will become an issue for grid oper-ators. While customers without experience believe they need to charge in very short times, experienced EV drivers find charging overnight to be quite convenient. However, battery size is a very important consideration for the cost of the vehicle, the time required to recharge, and the amount of peak power consumed to charge the battery. Bigger is not necessarily better.

To understand this issue, let us consider a battery pack of 1-kWh capacity. Charging this battery in 1 h would

Traditional autos meet the needs of both short commutes and longer-distance travel with one vehicle and have thus set that expectation for most of the 1 billion auto owners around the world.

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require 1 kW of power for 1 h. However, if we wanted to charge the same battery in 6 min (ten times faster), it would require 10 kW for 6 min. This example may not mean much to someone not used to computing power consumption, but to a grid operator, the peak power required to accom-modate recharging large batteries rapidly means knowing there is enough grid capacity at the location where charg-ing is to occur so as to accommodate the charging without overheating the nearest transformer. For a light vehicle with a relatively small battery pack, the problem is mini-mal. However, for a vehicle with a larger-capacity battery pack, the problem can be serious and costly for the grid operator and, therefore, the ratepayer.

As an example, the Model S from Tesla Motors is an attractive, high-performance all-EV with a high-capacity battery pack (either 60 kWh or 85 kWh). Consider that if you want to charge the battery to one-half of its capacity in 30 min, the power required to charge at that rate (assuming 100% efficiency) is 60 kW (60 kWh/2*2 = 60 kW). This amount of power needs to be carefully considered based on the capacity of the grids nearest trans-former, the time of day, and other local usage. However, if you want to charge the battery to 100% of its capacity overnight, it would be a more moder-ate charging rate, which would be handled much more easily by the elec-tric utility. In fact, charging over 8 h (overnight), when grid usage is lower and transformers are generally cooler, reduces the power requirement and the risk of transformer damage considerably.

Plug-In Range-Extended AlternativeThe Chevy Volt, with a 16-kWh battery pack, has a roughly 38-mi electric range under good conditions before a range-extender engine starts generating electricity for longer-range travel. The range-extender feature is intended to counteract the fear of running out of electricity, sometimes referred to as range anxiety. This strategy accommodates local electric driving with the ability to travel longer dis-tances by burning petroleum. It is aimed at a customer seeking all-purpose usage within one vehicle. Volt customers report that they fill their gas tanks about once per month or even once in several months, so petroleum fuel consumption is minimal. The battery capacity is noticeably smaller than that of the Model S and the car is less than half the price, but less often noticed is the demand on the electrical grid. The Volt can be charged at night, demanding less than one houses daytime peak energy requirement, and is therefore much more accommodating of the grid. In fact, if these cars were charged only at night, no upgrades would be required of the grid in most places for quite some time.

Charging ConsiderationsThere are several considerations for charging plug-in vehi-cles that impact the power required of the grid. First among them is the time of day when the battery is charged. In general, if vehicle batteries are charged between the hours of 10 p.m. and 6 a.m., when grid usage is at its minimum and local transformer temperatures are lower, they have the least negative impact on the grid. Charging during peak grid usage may require upgrades to the grid to handle peak demand that is higher than is experienced today. Second, chargers should be designed with a nighttime mode, to draw only enough power to charge the battery in the 8-h nighttime period, based on the known state of charge of the battery. Charging at night, when usage is low, improves the economics of the

utility grids, potentially reducing the cost of electricity. Third, with the cur-rent technology, optimal battery usage suggests plug-in hybrid and range-extended EVs are of great inter-est because of their high energy and power efficiency. All-electric propul-sion is exceptionally more attractive when employed for short local trips, while hybrid electric propulsion is more practical for longer-range trips. This strategy reduces petroleum usage dramatically (often 5070%, depending on usage patterns) and optimizes the usage of electric local travel without prompting high grid-upgrade costs. It optimizes petroleum usage, electric efficiency, battery capacity, and grid power demand

the key elements in an electrified transportation system. Some 85% of trips are 20 mi or shorter in the United States and, thus, would consume only electricity, with petroleum available for longer trips. With this configuration, the bat-tery size can be constrained to 8 kWh on a traditional vehicle, providing 20 mi of range, or 6 kWh, providing the same 20 mi of range, on a vehicle weighing 2,000 lb or less.

Sensible Design StrategiesSensible design strategies using current technologies sug-gest that vehicles should provide a balance of efficient energy usage for each type of drive cycle while minimiz-ing the impact on the energy source, whether electricity from the grid or petroleum from the pump. Electricity con-sumed from the grid is typically 1020% of the cost of petroleum and offers considerable improvements in air quality, so it should be the primary source of energy for transportation where practical. At the current state of technology, petroleum is still the primary source of energy for long-distance travel and will likely remain so for a decade or more; HEVs offer the best powertrain options for long-distance travel.

Developing the right mix of elements to appeal to the senses and emotions of customers is as important as engineering the right blend of power during acceleration.

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The scenario in which plug-in hybrid or range-extended EVs incorporating moderate-sized batteries for local elec-tric travel and small ICEs for long-distance travel are used offers an exceptionally attractive and efficient single-vehicle travel solution. This approach has the potential to reduce petroleum consumption and the associated pollution by as much as 80% per vehicle, dramatically reducing the overall cost of energy consumed by transportation. This strategy also reduces or delays infrastructure investment in grid upgrades and charging stations, which are required for all-EVs with higher-capacity batteries, while using the strengths of the current and emerging technologies to their fullest.

SummaryMost currently available conventional automobiles have very inefficient propulsion systems that waste most of the fuel burned for forward motion. The wasted fuel contrib-utes to air pollution and has a high economic cost. Electri-cally powered vehicles can improve transportation efficiency, dramatically lowering fuel consumption and reducing emissions in the process. Yet, burning some fossil fuel to extend the range of current battery storage is a practical option that is cost effective. Plug-in hybrid and range-extended EV designs that draw much of their energy from the grid are a great way to use energy from the grid, partic-ularly if their battery packs are charged overnight. This approach allows for gradual electric utility grid infrastruc-ture upgrades over a period of a decade or two to accom-modate a gradual shift toward electricity as the primary fuel for transportation. Plug-in hybrids also avoid the need for fast charging during the day because they can burn small amounts of fossil fuels to fill the gap beyond the bat-tery capacity instead of stopping for a rapid charge at peak power loads. Plug-in hybrid architectures best blend the usage of electric motors with smaller, more efficient ICEs to achieve very efficient and sensible transportation solutions that appeal to consumers.

While consumers often make purchase decisions based on emotional considerations or, perhaps, on perceptions based on their past experiences, consumers and vehicle designers alike should think beyond the notion of style and luxury to optimize a broader set of considerations related to efficiencies, economic impact, and the vehicles impact on its infrastructure. Keep in mind that roughly 1 billion vehi-cles are in use on our planet today, and any inefficiency, multiplied by a billion, results in a huge waste. Well-thought-out designs can result in improved efficiency of our transportation system globally and represent a huge economic opportunity for those who view and understand the impact of our global transportation system on the

health of our planet and our way of life. A growing number of people are being excluded from personal transportation, either by economics or increased urbanization, but the utili-zation of the available technologies and better design strat-egies offers the promise of better transportation solutions for generations to come.

For Further ReadingA. Emadi, Transportation 2.0: ElectrifiedEnabling cleaner, greener, and more affordable domestic electricity to replace petroleum, IEEE Power Energy Mag., vol. 9, no. 4, pp. 1829, July/Aug. 2011.

A. Emadi, Handbook of Automotive Power Electronics and Motor Drives. New York: Marcel Dekker, 2005.

Electrification Roadmap: Revolutionizing Transporta-tion and Achieving Energy Security, Electrification Coali-tion, Nov. 2009.

A. Emadi, S. S. Williamson, and A. Khaligh, Power elec-tronics intensive solutions for advanced electric, hybrid electric, and fuel cell vehicular power systems, IEEE Trans. Power Electron., vol. 21, no. 3, pp. 567577, May 2006.

S. M. Lukic, J. Cao, R. C. Bansal, F. Rodriguez, and A.Emadi, Energy storage systems for automotive applica-tions, IEEE Trans. Ind. Electron., vol. 55, no. 6, pp. 22582267, June 2008.

S. G. Wirasingha and A. Emadi, Classification and review of control strategies for plug-in hybrid electric vehicles, IEEE Trans. Veh. Technol., vol. 60, no. 1, pp. 111122, Jan. 2011.

BiographiesRandy Reisinger ([email protected]) is the industry liaison manager for the Canada Excellence Research Chair in Hybrid Powertrain Program at McMaster University in Hamilton, Ontario, Canada. In 2006, he was senior advisor for CalCars.org, promoting the first plug-in Prius devel-oped by CalCarss Ron Gremban. In 2010, he cofounded Sugar Rides Inc., which built an 800-mi/gal-equivalent EVprototype, then participated in the Automotive X-Prize Competition with finalist Team TW4XP.

Ali Emadi ([email protected]) is the Canada Excel-lence Research Chair in Hybrid Powertrain and director of the McMaster Institute for Automotive Research and Technology at McMaster University in Hamilton, Ontario, Canada. Before joining McMaster University, he was the Harris Perlstein Endowed Chair Professor of Engineering and director of the Electric Power and Power Electronics Center and Grainger Laboratories at Illinois Institute of Technology, Chicago.

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2325-5987/13/$31.002013IEEE IEEE Electr i f icat ion Magazine / DECEMBER 2013 13

INCE 2002, THE RAIL transport industry in Spain has increased its consumption of electricity by 6% annu-

ally. Although one of the arguments used to defend the increased con-sumption is that rail is the transport type that consumes the least energy per passenger, it does require an enormous input of energy, which demands continuous improvement, above all with regard to safety and energy saving.

The saving of energy, quality of sup-ply, reliability, reduction of power surg-es, capacity to accelerate during start-up, and losses of voltage in the over-head power line are some of the topics that must be taken into account when discussing research in railway trans-port. These aspects become especially problematic when dealing with railway dc power lines, that is, regional net-works, light railways, and metropolitan network trains. In these cases, due to the technology in the diode rectifiers contained in electrical substations, the flow of power is only admitted from the network toward the power line and not in the opposite direction. On the one hand, this simplifies the electrical substation and its connection to the transport network; on

the other hand, it renders impossible the option of per-forming a regenerative braking (returning the electrical energy), with the consequential loss of energy that the Spanish Manager of Railway Infrastructures (ADIF) has estimated can be around 20% of the energy consumed.

The first alternative to this problem is the revision of the technology in substations, converting them into


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Improving railway efficiency with energy storage.

By Marcos Lafoz Pastor, Luis Garca-Tabars Rodrguez, and Cristina Vzquez Vlez

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reversible substations that are capable of injecting power surges into the network from the power line whenever there is energy from trains braking, which would mean a reduction of the electrical security problems within the system. The second alternative of interest is the storage of energy, which allows for a much wider energy utiliza-tion scenario, with the railway substation eventually becoming in a sense, a micronetwork. In railway lines with a lot of traffic, the energy from braking is usually consumed by another train that is accelerating at that moment. So in those cases, energy storage is not a reli-able option, but in railway lines with less traffic, it results in an improve-ment in the efficiency.

The recommended levels of power and energy in energy storage for short-distance trains depend on the braking conditions and the type of machine. The power is between 500 and 2,000 kW. On the other hand, braking time is around 10 s. So high-power and low-energy systems would be required.

Energy-Storage Systems Suitable for RailwaysStoring energy is a way to modify the basic equation of electrical energy production, which states that the energy produced must equal the consumed energy. If an energy-storage device (ESD) is present in the network, the equation is modi-fied; the produced energy is now the sum of the con-sumed and the stored energies with their correspond-ing signs: plus when storing and minus when pump-ing back. This means that storing is a way to decouple the offer and the demand at a certain moment. ESDscan be classified depending on the power and energy densities. Massive energy storage, such as hydropump-ing power or compressed air, usually requires certain geographical conditions, whereas fast ESDs such as some types of batteries, super capacitors, flywheels [also called kinetic energy-storage systems (KESSs)], and

superconducting magnetic energy storage (SMES) do not have those restrictions. In the case of capacitors, the energy is stored in the electrical field; in the case of a flywheel it is stored as kinetic energy in a rotating ele-ment; and in a superconducting magnet, it is stored in the magnetic field of a lossless inductor.

Any ESD can be defined by two basic parameters, the power and the energy. Usually, both are independent, resembling the flow rate and the volume of a water tank. For instance, one can imagine a big deposit (high energy) with a small drain (low power) or any other combination

of these two variables. It is also very common to speak in terms of energy and power densities, normalizing with the mass or the volume of the device.

The key question here is: what type of energy storage is the most appropriate for a certain application? This is not an easy question to answer, given that it depends on the specific problem. The majority of the work published on this matter and the research in recent years have been based on battery technology and ultracapacitors (condensers with a capacity of thousands of farads), although other interesting options can be considered, such as flywheels. This type of energy storage is called

kinetic energy storage in general and, more specifically, a flywheel energy storage system (FESS) when such devic-es are used. This provides an attractive solution for two main reasons: high-density levels of power and energy can be achieved, and the total cost does not grow linearly with the increase of power and energy, for example, in the case of batteries. Therefore, flywheels will be the technology discussed in this article.

Another issue to be discussed is whether to install the ESD onboard or trackside. The options for onboard storage are normally more efficient, taking into account the fact that they work closer to the site of consumption and less power is lost during transmission. On the other hand, they

Flywheel

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(a) (b) (c)Figure 1. The kinetic energy-storage components: (a) the flywheel, (b) the electrical machine, and (c) the power electronics.

Storing energy is a way to modify the basic equation of electrical energy production, which states that the energy produced must equal the consumed energy.

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bring with them extra weight and require space, which sometimes is unavailable, and so offboard storage applica-tions, connected to the railway power line, may be almost a necessity.

Kinetic Energy Storage Based on FlywheelsA kinetic energy-storage system is a flywheel driven by an electrical machine, able to work as a motor or a generator. When the machine (acting as a motor) exerts a positive torque on a flywheel with a certain moment of inertia, it increases its speed at a rate torque/inertia, until it reaches maximum velocity, storing a given kinetic ener-gy. At this stage, the energy can be kept constant by just supplying the idle losses with the motor. For releasing the energy, the electrical machine (acting as a generator) applies a negative torque to the fly-wheel, braking at a rate (torque/inertia) and pumping the energy back to the source to where it is connected.

The relevant issue in the storage system is how much energy can be stored per mass or volume unit (some-times preference is given to cost unit). In the particular case of the flywheel, in theory, all the energy one would want can be stored, provided that the flywheel is large enough. The question is how to do so in an efficient manner and within the limits allowed by the technolo-

gy currently in use. Two main groups of technologies are being developed for flywheels: metallic and compound materials. The for-mer is relatively slow (below 10,000 r/min). The wheel is metallic and often has magnetic levitation sys-tems, which offsets its weight. These slow storage systems are, in theory, simpler in a technological sense, and their main use is in sta-tionary applications, where their weight is not an obstacle. There is also another family of flywheels,

rapid ones, that can achieve a velocity of 50,000 r/min and use wheels made of composite materials such as

Material m (MPa) l (kg/m3) eM (kJ/Kg)

Steel (AISI 4340)

1,800 7,800 140

Alloy (AlMnMg)

600 2,700 135

Titanium (TiAl62r5)

1,200 4,500 162

Fiberglass (60%)

1,600 2,000 485

Carbon fiber (60%)

2,400 1,500 970

Transport Power UPS Industry

Railway (light trains, intercity)

Frequency regulation and support

Critical loads (communica-tion centers, airports, hospitals, etc.)

Cranes, motor starting

Trams, buses

Renewable energies

Ferries

Automobiles

TABLE 2. Applications of Flywheels.

The efficiency during the power exchange between the flywheel and the load depends highly on the power level.

CLR (dc/ac)

CLM (dc/ac)230 V

SRM + Flywheel750 V

400 V

Catenarydc/dc

Figure 2. The connection schema for the flywheel equipment. CLM: machine-side converter; CLR: grid-side converter; SRM: switched reluctance machine.

TABLE 1. Materials Used in Flyweels.

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carbon fibers, which offer high lev-els of mechanical resistance and low density. The elevated cost of the wheel and the difficulty of manu-facturing mean that its use is restricted in general to applications of limited energy in which the sys-tem price is not a critical issue.

Table 1 shows that greater densi-ties per mass unit are achieved using compound materials (ideally carbon fiber). However, if the concern is to achieve energy per volume unit, metals such as steel can be as effective as fibers and are much more economical. Table 2 displays some applications of these storage sys-tems, classified bysectors.

Description of the Technology The flywheel developed for the railway application [Advance Energy Storage Systems Project (SA2VE)] is made of high-resistance forged steel, weighing 6 t and capable of

spinning at a speed of 6,500 r/min, and it has an energy-storage capacity of 200 MJ (55 kWh). The mechanical part is also formed by conventional ceramic bearings. Given the load on the axle and the highly elevated rota-tion speed, a magnetic levitation sys-tem has been put in place using per-manent magnets and an electromag-net to give regulation capacity, reducing the stress on the axle and therefore the sizing of the bearings.

The electrical machine, which serves for acceleration and braking (depending on whether one wishes to store or release energy), is a switched reluctance machine operat-ing at 350 kW and that turns at the same speed as the wheel. This machine is connected to the electrical network through a series of power electronics equipment, which are given the role of properly adapting voltage and fre-quency levels between the supply and the machine and the network to which the system is connected. The control

(a) (b)

(c) (d)

Figure 3. The equipment associated with the flywheel energy storage at the ADIFs railway substation in Madrid, Spain. The (a) entrance to the security pit at the ADIF substation, (b) power electronics converters, (c) the flywheel energy storage device, and (d) the control room. (Photos courtesy of ADIF.)

A kinetic energy-storage system is a flywheel driven by an electrical machine, able to work as a motor or a generator.

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hardware has been developed specifically by the Centre for Energy, Environment and Technology Research (CIEMAT) for this application and is based on a distributed network of microcontrollers, which manage all the electronic devic-es and inform the system operators command center, from where they can be controlled.

The system developed for this application offers a series of advantages in comparison with other commer-cial devices, for example, simplicity and robustness, and it has been designed with aim of providing competitive

technology as opposed to other types of technology such as chemical batteries and ultracondensers (Figure 1).

Experimental Commissioning of the Energy-Storage System in a Railway SubstationThe complete system was first installed and tested in the CEDEX Laboratory in Madrid, Spain, in conditions similar to those of the substation in terms of voltage and power levels. Different operation modes, the thermal behavior,

Figure 5. The speed versus time operation profile.

3,500 r/min

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Figure 6. The speed maintenance around the reference value, waiting for a load to be absorbed or delivered (the time scale is not real).

Maximum Speed

Recovery Speed

t

P

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Figure 7. Monitoring environment: (a) voltage and (b) current evolution of the railway power line.

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the response capacity, and the reli-ability of the communication and controls were checked during this stage. Finally, the system was moved to the Cerro Negro railway substation in Madrid, owned by ADIF, where it would be commissioned. The connec-tion schema for the system is shown in Figure 2.

For safety reasons, the flywheel was installed in a protected pit locat-ed in the high-voltage equipment area of the railway substation. The power electronics and control equip-ment were located in the substation building, as was the system control post, though it is not necessary to operate it from there, as it can be accessed using a remote connection from the ADIF substation control cen-ter. Figure 3 shows the equipment installed in the substation.

The automatic operation mode of the system has two control vari-ables: the current consumed by the railway power line from the substa-tion and its voltage. Depending on these variables, the ESD operation is defined as shown in Figure 4.

If there is no current consumed from the substation and the overhead power line voltage exceeds a certain maximum limit, it can be concluded that the train is returning energy to the line. In this case, the flywheel must store this energy. On the other hand, if the voltage is below a certain limit and some level of current con-sumption from the substation is detected, this indicates that the train is demanding an input of energy. In this case, and if the storage device has available energy, this energy will be provided to the train, and so network consumption will be reduced.

The ESD will maintain a reference speed or status that will depend on the given operation criteria. For this partic-ular case, the flywheel is set at the low-est operation speed range (2,000 r/min) awaiting a train braking. In this scenar-io, the braking energy would be absorbed to increase the flywheel speed. That energy can be directly returned, a short time afterward to the same or another train accelerating, so the flywheel speed would be reduced again. This operation is described in Figure 5.

Speed will tend to fall after a certain amount of time due to energy losses, and so to maintain flywheel speed around a certain value, it should be recovered through a small power input from time to time. Figure 6 shows the

Figure 8. The details of electric currents at the electrical machine of the ESD in (a) storage mode, (b) supplying mode, and (c) transition during a flywheel demand.

(a)

(b)

(c)

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evolution of speeds over time with these additional power inputs.

Mechanical losses are the most important losses in a flywheel, so it is important to reduce them as much as possible. To minimize them, two important measures have been con-sidered: the use of magnetic levitation to lessen the bearings axial loada ring of permanent magnets and an additional electromagnet for fine reg-ulation have been disposed to do soand the reduction of the air pressure around the flywheel to make smaller the aerodynamic lossesan inner pressure of 100 mbar is accomplished. This results in an average efficiency of 98% when there is no power exchange. The efficiency during the power exchange between the flywheel and the load depends highly on the power level, and it is mainly associated with the copper losses at the electrical machine, being in the range of 8090% in the case of our study. Figures 7 and 8 show the results obtained with the flywheel during the tests in the railway substation.

Figure 7(a) shows the operating and monitoring envi-ronment while the system is working, and Figure 7(b) shows a sample of the power line voltage and current injected by the system for storage in a series of tests regarding absorption and power injection. Figure 8 shows the results of the dc voltage and the current managed by the electrical machine in motor mode (storage mode) and generator mode (recovery mode), and the transition between stand-by state and consumption state, which is produced in a few milliseconds. The dc-voltage oscilla-tion is lower than 10%, an acceptable value for the elec-tric system.

As their power lines are high-capacity transportation lines, a more extended concept of energy manage-ment can be considered for railways, by integrating the train traffic and the grid stability with renewable energy generation, electric vehicle recharging, and energy storage of different types to increase the reliability of the whole system. Some projects are currently being carried out in Spain based on this idea. Figure 9 shows a schema of an application example where the energy management is carried out from a railway power line, with mechanical energy storage, storage

with batteries and ultracapacitors, renewable energy gener-ation, electric vehicle charging, and the presence of the grid.

This article presents the technology of flywheels applied to railway transport, but it does not mean that it is the most appropriate for this application. Only the combi-nation of different technologies of energy storage and power supply, advanced control devices, and optimized strategies of operation will provide the increase in effi-ciency and reliability that such complex systems require.

BiographiesMarcos Lafoz Pastor ([email protected]) is with Cen-tro de Investigaciones Energticas, Medioambientales y Tecnolgicas in Spain.

Luis Garca-Tabars Rodrguez ([email protected]) is with Centro de Investigaciones Energticas, Medioambien-tales y Tecnolgicas in Spain.

Cristina Vzquez Vlez ([email protected]) is with Centro de Investigaciones Energticas, Medio-ambientales y Tecnolgicas in Spain.

Auxiliary Service Lab230 V45 kV

Power Line CLM1

:+ 1

CLM2

:+ 2

dc/dccat CLR

dc/dc1EV FastChargers

400 V700 V3,500 V

dc/dc2 ac/dc

PV PanelUC1 ES

Figure 9. A microgrid concept based on railway power lines. UC: ultracapacitors, EV: electric vehicle, PV: photovoltaic panel, and ES: energy storage.

For safety reasons, the flywheel was installed in a protected pit located in the high-voltage equipment area of the railway substation.

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2325-5987/13/$31.002013IEEE IEEE Electr i f icat ion Magazine / DECEMBER 2013 21

MICROGRID IS A CLUSTER OF DISTRIBUTED generation (DG), renewable sources, and local loads connected to the utility grid. Amicrogrid provides a solution to manage local generations and loads as a single grid-

level entity. It has the potential to maximize overall sys-tem efficiency, power quality, and energy surety for critical loads. The Microgrid Exchange Group, an ad hoc group of expert and imple-menters of microgrid

technology, has defined a microgrid as a group of inter-connected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island mode.

Microgrids can operate in parallel to the grid or as an island. The most compelling feature of a

microgrid is its ability to separate and isolate itself from the


The Role of Energy Storage in a Microgrid Concept

Examining the opportunities and promise of microgrids.

By Qiang Fu, Ahmad Hamidi, Adel Nasiri, Vijay Bhavaraju, Slobodan (Bob) Krstic, and Peter Theisen

TOWN COURTESY OF WIKIMEDIA COMMONS/SIMISA

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utilitys distribution system during grid events, i.e., faults, voltage collapses, and blackouts. It may also intentionally disconnect during grid maintenance and when the quality of power from the grid falls below certain standards. A microgrid can be reconnected to the utility grid without any interruption once grid stability is recovered.

Figure 1 shows the concept of a future microgrid for a small town. There are several types of genera-tion, including renewables and non-renewables. Renewable sources, e.g., solar photovoltaic (PV), can be more distributed. Energy-storage systems (ESSs) are also distributed, but their controls have to be coordinated to support frequency and voltage. Aproper mixture of energy sources and management of the resources thus becomes an important requirement for the future high-reliability industrial parks or campuses and townships with energy surety mandates.

Microgrid Drives: Benefits and BarriersThere are several technical drivers behind the idea of microgrids, including [ utility transmission constraints requiring that sup-plies be located closer to their loads [ demands for improved power reliability, efficiency, and quality [ a desire for energy security[ the integration of renewable energy and distributed energy resources[ military demands for enhanced energy securitysure-ty, survivability, supply, sufficiency, and sustainability[ higher system efficiency (e.g., use of generation waste heat in a combined heat and power installation).

In addition, the lower costs of solar PV installations, natu-ral gas, and energy-storage devices have been supporting further expansion of DG and microgrids.

The microgrid concept provides opportunities for eco-nomic development in the electric power and clean energy industry. According to Navigant Research, the microgrid mar-ket was at US$10 billion in 2013 and projected to increase to more than US$40 billion annually by 2020. In addition to eco-nomic development opportunities, microgrids are envi-sioned to be environmentally friendly and a promising way of building net zero energy communities, which have the ability to operate separately from bulk grid and sustain themselves in the event of a grid outage. This is crucial for critical infrastructure, such as hospitals, public facilities, mili-tary bases, and emergency-response facilities.

As envisioned, a microgrid would provide added value to society, the grid, and customers by improving energy

reliability; reducing the cost of energy; managing price vol-atility; assisting in the optimization of power delivery sys-

tems, including the provision for services, providing different levels of service quality and value to customers segments at different price points; helping to manage the intermittency of renewables; promoting the deploy-ment and integration of energy-effi-cient and environmentally friendly technologies; and increasing the resil-iency and security of the power deliv-ery system by promoting the dispersal of power resources.

However, some technical and non-technical barriers must be overcome to provide these benefits. The greatest technical challenges are monitoring, controls, and protection. A highly effi-cient and reliable supervisory and monitoring system has to be devel-

oped to accommodate a wide range of load and genera-tion variations. The communication and information layers play a critical role in the supervisory management for microgrids. A lack of standardized communication and controls has thus far limited microgrid development to custom designs and case studies. Other technical chal-lenges include microgrid sizing and planning, steady-state and dynamic performance, utility system and equipment upgrades, and interconnection requirements.

Additionally, to ensure that microgrids operate as legal entities, regulatory barriers need to be resolved, including regulatory policies, microgrid ownership models, the choice of voltage level, the legality of microgrids, service territories, utility tariffs, and environmental and sitting laws.

Ongoing Microgrid ActivitiesSeveral microgrid projects are currently under research and development in the United States, including the 100-kW Consortium for Electric Reliability Technology Solutions (CERTS) microgrid test bed near Columbus, Ohio, the 3-MW Santa Rita correctional facility test site in Alameda County, California, the Smart Power Infrastructure Demonstration for Energy Reliability and Security (SPIDERS) microgrids, the 700-kW Fort Sill microgrid project, and the Illinois Institute of Technologys Perfect Power System. The CERTS microgrid test bed uses advanced control techniques to perform seamless islanding and reconnection and apply the peer-to-peer and plug-and-play concepts for devices. One of the goals of the SPIDERS program is to provide reliable backup power during emergencies by integrating renewables and other DG sources into the microgrid and to ensure that crit-ical operations can be sustained during prolonged utility power outages. In the Fort Sill microgrid project, the objec-tive is to demonstrate a field-scale, renewable-focused, intelligent microgrid, which serves critical mission power

A highly efficient and reliable supervisory and monitoring system has to be developed to accommodate a wide range of load and generation variations.

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requirements in a sustainable, reliable, and secure manner. This microgrid includes two natural gas generators, one 500-kWh energy-storage element, small wind and solar PV systems, various loads, and a static switch. Worldwide, there are several other other experimental microgrid facili-ties under operation and construction as well.

Figure 2 shows the configuration of the Fort Sill microgrid, which has a rat-ing of 0.480 kV, 60 Hz, and 630 kW. It is connected to the utility grid through a 0.48-kV/13.20-kV transformer and a static switch. The generations in this microgrid include two natural gas gen-erators rated at 190 kW each, one 30-kW solar PV system, a 2.5-kW wind turbine, and a 250-kW energy-storage device. The solar PV and wind turbine generators are connected to the system through inverters operating in a cur-rent mode while the energy storage inverter operates in a voltage mode. The system also includes various motor loads and variable loads. The motor loads mainly include chillers, water pumps, and air compressors.

The microgrid concept has already been applied at the community level to provide benefits to customers. The first

microgrid in The Netherlands was built in Bronsbergen Holiday Park, Zutphen, mainly to improve power quality. The microgrid consists of solar PV systems on 108 houses, with a peak total generation of 315 kW and a peak load of 150 kW. Two battery storage systems with inverters and a grid tie switch have been added to convert the existing sys-

tem to a microgrid. An aerial photo of the park and the configuration of the microgrid are shown in Figure 3. The configuration is mesh type and cen-trally controlled.

Needs for ESSs in MicrogridsIt is well known that within an envi-sioned microgrid, various types of DG and customers create and demand varying active and reactive power profiles that may challenge the stability of the system. The ESSs, therefore, play a critical role in stabi-lizing the voltage and frequency of the microgrid for both short- and

long-term applications. From the device to system level, the ESS is a crucial element in the integration of DG into the microgrid. Researchers have employed various types of energy storage at the turbine and farm levels for wind

Wind TurbinesSubstation

GridSwitch

NaturalGas Generation

ESS

ESS ESS

Fuel Cell Energy

Solar Panels

Figure 1. A microgrid envisioned for a small town.

The inverter plays a critical role to regulate voltage and frequency and mange transitions to island andgrid-tie modes.

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energy to smooth the power intermittency and mak

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IEEE ELECTRIFICATION Dec 2013