Energy storage and in particular electrical storage of energy has become a very talked about topics in circles, ranging from lay person in regard to hybrid and battery electric vehicles, to professional and certainly by legislators and energy policy makers in government. But even to professional the distinction between physical and chemical forms of electric energy storage are unclear and at times poorly understood. if at all. This book takes a critical look at physical storage of electricity in the devices known collectively as electrochemical capacitors and particularly as ultracapacitors. In its 12 chapters, this text covers ultracapacitors and advances battery topics with emphasis on clear understanding of fundamental principles, models and applications. The reader will appreciates the case studies ranging from commercial to industrial to automotive applications of not only ultracapacitors but these power device components in combination with energy dense battery technologies.
Inspec keywords: supercapacitors; hybrid electric vehicles; automotive electronics; battery powered vehicles
Other keywords: ultracapacitor; electrochemical capacitors; hybrid electric vehicles; energy policy; energy dense battery technology; battery electric vehicles; automotive; electric energy storage; power device components
Subjects: Other energy storage; Automobile electronics and electrics
Ultracapacitors, or to be technically correct, electronic double layer capacitors, form a subset of the general category of electrochemical energy storage devices, in particular, that of electrochemical capacitors (ECs). This chapter introduces electrochemical energy storage devices including electrochemical couples or batteries.
This chapter summarizes activities performed at Maxwell Technologies Inc. that were aimed at providing equivalent circuit models of the carbon-carbon symmetric ultracapacitor for use in computer simulations. In particular, these models are for users of ANSYS Ansoft Simplorer v.8 and Mathlab Simulink v.10 simulation software packages. These are behavioral-level models based on laboratory char acterization of production cells. Each model consists of three components: (i) cell parasitic elements, (ii) electrode dynamic representation of ESRdc and ESRac, and (iii) main branch EDLC, C(U), and parallel resistance Rρ representing leakage.
The evolution of symmetric electrochemical capacitor (EC) specific energy has been incremental over the past 30 years, mainly because of the limits on cell potential by the available electrolytes and purity of activated carbon. Referring to Figure 3.1 it is evident that cell energy tracks closely to cell potential, which for organic electrolytes has trended from 2.3 to 2.7 V at present. This averages to 20 mV/year in cell potential and clearly revolutionary change in cell potential will not happen to this class of ultracapacitor other than what available materials can provide. It is anticipated that cell potential will increase to 2.85 V, perhaps 3.0 V or even 3.1 V at most, for high quality activated carbon. For example, Panasonic commercialized the power ultracapacitor in the mid-1980s rated 470 F, 2.3 V, and 3.9 mΩ, the increase in performance has been incremental with cell voltage increase to 2.5 V in 1999 and to 2.7 V by 2006. Power evolution, on the other hand, has been more dramatic and is projected to continue increasing with improvements in materials, manufacturing process, and cell potential gains. One projection is that specific power of ultracapacitors will reach 20 kW/kg by 2015.
In this chapter our emphasis shifts from electrochemical fundamentals and equivalent circuit modeling to the application of ultracapacitor products in com mercial systems. In the nontransportation-related applications to be discussed, the operating voltage and power levels will be high, on the order of 100 s of kW to 10 s of MW in scale. For example, a commercial uninterruptible power supply may consist of a 900-V battery bank supported by an equivalent rated ultracapacitor bank that is capable of fully supporting such high-power loads for 15 s to 15min. Details and examples will be presented to highlight the selection of appropriate energy and power of the energy storage system to meet application goals and the methodologies employed during a design-in phase.
The industrial application of energy storage is a very broad topical area that cannot be adequately treated in a short chapter. To convey the scope of such a diverse range of applications, this chapter focuses on just three representative industrial areas: (1) material handling trucks such as forklifts and front end loaders, (2) cranes and hoists such as the rubber tired gantry crane used for container loading and unloading in shipyards, and (3) earth moving equipment such as excavators and drag lines.
This chapter is a continuation of ultracapacitor application case studies with focus on heavy transportation. Public transportation systems and vehicles in particular are becoming more of a focus by regulatory agencies as a sector to promote energy efficiency. Energy efficiency now has the connotation of energy security, offsetting of imported oil, and moderation of climate change through reduced emissions. All of these reasons mesh well with the trend to hybridize heavy transportation. Consider for the moment transit buses, the type we are most familiar with as a city bus, and one that is typically powered by a large CIDI engine burning diesel fuel.
Ultracapacitors are now beginning to be applied in low-end hybrid electric vehicles for primarily idle-stop feature. In reality, an idle-stop system is not a true hybrid electric vehicle, rather a microhybrid, since it applies no electric torque to the vehicle-driven wheels. The PSA Peugeot Citroen system consists of a Valeo iStARS (integrated Starter-Alternator Reversible System) that provides engine stop-start function by way of the alternator belt. The iStARS delivers, on its own, a 15% fuel consumption reduction on the New European Driving Cycle (NEDC) when integrated into 1.4 and 1.6 L HDi diesels. The market appears to be approximately 1 million such units by 2012. This microhybrid represents two industry, a belt-driven system for a diesel and ultracapacitor for energy storage.
This chapter discusses the application of ultracapacitors in strong hybrid vehicle systems that are designed around power-split transmissions.
Possibly one of the most important innovations in electrically variable transmissions (EVTs) has been the introduction of the electric 2-mode by General Motors Allison for application in transit buses. This innovation went on to become the hybrid technology vehicle of choice in the General Motors-DaimlerChrysler BMW Hybrid Development Center collaboration during the era 2004-2008. A major advantage of the 2-mode is the uniform rating of the two electric machines needed to implement the electric variator function is shown in the chapter.
Life cycle evaluation of ultracapacitors is the subject of considerable industrial interest, especially in terms of elevated temperature conditions and power cycling. The authors have investigated ultracapacitor parameter changes due to electrical and thermal stress and have explored their aging characteristics. Ultracapacitor life can be evaluated under two different characterization methods: (1) power cycling where the unit under test is charged from zero to its rated voltage, then cycled from rated to half-rated voltage repeatedly and (2) d.c. life under constant voltage and temperature stress. When we speak of power cycling, we think of the current and voltage waveform are shown in the chapter, where the current is a quasi-square wave of discharge and charge such that the ultra capacitor voltage is maintained within its voltage window.
The chapter focuses on ultracapacitor abuse. But what exactly constitutes abuse conditions? The answer is one or a combination of the following factors: voltage, temperature, vibration, shock, extreme current, such as short circuit, and reverse polarity. Clinton Winchester of Naval Surface Warfare Center (NSWC) Caderock division, Maryland, presented results of ultracapacitor cell overvoltage and overtemperature abuse testing performed for the application of these cells in Navy equipment. This reference on abuse testing is an excellent introduction to the topic because it illustrates the response these cells have to abuse. The abuse testing presented was prefaced by proper preparation of the test cells, in this case Maxwell Technologies 3000 F ultracapacitors. First, the cell is wrapped with thermal tape and thermocouples are installed on center of cell and at terminal posts. Second, the cell is covered with insulation to obtain near adiabatic conditions, and lastly, the cell is connected to laboratory instrumentation. After this, the testing begins and electrical and thermal responses are measured and logged using data acquisition equipment.
This chapter highlights some of the more promising methods of transmitting electric power wirelessly to a vehicle while stationary or moving. Stationary systems are today being heavily researched, and several companies are beginning to market such systems. Techniques to transmit large amounts of power to a moving vehicle are perhaps further in the future, but systems already exist that are capable of such feats.