A PARADIGM CHANGE IN ENERGY STORAGE
Abstract: Ultracapacitor (UC) is useful in many pulse power applications. It finds applications in low end electronic systems like memory backups, mobiles, laptops and communication circuits as well as high end applications such as electric vehicles, rocket launchers and wind mill pitch control. In order to use it in any control or power circuit, it is necessary to understand technical and commercial aspects of this new kind of device. Various parameters of this device are required to be studied to induce it in the present energy storage system. Optimization of the ultra-capacitors based energy storage system is most important issue in the end product. This article presents brief over view various aspects of ultra-capacitor based system.
II. WORKING PRINCIPLE OF ULTRA-CAPACITOR
As shown in the figure 2, ultra-capacitor consists of two highly porous electrodes made up of activated carbon and metal oxides. These electrodes are immersed in the suitable electrolyte for charge transfer and storage mechanism. Typically electrode material has very high surface area of the order of 1000 square meter per gram of material. In the porous electrode, charges are accumulated in the pores resulting in formation of capacitance. Commercially available ultra-capacitors are carbon based with non-aqueous electrolyte.
III. ULTRA-CAPACITOR BASED SYSTEM
Electric energy storage requirements are fulfilled mainly by batteries and conventional capacitors. Electro chemical Double layer capacitor or Ultra-capacitor (UC) or supercapacitor is latest addition to it. Research work around the world is turning toward more advanced hybrid capacitors. Ultra-capacitors and hybrid capacitors both are essentially similar from the applications point of view. The difference lies in the effective use of space, construction and hence the cost. Ultra-capacitors are better than hybrid capacitors due to its life expectancy and higher energy density even though they are expensive. Very few industries are producing ultra-capacitors as commercial scale in the world and they offer ultra-capacitor based energy storage solution for many applications. For many of these companies, ultra-capacitor is ‘Add on” product or secondary product along with their main product. For faster growth of this technology it is necessary to that these ultra-capacitor manufacturing companies should focusing on ultra-capacitor development and some other UPS or power supply industries should take up the design and development of the ultra-capacitor based power source for various applications. This will result in more effective use of research and development fund towards technology development.
Applications of ultra-capacitors are mainly based on the combined battery- ultra-capacitor pack as energy storage system. Recent technological developments in consumer electronics, industrial automation, power sector and automobile sector are demanding advanced power source for proper function of system. For example it is required a “non battery” UPS with high power, high voltage, low response time and without maintenance to tackle short period voltage drops. Today, one can find electronic toys with ultra-capacitors to feed the energy to the internal circuit for short time duration. Some of Electric Vehicles (EVs) use this device along with battery. Thus ultra-capacitors can be used along with battery and suitable DC to DC converter or it can be used stand alone, depending on the system requirement. In advanced electrical energy storage systems, battery parameters, load parameters and system parameters are required to be studies to match it with characteristics of the ultra-capacitor. Table 1 show, some of the important parameters which are considered in ultra-capacitor based system. In some of the applications ultra-capacitors can replace batteries in near future. Hence charging it by solar cells and then using the stored energy in low voltage pulse power application such as photo studio camera, audio-video applications, X-ray machines, starting of automobile, wind mill pitch control and photo coping machines are gaining momentum. Defense sector has large number of applications such as electric gun and rocket launcher, where pulse power is required. Ultra-capacitor based power system offers good solution in these applications. High voltage (@ 220V) and high power ultra-capacitors can be widely used to solve various problems of power quality and control systems. In high voltage systems, series parallel connection of ultra-capacitors and battery packs becomes inevitable. Such series parallel connections can be in direct form or through electronic voltage balancing circuits. Control strategy of DC-DC converter and voltage balancing circuit plays important role in optimum use of ultra-capacitors in such system. Typical ultra-capacitor based energy storage system block diagram in shown in figure 3
PARAMETERS TO BE CONSIDERED FOR ULTRA-CAPACITOR BASED SYSTEM
|Size and weight||Size||Size and cost|
|Power rating and energy ratings||Power rating||Load current profiles|
|Self discharge||Self discharge||Power devices and their characteristics|
|Cost and maintenance||Cost||Switching strategy|
|Discharge characteristics||Charge and Discharge characteristics||Impedance matching issues|
|Internal resistance||ESR and time constant|
IV. PHOTOVOLTAIC SYSTEM AND ULTRA-CAPACITORS
Use of photovoltaic cells and ultra-capacitor is seriously considered in many applications due to the fact that photovoltaic system is energy rich-power poor system where as ultra-capacitor is power reach- energy poor device. Hence combine system can make a good power source. Today cost of photovoltaic system and that of ultra-capacitor is a main hurdle in their use in many consumer products. Charging with photovoltaic cells does not require any special circuit as ultra-capacitor takes the charges as much as we supply to it. These two devices are voltage compatible with each other and hence they can form stable power source. Ultra-capacitor with solar charging system can be used in many static pulse power applications. It can be used in dynamic load such as vehicles where small photovoltaic panel on the top of vehicle can keep SC in charged state for starting operation of vehicle. Here problem of self discharge of battery and ultra-capacitor can be overcome by charging them by photovoltaic system to keep them in ready to use condition. For effective use of these devices, one has to study different parameters of ultra-capacitor and photovoltaic system along with characteristics of load as mentioned earlier.
V. TESTING OF ULTRA-CAPACITOR BASED SYSTEM
In design and development of any ultra-capacitor based system, its testing becomes very important from reliability point of view. Battery testing and ultra-capacitor based energy storage systems testing are similar, except that later requires certain additional parameter measurement. Ultra-capacitor based system need to test power source and electronic convertor and ultra-capacitor parameters separately and in circuit. Some of the commonly used test/ measurements are- capacitance measurement, Equivalent Series Resistance (ESR) measurement, leakage current measurement, self discharge measurement, mechanical test related to passive inflammability, robustness of terminals, soldering quality, change of temperature, vibration, damp heat, life testing, storage, storage and immersion in cleaning solvents. Figure 4 shows the parameters to be tested for any ultra-capacitor based system.
Low ESR, low self-discharge ultra-capacitor can be very good device to store the electrical energy available from solar panels for pulse power applications. Today ultra-capacitor finds applications in energy storage systems of small portable applications to shave off the peak power demands. But with optimized ultra-capacitor battery power pack it can be used in many static as well as dynamic pulse power applications such as photo studio cameras, photo coping machines and X-ray machines and electric vehicles. In these applications, use of this device can enhance life of cells or battery. Due to its many advantages over battery it can give challenge to battery technology in near future. Cost, ESR and power density are main factors in its commercial use. Terminal voltage, current delivering capacity, self-discharge and reliability are very important parameters in photovoltaic based applications. For further technological development the material development, system simulation and modeling should progress together to develop smart electrical energy storage system for each application.