One example of our dependence on electronic equipment is shown by the growth in the market for back-up power supplies. So the uninterruptible power supply (UPS) has been developed from a basic battery back-up unit to a system that can deliver a megawatt or more with enough backup time for an ordered shutdown of the protected equipment.
Conventional batteries are facing competition in this application. Their drawbacks include the need for charging because the usual trickle charging of UPS systems significantly shortens battery life. It leads to dried up electrolyte and corroded platesreducing potential life by up to 50per cent.
But new technologies are emerging. Plurion Systems has developed a new class of Redox technology for large capacity electrical storage solutions that provide the highest power density of any battery on the market.
It is based on zinc/cerium technology in an enclosed contaminant-free benignorganic electrolyte solution that provides a non-polluting and low maintenance operating environment.
The Redox batteries can be configured to provide 250 to 5000kW of emission-freemodular capacity storage with a discharge time of 1-6hours. This can be set to meet customer’s requirements as it depends on the size of the electrolyte storage connected to the battery.
They also feature a higher cell voltage than other batteries – at 2.4v it is 1v higher; they offer greater power densityenabling more power with fewer cellsresulting in lower installation cost; and use environmentally benign chemicals that require no electrolyte management systems to maintain battery performance.
Ultracapacitors
Also emerging is the ultracapacitorwhich is best suited to mission-critical environments that require relatively short power support. Installations that need support of the order of 60 secondswhether to simply buffer poor power transientsperform an orderly shut down or to transfer to a secondary source such as a genset or fuel cellcan use ultracapacitors.
They are reliable devices because of their composition and construction. The energy-storage mechanism is a highly reversible processrelying on the movement of ions within an electrolyte. Unlike a batteryan ultracapacitor doesn’t make or break chemical bonds. So it is capable of fast response and can deliver hundreds of thousands of cycles with minimal loss of performance.
Cycle depth is unimportantso ultracapacitors can be micro-cycledwithin 5percent of their total energyor full-cycled to more than 80percent of their total energywith the same long life. The highest performance ultracapacitors use advanced organic electrolytes that provide excellent conductivity to temperatures as low as -40°C and stable operation to 65°C. The best ultracapacitors are constructed of hermetically sealed packagespreventing the electrolyte from drying out. Ultracapacitors offer cell voltages between 1V and 2.5Vdepending on the chemistry.
But there will be slight variations between cells as a result of the manufacturing processas with conventional batteries. Variations in capacitance and leakage current affect the voltage distribution. But a cell management circuit maximises the performance and life of ultracapacitor cells and cell balancing is the most effective method of managing cells in series.
Passive balancing is most appropriate for low duty-cycle applications that can tolerate higher leakagesuch as in backup power systems. A passive balancing system is designed to overcome the inherent variations in leakage current by installing a resistor in parallel with each cell. The resistor is typically sized at 10 times the average leakage current of the cell. The benefits to this balancing method are simplicity and low cost. The drawback of this technique is slow response due to the linearity of leakage current with voltage and high parasitic losses due to the 10-times additional leakage current.
Because of the higher leakage current and slow responsepassive balancing solutions are not appropriate for high duty-cycle applicationssuch as dynamic load levellingnor in standby applications where the higher leakage current could jeopardise the backup energy source.
Active version
An active balancing circuit is required in high duty-cycle applications and where low parasitic losses are necessary. They may be used anywhere a passive system is useddepending on cost effectiveness in the specific application. In contrast to passive solutionsan active balancing circuit behaves nonlinearly and forces the cells to deliver equal voltageresulting in the most effective use of the ultracapacitor string.
A number of schemes are used to achieve active balancingand many are patented or patent pending. The benefit of active balancing is fastaccurate equalisation of the voltage distribution and minimal parasitic losses. The drawback is higher cost.
In high duty-cycle applicationssome mechanism to cool the cellswhether by air cooling the cells directly or conducting the heat to some type of heat exchangeris necessary to maintain the ultracapacitors within rated operating temperatures.
Charging ultracapacitors is one of the simpler aspects of power management. Assuming a cell balancing circuit is in placean ultracapacitor bank can be charged with almost any sourceso long as the applied voltage is less than or equal to the maximum rated bank voltage. Constant current and constant power sources work welland the ultracapacitor can tolerate high charge currentsas long as thermal management considerations based on duty cycle are taken into consideration.
The ultracapacitor’s impedance is so low that a power supply that contains short-circuit fault detection may tripgoing into a current limit mode and limiting the recharge time of the system. As long as the supply can feed a near-short and has an output voltage that is limited to the ultracapacitor bank’s maximum voltagealmost any supply architecture should work.
Ultracapacitors deliver energy by reducing the electrostatic field across the electrolyte. When this occursthe voltage on the ultracapacitor drops. Ultracapacitors obey the standard capacitor law for energy storage.
New type
Amongst the developments coming on to the marketTavrima has developed a special type of high voltage ultracapacitorESCap 85/270to meet megapower UPS requirements.
Frank Levpresidentsays there are industrial applications where a ride-through time requirement is of short durationexpressed in seconds as opposed to minutesfor which batteries are less economically viable than ultracapacitors.
The ESCap85/270 is capable of on-float continuous application in the industrial type UPS to feed inverters of 250kW and higher at 540v. There are only two capacitors required to be connected in series to meet the voltage requirement and several parallel branches for energy and power. This renders the important benefit of reliability because loss of several parallel branches does not incapacitate the whole storage system
The system comprising 10 capacitors per rack has been developed and sold to the Canadian Company IPS (International Power Systems) for its 500kVA UPS. Capacitors have also been supplied to American Bonitron Company for its industrial 540v UPS.
Tavrima has also recently developed a 600v ultracapacitor which will eliminate series connections altogetherproviding the highest reliability of storage systemessential for UPS.
Cell banks
Maxwell Technologies offers a UPS systems with megawatt power capability based on a bank of its ultracapacitors and says it is a straightforward exercise to make a number of cells into a single system.
One application is found on several cities’ metro systems including Madrid and Cologne. Siemens Transportation Systems has fitted a 1MW peak power unit that can deliver at this level for around 30–40 seconds. It is used for regenerative braking and to stabilise the voltage on the network.
The company says the key issue when choosing UPS technologies is how long will power be needed. For ultracapacitors this timeeconomicallyis of the order of secondsup to around 30 seconds. So they are viable for bridging powerwhile changing from one power source to anotherfor example. And as take-up of the technology increasesprices are dropping. The largest ultracapacitors from Maxwell Technologies are in its MC series.
But whatever technology is usedsystem management is advisable to ensure reliability of supply. Eatonfor exampleoffers Powerware software that enables users to preserve data integrity system-wide and to test all networked UPS systems from one node.
And for its conventional battery UPSthe company says its exclusive ABM technology uses a three-stage charging technique that not only doubles battery service lifebut also optimises battery recharge time and provides a 60-day advanced notification of the end of useful battery life.
This applies to the Powerware9315UPS which is ideal for mission-critical applications such as data centresserver farmsand transportation facilities. It is based on conventional battery technology and can be paralleled for both redundancy and capacity using patented Powerware Hot Sync UPS technology.
The ultracapacitor has been in the background for a few years nowbut take up is increasing as prices drop due to the R&D costs being recouped and manufacturing costs coming down. And the technology appears to have none of the great environmental burden that comes with conventional lead acid batteries.
For more informationvisit www.powerware.comwww.maxwell.comwww.plurionsystems.com www.tavrima.com
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