By Bobby Maher
Backup power systems are an integral part of most, if not all, mission-critical installations. Services as varied as data centers, communications networks, and plant operations all rely on continuous availability of quality power. Traditionally batteries have been the Achilles heel of telecom sites and other power quality applications. The cost of replacement and the sub-par reliability required for full time energy availability and poor low temperature performance of batteries has always haunted telecom companies. Not to mention the environmental impact associated with the disposal of these devices. With the introduction of new backup energy storage systems, ultracapacitors are finding their ways into the power quality mainstream.
Ultracapacitors are energy storage devices with high power capability and long life. Desirable characteristics for use in high-power applications include: extremely long cycle life, wide operating temperature ranges, low weight, flexible packaging, zero maintenance, and environmental friendliness. Ultracapacitors are becoming the component of choice for engineers and designers with applications requiring short-term or peak (burst) power. They are ideally suited as stand-alone solutions for short-term power requirements ranging from a few seconds to a few minutes. Applications requiring many minutes to hours of back-up reserve energy require an additional supplemental energy source similar to a fuel cell.
Ultracapacitors, also known as electrochemical double layer capacitors (EDLC), are not a new technology. They have been around for decades and first appeared as a low-power, low-energy long life backup in consumer electronics such as VCR’s and alarm clocks. During the last 10 years there have been substantial advancements in this technology from material and construction as well as manufacturing processes that has made ultracapacitors an acceptable solution in many mission critical applications. The beneficial characteristics of ultracapacitors are possible due to their composition and construction. The activated carbon material used in ultracapacitors has a specific surface area of 2000m2/g and the charge separation is about 10 Angstrom or less.
The energy storage mechanism of an ultracapacitor is a highly reversible process, relying on the movement of ions within an electrolyte. With no chemical bonds being made or broken, cycle life of over 500,000 cycles has been demonstrated with minimal degradation. Wide operating temperatures of –40° to 65°C (or even higher for short durations) are possible due to the high conductivity low freezing point electrolyte. Long life is aided by hermetically sealed packages, preventing the electrolyte from drying out. With no mechanical moving parts and unlimited shelf life at temperatures up to 65°C ultracapacitors are completely maintenance free.
In today’s world performance and reliability are an essential part of every design. For engineers energy storage has always been the Achilles heal of their design. In the past, the solution to back up power has been batteries, predominately lead acid. Today, engineers have more options to attack back up power demands. These options include advanced battery technologies, fuel cells, solar cells, and double layer capacitors.
Li-ion, NiMH, and other battery technologies have made great strides in providing a reliable energy storage solution. They have found their way in many designs and worked out many of the early cost issues. But in the end, designers are faced with the same dilemma they had when using lead-acid batteries. All these technologies are based on a chemical reaction and suffer from limited life along with temperature restriction. High current demands will also directly affect their life. Therefore, for long lasting, reliable applications they posses some challenges.
On the horizon there is a very attractive technology called fuel cells. These devices have been publicized lately and are finding their way into many applications. Backup power markets have recently embraced these devices. The key issue with using fuel cells for backup power, as well as main powe,r is the startup time and the dynamic power reaction of these devices. Although they have excellent energy density they do suffer from low dynamic power. Therefore they need an augmenting technology for power assist and startup.
This opens the door for ultracapacitors. As mentioned earlier, ultracapacitors have a high energy density compared to typical capacitors and offer a long life solution compared to batteries. Therefore in applications requiring a limited energy storage and a high degree of reliability they have become very attractive.
Each energy storage solution has advantages and disadvantages. Ideally you would like to have a solution that hits the outside of this graph on all categories. No technology by itself can satisfy the entire spectrum; therefore, we have to look at the best possible combination. The best combination is the use of ultracapacitors and fuel cells. This combination results in an energy rich, reliable, and maintenance free solution that is environmental friendly as well.Fuel cells are high efficiency energy conversion devices that can operate continuously for as long as hydrogen fuel is available. They are environmentally benign and can provide a reliable source of backup power demanded by a wide variety of applications. For this reason, many fuel cell companies have looked into introducing a battery-less replacement unit. These units are typically in the 5 KW range, which seems to be in the sweet spot of stationary backup requirement.
One of the key disadvantages of fuel cells is the start up time and the lack of dynamic load response time. A typical fuel cell has a start up time of about 20 seconds with some being up to a minute to achieve full power. In order to fix this down fall fuel cells need a temporary energy storage unit as an augment.
Batteries would be the first obvious choice, but the reason to use a fuel cell has been to replace batteries. Therefore, introducing batteries in the system will defeat the main purpose of having a battery less solution. This is where ultracapacitors become an attractive choice. Since the bridge power needed is usually less than one minute, the energy density of ultracapacitors can easily satisfy this demand.
The combination of this cost reduction by the fuel cell companies along with the improvement from the ultracapacitor side should make the ultracapacitor/fuel cell combination a reliable and cost effective solution for the telecom and data center’s backup power applications.
Bobby Maher has been with Maxwell Technologies Inc. for seven years in various capacities. Prior joining Maxwell Technologies he worked at Cubic Corp. as a project engineer for several programs in the transportation industry. He holds a bachelor’s of science in electrical engineering from University of California San Diego and also holds an M.B.A. Maxwell Technologies can be reached at www.maxwell.com or by calling 858-503-3300
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