Electric double-layer capacitors (EDLCs), better known as ultracapacitors or supercapacitors, are now a primary energy storage option for design engineers. During the past 10 years, we have seen the emergence of this technology, especially with the move toward “green” and more energy-efficient options. These devices have an advantage over batteries, particularly in applications requiring long life or operation in low temperatures.
The basic construction of double-layer capacitors is similar to conventional capacitors, with the main difference being the material used in the manufacturing of the electrode. Ultracapacitors use a carbon-based electrode structure, which provides for a large surface area. The combination of this large surface area with a small charge separation gives these devices capacitances of 3000 F/m2. Unlike batteries, ultracapacitors do not generate energy, but rather store it, so there is no chemical reaction involved and the energy storage is based on charge separation.
With the emergence and market opportunity for this new energy storage technology, there is a pronounced expansion of manufacturers globally in this area. In addition to known market leaders like Nesscap and Maxwell Technologies, which concentrate primarily on the ultracapacitor technology, larger multi-national companies such as Panasonic, WIMA, Batscap (Bollore), Nippon Chemi-con, and LS Cable (LG) are making their way into this arena. This validates the anticipated level of market demand for these devices.
One of the biggest challenges for using ultracapacitors is their low cell voltage. These devices typically are rated from 2.5 to 2.7 V per cell. For most applications, multiple cells need to be placed in series to achieve a more useful higher voltage range. Several factors need to be addressed when placing cells in series:
Cell balancing: When placing any capacitors in series, especially ultracapacitors, designers need to ensure that cells are charged to the same voltage. This will prevent any overvoltage condition and allow for longer-life operation. The key contributor to this imbalance is the difference in the leakage current of the cells. One cell in a series string with a higher leakage current will force itself to a lower voltage, increasing the voltage on other cells. To resolve this, most users employ an active or passive circuit in parallel to each cell.
Charging: One of the major technical attributes of ultracapacitors is their low internal resistance, which contributes to their high power capability. This is seen both on the charge and discharge side. As attractive as this can be, it also causes some issues on the charging side. With the low internal resistance of the cells when placed across a power source, the ultracapacitors will look like a short and try to pull a high rate of current from the source. To address this issue, a current limit will need to be placed between the source and the ultracapacitor.
Voltage: Unlike batteries, ultracapacitors will drop their voltage as they deliver current, very similar to conventional capacitors. Some applications cannot handle a wide voltage range and will need to have a constant voltage available. For these applications, a dc-dc converter or a boost circuit will need to be installed between the ultracapacitor and the application.
Semiconductor Market Opportunity
To address these challenges, designers have to come up with unique circuits to balance cell voltages, reduce charge current, and smooth out the discharge voltage. Until recently, there has been no off-the-shelf IC available to perform any of these operations in support of the ultracapacitor market. Now, several semiconductor manufacturers offer ultracapacitor-specific circuits.
Linear Tech’s 150-mA LTC3225 ultracapacitor charger includes a cell balancing function. Although this is a good starting point, the industry needs more off-the-shelf components to address the needs of this sector. For example, the LTC3225 limits the charge current to 150 mA. When we look at the market demand for ultracapacitors, we see a large market for 10- to 400-F cells in high-power applications similar to automatic meter readers (AMRs), solid-state drives (SSDs), and other consumer and industrial systems. For many of these applications, a higher charge current is needed along with fast multiple cell balancing.
FullPower Inc. expects the market for ultracapacitors to grow to $250 million annually by 2010 with a growth rate of 10% to 15% per year. Controlling this energy storage technology represents a large opportunity for semiconductor companies and IC designers interested in participating in the green revolution. Providing standard energy storage management circuits to address these issues will lead to faster adoption of the technologies, improve design time, and ultimately benefit ultracapacitor suppliers and semiconductor manufacturers.
The next step in the evolution of this industry is the combination of ultracapacitors with various battery chemistries to achieve the balance of stored energy and delivered power as needed for critical applications. As energy generation technologies such as wind, solar, geothermal, tidal, and others become more available, the requirement to efficiently store and then deliver power to the user will become a major technological challenge.
The cyclical nature of many renewable energy sources creates new problems at the user end. Wind peaks and ebbs, and solar power cycles vary throughout the day and can be subject to the effects of clouds and other weather events. Users need a reliable and constant supply of power regardless of the conditions at the generator. The semiconductor industry can play a major role in managing the use of new battery and ultracapacitor technologies to ensure the most efficient use of our available resources.
Regardless of the energy source, our environment demands that we make the most efficient use of available energy. Great strides are being made through government, university, and commercial ventures to develop the chemistries and technologies that will improve both the generation and distribution of energy. It is now time for the semiconductor industry to contribute the logic and management products that will move the ultracapacitor and hybrid battery technologies into the mainstream of clean and green technology deployment.