The development of ultracapacitors suitable for high-power applications has raised much excitement and speculation throughout the electric industry. Manufacturers of adjustable-speed drives (ASDs), uninterruptible power supplies (UPSs), and fuel cells are keen on using the inherent advantages of ultracapacitors to create better products. Now utilities are looking to these devices for performance improvement and reliability in a variety of areas.
For over two decades, engineers have used ultracapacitors as low-voltage backup for memory within programmable logic controllers and computer mother boards because of their excellent energy-storage capability, high cycle life, and outstanding reliability. Today, advancements in the materials and design configurations for ultracapacitor systems have made it practical to apply these devices at much higher power levels and in voltage configurations typically up to 600VDC (see the photo).
What Is an Ultracapacitor?
Ultracapacitors are devices that store electrical energy as charge separation in porous electrodes with large surface areas. They are true capacitors in the sense that energy is stored via charge separation at the electrode-electrolyte interface, and they can withstand a large number of charge/discharge cycles without degradation. They also are similar to batteries in many respects, including the use of electrolyte and the practice of configuring various-sized cells into modules to meet the power, energy, and voltage requirements for a wide range of applications.
In ultracapacitors, the physics of capacitance have not changed. As in any capacitor, the amount of capacitance directly relates to the surface area of the electrodes. What has changed is the surface area of the electrodes. The surface area of a carbon electrode is an amazing 1000 m2 to 2000 m2 per gram.
While batteries store charges chemically, ultracapacitors store them electrostatically (see Fig. 1, on page 42). Ultracapacitors use an electrolyte solvent, typically potassium hydroxide or sulfuric acid (both aqueous electrolytes), and they consist of two capacitors connected in series via the electrolyte. They are often called double-layer capacitors because of the dual layers within the structure — one at each electrode. The combination of carbon and potassium hydroxide allows for inexpensive construction.
Some key benefits of ultracapacitors include the following:
- Highest capacitance density of any capacitor technology
- Lowest cost per farad
- Reliable, long life
- High cycle life
- Maintenance-free operation
- Environmentally safe
- Wide range of operating temperature
- High power density and good energy density
Of these features, the greater power and energy densities fill the gap between standard batteries and traditional capacitors for high-power, short-duration energy storage. The basic ultracapacitor technology, which is scalable in the cell form, is also scalable in module form, and ultracapacitor modules are now being configured for high-voltage, high-power applications. The flexibility and scalable nature of ultracapacitor technology enables a wide range of applications that directly interface with power-conversion systems, including DC-to-DC boost converters and DC-to-AC inverters.
In a time of constrained transmission corridors, increased concern for power quality, and obstacles to the construction of transmission lines, the search for innovative technologies promises significant risks and rewards. Ultracapacitors can be the solution.
EPRI PEAC engineers have been actively involved in investigating the use of ultracapacitors with utility-scale inverters. A simplified schematic is shown in Fig. 2, on page 44.
Utility applications don't always require large amounts of energy generation. Many require only a small amount of energy storage combined with advanced power electronics. The most promising applications are described below:
Transmission line stability
It's possible to increase the stability of a transmission system by adding energy storage. This serves to dampen oscillation through the successive generation and absorption of real (as opposed to reactive) power.
There is also transient stability — the stability required after a utility event (loss of substation or major line). During a transient event, achieving stability requires a substantial capability to absorb energy quickly. This is somewhat analogous to “dynamic braking” because generator turbines must be slowed. A typical specification is 100MW with 500 MJ (< 5 sec).
This is the generation capacity that a utility holds in reserve to prevent service interruptions if a generator fails. An ultracapacitor system can be built to supply power during the interruption, until quick-start diesels begin to supply power. A typical specification is 20MW to 100MW and 300 MJ to 1500 MJ.
Area and frequency control
Electricity generated by utilities must be produced and consumed at the same time. Any discrepancy between aggregate electrical energy production and aggregate energy consumption (including losses) appears in an AC system as a frequency variation. Because of its potential to react instantly, an ultracapacitor-based system would be considerably more effective than a generating plant in supplying frequency regulation. A system based on ultracapacitors can absorb or supply energy as required, freeing other generation sources from frequency regulation or tie-line control duties. A typical specification is 100MW to 1000MW and 0.1MWh to 10MWh.
Voltage regulation is a system's ability to maintain the voltage at either end of a transmission line within 5%. A voltage-source inverter is capable of regulating local-area voltage (when it's connected in shunt) or regulating power flow (when it's connected in series). It's capable of providing these two functions without a significant amount of energy storage. The need for voltage regulation and power flow control is site-specific. A typical specification is 1 Mvar for less than 15 minutes.
Power quality and uninterruptible power
Power outages can result in costly interruptions for large commercial and industrial customers. Energy storage has been used for decades in power quality applications at distribution and utilization voltages. The next logical step is power quality for wide areas at the transmission level. Examples of this already exist. Dynamic voltage restorer (DVR) technology can provide energy storage equivalent to ½ sec of peak load. Similar principles can be applied to large loads (>20MW) served from low-voltage transmissions. A typical specification is 1MW to 5MW and 10 MJ to 50 MJ.
Integration Design Issues
There are numerous key factors involved in the design and integration of ultracapacitors, as compared to lead-acid batteries and electrolytic capacitors. These factors include:
Obviously, the smaller the element, the better. However, it's important to note that the power requirement is strenuous, and current values to several hundreds of amps are common. Generally, the mechanics of the interconnect dictate the packaging. In addition, electrolytic capacitors or large snubbers will be required if the storage element is so large that more than a few inches of lead length are needed to connect the element to the power electronic switches. This would increase cost and decrease efficiency.
As market size increases, so must the availability of the energy source. Lead-acid batteries and electrolytic capacitors are readily available. Ultracapacitors are not.
Of course, lower costs are preferable to higher ones. However, it's imperative to include factors such as maintenance, cycle life, and design complexity in any cost analysis. Despite the fact that batteries are a mature technology, ultracapacitors compare favorably if you consider more than the initial cost.
Design complexity and useful voltage range
Design complexity defines the cost of integrating an ultracapacitor into a system. It also answers the question, “Is the energy in a form that is useful, or are power electronics required to convert the stored energy to the proper voltage and current levels for the application?” An important consideration is the tradeoff between depth of discharge, which determines the quality and rating of power-electronic switches, and the number of ultracapacitors placed in parallel.
Design complexity is closely related to the voltage range at which the energy is available. The cost of power electronics is driven upward with decreasing voltage and increasing current, while the cost of the energy source is generally driven upward with increasing voltage and decreasing internal resistance.
Ultracapacitors do not have flat discharge voltages like batteries. Thus, a balance is needed between the cost of power electronics and the size of the ultracapacitor bank.
There is a cost associated with the charge/discharge efficiency, which is a function of the equivalent series resistance (ESR) and charge acceptance of the ultracapacitor. A lead-acid battery typically requires 30% more energy to charge than to store. Capacitors require much less, with electrolytic being the best. Efficiency is a function of rate of charge. The faster the rate, the lower the efficiency.
Cycle life defines the number of charges/discharges that an ultracapacitor can withstand. In lead-acid battery systems, cycle life is closely related to depth of discharge. This is not so with ultracapacitors. Therefore, the inherent energy advantage in lead-acid batteries is, in part, offset if the application requires a large number of cycles.
The self-discharge rate describes the rate at which energy is lost (within an ultracapacitor) as a result of parasitic losses. A high self-discharge rate will increase design complexity by requiring an additional circuit to maintain a typically small but constant flow of energy to the element. Discharge rate varies by the type of technology used within an ultracapacitor, with the symmetrical (carbon/carbon) systems having a higher leakage current than the asymmetrical (metal/carbon) systems.
Cells are placed electrically in series to produce the voltage required for a particular application. During discharge, depending on the condition of each individual cell, an imbalance between cells can occur. If this situation remains uncorrected over time, it's possible for a single cell to completely discharge. Then, instead of producing energy, the cell accepts an excessive amount of energy, eventually causing the entire string to fail. The equalization process ensures that all cells contain approximately the same amount of energy. Having to equalize the ultracapacitor elements increases design complexity.
Excess discharge to 0V
During maintenance, it is safest to eliminate all potential sources of energy. Batteries and asymmetrical ultracapacitors cannot be completely discharged without damage. Electrolytic capacitors and symmetrical ultracapacitors can be completely discharged.
Ultracapacitors can be applied to utility applications. From improving area/frequency control to improving voltage regulation, ultracapacitor-based systems can provide and absorb real and reactive power. The appropriate configuration will depend on the application and site. Ultracapacitors are available today with the necessary power and energy characteristics to meet the application requirements.
EPRI PEAC Corp. is currently in the process of gathering more information about the design and application of systems sized for multimegawatt applications. Preliminary analysis indicates good performance, small footprint, and high reliability.
Thomas D. Geist is a senior power quality engineer with EPRI PEAC Corp. in Knoxville, Tenn. You can reach him at [email protected].
Douglas Dorr is the business development manager at EPRI PEAC Corp. You can reach him at [email protected].