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Energy Storage: Capacitors

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Many storage technologies have been considered in the context of utility-scale energy storage systems. These include:

Each of these technologies has its own particular strengths and operational characteristics. For example, pumped hydro is best suited for large-scale bulk electrical energy storage (if suitable geographic topology, geology and environmental conditions exist). Pumped hydro generating stations have been built capable of supplying 1800MW of electricity for four to six hours.

This ClimateTechWiki description focuses on capacitors. A capacitor is a device used for storing electrical charge. There are three distinct types of capacitors: electrostatic, electrolytic, and electrochemical. As electrochemical capacitors have the most potential for energy storage purposes, this CTW description focuses on electrochemical capacitors.

In general, capacitor systems store energy as an electric charge on two materials that are separated by a dielectric, as illustrated in Figure 1. Conventional capacitor systems function by having metallic plates separated by thin layers (10s to 100s of microns thick) of a dielectric that is usually a polymer. A variety of techniques are used in these capacitorsystems to obtain large areas.

illustration © climatetechwiki.org

Figure 1: Simple schematic of a standard capacitor with a dielectric. Source: Wikipedia, no date

Introduction top

Electrochemical capacitors go by a variety of names, such as supercapacitor, supercondenser, pseudocapacitor, electrochemical double layer capacitor, or ultracapacitor. All these terms describe an electrochemical capacitor with relatively high energy density, typically on the order of thousands of times greater than an electrolytic capacitor.

Electrochemical capacitors consist of two electrodes, a separator, electrolyte, two current collectors, and packaging. Within the electrochemical capacitor, charge is stored electrostatically, not chemically as in a battery. It has, as a dielectric, an electrolyte solvent, typically potassium hydroxide or sulfuric acid, and is actually two capacitors connected in series via the electrolyte. It is called a dual layer capacitor because of the dual layers within the structure, one at each electrode as shown in Figure 2.

illustration © climatetechwiki.org

Figure 2: Schematic of a double layer, i.e. an electrochemical, capacitor. Source: EPRI, 2002

The surface area is directly related to the amoung of capacitance. The higher the surface area, the higher the capacitance of the capacitor. The material used within electochemical capacitors for the electrodes is the element Carbon. When fabricated into felt or woven into a fabric, it makes an excellent electrode structure having both mechanical integrity and electrical conductivity. The surface area of a carbon electrode is very large at 1000 to 2000 m2/cm3.  This large surface area is the reason for very high characteristic capacity and energy density (EPRI, 2002).

Electrochemical capacitor technology has evolved through four distinct design types. Each of these has their own development time and characteristics. These types can be categorized into symmetric and asymmetric designs. Symmetric designs are designs where both positive and negative electrodes are made of the same material with approx. the same mass, and which are available with aqueous or organic electrolytes. On the other hand, asymmetric designs uses a different material for the two electrodes, with one of the electrodes having much higher capacity than the other. Currently, asymmetric designs only use aqueous electrolytes. However, asymmetric organic electrolytes are in development. As illustrated in Table 2, there are significant differences in the characteristics and performance of the four types which leads to a wide variety of products with many different possible applications (EPRI, 2002). The fourth type is not included in this table since the type has active research programs directed toward its development, but it is currently not available as a commercial product.

Table 1. Comparison of functionality of electrochemical capacitor designs. Source: EPRI, 2002 
Electrochemical capcitor type: Type I symmetric aqueous Type II Symmetric / organicType III Assymetric aqueous
Energy densityLow to moderateModerate to highHigh to very high
Power performanceHighHighLow to high
Cycle lifeHighHighHigh
Self discharge rateLowLowVery low
Low-temp dischargeExcellentGood to excellentExcellent
Packagingnon-hermeticHermeticNon-hermetic, resealable vent valve
Voltage balanceresistor/activeResistor/activeSelf limiting/active
Cell voltage< 1 V2.3/2.7 V1.4 -1.6 V

The first devices, type I, use a symmetric design with activated carbon for the positive and negative electrodes, each with approximately the same mass and similar capacitance values. The choice of electrolyte is an aqueous solution, usually high-concentration sulfuric acid or potassium hydroxide. Because of the aqueous electrolyte, operating voltages are limited to ~1.2 V per cell, with nominal ratings of 0.9 Vdc. (EPRI, 2002).

The second type is similar to the first, but has an organic rather than an aqueous electrolyte. The organic electrolyte is typically an ammonium salt which is dissolved in an organic solvent such as propylene carbonate or acetonitrile (EPRI, 2002). This electrolyte allows for operation at higher unit cell voltages. Type II products are the most common type in use today and are rated at voltages in the range of 2.3 to 2.7 V/cell, depending on the manufacturer (EPRI, 2002).

The type III design, referred to as asymmetric, is the most recent available. They are comprised of two capacitors in series, one being an electrostatic capacitor and the other a faradaic pseudocapacitor (EPRI, 2002). The electrostatic capacitor is the same as those used in the symmetric type I and II devices. The second, pseudocapacitor, relies on an electron charge transfer reaction at the electrode-electrolyte interface to store energy. This is very similar to an electrode in a rechargeable battery. (EPRI,2002)

Feasibility of technology and operational necessities top

Electrochemical Capacitor Construction

Since the surface area is directly related to the capacitance, it is essential to optimize the surface area. As mentioned, carbon electrodes used in both symmetric and asymmetric capcitors realize a high-surface-area of activated carbon. This area is on the order of 1000 square meters per gram in particulate or cloth form (EPRI, 2002). The electrode itself is in contact with a current collector. Another required material is something that prevents physical contact between the electrodes but that does allow for ion conduction (EPRI, 2002).

Another essential component of a electrochemical capacitor is the electrolyte. The main important properties in an electrolyte are conductivity and voltage stablitily. The higher, the better. Higher conductivity results in increased power performance, while higher voltage stability characteristics allow stable operation at high voltages. Conductivity and voltage stability are important for energy and power since both these measures scales as the square of the voltage. The upper voltage limit is also dependent on the solvent and salt, their levels of purity, the operating temperature, and component design life. It is expected that major improvements are possible in the electrolyte for type II products. However, little change is expected in the conductivity and voltage characteristics of the electrolyte types used in type I or type III products (EPRI, 2002). 

Also common to all electrochemical capacitors is the requirement that some pressure is applied to the cell so that the electrodes remain in contact with the separator and the current collectors. In addition, everything needs to be wetted with electrolyte. The amount of pressure required to realize this is dependent on the design and electrode form (EPRI, 2002).

Performance Features and Limitations

The performance features and limitations of electrochemical capacitors can be categorized into four groups: a) power energy relationships, b) temperature performance, c) combining cells into modules, and d) charging and discharging capabilities. 

a) power energy relationships

More energy is released at slow discharge rates than at faster rates. Losses increase and efficiency drops off significantly at high rates thus reducing the amount of energy that can be delivered in any particular application.

The asymmetric capacitor design can offer energy density advantages over symmetric designs. Another advantage of an asymmetric capacitor is that it can reliably operate above 1.2 V (the breakdown voltage of water) without gas evolution, even when employing an aqueous electrolyte. Operation above 1.2 V is possible because reaction kinetics for gas evolution are slow. Therefore available asymmetric capacitors products can operate at 1.4 to 1.6 Vdc for the same reason lead-acid batteries can operate at 2.05 V per cell with an aqueous electrolyte (EPRI, 2002).

b) temperature performance

Electrochemical capacitors provide good operating performance over a wide range of temperatures. Upper temperature limits are generally below 85 C, depending on the product. Lower temperature limits are as low as -55 C in some products. Capacitor properties, in particular leakage current, are affected by temperature (EPRI, 2002).

Self-discharge rates increase dramatically with temperature and often establish a practical upper operating temperature limit. Correspondingly, product life decreases at high temperatures since mechanisms responsible for the leakage current are often chemical side-reactions (EPRI, 2002).

Exceptional low-temperature performance can usually be expected in all electrochemical capacitors. This is possible because, unlike batteries, reaction kinetics do not limit the charge or discharge rate of an electrochemical capacitor. Instead, the limit is usually established by the electrolyte conductivity. Thus, capacitors can operate with good performance at very low temperatures (EPRI, 2002).

c) Combining cells into modules

Unlike conventional electrostatic and electrolytic capacitors, electrochemical capacitors are inherently low voltage devices. The maximum voltage of a single cell in a commercial product is 2.7 V. Thus, to meet the 600- to 800-V requirements of a utility application, hundreds of cells are series-connected and a dc-to-dc boost converter may also need to be employed (EPRI, 2002).

However, connecting cells into long series brings in a problem: the system becomes as strong as the weakest link. In other words, failure in just one cell might lead to a break down of the entire storage system (EPRI, 2002).

For long life, each cell in a series-string must remain below its maximum voltage rating under all conditions, which includes charge/discharge as well as float operation. The three key parameters affecting the cell voltage are variability in capacitance, internal resistance, and leakage current. Each of these parameters can lead to voltage imbalance among cells in a string. Thus, the construction of the cell, and its normal variability, will affect the reliability, and therefore lifetime, of a high-voltage string (EPRI, 2002).

d) Charging and discharging

A unique characteristic of a capacitive energy storage system, such as electrochemical capacitors, is that the state of charge of the system is always known as it is determined by the voltage. This is very different compared to most battery storage systems. This feature can be usefully exploited when charging and discharging a capacitor (EPRI, 2002).

A practical difference between the power source used for charging a capacitor and that used for charging a battery is the power level.  This feature results in the following characteristic of capacitors: charging of the capacitor can be performed much fasted than for a typical lead-acid battery design. Capacitors generally can be charged at any rate provided overheating does not occur. This means that higher power chargers can be effectively used for capacitors since they can be charged in seconds to minutes, not hours. Similarly, their discharge rate can be high and is only limited by the series resistance of the capacitor. However, high-rate charge and discharge, particularly with cycling, can lead to internal heating of the capacitor which, without dissipation, can lead to overtemperature conditions and system failure as described previously (EPRI, 2002).

Status of the technology and its future market potential top

While commercial applications, such as energy smoothing and backup power, have already been established for electrochemical capacitors, in general electrochemical capcitors for energy storage are in the developmental and demonstration phase, as illustrated in Table 2.

Table 2. Summary of development status of several key energy storage technologies. Source: APS, 2007
Commercial Pre-commercial prototype Demonstration stageDevelopmental phase

Pumped Hydro

Flywheels for power
quality applications at
the consumer site

CAES

Lead-Acid Battery

Ni-Cad battery

Sodium Sulfur battery

Flywheel (as load device)

Micro SMES (as load device)

Zinc- bromine battery

Flywheel (as grid device)

Vanadium Redox Battery

Electrochemical capacitor

Lithium - ion battery for grid applications

SMES (as grid device)

Electrochemical capacitors

Other advanced batteries

As can be seen in Figure xxx, capacitors do not posses a large system power rating. However, it is one of the energy storage technologies with very fast discharge times. This excludes the technology for large scale energy storage application, but it makes it especially suitable for energy applications such as power quality and system stability, i.e. they are attractive for high-power applications that require short or very short discharge durations) (Eyer and Corey, 2010). Figure 3 compares several energy storage technologies.

illustration &copy; climatetechwiki.org

Figure 3: Comparison of several key energy storage technologies in system power rating and discharge times. Source: APS, 2007

There is significant potential for this technology. A wide range of possible applications are available. For instance, Siemens has developed a storage system that utilizes capacitors to capture and store braking energy of trains. The concept has also been considerd for use in the automotive industry. A successful demonstration project of a large 1 MJ, 100 kW uninterruptible power supply system using electrochemical capacitors for bridging power was carried out by EPRI power electronics application center in 2003. However, experts argue that the technology requires more fundamental research before capacitors are ready for wide scale testing.

While capacitors are more capable than batteries for several applications, such as applications requiring very fast charge cycles (for instance, electric vehicles), they are also more expensive at the moment. In order to reduce costs, it is important to develop improved high-speed manufacturing methods for capacitor cell fabrication or the development of cheaper electrode materials (APS, 2007). 

The technology is suitable for fast-response, short duration applications, such as backup power during brief outages. The technology is excellent for stabilizing voltage and frequence. Proper management and organization might also make the technology suitable for longer time scale applications. Electrochemical capacitors have several advantages including a temperature-independent response, low maintenance and long projected lifetimes, but they suffer from relatively high cost (APS, 2007). The capabilities of several energy storage technologies is illustrated in Table 3.

Table 3. Current capabilities of energy storage technologies. Note: Due to their modular nature, multiple units of a given technology often can be networked to achieve greater power levels. Source: APS, 2007
Storage technologyPumped HydroCompressed Air energy storage (CAES) BatteriesFlywheelsSMES Capacitors
Energy storage capacity< 24 000 MWh400 - 7200 MWh< 200 MWh< 100 KWh0.6 KWh0.3 KWh
Duration of discharge at max. power level12 hours4 - 24 hrs1 -8 HrsMinutes to 1 hour10 sec10 sec
Power level< 2000 MW100 - 300 MW< 30 MW< 100 KW (each)200 kW100kW
Response time30 ms3 - 15 minutes (large scale)30 ms5 ms5 ms5 ms
Cycle efficiency0.870.80.70- 0.850.930.950.95
Lifetime40 years30 years2 - 10 yrs20 yrs40 yrs40 yrs
How the technology could contribute to socio-economic development and environmental protection top

A detailed description on the contributions of energy storage technologies to socio-economic development and environmental protection see the CAES CTW description (for utility scale benefits), and the flywheel CTW description (which concerns power quality and other small scale benefits of energy storage).

Financial requirements and costs top

Schoenung and Hasselzahn (2003) identified the lifecycle costs of several energy storage technologies, including electrochemical capacitors. Figure 4 illustrates the annual costs of electrochemical capacitors, in this Figure called supercapacitors, compared to several other energy storage technologies.

illustration &copy; climatetechwiki.org

Figure 4: Annual costs supercapacitors compared to several other fast discharge technologies (click image to enlarge) Source: Schoenung and Hasselzahn, 2003

Clearly, on the very short discharge times, electrochemical capacitors are cost effective. However, due to the characteristics of capacitors, this changes rapidly with higher required discharge times. Schoenung and Hasselzahn (2003) also identified the component costs of capacitors and compared these with several other technologies, as illustrated in Figure 5.

illustration &copy; climatetechwiki.org

Figure 5: Component costs capacitors compared to several other fast-discharge technologies for a 1 second discharge (click image to enlarge). Source: Schoenung and Hasselzahn, 2003

References top

 

APS, 2007. Challenges of Electric Energy Storage Technologies: A Report from the APS Panel on Public Affairs Committee on Energy and Environment. Document can be found online at: www.aps.org/policy/reports/popa-reports/upload/Energy_2007_Report_ElectricityStorageReport.pdf

EPRI, 2002. Handbook for Energy Storage for Transmission or Distribution Applications. Report No. 1007189. Technical Update December 2002. Document can be found at: www.epri.com

Schoenung, S., M., & Hassenzahn, W., V., 2002. Long- vs Short-Term Energy Storage Technology Analysis: A life cycle cost study. A study for the Department of Energy (DOE) Energy Storage Systems Program. Document can be found online at: http://www.ntis.gov/help/ordermethods.asp?loc=7-4-0#online

Butler, P., Miller, J. L., Taylor, P. A., 2002. Energy Storage Opportunities Analysis Phase II Final Report A Study for the DOE Energy Storage Systems Program. Document can be found online at: prod.sandia.gov/techlib/access-control.cgi/2002/021314.pdf

Eyer, J., M., Corey, G.P., 2004. Energy Storage Benefits and Market Analysis Handbook. A study for the DOE Energy Storage Systems Program. Document can be found online at: prod.sandia.gov/techlib/access-control.cgi/2004/046177.pdf 

Images:

Wikipedia, no date. Image retrieved from: http://nl.wikipedia.org/wiki/Bestand:Capacitor_schematic_with_dielectric.svg