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EPRI Proprietary Licensed Material There is a second factor that is even more significant in reducing battery cycle efficiency but does not play a role in capacitor efficiency. This is due to the thermodynamic effect that requires a battery be recharged at a higher voltage than its discharge voltage. For example, a lead acid cell must be charged at a potential of 2.3 to 2.8 V per cell while its theoretical maximum discharge voltage is 2.1 V, and it often is below 2.0 V. This energy loss is independent of the charge rate, and it inherently reduces lead-acid battery cycle efficiency to values below those of a capacitor. Table 4 shows other characteristics of electrochemical capacitors compared with conventional lead-acid batteries. Table 4 Comparison of batteries. electrochemical capacitor characteristics to lead-acid batteries Parameter Electrochemical-Capacitors Lead-Acid Batteries 1 Discharge Time Range .1 seconds or minutes seconds Seconds to hours Recharge Time Range Seconds to Minutes Minutes to Hours Roundtrip Efficiency (1) 90-97% 80% Typical Cycle Life >100,000 cycles 2,000 cycles Operating Temp. Range, °C -50 / +50 0 to 26 Cost $/kJ Range (2) $5 – 40 $0.1 - 1 Technology Status Emerging Mature 1. Higher efficiencies occur for longer discharge/recharge cycles 2. First cost over rated discharge time range from longest (lowest cost) to shortest time (highest cost) Short-term Power Delivery Applications The required “duration” of energy storage is key to an application and determines whether the application requires a short burst or a longer-term delivery of energy. For example, utility power fault mitigation and transient stability control may require storage of only a few cycles of energy storage. Momentary interruption mitigation requires several seconds to minutes of stored energy. System support functions such as peak shaving and load leveling, where stored energy is dispatched to offset the load during peak periods, may require hours of stored energy. Rescheduling energy for cost or environmental reasons would involve 8 to 12 hours of energy storage. Capacitors are well suited to deliver short-term power, 15 seconds or less, and may be the preferred choice when the application requires higher specific power, low roundtrip losses, higher cycle life, and wider tolerance to temperature. Figure 17, a comparison of the operating range for high-energy capacitors and lead-acid battery, shows the specific power versus discharge time to the end voltage for a 250 kW system comparing highenergy capacitors and lead-acid battery. Electrochemical Capacitors 38
EPRI Proprietary Licensed Material 900 800 Power (Watts) per Kilogram 700 600 500 400 300 200 100 Ultracapacitor Lead-Acid Battery 0 1 10 100 1,000 10,000 100,000 Discharge Time to End Voltage (seconds) Figure 17 Specific power in watts per kilogram versus discharge time to end voltage in seconds. For longer-term energy storage applications (greater than 15 seconds), the lead-acid battery is lower cost and the preferred solution. The amount of energy available in the lead-acid battery for these longer durations is more than in capacitors, as shown in Figure 18, and the cost is about 10 times lower. 100 Lead-Acid Battery Energy per kG (kJ) 10 1 Ultracapacitor 0 1 10 100 1000 10000 100000 Discharge time to end voltage (s) Figure 18 Specific energy in kiloJoules per kilogram versus discharge time to end voltage in seconds. Application of Electrochemical Capacitors for Utility Power Delivery Two electrochemical capacitor applications for utility power delivery are grid stabilization in T&D and station battery replacement or reduction in substations. Both applications require short-time delivery of energy for limited time. For grid stabilization stored energy is combined with electronic control and acts to source or sink power for momentary grid support. In this application, energy storage supplements reactive compensation and can improve system stability and power through-put. The second Electrochemical Capacitors 39
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