Handbook of Energy Storage for Transmission or ... - W2agz.com

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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