Sunday, September 1, 2013

Energy Storage Metrics: Time Dependent Parameters

Beyond discharge rate, many other metrics of energy storage devices are measured in units of time. “Response Time” is the amount of time it takes for an energy storage system to go from no discharge to full discharge. An associated metric is the “Ramp Rate”, or the rate at which a storage device can change its output once it has begun discharging. Energy storage systems are characterized by response times and ramp rates that are considered very short, measured in seconds, or at most, a few minutes. By contrast many current electricity generation systems take minutes or even hours to begin supplying electricity and often require intervals far greater than those of storage devices to change their output (EPRI, 2003).
“Charge Rate” is the amount of time it takes to recharge an energy storage system. It is very important that the system recharge sufficiently in the interval between discharges in order to reliably meet its output requirements. Many variables affect charge rate with some systems having charge rates that are faster than the system’s discharge rate (Sandia Corporation, 2012). Charge rate is not a metric shared with traditional electricity generation systems, as these systems can stay online indefinitely provided there is sufficient fuel.
Somewhat tied with the charge rate is the metric of “Energy Retention Time”. Energy storage devices dissipate energy to some extent when they are not in use. This dissipation affects how long the energy storage device can store energy (i.e. the retention time) at a specific power and energy rating before needing to be recharged (Droste-Franke, et al., 2012).
“Lifetime Discharges”, the expected lifetime of the storage technology in units of time, can be derived from the number of lifetime discharges that are possible for the device. The storage media in all devices degrades with use, forcing eventual replacement or repair. For electrochemical storage the average extent of discharge or “Discharge Depth” can be a predictor of lifetime discharges. Often a “deeper” discharge causes more degradation to the system compared to a “shallow” discharge (EPRI, 2003).

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Works Cited

Droste-Franke, B., Paal, B. P., Rehantz, C., Sauer, D. U., Schneider, J. P., Schreurs, M., et al. (2012). Balancing Renewable Electricity; Energy Storage, Demand Side Management, and Network Extension from an Interdisciplinary Perspective. Verlag Berlin Heidelberg: Springer.

EPRI. (2003). EPRI-DOE Handbook of Energy Storage for Transmission & Distribution Applications. Washington DC: EPRI, Palo Alto, CA, and the U.S. Department of Energy
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Sandia Corporation. (2012). Energy Storage Systems - Technology; Power Electronics. Retrieved January 5, 2012, from Sandia National Laboratories: http://www.sandia.gov/ess/tech_power.html



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