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) .
<<Power and Energy Metrics Cost of Energy Storage>>
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
.
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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